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Title: The Cambridge natural history, Vol. 07 (of 10)
Editor: S. F. Harmer
Author: George Albert Boulenger
Thomas William Bridge
William Abbott Herdman
Editor: Sir A. E. Shipley
Release date: May 26, 2024 [eBook #73702]
Language: English
Original publication: London: Macmillan and Co, 1904
Credits: Keith Edkins, Peter Becker and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)
*** START OF THE PROJECT GUTENBERG EBOOK THE CAMBRIDGE NATURAL HISTORY, VOL. 07 (OF 10) ***
Transcriber's note: Text enclosed by underscores is in italics (_italics_).
A single underscore introduces a subscript (CO_2), and a caret a
superscript (B^1).
Page numbers enclosed by curly braces (for example: {25}) have been
incorporated to facilitate the use of the Alphabetical Index and other page
references in the text.
* * * * *
THE
CAMBRIDGE NATURAL HISTORY
EDITED BY
S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge;
Superintendent of the University Museum of Zoology
AND
A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge;
University Lecturer on the Morphology of Invertebrates
VOLUME VII
[Illustration]
HEMICHORDATA
By S. F. HARMER, SC.D., F.R.S., Fellow of King's College, Cambridge.
ASCIDIANS AND AMPHIOXUS
By W. A. HERDMAN, D.SC. (Edinb.), F.R.S., Professor of Natural History in
the University of Liverpool.
FISHES (Exclusive of the Systematic Account of Teleostei)
By T. W. BRIDGE, SC.D., F.R.S., Trinity College, Cambridge; Mason
Professor of Zoology and Comparative Anatomy in the University of
Birmingham.
FISHES (Systematic Account of Teleostei)
By G. A. BOULENGER, F.R.S., of the British Museum (Natural History).
London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1904
_All rights reserved_
_Third Fisherman_.—Master, I marvel how the fishes live in the sea.
_First Fisherman_.—Why, as men do a-land,—the great ones eat up the
little ones.
_Pericles_, Act II. Scene i.
{v}PREFACE
Owing to unforeseen circumstances, not unconnected with the foundation of a
new University, the publication of this volume has been unduly delayed.
Some parts of the work have actually been in type for more than four years;
and although the authors have made every effort to keep them up to date,
the arrangement is naturally not quite what it might have been if the
articles had been written immediately before publication.
In view of the novelty of Mr. Boulenger's classification of the
Teleosteans, and of the fact that several independent workers have been
occupying themselves with the subject during the last year or two, it is
fair to state that this part of the volume was completed in 1902. Professor
Herdman's account of the Ascidians was ready for publication two years
earlier.
Professor Bridge wishes to express his best thanks to Dr. R. H. Traquair.
F.R.S., for his kindness in reading the proofs of the pages which deal with
the fossil Crossopterygii, Chondrostei, Holostei and Dipnoi, and for much
helpful advice and criticism; to Mr. G. A. Boulenger, F.R.S., for his
valuable and suggestive criticism on certain points; and to Mr. Edwin
Wilson, for the care which he has taken in the preparation of the figures.
_July 1904_.
{vii}CONTENTS
PAGE
PREFACE v
SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK xi
HEMICHORDATA
CHAPTER I
CHORDATA AND VERTEBRATA-HEMICHORDATA-ENTEROPNEUSTA-EXTERNAL
CHARACTERS AND HABITS-STRUCTURE-GENERA-DEVELOPMENT-PTEROBRANCHIA-
_CEPHALODISCUS_ AND _RHABDOPLEURA_-PHORONIDEA-_PHORONIS_ AND
ACTINOTROCHA-AFFINITIES OF THE HEMICHORDATA 3
TUNICATA
CHAPTER II
INTRODUCTION-OUTLINE OF HISTORY-STRUCTURE OF A TYPICAL ASCIDIAN-
EMBRYOLOGY AND LIFE-HISTORY 35
CHAPTER III
CLASSIFICATION: LARVACEA-APPENDICULARIANS-STRUCTURE, ETC.-ASCIDIACEA-
SIMPLE ASCIDIANS-SPECIFIC CHARACTERS-COMPOUND ASCIDIANS-GEMMATION-
MEROSOMATA-HOLOSOMATA-PYROSOMATIDAE-THALIACEA-DOLIOLIDAE-
SALPIDAE-GENERAL CONCLUSIONS-PHYLOGENY 63
CEPHALOCHORDATA
CHAPTER IV
INTRODUCTION-GENERAL CHARACTERS-ANATOMY OF AMPHIOXUS-EMBRYOLOGY AND
LIFE-HISTORY-CLASSIFICATION OF CEPHALOCHORDATA-SPECIES AND
DISTRIBUTION 112
CYCLOSTOMATA AND FISHES {viii}
CHAPTER V
THE SYSTEMATIC POSITION AND CLASSIFICATION OF FISHES 141
CHAPTER VI
EXTERNAL CHARACTERS OF CYCLOSTOMATA AND OF FISHES: EXTERNAL CHARACTERS-
COLORATION-POISON GLANDS AND POISON SPINES-PHOSPHORESCENT ORGANS 150
CHAPTER VII
THE SKIN AND SCALES 182
CHAPTER VIII
THE SKELETON 193
CHAPTER IX
THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS - 247
CHAPTER X
THE RESPIRATORY ORGANS 277
CHAPTER XI
THE AIR-BLADDER 297
CHAPTER XII
THE VASCULAR SYSTEM, THE LYMPHATICS, AND THE BLOOD-GLANDS 313
CHAPTER XIII
MUSCULAR SYSTEM-LOCOMOTION-SOUND-PRODUCING ORGANS-ELECTRIC ORGANS 349
CHAPTER XIV
NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE 367
CHAPTER XV {ix}
THE KIDNEYS AND THE REPRODUCTIVE ORGANS-BREEDING 397
CHAPTER XVI
CYCLOSTOMATA (SYSTEMATIC) 421
CHAPTER XVII
ELASMOBRANCHII: GENERAL CHARACTERS-PLEUROPTERYGII-ICHTHYOTOMI-
ACANTHODEI-PLAGIOSTOMI-SELACHII-BATOIDEI-HOLOCEPHALI 431
CHAPTER XVIII
TELEOSTOMI: GENERAL CHARACTERS-CROSSOPTERYGII-CHONDROSTEI-HOLOSTEI 475
CHAPTER XIX
DIPNEUSTI 505
CHAPTER XX
APPENDIX TO THE FISHES: PALAEOSPONDYLIDAE-OSTRACODERMI-HETEROSTRACI-
OSTEOSTRACI-ANASPIDA-ANTIARCHI-ARTHRODIRA 521
CHAPTER XXI
TELEOSTEI: GENERAL CHARACTERS-MALACOPTERYGII-OSTARIOPHYSI 541
CHAPTER XXII
TELEOSTEI (_CONTINUED_): SYMBRANCHII-APODES-HAPLOMI-HETEROMI-
CATOSTEOMI-PERCESOCES-ANACANTHINI 597
CHAPTER XXIII
TELEOSTEI (_CONTINUED_): ACANTHOPTERYGII-OPISTHOMI-PEDICULATI-
PLECTOGNATHI 650
INDEX 729
{xi}SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK
_The names of extinct groups are printed in italics_.
CHORDATA (p. 3).
I. HEMICHORDATA (p. 3).
Order. Family.
–––––– –––––––
ENTEROPNEUSTA (p. 5)
Glandicipitidae (p. 17).
Ptychoderidae (p. 17).
Harrimaniidae (p. 17).
PTEROBRANCHIA (p. 21)
PHORONIDEA (p. 27)
II. UROCHORDATA = TUNICATA (pp. 4, 35, 63).
Order. Sub–Order. Family. Sub–Family.
–––––– –––––––––– ––––––– –––––––––––
LARVACEA (p. 64)
Kowalevskiidae (p. 68).
Appendiculariidae (p. 68).
ASCIDIACEA (p. 70)
ASCIDIAE SIMPLICES (p. 71).
Clavelinidae (p. 71).
Ascidiidae (p. 72).
Hypobythiinae (p. 72).
Ascidiinae (p. 72).
Corellinae (p. 73).
Cynthiidae (p. 74).
Styelinae (p. 74).
Cynthiinae (p. 75).
Bolteninae (p. 75).
Molgulidae (p. 77).
ASCIDIAE COMPOSITAE(p. 80)
MEROSOMATA (p. 85)
Distomatidae (p. 85).
Coelocormidae (p. 86).
Didemnidae (p. 86).
Diplosomatidae (p. 87).
Polyclinidae (p. 87).
HOLOSOMATA (p. 88)
Botryllidae (p. 88).
Polystyelidae (p. 89).
ASCIDIAE LUCIAE (p. 90)
Pyrosomatidae (p. 91).
THALIACEA (p. 95)
CYCLOMYARIA (p. 95).
Doliolidae (p. 96).
HEMIMYARIA (p. 101).
Salpidae (p. 101).
Octacnemidae (p. 108).
III. CEPHALOCHORDATA (pp. 4, 112).
Branchiostomatidae (p. 137).
IV. CRANIATA (pp. 4, 141).
CLASS––CYCLOSTOMATA (pp. 145, 150, 421).
Sub–Class. Order. Sub–Order. Family. Sub–Family.
–––––––––– –––––– –––––––––– ––––––– –––––––––––
MYXINOIDES (p. 421)
Myxinidae (p. 422).
Bdellostomatidae (p. 423).
PETROMYZONTES (p. 425)
Petromyzontidae (p. 426).
CLASS––PISCES (pp. 145, 431).
ELASMOBRANCHII(p. 431)
_PLEUROPTERYGII_ (p. 436)
_Cladoselachidae_ (p. 438).
_ICHTHYOTOMI_ (p. 440)
_Pleuracanthidae_ (p. 440).
_ACANTHODEI_ (p. 440)
_Diplacanthidae_ (p. 441).
_Acanthodidae_ (p. 441).
PLAGIOSTOMI (p. 442)
SELACHII (p. 442)
Notidanidae (p. 442).
Chlamydoselachidae (p. 443).
Heterodontidae (p. 444).
_Cochliodontidae_ (p. 445).
_Psammodontidae_ (p. 446).
_Petalodontidae_ (p. 446).
Scylliidae (p. 446).
Carchariidae (p. 448).
Sphyrnidae (p. 449).
Lamnidae (p. 450).
Cetorhinidae (p. 453).
Rhinodontidae (p. 454).
Spinacidae (p. 454).
Rhinidae (p. 456).
Pristiophoridae (p. 457).
BATOIDEI (p. 457)
Pristidae (p. 459).
Rhinobatidae (p. 460).
Raiidae (p. 461).
_Tamiobatidae_ (p. 462).
Torpedinidae (p. 462).
Trygonidae (p. 464).
Myliobatidae (p. 465).
HOLOCEPHALI (p. 466)
_Ptyctodontidae_ (p. 468).
_Squaloraiidae_ (p. 468).
_Myriacanthidae_ (p. 468).
Chimaeridae (p. 468).
TELEOSTOMI(p. 475)
CROSSOPTERYGII (p. 476)
_OSTEOLEPIDA_ (p. 437)
_Osteolepidae_ (p. 477).
_Rhizodontidae_ (p. 478).
_Holoptychidae_ (p. 479).
_Coelacanthidae_ (p. 480).
CLADISTIA (p. 481)
Polypteridae (p. 481).
CHONDROSTEI (p. 485)
_Palaeoniscidae_ (p. 486).
_Platysomidae_ (p. 487).
_Belonorhynchidae_ (p. 488).
_Catopteridae_ (p. 488).
_Chondrosteidae_ (p. 489).
Polyodontidae (p. 491).
Acipenseridae (p. 492).
HOLOSTEI (p. 495)
_Semionotidae_ (p. 497).
_Macrosemiidae_ (p. 498).
_Pycnodontidae_ (p. 498).
_Eugnathidae_ (p. 498).
Amiidae (p. 499).
_Pachycormidae_ (p. 501).
_Aspidorhynchidae_ (p. 502).
Lepidosteidae (p. 502).
TELEOSTEI (pp. 504, 541)
MALACOPTERYGII (p. 543)
_Pholidophoridae_ (p. 545).
_Archaeomaenidae_ (p. 545).
_Oligopleuridae_ (p. 545).
_Leptolepididae_ (p. 546).
Elopidae (p. 546).
Albulidae (p. 547).
Mormyridae (p. 549)
Mormyrinae (p. 551).
Gymnarchinae (p. 551)
Hyodontidae (p. 552).
Notopteridae (p. 554).
Osteoglossidae (p. 555).
Pantodontidae (p. 558).
_Ctenothrissidae_ (p. 559).
Phractolaemidae (p. 560).
_Saurodontidae_ (p. 561).
Chirocentridae (p. 561).
Clupeidae (p. 562)
Thrissopatrinae (p. 562).
Engraulinae (p. 563).
Clupeinae (p. 563).
Chaninae (p. 563).
Salmonidae (p. 565).
[_Pachyrhizodontidae_ (p. 569).]
Alepocephalidae (p. 569).
Stomiatidae (p. 570)
Chauliodontinae (p. 571).
Sternoptychinae (p. 571).
Stomiatinae (p. 571).
Gonorhynchidae (p. 572).
Cromeriidae (p. 573).
OSTARIOPHYSI (p. 573)
Characinidae (p. 575)
Erythrininae (p. 575).
Hydrocyoninae (p. 575).
Serrasalmoninae (p. 576).
Ichthyoborinae (p. 576).
Xiphostominae (p. 576).
Anostominae (p. 576).
Hemiodontinae (p. 576).
Distichodontinae
(p. 576).
Citharininae (p. 576).
Gymnotidae (p. 579).
Cyprinidae (p. 581)
Catostominae (p. 581).
Cyprininae (p. 582).
Cobitidinae (p. 582).
Homalopterinae (p. 582).
Siluridae (p. 586)
Clariinae (p. 588).
Silurinae (p. 588).
Bagrinae (p. 588).
Doradinae (p. 588).
Malopterurinae (p. 588).
Callichthyinae (p. 588).
Hypophthalminae (p. 589).
Trichomycterinae
(p. 589).
Loricariidae (p. 594)
Arginae (p. 595).
Loricariinae (p. 595).
Aspredinidae (p. 596).
SYMBRANCHII (p. 597)
Symbranchidae (p. 597).
Amphipnoidae (p. 598).
APODES (p. 599)
Anguillidae (p. 600).
Nemichthyidae (p. 603).
Synaphobranchidae (p. 603).
Saccopharyngidae (p. 603).
Muraenidae (p. 604).
HAPLOMI (p. 605)
Galaxiidae (p. 607).
Haplochitonidae (p. 608).
_Enchodontidae_ (p. 608).
Esocidae (p. 609).
Dalliidae (p. 610).
Scopelidae (p. 611).
Alepidosauridae (p. 614).
Cetomimidae (p. 614).
_Chirothricidae_ (p. 615).
Kneriidae (p. 615).
Cyprinodontidae (p. 616).
Amblyopsidae (p. 618).
Stephanoberycidae (p. 619).
Percopsidae (p. 620).
HETEROMI (p. 621)
_Dercetidae_ (p. 623).
Halosauridae (p. 623).
Lipogenyidae (p. 624).
Notacanthidae (p. 624).
Fierasferidae (p. 625).
CATOSTEOMI (p. 626)
Gastrosteidae (p. 629)
Aulorhynchidae (p. 631)
_Protosyngnathidae_ (p. 631)
Aulostomatidae (p. 632)
Fistulariidae (p. 632)
Centriscidae (p. 633)
Amphisilidae (p. 633)
= Hemibranchii (p. 627).
Solenostomidae (p. 633)
Syngnathidae (p. 634)
= Lophobranchii (p. 628).
Pegasidae (p. 635)
= Hypostomides (p. 628).
PERCESOCES (p. 636)
Scombresocidae (p. 637).
Ammodytidae (p. 639).
Atherinidae (p. 639).
Mugilidae (p. 640).
Polynemidae (p. 640).
Chiasmodontidae (p. 641).
Sphyraenidae (p. 642).
Tetragonuridae (p. 642).
Stromateidae (p. 643).
Icosteidae (p. 644).
Ophiocephalidae (p. 644).
Anabantidae (p. 645).
ANACANTHINI (p. 646)
Macruridae (p. 647).
Gadidae (p. 647).
Muraenolepididae (p. 649).
Sub–Order. Division. Family.
–––––––––– ––––––––– –––––––
ACANTHOPTERYGII (p. 650)
PERCIFORMES (p. 652)
Berycidae (p. 655).
Monocentridae (p. 656).
Pempheridae (p. 656).
Centrarchidae (p. 657).
Cyphosidae (p. 657).
Lobotidae (p. 658).
Toxotidae (p. 658).
Nandidae (p. 658).
Percidae (p. 658).
Acropomatidae (p. 659).
Serranidae (p. 659)
Subfamilies:
* Serraninae (p. 659).
* Grammistinae (p. 660).
* Priacanthinae (p. 660).
* Centropominae (p. 660).
* Pomatominae (p. 660).
* Ambassinae (p. 660).
* Chilodipterinae
(p. 660).
* Lutjaninae (p. 660).
* Cirrhitinae (p. 660).
* Pentacerotinae
(p. 660).
Anomalopidae (p. 660).
Pseudochromididae
(p. 661).
Cepolidae (p. 661).
Hoplognathidae (p. 662).
Sillaginidae (p. 662).
Sciaenidae (p. 663).
Gerridae (p. 663).
Lactariidae (p. 663).
Trichodontidae (p. 663).
Latrididae (p. 663).
Haplodactylidae (p. 664).
Pristipomatidae (p. 664).
Sparidae (p. 664).
Mullidae (p. 665).
Scorpididae (p. 666).
Caproidae (p. 666).
Chaetodontidae (p. 667).
Drepanidae (p. 668).
Acanthuridae (p. 668).
Teuthididae (p. 668).
Osphromenidae (p. 669).
Embiotocidae (p. 670).
Cichlidae (p. 670).
Pomacentridae (p. 672).
Labridae (p. 673).
Scaridae (p. 674).
SCOMBRIFORMES (p. 675)
Carangidae (p. 677).
Rhachicentridae (p. 677).
Scombridae (p. 678).
Trichiuridae (p. 679).
Histiophoridae (p. 679).
_Palaeorhynchidae_
(p. 680).
Xiphiidae (p. 681).
Luvaridae (p. 681).
Coryphaenidae (p. 681).
Bramidae (p. 682).
ZEORHOMBI (p. 682)
Zeidae (p. 683).
_Amphistiidae_ (p. 684).
Pleuronectidae (p. 684).
KURTIFORMES(p. 687)
Kurtidae (p. 687).
GOBIIFORMES(p. 688)
Gobiidae (p. 689).
DISCOCEPHALI(p. 691)
Echeneididae (p. 691).
SCLEROPAREI (p. 692)
Scorpaenidae (p. 694).
Hexagrammidae (p. 696).
Comephoridae (p. 696).
Rhamphocottidae (p. 697).
Cottidae (p. 697).
Cyclopteridae (p. 698).
Platycephalidae (p. 699).
Hoplichthyidae (p. 699).
Agonidae (p. 700).
Triglidae (p. 700).
Dactylopteridae (p. 701).
JUGULARES (p. 702)
Trachinidae (p. 704).
Percophiidae (p. 705).
Leptoscopidae (p. 705).
Nototheniidae (p. 705).
Uranoscopidae (p. 706).
Trichonotidae (p. 706).
Callionymidae (p. 706).
Gobiesocidae (p. 707).
Blenniidae (p. 709).
Batrachidae (p. 710).
Pholididae (p. 711).
Zoarcidae (p. 712).
Congrogadidae (p. 713).
Ophidiidae (p. 713).
Podatelidae (p. 713).
TAENIOSOMI (p. 714)
Trachypteridae (p. 715).
Lophotidae (p. 716).
OPISTHOMI (p. 716)
Mastacembelidae (p. 716).
PEDICULATI (p. 717)
Lophiidae (p. 718).
Ceratiidae (p. 719).
Antennariidae (p. 720).
Gigantactinidae (p. 720).
Malthidae (p. 720).
PLECTOGNATHI (p. 721)
SCLERODERMI (p. 722)
Triacanthidae (p. 722).
Triodontidae (p. 723).
Balistidae (p. 723).
Ostraconiidae (p. 722).
GYMNODONTES (p. 725)
Tetrodontidae (p. 726).
Diodontidae (p. 726).
Molidae (p. 726).
DIPNEUSTI = DIPNOI (p. 505)
_Ctenodontidae_ (p. 505).
_Uronemidae_ (p. 507).
Ceratodontidae (p. 507).
Lepidosirenidae (p. 511).
OF UNCERTAIN POSITION
Order. Family.
–––––– –––––––
_PALAEOSPONDYLIDAE_ (p. 521).
_OSTRACODERMI_ (p. 522) _HETEROSTRACI_ (p. 524) _Coelolepidae_ (p. 524).
_Drepanaspidae_ (p. 525).
_Psammosteidae_ (p. 526).
_Pteraspidae_ (p. 527).
_OSTEOSTRACI_ (p. 527) _Ateleaspidae_ (p. 528).
_Cephalaspidae_ (p. 528).
_Tremataspidae_ (p. 530).
_ANASPIDA_ (p. 531) _Birkeniidae_ (p. 531).
_ANTIARCHI_ (p. 532) _Asterolepidae_ (p. 534).
_ARTHRODIRA_ (p. 535) _Coccosteidae_ (p. 536).
HEMICHORDATA
BY
SIDNEY F. HARMER, Sc.D., F.R.S.
Fellow of King's College, Cambridge.
{3}CHAPTER I
HEMICHORDATA
CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA—EXTERNAL CHARACTERS AND
HABITS—STRUCTURE—GENERA—DEVELOPMENT—PTEROBRANCHIA—_CEPHALODISCUS_ AND
_RHABDOPLEURA_—PHORONIDEA—_PHORONIS_ AND ACTINOTROCHA—AFFINITIES OF THE
HEMICHORDATA.
The Hemichordata, a marine group which includes the worm-like
Balanoglossus, owe much of their interest to the fact that they are
believed by many zoologists to be related to the lower Vertebrates. This
view is one of a number of mutually exclusive hypotheses, which seek to
derive Vertebrate animals from various Invertebrate ancestors. It is
supported by many striking resemblances between Balanoglossus and the
lowest forms which are by common consent regarded as belonging to the
Vertebrate alliance; but it must be distinctly understood that
Balanoglossus is at most the much modified modern representative of extinct
forms which were also the ancestors of Vertebrates.
The axis of the backbone of all Vertebrates is formed by an elastic rod
known as the "notochord" (Figs. 72, 115), which lasts throughout life in
some of the lowest forms, but in the higher forms appears only in the
embryo. The universal occurrence of this structure has been regarded as the
most important characteristic of the Vertebrata and their allies, which are
accordingly grouped together in the Phylum CHORDATA. The members of this
Phylum are further distinguished from other animals by several important
features. Of these one of the most important appears to be the existence of
lateral {4}outgrowths of the pharynx, which unite with the skin of the neck
and form a series of perforations leading to the exterior. These structures
are the gill-slits, and in the Fishes their walls give rise to vascular
folds or gills. With the assumption of a terrestrial life, the higher
Vertebrates lost their gills as functional organs, respiration being then
performed by entirely different organs, the lungs. But even in these cases,
the gill-slits appear in the embryo; and remains of one pair can usually be
recognised in the adult state of even the highest Vertebrates. Another
fundamental characteristic of the Chordata is given by the central nervous
system, which lies entirely above the alimentary canal, just dorsal to the
notochord. Not only does this position of the nerve-centres distinguish the
Chordata from Invertebrates, but a further point of difference is found in
the development. While in Invertebrates the ventral nerve-cord is formed as
a thickening of the ectoderm or outermost layer of the embryo, in the
Chordata the nervous system is usually formed as a longitudinal groove
running medianly along the back of the embryo. This groove closes to form a
tube of nervous matter, the cavity of which always persists throughout life
as the "central canal" of the spinal chord and its anterior prolongation
which constitutes the "ventricles" of the brain.
Although the animals which are considered in this chapter are not admitted
by all zoologists to be related to the Vertebrates, there can be no
question that their respiratory organs closely resemble typical gill-slits.
Since, moreover, they possess structures which can be regarded, with a fair
amount of probability, as agreeing in essential respects with the notochord
and the tubular dorsal nervous system of Vertebrates, it appears
justifiable to include them in the Chordata, which are then subdivided into
(1) HEMICHORDATA, in which a "notochord" occurs in the anterior end of the
body only; (2) UROCHORDATA (Tunicata or Ascidians), in which the notochord
is restricted to the tail; (3) CEPHALOCHORDATA (Amphioxus), in which the
notochord extends the entire length of the body _and of the head_; (4)
CRANIATA, in which a brain is developed as an enlargement of the central
nervous system, the notochord does not extend farther forward than the
middle of the brain, and a vertebral column is present. These last are thus
usually known as Vertebrata, although in distinguishing an "Invertebrate"
from a "Vertebrate" it is more {5}logical to regard all Chordata as
Vertebrates, since the Invertebrata are in no sense a natural group with
common characteristics, their union under one name merely implying that
they have no close affinity to the Vertebrates. It is often convenient in
practice to divide animals into Vertebrates and Invertebrates, but from a
zoological point of view a division of the animal kingdom into Molluscs and
Non-Molluscs would have as much or as little significance.
The sub-phylum Hemichordata[1] consists of the Orders:—(I.)
ENTEROPNEUSTA,[2] including Balanoglossus (Fig. 1); (II.) PTEROBRANCHIA,[3]
represented by the genera _Cephalodiscus_ (Fig. 9) and _Rhabdopleura_ (Fig.
12). To these should possibly be added (III.) PHORONIDEA, for the reception
of _Phoronis_ (Fig. 13).
ORDER I. ENTEROPNEUSTA.
_Worm-like Hemichordata, with numerous gill-slits, a straight intestine,
and a terminal anus. Proboscis separated by a narrow stalk from the simple
ring-shaped collar, which is succeeded by an elongated trunk_.
The structure of Balanoglossus, formerly the sole genus belonging to this
Order, but now divided[4] into the genera _Ptychodera_, _Balanoglossus_,
_Glossobalanus_, _Glandiceps_, _Spengelia_, _Schizocardium_, _Harrimania_,
_Dolichoglossus_, and _Stereobalanus_, has of recent years formed the
subject of elaborate investigations by Spengel,[5] Bateson,[6] and
Willey.[7] More than thirty species are known, ranging in length from 25
mm.[8] (_Pt. bahamensis_) to 2500 mm. (_B. gigas_), and for the most part
inhabiting shallow water; _Glossobalanus sarniensis_ occurring between
tide-marks in the Channel Islands. _Glandiceps talaboti_ has, however, been
dredged near Marseilles from as much as 190 fathoms, while _G. abyssicola_
was found by the "Challenger" at a depth of 2500 fathoms, off the West
Coast of Africa.
{6}[Illustration: FIG. 1.—Forms of Balanoglossus. A, _Balanoglossus
clavigerus_, Eschsch., Naples, × ½; B, _Glandiceps hacksi_, Mar.
(incomplete), Japan, × 1; C, _Schizocardium brasiliense_, Speng., Rio de
Janeiro, × 1; D, _Dolichoglossus kowalevskii_, A.Ag., Chesapeake Bay, ×
1. _a_, Anus; _ab_, abdominal and caudal regions; _b_, branchial region;
_c_, collar; _g_, genital region; _g.p_, gill-pore; _g.w_, genital wing;
_h_, hepatic region; _m_, position of mouth; _p_, proboscis; _t_, trunk.
(A, B, and C from Spengel; D from Bateson.)]
_Balanoglossus_, the largest genus now recognised by Spengel, appears to be
practically world-wide in its distribution; _Schizocardium_ is recorded
from both sides of S. America; _Glandiceps_ from the Atlantic, the
Mediterranean, Japan, and the Indian Ocean; _Spengelia_ from the South
Pacific; and other species from the White Sea to New Zealand. The habitat
is usually sand or gravelly sand, in which the animal forms a kind of tube
by means of the abundant mucus secreted by its skin. _Dolichoglossus
kowalevskii_ (Fig. 1, D), according to Bateson,[9] lives between tide-marks
at a {7}depth of about eight inches. The greater part of the body is coiled
in an even, cork-screw-like spiral, while the anterior end, including the
front part of the branchial region, is maintained in a vertical position.
The posterior end is also kept upright, and can be moved up and down in a
vertical shaft opening on the surface, thus enabling the animal to eject
the undigested sand from its anus.
The coloration of Balanoglossus is often brilliant. That of _D.
kowalevskii_[10] is as follows:—The "proboscis" (cf. Fig. 1, B, _p_) is
yellowish white; the "collar" (_c_) is brilliant red-orange (especially in
males), with a white ring posteriorly; the "trunk" (_t_), the subdivision
of which into "branchial," "genital," "hepatic," "abdominal," and "caudal"
regions is better indicated in other species (Fig. 1, A, _b_, _g_, _h_,
_ab_), is orange-yellow, shading to green-yellow in the semi-transparent
caudal region. The genital region is grey in females and yellow in males, a
sexual difference in colour being common in Enteropneusta. The hepatic
papillae of other species may be bright green.
The odour of _D. kowalevskii_ resembles that of "chloride of lime with a
faecal admixture," while that of _Balanoglossus aurantiacus_ suggests
iodoform. All Enteropneusta are said to have a more or less offensive
smell. A species of Balanoglossus is known to be intensely
phosphorescent.[11]
The mouth (Fig. 7, _m_) is situated on the ventral side, at the base of the
proboscis, and is concealed by the free anterior edge of the collar, which
encircles the thin "proboscis-stalk" (Fig. 3, _p.s_). The animal has the
singular peculiarity of being unable to close its mouth;[12] and thus, as
it burrows through the ground, the sand which passes into the alimentary
canal leaves it in a continuous column through the terminal anus.[13] The
large coiled "castings" formed in this way between tide-marks enable the
experienced collector to infer the presence of Balanoglossus; and in a West
Indian species described by Willey[14] they are so large as to form "an
important feature in the landscape at low tide."
The principal agents in burrowing are the proboscis and collar. An animal
observed by Spengel pushed the tip of its proboscis into the sand, waves of
muscular contraction meanwhile passing {8}over the surface of the
proboscis. At first the animal made slow progress; but the collar, becoming
surrounded by sand, soon became a point of resistance by means of which the
proboscis could bury itself yet more deeply. The animal quickly disappeared
as soon as the first two regions of its body were engaged in the task of
burrowing[15]
This action is due partly to the muscles of the body-wall, but largely to
the power possessed by the proboscis and collar of becoming swollen and
turgid. Spengel has observed that these parts become flaccid when the
animal is taken out of water, and can only swell again when it is replaced
therein; and it may thus fairly be concluded that the enlargement is due to
the taking in of water. This is probably in fact the most important
function of the "proboscis-pore" and of the "collar-pores" which are
described below.
[Illustration: FIG. 2.—Diagram of a dorsal view of a Balanoglossus-embryo,
after the formation of the body-cavities, _a_, Alimentary canal; _b.c_^1,
body-cavity of the proboscis; _b.c_^2, of the collar; _b.c_^3, of the
trunk. (From Bateson.)]
BODY-CAVITIES.—The existence of five separate body-cavities (Fig. 2) is one
of the most fundamental facts in the anatomy of Balanoglossus. The first
body-cavity, or cavity of the proboscis (_b.c_^1), is single and unpaired;
the second body-cavities (_b.c_^2) are paired spaces, one belonging to each
side of the collar; the third body-cavities (_b.c_^3) are similarly paired,
and correspond with the trunk. While there is no connection between
successive body-cavities, there are in certain regions communications
between the two cavities of the same pair. Each of the paired cavities is
at one time a closed lateral space between the skin and the alimentary
canal. As the two spaces which constitute the pair grow towards one
another, both above and below the alimentary canal, they come into such
close apposition that they remain separated only by their conjoined walls.
In this way are formed the dorsal and ventral mesenteries (Fig. 4, _d.m_,
_v_), the former being the only one to persist in the higher Vertebrates.
The body-cavities of the adult become to a large extent disguised by being
traversed by connective tissue and muscles.
{9}The hinder part of the proboscis-cavity is divided by the forward growth
of the notochord (Fig. 3, _n_) into dorsal and ventral portions. The dorsal
cavity in extending backwards becomes further subdivided into right and
left halves, the latter typically opening dorsally to the exterior on the
proboscis-stalk by an asymmetrical "proboscis-pore" (_p.p._). Two
symmetrical proboscis-pores may, however, occur, or a median pore connected
with the left division of the proboscis-cavity. These may be individual
variations within the limits of a single species, or may occur as a normal
feature of a species.
[Illustration: FIG. 3.—Dorsal view of the anterior end of the body of
_Dolichoglossus kowalevskii_, × 3. _c_, Collar; _c.n_, circular nerve;
_c.p_, collar-pore; _d_, dorsal nerve; _g_, gill-pore; _n_, notochord;
_n.s_, central nervous system, showing the anterior and posterior
neuropores; _p_, proboscis; _p.p_, proboscis-pore; _p.s_, proboscis-stalk;
_t_, trunk; _v_, ventral nerve. The nerve-plexus of the proboscis is
represented as a black line. (After Bateson.)]
The collar-cavities open by two "collar-pores" (Fig. 3, _c.p._), situated
at the posterior end of the collar, into the first pair of gill-pouches,
near their external opening. Willey has recently described[16] vestigial
pores in relation with the "perihaemal spaces," a pair of dorsally situated
outgrowths of the third body-cavities into the collar-region. Narrow
"peripharyngeal spaces," also a forward growth of the third body-cavities,
closely invest the pharynx in some species.
BODY-WALL AND NERVOUS SYSTEM.—The body-wall (Fig. 4) consists externally of
a thick ciliated epidermis (_e_), containing numerous gland-cells which
secrete an abundant mucus. Beneath the epidermis is a basement-membrane,
while more internally are layers of muscles, whose arrangement differs in
different parts of the body and in different species.
The nervous system consists of a plexus of cells and fibres which lie in
the basal part of the epidermis of all parts of the animal, outside the
basement-membrane; the thicker portions of the plexus forming definite
nerve-tracts. This intimate connexion {10}between the epidermis and the
nervous system is usually restricted to embryonic life in other animals.
[Illustration: FIG. 4.—_Ptychodera bahamensis_, Bahama Is. Transverse
section through the branchial region. _b_, Branchial part of pharynx;
_b.c_^3, third body-cavity; _d.m_, dorsal mesentery; _d.n_, dorsal nerve;
_d.v_, dorsal vessel; _e_, epidermis, with nerve-layer (black) at its base;
_g_, genital wing; _g.p_, gill-pore, encroached on by the tongue-bar (_t_);
_l_, lateral septum; _m_, longitudinal muscles; _o_, oesophageal or
alimentary part of pharynx; _r_, reproductive organ; _t_, tongue-bar; _v_,
ventral mesentery and ventral vessel; _v.n_, ventral nerve. (After
Spengel.)]
The main nerves of Balanoglossus are a dorsal and a ventral tract in the
trunk region (Fig. 4, _d.n_, _v.n_), a circular tract (Fig. 3, _c.n_)
connecting these two at the posterior edge of the collar, and a strong
concentration of nerve-tissue round the whole of the proboscis-stalk, and
of the posterior end of the proboscis (Fig. 3). In the region of the collar
the nervous system attains its highest development, taking the form of a
median cord passing above the alimentary canal. This cord, known as the
central nervous system (Fig. 7, _n.s_), runs through the cavity of the
collar, but is connected with the epidermis at each end. It thus becomes
continuous in front with the nerve-layer on the proboscis-stalk, while
posteriorly it passes into the dorsal and the circular nerve-tracts. In
nearly all cases the epidermis is pushed into the cord at the points where
it passes into the skin, in the form of an anterior and a posterior
"neuropore" (Fig. 3). A transverse section through the extreme front or
hind end of the collar accordingly shows a tubular nervous system. In
certain species, as in _Glossobalanus sarniensis_ and _Ptychodera flava_, a
central canal, opening in front and behind, exists throughout the entire
length of the central nervous system, while in _G. minutus_ a canal of this
kind occurs in the young animal, but not in the adult. The central nervous
system is developed as a longitudinal {11}dorsal groove in the larva,[17]
and in a similar manner in the collar which is formed as the result of
regeneration after injury.[18] Balanoglossus is thus typically provided
with a dorsal, tubular, central nervous system, and although this
arrangement does not extend beyond the limits of the collar, it shows a
noteworthy resemblance to Vertebrate animals.
In some cases the central nervous system is connected with the dorsal
epidermis by a varying number (1-17) of median "roots," which have been
compared by Bateson with the dorsal roots of the spinal nerves of
Amphioxus, and are probably remains of the embryonic connexion of the
collar nervous system with the dorsal epidermis.
ALIMENTARY CANAL.—The mouth (Fig. 7, _m_) leads widely into the alimentary
canal, which, passing through the collar, enters the branchial region,
where it is characterised by the existence of communications with the
exterior. These, the gill-slits, are developed, as in Vertebrates, as
paired outgrowths of the alimentary canal, and new gill-slits are
constantly being formed at the posterior end of the branchial region with
advancing age. The maximum number of the gill-slits, and the extent of the
branchial region, are by no means uniform throughout the Enteropneusta.
Thus _Dolichoglossus otagoensis_ is said to have no more than 12 pairs,
_Glossobalanus minutus_ only 40 pairs, while _Balanoglossus aurantiacus_
may have as many as 700 pairs. In _Ptychodera flava_ the variation is so
great that Willey distinguishes[19] two extreme conditions as
"macrobranchiate" and "brachybranchiate" respectively, although
intermediate conditions are also found. It should be noted that
Balanoglossus agrees with Amphioxus in the indefinite number of the
gill-slits.
The gill-slits usually have the form of the so-called "branchial pouches"
or "gill-sacs" (Figs. 5, 6, _g.s_). Each ordinarily opens to the exterior
by a small pore (Fig. 1, D, 5, _g.p_) or slit, situated on the dorsal side,
in a shallow longitudinal groove not far from the middle line. The gill-sac
has a complete wall of its own, and lies between the alimentary canal and
the body-wall, communicating with the former by a U-shaped slit. While a
dorsal {12}view of the animal thus shows a linear series of simple pores, a
view of the pharynx from the inside appears as in Fig. 5.
At the hind end of the pharynx the inner opening of the developing gill-sac
is circular. Slightly further forward the dorsal side of the pore is
indented into a crescent, which grows longer in a dorso-ventral direction,
and becomes a U, whose two limbs are nearly separated by a mass of tissue,
the so-called "tongue-bar" (Fig. 5, _t_). The special interest of this mode
of development is that it is identical with what occurs in Amphioxus (p.
120), which is universally admitted to belong to the Chordata.
The gill-sacs of Balanoglossus follow one another closely, the hind wall of
one being in contact with the front wall of the next, and constituting a
"branchial septum" (_b.s_). Both septa and tongue-bars are supported by
chitinous rods, which are special thickenings of the membrane at the base
of their epithelium. Two rods occur in each tongue-bar, separated by an
interval of body-cavity (Figs. 5, 6), and only one rod in each septum.
Originally of this form—∩∩ ∩∩—the rods have joined in pairs, the united
limbs forming the single rod of each branchial septum. In this respect
again we have a similarity between Balanoglossus and Amphioxus, except that
in the latter the concrescence proceeds one step farther, and the two rods
of the tongue-bar unite, like those of the branchial septum. The latter,
the so-called "primary" skeletal rods of Amphioxus, are forked ventrally as
in Balanoglossus (Fig. 5).
[Illustration: FIG. 5.—Diagram of two gill-sacs of Balanoglossus, seen from
the inside of the pharynx. _b_, Branchial skeleton, consisting of a single
forked bar in each branchial septum (_b.s_), and of two bars in each
tongue-bar; _g.p_, gill-pore, opening on the dorsal surface of the trunk;
_g.s_, gill-sac; _s_, synapticulum (only one or two shown); _t_,
tongue-bar. The arrows indicate the communications of the gill-sacs with
the exterior and with the pharynx.]
In Amphioxus, as in most Enteropneusta, adjacent rods are {13}connected at
intervals by chitinous "synapticula" (Fig. 5, _s_), which traverse one or
the other of the halves of the gill-slit. In _Dolichoglossus_, where no
synapticula occur, the tongue-bars may be turned inside out by slight
pressure, and then project to the exterior through the gill-pores.
The subdivision of the branchial region of the alimentary canal into two
parts, as shown in Fig. 4, is characteristic of _Glossobalanus_ and its
allies. In _Dolichoglossus_ and _Glandiceps_ there is no such constriction,
the region occupied by the gill-slits being merely the dorsal half of a
tube with a simple circular section. _Schizocardium_ (Fig. 6) agrees with
Amphioxus in the fact that the gill-slits occupy nearly the whole of the
wall of the pharynx; the only parts not perforated by gill-slits being the
small dorsal and ventral portions.
In _Ptychodera_ (Fig. 4), the gill-sacs are practically absent. The
U-shaped slits of the pharyngeal wall thus open directly to the
exterior,[20] and can be seen from the outside. In species which have this
arrangement, the genital wings are greatly developed, so as to arch over
the back of the branchial region. The gill-slits thus open into a kind of
"atrium," resembling that of Amphioxus in its relation to the gill-slits,
and in having the generative organs on its outer side, but differing from
it in being dorsal to the pharynx.
[Illustration: FIG. 6.—_Schizocardium brasiliense_; transverse section
through the branchial region, showing the great extent of the branchial
part (_b_) of the pharynx; the oesophageal part (_o_) is reduced to a mere
groove; _g_, gill-pore; _g.s_, gill-sac; _r_, reproductive organ; _s_,
synapticula (cf. Fig. 5); _t_, tongue-bar. The muscles of the body-wall are
not indicated: in other respects the figure corresponds with Fig. 4, except
for the absence of genital wings in this region of the body. (After
Spengel.)]
At a certain distance behind the branchial region, the alimentary canal in
_Balanoglossus_ and _Schizocardium_ is produced into a series of
outgrowths, into which food does not pass. These "liver-sacs" give rise to
corresponding folds (Fig. 1, A, _h_) {14}of the dorsal body-wall, a
conspicuous external feature of the species in which they are present. The
most interesting peculiarity of the digestive tract in this region is the
existence, in certain species, of pores, possibly vestigial gill-slits,
leading from it to the exterior.
NOTOCHORD AND SKELETON.—The structure compared by Bateson with the
Vertebrate notochord is a hollow dorsal outgrowth of the alimentary canal
of the collar-region (Fig. 7, _n_). Near its origin it is slender, but in
the proboscis it dilates into a comparatively large organ, which in most
cases retains its cavity. Its cells have a vacuolated appearance, which
recalls the fine structure of the Vertebrate notochord. In _Schizocardium_
and _Glandiceps_, the organ is produced into a slender "vermiform process"
(_v_), which extends nearly to the tip of the proboscis.
[Illustration: FIG. 7.—_Schizocardium brasiliense_; longitudinal, median
section through the proboscis, the collar, and the first part of the trunk;
_b_, main blood-space of the proboscis; _b.c_^1, _b.c_^2, _b.c_^3, first,
second and third body-cavities; _c.m_, circular muscles of proboscis; _e_,
epidermis; _l.m_, longitudinal muscles of proboscis; _m_, mouth; _n_,
notochord; _n.s_, central nervous system, continuous with the subepidermic
nerve-plexus (black) of the proboscis, and with the dorsal nerve (_d_);
_p.c_, pericardium; _p.s_, proboscis-stalk; _s_, proboscis-skeleton; _v_,
vermiform process of notochord. (After Spengel.)]
The main support of the proboscis-stalk is the "proboscis-skeleton" (_s_),
a Y-shaped organ whose median part lies beneath the base of the notochord,
its diverging legs extending backwards along the outer side of the
alimentary canal of the collar. The proboscis-skeleton, like the branchial
skeleton, is a special development of the structureless membrane which is
found at the base of the layers of cells {15}of Balanoglossus, and in most
species it grows merely by the deposition of laminae of chitin from the
notochord, and from the ventral epidermis of the proboscis-stalk.
In some species, however, and particularly in _Balanoglossus aurantiacus_
and _Glandiceps_, the primary skeleton becomes surrounded by an extensive
development of a secondary cartilaginoid skeleton, consisting of a
structureless substance into which the adjacent body-cavities of the
proboscis and collar send cellular outgrowths. The possibility of a
relation between this tissue, more or less surrounding a part of the
notochord, and the cartilage of Vertebrates cannot be overlooked.
The caudal region may be stiffened (?) by a "pygochord"[21] which is a
median derivative of the alimentary canal on its ventral side.
VASCULAR SYSTEM AND PROBOSCIS-GLAND.—The main vessels are a dorsal and a
ventral vessel (Fig. 4, _d.v_, _v_), lying in their respective mesenteries.
The details of the vascular system are complicated, and have not been
thoroughly made out, the nearly colourless character of the blood making
their investigation a difficult matter. The following points may, however,
be noted. The blood is said to pass forwards in the dorsal vessel, which,
like the ventral vessel and a pair of lateral vessels in the hepatic
region, is contractile. In the collar the dorsal vessel lies between the
two perihaemal spaces, on the dorsal side of the base of the notochord. The
principal blood-space in the proboscis (Fig. 7, _b_) lies between the
notochord (_n_) and an organ known as the "heart-vesicle" or "pericardium"
(_p.c_). The latter has muscular walls and it contracts rhythmically in the
larva. Its behaviour in the adult is not so easily made out, but it is
probable that, although it does not communicate with the vascular system,
its contractions propel the blood contained in the space immediately
beneath it. The blood, after passing to a glandular organ, the
"proboscis-gland" or "glomerulus," which lies at the sides and in front of
the notochord, appears to pass round the collar to the ventral vessel.
Various systems of vessels are connected with the skin, the gills, the
alimentary canal and the generative organs.
The function of the proboscis-gland is possibly excretory. In this case it
is probable that the proboscis-pore eliminates the {16}waste products
discharged by the gland into the anterior body-cavity, though this view is
not favoured by Willey.
REPRODUCTIVE ORGANS.—The sexes are separate, the reproductive organs
consisting of a series of simple or branched glands which occur along the
dorso-lateral lines of the anterior part of the body; being usually found
throughout the branchial and generative regions and ending at the beginning
of the hepatic region. The reproductive organs may pass into great
extensions of the body-wall known as the "genital wings," specially
developed in some species of _Balanoglossus_ and _Ptychodera_ (Figs. 1 A,
4).
_Stereobalanus canadensis_, a species with long slit-like external
gill-pores, is interesting in possessing a well-developed genital wing both
dorsally and ventrally to the series of gill-pores of each side.
Each reproductive gland opens by its own pore or pores directly to the
exterior. Several glands and pores may occur in the same transverse
section.
According to Spengel there is no definite relation between the number of
the reproductive organs and that of either the gill-sacs or the
liver-outgrowths. The only definite segmentation exhibited by Balanoglossus
is thus the division into three regions which is so distinctly shown by the
arrangement of the body-cavities; though the gill-sacs may indicate an
incipient further segmentation of the major part of the body. In this
connexion it is interesting to notice MacBride's statement[22] that the
body-cavity of Amphioxus develops in the embryo as five cavities, just as
in Balanoglossus; the segmented part of the body being formed by a
secondary segmentation of the third body-cavities.
REGENERATION.—Balanoglossus, like _Phoronis_ (p. 30), possesses great
powers of regenerating lost parts. The posterior part of the body is
readily re-formed, while Spengel has shown[23] that even the proboscis,
collar and branchial region can be regenerated, apparently from a fragment
of the body.
GENERA OF ENTEROPNEUSTA.—Spengel, whose Monograph is indispensable to every
student of the Enteropneusta, formerly {17}proposed to divide the old genus
_Balanoglossus_ into four; but he now recognises no less than nine.[24]
Some of the more important characters are given below, but for the
arrangement of the muscles, important from a systematic point of view,
reference must be made to the original sources.
A. Notochord with a vermiform process (Fig. 7, _v_); pericardium
with anterior diverticula more or less developed. GLANDICIPITIDAE
(_a_) Liver-sacs and synapticula present; gill-slits almost equalling
the pharynx in depth, so that the ventral, non-branchial part of
the pharynx is reduced to a mere groove (Fig. 6); nerve-roots
absent; pericardial diverticula long. _Schizocardium_, Speng.
(_b_) Liver-sacs absent;[25] ventral part of pharynx well developed;
pericardial diverticula short.
(i.) Synapticula and nerve-roots absent. _Glandiceps_, Speng.
(ii.) Synapticula present; nerve-roots present or absent;
genital region with dermal pits. _Spengelia_, Willey.
B. Notochord with no vermiform process; pericardium simple; ventral
part of pharynx large, and sometimes more or less separated from
the branchial part (Fig. 4).
(_a_) Liver-sacs,[26] synapticula and nerve-roots present.
PTYCHODERIDAE
(i.) Genital wings well developed.
(α) Gill-sacs opening by long slits. _Ptychodera_, Eschsch.
(β) Gill-sacs opening by small pores.
_Balanoglossus_, Delle Chiaje.
(ii.) Genital wings hardly developed. _Glossobalanus_, Speng.
(_b_) Liver-sacs, synapticula and nerve-roots absent. HARRIMANIIDAE
(i.) Proboscis long; one proboscis-pore. _Dolichoglossus_, Speng.
(ii.) Proboscis short; two proboscis-pores.
(α) Two pairs of genital wings. _Stereobalanus canadensis_, Speng.
(β) No genital wings. _Harrimania_, Ritter.
The name _Balanoglossus_ was introduced by Delle Chiaje in 1829 for _B.
clavigerus_ (Fig. 1, A), from the neighbourhood of Naples. As Spengel has
shown, its etymology has been much misunderstood. The second half of the
name refers to a fancied resemblance between the Balanoglossus, with its
largely developed genital wings, and the tongue of an ox. Βάλανος means
"acorn," and it has usually been supposed that this name was suggested by
the resemblance of the proboscis, projecting from the collar, to an acorn
in its cup, a view which finds its expression in the {18}name "Eichelwurm"
used by German zoologists. But the idea expressed by Delle Chiaje was
really a similarity between the collar of _Balanoglossus_ and the outer
shell of _Balanus_, the barnacle or "acorn-shell" found everywhere on rocks
between tide-marks.
[Illustration: FIG. 8.—Metamorphosis of Balanoglossus, probably of
_Balanoglossus biminiensis_ Willey, Bahama Islands. All the figures are
magnified to the same scale (× 14). A, fully developed free-swimming larva,
or _Tornaria_, side view; B, commencement of metamorphosis, side view; C,
later stage, dorsal view. Increase in size takes place after this stage;
_a_, anus; _b.c_^1, body-cavity of proboscis; _c_, collar; _c.r_,
transverse ciliated ring; _d.p_ (in A), dorsal pore (= proboscis-pore),
seen also in C on the left side, just behind the reference line _p.c_; _e_,
eyes and sensory thickening of skin (in A); _g_, gill-pore; _g.s_,
gill-sacs, developing as outgrowths of the alimentary canal; three are
already present in B, but are better seen in C, in which they are still
without openings to the exterior; _l_, postoral part of the longitudinal
band of cilia; _l_′, its praeoral part; both _l_ and _l_′ are produced (in
A) into tentacles, over which the band of cilia is looped; the groove in
the middle of the figure, between _l_ and _l_′, conducts the food by the
transverse groove to the mouth (_m_); _p.c_, blood-space of proboscis and
pericardium ("heart" of larva); _s_, stomach. (After Morgan).]
DEVELOPMENT.—The free-swimming, larval stage of Balanoglossus is known as
_Tornaria_ (Fig. 8, A). Several distinct forms of the larva are known,[27]
although it is not yet possible to refer them with certainty to their
respective adults.
{19}Tornaria was described and named by Johannes Müller, who regarded it as
the larva of a Starfish,[28] in spite of his intimate knowledge of the
development of these animals. Its correct systematic position was first
demonstrated by Metschnikoff in 1869.
The larva agrees with many other pelagic forms in being excessively
transparent. The form described by Spengel as _T. grenacheri_ attains the
remarkable length of 9 mm. (nearly ⅖th inch).
The full-grown larva is usually ovoid, and a complicated "longitudinal"
band of cilia runs in several loops over its anterior two-thirds. In side
view, part of the surface limited by the ciliated band appears like a T
with a double outline, the cross piece being bent downwards on each side,
so as to form an anchor-like curve, the middle of which is at the anterior
pole of the larva. In _T. krohni_, which occurs on our south coast,[29] the
ciliated band has a wavy course. In the West Indian larva[30] shown in Fig.
8 A, the ciliated band is produced into numerous tentacles, which fringe
the sides of the T-shaped areas or grooves of the surface. These grooves
and the cilia which border them are used for conveying food to the
mouth.[31] At the apex of the larva is a thickening (_e_) of the ectoderm,
bearing two eye-spots. The main locomotor organ is a simple transverse band
(_c.r_) of "membranellae," vibratile structures composed of fused cilia.
The mouth (_m_), on the ventral side, leads into the oesophagus, and this
into the stomach (_s_). The latter is separated by a marked constriction
from the intestine, which opens by the anus (_a_) at the posterior pole.
On the dorsal side is a pore, the "dorsal pore" (_d.p._), which leads into
a thin-walled sac (_b.c_^1) destined to become the proboscis-cavity of the
adult. To the right of the dorsal pore lies the pulsating "heart," which
apparently becomes the pericardium of the adult. Bourne and Spengel regard
it as a right proboscis-cavity. In the older larvae, the second and third
body-cavities appear as paired thin-walled sacs in close contact with the
hinder part of the stomach. The skin is very thin, and the five
body-cavities do not nearly fill the space between it and the
{20}alimentary canal. This space becomes obliterated for the most part by
the enlargement of the body-cavities, and its last remains persist, as in
many other animals,[32] as the vascular spaces of the adult.
In _Dolichoglossus kowalevskii_, and probably in other species with large
eggs,[33] development proceeds by gradual stages to the adult form, and no
Tornaria-stage is passed through. The opaque young animal, on being
hatched, creeps about in the muddy sand in which the adult is found, later
moving in a leech-like manner, by alternately attaching itself by its two
ends. The young stages were ingeniously obtained by Bateson, to whom our
knowledge of the development of this species is due,[34] by allowing a
large quantity of the mud to settle after being stirred up, the layer of
the specific gravity corresponding with that of the young Balanoglossus
being then separated by means of a siphon. The young stages previously
contained in several hundredweight of mud were thus easily collected into a
pint of water. Morgan recommends treating the layer obtained by a similar
process with picric acid, which stains the young Balanoglossus yellow.
The embryo early becomes a "blastosphere" or hollow vesicle formed of a
single layer of cells. One half of this is invaginated, or pushed into the
other half, and a "gastrula" is thus developed, the cavity of which is the
"archenteron," and the two cell-layers respectively "ectoderm" and
"endoderm." The "blastopore," or orifice of invagination, is at the
posterior pole of the larva, where it narrows and closes, the locomotor,
transverse band of cilia developing round it. No other bands of cilia
appear in this form of development. The proboscis becomes marked out
externally by the appearance of a circular groove, near the middle; and
behind this groove a second one appears, which forms the posterior boundary
of the collar. The larva, which now resembles Fig. 8 C, is usually hatched
at this stage. Two gill-slits make their appearance, and the mouth and anus
are perforated; the anus being in the position of the blastopore.
{21}The body-cavities are formed as five derivatives of the archenteron.
One of these is unpaired, and becomes the proboscis-cavity; while the
others are the paired cavities of the collar and trunk (cf. Fig. 2). There
is some uncertainty about the origin of the body-cavities of the
free-swimming Tornaria, although it seems most probable that they are
developed either from the wall of the stomach or intestine,[35] or from
scattered mesoderm cells[36] which lie in the segmentation-cavity.
The metamorphosis of Tornaria is accompanied by a great diminution in
size,[37] probably due to the loss of water; by this cause and by the
simultaneous thickening of the skin, the larva loses its transparency.
The external features of the metamorphosis are sufficiently indicated by
Fig. 8, the ciliated bands finally disappearing. The dorsal pore persists
as the proboscis-pore; the notochord and numerous gill-slits are developed
as outgrowths of the alimentary canal, the reproductive organs make their
appearance, probably from the mesoderm,[38] the trunk meanwhile elongating
so that the proportions of the adult are acquired.
ORDER II. PTEROBRANCHIA.
_Tubicolous Hemichordata, with one pair of gill-slits or none, a_
U-_shaped alimentary canal, and a dorsal anus situated near the mouth.
Proboscis flattened ventrally into a large "buccal disc," its base
covered dorsally by the collar, which is produced into two or more
tentaculiferous arms. Trunk short, prolonged into a stalk. Reproduction
by budding occurs_.
This group consists of the two genera _Cephalodiscus_ (Fig. 9) and
_Rhabdopleura_ (Fig 12). The latter, first dredged by G.O. Sars, in 1866,
from 120 fathoms off the Lofoten Islands, was included in a catalogue of
deep-sea animals published by his father, M. Sars, in 1868 as _Halilophus
mirabilis_, a name which has been superseded by _Rhabdopleura normani_,
Allman,[39] based on specimens dredged by Canon Norman in 90 fathoms, off
the Shetland Islands.
{22}[Illustration: FIG. 9.—_Cephalodiscus dodecalophus_, M‘Intosh, Straits
of Magellan; A, small portion of the common "house," × 1; _a_, a single
individual, shown also as B, × 65; six of the tentacular arms, belonging to
the collar, are seen springing from behind the proboscis or "buccal disc."
This has a crescentic band of pigment parallel with its posterior border,
which conceals the mouth. The stalk, bearing a bud, which already shows the
beginning of two tentacular arms, is seen to the right. (After M‘Intosh, B
from Parker and Haswell.)]
{23}The structure of _Rhabdopleura_ has been described by Sars,[40]
Lankester,[41] and Fowler.[42] _R. normani_ is common in certain Norwegian
Fjords, at depths of 40 fathoms or more, and has been recorded by Fowler
from the Tristan d'Acunha group in the S. Atlantic; _R. compacta_ has been
found off the N.E. coast of Ireland[43] and near Roscoff, on the N. coast
of Brittany; while forms described by Jullien[44] as _R. grimaldii_ and _R.
manubialis_ have been dredged off the Azores. I have recently found a
fragment of _Rhabdopleura_ from South Australia. It is doubtful how far
these species are distinct.
_Cephalodiscus dodecalophus_[45] was found in the Straits of Magellan,
during the "Challenger" voyage, at a depth of 245 fathoms, and has recently
been rediscovered in shallower water in the same neighbourhood by the
Swedish Antarctic Expedition. Another _Cephalodiscus_, at present
undescribed, has been obtained by Dr. Levinsen from 100 fathoms off the
coast of Japan; while the Dutch expedition carried out by the "Siboga" has
resulted in the discovery of two other specimens, one from a reef close to
low-tide mark on the coast of Borneo, the other from 41-52 fathoms off
Celebes. These three specimens differ markedly from one another and from
the "Challenger" specimen of _C. dodecalophus_, and it is probable that
they all belong to new species. The occurrence of a deep-sea animal at a
great distance from the locality at which it was first found is not in
itself a matter for great surprise; but in the present instance two of the
newly discovered forms are from shallow water, and one of them is actually
littoral. The occurrence of so many species of _Cephalodiscus_ in Oriental
waters suggests that the Pacific or the Indian Ocean may be the
headquarters of the genus, which may prove to be far less of a rarity than
has hitherto been {24}supposed. There is evidence derived from the results
of the "Siboga" expedition that abyssal animals may migrate into
comparatively shallow water in the Malay Archipelago.
_Cephalodiscus_ and _Rhabdopleura_ are remarkable for their power of
producing buds. In the former these arise from the apex of a stalk which is
given off on the ventral side of the body, and they break off when they
reach a certain age; in the latter they do not become free, and a colony
results, which consists of a creeping "stolon" from which vertical branches
are given off at intervals, each ending in an individual of the colony.
_Cephalodiscus_ forms a gelatinous "house" (Fig. 9, A), in the passages of
which are found large numbers of the free individuals, together with their
eggs and embryos. _Rhabdopleura_ (Fig. 12) is protected by cylindrical
tubes, one of which corresponds with each individual.
[Illustration: FIG. 10.—Longitudinal median section of _Cephalodiscus
dodecalophus_. _a_, Anus; _b.c_^1, _b.c_^2, _b.c_^3, first, second, and
third body-cavities; _int_, intestine; _m_, mouth; _nch_, notochord; _n.s_,
central nervous system; _oes_, oesophagus; _op_, operculum, the
ventro-lateral part of the collar; _ov_, ovary; _ovd_, pigmented oviduct;
_ph_, pharynx; _p.p_, proboscis-pore; _ps_, proboscis; _st_, stomach;
_stk_, stalk.]
_Cephalodiscus_, though no more than two or three millimetres in length, is
provided with practically all the important organs possessed by
Balanoglossus. Its proboscis or "buccal shield" (Fig. 10, _ps_) is a large
flattened structure, which overhangs and entirely conceals the mouth. The
anterior body-cavity opens to the exterior by two symmetrically placed
proboscis-pores (_p.p_), just in front of the tip of the notochord (_nch_).
The collar, which has paired body-cavities, is produced dorsally into 4-6
{25}pairs of plume-like arms, which bear an immense number of
pinnately-arranged tentacles. The arms, which may end in a swollen
bulb,[46] have ventral grooves along which food doubtless travels to the
mouth by ciliary currents. The anterior edge of the ventral half of the
collar is drawn out into a narrow flap or operculum (Fig. 11, _op_), in
front of which is the mouth, and behind it the gill-slits (_g_) and
collar-pores (_c_). The central nervous system (_n.s_) is a thick mass of
nerve-tissue in the dorsal epidermis of the collar; it is not sunk beneath
the skin as in Balanoglossus. The details of the nervous and vascular
systems, and the development of the buds, have been described by Masterman.
In the dorsal region of the collar the alimentary canal has a slender
diverticulum, the notochord, which passes into the base of the proboscis;
it is believed by Masterman to have a function similar to that of the
neural gland (cf. p. 52) of Tunicates.
The next part of the alimentary canal, the pharynx,[47] has a single pair
of simple gill-slits opening to the exterior immediately behind the
collar-pores. The short oesophagus (Fig. 10, _oes_) is followed by the wide
stomach (_st_), and this by the intestine (_int_), which opens by the anus
(_a_) near the front end of the body.
[Illustration: FIG. 11.—Longitudinal section through _Cephalodiscus
dodecalophus_, passing through the two sides of the body; _a_, tentacular
arm; _b.c^2_, collar-cavity; _b.c^3_, trunk-cavity; _c_, collar-pore; _g_,
gill-slit; _i_, intestine; _n.s_, central nervous system; _o_, oesophagus;
_op_, operculum; _p_, pharynx; _s_, stomach.]
The trunk contains paired third body-cavities (_b.c^3_), the septum between
which and the collar-cavities is slightly behind the line of origin of the
operculum. Two ovaries (_ov_) are situated between the pharynx and the last
part of the intestine, each opening to the exterior dorsally between the
central nervous {26}system and the anus. Each oviduct (_ovd_) contains dark
pigment, which is seen through the dorsal skin on removing the tentacular
arms. Eggs, each enclosed in a stalked membrane, occur in numbers in the
cavities of the gelatinous house. The early stages of the development are
passed through inside the tubes; but there is at present little other
information with regard to the embryonic development of the Pterobranchia.
The specimen obtained by the "Siboga" from Celebes is a male colony with
dimorphic individuals, the reproductive organs being confined to two-armed
zooids with vestigial alimentary canal.
[Illustration: FIG. 12.—Small portion of colony of _Rhabdopleura normani_,
Allman, Lofoten Islands, × 16. _a_, Anus; _p_, proboscis (= buccal disc);
_r_, rod-like axis of the adherent part of the colony, prolonged into _s_,
the stalks of the individuals; _st_, stomach; _t_, the two tentacular arms
of the collar. (After Sars.)]
_Rhabdopleura_ differs from _Cephalodiscus_ in its much smaller size,[48]
and it is perhaps due to its minuteness that it does not possess certain
organs found in the latter. The stalk is represented by a long muscular
cord, which is merely a narrow part of the body. Basally the stalk of each
individual passes into a common axis, which is for the most part attached
to the substance on which the colony is growing, and is to some extent
branched. The muscular stalk can be contracted into a spiral, thereby
retracting the animal into its tube. The stalks and the younger parts of
the axis which connects them are soft, but the older parts secrete a dark
brown cuticle, forming a narrow {27}tube which becomes embedded in the
adherent wall of the outer tube. The thin dark axis, to which the name
_Rhabdopleura_ refers, is the feature by which the animal can most readily
be recognised without magnification.
The outer transparent tube is constructed by the proboscis, or buccal
shield, the secretion of which appears to be intermittent, so that the tube
consists of a series of rings piled on one another. The animal crawls up
the inside of its tube by means of its proboscis, while it is retracted by
means of the muscles of its stalk.
The growing axis ends in a row of young buds, the buccal shields of which
early reach a relatively large size. The terminal bud gives rise to
tube-rings, so that the axis is surrounded by a cylindrical outer tube,
which becomes interrupted by transverse septa, each bud, except the end
one, thus lying in a closed chamber. The wall of each chamber becomes
perforated, and the buccal shield then prolongs this perforation by adding
tube-rings, the formation of which continues till the tube reaches a
considerable length. The bud remains connected with the axis by means of
its narrow proximal region, which forms its stalk. The adherent part of the
adult colony thus consists of a row of short tubes, traversed by the common
axis of the colony. Each tube is produced laterally into the upright tube
of an individual.
The general anatomy closely resembles that of _Cephalodiscus_.[49] There
are five body-cavities and a notochord. Collar-pores exist, but
proboscis-pores and gill-slits have not been described. The dorsal region
of the collar bears only a single pair of arms.
ORDER III. PHORONIDEA.
The structure and development of _Phoronis_ (Fig. 13), have already been
described in Vol. II.[50] of this series; and Masterman's investigations,
then published in a preliminary form only, are there alluded to. Since then
this author has published fuller accounts[51] of his results, which, if
substantiated, {28}would indicate a near relationship between
_Cephalodiscus_ and _Phoronis_.
_Phoronis_ is a small tubicolous animal, of gregarious habits, which has
usually been regarded as related to the Gephyrea. Its body ends in a plume
of ciliated tentacles, which can be protruded from its tube, and the anus
is on the dorsal side, not far from the mouth. In both these respects it
agrees with _Cephalodiscus_, but a more striking similarity is asserted by
Masterman to exist between the latter and _Actinotrocha_, the larval stage
of _Phoronis_. The prae-oral ciliated hood (Fig. 14) of Actinotrocha is
regarded as the proboscis, and it contains a median cavity, traversed, like
that of Balanoglossus, by muscular fibres. The collar is the region between
the constricted neck and an oblique line, parallel to and immediately
behind the series of tentacles, which thus belong to the collar. This
division has a collar-cavity which is said to be distinct from the
prae-oral cavity, and is separated by a septum from the posterior
body-cavity. Its dorsal epidermis contains the central nervous system
(_n.s_), which is connected with a system of nerves resembling those of
Balanoglossus. A median diverticulum of the alimentary canal of this part
may be compared with the notochord of that animal, but there are no
gill-slits.
[Illustration: FIG. 13.—_Phoronis buskii_, M‘Intosh, Philippine Islands, x
about 2. (After M‘Intosh, from Shipley.)]
The remainder of the body of Actinotrocha corresponds with the trunk of
Balanoglossus. Its body-cavity is distinct from that of the collar, and is
divided by a ventral mesentery, though not by a dorsal mesentery. A
noteworthy fact is that both Actinotrocha and Tornaria swim by means of a
ring of strong cilia or membranellae[52] which surrounds the anus.
{29}[Illustration: FIG. 14.—_Actinotrocha_-larva of _Phoronis_. _a_, Anus;
_b.c^1_, _b.c^2_, _b.c^3_, first, second and third body-cavities; _c_,
circular nerve, running in the posterior boundary of the collar,
immediately behind the ring of tentacles; _c.r_, ciliated ring; _d_,
diverticulum (paired) of alimentary canal; _m_, mouth; _n.s_, central
nervous system; _p_, nerve running round the ventral border of the
proboscis; _s_, sense-organ; _s.s_, subneural sinus, a vascular space whose
hind wall is constituted by the front boundary of _b.c^2_, its front wall
being formed by the hind wall of _b.c^1_; in this region is seen a median
outgrowth of the alimentary canal, which may be compared with the notochord
of _Cephalodiscus_, or of the young Tornaria (cf. Morgan, _J. Morphol_. v.
1891, Plate xxvi. Fig. 40.) (After Masterman.)]
Important memoirs on the structure of Actinotrocha have recently been
published by Ikeda,[53] de Selys Longchamps,[54] Goodrich,[55] and
Schultz,[56] who criticise many of Masterman's statements. While it is
admitted on all sides that an oblique septum following the line of the
bases of the tentacles completely subdivides the body-cavity, Masterman's
account of the anterior cavities is not confirmed, the spaces indicated by
_b.c^1_ and _b.c^2_ in Fig. 14 being stated to be really continuous with
one another, while the "subneural sinus" (_s.s_) is regarded as a part of
this space. It appears, however, from the account given by Ikeda, and
followed by Goodrich, that the old Actinotrocha has two distinct spaces in
front of the septum. The first of these corresponds with _b.c^1_ + most of
_b.c^2_ in Fig. 14, and is continuous with the cavities of the larval
tentacles. Into it project the blind ends of the larval excretory organs,
which, according to Goodrich, bear numerous "solenocytes" similar to those
described by the same author in Amphioxus and in Polychaet worms (Fig. 79,
p. 127). The second cavity is a relatively small crescent (not shown in
Fig. 14), lying on the anterior face of the septum, {30}the tips of the
crescent nearly meeting dorsally, so as to constitute an almost complete
ring following the bases of the tentacles, into each of which it gives off
a blind outgrowth. At the metamorphosis, the crescentic space becomes the
prae-septal body-cavity and the cavities of the tentacles of the adult, the
circular blood-vessel of which is formed from the remains of the large
prae-septal space of the larva. Schultz, in calling attention to the fact
that both _Phoronis_ and its larva have a striking power of regenerating
lost parts, confirms the conclusion that this animal belongs to the
Hemichordata. He gives reasons, however, for believing that it is in the
adult _Phoronis_ rather than in the larval Actinotrocha that it is possible
to discover the most satisfactory evidence of this affinity.
The metamorphosis[57] of Actinotrocha is very remarkable, and is
accompanied by the eversion of a ventral ingrowth of the body-wall. A loop
of the alimentary canal passes into this eversion, which becomes the main
part of the body of the adult; and the anus is thereby brought relatively
nearer the mouth than in the larva. The occurrence of this process may help
to explain the position of the anus in the Pterobranchia.
AFFINITIES OF THE HEMICHORDATA.—There can be no doubt that some of the
resemblances, in structure and in development, between Balanoglossus and
certain Vertebrates are extremely striking. The view that Balanoglossus is
related to the ancestors of Vertebrates[58] appears to exclude other
views[59] which have been suggested with regard to the same question. The
Balanoglossus-theory does not explain the similarity between the
segmentation and the excretory systems of Vertebrates and Chaetopods; but,
on the contrary, there are important characters which Vertebrates share
with Balanoglossus but with no other "Invertebrates." Of these the most
important appear to be the resemblances between the gill-slits and
gill-bars of Balanoglossus and Amphioxus; the position, structure and mode
of development of the central nervous system; and the presence of a
structure in the Hemichordata, which may be regarded as the notochord.
{31}There are other points in which Balanoglossus specially resembles
Amphioxus, such as the early development, the mode of formation of the
body-cavities,[60] and the presence of numerous generative organs.
All these, taken together, make it necessary to consider carefully the
claims of Balanoglossus to relationship with the ancestors of Vertebrates
in making any speculations on this interesting problem.
However improbable it may appear at first sight, it is possible to hold the
view that Balanoglossus is related at the same time to Vertebrates and to
Starfishes and other Echinoderms. The similarity between a young Tornaria
and a young Bipinnaria-larva of a Starfish is so great as to have misled
even Johannes Müller. The more obvious resemblances are the almost
identical course of the longitudinal ciliated band in the young stages, and
the presence of a dorsal pore. The Echinoderm-larva is not, however,
provided with eye-spots, nor has it the posterior, or transverse, ciliated
band of Tornaria.
Recent studies on the development of Echinoderms[61] have made it probable
that the five body-cavities of Balanoglossus are represented in the larvae
of those animals; and this materially strengthens the probability of the
view that the respective adults are also allied.[62] It may be added that
the relationship which appears to be indicated is between Balanoglossus and
the bilateral ancestors from which the radially-symmetrical Echinoderms are
probably descended.
In comparing the Enteropneusta with the Pterobranchia, the disproportionate
size of the trunk of Balanoglossus may perhaps be explained by assuming
that the region of the third body-cavities has been enlarged since
Balanoglossus branched off from the ancestral stock.[63] The approximation
of the anus to the mouth in Pterobranchia is perhaps the result of their
tubicolous habits.[64] In the position of the central nervous system in the
skin of the collar, _Cephalodiscus_ appears to be more primitive {32}than
Balanoglossus, as has been pointed out by Morgan.[65] It is not impossible
that the presence of one pair of gill-slits in _Cephalodiscus_ indicates
that this animal diverged from the ancestors of Balanoglossus before the
gill-slits were metamerically repeated.
ASCIDIANS AND AMPHIOXUS
BY
W. A. HERDMAN, D.SC. (EDINB.), F.R.S.
Professor of Natural History in the University of Liverpool
{35}CHAPTER II
TUNICATA (ASCIDIANS AND THEIR ALLIES)
INTRODUCTION—OUTLINE OF HISTORY—STRUCTURE OF A TYPICAL ASCIDIAN—EMBRYOLOGY
AND LIFE-HISTORY
The TUNICATA are marine animals found in practically all parts of the sea,
and at all depths. They extend from the Arctic and Antarctic regions to the
tropical waters, and from the littoral zone down to the abyssal depths of
over three miles. They are abundant in British seas. They vary greatly in
shape and colour, and range in size from an almost invisible hundredth of
an inch to large masses a foot or more in diameter. And yet most Tunicata
have a characteristic appearance by which they can be readily distinguished
from other animals. They form a well-defined group, with definite
anatomical characters, and there are no known forms intermediate between
them and other groups. The Tunicata were formerly regarded as constituting,
along with the Polyzoa and the Brachiopoda, the Invertebrate Class
"MOLLUSCOIDEA." They are now known to be a degenerate branch of the lower
CHORDATA, and to be more nearly related to the Vertebrata than to any group
of Invertebrates.
Tunicata occur either fixed or free, solitary, aggregated or in colonies
(see Fig. 27, p. 64). The fixed forms, found on the sea-bottom, are usually
termed "Ascidians," those that are solitary or merely aggregated being
"Simple Ascidians" or Monascidiae, and those that are organically united
into a colony being "Compound Ascidians" or Synascidiae. The colonies have
been produced by budding, a process which is very general in the group, and
the members of the colony are conveniently known as "Ascidiozooids." Some
exhibit {36}alternation of generations, and all pass through remarkable
changes in their life-history, nearly all of them undergoing a
retrogressive metamorphosis.
OUTLINE OF HISTORY.
More than two thousand years ago Aristotle gave a short account of a Simple
Ascidian under the name of _Tethyum_. He described the appearance and some
of the more important points in the anatomy of the animal. From that time
onwards comparatively little advance was made until Schlosser and Ellis, in
a paper on _Botryllus_, published in the _Philosophical Transactions_ of
the Royal Society for 1756, first brought the Compound Ascidians into
notice. It was not, however, until the commencement of the nineteenth
century, as a result of the careful anatomical investigations of Cuvier[66]
upon the Simple Ascidians, and of Savigny[67] upon the Compound Ascidians,
that the relationship between these two groups of Tunicata was conclusively
demonstrated. Up to 1816, the date of publication of Savigny's great work,
the few Compound Ascidians previously known had been generally regarded as
Alcyonaria or as Sponges; and although many new Simple Ascidians had been
described by O. F. Müller[68] and others, their internal structure had not
been investigated. Lamarck[69] in 1816, chiefly as the result of the
anatomical discoveries of Savigny and Cuvier, instituted the class
TUNICATA, which he placed between the Radiata and the Vermes in his system
of classification. The Tunicata included at that time, besides the Simple
and the Compound Ascidians, the pelagic forms _Pyrosoma_, which had been
first made known by Péron in 1804, and _Salpa_ described by Forskål in
1775.
Chamisso, in 1819, made the important discovery that _Salpa_ in its
life-history passes through the series of changes which were afterwards
more fully described by Steenstrup in 1842 as "alternation of generations";
and a few years later Kuhl and Van Hasselt's investigations upon the same
animal resulted in the discovery of the alternation in the directions in
which the wave of contraction passes along the heart, and in which the
{37}blood circulates through the body. It has since been found that this
observation holds good for all groups of the Tunicata. In 1826, H.
Milne-Edwards[70] and Audouin made a series of observations on living
Compound Ascidians, and amongst other discoveries they found the
free-swimming tailed larva and traced its development into the young
Ascidian.
In 1845, Carl Schmidt[71] first announced the presence in the test of some
Ascidians of "tunicine," a substance very similar to cellulose; and in the
following year Löwig and Kölliker[72] confirmed the discovery, and made
some additional observations upon this substance and upon the structure of
the test in general. Huxley,[73] in an important series of papers published
in the _Transactions_ of the Royal and Linnean Societies of London from
1851 onwards, discussed the structure, embryology, and affinities of the
pelagic Tunicates, _Pyrosoma_, _Salpa_, _Doliolum_ and _Appendicularia_.
These important forms were also investigated about the same time by
Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart.
The most important epoch in the history of the Tunicata is the date of the
publication of Kowalevsky's celebrated memoir[74] upon the development of a
Simple Ascidian. The tailed larva had been previously discovered and
investigated by several naturalists, notably by H. Milne-Edwards,[75] P. J.
van Beneden, and Krohn; but its minute structure had not been sufficiently
examined, and the meaning of what was known of it had not been understood.
It was reserved for Kowalevsky in 1866 to demonstrate the striking
similarity in structure and in development between the larval Ascidian and
the Vertebrate embryo. He showed that the relations between the nervous
system, the notochord, and the alimentary canal are practically the same in
the two forms, and have been brought about by a very similar course of
embryonic development. This discovery clearly indicated that the Tunicata
are closely allied to Amphioxus and the Vertebrata, and that the tailed
larva represents the primitive or ancestral form from which the adult
Ascidian has been evolved by degeneration. This led naturally to the view
usually accepted at the present day, that the group is a degenerate
{38}side-branch from the lower end of the phylum CHORDATA, which includes
the Tunicata (or Urochordata), Balanoglossus and its allies (Hemichordata),
Amphioxus (Cephalochordata), and the Vertebrata (or Craniata). Kowalevsky's
great discovery has since been confirmed and extended to all other groups
of the Tunicata by Kupffer,[76] Giard, and others.
In 1872 Fol[77] added largely to the knowledge of the Appendiculariidae,
and Giard[78] to that of the Compound Ascidians. The latter author
described a number of new forms and remodelled the classification of the
group. The most important additions which have been made to the Compound
Ascidians since Giard's work have been the species described by von
Drasche,[79] from the Adriatic, and those discovered by the "Challenger"
expedition.[80] The structure and the systematic arrangement of the Simple
Ascidians have been discussed of recent years mainly by Alder[81] and
Hancock, Heller,[82] Lacaze-Duthiers,[83] Traustedt,[84] Roule, Hartmeyer,
Sluiter[85] and Herdman.[86] In 1874 Ussoff investigated the minute
structure of the nervous system and of the underlying gland, which was
first discovered by Hancock, and showed that the gland has a duct which
communicates with the front of the branchial sac or pharynx by an aperture
in the dorsal (or "olfactory") tubercle. In an important paper published in
1880, Julin[87] drew attention to the similarity in structure and relations
between this gland and the "hypophysis cerebri" of the Vertebrate brain,
and insisted upon their homology. Metcalf has recently added further to our
knowledge on this and related matters.
The Thaliacea or pelagic Tunicata have of late years been the subject of
several very important memoirs. The researches {39}of Todaro, Brooks,[88]
Salensky,[89] Seeliger,[90] Korotneff,[91] and others have elucidated the
embryology, the gemmation and the life-history of the Salpidae; and
Grobben, Barrois,[92] and more especially Uljanin,[93] have elaborately
worked out the structure and the details of the complicated life-history of
the Doliolidae. Finally we owe to the labours of Metschnikoff, Kowalevsky,
Giard, Hjort, Seeliger, Ritter, Van Beneden and Julin, much detailed
information as to development and life-history, the process of gemmation
and the formation of colonies, which has added greatly to our knowledge of
the position and affinities of the Tunicata and of their natural
classification.
STRUCTURE OF A TYPICAL ASCIDIAN.
If a typical "Simple Ascidian," such as the common British _Ascidia
mentula_ (Fig. 15), or _Ascidia virginea_, be examined alive and expanded
in sea-water it will be seen to bear on the upper surface two short
projections, each terminated by a wide tubular opening, through which the
animal, when touched, can emit jets of water with considerable force—thus
accounting for the popular name "sea-squirts." The rest of the body is
covered by the dull grey tough cuticular outer "test" or "tunic" (hence
TUNICATA) by means of which the animal is attached to a rock or other
foreign body. One of the tubular openings, the mouth or "branchial
aperture," is terminal, and indicates the morphological anterior end; it is
surrounded by eight lobes. The other opening, the cloaca or "atrial
aperture," is on the dorsal edge, from one-third to one-half way down the
body, and is bounded by six lobes only; consequently the two apertures, and
so the ends of the body, can be distinguished externally by the number of
lobes—an important matter. The area of attachment is usually the posterior
part of the left side; in Fig. 15 the animal is seen from the right hand
side.
If a little carmine-powder, or some other insoluble particles be scattered
in the water in which the Ascidian is living, the {40}particles will be
seen to converge to the branchial aperture and be sucked in by the inhalent
current entering the body. After a short interval a certain proportion of
the particles will be shot out from the atrial aperture with the exhalent
current.
These particles have passed through the pharyngeal portion of the
alimentary canal and the cloacal passages, with the water used in
respiration, but a considerable amount of such particles taken in with the
water do not reappear, as they are retained by the nutritive organs and
pass along the remainder of the alimentary canal with the food. The current
of water passing in at the branchial and out at the atrial aperture is of
primary importance in the life of the Ascidian. Besides serving for
respiratory purposes it conveys all the food into the body and removes
waste matters both intestinal and renal, and also expels the reproductive
products from the body.
[Illustration: FIG. 15.—_Ascidia mentula_ Linn. from the right side
(natural size), Loch Fyne, N.B.; _Br_, Branchial aperture; _At_, atrial
aperture. Arrows show the direction of the water currents.]
THE TEST.—The test is notable amongst animal structures for containing
"tunicine," a substance which appears to be identical in composition, and
in behaviour under treatment with various reagents, with cellulose. It is
cartilaginous in appearance and consistency, and to some extent in
structure, as it consists of a clear (or in some cases fibrillated) matrix
in which are embedded many corpuscles or cells. It is the matrix that
contains the cellulose, which may form over sixty per cent by weight of the
entire test. As the test is morphologically a cuticle, being a secretion on
the outer surface of the ectoderm (Fig. 16, _ec_), the cells it contains
have immigrated to it from the body, and it has recently been shown that
many of these are mesodermal cells (leucocytes or connective tissue
wandering cells, amoebocytes, and in some cases embryonic "kalymmocytes,"
or {41}egg-follicle cells, see below, p. 56), which have passed through the
ectoderm. This process commences in the larval state with the migration of
mesenchyme cells from the blastocoele through the epiblast. Ectoderm cells,
and possibly also some primitive endoderm cells, also take part in forming
the test. Many of these cells in the test remain small and simple, as the
fusiform and stellate test-cells; some become pigment-cells, while others
enlarge and become vacuolated to form the large (up to 0.15 mm. in
diameter) vesicular or "bladder" cells—this is especially the case in the
outer layer of the test in _Ascidia mentula_ (see Fig. 17, _bl_) where
there are innumerable clear vesicles, each surrounded by a thin film of
protoplasm and having the nucleus still visible at one point of the
surface. In some of the Tunicata the test-cells produce calcareous spicules
of various shapes (see below, p. p. 86).
[Illustration: FIG. 16.—Diagrammatic section through test and mantle of
_Ascidia_ to show the relations of ectoderm to body-wall and cuticle.
_bl.c_, Bladder-cells; _bl.s_, blood-sinus; _c.t.c_, connective tissue
cells; _ec_, ectoderm; _mes.c_, wandering mesoblast cells; _m.f_, muscle
fibres; _t.c_, test-cells; _t.v_, "vessel" of the test."]
The test also becomes organised by the growth into it of the so-called
"vessels." These are outgrowths of the body-wall covered by ectoderm and
containing prolongations of blood-channels from the connective tissue of
the "mantle" (body-wall). Fig. 16, _t.v_ shows such an outgrowth, and
exhibits the general relations of test (cuticle), ectoderm, and mesoderm.
It also explains how it is that the blood-channel being pushed out as a
{42}loop gives rise to the double or paired "vessels" seen branching
through the test (see Fig. 17, _v_). The two vessels of a pair are one
blood-channel imperfectly divided by a connective-tissue septum. The blood
courses out along one side, round the communication in a "terminal knob" at
the end, and back down the other side. The "terminal knobs" are very
numerous, and form a marked feature in the outer layer of the test (Fig.
17, _t.k_); in some cases (_Culeolus murrayi_), they probably form an
accessory organ of respiration, while in others (Botryllidae), they pulsate
and aid in keeping up the circulation.
The ectoderm is a simple epithelial layer (Fig. 16, _ec_). It is turned in
for a short distance at the branchial aperture (mouth), and atrial aperture
(cloaca), as a short stomodaeum and proctodaeum respectively, lined in each
case by a delicate prolongation of the test.
[Illustration: FIG. 17.—Section through the surface layer of test of
_Ascidia mentula_, × 50. _bl_, Bladder cells; _t.c_, test cell; _t.k_,
terminal knobs of vessels; _v_, vessels of test.]
Fig. 24, A, p. 52, shows the relations of ectoderm, mesoderm, and endoderm
in a section through the antero-dorsal part of the body. The cavity marked
_p.br_ is a portion of the atrial cavity lined by ectoderm, and must not be
confounded with a coelom. The absence of a true coelom in the mesoderm will
be noticed in this and other figures, and yet the Tunicata are Coelomata,
although it is very doubtful whether the enterocoel which has been
described in the development of some is ever found. The coelom is in any
case largely suppressed later, and is only represented in the adult by the
pericardium and by small cavities in the renal and reproductive organs and
ducts.
BODY-WALL AND CAVITIES OF THE BODY.—The name "mantle" is given to the
ectoderm with the parietal mesoderm which form the body-wall inside the
test. It is largely formed of connective {43}tissues—both homogeneous and
fibrous—with cells, blood-sinuses, and many muscle-bundles large and small
running circularly, longitudinally, and obliquely, and interlacing in all
directions (Fig. 18, _m_). The muscles are all formed of very long fusiform
non-striped fibres. The mantle in some Ascidians is often brilliantly
pigmented—red, yellow and opaque white, the coloured cells being exactly
like those found in the blood.
[Illustration: FIG. 18.—Dissection of _Ascidia_, from right side, to show
anatomy. _a_, Anus; _At_, atrial aperture; _Br_, branchial aperture;
_br.s_, _br.s′_, branchial sac; _end_, endostyle; _g.d_, genital ducts;
_gon_, ovary; _hyp_, neural gland; _hyp.d_, the duct leading to dorsal
tubercle; _m_, mantle; _n.g_, ganglion; _oes_, oesophagus; _p.br.c_,
peribranchial cavity; _ren_, renal vesicles; _st_, stomach; _t_, test;
_tn_, tentacles; _ty_, typhlosole.]
The mantle forms two well-marked siphons or short wide tubes, which lead in
from the branchial and atrial apertures. These are surrounded by strong
sphincter muscles,[94] and are lined by the invaginated ectoderm and test.
The one leads into the branchial sac or modified pharynx, and the other
into the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, _p.br_).
Figs. 18 and 19 show the relations of the branchial and peribranchial
cavities to one another. The peribranchial cavity {44}opens to the exterior
dorsally by the atrial aperture, forms the cloaca along the dorsal edge of
the body, and has extensions laterally on each side of the branchial sac,
with the interior of which it is placed in communication by the secondary
gill-slits or "stigmata" (Fig 19, _sg_). Along the ventral edge the mantle
is united to the wall of the branchial sac, and it is only this union (Fig.
19, _end_) that prevents the peribranchial cavity from completely
surrounding the branchial sac.
The following list of the cavities present in the body of the adult
_Ascidia_ may be useful at this point:—
1. The alimentary canal, including the branchial sac. This is derived from
the archenteron of the embryo, is lined throughout by endoderm, and the
system of cavities of the intestinal gland is to be regarded merely as an
outgrowth from the alimentary canal.
2. The peribranchial (atrial) cavity, derived from two lateral ectodermal
invaginations which join dorsally to form the cloaca and open to the
exterior by the atrial aperture.
3. The original embryonic segmentation cavity (blastocoele) remains, where
not obliterated by the development of the mesodermal connective tissue, as
the irregular system of blood spaces, with its outgrowths in test and
branchial sac. The heart, which has differentiated muscular walls, becomes
secondarily connected at its ends with these blood spaces.
4. The pericardium and epicardium (see p. 83) originate as outgrowths from
the archenteron. They may therefore be regarded as enterocoelic spaces. The
pericardium becomes completely closed off and separated from the alimentary
canal. The epicardium may form paired tubes of great length, and may
remain permanently connected with the branchial sac.
5. The cavities of the renal vesicles and of the gonads and ducts are
spaces formed in the mesoblast. They have been variously interpreted:—
(_a_) As of the same nature as the blood spaces (blastocoelic), or
(_b_) As formed by a splitting of the mesoblast (coelomic).
6. The cavity of the neural gland and of its duct opening at the dorsal
tubercle is derived from the primitive dorsal neural tube of the embryo,
and so may be regarded as a part of the lumen of the cerebro-spinal nervous
system.
TENTACLES, ETC.—The branchial aperture leads through the {45}branchial
siphon into the branchial sac. At the base of the siphon, just about the
line of junction of the ectoderm of the stomodaeum with the endoderm of the
mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18,
_tn_) which stand out at right angles to the wall, and more or less
completely meet in the centre to form a delicate, sensory grid or sieve
through which all the water entering the body has to pass. These tentacles
not only act mechanically, but are also sensitive although only scattered
sensory cells, and no specially differentiated sense-organs are found upon
them. Behind the tentacles lies the plain, or papillated, prebranchial zone
(Fig. 21, _p.br.z_), bounded behind by a pair of parallel and closely
placed ciliated ridges with a groove between—the peripharyngeal bands—which
encircle the anterior end of the branchial sac.
[Illustration: FIG. 19.—Semi-diagrammatic transverse section of _Ascidia_,
passing through the atrial aperture, seen from anterior surface, left side
uppermost. _At_, Atrial aperture; _at.l_, atrial lobe; _Br.s_, branchial
sac; _cl_, cloaca; _con_, connective; _d.bl.s_, dorsal blood-sinus; _d.l_,
dorsal lamina; _end_, endostyle; _g.d_, genital ducts; _i, i′_, intestine;
_l.v_, interstigmatic vessel; _m_, mantle; _m.b_, muscle-bundles; _ov_,
ovary; _p.br_, peribranchial cavity; _r_, rectum; _ren_, renal vesicles;
_sg_, stigmata; _sph_, atrial sphincter; _t_, test; _tr_, transverse
vessel; _ty_, typhlosole; _v.bl.s_, ventral blood-sinus.]
The branchial sac is very large—much the largest organ of the body—and
extends almost to the posterior end of the body, while the rest of the
alimentary canal lies upon its left side. The food particles, consisting of
microscopic plants and animals, are carried in through the branchial
aperture by the current of water, but most of them do not pass out through
the gill-slits to the atrium, being entangled in the viscid mucus which
passes by ciliary action along the groove between the peripharyngeal bands.
{46}ENDOSTYLE.—The mucus just referred to is produced in the long
canal-shaped gland called the endostyle or hypobranchial groove, which runs
along the entire ventral edge of the branchial sac (Fig. 18, _end_). The
sides, and especially the floor of the endostyle, are richly ciliated,
while there are four (or six) strongly-marked, peculiarly-shaped glandular
tracts, two (or three) on each side (Fig. 20, _gl_) running along its
length, and separated by areas of closely-packed fusiform cells with short
cilia, amongst which are found some bipolar sensory cells.
[Illustration: FIG. 20.—Transverse section of the endostyle of _Ascidia
mentula_, × 350. _bl.s_, Blood-sinus; _end.l_, lips of the endostyle; _gl_,
glandular tracts; _i.l_, internal longitudinal bar; _l.v_, interstigmatic
vessels; _m_, mantle; _p.br_, peribranchial cavity; _sg_, stigmata;
_v.bl.s_, ventral blood-sinus.]
This organ corresponds to the hypopharyngeal groove of Amphioxus and the
median part of the thyroid gland of Vertebrata. It is interesting to notice
that the (at least) four longitudinal tracts of gland-cells are of
remarkable constancy, being found not only in all groups of Tunicata,
including even the pelagic, tailed Appendicularians, but also in Amphioxus
and in the young thyroid gland of the Ammocoete. When, in Ascidians, a
third marginal glandular tract is added it has a different appearance from
the two characteristic tracts. The mucus is carried forward by the action
of the large floor-cilia of the endostyle (Fig. 20) to the groove between
the peripharyngeal bands, and after encircling the anterior end of the
branchial sac and collecting the food particles, it passes backwards along
the dorsal edge {47}of the branchial sac to the oesophagus, guided by a
membranous fold, the dorsal lamina (Fig. 21, _d.l_), which is more or less
ridged or corrugated, and may be armed with marginal tags or even replaced
by larger processes (the "languets") in some species of Ascidians. In the
living animal the lamina has its free edge curved to the right hand side in
such a manner as to constitute a fairly perfect tube along which the train
of food passes.
[Illustration: FIG. 21.—Antero-dorsal part of pharynx in _Ascidia mentula_,
× 15. _br.s_, Part of branchial sac; _d.l_, dorsal lamina; _d.t_, dorsal
tubercle; _p.br.z_, prebranchial zone; _p.p_, peripharyngeal bands; _sph_,
sphincter of branchial aperture; _tn_, tentacle.]
BRANCHIAL SAC.—Thus we have the dorsal lamina (or the languets) along the
dorsal edge, the endostyle along the ventral edge, and the peripharyngeal
bands around the anterior end. The wall of the branchial sac itself is
penetrated by a large number of channels through which blood flows. Some of
these run in one direction and some in another, so as to form complicated
networks, which differ greatly in their arrangement in different Ascidians.
Between these blood-channels there are clefts ("stigmata"), the secondary
or subdivided gill-slits, by means of which the current of water passes
from the branchial sac to the large external peribranchial or atrial
cavity. All the stigmata (of which there may be several hundred thousand)
in the wall of the branchial sac are bounded by cubical or columnar
epithelial cells, which are ciliated. These cilia, so long as the animal is
alive, are in constant motion, so as to drive the water onwards, and it is
this constant ciliary action in the walls of the branchial sac that gives
rise to the all-important current of water streaming through the body. In
addition to the stigmata there are generally one or two much larger
elongated slits (Garstang's {48}pharyngo-cloacal slits) placed close to the
dorsal lamina and leading direct to the cloaca.
[Illustration: FIG. 22.—A mesh of the branchial sac of _Ascidia_, seen A,
from inside; B, in horizontal section. _c.d_, Connecting duct; _h.m_,
horizontal membrane; _i.l_, internal longitudinal bars; _l.v_,
interstigmatic vessels; _p_, _p′_, papillae; _sg_, stigmata; _tr_,
transverse vessels.]
Fig. 22 shows a small part of the wall of the branchial sac, in which it
may be seen that the bars containing the blood-channels are arranged in
three regular series:—(1) The "transverse vessels" which run horizontally
round the wall and open at their dorsal and ventral ends into large median
longitudinally running tubes, the dorsal blood-sinus (or "dorsal aorta")
behind the dorsal lamina, and the ventral blood-sinus (or "branchial
aorta") beneath the endostyle; (2) the fine longitudinal or "interstigmatic
vessels" which run vertically between adjacent transverse vessels and open
into them, and which therefore bound the stigmata; and (3) the "internal
longitudinal bars" which run vertically, in a plane internal to that of the
transverse and fine longitudinal vessels. These bars (Fig. 22, _i.l_)
communicate with the transverse vessels by short side branches where they
cross, and at these points are prolonged into the cavity of the sac in the
form of hollow papillae. In some Ascidians (e.g. _Corella_ and most of the
Molgulidae) the interstigmatic vessels are curved so that the stigmata form
more or less complete spirals (see Figs. 35 and 41). In some species of
_Ascidia_, and other Ascidians, the interstigmatic vessels are inserted
into the transverse vessel in an undulating course in place of the straight
line seen in Fig. 22, B, _l.v_, the result being that the stigmatic part of
the wall of the branchial sac seems to be folded or thrown into microscopic
crests and troughs. This is known as "minute plication." In some cases,
again (Cynthiidae), the whole wall of the sac is pushed inwards at
intervals to form large folds visible to the eye (see Fig. 36, A and B).
The intersections of the internal longitudinal bars with the transverse
vessels divide up the inner surface of the branchial {49}sac wall into
rectangular areas called "meshes." One such mesh, containing eight stigmata
in a row, is seen in Fig. 22, A. The internal longitudinal bars bear
papillae at the angles of the meshes, and occasionally in intermediate
positions. There are frequently horizontal membranes (Fig. 22, B, _h.m_)
attached to the transverse vessels between the papillae. There are many
"connectives" running from the outer wall of the branchial sac to the
mantle outside, and allowing the blood in the transverse vessels to
communicate with that in the sinuses of the mantle (see Fig. 19, _con_).
HEART AND CIRCULATION.—It is one of the notable features of the Tunicata
that the circulation is not constant in direction, but is periodically
reversed.
The blood of Ascidians is in the main transparent, but usually contains
certain pigmented corpuscles in addition to many ordinary leucocytes or
colourless amoeboid cells. The pigment in the coloured cells may be red,
yellow, brown, or in some cases blue or opaque white. The blood may reach
the branchial sac either from the dorsal or from the ventral median sinus
according to the direction in which the heart is beating at the moment (see
below); and it is a most interesting and beautiful sight to see the
circulation of the variously coloured corpuscles through the transparent
vessels, and the lashing of the cilia along the edges of the neighbouring
stigmata in a small Ascidian under the microscope.
In _Ascidia_ (Fig. 23) the heart is an elongated fusiform tube placed on
the ventral and posterior edge of the stomach, projecting into a space (the
pericardium) which is a part of the original coelom, the remainder of which
is represented in the adult by the reproductive and renal cavities. The
wall of the heart is continuous along one edge with that of the
pericardium, and the heart is to be regarded as a tubular invagination of
the pericardial wall, shutting in a portion of the surrounding space (the
blastocoel of the embryo), and having open ends which communicate with the
large blood sinuses leading to the branchial sac, to the viscera, and to
the body-wall and test. The cavity of the heart is not divided and there
are no valves. Its wall is formed of a single layer of epithelio-muscular
cells, the inner, muscular, ends of which are cross-striated fibres running
round the heart—the only striated muscular tissue found in the body. Waves
of {50}contraction pass along the heart from end to end, first for a
certain number of beats in one direction, and then, after an interval, in
the other. If a small or young _Ascidia_ be placed alive, left side
uppermost, in a watch-glass or small trough of sea-water, and examined with
a low power of the microscope, the heart will be readily seen near the
posterior end of the transparent body. It will be noticed that the
"beating" looks like successive waves of blood pressed through the tubular
heart from one end to the other by its contractions. After watching the
waves passing, let us say, from the right hand end of the heart to the left
for about a minute and a half (perhaps 60 or 80 to 100 beats), it will be
seen that they gradually become slower and then stop altogether. But after
seven or eight seconds a faint wave of contraction will start from the
_left_ end of the heart and pass over it to the right; and this will be
followed by larger ones for a minute and a half, and then again a pause
will occur and the direction change. It has been suggested that the cause
of this remarkable reversal may possibly be that the heart being on the
ventral vessel, which is wider than the corresponding dorsal trunk, pumps
the blood into either the lacunae of the branchial sac or those of the
viscera in greater volume than can possibly get out through the smaller
branchio-visceral vessel in the same time, the result being that the
lacunae in question soon become engorged, and by back pressure cause the
stoppage, and then reversal of the beat. The absence of any valves in the
heart to regulate the direction of flow obviously facilitates this
alternation of the current.
The larger channels through which the blood flows may be lined with a
delicate endothelium, but the smaller passages are merely spaces in the
connective tissue. The heart, although anatomically a "ventral vessel,"
runs in the main dorso-ventrally. The blood-channel leaving the ventral end
of the heart is the "branchio-cardiac vessel" (Fig. 23, _b.c_). This gives
off a branch which, along with a corresponding branch from the
"cardio-visceral" vessel (_c.v_) at the other end of the heart, goes to the
test, and then runs along the ventral edge of the branchial sac as the
branchial aorta (_b.a_), external to the endostyle, communicating laterally
with the ventral ends of all the transverse vessels of the branchial sac.
The cardio-visceral vessel (Fig. 23, _c.v_) after giving off its branch to
the test breaks up into a number of sinuses which {51}ramify over the
alimentary canal and the other viscera. These visceral lacunae finally
communicate with a third great sinus, the "branchio-visceral" vessel
(_b.v_) which runs forward along the dorsal edge of the branchial sac as
the dorsal aorta (_d.a_), externally to the dorsal lamina, and joins the
dorsal ends of all the transverse vessels of the branchial sac. Besides
these three chief systems—the branchio-cardiac, the cardio-visceral, and
the branchio-visceral—(see Fig. 23), there are numerous lacunae in all
parts of the body by means of which anastomoses are established between the
different currents of blood.
[Illustration: FIG. 23.—Diagrammatic dissection of _Ascidia_, from left
side, to show course of circulation. Front part of branchial sac opened,
back part covered by viscera. _b.a_, Branchial (ventral) aorta; _b.c_,
branchio-cardiac vessel; _b.v_, branchio-visceral vessel; _c.v_,
cardio-visceral vessel; _d.a_, dorsal aorta; _ht_, heart. _A_, anterior;
_P_, posterior; _D_, dorsal; _V_, ventral.]
When the heart contracts ventro-dorsally the course of the circulation is
as follows:—the blood which is flowing through the vessels of the branchial
sac is collected in an oxygenated condition in the branchio-cardiac vessel,
and after receiving a stream of blood from the test enters the ventral end
of the heart. It is then propelled from the dorsal end into the
cardio-visceral vessels, and so reaches the test and the digestive and
other viscera; then, after circulating in the visceral lacunae it passes
into the branchio-visceral vessel in an impure condition, and is
distributed to the branchial vessels to be purified again. When the heart,
on the other hand, contracts dorso-ventrally, this course of the
circulation is reversed, the "veins" and "arteries" exchange functions, and
what a minute before was a "systemic," is now a "respiratory" heart. This
is a phenomenon without parallel in the animal kingdom.
{52}All the blood-spaces and lacunae are probably derived, like the cavity
of the heart, from the blastocoel of the embryo, and are not, like the
cavity of the pericardium, a part of the coelom (of endodermal origin).
NEURAL GLAND AND DORSAL TUBERCLE.—In the dorsal median line near the
anterior end of the body, and imbedded in the mantle on the ventral[95]
surface of the nerve-ganglion, there lies a small glandular mass—the neural
gland—which, as Julin first showed, there is some reason to regard as the
homologue of the hypophysis cerebri of the Vertebrate brain. Metcalf has
recently shown that the neural gland may be a double structure—partly
cerebral and partly stomodaeal—as in Vertebrates.
[Illustration: FIG. 24.—Antero-dorsal part of _Ascidia_ showing the
relations of the layers of the body, and of the nervous system. A, in
sagittal section; B, in transverse section. _d.bl.s_, Dorsal blood-sinus;
_d.l_, dorsal lamina; _d.n_, dorsal nerve; _d.t_, dorsal tubercle; _ect_,
ectoderm; _en_, endoderm; _e.p.br_, epithelium of peribranchial cavity;
_gl.d_, duct of subneural gland; _l.v_ points to the ciliated epithelium
covering a longitudinal vessel of branchial sac; _m_, mantle; _n_, nerve;
_n.g_, ganglion; _n.gl_, neural gland; _p.br_, peribranchial cavity;
_pp.b_, peripharyngeal bands; _sph_, branchial sphincter; _t_, _t′_, test;
_tn_, tentacle.]
The function of this gland is still somewhat mysterious. It may merely form
the viscid secretion which is carried along the peripharyngeal bands and
down the dorsal lamina. On the other hand, it has been suggested that the
function of the organ may possibly be renal, for the removal of nitrogenous
waste matters in the neighbourhood of the nervous system. Finally, it may
be a lymph gland.
{53}The neural gland, which was first noticed by Hancock, may be continued
backwards along with the dorsal nerve, and it communicates anteriorly by
means of a narrow duct with the front of the branchial sac (pharynx). The
opening of the duct is enlarged to form a funnel-shaped cavity (Fig. 24,
A), which may be folded upon itself, convoluted, or even broken up into a
number of smaller openings (see Fig. 43, p. 79), so as to form a
complicated projection called the dorsal tubercle, situated in the dorsal
part of the prebranchial zone. The dorsal tubercle in _Ascidia mentula_ is
somewhat horse-shoe shaped (Fig. 21, _d.t_); it varies in most Ascidians
(see Fig. 43) according to the genus and species, and in some cases in the
individual also. Sensory cells are found in the epithelium, and so it is
highly probable that besides being the opening of the duct from the neural
gland, this convoluted ciliated ridge may be a sense-organ for testing the
quality of the water entering the branchial sac.
NERVOUS SYSTEM AND SENSE-ORGANS.—The single elongated ganglion (Fig. 24,
_n.g_), in the median dorsal line of the mantle, between the branchial and
atrial siphons, is the only nerve-centre in _Ascidia_ and most other
Tunicata. It is the degenerate remains of the dorsal wall of the tubular
cerebro-spinal nervous system of the trunk-region of the tailed larval
Ascidian—the ventral wall opposite having given rise to the subneural
gland. The more posterior or spinal part of the larva has almost entirely
disappeared in most adult Tunicata. It persists, however, in the
Appendiculariidae, and traces of it have been found in the dorsal nerve
running backwards towards the oesophagus in some Ascidians (e.g.
_Clavelina_). It may be ganglionated in Molgulidae.
The ganglion has small rounded nerve-cells on its surface, and interlacing
nerve-fibres inside. It gives off distributory nerves at both ends (Fig.
24, A), which run through the mantle to the neighbourhood of the apertures,
where they divide up to supply the lobes and the sphincter muscles. The
only sense-organs are the pigment spots ("ocelli," formed of modified
ectoderm cells imbedded in red and yellow pigment), between the branchial
and atrial lobes, the tentacles at the base of the branchial siphon, and
probably the dorsal tubercle and the languets or dorsal lamina, in all of
which, as well as in the endostyle and peripharyngeal bands and in papillae
on the ectoderm and in the branchial sac, sensory cells have been found.
{54}These, considered as sense-organs, are all in a lowly-developed
condition. The larval Ascidians, on the other hand, have well-developed
intra-cerebral optic and otic sense-organs (see Fig. 26, p. 60), and in
some of the pelagic Tunicata, otocysts and pigment-spots are found in
connexion with the ganglion.
ALIMENTARY CANAL.—The mouth and pharynx (branchial sac) have already been
described. The remainder of the alimentary canal is a bent tube, which in
_A. mentula_ and most other Ascidians lies imbedded in the mantle on the
left side of the body, and projects into the peribranchial cavity (see
Figs. 18 and 19). The oesophagus leaves the branchial sac in the dorsal
middle line, near the posterior end of the dorsal lamina. It is a short
curved tube which leads ventrally to the large fusiform thick-walled
stomach, ridged internally. The intestine emerges from the ventral end of
the stomach and soon turns anteriorly, then dorsally, and then posteriorly,
so as to form a curve, the intestinal loop, in which the ovary lies, open
posteriorly. The intestine now curves anteriorly again, and from this point
runs nearly straight forward as the rectum, thus completing a second curve,
the rectal loop, in which the renal vesicles lie, open anteriorly. The wall
of the intestine is thickened internally to form the typhlosole (Fig. 18,
_ty_), a pad which runs along its entire length, so as to reduce the lumen
of the tube to a crescentic slit. The anus opens into the dorsal or cloacal
part of the peribranchial cavity near the atrial aperture. The walls of the
stomach are glandular, and most of the endoderm cells lining the tube are
ciliated. A system of delicate, microscopic, branched tubules with dilated
ends (the "refringent organ"), which ramifies over the outer wall of the
intestine, and communicates with the cavity of the stomach at the pyloric
end by means of a duct is probably a digestive gland. There is in _Ascidia_
no separate large gland to which the name "liver" can be applied, as in
some other Tunicata.
RENAL ORGAN.—A mass of large clear-walled vesicles which occupies the
rectal loop (Figs. 18 and 19, _ren_), and may extend over the adjacent
walls of the intestine, is a renal organ without a duct. Each vesicle is
the modified remains of a part of the primitive coelom or body-cavity, and
is formed of cells which eliminate nitrogenous waste matters from the blood
circulating in the neighbouring blood-lacunae, and deposit them in the
cavity of {55}the vesicle, where they form one or more concentrically
laminated concretions of a yellowish or brownish colour, sometimes coated
with a chalky deposit. These concretions contain uric acid, and in a large
Ascidian are very numerous. The nitrogenous waste products are thus
deposited and stored up in the renal vesicles in place of being excreted
from the body. In other Ascidians the renal organs may differ from the
above in position and structure; but in no case have they any excretory
duct, unless the neural gland is to be regarded as one of the renal
organs—which has not yet been proved.
REPRODUCTIVE ORGANS.—_Ascidia mentula_ is hermaphrodite, and the
reproductive organs lie with the alimentary canal, on the left side of the
body (Fig. 19, _ov_). The ovary is a ramified gland which occupies the
greater part of the intestinal loop. It contains a cavity which, along with
the cavities of the testis, is derived from an embryonic coelom; the ova
are formed from its walls, and fall when mature into the cavity. The
oviduct is continuous with the cavity of the ovary, and leads forward
alongside the rectum, finally opening near the anus into the peribranchial
cavity (Fig. 18, _g.d_). The testis is composed of a great number of
delicate, branched tubules, which ramify over the ovary and the adjacent
parts of the intestinal wall. These tubules terminate in ovate swellings.
Near the commencement of the rectum the larger tubules unite to form the
vas deferens, a tube of considerable size, which runs forward alongside the
rectum, and, like the oviduct, terminates by opening into the peribranchial
cavity close to the anus. The lumen of the tubules of the testis, like the
cavity of the ovary, is a part of the embryonic mesoblastic space, and the
spermatozoa are formed from the cells lining the wall. In some Ascidians
(certain Molgulidae and Cynthiidae), reproductive organs are present on
both sides of the body, and in others, as in _Polycarpa_, there are many
complete sets of both male and female systems attached to the inner surface
of the mantle on both sides of the body and projecting into the
peribranchial cavity.
EMBRYOLOGY AND LIFE-HISTORY OF A TYPICAL ASCIDIAN.
The eggs of Tunicata are for the most part of small size, nearly colourless
and transparent, and with little or no food-yolk. {56}In some, however
(such as some of the Cynthiidae, and some Compound Ascidians), the eggs are
larger, more opaque, and have a fair amount of food-yolk. Ova of this type
are not expelled from the body of the parent as ova, but are fertilised,
and remain in the atrial cavity or in a special diverticulum thereof—the
incubatory pouch—until they are far advanced in development; and usually
leave the body as tailed larvae. In many species, the ova and spermatozoa
mature at different times in the life-history, and so self-fertilisation is
prevented. Some species (such as many Botryllidae and Distomatidae) are
protogynous, the ova being produced and shed before the testes have
matured, while other species (_Coelocormus huxleyi_) are protandrous, being
male while young and female later. But there is no doubt that in other
cases (e.g. _Ascidia mentula_) self-fertilisation is not only possible, but
does take place. After maturation certain of the follicle-cells which
invest the ovum in the ovary migrate into the egg and proliferate so as to
form a layer in the superficial part of the egg, where they appear as the
so-called "testa-cells" or "kalymmocytes" (Fig. 25, A, _t.c_). The
remaining follicle-cells may form two or more layers, usually one of large
cubical cells, which may become greatly vacuolated, next to the ovum, and
an external flattened layer which is cast off when the egg escapes from the
ovary.
Segmentation is complete and results in the formation of a spherical
blastula with a small segmentation-cavity (Fig. 25, C). The blastula grows
larger and begins to differentiate.[96] There are slightly smaller cells
which divide more rapidly at one end of this embryo, the future ectoderm,
and slightly larger and more granular cells at the other, which become
chiefly endoderm (hypoblast). Invagination of the larger cells then takes
place (Fig. 25, D), resulting in the formation of a gastrula with an
archenteron. The hypoblast cells lining the archenteron become columnar
(_hy_). The curving and more rapid growth at the anterior end of the embryo
narrow the primitively wide open blastopore, and carry it to the posterior
end of the future dorsal surface (Fig. 25, E). The orientation of the body
is now clear.
{57}[Illustration: FIG. 25.—Embryology of Ascidian. A, mature ovum: _foll_,
follicle-cell; _m_, membrane; _n_, nucleus; _p_, protoplasm; _t.c_,
test-cell; B, mature spermatozoon; C, segmentation-stage in section to show
blastocoel; D, early gastrula-stage; E, later gastrula-stage; F, later
embryo showing rudiments of notochord and neural tube; G, transverse
section of body of embryo showing mesoblast and formation of neural canal;
H, late embryo showing body and tail, notochord, neural canal, and
mesenteron; I, young larva ready to be hatched; K, transverse section of
tail of larva. _ar_, Archenteron; _at_, atrial invagination; _au_, otocyst;
_b.c_, blastocoel; _b.p_, blastopore; _ch_, notochord; _ep_, epiblast; _f_,
tail-fin; _hy_, hypoblast; _m.b_, mesoblast; _mes_, mesenteron; _musc_,
muscle-cell; _n.c_, neural canal; _ne.c_, neurenteric canal; _n.v_, neural
vesicle; _oc_, ocellus. (Modified from Kowalevsky and others.)]
The embryo is elongated antero-posteriorly, the dorsal surface is
flattened, and the blastopore indicates its posterior end. Around the
blastopore the large ectoderm cells form a medullary plate, along which a
groove (the medullary groove), runs forwards, bounded at the sides by
medullary folds which meet behind the blastopore. Underneath the posterior
part of the medullary groove certain of the hypoblast cells from the dorsal
wall of the archenteron, in the median line, form a band extending forwards
(Fig. 25, E, _ch_). This band separates off from the hypoblast, which
closes in beneath it, and thus gives rise to the notochord (Fig. 25, F).
The more lateral and posterior cells become mesoblast, and separate off as
lateral plates, which show no trace of metameric segmentation (Fig. 25, G).
The remainder of the archenteron {58}becomes the branchial sac, and by
further growth buds off the rest of the alimentary canal.
The medullary groove now becomes converted into the closed neural canal by
the growing up and arching inwards (Fig. 25, G, _n.c_) of the medullary
folds, which unite with one another from behind forwards in such a way that
the blastopore now opens from the enteron into the floor of the neural
canal, forming the neurenteric passage (Fig. 25, F, _n.e.c_). For a time
the anterior end of the neural canal remains open as a neuropore. By this
time the posterior end is elongating to form a tail, and the embryo is
acquiring the tadpole-shape (Fig. 25, H) characteristic of the free larva.
The tail grows rapidly, curves round the body, and also undergoes torsion,
so that its dorsal surface comes to lie on the left side. It contains
ectoderm cells on its surface, notochordal cells (in single file) up the
centre (see Fig. 25, H, _ch_), a neural canal dorsally, and a row of
endoderm cells representing the enteron ventrally to the notochord. Later
on the mesoblast also is prolonged into the tail, where it forms a band of
striated muscle-cells at each side of the notochord. When the ectoderm
cells begin to secrete the cuticular test this forms two delicate
transparent longitudinal (dorsal and ventral) fins in the tail (Fig. 25, K,
_f_), and especially at its extremity where radial thickenings form striae
resembling fin-rays. The ectoderm on the anterior end of the body grows out
into three adhering papillae (Fig. 26, A).
The neural canal now differentiates into a tubular dorsal nervous system.
The anterior end dilates to form the thin-walled cerebral vesicle (see
Figs. 25, I, and 26, A), containing later the intra-cerebral, dorsal,
pigmented eye (_oc_), and the ventral otolith (_au_) of the larva. The next
part of the canal thickens to form the trunk-ganglion, and behind that is
the more slender "spinal cord," which runs to the extremity of the tail. A
ciliated diverticulum of the anterior end of the enteric cavity (future
pharynx) which enters into close relations with the front of the cerebral
vesicle,[97] and later opens into the ectodermic invagination which forms
the mouth at that spot, is evidently the rudiment of the neural duct or
hypophysial canal. The future branchial sac (pharynx), with a ventral
median thickening which will be the endostyle, is by this time clearly
distinguishable by its large size {59}from the much narrower posterior part
of the enteron, which grows out to become the oesophagus, stomach, and
intestine. The notochord does not extend forward into the pharyngeal
region, but is confined to the posterior or caudal part of the embryo. It
now shows lenticular pieces of a gelatinous intercellular substance
secreted by the cells and lying between them (Fig. 25, I). The mouth forms
as a stomodaeum, or ectodermal invagination, antero-dorsally in the region
where the neuropore has closed, and about the same time two lateral
ectodermal involutions form (Fig. 26, A, _at_), which become the atrial or
peribranchial pouches, at first distinct, afterwards united in the
mid-dorsal line to form the adult cloaca and atrial aperture. Ingrowths
from the atrial pouches and outgrowths from the wall of the pharynx
coalesce to form the proto-stigmata (primary gill-slits) by which the
cavity of the branchial sac is first placed in communication with the
exterior through the atrial apertures. Opinions differ as to whether only
one or a few pairs of true gill-clefts are represented in the young
Ascidian; and the actual details of their formation and subdivision, to
form the stigmata of the adult, differ considerably in different forms. In
_Clavelina_ the stigmata are formed as independent perforations of the
pharyngeal wall; in _Ascidia_ two pairs of protostigmata increase to six
pairs, which are subdivided into stigmata; _Botryllus_ and other forms are
intermediate in some respects. No doubt the subdivision of proto-stigmata
is primitive, but has been lost from the ontogeny in some cases. To what
precise extent the walls of the atrial or peribranchial cavities are formed
of ectoderm, or of endoderm, is still doubtful.
The embryo is hatched about two or three days after fertilisation, as a
larva or Ascidian tadpole (Fig. 26, A) which leads a free-swimming
existence for a short time, during which it develops its nervous system and
cerebral sense-organs, and the powerful mesoblastic muscle-bands lying at
the sides of the notochord (now a cylindrical rod of gelatinous nature
surrounded by the remains of the original cells) in the tail which form the
locomotory apparatus. Fig. 26, A, shows this stage, the highest in its
chordate organisation, when the larva swims actively through the sea by
vibrating its long tail with the dorsal and ventral fins.
In addition to the structures already mentioned, the mesoderm {60}has
formed the beginning of the muscular body-wall, the connective tissue
around the organs, and the blood; the endostyle has developed as a
thick-walled groove along the ventral edge of the pharynx, which has become
the branchial sac; and the pericardial sac and its invagination the heart
have formed in the mesoblast between the endostyle and stomach. The
"epicardiac tubes" grow out from the posterior end of the endostyle to join
the pericardium. They play an important part in the formation of buds in
the colonial Tunicata. The heart acquires a connexion with blastocoelic
blood-spaces at its two ends. The heart and pericardium show the same
relations in Tunicata as in Enteropneusta, but it is very doubtful whether
these organs are genetically related to the Vertebrate heart.
[Illustration: FIG. 26.—Metamorphosis of an Ascidian. A, free-swimming
tailed larva; B, the metamorphosis—larva attached; C, tail and nervous
system of larva degenerating; D, further degeneration and metamorphosis of
larva into E, the young fixed Ascidian. _at_, Atrial invagination; _ch_,
notochord; _hy_, hypoblast cells; _i_, intestine; _m_, mouth; _mes_,
mesenteron; _n.c_, neural canal; _n.v_, neural vesicle with sense-organs.
(Modified from Kowalevsky and others.)]
The unpaired optic organ in the cerebral vesicle when fully formed has a
retina, pigment layer, lens and cornea; while the ventral median organ is a
large, spherical, partially-pigmented otolith attached by delicate
hair-like processes to the summit of a hollow "crista acustica" (Fig. 26,
A). After a few hours, or at most a day or so, the larva attaches itself
{61}by one or more of the three anterior ectodermal glandular papillae (one
dorsal and two lateral) to some foreign body, and commences the
retrogressive metamorphosis which leads to the adult state. The adhering
papillae, having performed their function, begin to atrophy, and their
place is taken by the rapidly increasing test. The tail which at first
vibrates rapidly is partly withdrawn from the test and absorbed, and partly
cast off in shreds (Fig. 26, B, C, D). The notochord, nerve-tube, muscles,
etc., are withdrawn into the body, where they break down and are absorbed
by phagocytes. The posterior part of the nerve cord and its anterior end
with the large sense-organs disappear, and the middle part or
trunk-ganglion is reduced to form the relatively small ganglion of the
adult, underneath which the hypophysial tube gives rise to the neural
gland. While the locomotory, nervous and sensory organs are thus
disappearing, or being reduced, the alimentary canal and reproductive
viscera are growing largely. The branchial sac enlarges, its walls become
penetrated by blood-channels, and grow out to form bars and papillae, and
the number of openings greatly increases by the primary gill-slits being
broken up into the transverse rows of stigmata. The stomach and intestine,
which developed as an outgrowth from the back of the branchial sac at the
right side, become longer and curve, so that the end of the intestine
acquires an opening into at first the left hand side, and eventually the
cloacal or median part of the atrial cavity. The adhering papillae have now
disappeared, and are replaced functionally by a growth of the test over
neighbouring objects; and at the same time the region of the body between
the point of fixation and the mouth (branchial aperture) increases rapidly
in extent, so as to cause the body of the Ascidian to rotate through about
180°, and thus the branchial siphon is carried to the opposite end from the
area of attachment (see Fig. 26, B, C, D, E). Finally the gonads and their
ducts form in the mesoderm between the stomach and intestine. We thus reach
the sedentary degenerate fixed adult Ascidian with little or no trace of
the Chordate characteristics so marked in the earlier larval stage (see E
and A, Fig. 26). The free-swimming tailed larva shows the Ascidian at the
highest level of its organisation, and is the stage that indicates the
genetic relationship of the Tunicata with the Vertebrata.
In some Ascidians with more food-yolk in the egg, or in which {62}the
development takes place within the body of the parent, the life-history as
given above is more or less modified and abbreviated, and in some few forms
the tailed larval stage is missing. Some exceptional cases of development
will be noted below under the groups to which they belong.
The remarkable life-history of the typical Ascidian, of which the outlines
are given above, is of importance from two points of view:—
1. It is an excellent example of degeneration. The free-swimming larva is a
more highly developed animal than the adult Ascidian. The larva is, as we
have seen, comparable with a larval fish or a young tadpole, and is thus a
Chordate animal showing evident relationship to the Vertebrata; while the
adult is in its structure non-Chordate, and is _on a level_ with some of
the worms, or with the lower Mollusca, in its organisation, although of an
entirely different type.
2. It shows us the true position of the Ascidians (Tunicata) in the animal
series. If we knew only the adult forms we might regard them as being an
aberrant group of Worms, or possibly as occupying a position between worms
and the lower Mollusca, or we might place them as an independent group; but
we should certainly have to class them as Invertebrate animals. But when we
know the whole life-history, and consider it in the light of
"recapitulation" and "evolutionary" views we recognise that the Ascidians
are evidently related to the Vertebrata, and were at one time free-swimming
Chordata occupying a position somewhere below the lowest Fishes.
{63}CHAPTER III
TUNICATA (_CONTINUED_)
CLASSIFICATION: LARVACEA—APPENDICULARIANS—STRUCTURE, ETC.—ASCIDIACEA—SIMPLE
ASCIDIANS—SPECIFIC CHARACTERS—COMPOUND ASCIDIANS—GEMMATION—MEROSOMATA—
HOLOSOMATA—PYROSOMATIDAE—THALIACEA—DOLIOLIDAE—SALPIDAE—GENERAL
CONCLUSIONS—PHYLOGENY.
We now turn to the systematic classification of the group; and further
details of structure or function, points of interest in the life-history
such as budding and the formation of colonies, the habits and occurrence,
and other peculiarities such as phosphorescence, will all be noted under
the orders, sub-orders, families and genera in which they occur.
CLASS TUNICATA.
The Tunicata or Urochordata are hermaphrodite marine Chordate animals,
which show in their development the essential Vertebrate characters, but in
which the notochord is restricted to the posterior part of the body, and is
in most cases present only during the free-swimming larval stages. The
adult animals are usually sessile and degenerate, and may be either
solitary or colonial, fixed or free. The nervous system is, in the larva,
of the elongated, tubular, dorsal, Vertebrate type, but in most cases it
degenerates in the adult to form a small ganglion placed above the pharynx.
The body is completely covered with a thick cuticular test ("tunic") which
contains a substance similar to cellulose. The alimentary canal has a
greatly enlarged respiratory pharynx or branchial sac, which is perforated
by two or many more or less modified gill-slits opening into a
peribranchial or atrial cavity, which communicates with the exterior by a
single dorsal exhalent aperture (rarely {64}two ventral apertures). The
ventral heart is simple and tubular, and periodically reverses the
direction of the blood-current.
[Illustration: FIG. 27.—Sketch of the chief kinds of Tunicata found in the
sea.]
This Class is divided into three Orders:—The Appendicularians, the
Ascidians, and the Salpians (see Fig. 27).
ORDER I. LARVACEA (APPENDICULARIANS).
Free-swimming pelagic forms, in which the posterior part of the body takes
the form of a large locomotory appendage, the {65}"tail," in which there is
a skeletal axis, the urochord. A relatively large cuticular test, the
"house," may be formed with great rapidity (in an hour or so) as a
secretion from a part of the ectoderm; it is, however, merely a temporary
structure which is soon cast off and replaced by another. The branchial sac
is simply an enlarged pharynx with two ventral ciliated openings (stigmata)
leading to the exterior. These may be regarded as the representatives of
the primary gill-slits (undivided) of the Ascidian. There are thus a single
pair. There is no separate peribranchial, atrial, or cloacal cavity. The
nervous system consists of a large dorsally placed ganglion and a long
nerve-cord, which stretches backwards over the alimentary canal to reach
the tail, along which it runs on the left side (morphological dorsal edge)
of the urochord. The anus opens ventrally on the surface of the body,
usually in front of the stigmata. No reproduction by gemmation or
metamorphosis is known in the life-history.
STRUCTURE AND MODE OF LIFE.—This is one of the most interesting groups of
the Tunicata, as it shows more completely than any of the rest the probable
characters of the ancestral forms. It has undergone little or no
degeneration, and consequently corresponds more nearly to the tailed,
larval condition than to the adult forms of the other groups. It retains,
in fact, the originally posterior, chordate, part of the body which is lost
in the metamorphosis of all the other Tunicata. Hence the Appendicularians
have been described as permanent, or sexually mature, larval forms, and
hence also the adult _Ascidia_ may be said to correspond to the trunk alone
of the Appendicularian. The Order includes a single group, the
APPENDICULARIIDA, all the members of which are minute (usually about 5 mm.
in total length) and free-swimming (Fig. 28). They occur near the surface
of the sea (and exceptionally in deeper water) in most parts of the world,
moving in a characteristic vibratory manner by the contractions of the
powerful tail (see Fig. 27). They possess the power of forming with great
rapidity, from tracts of specially large glandular ectoderm cells, the
"oikoplasts," an enormously large (many times the size of the body)
investing gelatinous layer, which probably corresponds to the test of other
groups, although it is doubtful whether it contains cellulose, and it
differs also in having no immigrated cells and in its temporary nature.
This structure (Fig. 28) was first described by Von {66}Mertens, and by him
named "Haus"; it has recently been more minutely investigated by Lohmann.
It is only loosely attached to the body, and is frequently thrown off soon
after its formation. Its function is probably protective, and possibly to
some extent hydrostatic, and it may also be of use in straining the
nutritive particles from the large volumes of water which filter through
its complicated passages and perforated folds.[98] The long, laterally
compressed "tail" in the Appendiculariida is attached to the ventral
surface of the body (Fig. 30), and is bent downwards and forwards, so that
it usually points more or less anteriorly; and is twisted through an angle
of 90°, so that the dorsal edge lies to the left. It shows what have been
interpreted as traces of metameric segmentation, having its lateral
muscle-bands broken up into successive pieces (supposed myotomes, probably
only cells), while the nerve-cord presents a series of enlargements formed
of groups of nerve-cells from which distributory nerves are given off. In
_Oikopleura_ the muscle-band in the tail is formed of ten cells fused on
each side. Near the base of the tail there is a distinctly larger elongated
ganglion. The urochord in the tail consists of a homogeneous rod surrounded
by a sheath containing nuclei.
[Illustration: FIG. 28.—Appendiculariida. A, _Appendicularia sicula_, Fol,
with house; B, _Megalocercus abyssorum_, Chun, nat. size; C, _Oikopleura
cophocerca_, Gegenb., with house; D, _Fritillaria megachile_, Fol, with
vesicle; E, Appendicularian in its house; F and G, two stages in the
formation of the house. (A to D from Seeliger; E to G from Lohmann.)]
The anterior (cerebral) ganglion has connected with it an otocyst (Fig.
29), a pigment spot, and a tubular richly ciliated process opening into the
branchial sac, and representing the dorsal tubercle and associated parts of
an ordinary Ascidian. The tube ends in a plain or coiled cellular mass
lying to the right of the ganglion. No neural gland is found.
{67}[Illustration: FIG. 29.—Transverse section through anterior part of
_Oikopleura_ to show ganglion, sense-organs, endostyle, etc. × 300. _br.s_,
Branchial sac; _c.f_, ciliated funnel; _ec_, dorsal ectoderm; _end_, closed
anterior end of endostyle; _hy_, hypobranchial groove in floor of branchial
sac; _n.g_, nerve-ganglion; _or.gl_, oral gland; _ot_, otocyst; _x_,
opening of ciliated funnel into pharynx.]
The branchial aperture or mouth leads into the simple branchial sac or
pharynx (Fig. 30, _br.s_). There are no tentacles. The endostyle is short,
is a closed tube both anteriorly and posteriorly (Fig. 29), and has about
four longitudinal rows of gland-cells. There is no dorsal lamina, and the
peripharyngeal bands run dorsally and posteriorly to unite close in front
of the oesophageal opening. The wall of the branchial sac does not show the
complex structure usual in Tunicata, and has only two ciliated apertures
(Figs. 30, 31, 32, _sg_). These are homologous with the primary stigmata of
the typical Ascidians, and with a pair of the gill-clefts of Vertebrates.
They are placed far back on the ventral surface, one on each side of the
middle line, and lead into short funnel-shaped tubes which open on the
surface of the body behind the anus (Fig. 30, _at_). These tubes correspond
to the right and left atrial involutions, which in an ordinary Ascidian
fuse to form the peribranchial cavity. The remainder of the alimentary
canal consists of oesophagus, stomach (which may have a glandular
diverticulum), intestine and rectum (Fig. 30). The heart, surrounded
ventrally by a delicate pericardial membrane, lies below and in front of
the stomach, and is formed by the differentiation of the outer ends of
epithelial cells into muscular fibrillae. Two specially large glandular
cells are placed at the opposite ends of the heart. There are no
blood-vessels except the remains of the primary body-cavity (blastocoel).
No heart can be seen in some of the smaller species of _Oikopleura_. Nearly
all the species are hermaphrodite, and the large ovary and testis are
placed at the posterior end of the body. There is no proper oviduct, the
genital {68}products merely breaking through to the exterior at the point
marked _g.d_ in Fig. 30. The spermatozoa are generally matured and shed
before the ova, and thus self-fertilisation is prevented. The ova are very
small, and little is known of the development.
[Illustration: FIG. 30.—Longitudinal optical section of _Oikopleura_. Part
of the tail is cut off. _a_, Anus; _at_, atrial opening; _br.s_, branchial
sac; _c.f_, ciliated funnel; _ec_, ectoderm; _end_, endostyle; _ep.p_,
epipharyngeal ridge; _g.d_, opening of gonads to exterior; _ht_, heart;
_hy.p_, hypopharyngeal ridge; _i_, intestine; _m_, mouth; _mus_,
muscle-bands in tail; _n_, nerve-cord; _n′_, nerve in tail; _n.ch_,
urochord; _n.g_, nerve-ganglion; _n.g′_, ganglion in tail; _oes_,
oesophagus; _or.gl_, oral gland; _ot_, otocyst; _ov_, ovary; _sg_,
stigmata; _so_, sense-organ; _sp_, testis; _st_, stomach; _t_, test. (After
Herdman.)]
CLASSIFICATION.—There are two Families of Larvacea: First, the
KOWALEVSKIIDAE, including only the remarkable genus _Kowalevskia_, Fol, in
which the heart and endostyle are absent, and the branchial sac is provided
with four rows of ciliated tooth-like processes. The two known species have
been found in the Mediterranean and in the Atlantic.
The second family APPENDICULARIIDAE comprises about eight genera, amongst
which may be mentioned:—(1) _Oikopleura_, Mertens, and (2)
_Appendicularia_, Fol, in both of which the body is short (1 or 2 mm. in
length) and compact (Fig. 30), and the tail relatively long, while the
endostyle is straight. (3) _Megalocercus_, Chun, from deep water in the
Mediterranean; _M. abyssorum_ is the largest Appendicularian known, having
a total length of {69}3 cm.—it is of a bright red colour. (4)
_Fritillaria_, Q. and G., in which the body is elongated (Fig. 32) and
composed of anterior and posterior regions, the tail relatively short, the
endostyle recurved, the stigmata opening far in front of the anus, and an
ectodermal hood is formed over the front of the body.
In all nearly forty species of Larvacea are known.
[Illustration: FIG. 31.—Transverse section of body and tail of _Oikopleura
flabellum_ (?) _at_, Atrial tube; _bl.s_, blood-space; _br.s_, cavity of
pharynx or branchial sac; _ec_, ectoderm; _en_, endoderm; _ep.p_,
epipharyngeal ciliated bands; _gel_, gelatinous layer between ectoderm and
endoderm; _hy.p_, hypopharyngeal ciliated band; _mus_, muscular tissue on
inner surface of ectoderm of tail; _n_, nerve-cord; _n′_, its continuation
in the tail; _n.ch_, notochord in tail; _r_, rectum; _sg_, one of the
stigmata or ciliated openings from the branchial sac to the atrial tube;
_t_, test (= young "house"); _x_, bridge of gelatinous tissue in front of
stigma closing branchial sac off from atrial tube. (After Herdman.)]
OCCURRENCE.—Although for the most part transparent, and usually almost
invisible in sea-water, some Appendicularians may have certain parts of the
body (alimentary canal, endostyle, gonads, etc.) brilliantly pigmented
(orange, violet, etc.), and may under exceptional circumstances be present
in such profusion as to colour tracts of the sea. Appendicularians are
widely distributed, having been found in all seas from the Arctic to the
Antarctic, both round coasts and in the open ocean. Although a few species
have been found at considerable depths in the Mediterranean, still in the
Atlantic they are not deep-water animals, and as a group must be regarded
as surface-forms. They are fairly abundant to a depth of 100 fathoms, and
some few reach 1500. Species of _Oikopleura_ and _Fritillaria_ are
{70}frequent round the British coasts, our commonest species being probably
_O. dioica_, Fol, and _F. furcata_, Moss. Young specimens appear in the
plankton about February and March, and larger forms are as a rule found
later in the summer. Several instances have been recorded of swarms of
especially large forms, provided with massive tests (the "house"), having
appeared suddenly on our coast in such abundance as to form an important
element in the surface life of the sea.
[Illustration: FIG. 32.—Diagram of _Fritillaria_ seen from the right side
to show the elongated body, the hood, and the relative positions of anus,
atrial opening, and gonads. (Compare with _Oikopleura_, Fig. 30.) _a_,
Anus; _at_, opening of atrial tube; _br.s_, branchial sac; _end_,
endostyle; _ht_, heart; _m_, mouth; _n.ch_, notochord; _n.g_,
nerve-ganglion; _oes_, oesophagus; _ov_, ovary; _sg_, stigma; _sp_, testis;
_st_, stomach.]
ORDER II. ASCIDIACEA (ASCIDIANS).
Fixed or free-swimming Simple or Compound Ascidians, which in the adult are
never provided with a locomotory appendage or tail, and have no trace of a
notochord. The free-swimming forms are colonies, the Simple Ascidians being
always sedentary and usually fixed. The test is permanent and well
developed, and becomes organised by the immigration of cells from the body;
as a rule it increases in size with the age of the individual. The
branchial sac is large and well developed. Its walls are perforated by
numerous slits (stigmata) opening into the peribranchial cavity, which
communicates with the exterior by the single atrial aperture. Many of the
Ascidiacea, both fixed and free, reproduce by gemmation to form colonies,
and in most of them the sexually produced embryo develops into a tailed
larva.
The Ascidiacea includes three groups, the Simple Ascidians, the Compound
Ascidians, and the free-swimming colonial _Pyrosoma_, which in some
respects connects this Order with the Thaliacea.
{71}SUB-ORDER 1. ASCIDIAE SIMPLICES.
Fixed Ascidians, which are solitary, and very rarely reproduce by
gemmation; if, as in a few cases, small colonies are formed, the members
are not buried in a common investing mass, but each has a distinct test of
its own. No strict line of demarcation can be drawn between the Simple and
Compound Ascidians; and one of the families of the former group, the
Clavelinidae (the "Social" Ascidians of Milne-Edwards), forms a transition
from the typical Simple forms which never reproduce by gemmation, to the
Compound forms which always do. Over 500 species of Ascidiae Simplices are
now known, but there are probably very many more still undescribed. The
sub-order may be divided into the following families:—
FAM. 1. CLAVELINIDAE.—Simple Ascidians which reproduce by gemmation to form
small colonies (Fig. 33), in which each member, or ascidiozooid, has a
distinct test, but all are connected by a common blood-system, and by a
prolongation of the "epicardiac tubes" (see p. 83) from the branchial sac.
Buds are formed on the stolons (Fig. 33), which are vascular outgrowths
from the posterior end of the body, containing prolongations from the
ectoderm, mesoderm, and endoderm (the epicardium) of the Ascidiozooid.
Branchial sac not folded; internal longitudinal bars usually absent;
stigmata straight; tentacles simple. The Clavelinidae are the simplest of
the Ascidiae Simplices. They are the forms that come nearest to the
Compound Ascidians, and are closely related to the Distomatidae. They are
probably the nearest representatives now existing of the ancestral forms
from which both Simple and Compound Ascidians are descended.
[Illustration: FIG. 33.—Colony of _Clavelina lepadiformis_ (nat. size).]
This family contains amongst others the following three
genera:—_Ecteinascidia_, Herdman, with internal longitudinal bars in the
branchial sac; _Clavelina_, Savigny, with a long body and intestine
extending behind the branchial sac (Fig. 33); and {72}_Perophora_,
Wiegmann, with a short compact body and intestine alongside the branchial
sac. _Clavelina lepadiformis_ and _Perophora listeri_ are common British
species found at a few fathoms depth off various parts of our coast. Both
occur round the south end of the Isle of Man. In autumn _Clavelina_
accumulates reserve-material in the ectoderm cells of parts of the stolon,
which remain when the rest of the colony dies away, and then form new buds
in spring.
FAM. 2. ASCIDIIDAE.—Solitary fixed Ascidians, never forming colonies; with
gelatinous or cartilaginous test; branchial aperture usually eight-lobed,
atrial aperture usually six-lobed; branchial sac not folded; internal
longitudinal bars usually present; stigmata straight or curved; tentacles
simple; gonads in or around the intestinal loop. This family is divided
into three sections:—
SUB-FAM. 1. HYPOBYTHIINAE.—Branchial sac with no internal longitudinal
bars, test strengthened with curious symmetrically placed nodules.
The one genus _Hypobythius_, Moseley, contains two stalked deep-water forms
found by the "Challenger;" _H. calycodes_ (Fig. 34, A), from the North
Pacific, 2900 fathoms, and _H. moseleyi_ from the South Atlantic, 600
fathoms.
[Illustration: FIG. 34.—A, _Hypobythius calycodes_, Moseley; B, _Chelyosoma
macleayanum_, Brod. and Sowb.; C, _Corynascidia suhmi_, Herdman; D,
_Rhodosoma callense_, Lac.-Duth.]
SUB-FAM. 2. ASCIDIINAE.—Internal longitudinal bars present; stigmata
straight. Many genera, of which the following are the more
important:—_Ciona_, Fleming, dorsal languets present; _Ascidia_, Linnaeus
(in part _Phallusia_, Savigny), dorsal lamina {73}present (Fig. 15, p. 40);
_Rhodosoma_, Ehrenberg, anterior part of test modified to form operculum
(Fig. 34, D); _Abyssascidia_, Herdman, intestine on right side of branchial
sac. The type genus of this section, _Ascidia_, has been described in
detail above (Chapter II. p. 39), and Figs. 15 to 26 illustrate its
structure and life-history. There are many species. _Ciona intestinalis_,
Linn. (Fig. 40, B), is one of the commonest of British Ascidians, and lives
readily in aquaria.
SUB-FAM. 3. CORELLINAE.—Stigmata curved and forming spirals (Fig. 35).
Three genera:—_Corella_, Alder and Hancock, test gelatinous, body sessile;
_Corynascidia_, Herdman, test gelatinous, body pedunculated (Fig. 34, C), a
remarkable deep-sea form with very delicate spirally-coiled vessels in the
branchial sac (Fig. 35, A), found in the Pacific (2160 faths.) and the
Southern Ocean; _Chelyosoma_, Brod. and Sowb., upper part of test modified
into horny plates (Fig. 34, B).
[Illustration: FIG. 35.—A, branchial sac of _Corynascidia suhmi_, Herdman;
B, branchial sac of _Corella japonica_, Herdman. _i.l_, Internal
longitudinal bars; _tr_, transverse vessels. (After Herdman.)]
_Corella_ contains several British species, one of which, _C.
parallelogramma_, O. F. Müll., is one of the commonest and most handsome
Ascidians in our coralline zone (about 20 faths.). Through its clear
crystalline test the lemon-yellow and carmine pigmentation of the mantle,
and even (with a lens) the working of the cilia along the spiral stigmata
of the branchial sac (compare Fig. 35, B), can readily be seen. The beating
of the heart can be seen just in front of the viscera upon the _right_ side
of the branchial sac (compare with _Ascidia_, Fig. 23).
In the family Ascidiidae the eggs are minute and contain {74}little or no
food-yolk, and the tailed larvae (Figs. 26, 42, A) are of the typical form
and structure described in Chapter II.
FAM. 3. CYNTHIIDAE.—Solitary fixed Ascidians (Fig. 39), sometimes occurring
in aggregations, but never forming colonies; usually with leathery or
fibrous, opaque test, which is sometimes encrusted with sand; branchial and
atrial apertures usually both four-lobed. Branchial sac longitudinally
folded (Fig. 36, A); stigmata straight; tentacles simple or compound (Fig.
37); neural gland dorsal to ganglion; gonads attached to body-wall. This
family is divided into three sections:—
[Illustration: FIG. 36.—Diagrammatic transverse sections of branchial sacs
of Cynthiidae. A, _Cynthia_; B, _Styela_; C, _Styelopsis_; D, _Pelonaia_.
_Br.f_ 1-7, First to seventh branchial fold; _d.l_, dorsal lamina; _end_,
endostyle; _mh_, meshes.]
SUB-FAM. 1. STYELINAE.—Not more than four folds (Fig. 36, B) on each side
of branchial sac; tentacles simple (Fig. 37, A). The more important genera
are—_Styela_, Macleay, and _Polycarpa_, Heller (Fig. 39), with stigmata
normal; and _Bathyoncus_, Herdman, with stigmata absent or modified. There
are a very large number of species of both _Styela_ and _Polycarpa_ from
all parts of the world, including our own seas. A very abundant British
littoral form has been placed in an allied genus under the name _Styelopsis
grossularia_ (Fig. 39, A). It is known in some places round our coasts as
"the red-currant squirter." This species has only one well-marked fold in
the branchial sac (Fig. 36, C). Another exceptional British Styelid is
_Pelonaia corrugata_, Forb. and Goods. (Fig. 39, I), with no branchial
folds (Fig. 36, D).
{75}SUB-FAM. 2. CYNTHIINAE.—More than eight folds in branchial sac (Fig.
36, A); tentacles compound (Fig. 37, B); body sessile or with a short stalk
(Fig. 39, F). The chief genus is _Cynthia_, Savigny, with a large number of
species, some of which are British. _Rhabdocynthia_ has echinated
calcareous spicules in the mantle (see Fig. 50, D, p. 87).
_Forbesella tessellata_ is a remarkable British species, having the test
marked out into plates (Fig. 39, B). It is intermediate in some characters
between Styelinae and Cynthiinae.
[Illustration: FIG. 37.—Tentacles of Cynthiidae. A, Simple, in Styelinae;
B, Compound, in Cynthiinae.]
[Illustration: FIG. 38.—_Culeolus wyville-thomsoni_, Herdman. A, from left
side (half-nat. size); B, part of branchial sac. _At_, Atrial aperture;
_Br_, branchial aperture; _br.f_, branchial fold; _i.l_, internal bar;
_sp_, spicules; _tr_, transverse vessel. (After Herdman.)]
SUB-FAM. 3. BOLTENINAE.—More than eight folds in branchial sac; tentacles
compound; body pedunculated (Fig. 38, A). The chief genera are—_Boltenia_,
Savigny, with the branchial aperture four-lobed, and the stigmata normal;
and _Culeolus_, Herdman (Fig. 38), with branchial aperture having less than
four lobes, and the stigmata absent or modified (Fig. 38, B), the branchial
sac showing a wide mesh-work of vessels stiffened by branched calcareous
spicules. _Culeolus_ is a deep-sea genus discovered by the {76}"Challenger"
expedition; eight or nine species are now known from various parts of the
world, ranging in depth from 630 to 2425 fathoms. Most of the species are
from the Pacific; only one from the North Atlantic. The curiously curved
type of spicule found in the branchial sac and other organs is shown at
Fig. 50, C (p. 87).
Amongst the Cynthiidae are found most varied conditions of the reproductive
organs. The gonads are sometimes on both, sometimes on only one side of the
body, sometimes in one or several branched masses, and sometimes
distributed as a large number of minute "polycarps" over the inner surface
of the mantle.
[Illustration: FIG. 39.—Various Cynthiidae. A, two forms of _Styelopsis
grossularia_, Van Ben.; B, _Forbesella tessellata_, Forb.; C, _Polycarpa
aurata_, Q. and G.; D, _Styela clava_, Herdman; E, _Polycarpa tinctor_, Q.
and G.; F, _Cynthia formosa_, Herdman; G, _Polycarpa comata_, Alder; H,
_Polycarpa pedata_, Herdman; I, _Pelonaia corrugata_, Forb. and Goods.
(After Herdman.)]
The family Cynthiidae is the largest section of the Simple Ascidians. The
species range from the size of a pea to that of a large cocoa-nut. They are
for the most part opaque, and often richly coloured—reds, yellows and rich
browns predominating—and so look very different to the grey gelatinous
Ascidiidae, and to the sand-encrusted Molgulidae. They extend from between
tide-marks (_Styelopsis grossularia_), down to the abysses (_Styela bythia_
and _S. squamosa_ at 2600 fathoms). Some genera (_Styela_ and the closely
related _Dendrodoa_), extend far into Arctic seas, but many allied forms
(_Styela_ and _Polycarpa_) are also found in the tropics.
{77}[Illustration: FIG. 40.—Three simple Ascidians with vascular adhering
processes from the test (nat. size). A, _Ascidiella aspersa_, O. F. Müller;
B, _Ciona intestinalis_, Linn.; C, _Molgula oculata_, Forb.]
[Illustration: FIG. 41.—Branchial sacs of Molgulidae showing curved
stigmata. A, _Ascopera gigantea_, Herdman; B, _Molgula pyriformis_,
Herdman; C, _Eugyra kerguelenensis_, Herdman.]
FAM. 4. MOLGULIDAE.—Solitary sessile Ascidians, sometimes not fixed;
branchial aperture six-lobed, atrial four-lobed. Test usually encrusted
with sand, which is generally attached to branched hair-like processes from
the test (Fig. 40, C). Branchial sac longitudinally folded; stigmata more
or less curved, usually arranged in spirals (Fig. 41); tentacles compound.
The chief genera are—_Molgula_, Forbes (Fig. 40, C), with distinct folds in
the branchial sac (Fig. 41, B), and _Eugyra_, Ald. and Hanc., with no
distinct folds, but merely broad internal longitudinal bars in the
branchial sac (Fig. 41, C). In some of the Molgulidae {78}(genus
_Anurella_, Lacaze-Duthiers), the embryo does not become converted into a
tailed larva, the development being direct without metamorphosis (see Fig.
42, C). The embryo when hatched gradually assumes the adult structure, and
never shows the features characteristic of larval Ascidians, such as the
urochord and the median sense-organs. Fig. 42 shows an Ascidiid (A), a
Cynthiid (B), and this exceptional Molgulid (C), type of larva, and three
forms of Compound Ascidian larvae, the Distomatid (D), the Botryllid (E),
and the Diplosomatid (F).
[Illustration: FIG. 42.—Larvae of various Ascidians. A, _Ascidia mentula_,
Linn.; B, _Polycarpa glomerata_, Alder; C, _Anurella roscovita_,
Lac.-Duth.; D, _Distaplia magnilarva_, Della Valle; E, _Polycyclus
renieri_, Lamk.; F, _Diplosomoides lacazii_, Giard. (Mostly after
Lahille.)]
In the Molgulidae the viscera are characteristic in position and
appearance. The alimentary canal lies on the left side of the branchial
sac, and the intestine forms a long narrow loop directed in the main
transversely. The pericardium and heart are on the middle of the right
side, and behind them is placed the single sac-like ductless renal organ,
generally occupied by one or more concretions. The gonads are in most cases
on both sides of the body, in front of the intestine on the left, and in
front of the heart on the right; but in _Eugyra_ there is no gonad on the
right side, and in some other forms the gonad on the left side is absent.
(For _Oligotrema_, see p. 111, note.)
There are a number of British Molgulidae, the two commonest {79}of which
are—_Molgula oculata_, Forbes, thickly covered with gravel or broken
shells, and forming an ovate mass as large as a walnut; and _Eugyra
glutinans_, Möller, a smaller more globular body, the size of an acorn, and
covered with fine sand, except at one circular area near the posterior end,
where the leaden grey test shows through. Both these species are obtained
by dredging in from 10 to 30 fathoms, and lie freely on the bottom. A
rather rarer littoral species _Molgula citrina_, Hancock, found on some
parts of our coast (_e.g._ in the Firth of Forth, at Arran, and at Port
Erin), is exceptional in having the test free from sand, and in being fixed
like an _Ascidia_, generally to the lower surfaces of large stones near low
tide.
SPECIFIC CHARACTERS AND DORSAL TUBERCLE.—The chief points in which the
various genera and species of Simple Ascidians differ are the details of
the branchial sac (see Figs. 22, 35, 36, 38, and 41), the condition of the
tentacles (Fig. 37), the dorsal lamina or languets, and the dorsal
tubercle, in addition to form, colour, and other external features.
[Illustration: FIG. 43.—Various forms of dorsal tubercle in Simple
Ascidians. 1. _Molgula pyriformis_; 2. _Forbesella tessellata_; 3. _Ascidia
meridionalis_; 4. _Cynthia formosa_; 5. _Cynthia papietensis_; 6. _Ascidia
challengeri_; 7. _Polycarpa tinctor_; 8. _Cynthia cerebriformis_;
9. _Ascopera gigantea_; 10. _Boltenia tuberculata_; 11. _Ascidia
translucida_; 12. _Culeolus moseleyi_; 13. _Ascidia pyriformis_;
14. _Boltenia pachydermatina_; 15. _Microcosmus draschii_; 16. _Styela
etheridgii_; 17. _Styela whiteleggii_; 18. _Polycarpa aurata_. (After
Herdman.)]
Fig. 43 shows some of the more remarkable forms of dorsal tubercle.
Starting with a simple circular opening (1) surrounded by a thickened
ciliated ring, the anterior border becomes pushed in to form a crescentic
slit (2 and 3). The horns of the crescent then grow longer and may be
turned in (4 and 5) or out (6 and 7), and so give rise to the many
varieties of horse-shoe (such as 6), perhaps the commonest form of dorsal
tubercle in Simple Ascidians. In many Cynthiidae the central part of the
{80}horse-shoe remains small, while the horns become long and much coiled
so as to constitute two prominent spirals (8, 9, 10). In other exceptional
forms again the curved slit becomes straightened out, undulating (11),
irregularly bent (12 and 13), elaborately folded (14 and 15), or broken up
into pieces (16), so that there come to be several or even a large number
(17 and 18) of minute openings in place of the original single aperture.
It cannot be said that any form of dorsal tubercle is characteristic of any
of the families or genera of Ascidians, and in the case of some species the
organ is liable to great individual variation; but still in most species
there is found to be a characteristic shape or appearance of tubercle which
is a useful diagnostic feature.
SUB-ORDER 2. ASCIDIAE COMPOSITAE.
Fixed Ascidians which reproduce by gemmation so as to form colonies (Fig.
44) in which the ascidiozooids are buried in a common investing mass (Fig.
45) and have no separate tests—hence "Synascidiae," a name they often
receive from foreign writers.
[Illustration: FIG. 44.—Colonies of Compound Ascidians (nat. size). A,
_Colella quoyi_, Hrdn. Antarct.; B, _Leptoclinum neglectum_, Hrdn.; C,
_Pharyngodictyon mirabile_, Hrdn. Southern Ocean; D, _Botryllus
schlosseri_, Sav. Europe. (After Herdman.)]
This is probably a somewhat artificial assemblage formed of those two or
three groups of Ascidians which produce colonies, in which the
ascidiozooids are so intimately united that they possess a common test or
investing mass. This is the only character which distinguishes them from
the Clavelinidae, but the property of reproducing by gemmation separates
them from the rest of the Ascidiae Simplices. In some cases the atrial
apertures of several neighbouring ascidiozooids join to open to the
exterior by a common cloacal aperture (Fig. 45, _c.c_). Such {81}groups of
the ascidiozooids of a colony are known as "systems" or coenobia (see Fig.
44, D; also Fig. 53, p. 89).
The Ascidiae Compositae may be divided into seven families, which seem to
fall into two well-marked sets:—(1) MEROSOMATA, in which the heart and
alimentary and reproductive viscera are placed behind the branchial sac, so
as to constitute a more or less extended body divided into at least two
regions (Fig. 46, B), and sometimes three (Fig. 46, C)—thorax, abdomen, and
post-abdomen; and (2) HOLOSOMATA, in which the body of the ascidiozooid is
short, compact, and not divided into regions (Fig. 46, A). The latter group
comprises the two families Botryllidae and Polystyelidae, which agree both
in points of structure and in having the same type of budding, and are
probably derived from ancestral Cynthiidae amongst Simple Ascidians; while
the Merosomata seem more nearly related to the Clavelinidae.
[Illustration: FIG. 45.—Vertical section through a small part of a compound
Ascidian colony. _Asc_. 1 and _Asc_. 2, Parts of two ascidiozooids whose
cloacas (_cl_) open into the common cloacal cavity (_c.c_) of the colony;
_at.l_, atrial lobes; _t_, _t_, _t_, common test of the colony. The
structure of the posterior parts of the ascidiozooids would depend upon the
family (see Fig. 46). The arrows show the direction of the water currents.]
GEMMATION takes place in the Compound Ascidians in a variety of ways, being
sometimes very different in its details in closely allied forms. There are,
however, two main types of budding, to one or other of which most of the
described methods may be referred. These are:—
1. The STOLONIAL, or "epicardiac" type—seen in the Merosomata, typically in
Distomatidae and Polyclinidae, and comparable with the gemmation in
Clavelinidae, Pyrosomatidae, and Thaliacea outside this group.
{82}2. The PARIETAL, or "peribranchial" type—seen in the Holosomata,
typically in the Botryllidae.
The remarkable process of gemmation seen in the families Didemnidae and
Diplosomatidae, where the bud arises from at least two rudiments, the one
stolonial or epicardiac in origin, and the other formed by one or more
oesophageal or intestinal outgrowths, has been called "entero-epicardiac,"
but it may probably be regarded as a modification of the stolonial type.
[Illustration: FIG. 46.—A, Ascidiozooid from a Botryllid colony; B,
ascidiozooid from a Distomid colony; C, ascidiozooid from a Polyclinid
colony. _a_, Anus; _at_, atrial aperture; _at.l_, atrial languet; _br_,
branchial aperture; _cl_, cloaca; _d.l_, dorsal languet; _ec_, ectoderm;
_end_, endostyle; _ep.c_, epicardiac tube; _gl_, intestinal gland; _h_,
heart; _i_, intestine; _n.g_, nerve-ganglion; _oes_, oesophagus; _ov_,
ovary; _p.c_, pericardium; _r_, rectum; _sg_, stigmata of branchial sac;
_sp_, spermatic sacs; _sph_, sphincter; _st_, stomach; _t_, tentacle;
_t.k_, terminal ampullae of vessels in test; _v_, colonial vessels;
_v.app_, "vascular appendage" (stolon).]
The marked differences in the appearance of the colonies of Compound
Ascidians is largely due to the methods of budding; and even in those of
the stolonial type, where the budding is practically the same in essential
nature, the results may be very different in superficial appearance,
according as the buds are {83}formed on a short stolon close to the parent
body, or from the extremity of the post-abdomen (as in the Polyclinidae),
or from a long epicardiac tube (as in _Colella_, Fig. 47), which may extend
for some inches from the ascidiozooid. The post-abdomen of the Polyclinidae
may be regarded as a stolon invaded by the gonads and the heart (see Fig.
46, C), and traversed by the epicardium in the form of a flattened tube
dividing a dorsal blood-sinus containing the gonads from a ventral sinus
which has merely the one extremity of the tapering pericardium. The whole
of this post-abdomen segments to form the buds, the heart at the extremity
being absorbed, and a new one formed from the anterior end of the
pericardium.
The epicardium, which supplies the endodermal element to each bud, was
first described by E. van Beneden and Julin in the envelopment of
_Clavelina_,[99] as a structure concerned in the formation of the
pericardium and heart—hence its unfortunate name. It grows backwards in the
larva, from the posterior wall of the branchial sac, close to the
endostyle, as a tube which usually divides into two lateral branches to be
united again eventually so as to form the single tubular flattened
partition of the stolon in Polyclinidae, Distomatidae, Clavelinidae, etc.
In some Compound Ascidians the epicardium is, from its origin, two distinct
lateral tubes, which grow back from the inner vesicle of the embryo (later
the branchial sac). These unite in the post-abdomen to form the flattened
tube, which in its turn forms the inner vesicle of the future buds, and so
the endodermal element is handed on from generation to generation. In
addition to the epicardium, the stolon contains also a prolongation of the
ovary of the parent, or at least a string of migrating germ cells, so that
the reproductive elements are also handed on.
It is clear from the recent researches of Hjort, Ritter, Lefevre,[100] and
others, that the development of the bud (blastozooid) and that of the
embryo (oozooid) do not proceed along parallel lines. It is evidently
impossible to harmonise the facts of gemmation with the germ-layer theory;
and attempts to explain budding in Ascidians solely as a process of
regeneration by which the organs of the parent or their germ-layers give
rise to the corresponding organs in the bud have in many cases failed.
{84}The rudiment of the bud is in typical cases composed of two vesicles,
an outer derived from the ectoderm of the parent and enclosing free
blood-cells (mesodermal) between its wall and that of the inner
vesicle—which is usually of endodermal origin, but in Botryllidae is
derived from the peribranchial sac, an ectodermal structure. The inner
vesicle, derived in the two cases from different germ-layers, forms the
same organs of the bud, and these organs may be of widely different origin
in the larva. Moreover, free cells of the blood may play in the bud a very
important part, and give rise (_Perophora_) to such important systems as
pericardium and heart, neural tube and ganglion, the gonads and their
ducts, some of which are of ectodermal and others of endodermal origin in
the larva.
In some cases of precocious budding (blastogenetic acceleration) the young
buds begin to appear during the tailed larval stage. The larva may even
contain a first blastozooid (bud) with a branchial sac as large as that of
the oozooid (derived from the egg); and in the Diplosomatidae the larva
(see Fig. 42, F), when it settles down, may be already a small colony of
three young ascidiozooids.
The larvae in most Compound Ascidians, in place of adhering papillae, have
several or even a considerable number of ectodermal tubes or prolongations
from the body (see Fig. 42, E and F) into the surrounding test. These
apparently aid in the formation of the common test of the young colony,
which grows over and adheres to foreign objects.
There are many irregularities in the larval development of Compound
Ascidians, due to the very different amount of food-yolk present in the ova
in different genera. In some cases there is even dimorphism, two forms of
larvae being found in the same colony.
Compound Ascidians are amongst the most varied and brilliant of sessile
animals seen at low tide on our own and most other coasts. Some are stalked
and form club-shaped or knob-like outgrowths. Others again form flat
gelatinous expansions attached to sea-weeds or stones, and are
symmetrically marked with bright spots of colour in the form of circles,
meandering lines, or star-like patterns. In such colonies each spot of
colour or ray of a star represents an ascidiozooid or member of the colony,
equivalent to the whole animal in the case of the solitary Simple Ascidian.
{85}GROUP A. _MEROSOMATA_.
Viscera posterior to branchial sac; budding stolonial.
[Illustration: FIG. 47.—A, Colony of _Colella pedunculata_, Q. and G., nat.
size: _a_, zone of buds; _b_, zone of young ascidiozooids; _c_, zone of
reproducing adults; _d_, old decaying adults and incubatory pouches with
larvae. B, Ascidiozooid, with incubatory pouch enlarged: _At_, atrial
aperture; _Br_, branchial aperture; _emb_, embryos; _end_, endostyle;
_ep.c_, epicardium; _inc.p_, incubatory pouch; _od_, oviduct; _od′_, its
prolongation into _inc.p_; _od″_ its termination at tip of _inc.p_; _ov_,
ovary; _p.br_, peribranchial opening of _inc.p_; _st_, stomach.]
FAM. 1. DISTOMATIDAE.—Ascidiozooids divided into two regions, a thorax,
containing the branchial sac, and an abdomen, with the remaining viscera
(Fig. 47, B); testes numerous; vas deferens not spirally coiled. The chief
genera are—_Distoma_, Gaertner, with some British species;
_Chondrostachys_, Macdonald, _Cystodytes_, v. Drasche, with calcareous
plate-like spicules in the test (Fig. 50, A); _Distaplia_, Della Valle, and
_Colella_, Herdman, forming a pedunculated colony (Fig. 47, A), in which
the ascidiozooids (Fig. 47, B) are provided with large incubatory pouches,
opening from the peribranchial cavity, but also connected, as Bancroft[101]
has recently shown, with the end of the oviduct (see Fig. 47, B). In these
pouches the embryos undergo their development, and are set free by the
decay of the top of the colony. The stolons pass from the ascidiozooids in
the upper part of the colony down into the stalk, and there produce buds
which gradually work up to the top of the stalk, where they take their
places as young ascidiozooids. At the top of the colony the old
ascidiozooids die and are removed (see Fig. 47, A). Caullery has shown that
in {86}this genus there may be dimorphism in the buds, some of them placed
deeply in the stalk having a large amount of reserve food-matter in their
ectoderm, and remaining dormant until required to regenerate the "head" or
upper part of the colony when it is lost. This genus was made known by the
"Challenger" expedition. The species are mostly tropical, or from southern
seas.
FAM. 2. COELOCORMIDAE.—Colony not fixed, having a large axial cavity with a
terminal aperture. Branchial apertures five-lobed. This includes one
species, _Coelocormus huxleyi_, Herdman, which is in some respects a
transition-form between the ordinary Compound Ascidians (_e.g._
Distomatidae) and the Ascidiae Luciae (_Pyrosoma_, see p. 90).
[Illustration: FIG. 48.—Transverse section of the abdomen of a Distomid.
_bl.s_, Blood-sinus; _ec_, ectoderm; _ep.c_, epicardium; _gl_, intestinal
glands; _h_, heart; _i_, intestine; _l.m_, longitudinal muscles; _mes_,
mesoderm; _o.d_, oviduct; _p.c_, pericardium; _st_, stomach; _v.d_, vas
deferens. (After Seeliger.)]
[Illustration: FIG. 49.—Section of _Leptoclinum_ colony, showing the
distribution of spicules and parts of the ascidiozooids. _b_, Base of
colony; _br_, branchial aperture; _br.s_, branchial sac; _sp_, spicules;
_st_, stomach; _tes_, testis; _v.d_, vas deferens.]
FAM. 3. DIDEMNIDAE.—Colony usually thin and incrusting. Test containing
stellate calcareous spicules (Figs. 49 and 50, B). {87}Testis single,
large; vas deferens spirally coiled (Fig. 49). The chief genera
are—_Didemnum_, Savigny, in which the colony is thick and fleshy, and there
are only three rows of stigmata on each side of the branchial sac; and
_Leptoclinum_, Milne-Edwards, in which the colony is thin and incrusting
(Fig. 49), and there are four rows of stigmata. Colonies of _Leptoclinum_,
forming thin white, grey, or yellow crusts under stones at low water, are
amongst the commonest of British Compound Ascidians.
[Illustration: FIG. 50.—Calcareous spicules of the Tunicata, enlarged. A,
From _Cystodytes_; B, from _Leptoclinum_; C, from _Culeolus_; D, from
_Rhabdocynthia_.]
FAM. 4. DIPLOSOMATIDAE.—Test reduced in amount (Fig. 51), rarely containing
spicules. Vas deferens not spirally coiled. In _Diplosoma_, Macdonald, and
other allied genera (Fig. 51), the larva is gemmiparous (Fig. 42, F). Some
species are common British forms, especially on _Zostera_-beds and amongst
seaweeds.
[Illustration: FIG. 51.—Section of a colony of _Diplosoma_ (enlarged) to
show the small amount of test present. _br_, Branchial aperture; _c.cl_,
common cloaca; _t_, test.]
FAM. 5. POLYCLINIDAE.—Ascidiozooids divided into three regions—thorax,
abdomen, and post-abdomen (Fig. 46, C). Testes numerous; vas deferens not
spirally coiled. The chief genera are—_Pharyngodictyon_, Herdman, with
stigmata absent or modified, containing one species, _Ph. mirabile_ (Fig.
44, C), the {88}only Compound Ascidian known from a depth of 1000 fathoms;
_Polyclinum_, Savigny, with a smooth-walled stomach (Fig. 52, A);
_Aplidium_, Savigny, with the stomach-wall longitudinally folded (Fig. 52,
B); _Morchellium_, Giard, with an "areolated" stomach (Fig. 52, D), bearing
knobs on the outside; and _Amaroucium_, Milne-Edwards, in which the
ascidiozooid has a long post-abdomen and a large atrial languet, and where
the stomach-wall shows longitudinal ridges breaking up into knobs
(pseudo-areolated, Fig. 52, C). The last four genera contain many common
British species.
[Illustration: FIG. 52.—Various conditions of stomach in Polyclinidae. A,
_Polyclinum molle_, Herdman; B, _Aplidium zostericola_, Giard; C,
_Amaroucium proliferum_, M.-Edw.; D, _Morchellium argus_, M.-Edw.]
Many of the Compound Ascidians die down in winter; but amongst
Polyclinidae, as in _Clavelina_, a form of hibernation is found, the old
ascidiozooids dying, but some of the buds in the basal part of the colony
accumulating a large store of reserve-material in their ectoderm, and lying
dormant until spring, when they regenerate the colony.
GROUP B. _HOLOSOMATA_.
Body short, compact, with viscera by the side of branchial sac; budding
parietal
FAM. 6. BOTRYLLIDAE.—Ascidiozooids grouped in systems round common cloacal
apertures (Fig. 53). Ascidiozooids having the intestine and reproductive
organs by the side of the branchial sac (Fig. 46, A, p. 82). Dorsal lamina
and internal longitudinal bars present in the branchial sac. Neural gland,
as in Cynthiidae, dorsal to the ganglion in place of ventral as in the
majority of Tunicata. The chief genera are—_Botryllus_, Gaertn. and Pall,
with simple stellate systems (Fig. 53), and _Botrylloides_, Milne-Edwards,
with elongated or ramified systems. There are {89}many species of both
these genera, which form brilliantly coloured fleshy crusts under stones
and on sea-weeds at low tide. They are amongst the commonest and the most
beautiful of British Ascidians. Both genera contain species remarkable for
the rich profusion of ectodermal "vessels" which ramify and anastomose in
the colonial test. On the margins of the colony these vessels end in
knob-like dilatations, the ampullae (Fig. 46, A, _t.k_), which are said by
Bancroft to pulsate rhythmically, and so aid in keeping up the colonial
circulation. They are also storage reservoirs for the blood, doubtless help
in respiration, and are organs for the secretion of the test-matrix.
[Illustration: FIG. 53.—Two "systems" from a colony of _Botryllus
violaceus_, M.-Edw. _cl_, Common cloaca of a system; _or_, branchial
apertures of ascidiozooids, magnified. (After H. Milne-Edwards.)
FIG. 54.—_Goodsiria placenta_, Herdman. A, Colony (half nat. size); B,
section of colony showing ascidiozooids. (After Herdman, from _Challenger
Reports_.)]
FAM. 7. POLYSTYELIDAE.—Ascidiozooids not grouped in systems; branchial and
atrial apertures four-lobed; branchial sac may be folded; internal
longitudinal bars present. The chief genera are—_Thylacium_, Carus, with
the ascidiozooids projecting above the general surface of the colony;
_Goodsiria_, Cunningham, with the ascidiozooids completely imbedded in the
investing mass (Fig. 54); and _Chorizocormus_, Herdman, with the
ascidiozooids {90}united in little groups which are connected by stolons.
The last genus contains one species, _Ch. reticulatus_, in some respects a
transition-form between the other Polystyelidae and the Styelinae amongst
Simple Ascidians.
BUDDING IN HOLOSOMATA—In the Polystyelidae, according to Ritter,[102] the
budding is of the same type as in Botryllidae, the bud arising in each case
from the lateral body-wall of the parent.
In _Botryllus_[103] the oozooid formed from the larva gives rise at a very
early period to the first blastozooid of the future colony. This then forms
the two buds of the second generation on its sides (see Fig. 55), and these
in their turn form the third, and these the fourth generation, in which
there are thus eight blastozooids; and so the process goes on, the buds of
each generation arranging themselves in a circle to form a system. As each
new generation makes its appearance, the preceding one undergoes
degeneration, and is eventually absorbed. Consequently, in a system there
can usually be seen, in addition to the adult members, certain older ones
in various stages of degeneration and removal, and certain younger ones
arising as buds on the sides of their predecessors, or just separated from
them, and ready to take their places as young ascidiozooids in the system.
Three distinct generations are thus commonly seen in a system. Now and
again one or two young ascidiozooids become squeezed by the pressure of
their neighbours out of a system into the surrounding test, and so give
rise to new systems which add to the extent of the colony.
[Illustration: FIG. 55.—Diagram to illustrate the budding and formation of
a system in _Botryllus_. _Ooz_; oozooid; _Bl_ 1, first blastozooid; 2, 2,
etc., successive generations of buds.]
SUB-ORDER 3. ASCIDIAE LUCIAE.
Free-swimming pelagic colonies having the form of a hollow cylinder closed
at one end (Fig. 56). The ascidiozooids forming the colony are imbedded in
the common test in such a manner that the branchial apertures open on the
outer surface and the {91}atrial apertures on the inner surface next to the
central cavity of the colony. They are placed with their ventral surfaces
towards the closed end (Fig. 56, C). The first ascidiozooids of a colony
are produced by gemmation from a stolonic prolongation of an imperfect
oozooid or rudimentary larva (the "cyathozooid"), developed sexually. The
subsequent ascidiozooids are formed from these as buds on a ventral stolon.
This sub-order includes a single family, the PYROSOMATIDAE, containing one
well-marked genus _Pyrosoma_, Péron, with about six species. They are found
swimming near the surface of the sea, chiefly in tropical latitudes, and
are brilliantly phosphorescent. A fully developed _Pyrosoma_ colony may be
from an inch or two to upwards of twelve feet in length.
[Illustration: FIG. 56.—_Pyrosoma_. A, lateral view (nat. size); B, end
view; C, diagram of longitudinal section. _at_, Atrial apertures; _br_,
branchial apertures; _c.cl_, common cloaca; _end_, endostyle; _t_, test;
_v_, velum or diaphragm at terminal opening.]
THE COLONY.—The shape of the colony is seen in Fig. 56, A. It tapers
slightly towards the closed end, which is rounded. The opening at the
opposite end may be reduced in size (see B and C), by the presence of a
membranous prolongation of the common test, which can be contracted or
expanded by means of the muscle-bands it receives from the atrial siphons
of neighbouring zooids. The branchial apertures of the ascidiozooids are
mostly placed upon short (in some cases longer) papillae projecting from
the general surface, and many of the ascidiozooids have long conical
processes of the test extending outwards beyond their branchial
{92}apertures (Fig. 57, _t′_). There is only a single layer of adult
ascidiozooids in the wall of the _Pyrosoma_ colony, as all the fully
developed ascidiozooids are placed with their antero-posterior axes at
right angles to the surface and communicate by their atrial apertures with
the central cavity (Fig. 56, C). Their dorsal surfaces are turned towards
the open end of the colony, and the buds are given off from their ventral
edges (Fig. 57).
[Illustration: FIG. 57.—Ascidiozooid of _Pyrosoma_ from the right side.
_a_, Anus; _At_, atrial aperture; _at.m_, atrial muscles; _Br_, branchial
aperture; _br.s_, branchial sac; _cl_, cloaca; _d.l_, dorsal lamina; _d.t_,
dorsal tubercle; _ec_, ectoderm; _en_, endoderm; _end_, endostyle; _Ht_,
heart; _l.o_, luminous organ; _mes_, mass of mesoderm cells; _m.f_, muscle
fibre; _n.g_, nerve-ganglion; _oes_, oesophagus; _sg_, stigmata; _st_,
stomach; _stol_, stolon; _t_, test; _t′_, projection of test near branchial
aperture; _tes_, testis; _tn_, tentacle; 1, 2, 3, buds.]
ANATOMY.—The more important points in the structure of the ascidiozooid of
_Pyrosoma_ are shown in Fig. 57. A circle of tentacles, of which one,
placed ventrally (_tn_), is larger than the rest, is found just inside the
circular branchial aperture. From this point a wide cavity, with a few
circularly placed muscle-bands running round its walls, leads back to the
large branchial sac (_br.s._), which occupies the greater part of the body.
The large stigmata are elongated transversely (dorso-ventrally), and are
crossed by internal longitudinal bars running antero-posteriorly. The
dorsal lamina is represented by a series of eight or ten {93}languets. The
nerve-ganglion (on which is placed a small pigmented sense-organ, the
unpaired "eye"), the neural gland, the dorsal tubercle, the peripharyngeal
bands and the endostyle are placed in the usual positions. On each side of
the anterior end of the branchial sac, close to the peripharyngeal bands is
a mass of rounded mesodermal gland-cells (_l.o_), which are the source of
the phosphorescence. They are apparently modified leucocytes lying in
blood-sinuses. The alimentary canal is placed posteriorly to the branchial
sac, and the anus opens into a large peribranchial or atrial cavity, of
which only the median posterior part (_cl_), is shown in Fig. 57. The heart
(_Ht_) lies between the posterior end of the branchial sac and the
intestine, close to where the endostyle is prolonged outwards to form the
inner tube of the ventral stolon. The reproductive organs are developed
from a cord of germinal tissue which forms a part of every budding stolon,
and so establishes a continuity of origin between the ova of successive
generations of _Pyrosoma_. On the ventral edge of the body, immediately
behind the stolon, with part of which it is continuous, a portion of this
germinal tissue gives rise to a lobed testis (_tes_), and to a single ovum
surrounded by indifferent or follicle-cells.
DEVELOPMENT AND LIFE-HISTORY.—The development takes place within the body
of the parent, in a part of the peribranchial cavity. It is a "direct"
development, the tailed larval stage being omitted. The segmentation is
incomplete or "meroblastic," and an elongated embryo is formed on the
surface of a mass of food-yolk. Follicle-cells, or kalymmocytes, migrate
into the embryo, where they aid in its nutrition. The embryo (or young
oozooid),[104] after the formation of an alimentary cavity, a tubular
nervous system, and a pair of laterally placed atrial tubes, divides into
an anterior and a posterior part (see Fig. 58). The anterior and ventral
part, or stolon, then segments into four pieces (the tetrazooids or first
blastozooids),[104] which afterwards develop into the first ascidiozooids
of the colony, while the posterior part remains in a rudimentary condition,
and is what was called by Huxley the "cyathozooid" (Fig. 58, _cy_). This is
really the degenerate oozooid, and eventually atrophies without having
{94}completed its development, but having precociously given rise to the
budding stolon.
As the four ascidiozooids increase in size, they grow round the cyathozooid
and soon encircle it (Fig. 58, B). In this condition the young colony
leaves the body of the parent and becomes free. The cyathozooid absorbs the
nourishing yolk upon which it lies, and distributes it to the ascidiozooids
by means of a heart and system of vessels which have been meanwhile formed.
When the cyathozooid atrophies and is absorbed, its original atrial
aperture remains and deepens to become the central cavity[105] of the young
colony, which now consists of four ascidiozooids placed in a ring, around
where the cyathozooid was, and enveloped in a common test. The test is at
first formed by the ectoderm cells of the cyathozooid. Later it becomes
invaded by mesoblast cells from the ascidiozooids in the usual manner. The
colony gradually increases by the formation of buds from these four
original ascidiozooids. The young colony is, in some species, at first
male, and only becomes hermaphrodite when it has attained to some size.
[Illustration: FIG. 58.—Development of _Pyrosoma_ colony. A, young stage
showing oozooid or cyathozooid, _cy_, with stolon divided into four
blastozooids (I.-IV.): _v_, vitellus. B, older stage showing the four
blastozooids in a ring around the remains of the cyathozooid. (After
Salensky.)]
OCCURRENCE.—The half-dozen known species of _Pyrosoma_ are widely
distributed over the great oceans, although they are probably most abundant
in tropical waters. _Pyrosoma atlanticum_, Péron, and _P. giganteum_,
Lesueur, are the commonest forms. Although sometimes abundant in the
Mediterranean and the North Atlantic they have apparently not been found in
British seas. _P. elegans_, Lesueur, is a Mediterranean form allied to the
last two; and _P. minatum_ and _P. aherniosum_, Seeliger, were discovered
during the German "Plankton" expedition in the tropical Atlantic. Finally,
the enormous _P. spinosum_, Herdman, was found by the "Challenger" in both
North and South Atlantic in 1873; and {95}some years later (Perrier's _P.
excelsior_) by the French "Talisman" expedition in the tropical Atlantic.
The late Professor Moseley said of this ("Challenger") species, "I wrote my
name with my finger on the surface of the giant _Pyrosoma_ as it lay on
deck in a tub at night, and my name came out in a few seconds in letters of
fire." Bonnier and Pérez have recently recorded that they saw an enormous
profusion of a large _Pyrosoma_ (up to four metres in length) in the
Arabian part of the Indian Ocean.
ORDER III. THALIACEA (SALPIANS).
Free-swimming pelagic forms of moderate size, which may be either simple or
compound, and in which the adult is never provided with a tail or
notochord. Consequently the whole body here corresponds to the trunk only
of the Appendicularian without the tail. The test is permanent, and may be
either well developed or very slight. In all cases it is clear and
transparent. The musculature of the body-wall is in the form of more or
less complete circular bands, by the contraction of which water is ejected
from the body, and so locomotion is effected. The branchial sac has either
two large, or many small, stigmata, leading to a single peribranchial
cavity, into which the anus also opens. Blastogenesis takes place from a
ventral, endostylar stolon. Alternation of generations occurs in the
life-history, and may be complicated by polymorphism. The Order Thaliacea
comprises two groups, CYCLOMYARIA (such as _Doliolum_) and HEMIMYARIA (such
as _Salpa_).
SUB-ORDER 1. CYCLOMYARIA.
Free-swimming pelagic forms which exhibit alternation of generations in
their life-history, but never form permanent colonies. The body is
cask-shaped, with the branchial and atrial apertures at the opposite ends.
The test is moderately well developed, never much thickened. The
musculature is mostly in the form of complete circular bands surrounding
the body. The branchial sac is fairly large, occupying the anterior half or
more of the body. Stigmata are usually present in its posterior part only.
The peribranchial cavity is mainly posterior to the branchial sac. The
alimentary canal is placed ventrally, close to {96}the posterior end of the
branchial sac. Hermaphrodite reproductive organs lie ventrally near the
intestine.
This group is clearly distinguished from the second sub-order, the
Hemimyaria, by the condition of the muscle-bands and of the branchial sac,
and by the life-history. The muscle-bands are complete rings (except in
_Anchinia_), while in the Hemimyaria they are always more or less
incomplete. The branchial sac in the Cyclomyaria is a distinct cavity, and
communicates with the peribranchial cavity only by small slits or stigmata.
The life-history is also very characteristic, as the sexual generation in
the Cyclomyaria is always polymorphic, while in the Hemimyaria it consists
of one form only.
[Illustration: FIG. 59.—Sexual generation of _Doliolum tritonis_, Herdman,
from left side, × 10. _at_, Atrial aperture; _at.l_, atrial lobes; _at.m_,
wall of atrium; _br_, branchial aperture; _br.l_, branchial lobes; _br.s_,
branchial sac; _d.t_, dorsal tubercle; _end_, endostyle; _h_, heart; _i_,
intestine; _m_, mantle; _m_^1-_m_^8, circular muscle-bands; _n_, nerve;
_n.g_, nerve-ganglion; _ov_, ovary; _p.br_, peribranchial cavity; _p.p_,
peripharyngeal bands; _sg_, stigmata; _s.gl_, neural gland; _s.o_,
sense-organ; _st_, stomach; _t_, test; _tes_, testis; _z_, prebranchial
zone. (After Herdman.)]
STRUCTURE OF DOLIOLUM.—The single family DOLIOLIDAE includes three genera,
_Doliolum_, Quoy and Gaimard, _Dolchinia_, Korotneff, and _Anchinia_,
Eschscholtz. _Doliolum_, of which about a dozen species are known, from
various seas, has a cask-shaped body (Fig. 59), usually from 1 to 2 cm. in
length. The terminal branchial and atrial apertures are lobed, and the
lobes are provided with sense-organs. The test is a thin but tough
transparent layer, and contains no "test" cells. It is merely a cuticle
covering the surface of the squamous ectoderm. The body-wall has eight or
nine circular muscle-bands surrounding the body. The most anterior and
posterior of these form the branchial and atrial sphincters. The wide
branchial and atrial apertures lead respectively into branchial and
peribranchial cavities separated by the posterior and postero-lateral walls
of the branchial sac which are pierced by a considerable number of
{97}small stigmata; consequently there is a free passage for the water
through the body along its long axis, and the animal swims by contracting
its ring-like muscle-bands so as to force out the contained water
posteriorly. When stigmata are found on the lateral walls of the branchial
sac (see Fig. 59) there are corresponding anteriorly directed diverticula
of the peribranchial cavity. There is a distinct endostyle on the ventral
edge of the branchial sac and a peripharyngeal band surrounding its
anterior end, but there is no representative of the dorsal lamina along its
dorsal edge; and there are neither branchial nor atrial tentacles. The
oesophagus commences rather on the ventral edge of the posterior end of the
branchial sac, and runs backwards to open into the stomach, which is
followed by a curved intestine opening into the peribranchial cavity. The
alimentary canal as a whole is to the right of the middle line. The
hermaphrodite reproductive organs are to the left of the middle line
alongside the alimentary canal. They open into the peribranchial cavity.
The ovary is nearly spherical, while the testis is elongated, and may be
continued anteriorly for a long distance. The heart is placed in the middle
line ventrally, between the posterior end of the endostyle and the
oesophageal aperture. The nerve-ganglion lies about the middle of the
dorsal edge of the body, and gives off many nerves. Under it is placed the
neural gland, the duct of which runs forward and opens into the anterior
end of the branchial sac by a simple aperture surrounded by the spirally
twisted dorsal ends of the peripharyngeal bands.
LIFE-HISTORY.—The ova produced by the _Doliolum_ of the sexual generation,
after a complete or "holoblastic" segmentation, and normal invagination,
produce tailed larvae with a relatively small caudal appendage, and a large
body in which the characteristic musculature begins to appear (Fig. 60, A).
These larvae after metamorphosis lose their tails and develop into
oozooids, known as "nurses," which are asexual, and are characterised (Fig.
60, B) by the possession of nine muscle-bands, by the stigmata being few in
number and confined to the posterior end of the branchial sac, by an
otocyst on the left side of the body, by a ventrally-placed complex stolon
or "rosette organ" near the heart, from which primary buds are produced by
constriction, and by a dorsal outgrowth ("the cadophore") near the
posterior end of the body. The buds (blastozooids) give rise eventually,
after {98}further division, to the sexual generation, which is
polymorphic—having three distinct forms, in two of which the reproductive
organs remain undeveloped.
[Illustration: FIG. 60.—Life-history of _Doliolum_. A, tailed larval stage;
B, "nurse" or oozooid, showing buds (blastozooids) migrating from the
ventral stolon to the dorsal process; C, posterior part of much later
oozooid to show buds arranged in three rows on dorsal process; D, stolon
segmenting; E, young migrating bud; F, trophozooid developed from one of
the buds of a lateral row. _At_, Atrial aperture; _b_, buds; _Br_,
branchial aperture; _cl_, cloaca; _d.p_, dorsal process; _end_, endostyle;
_ht_, heart; _l.b_, lateral buds; _m.b_, median buds; _n.g_,
nerve-ganglion; _ot_, otocyst; _p.c_, pericardium; _sk_, stalk; _sto_,
stolon. (After Uljanin and Barrois.)]
The primary buds are constricted off while still very young and undeveloped
(Fig. 60, D, B, and E); they migrate from their place of origin on the
stolon, over the surface (aided by large amoeboid test-cells which become
attached to the buds) (Fig. 60, B), multiply by fission, and become
attached (again by the help of amoeboid test-cells and ectoderm cells which
form a slight "placenta") in three rows—a median and two lateral—to the
dorsal outgrowth (Fig. 60, C) of the body of the nurse. This parent-form by
this time has become greatly modified, and its structure is largely
sacrificed for the good of the buds or growing zooids, for which it really
forms a locomotory organ. Its muscle-bands become greatly developed in
width (Fig. 60, C), and the branchial meshwork, endostyle, and alimentary
canal disappear.
The three forms produced in the second generation are as follows:—(1)
Nutritive forms ("trophozooids") derived from the lateral rows of buds,
which remain permanently attached to the {99}oozooid, and are sacrificed
for the benefit of the rest of the colony. They serve merely to aid in
respiration, and to provide the food for the nurse and the median buds.
Their development is arrested; they have the body elongated dorso-ventrally
with a large funnel-like branchial aperture (Fig. 60, F), and the
musculature is very slightly developed.
(2) Some of the median buds become foster forms ("phorozooids"), which,
like the preceding trophozooids, do not become sexually mature, but, unlike
them, are eventually set free as cask-shaped bodies having the _Doliolum_
appearance, with eight encircling muscle-bands, and having, moreover, a
ventral outgrowth (not a stolon), which is formed of the stalk by which the
body was formerly attached to the dorsal process of the oozooid. On this
ventral outgrowth the "gonozooids" (3) are attached while still very young
buds, and after the phorozooids are set free these reproductive forms
gradually attain their complete development, become sexually mature, and
are eventually separated off, finally losing all trace of their temporary
connexion with the foster-forms. They resemble the foster-forms in having a
cask-shaped body with eight muscle-bands, but differ in the absence of a
ventral process, and in having the sexual reproductive organs fully
developed.
OCCURRENCE.—The best-known member of the genus is _Doliolum tritonis_,
Herdman, which was captured in the tow-nets in thousands by Sir John Murray
during the cruise of H.M.S. "Triton" in the summer of 1882 in the North
Atlantic. Since then that species, or the closely allied _D. nationalis_,
Borgert, have been found on more than one occasion in the English Channel
and other parts of our south-west coast, and so _Doliolum_ may be regarded
as an occasional member of the British surface fauna.
It is probable that the occasional phenomenal swarms of _Doliolum_ which
have been met with in summer in the North Atlantic are a result of the
curious life-history which, under favourable circumstances, allows of a
small number of oozooids producing from minute buds an enormous number of
phorozooids and gonozooids.
As the result of the careful quantitative work of the German "Plankton"
expedition, Borgert thinks that the temperature of the water has more to do
with both the horizontal and the {100}vertical distribution of these
Thaliacea in the sea than any other factor.
OTHER GENERA.—_Anchinia_, of which only one species is known, _A. rubra_,
Vogt, from the Mediterranean, has the sexual forms permanently attached to
portions of the dorsal outgrowth from the body of the unknown oozooid
("nurse"). The stolon is probably much longer than in _Doliolum_, and
curves round so as to reach and lie along the dorsal outgrowth, upon which
it places the buds.
The body of the adult is elongated dorso-ventrally. The test is well
developed and contains branched cells. The musculature is not so well
developed as in _Doliolum_. There are two circular bands at the anterior
end, two at the posterior, and two muscles on the middle of the body, which
unite to form the characteristic S-shaped lateral bands. The stigmata are
confined to the obliquely-placed posterior end of the branchial sac. The
alimentary canal forms a U-shaped curve. The reproductive organs are placed
on the right side of the body. The life-history is still imperfectly known.
As in the case of _Doliolum_ the sexual generation is polymorphic, and has
three forms, two of which remain in a rudimentary condition so far as the
reproductive organs are concerned. They are known as the first and second
sterile forms, or "trophozooids." In _Anchinia_, however, the three forms
do not occur, so far as we know, together at the same time on the one
outgrowth, but are produced successively, or in different regions, the
reproductive forms of the sexual generation being independent of the
"foster-forms."[106]
The third genus, _Dolchinia_, contains also only a single species, _D.
mirabilis_, found by Korotneff[107] in the Gulf of Naples. It must have
three different forms in its life-history—oozooid, phorozooid, and
gonozooid, but the first of these is still unknown. On what must be body
processes detached from the oozooid are found phorozooids somewhat like
those of _Doliolum_, bearing sexual forms attached to ventral stalks.
_Dolchinia_ is intermediate on the whole between _Anchinia_, the most
simple member of the family, and _Doliolum_ the most complex; and may
eventually come to be united with the latter genus.
{101}SUB-ORDER 2. HEMIMYARIA.
Free-swimming pelagic forms which exhibit alternation of generations in
their life-history, and in the sexual condition form colonies. The body is
more or less fusiform, with the long axis antero-posterior, and the
branchial and atrial apertures nearly terminal and opposite. The test is
well developed but transparent. The musculature of the body-wall is in the
form of a series of transversely-running bands which do not usually form
complete independent rings as in the CYCLOMYARIA. These
partially-encircling muscles in the Salpidae (see Fig. 61, _m.b_) are
probably to be regarded as modified branchial and atrial sphincters which
have spread over the intervening body. The branchial and peribranchial
(cloacal) cavities form a continuous space in the interior of the body,
opening externally at the ends by the branchial and atrial apertures, and
traversed obliquely from the dorsal and anterior to the ventral and
posterior end by a long narrow vascular ciliated band, which represents the
dorsal lamina, the dorsal blood-sinus, and the neighbouring parts of the
dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary
canal is placed ventrally. It may either be stretched out so as to extend
for some distance anteriorly, or, as is more usual, be concentrated to form
along with the testis a rounded opaque mass near the posterior end of the
body, known as the visceral mass or "nucleus." The embryonic development is
direct, no tailed larva being formed. The embryo is united to the parent
for a time by a "placenta."
This sub-order contains, in addition to its typical members, the SALPIDAE,
another still somewhat problematical family the OCTACNEMIDAE, including a
single very remarkable deep-water genus (_Octacnemus_), which in some
respects does not conform with the characters given above, and exhibits a
certain amount of affinity with the primitive fixed forms from which
Salpidae have been derived.
{102}[Illustration: FIG. 61.—_Salpa runcinata-fusiformis_. A, aggregated or
"chain" form; B, solitary form. _At_, Atrial aperture; _at.m_, atrial
muscles; _Br_, branchial aperture; _br.m_, branchial muscles; _d.l_, dorsal
lamina or "gill"; _d.t_, dorsal tubercle; _emb_, embryo; _end_, endostyle;
_m_, mantle; _m.b_, muscle-bands; _n.g_, nerve-ganglion; _p.p_,
peripharyngeal bands; _st_, stolon; _st″_, "chain" of buds; _t_, test; _v_,
visceral "nucleus."]
[Illustration: FIG. 62.—Diagram to show the arrangement and connexion of
the aggregated zooids in a young chain of Salps. 1, 3, 5, zooids on the
right; 2, 4, 6, zooids on the left. _At_, Atrial aperture of a zooid; _Br_,
branchial aperture of another; _c.t_ at the top of the figure points to
three pairs of connecting tubes; _c.t_ at the foot, to two pairs. Each
zooid is united to each of the four neighbours it touches by a pair of
connecting tubes, and so has eight such tubes in all.]
OCCURRENCE AND REPRODUCTION.—The family SALPIDAE[108] includes the single
genus _Salpa_, Forskål, which, however, may be divided into two well-marked
groups of species—(1) those such as _S_. (_Cyclosalpa_) _pinnata_, in which
the alimentary canal is stretched out ("ortho-enteric" condition) along the
ventral surface of the body, and (2) those such as _S.
runcinata-fusiformis_, in which the alimentary canal forms a compact
globular mass (Fig. 61, _v_), the "nucleus" ("caryo-enteric" condition),
near the posterior end of the body. About fifteen species altogether are
known; they are all pelagic in habit, and are found in nearly all seas.
Each species occurs in two forms (Fig. 61, A and B), the solitary asexual
(_proles solitaria_), and the aggregated sexual (_proles gregaria_), which
are in most species quite unlike one {103}another, the aggregated form
being usually more rounded, ovoid, or fusiform (Fig. 61, A), and the
solitary more quadrangular, and often provided with conical processes or
projecting points.
[Illustration: FIG. 63.—Diagram to show the relations of the groups of
young buds, when first formed on the stolon of _Salpa_. _at_, Atrial
aperture; _br_, branchial aperture; _el_, elaeoblast; _end_, endostyle;
_h_, heart; _n.g_, nerve-ganglion; _ov_, ovum; _s_, stolon; _st_, stomach;
I, II, III, groups of buds. (After Brooks.)]
[Illustration: FIG. 64.—Transverse section through endostyle and young
stolon of _Salpa pinnata_. _ec_, Ectoderm of parent reflected at _ec′_ to
cover base of stolon; _ec″_, ectoderm of stolon; _end_, endoderm of stolon;
_g_, ovary; _mes_, mesoderm cells; _n_, nerve-tube of stolon; _p.br_,
peribranchial tubes of stolon. (After Brooks.)]
The solitary form gives rise, by gemmation at the posterior end of the
endostyle (Fig. 63), to a complex tubular stolon, containing processes from
the more important organs of the parent-body, which give rise to an
endodermal tube, two peribranchial tubes, a neural tube, two blood-sinuses
and mesoblast cells, a genital cord, and over all the ectodermal covering
(see Fig. 64). This stolon becomes segmented (Fig. 63) into a series of
buds or young "chain" individuals, of which there may be several hundreds.
As the stolon elongates (Fig. 61, B, _st″_), the buds {104}undergo lateral
shifting, and rotation round their longitudinal axis, so as to acquire the
relations seen in the "chain," which then emerges from the tube in the test
through which it has been growing, so as to project to the exterior near
the atrial aperture. The buds at its free end which have now become far
advanced in their development are set free in groups, which remain attached
together by processes of the test, each enclosing a diverticulum from the
body-wall (Fig. 62), so as to form "chains." Each member of the chain is a
_Salpa_ of the sexual or aggregated form, and when mature may—either still
attached to its neighbours or separated from them—produce one or several
embryos (Fig. 61, A, _emb_), which develop into the solitary form of
_Salpa_. Thus the two forms, different in appearance and structure and
different in mode of origin, alternate regularly in the life-history of
_Salpa_.
STRUCTURE.—The more important points in the structure of a typical _Salpa_
are shown in Fig. 65. The branchial and atrial apertures are at opposite
ends of the body, and lead into large cavities, the branchial and
peribranchial sac respectively, which are in free communication at the
sides of the obliquely-running dorsal lamina or "gill" (_d.l_). The
transparent test is usually thick, and varies from a gelatinous to a stiff
cartilaginous condition; it adheres closely to the surface of the mantle
(ectoderm and body-wall). The muscle-bands (from 4 to about 20—usually 8 or
10) of the mantle do not in most cases completely encircle the body. They
are present dorsally (Fig. 65, _mus.bds_) and laterally, but the majority
do not reach the ventral surface. In many cases neighbouring bands join in
the median dorsal line (Fig. 61). The muscle fibres are striated, and have
rows of large equidistant nuclei. The anterior end of the dorsal lamina is
in some cases prolonged to form a prominent tentacular organ, the languet
or dorsal tentacle, projecting into the branchial sac, while near this
opens a ciliated funnel corresponding to the dorsal tubercle, but having no
connexion in the adult with either ganglion or subneural gland. The
conjoined ganglion and subneural gland, the dorsal lamina, the
peripharyngeal bands and the endostyle are placed in the usual positions.
Eyes in the form either of a continuous horse-shoe-shaped pigmented ridge
on the dorsal surface of the ganglion immediately below the ectoderm, or of
one larger median and several smaller lateral ocelli are found in the
various species of _Salpa_. These eyes have in {105}most cases a retina
formed of elongated cells, and a pigment-layer placed upon the ganglion.
The so-called otocysts of _Salpa_ have been shown by Metcalf to be really
glandular organs. They have been called lateral neural glands; they do not
open at the dorsal tubercle, but separately into the pharynx. These lateral
neural tubular glands have also been regarded as nephridia.
The large spaces at the sides of the dorsal lamina (often called the gill
or branchia of _Salpa_), by means of which the cavity of the branchial sac
is placed in free communication with the peribranchial cavity, are to be
regarded as gigantic gill-slits formed by the suppression of the lateral
walls and small stigmata of the branchial sac. The alimentary canal at the
posterior end of the "gill" consists of oesophagus, stomach, and intestine,
with a pair of lateral gastric glands or caeca. These viscera along with
the reproductive organs, when present, make up the "nucleus" (Fig. 66,
_v_).
[Illustration: FIG. 65.—Diagrammatic sagittal section of a "chain" _Salpa_.
_an_, Anus; _at_, atrial aperture; _at.m_, muscles of atrial aperture;
_atr.cav_, atrial cavity; _br_, branchial aperture; _br.m_, muscles of
branchial aperture; _br.s_, branchial sac; _d.l_, dorsal lamina or "gill";
_d.t_, dorsal tubercle; _end_, endostyle; _ht_, heart; _int_, intestine;
_l_, sensory languet; _mus.bds_, muscle-bands; _n.g_, nerve-ganglion; _oc_,
eye-spot; _oe_, oesophagus; _ov_, ovary; _p.p.b_, peripharyngeal band;
_s.gl_, neural gland; _stom_, stomach; _t, t′_, test; _tes_, testis; _z_,
prebranchial zone. (After Herdman.)]
ALTERNATION OF GENERATIONS.—Fig. 66 represents an aggregated or sexual
_Salpa_, which was once a member of a chain, since it shows a testis and a
developing embryo. The ova (always few in number, usually only one) appear
at a very early period in the developing chain _Salpa_, while it is still a
part of the gemmiparous stolon in the body of the solitary _Salpa_. This
gave rise to the view put forward first by Brooks that the ovary
{106}really belongs to the solitary stolon-bearing _Salpa_, which is
therefore a female producing a series of males by asexual gemmation, and
depositing in each of these an ovum, which will afterwards, when
fertilised, develop in the body of the male into a solitary or female
_Salpa_. This idea, if adopted, would profoundly modify our conception of
_Salpa_ as an example of a life-history showing alternation of generations,
but it seems to me to give a distorted view of the sequence of events. The
fact that the stolon while in the solitary _Salpa_ contains, along with
representatives of other important systems of the body, a row of germinal
cells, does not constitute that solitary _Salpa_ the parent of the ova
which these germinal cells will afterwards become in the body of an
independent bud. We must regard as the parent the body in which the ova
become mature and fulfil their function. The sexual or chain _Salpa_,
although really hermaphrodite in its life-history, is usually[109]
protogynous, _i.e._ the ova mature at an earlier period than the male organ
or testis. This prevents self-fertilisation. The ovum is presumably
fertilised by the spermatozoa of an older _Salpa_ belonging to another
chain, and the embryo is far advanced in its development before the testis
is formed. The development takes place inside the body of the parent, and
is "direct"—no tailed larval form being produced.
[Illustration: FIG. 66.—_Salpa hexagona_, Q. and G. Chain form dissected
from the left side. _a_, Anus; _at_, atrial aperture; _br_, branchial
aperture; _d.l_, dorsal lamina ("gill"); _d.t_, dorsal tubercle; _emb_,
embryos; _end_, endostyle; _m.b_ 2, _m.b_ 7, second and seventh
muscle-bands; _n.g_, nerve-ganglion; _v_, visceral "nucleus." (After
Traustedt.)]
DEVELOPMENT AND LIFE-HISTORY.—The segmentation of the egg is holoblastic,
and gives rise to a number of blastomeres, {107}which are for a time masked
by the phenomenal activity of certain cells of extraneous origin, the
"kalymmocytes," derived from the follicular epithelium surrounding the
ovum. These follicular kalymmocytes migrate into the ovum, surround groups
of blastomeres, and arrange themselves so as to reproduce the essential
structure of the future embryo for which they form what may be termed a
scaffolding or temporary support. After a time the blastomeres become
active, proliferate rapidly, and finally press upon and absorb the
kalymmocytes, and so eventually take their proper place in building up the
organs. Some observers regard the kalymmocytes as being passive and
nutritive only in function.
[Illustration: FIG. 67.—Young solitary _Salpa democratica-mucronata_
attached to the parent by the placenta. _atr.ap_, Atrial aperture; _br_,
dorsal lamina; _cil.gr_, dorsal tubercle; _ebl_, elaeoblast; _end_,
endostyle; _n.gn_, nerve-ganglion; _oes_, oesophagus; _or.ap_, branchial
aperture; _peric_, pericardium; _pl_, placenta; _rect_, intestine; _stol_,
stolon; _stom_, stomach. (From Parker and Haswell, after Salensky.)]
At an early period in the development a part of the surface of the embryo,
on its ventral edge, becomes separated off, along with a part of the wall
of the cavity ("oviduct"—a diverticulum from the atrium) in which it lies,
to form the "placenta" (Fig. 67, _pl_) in which the embryonic and maternal
blood-streams circulate in close proximity, and so allow of the conveyance
of nutriment to the developing embryo by means of large migrating placental
cells. At a somewhat later stage a number of cells placed at the posterior
end of the body alongside the future nucleus become filled up with
oil-globules to form a mass of nutrient material—the "elaeoblast" (Fig. 67,
_ebl_)—which is used up later in the development. Many suggestions have
been made as to the homology and meaning of the elaeoblast; but it may now
be regarded as most probable that it is reserve food-material associated
with the disappearing rudiment of the {108}tail found in the larval
condition of most Ascidians. The development is direct; and it may be said,
then, that this young asexual (solitary) _Salpa_ differs from the
corresponding form in the life-history of _Doliolum_ (Fig. 60, A) in that
its tail is no longer a locomotory organ, but is represented by a nutritive
mass, the elaeoblast, while the body, in place of being free, is attached
by its ventral surface to a special organ of nutrition—the "placenta"—in
connexion with the blood-stream of the parent.
This embryo sexually produced inside the body of an aggregated form becomes
a solitary _Salpa_ (such as Fig. 61, B), which differs in appearance,
structure, and habits from its parent, and has no reproductive organs.
After swimming for a time, however, it develops the ventral stolon on which
buds form which are eventually sexual Salpae. These are set free from the
solitary form in sets, still connected together, and they may swim about
together for a time as a chain of aggregated Salpae before separating to
become the adult sexual individuals (such as Fig. 61, A).
CLASSIFICATION.—_Salpa_ may be divided into the following
subgenera:[110]—_Cyclosalpa_, Blainville, in which the alimentary canal is
ortho-enteric, and the "chain" consists of individuals united in a circle;
_Iasis_, Savigny, with several embryos formed at a time; and _Pegea_, Sav.,
_Thalia_, Blumenbach, and _Salpa_, Forskål, all with one embryo only, and
differing from one another in the condition of the "gill" and other
details: all except _Cyclosalpa_ have the alimentary canal caryo-enteric.
_Cyclosalpa_ has three species, the best known of which is _C. pinnata_ of
the Mediterranean, a form possessing light-producing organs like those of
_Pyrosoma_, but placed along the sides of the body. _Salpa_ has four or
five species, one of which, _S. runcinata-fusiformis_ (Fig. 61), has
occasionally been found in British seas; _Thalia_ includes the species _T.
democratica-mucronata_, which has been sometimes obtained in swarms in the
Hebridean seas, or cast ashore on our southern or western coasts; _Pegea_
has the species _P. scutigera-confoederata_; and _Iasis_ contains the
remaining half-dozen species, the best known of which is _I.
cordiformis-zonaria_, the only other Salpian which has been found in
British seas.
{109}[Illustration: FIG. 68.—A, solitary form of _Octacnemus bythius_
(after Moseley); B, diagram of structure of _Octacnemus_ (after Herdman);
C, aggregated form of _O. patagoniensis_ (after Metcalf). 1, from outside;
2, with test removed; and 3, with mantle removed. _a_, Anus; _adh_, area of
attachment; _at_, atrial, and _br_, branchial aperture; _br.s_, branchial
sac; _end_, endostyle; _g.s_, gill-slits; _i_, intestine; _n.y_,
nerve-ganglion; _oe_, oesophagus; _ov_, ovary; _p.br_, peribranchial
cavity; _st_, stomach; _stol_, stolon.]
The family OCTACNEMIDAE includes the single remarkable genus _Octacnemus_,
now known in a solitary and an aggregated form. It was found during the
"Challenger" expedition, and was first described by Moseley. It is
apparently a deep-sea representative of the pelagic Salpidae, and may
possibly be fixed at the bottom. The body in the solitary form is somewhat
discoid, with its margin prolonged to form eight tapering processes, on to
which the muscle-bands of the mantle are continued. The alimentary canal
forms a compact nucleus, which is attached to an apparently imperforate
membrane which stretches across the body, separating the branchial from the
atrial cavities. The endostyle is very short, and the dorsal lamina is also
much reduced. The reproduction and life-history are entirely unknown. The
aggregated form consists of a small number of individuals united by a
slender cord composed of test, body-wall, and endodermal tissue.
_Octacnemus_ has been found[111] in the South Pacific from depths of 1070
and 2160 fathoms, and off the Patagonian coast from 1050 fathoms. Two
species have been described: _O. bythius_, Moseley, and _O. patagoniensis_,
Metcalf. Metcalf, who has recently investigated the aggregated form (_O.
patagoniensis_), considers that the genus is more nearly related to the
Clavelinidae than to the Salpidae. Possibly its position might be best
{110}indicated by a line diverging from near the point (3) in the
phylogenetic diagram below.
GENERAL CONCLUSIONS.
The following diagram is a graphic representation of the genetic
affinities, or what is now generally supposed to have been the probable
course of phylogeny of the Tunicata. It will be noticed that it shows (1)
the Proto-Tunicates arising from Proto-Chordata, not far from the ancestors
of Amphioxus (see also, this vol. p. 112); (2) that the Larvacea are
regarded as the most primitive section of the group; (3) that the Thaliacea
(Doliolidae and Salpidae) are supposed to be derived not directly from
primitive pelagic forms, but through the early fixed Ascidians, not far
from (4) the ancestral compound Ascidians, which gave rise to the
Pyrosomatidae; (5) that the Ascidiidae and other higher Simple Ascidians
are derived, like the Compound Ascidians, from ancestral Clavelinidae; and
(6), that the Ascidiae Compositae are polyphyletic, the Holosomata
(Botryllidae and Polystyelidae) being derived from ancestral Simple
Ascidians independently of the Merosomatous families.
↑
| Molgulidae
| Ascidiidae |
+––––––––Amphioxus (5) | |
P | +–––––––+–––––+–––––+––––––+––– Cynthiidae.
r | | | |
o | +– C | |
t | Larvacea. | l | (6) Polystyelidae
o | | | a |
c | (1) (2) | +– v Botryllidae
h +––––––––––––––+––––––––––––| e
o | Proto–Tunicata. | l
r | +– i Distomatidae
d | Doliolidae–+ | n |
a | | | i | Didemnidae and
t | | | d | | Diplosomatidae
a | | | a | |
| | (3)| e (4) | |
| +–––––––+–––––––+––––+––––––––––+––––Polyclinidae
| Salpidae–––+ |
| Pyrosomatidae
The Tunicata are remarkable for the variety in appearance, structure, and
life-history which they present. No group illustrates in a more instructive
manner so large a number of important biological principles and phenomena.
They show solitary and colonial forms, fixed and free, pelagic and abyssal.
The development is in some cases larval and with metamorphosis, in others
abbreviated and direct. Persistent traces of ancestral characters are seen
in the embryonic and larval stages, while the adults present the most
varied secondary adaptations to littoral, {111}pelagic, and deep-sea,
free-swimming and sessile modes of existence. In the details of their
classification they demonstrate both stable and variable species,
monophyletic and polyphyletic groups. They exhibit the phenomena of
gemmation and of embryonic fission, of polymorphism, hibernation,
alternation of generations, and change of function. They have long been
known as a stock example of degeneration; but in fact they lend themselves
admirably to the exposition of more than one "Chapter of Darwinism."
* * * * *
NOTE TO P. 78.—_Oligotrema_, Bourne (_Quart. J. Micr. Sci_. xlvii. Pt. ii.
1903, p. 233), a Molgulid from the Loyalty Islands, has a reduced branchial
sac and greatly developed pinnate, muscular branchial lobes, probably used
in capturing food.
{112}CHAPTER IV
CEPHALOCHORDATA
INTRODUCTION—GENERAL CHARACTERS—ANATOMY OF AMPHIOXUS—EMBRYOLOGY AND
LIFE-HISTORY—CLASSIFICATION OF CEPHALOCHORDATA—SPECIES AND DISTRIBUTION
The CEPHALOCHORDATA comprise only a small group of little fish-like forms,
the Lancelets, usually known as "Amphioxus," and referable to about a dozen
species arranged in several closely allied genera under the single family
Branchiostomatidae. The best known form is _Branchiostoma lanceolatum_
(Pallas), the common Amphioxus or Lancelet, which has been found in British
seas, and even as far north as the coast of Norway, but is much more common
in warmer waters, such as the Mediterranean, and is also found in the
Indian Ocean. It is abundant in the Bay of Naples, and lives and breeds in
great numbers in a salt lagoon, the "Pantano," near Messina, and from these
localities most of the specimens have been obtained for the numerous recent
researches upon its structure and development.
Amphioxus was first discovered and described (1778) by Pallas, who regarded
it as a Mollusc, and named it _Limax lanceolatus_. It was first correctly
diagnosed as a low Vertebrate, and named _Branchiostoma_, by Costa, in
1834. The term _Amphioxus_, under which it has become so well known, was
applied to it a couple of years later by Yarrell.
The anatomy was for the first time fully investigated by Johannes Müller in
1841, and this important memoir has been supplemented in regard to special
systems and histological details by numerous papers by many leading
zoologists, such as those by Huxley in 1874, Langerhans in 1876, Lankester
in {113}1875 and in 1889, Retzius in 1890, and Boveri and Hatschek, both in
1892. Important papers on special points have also been written by Rolph,
Rohde, Benham, Andrews, Goodrich, and others. The development was first
elucidated by Kowalevsky in 1867, at about the same time when he studied
the development of the Ascidians, and later again in 1877. Further papers
on the development and metamorphosis we owe to Hatschek in 1881, Lankester
and Willey in 1890 and 1891, Wilson in 1893, and quite recently to
MacBride. Dr. Willey's book, _Amphioxus and the Ancestry of the Vertebrata_
(1894), contains a summary of investigations on structure and development,
an interesting discussion of the relations of Amphioxus to the other
Chordata, and a full bibliography.
In addition to such original researches, Amphioxus is studied in more or
less detail every year by countless senior and junior students in
zoological laboratories and marine stations throughout the civilised world.
The value of this primitive form as an object of biological education
depends upon the fact that it shows the essential Vertebrate characters,
and their mode of formation, in a very simple and instructive condition.
Although no doubt somewhat modified, and possibly degenerate in some
details of structure, in its general morphology it presents us with a
persistent type probably not far removed from the ancestral line of early
Chordata. There are no sufficient grounds for the view that Amphioxus is a
very degenerate representative of fish-like Vertebrata.
GENERAL CHARACTERS.—The Cephalochordata (or Acrania, in contradistinction
to the Craniata or Vertebrata) are marine, non-colonial Chordata, in which
the notochord extends the entire length of the body, running forward into
the snout beyond the nervous system. There is no skull, and the notochord
is not surrounded by any vertebral column. There are no limbs nor paired
fins. There is no exoskeleton, and the ectoderm is a single layer of
non-ciliated columnar cells. The mouth is ventral and anterior, the anus is
ventral, posterior, and asymmetrically placed on the left side. The pharynx
is a large branchial sac, having its sides perforated by many gill-slits,
and is surrounded by an ectodermal enclosure, the atrium, which opens to
the exterior by a median ventral atriopore. The stomach gives off a simple
saccular pouch, the liver, which has {114}connected with it a simple
hepatic portal blood system. There is a respiratory circulation, the
contractile ventral vessel which represents the heart sending the
colourless blood forward to the respiratory pharynx to be purified. The
body-wall is segmented into over fifty myotomes. There are numerous
separate nephridia which develop from the mesoderm and open into the
atrium. The brain remains undeveloped, being scarcely distinct from the
spinal cord. There are two pairs of cerebral nerves, and many spinal, in
which the dorsal and ventral roots or nerves do not unite. The sense-organs
are simple; there are no paired eyes and no auditory organs. The sexes are
separate; the gonads are metamerically arranged on the body-wall, and have
no ducts: they burst into the atrium. In the development the segmentation
is complete, a gastrula is formed by invagination, the nervous system is
formed from the dorsal epiblast, the notochord from the hypoblast, and the
mesoderm arises from metameric coelomic pouches. The body-cavity is an
enterocoele. The gill-slits are at first perforations of the body-wall
opening from the pharynx to the exterior, which later become enclosed by
the development of the atrium.
ANATOMY.
EXTERNAL CHARACTERS.—Amphioxus[112] is about 1½ to 2½ inches in length,
slender, somewhat translucent, and pointed at both ends (Fig. 69). It lives
in shallow water and burrows in the sand, head first, with great rapidity.
It frequently remains with the anterior end protruding from the sand. When
on the surface it lies on one side. It is said to swim freely at night. The
head end is rather the thicker, and the anterior two-thirds of the ventral
surface are flattened (Fig. 70, A), and may be slightly ridged
longitudinally. The lateral edges of this flat area project as metapleural
folds (Fig. 70, _mt.pl_), which begin anteriorly at the edges of the
external mouth, and die away in the middle line posteriorly behind a median
opening, the atriopore (Fig. 70, _atrp_). From this point a ventral median
fin (_vent.f_) extends backwards around the pointed posterior end
{115}(caudal fin, _cd.f_), and then forwards along the upper surface
(dorsal fin, _dors.f_) to the anterior end of the body. These fins thus
constitute a continuous median fold around a great part of the animal (Fig.
70, B, and Fig. 71).
[Illustration: FIG. 69.—Amphioxus (_Branchiostoma lanceolatum_) in the
Pantano at Messina. (After Willey.)]
[Illustration: FIG. 70.—_Branchiostoma lanceolatum_. A, ventral; B, side
view of the entire animal. _an_, Anus; _atrp_, atriopore; _cd.f_, caudal
fin; _cir_, cirri; _dors.f_, dorsal fin; _dors.f.r_, dorsal fin-rays;
_gon_, gonads; _mtpl_, metapleure; _myom_, myomeres; _nch_, notochord;
_or.hd_, oral hood; _vent.f_, ventral fin; _vent.f.r_, ventral fin-rays.
(After Kirkaldy.)]
The surface is soft all over, there being no exoskeleton. The epidermis or
ectoderm is formed by a single layer of {116}epithelial cells (see Fig. 72,
p. 118), some of which bear sensory processes, while others have a striated
cuticular border. There is no general ciliation of the surface in the
adult.
[Illustration: FIG. 71.—Diagram of the anatomy of Amphioxus. A, anterior;
B, posterior part. _an_, Anus; _atr_, atrium; _atr′_, its posterior
prolongation; _atrp_, atriopore; _br_, brain; _br.cl_, branchial clefts;
_br.f_, brown funnel; _br.sep.1_, primary, _br.sep.2_, secondary branchial
lamella; _br.r.1_, primary, _br.r.2_, secondary branchial rod; _caud.f_,
caudal fin; _cent.c_, central canal; _cir_, cirri; _coel_, coelom;
_dors.f_, dorsal fin; _dors.f.r_, dorsal fin-ray; _en.coe_, cerebral
vesicle; _e.sp_, eye-spot; _gon_, gonad; _int_, intestine; _lr_, liver;
_mth_, mouth; _myom_, myotomes; _nch_, notochord; _nph_, nephridia;
_olf.p_, olfactory pit; _or.f.hd_, oral hood; _ph_, pharynx; _sk_, skeleton
of oral hood and cirri (dotted); _sp.cd_, spinal cord; _vent.f_, ventral
fin; _vent.f.r_, ventral fin-ray; _vl_, velum; _vl.t_, velar tentacles.
(From Parker and Haswell.)]
The true mouth is a small pore at the bottom of a large vestibule (the
stomodaeum), placed at the anterior end of the ventral surface (Figs. 70
and 71), and formed by the "oral hood," which may be a prolongation
forwards of the atrial or metapleural folds at each side. The edges of the
oral hood bear 12 to 20 pairs of cirri (Fig. 70, _cir_) or ciliated
tentacles (strengthened by skeletal rods), which form a sensory fringe
around the opening. The anus (Figs. 70 and 71, _an_), is asymmetrical,
being {117}placed on the left side of the ventral fin, some distance behind
the atriopore, and not far from the posterior end of the body. The short
region behind the anus and surrounded by the caudal fin may properly be
called "tail." The current of water for respiratory and nutritive purposes,
and which may carry the ova and spermatozoa to the exterior, usually passes
in at the mouth and out at the atriopore, as in the Tunicata. On occasions,
however, it is said to be reversed.
GENERAL STRUCTURE.—The general plan of organisation of the body (see Fig.
71) is that a longitudinal skeletal axis, the notochord (_nch_), separates
a dorsal nervous system (_sp.cd_) from a ventral reduced coelom (_coel_),
in which lie the alimentary canal (_int_), the gonads (_gon_), and other
organs. Thus a transverse section of the body (see Fig. 72) shows the
typical Chordate arrangement of parts, and is comparable with a transverse
section of a tadpole, a young fish, or a larval Ascidian. A peribranchial
(_atr_) or atrial cavity (which is morphologically a part of the external
world shut in) lies external to the coelom and body-wall around the pharynx
and the greater part of the alimentary canal, and opens to the exterior by
the atriopore. As in the Tunicata, the perforations (gill-slits) in the
wall of the pharynx (_br.cl_) open into the atrial cavity and so indirectly
to the exterior.
MUSCULATURE.—The thick body-wall is largely formed by muscular tissue
metamerically segmented into about 60 myotomes (Fig. 71, _myom_). These
muscle-masses, which (as is usual in Vertebrata) are thickest dorsally at
the sides of the notochord and spinal chord (Fig. 72, _m_), are so arranged
as to present the appearance in a lateral view of the body of a series of
shallow cones (<<) fitting into one another and with their apices directed
forwards. The muscle fibres are striated, and run longitudinally along the
body from the anterior to the posterior edge of each myotome, so as to be
attached at their ends to the two septa of connective tissue which form the
boundaries of the myotomes. These septa, the myocommas, are conspicuous
features in the external appearance of the body (Fig. 70, B). They are not
arranged so as to be opposite one another on the two sides, but the
myotomes on the right and left sides alternate, as can be seen in a
transverse section (Fig. 74, A, p. 121).
{118}[Illustration: FIG. 72.—_Branchiostoma lanceolatum_. Diagrammatic
transverse section of the pharyngeal region, passing on the right through a
primary, on the left through a secondary branchial lamella. _ao_, Dorsal
aorta; _c_, dermis; _ec_, endostylar portion of coelom; _f_, fascia, or
investing layer of myotome; _fh_, compartment containing fin-ray; _g_,
gonad; _gl_, glomerulus; _k_, branchial artery; _kd_, pharynx; _ld_,
combined atrial and coelomic wall (ligamentum denticulatum); _m_, myotome;
_mt_, transverse muscle; _n_, nephridium; _n.ch_, notochord; _of_,
metapleural lymph space; _p_, atrium; _sc_, coelom; _si_, ventral aorta;
_sk_, sheath of notochord and spinal cord (_sp.cd_); _uf_, spaces in
ventral wall. (From Korschelt and Heider, after Boveri and Hatschek.)]
It is by means of these lateral muscle-bundles that the rapid vibration or
alternate bending of the body from side to side in swimming or burrowing
can be performed. There are usually, on each side, 35 myotomes in front of
the atriopore, 14 between the atriopore and the anus, and 11 postanal,
making 60 in all: some species have only about 50 myotomes, and some as
many as 85. (See {119}Classification, p. 137, where a list of the species
with the number of myotomes in each is given.)
There are also transverse muscles (Fig. 72, _mt_) extending across the
ventral surface in the region of the body enclosed by the metapleural
folds, and serving to compress the atrial cavity, and so aid in the
expulsion of its contents.
Outside the muscular layer of the body-wall the thin integument is formed
of a dermal layer of soft connective tissue, covered by the epidermis, a
single layer of columnar cells, many of which, especially on the oral
cirri, have sensory bristles.
SKELETON.—The endoskeleton consists of the notochord and some tracts of
modified connective tissue which support various parts of the body.
The notochord of this animal is noteworthy amongst Chordata for extending
practically the entire length of the body, including the head, from snout
to tip of tail (Fig. 71). It lies in the median plane, but nearer the
dorsal than the ventral surface (Fig. 72), and has the myotomes at its
sides, the nervous system above and the alimentary canal below. It is
elliptical in section, and tapers to the two ends. The nuclei of the
original notochordal cells are displaced to the dorsal and ventral edges,
and the greater parts of the cells, in the adult, are occupied by large
vacuoles filled with a fluid secretion, so as to form by their distended
condition a stiff elastic structure. This state of the cells, and the
appearance it gives rise to (Fig. 73), seen best in young specimens, is
very characteristic of notochordal tissue. Around the notochord lies a
sheath of connective tissue which is continuous with the similar sheath
around the nervous system and with the septa between the myotomes.
[Illustration: FIG. 73.—Median sagittal section of notochord of an
Amphioxus of 32 mm.]
In addition to these skeletal layers of connective tissue there is a
cartilage-like tract in the oral hood. This is jointed, or made up of
separate rod-like pieces, one at the base of each cirrus, into which it
sends a prolongation (Fig. 71, _sk_). The dorsal and ventral fins are
supported by single and double rows {120}respectively of what have been
called "fin-rays." They are short rods of gelatinous connective tissue,
each enclosed in a lymph space. Finally, the bars constituting the walls of
the pharynx between the gill-slits contain slender skeletal rods which run
obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see Fig.
75, p. 122). This skeletal connective tissue consists in all cases of a
fibrous deposit or matrix produced by the layer of epithelium (ectodermal,
endodermal, or mesodermal) which adjoins the tissue.
ALIMENTARY CANAL.—This has, as its most noteworthy feature, the Chordate
characteristic that the pharynx gives rise to the respiratory organ (see
Figs. 71 and 74, A); and in size and prominence, both in side view and in
sections, the modified pharynx of Amphioxus is fairly comparable with the
branchial sac (pharynx) of many Tunicata (see Fig. 23, p. 51), and might be
called by the same name.
The small primitive mouth, at the bottom of the cavity bounded by the oral
hood (stomodaeum), has a membranous border, the velum (Fig. 71, _vl_), the
edges of which are prolonged into a circle of 10 or 12 (up to 16 in some
species) simple oral tentacles turned inwards towards the pharynx (compare
tentacles of Ascidians, p. 45).
The pharynx, by far the largest part of the alimentary canal, and extending
nearly half-way along the body, is more important as a respiratory than as
a nutritive organ. Its walls over nearly the whole extent are perforated by
a large, and indefinite, number (100 or more on each side) of gill-slits
which run on the whole dorso-ventrally, but in the contracted condition
seen in preserved specimens have their lower ends directed obliquely
backwards, so that a vertical transverse section may cut through a number
of such slits and the intervening branchial bars (Fig. 74, A, _kb_). These
bars, and therefore the slits between them, are of two orders, primary and
secondary, the latter being developed later in larval life as downgrowths
or "tongue-bars," one from the top of each primary gill-slit, so as to
divide it into two secondaries. The primary and the secondary (or tongue-)
bars can be distinguished from one another by their structure in the adult
animal (Fig. 75, A and B).
{121}[Illustration: FIG. 74.—_Branchiostoma lanceolatum_. A, transverse
section of the pharyngeal region. _a_, Dorsal aorta; _b_, atrium; _c_,
notochord; _co_, coelom; _e_, endostyle; _g_, gonad (ovary); _kb_,
branchial septa; _kd_, pharynx; _l_, liver; _my_, myotome; _n_, nephridium;
_r_, spinal cord; _sn_, _sn_, dorsal and ventral spinal nerves. B,
Transverse section of the intestinal region. _atr_, Atrium; _coel_, coelom;
_d.ao_, dorsal aorta; _int_, intestine; _myom_, myotome; _nch_, notochord;
_neu_, spinal cord; _s.int.v_, sub-intestinal vein. (From Parker and
Haswell's _Zoology_. A, From Hertwig, after Lankester and Boveri; B, partly
after Rolph.)]
It must be remembered that these branchial bars, or septa between the
gill-slits, are not merely portions of the wall of the pharynx, but are in
a sense portions of the body-wall as well, and correspond in nature, though
not in number, to the visceral arches in a Vertebrate lying between the
visceral clefts which open on the exterior. In the adult Amphioxus the
clefts in the wall of the pharynx do not open directly to the exterior, but
into the peribranchial cavity or atrium, which, however, is only formed at
a late larval period as an invagination or enclosure of ectoderm. Previous
to that the first formed gill-slits opened to the exterior in Amphioxus
(see larva, Fig. 86, p. 134), just as they do in a fish or a young tadpole.
The atrial cavity is therefore, from its origin, lined by ectoderm, and the
outer surface of a branchial bar is virtually a part of the outer surface
of the body. It is only natural then to find that each bar contains a small
section of the coelom in its interior, communicating dorsally and ventrally
with other parts of that cavity (see Figs. 75 and 76). There are also
blood-vessels which run in the branchial bars and their junctions. The
greater part of the epithelium covering a branchial bar is pharyngeal
epithelium or endoderm {122}(Fig. 75, _br.ep_), but the external, wider,
non-ciliated cells (Fig. 75, _at.ep_) are ectodermal cells lining the
atrium. The gelatinous skeletal rods in the primary bars are forked
ventrally, while those in the secondary bars are simple; and there are
other points of detail in which the two kinds of bar differ. These bars are
obviously more numerous in the adult than the myotomes, but in the young
larva the first formed gill-clefts are metamerically arranged, and then
later they increase greatly in number. It is the cilia covering the
pharyngeal epithelium on the branchial bars, possibly aided by the ciliated
tracts of the oral hood, which cause the current of water already alluded
to.
[Illustration: FIG. 75.—Transverse sections through primary (A) and
secondary (B) branchial bars of Amphioxus. _at.ep_, Atrial epithelium;
_bl.s_, blood spaces or "vessels"; _br.ep_, branchial epithelium; _coel_,
coelomic cavity in primary bar; _sk_, skeletal rods. (From Willey, after
Benham.)]
Transverse branchial junctions (synapticula) run across the branchial bars,
connecting them at frequent intervals, and these transverse connexions,
like the branchial bars, are supported by skeletal rods. Along the ventral
median line of the pharynx runs a groove, the endostyle or hypopharyngeal
groove, comparable with the similar structure in the branchial sac of
Tunicata. This longitudinal groove (Fig. 76, _gl_) is lined by ciliated
epithelium containing four tracts of gland cells (compare endostyle in
Ascidians, Fig. 20, p. 46). There is reason to believe that this organ is
the homologue of the thyroid gland of Vertebrata. As in the case of
Tunicata the endostyle secretes mucus, which is carried forwards by the
cilia to constitute a train with entangled food particles which pass back
dorsally to the stomach. At the anterior end the ciliated lips of the
endostyle diverge to {123}the right and left to encircle the front of the
pharynx as the peripharyngeal bands. These unite again dorsally to form the
epipharyngeal (or hyperpharyngeal) groove which leads backwards,
corresponding to the hypopharyngeal groove below (see Fig. 74, A), till the
posterior end of the pharynx is reached.
[Illustration: FIG. 76.—Transverse section of the ventral part of the
pharynx of Amphioxus. _c_, Coelom; _e_, endostyle; _gl_, endostylar glands;
_m.b.a_, median branchial artery; _p.b_, primary bar; _sk_, endostylar and
branchial rods and skeletal plates; _t.b_, tongue-bar. (After Lankester.)]
The remainder of the simple alimentary canal is straight, and is scarcely
differentiated into regions. A slight narrowing of the tube behind the
pharynx has been called the oesophagus, and a slight enlargement which
follows, the stomach. From this point the intestine tapers backwards to the
anus (Fig. 71, p. 116). The ventral edge of the stomach gives off a blind
pouch, the hepatic caecum or saccular liver, which runs forwards on the
right-hand side of the pharynx (Fig. 74, A, _l_). This is a digestive
gland, is lined with glandular epithelium, and apparently corresponds with
the liver of Vertebrata. There are no other digestive glands in connexion
with the alimentary canal of Amphioxus.
COELOM.—In the young larva there are at first (as in Balanoglossus) five
coelomic spaces, a median anterior "head-cavity," a pair of antero-lateral
"collar-cavities," and a pair of more posterior long lateral grooves from
which arise, in the later larva, the segmented myotomes and ventrally a
large coelomic space surrounding the alimentary canal and separating it
from the body-wall. In the adult animal, however, the coelom has been so
much displaced by the formation of the spacious atrium that in front of the
atriopore it can only be recognised as a series of canals and crevices. The
relations of coelom to atrium in the region of the intestine are {124}seen
in Fig. 74, B, and in the region of the pharynx in Fig. 74, A. Fig. 72
shows the distribution of the spaces more in detail (see also Fig. 71).
Beginning anteriorly, along the dorsal surface of the pharynx and beneath
the notochord run a pair of dorsal coelomic canals, one at each side of the
epipharyngeal groove; these give off ventral diverticula which pass down
the primary branchial bars of the pharyngeal wall and unite ventrally in a
median tube, the endostylar coelom (see Fig. 72, _ec_). At the posterior
end of the pharynx these dorsal and ventral canals unite in a narrow
coelomic space encircling the stomach, inside the wall of the atrium, and
sending an extension forwards around the liver (Fig. 74, A, _l_). In the
region of the intestine, behind the atriopore, the coelom is allowed to
expand to its primitive condition on the left-hand side (Fig. 74, B), but
is still reduced on the right side, where there is a prolongation of the
atrial cavity reaching nearly to the anus. All these coelomic spaces are
lined by a coelomic epithelium.
The BLOOD SYSTEM of Amphioxus, although as simple as that of a Chaetopod
worm, is undoubtedly laid down on the Vertebrate plan—even though there is
no distinct heart and the vessels are few and of simple structure.
Capillary networks are formed in some places, but the colourless blood also
extends into many lacunae or lymph spaces, such as those around the
fin-rays and in the metapleura. As in a typical lower Vertebrate, there is
a contractile ventral vessel (the ventral or branchial aorta, Fig. 77,
_v.ao_) running forwards under the alimentary canal to the pharynx, and
giving off on each side afferent branchial vessels, which pass up the
primary branchial bars and give off branches joining the vessels in the
secondary bars. These latter do not communicate directly with the ventral
aorta, but the vessels in all the branchial bars open dorsally by efferent
branchial vessels into the paired dorsal aortae (Fig. 77, _d.ao_), which
run backwards along the top of the pharynx, one at each side of the
epipharyngeal groove. In the vessels of the branchial bars and their
connectives the blood is aerated by the current of water passing through
the gill-slits, and so reaches the dorsal aortae in a purified condition.
The right-hand dorsal aorta is continued forward further into the snout
than its fellow of the other side, and is dilated at its extremity (Fig.
77). At the posterior end of the pharynx the paired dorsal aortae unite to
form the median dorsal aorta {125}which runs backwards, lying between
notochord and alimentary canal. This vessel gives off branches to the wall
of the intestine, and these break up into capillary networks (Fig. 77,
_cp_), from which the blood is collected by the median sub-intestinal vein.
This then flows forwards to pass by the hepatic portal vein to the ventral
edge of the saccular liver, in the wall of which it is distributed in a
capillary network. The blood is collected on the dorsal edge of the liver
by the hepatic vein, which runs posteriorly and then turns downwards and
forwards to become continuous with the posterior end of the ventral aorta
or "heart."
[Illustration: FIG. 77.—Diagram of the vascular system of Amphioxus.
_af.br.a_, Afferent branchial arteries; _af.br.a′_, similar vessels of the
secondary (tongue) bars; _br.cl_, gill-slits; _cp_, intestinal capillaries;
_d.ao_, paired dorsal aortae; _d.ao′_, median dorsal aorta; _ef.br.a_,
efferent branchial arteries; _hep.port.v_, hepatic portal vein; _hep.v_,
hepatic vein; _int_, intestine; _lr_, liver; _ph_, pharynx; _s.int.v_,
sub-intestinal vein; _v.ao_, ventral aorta. (From Parker and Haswell.)]
It is clear that this course of the circulation agrees with that of a
typical lower Vertebrate in all essential points:—(1) in having the main
artery a dorsal aorta in which the blood flows backwards; (2) in having a
ventral vessel representing the heart, and sending impure blood forwards to
the respiratory region of the alimentary canal to be aerated; and (3) in
having a hepatic portal system consisting of the capillaries of the liver,
through which the blood from the intestinal wall has to pass before
reaching the ventral vessel (heart).
RENAL EXCRETORY functions have been attributed to various organs in
Amphioxus, and it is quite possible that, in addition to the true nephridia
which are now known, other tracts of tissue in the body may be able to
eliminate nitrogenous waste matters. Such are certain clumps of columnar
epithelial cells on the floor of the atrium, and the single pair of large
brown atrio-coelomic funnels lying on the dorsal edge of the posterior end
of the {126}pharynx (Fig. 71, _br.f_). There are, however, a large number
(about 100 pairs) of minute nephridia, discovered (1890) by Weiss and by
Boveri independently, lying at the sides of the dorsal coelomic canals
above the pharynx, which must be regarded as the chief functional renal
organs. These are bent tubules, partly glandular and partly ciliated, each
giving off several caecal knobs (at first supposed to be open nephrostomes,
one shown at each end of the tubule and three along its upper surface in
Fig. 78), which project into the coelom, and opening by one nephridiopore
(on the lower surface, and opposite a tongue bar of the pharynx) into the
atrial cavity. The knobs, or closed nephrostomes, are surrounded by
peculiar, slender, club-shaped tubular and flagellated cells—which
Goodrich[113] has shown to correspond to the "solenocytes" in the nephridia
of Polychaete worms (see Fig. 79).
[Illustration: FIG. 78.—_Branchiostoma lanceolatum_. A nephridium of the
left side with part of the wall of the pharynx, as seen alive, highly
magnified. (From Willey, after Boveri.)]
{127}The CENTRAL NERVOUS SYSTEM is dorsal and tubular as in Vertebrates,
and lies in a connective-tissue sheath immediately above the notochord
(Figs. 71, etc., and 80, A). Posteriorly it tapers to a fine point a little
in front of the end of the notochord, but anteriorly it ends abruptly some
distance behind the anterior extremity of the notochord. The central canal
is connected with the dorsal surface by a median longitudinal cleft (Fig.
80, C), and at the anterior end it dilates to form the cerebral vesicle
(_c.v_) with which two simple sense-organs, an eye-spot (_e_) and an
olfactory pit (_olf_), are connected. A patch of ciliated epithelium in the
floor of the vesicle has been described as an "infundibular-organ." There
is also a surface dilatation of the dorsal cleft behind the cerebral
vesicle (_dil_). The nervous system as far back as this point may be
regarded as the brain, though scarcely distinguishable externally (Figs. 71
and 80, A) from the spinal chord behind. From this "brain" arise two pairs
of "cranial" nerves, the first (I.) from the anterior end, and the second
(II.) from the dorsal surface of the cerebral vesicle; both are in front of
the first myotomes of the body, and supply the pre-oral snout with nerves.
[Illustration: FIG. 79.—Nephridia. A, portion of a nephridium of
_Phyllodoce_, a marine Polychaete, for comparison with B, portion of a
nephridium of Amphioxus. These figures show the solenocytes with their
flagella projecting through the long tubes into the lumen of the excretory
organ, and demonstrate the essential similarity of the nephridia of
Amphioxus with those of Polychaete worms (after Goodrich).]
The spinal cord gives off a large number of spinal nerves segmentally
arranged, but, like the myotomes, not opposite and symmetrical on the two
sides, but placed alternately (Fig. 81). Moreover, the spinal nerves arise
on each side at two levels, there being a more dorsal series each arising
by a single root and {128}supplying the integument as well as the
transverse muscles, so as to be sensory as well as motor, and a ventral
series arising each by a number of roots (Fig. 81) and wholly motor in
function, as they supply only the myotomes. These two series may be
compared to the dorsal and ventral roots which in the Vertebrata join to
form a mixed spinal nerve.
[Illustration: FIG. 80.—_Branchiostoma lanceolatum_. A, brain and cerebral
nerves of a young specimen; B, transverse section through neuropore; C,
behind cerebral vesicle; D through dorsal dilatation. _ch_, Notochord;
_cv_, cerebral vesicle; _dil_, dorsal dilatation; _e_, eye-spot; _np_,
neuropore; _olf_, olfactory pit; _I_ and _II_, cranial nerves. (From
Willey, after Hatschek.)]
In addition to ordinary small nerve cells the central nervous system
contains certain large nerve cells with very long processes, the "giant
fibres," which extend through the greater part of the length of the spinal
cord. No trace of a sympathetic nervous system has been found.
The SENSE-ORGANS connected with the nervous system are few and simple.
There are sensory cells in the ectoderm, on the margin of the velum, on the
velar tentacles, and especially in clumps on papillae of the cirri around
the mouth, which are probably tactile. In the roof of the oral hood there
is a sensory structure, the "groove of Hatschek," which is supposed to be
an organ of taste. The olfactory pit alluded to above opens externally on
the left-hand side of the snout. It is ciliated internally and leads to the
so-called olfactory lobe, an {129}antero-dorsal hollow outgrowth from the
brain. In the young animal the olfactory pit opens by the neuropore into
the central canal (Fig. 80, A), but that passage is closed in the adult.
Possibly the olfactory pit is homologous with the hypophysis or pituitary
body of Vertebrates, the homologue of which in Tunicata has a ciliated
funnel. Finally, the median cerebral eye (Figs. 80 and 81) is a mere
pigment spot in the anterior wall of the cerebral vesicle, and a series of
somewhat similar pigment spots occurs along the floor of the central canal
in the spinal cord.[114] There is no known auditory organ. On the under
surface of the oral hood patches of ciliated epithelium drawn out into
rounded lobes were called by Johannes Müller the "Räder-organ." This is
probably of use in drawing water inwards to the pharynx, but it may also be
a sense-organ.
[Illustration: FIG. 81.—_Branchiostoma lanceolatum_. Anterior portion of
central nervous system from above, showing dorsal and ventral spinal
nerves. (From Willey, after Schneider.)]
The GONADS are segmentally arranged along the sides of the body, projecting
into the atrial cavity at the sides of the pharynx and intestine. In some
species the gonads are paired, but in others belonging to the genus
_Asymmetron_ (p. 137) only a single series, that of the right side, is
present. In the common Amphioxus (_Branchiostoma lanceolatum_) there are
about 26 pairs (Fig. 70, B), lying in somites 25 to 51; and ovaries and
testes are found in separate individuals in all other respects. Each gonad
is surrounded by a layer of coelomic epithelium. The gonad must therefore
be regarded as having grown down from a myotome of the body-wall into a
coelomic pouch, carrying before it the coelomic and then the atrial
epithelium (Figs. 72, and 74, A, _g_). Eventually the gonads, when ripe,
burst through the layers of epithelium, and the ova and sperms are shed
into the atrium and escape to the exterior by the atriopore, or it may be
in some cases by the mouth.
{130}EMBRYOLOGY AND LIFE-HISTORY.
Development takes place in the sea-water where the egg is
fertilised—apparently always about sunset, the embryonic stages being
passed through during the night, and the larva hatched in the early
morning.
[Illustration: FIG. 82.—Stages in the segmentation of Amphioxus. A
represents the eight-celled stage; B, the sixteen-celled; D, vertical
section of C; F, vertical section of the blastosphere or blastula stage
(E). (From Korschelt and Heider, after Hatschek.)]
The egg is small (0.105 mm. in diameter when shed) and contains very little
food-yolk. Segmentation is complete (Fig. 82, A), is nearly regular, and
results in the formation of a hollow blastosphere (Fig. 82, E, F), the wall
of which is one cell thick. The lower cells (Fig. 82, B, C, D) are slightly
larger than the upper. Invagination of the lower cells then takes place
(Fig. 83, A), resulting in the suppression of the blastocoele or
segmentation cavity and the formation of an archenteron, at first shallow
and opening widely to the exterior (Fig. 83, B), and then deeper and with
the opening narrowed to a small posterior blastopore (Fig. 83, C). This
"gastrula" stage differs from the blastosphere in having a mouth or
blastopore, and in being two cell-layers thick—epiblast (ectoderm) on the
outside and hypoblast (endoderm) within. It soon shows the future aspects
of the body by its {131}elongation and shape (Fig. 83, C), as the dorsal
surface becomes flat and the ventral convex, while the blastopore is at the
posterior end of the dorsal surface. The blastopore soon closes, and the
mouth and anus are formed independently later.
[Illustration: FIG. 83.—Three stages in the formation of the gastrula of
Amphioxus. In A the nuclei of the endoderm have been omitted; C has the
dorsal surface uppermost, and the posterior end to the right (From
Korschelt and Heider, after Hatschek.)]
The epiblast cells become ciliated all over the surface, so that the embryo
rotates within the thin covering which still surrounds it. And now all the
chief systems of the body begin to be marked out. The tubular nervous
system develops from a depression of the epiblast (the medullary plate) in
the middle line of the flattened dorsal surface (Fig. 84, A, _mp_). The
edges of the depressed area grow inwards and unite over the deeper layer of
epiblast, which becomes the wall of the neural canal or embryonic nervous
system (Fig. 84, D, _n_); and further back these edges of the medullary
plate join one another behind the blastopore, so that the latter comes to
open into the floor of the neural canal, thus forming the neurenteric canal
(Fig. 85, A, _cn_). Anteriorly the neural canal (_n_) opens to the exterior
for some time by the neuropore.
The hypoblastic walls of the archenteron give off a long median dorsal
groove which becomes the notochord (Fig. 84, C and D, _ch_); and also an
anterior pouch and certain lateral pairs {132}of diverticula which are the
enterocoeles or coelomic pouches, and give rise to the mesoblastic somites
(Fig. 84, B and C, _mk_). The notochord (Fig. 84, D, _ch_) is at first a
longitudinal cellular ridge, which becomes segmented off from the hypoblast
as a rod lying below the neural canal. It is seen in various stages of
development in Figs. 84 and 86, leading to the vacuolated condition of the
adult.
[Illustration: FIG. 84.—Four stages in the development of the notochord,
nervous system, and mesoderm of Amphioxus. _ak_, Ectoderm; _ch_, notochord;
_dh_, cavity of archenteron; _hb_, ridge of ectoderm growing over medullary
plate; _ik_, endoderm; _lh_, coelom; _mk_, coelomic pouch; _mk^1_, parietal
layer of mesoderm; _mk^2_, visceral layer; _mp_, medullary plate; _n_,
neural canal; _ns_, protovertebra. (From Korschelt and Heider, after
Hatschek.)]
The coelomic pouches are five in number—(1) one median, anterior, which
gives rise to the two head cavities, the left-hand one of which opens to
the exterior by means of the pre-oral pit; (2) a pair of small lateral
pouches, placed anteriorly and dorsally, which do not divide but give rise
to the first pair of myotomes only and their outgrowths which extend back
into the metapleural folds, where, however, they are later replaced by
lymph spaces; and {133}(3) a second pair of diverticula, more posteriorly
placed, which continue to grow back towards the blastopore, and have paired
mesoblastic somites, the cavities in which are the beginnings of the coelom
in the body, constricted off from them successively from before backwards
(Fig. 85, A, _ush_) to form all the remaining myotomes.[115] This is the
first sign of segmentation in the animal, and at this stage, when it has
about five pairs of mesoblastic somites, it breaks out of its covering and
becomes a free-swimming larva.
[Illustration: FIG. 85.—Embryo of Amphioxus. A, in vertical section,
slightly to the left of the middle line. B, in horizontal section. _ak_,
Ectoderm; _cn_, neurenteric canal; _dk_ and _ud_, archenteron; _ik_,
endoderm; _mk_, mesodermal folds; _n_, medullary canal; _us_, first
coelomic pouch; _ush_, coelomic cavity; _V_, anterior, _H_, posterior, end.
(From Korschelt and Heider, after Hatschek.)]
The mouth now appears, and soon grows to a large opening on the left side
of the now pointed anterior end (Fig. 86, A, _m_), and the first gill-slit
(_ks_) forms as a direct communication from the front of the mesenteron
(pharynx) to the exterior. It is ventral at first, and then shifts over to
the right side.
The anus forms posteriorly, and the neurenteric canal closes {134}up. A
depression on the floor of the enteron close to the mouth gives rise to the
"club-shaped gland" (Fig. 86, B, _k_), which is probably a gill-cleft in
its nature.
[Illustration: FIG. 86.—A, young larva of Amphioxus. B, anterior end
enlarged. _c_, Provisional tail-fin; _ch_, notochord; _cn_, neurenteric
canal; _d_, enteron; _h_, coelom of snout; _k_, club-shaped gland; _k_′,
its external aperture; _ks_, first gill-slit; _m_, mouth; _mr_, nerve-tube;
_np_, neuropore; _sv_, sub-intestinal vein; _w_, pre-oral pit. (After
Hatschek.)]
[Illustration: FIG. 87.—More advanced larva of Amphioxus. _an_, Anus; _au_,
eye-spot; _c_, larval tail-fin; _ch_, notochord; _d_, enteron; _fl_,
rudiment of endostyle; _k_, club-shaped gland; _k′_, its external aperture;
_m_, mouth; _np_, neuropore; _w_, pre-oral pit; _x_, provisional
nephridium; 1-4, gill-slits. (From Korschelt and Heider, after Lankester
and Willey.)]
The walls of the coelomic pouches, which have been extending both dorsally
and ventrally (Fig. 84, D), become the {135}mesoderm, the outer the somatic
and the inner the splanchnic layer; and the ventral parts of their cavities
unite to form the coelom. The cells of the dorsal parts become muscle
fibres, and constitute the myotomes internally and the connective tissue of
the skin externally.
The larva (Fig. 87) is now long and narrow with many segments, pointed
ends, and a caudal fin. The gill-slits all appear first in the mid-ventral
line and then shift over to the right side (Fig. 87, 1-4): they are
metamerically arranged. After fourteen have been so formed a series of
eight appear dorsally to those on the right side, and then the first set,
originally ventral, move over to the left side, and by the suppression of
some they become equal in number and segmentally arranged on the two sides
of the body. This is perhaps the stage at which Amphioxus shows the nearest
approach to the typical embryo of a higher Vertebrate. The gill-slits are
here seven to nine on each side, and the Vertebrate embryo has usually five
to seven on each side. These first gill-slits in Amphioxus are later
subdivided by the downgrowth of the tongue-bar from the dorsal edge.
[Illustration: FIG. 88.—Ventral aspect of three larvae of Amphioxus,
showing the metapleural folds and the formation of the atrium. _ap_,
Atriopore; _k_, gill-slits; _lf_ and _rf_, left and right metapleural
folds; _m_, mouth; _w_, pre-oral pit. (From Korschelt and Heider, after
Lankester and Willey.)]
The atrium is an ingrowth of the external space between the two ventral
metapleural or atrial folds (Figs. 88 and 89), paired lateral ridges of the
body-wall, and so is lined by ectoderm. This ingrowth is shut off from the
exterior by the {136}growth towards each other of sub-atrial ridges on the
inner sides of the metapleural folds (see Fig. 89, A, _sl_), and then
becomes greatly enlarged by the increased relative growth of the
ventro-lateral part of the body-wall (Fig. 89, B, C). The posterior opening
between the metapleural folds remains as the atriopore (Fig. 88, C, _ap_);
while the anterior end (Fig. 88) also remains open for some time, but
eventually closes. As the metapleural folds lie outside the gill-slits
(Fig. 88, A) when these folds close in (B and C), it comes about that the
gill-slits which formerly opened freely to the exterior now open into the
cavity of the atrium (compare Figs. 87 and 88).
[Illustration: FIG. 89.—Diagrammatic transverse sections of three larvae of
Amphioxus to show the development of the atrium. _ao_, Aorta; _c_, dermis;
_ch_, notochord; _d_, intestine; _f_, connective tissue; _fh_, cavity of
dorsal fin-ray; _m_, myotome; _n_, nerve-tube; _p_, atrium; _sf_,
metapleural folds; _sfh_, lymph space in metapleural folds; _si_,
sub-intestinal vein; _sk_, sheaths of notochord and nerve-tube; _sl_,
sub-atrial ridge; _sp_, coelom. (From Korschelt and Heider, after Lankester
and Willey.)]
The mouth now becomes median and ventral, and is reduced in size, the oral
hood (stomodaeum) is formed in front of it, the gill-slits become more
numerous and vertically elongated, the endostyle forms along the floor of
the pharynx, and the gonads grow as paired pouches from the body-wall. This
brings the animal to the young adult condition, reached at a period of
probably about three months after the fertilisation of the egg.
The development as a whole shows a very marked resemblance {137}to that of
the Tunicata (see p. 55), but lends no support to the view that Amphioxus
has degenerated from a higher group of the Vertebrata.
CLASSIFICATION OF THE CEPHALOCHORDATA.
The known species of Amphioxus may be classified as follows[116]:—
FAMILY BRANCHIOSTOMATIDAE.
Genus 1. _Branchiostoma_ (Costa, 1834).
Having biserial gonads and symmetrical metapleura.
_B. lanceolatum_ (Pallas)—Myotomes 36 + 14 + 12, gonads 23-29 pairs:
Mediterranean, N.W. Europe, Ceylon, E. of United States.
[_B. belcheri_, Gray—Myotomes 38 + 17 + 9: Torres Straits, Singapore,
Borneo, Ceylon.
[_B. nakagawae_, Jord. and S.—Myotomes 37 + 16 + 11: Japan.
[_B. caribbaeum_, Sundevall—Myotomes 37 + 14 + 9: West Indies, Atlantic,
N. and S. America.
_B. capense_, Gilchrist—Myotomes 47 + 19 + 9: S. Africa.
_B. californiense_, J. G. Cooper—Myotomes 45 + 17 + 9: California.
_B_. (_Dolichorhynchus_) _indicum_ (Willey)—Myotomes 42 + 14 + 15: India
and Ceylon.
(?) _B. elongatum_, Sundevall—Myotomes 49 + 18 + 12: Peru.
(?) _B. pelagicum_, Günther—Myotomes 36 + 16 + 15: Honolulu, Gulf of
Manaar, South Indian Ocean.
Genus 2. _Asymmetron_ (Andrews, 1893).
With uniserial (right) gonads and asymmetrical metapleura.
_A. lucayanum_, Andrews—Myotomes 44 + 9 + 13: Bahamas, Maldives,
Zanzibar.
_A. caudatum_ (Willey)—Myotomes 40 + 9 + 11: Louisiade Archipelago.
_A_. (_Heteropleuron_) _bassanum_ (Günther)—Myotomes 45 + 16 + 14: Bass
Straits, Australia.
" _cingalense_ (Kirkaldy)—Myotomes 39 + 16 + 8: Ceylon.
" _cultellum_ (Peters)—Myotomes 32 + 10 + 10: Torres
Straits, Australia, Ceylon.
" _maldivense_ (F. Cooper)—Myotomes 45 + 16 + 12: Maldive
Archipelago, Zanzibar.
" _hectori_ (Benham)—Myotomes 53 + 19 + 12: New Zealand.
{138}Thus sixteen species have been described, of which the three under
_Branchiostoma_ placed after square brackets, seem to be merely varieties
of _B. lanceolatum_, and _B. nakagawae_ is probably identical with _B.
belcheri_; while it is a question whether _Asymmetron caudatum_ is more
than a variety of _A. lucayanum_, thus leaving eleven or twelve species
that seem fairly well characterised. The exact positions of the two marked
(?), viz. _B. elongatum_ and _B. pelagicum_, cannot be determined in the
absence of fuller descriptions of these species.
[Illustration: FIG. 90.—Sketch-map showing geographical distribution of the
Cephalochordata. + indicates _Branchiostoma_; o indicates _Asymmetron_.]
The list above, and the map (Fig. 90), give some indication of the
geographical distribution of the group, and show that, although the few
species are widely distributed over the shallow waters of the globe, most
of the records lie between 40° N. and 40° S. latitudes. In fact the group
is mainly a tropical one, and is most abundant in the Indo-Pacific region.
The crosses indicate records of species of _Branchiostoma_, and the circles
those of _Asymmetron_ (including _Heteropleuron_); the latter are confined
to the Indo-Pacific seas, with the exception of _A. lucayanum_ from the
Bahamas—one of the numerous cases of interesting similarity between the
marine faunas of the East and West Indies.
FISHES
(EXCLUSIVE OF THE SYSTEMATIC ACCOUNT OF TELEOSTEI)
BY
T. W. BRIDGE, Sc.D., F.R.S.
Trinity College, Cambridge; Mason Professor of Zoology and Comparative
Anatomy in the University of Birmingham
{141}CHAPTER V
THE SYSTEMATIC POSITION AND CLASSIFICATION OF FISHES
In the first chapter of this volume it was pointed out that the Craniata,
of which the Fishes form a subordinate group, is the last of the four
principal divisions into which the Chordata are divided. The animals
included in the first three, viz. the Hemichordata, the Urochordata, and
the Cephalochordata, have already been dealt with in the earlier chapters,
and it now remains for us briefly to consider the diagnostic characters of
the Craniata, and then, more in detail, the organisation of the Fishes.
The Craniata, often termed Vertebrata, form one of the best defined and
most easily recognisable divisions of the animal kingdom. As the name
implies, they are distinguished from the more primitive Chordata by the
formation of a definite "head," as the result of the modification of the
anterior portion of the central nervous system to form a complex brain,
round which are concentrated the chief organs of special sense. This is
combined with the evolution of a skull, which, in addition to providing a
"cranium" for the enclosure and protection of the brain, and partial or
complete capsules for the sense-organs, is connected behind with a system
of bony or cartilaginous visceral arches, which loop round the pharynx
between the gill-clefts. Besides supporting the breathing organs (gills) in
the lower aquatic Craniata, or existing as embryonic vestiges in the higher
lung-breathing forms, these arches usually form the basis of jaws for the
mouth. The epidermal portion of the superficial skin is always composed of
several layers of cells. The notochord, which is always present in the
embryo, and in a few Craniates, both living and extinct, may even be
retained in its entirety in the adult, fails to reach {142}the anterior end
of the brain. In most Craniates, however, the notochord becomes more or
less completely replaced in the adult by the development round it of a
series of vertebrae, forming the backbone or vertebral column. Two pairs of
limbs, and cartilaginous or bony limb-girdles for their support, are very
generally present.
The segmentation, or serial repetition of certain organs of the body, which
is so marked a feature in the Cephalochordata, is also characteristic of
the Craniata. Examples of this may be seen in the division of the lateral
longitudinal muscles of the body wall into muscle-segments or myotomes by a
series of transverse fibrous septa; in the formation of the vertebral
column by a series of successive joints or vertebrae; in a similar serial
repetition of the cranial and spinal nerves, the gill-clefts and branchial
arches, certain blood-vessels, and the renal tubules. There is sometimes,
however, no precise regional or numerical correspondence between the
different organs which are successively repeated in this way, and hence it
is probable that, in at least some of the organs of the Craniate body, the
segmentation has been independently evolved in each case.
The pharynx is relatively much shorter than in other Chordata. The
gill-clefts are few in number, whether, as in the lower Craniata, they are
retained as the functional breathing organs, or are present, as vestiges
only, in the embryos of the higher members of the group. In no instance are
they subdivided by the growth of "tongue-bars" or "synapticula," nor do
they open externally into an atrial or peribranchial cavity. The liver is a
massive compound tubular gland, never, in the adult at all events, a simple
caecal sac; and usually there is a pancreas and a spleen.
A spacious epithelium-lined body cavity or coelom, which, as regards its
origin, may be regarded as a "syncoelom,"[117] surrounds the alimentary
canal and separates it from the body wall. From the epithelial walls of the
coelom are derived the gonads (ovaries and testes), which in the adult are
limited to a single pair; while paired and often segmentally-arranged
lateral tubular outgrowths from it (renal tubuli) acquire a glandular
character and form the basis of the excretory or kidney system. A special
portion {143}of the coelom also surrounds the heart and forms a pericardial
cavity, and in some Craniata the genital ducts may be formed from its
lining membrane.
There is always a muscular heart, consisting of at least three chambers, a
sinus venosus, an auricle and a ventricle, and formed by a modification of
the initial portion of the ventral or cardiac aorta of the Cephalochordata.
The disposition of the great blood-vessels is based on a common plan in all
Craniata, and the blood which circulates in them is red in colour owing to
the presence of red, haemoglobin-containing corpuscles in addition to the
colourless leucocytes which alone are present in the Cephalochordata.
Ductless blood-glands of various kinds (spleen, thyroid, thymus, inter- and
ad-renal bodies) are very generally present, and modify in different ways
the character of the blood as it circulates through them. Besides
blood-vessels there is also a somewhat similar system of lymphatic vessels
distributed throughout the organs and tissues of the body, which serves the
purpose of re-collecting the fluid portion of the blood that has diffused
from the blood-vessels for the nutrition of the tissues, and conveying it
back to the blood vascular system. These lymphatics contain lymph, a fluid
comparable to dilute blood plasma, in which leucocytes float. In addition
to their continuity with the blood-vessels at certain points, the lymphatic
vessels may also communicate with the coelom, and hence the Craniata must
be included among those somewhat rare exceptions to the general rule that
no connexion exists between the series of blood-containing channels and the
coelom.
In the excretory system the renal tubuli in the adult Craniata rarely
retain their primitive embryonic communication with the coelom, and in no
instance have they separate and independent external apertures; on the
contrary, by the union of their outer or distal extremities, common
efferent ducts are formed, which either open into a "cloaca," or directly
on to the exterior of the body near the anus.
In all Craniates the dorsally-placed and tubular central nervous system has
its anterior portion enlarged and otherwise modified to form a "brain,"
while the remaining portion, retaining a simpler and more uniform
structure, forms the spinal cord. In the embryo the brain always consists
of three successive sac-like enlargements known as the fore-, mid-, and
hind-brain, and from {144}these are developed the various parts of the
complex adult brain, which in the disposition and mutual relations of its
parts conforms to a common plan in all the members of the group. There are
at least ten pairs of cranial nerves having their origin from the brain,
and, in addition, a varying number of spinal nerves arising from the spinal
cord, and as a rule formed in each case by the union of a mainly sensory,
ganglionated, dorsal root with a mainly motor, non-ganglionated, ventral
root.
The median and usually vestigial, parietal, or pineal eye may sometimes be
retained as a functional organ, but there exist in all Craniates, in
addition, paired eyes, the sensory portion of which, the retina, is derived
as an outgrowth from the first of the primary embryonic brain-vesicles. To
these organs of special sense are added a pair of auditory organs, and a
pair of olfactory organs, besides, in the lower aquatic Craniates, the
peculiar sensory organs of the "lateral line."
The gonads are reduced to a single pair in the adult, although it is
possible that they may have a multiple origin in the embryo. Gonoducts for
the discharge of the sex-cells are almost invariably present, and may owe
their origin either to a change of function on the part of certain
kidney-ducts, or to independent evolution from the lining membrane of the
coelom. The ova are generally provided with a large amount of nutritive
reserve in the shape of food-yolk, and hence the process of segmentation is
frequently partial or "meroblastic," but in some groups, in which the ova
have less food-yolk, it is complete or "holoblastic." The typical
invaginate gastrula stage, which is so striking a feature in the embryonic
history of the lower Chordata, occurs also in a few of the lower Craniates,
but in most of them it is apt to become masked or modified in various ways
by the presence of a superabundant amount of food-yolk.
Functional hermaphroditism is of very rare occurrence in Craniates, and, as
in the Cephalochordata, reproduction by budding and the formation of
colonies are unknown.
Thus distinguished from other Chordata, the Craniata are divided into six
"classes," which may be variously grouped, as the following table shows:—
{ } {145}
{ I. CYCLOSTOMATA. } AGNATHOSTOMATA.
{ Lampreys and } Without biting
{ Hag-Fishes. } jaws.
ICHTHYOPSIDA. {
Breathing by gills at some { }
period of life. { }
{ II. PISCES. }
{ True Fishes. }
ANAMNIOTA { }
No embryonic covering { }
or amnion. { }
{ III. AMPHIBIA. }
{ Newts, Frogs, and }
{ Toads. }
ANALLANTOIDEA. { }
No embryonic respiratory { }
organ or allantois. { } GNATHOSTOMATA.
{ } With biting
} jaws.
{ IV. REPTILIA. }
{ Lizards, Snakes, }
{ Turtles, and }
{ Crocodiles. }
{ SAUROPSIDA. { }
AMNIOTA. { { }
Amnion present. { { V. AVES. }
{ { Birds. }
{ }
ALLANTOIDEA. { }
Allantois present.{ VI. MAMMALIA. }
{ Hairy Quadrupeds. }
Apart from the distinctive characters of the six "classes" into which the
Craniata are divided, two or three of these classes may possess important
structural features in common by which they are distinguished from others.
Thus, Cyclostomata, Fishes and Amphibia agree with one another, and differ
from all the remaining groups in breathing by gills and in possessing
lateral line sensory organs during part, or the whole, of life. Their
embryos have no investing amnion, neither does the sac-like outgrowth from
the hind-gut, which is known as the allantois, if present at all, ever
extend beyond the coelom to form an embryonic investment or to act as a
primitive breathing organ. Hence, therefore, the terms Ichthyopsida,
Anamniota, and Anallantoidea have been applied to these three classes.
Similarly, the term Sauropsida, as applied to Reptiles and Birds, is a
convenient means of giving expression to the fact that, underlying the most
striking diversity of outward form and habits, there is a community of
inward structure which justifies the conclusion that these animals are more
closely related to one another than either group is to any other class of
Craniates. And again, the application of the terms Agnathostomata and
Gnathostomata brings into sharp relief the fundamental distinction between
the Cyclostomata and all the remaining groups of Craniata which is
{146}only partially illustrated by the presence or absence of biting jaws.
In a general and popular sense the Cyclostomata are usually regarded as
"Fishes," but this usage rests on no better foundation than a certain
agreement between the Cyclostomata and the true Fishes in outward form and
habits, and in their method of respiration by gills. On the other hand, it
has been maintained that the distinctive features of the Cyclostomata are
of sufficient importance not merely to separate them from the true Fishes,
but possibly even (as is to some extent expressed by the use of the terms
Agnathostomata and Gnathostomata) to warrant their elevation to a group
equal in taxonomic value to all the remaining living Craniata taken
collectively. The organisms included in the Cyclostomata, the Lampreys, and
especially the Hag-Fishes, exhibit in many respects an extremely low grade
of Craniate structure; but how far the simplicity or archaic nature of some
of their organs is primitive, or has been acquired through degeneration, it
is difficult, and is sometimes impossible, to determine with any degree of
satisfaction. In other respects, such as the presence of a rasping
"tongue," it is obvious that the Cyclostomata have attained a high degree
of specialisation. As one of several illustrations which might be given of
difficulties of this kind, it may be mentioned that it is by no means
certain that the Cyclostomata are not the degenerate descendants of
primitive but now extinct Gnathostomata. At all events the presence of
paired cartilages in the skull of the Lamprey, which, with some show of
reason, may be regarded as representatives of the primitive upper and lower
jaws of the latter group, would seem to suggest this conclusion. If this be
correct, we must regard the formation of a suctorial buccal funnel, with
its complex system of supporting cartilages—one of the most striking
features in the structure of this animal—as a secondary and adaptive
specialisation of a mouth originally provided with biting jaws. But in
spite of such difficulties there can be no question that the Cyclostomata
are the most primitive of all existing Craniates, and so far differ from
the true Fishes and from all other classes of Craniate animals, that their
inclusion in a class by themselves is the least that can be done to give
graphic expression to their isolated position, even if we do not fully
accept the dictum of {147}Haeckel that "they are further removed from
Fishes than Fishes from Man."
Briefly stated, the Cyclostomata or Agnathostomata are distinguished from
"Fishes" and all the remaining Craniata (Gnathostomata) by the following
characters:—
The mouth is either nearly terminal, as in the Hag-Fishes (_Myxine_); or,
as in the Lampreys (_Petromyzon_), it opens out of a spacious, pre-oral,
suctorial, buccal funnel, which, in its relations to the hypophysis or
pituitary body, recalls the pre-oral buccal cavity of the Cephalochordata.
As in Amphioxus, the hypophysis[118] is displaced dorsally by the forward
growth of the pre-oral portion of the head in the embryo, and consequently
it only attains its normal relations to the infundibular downgrowth[119]
from the ventral surface of the fore-brain by perforating the floor of the
skull from above instead of from below as in all other Craniates. In one
section of the group (e.g. _Myxine_) the hypophysis opens into the oral
cavity, and serves as a tubular passage for the inspiratory water-current
to the gill-sacs, a feature in which these Cyclostomes are unique. The
apparently median olfactory organ is carried inwards with the hypophysial
involution, and communicates with the latter throughout life. A primitive
upper jaw (palato-quadrate cartilages or sub-ocular arches) is present, and
in at least some Cyclostomes (_e.g._ the Lampreys), and possibly in all,
there are structures which very probably represent a primitive lower jaw
(Meckel's cartilages); but such structures are always non-biting, and
merely form skeletal supports for other portions of the skull. In place of
biting jaws the mouth is provided with a complex rasping lingual apparatus
supported by special cartilages, the so-called tongue, which bears horny
teeth and is capable of protrusion and retraction. Paired limbs are
entirely wanting.
In the Gnathostomata, on the contrary, there is no buccal funnel, and the
mouth, whether terminal or ventral in position, opens directly outwards.
The hypophysis is usually carried inwards with the stomatodaeal
invagination which in the embryo gives rise to the mouth, and is therefore
from the first in relation with the ventral surface of the brain. Biting
jaws (palato-quadrate and Meckelian cartilages), formed by the modification
of an anterior and primitively gill-bearing visceral arch, are
{148}invariably present. The olfactory organs are obviously paired, and
they are distinct from the hypophysis. Paired limbs are present.
As previously stated, the true Fishes form the second of the six "classes"
into which the Craniata are divided. As compared with the higher Craniata,
their distinctive characters may be concisely stated as follows:—
Fresh water or marine Gnathostomata, which in their shape and in method of
breathing are adapted for an aquatic life. Throughout life their
respiratory organs are in the form of vascular processes (gills) derived
from the walls of the branchial clefts, and supported by a series of
branchial arches. The principal organ of locomotion is the powerful
muscular tail; in addition, however, there are paired fins, pectoral and
pelvic, corresponding to the fore- and hind-limbs of the terrestrial
Craniata, and possessing a supporting cartilaginous or bony skeleton
("ichthyopterygium") which cannot readily be compared with the
limb-skeleton of the latter. Fishes also possess a system of median fins,
supported by a special skeleton of their own. An exoskeleton of dermal
spines or denticles, scales or bony plates, is usually present. Except in
one group, the Dipnoi, the heart has but one auricle, and receives only
venous blood, which it forces, first, through the blood-vessels of the
gills, and thence, as arterial blood, through the vessels of the body
generally. An air-bladder is frequently present, and serves as a
hydrostatic organ or float, but in a few cases it may act as a lung, and
helps the gills in the work of respiration. The paired olfactory organs
rarely communicate with the oral cavity by internal nostrils. Peculiar
cutaneous sense-organs are disposed in linear tracts along the sides of the
body (lateral line sensory organs), and on the head, and appear to be
specially associated with a life in water.
Fishes may be divided into the following "sub-classes," and these in turn
may be subdivided into various "orders" and "sub-orders":—
(i.) ELASMOBRANCHII; _e.g_. Sharks, Dog-Fishes, Skates, and Rays.
(1) Pleuropterygii†; e.g. _Cladoselache_.
(2) Ichthyotomi†; e.g. _Pleuracanthus_.
(3) Acanthodei†; e.g. _Acanthodes_.
(4) Plagiostomi.
(_a_) Selachii; _e.g_. many extinct and all living Sharks and
Dog-Fishes.
(_b_) Batoidei; _e.g_. Skates and Rays.
(5) Holocephali; e.g. _Chimaera_ and _Callorhynchus_.
(ii.) TELEOSTOMI; _e.g_. such well-known Fishes as the Perch, Cod,{149}
Salmon, and Herring, and also the less familiar "Ganoids,"
living and extinct.
(1) Crossopterygii; e.g. _Polypterus_.
(2) Chondrostei; _e.g_. the Sturgeons (_Acipenser_).
(3) Holostei; _e.g_. the Bow-fin (_Amia_), and the Gar Pike
(_Lepidosteus_).
(4) Teleostei; _e.g_. the Perch, Cod, Salmon, etc.
(iii.) DIPNOI; e.g. _Neoceratodus, Protopterus_, and _Lepidosiren_.
_Appendix to the Class Pisces_.
(i.) PALAEOSPONDYLIDAE†; e.g. _Palaeospondylus_.
(ii.) OSTRACODERMI†.
(1) Heterostraci; e.g. _Pteraspis_.
(2) Osteostraci; e.g. _Cephalaspis_.
(3) Anaspida; e.g. _Birkenia_.
(iii.) ANTIARCHI†; e.g. _Pterichthys_.
(iv.) ARTHRODIRA†; e.g. _Coccosteus, Dinichthys_.
† Entirely extinct.
The Fishes included in the Teleostomi were formerly arranged in two groups:
the Ganoidei, including the Crossopterygii, Chondrostei, and the Holostei,
with their numerous fossil allies; and the Teleostei. Living Ganoids agree
with one another, and differ from Teleosts in possessing an intestinal
spiral valve and a conus arteriosus. It is difficult, however, to separate
the two groups, inasmuch as in each group there are living forms which tend
to approximate to the other; and numerous fossil genera, of whose soft
parts nothing is known, are in many respects intermediate between the two.
The position and relationships of the Palaeospondylidae, Ostracodermi,
Antiarchi, and Arthrodira are very uncertain. The Palaeospondylidae have
been included in the Cyclostomata, or at all events have been regarded as
more or less closely related to that group, while the absence of paired
fins and the apparent want of jaws have suggested that the Ostracodermi
occupy an intermediate position between the Cyclostomata and the
Gnathostomata.[120] On the other hand, the Arthrodira are either regarded
as an independent group of Fishes, or are included amongst the Dipnoi. In
the latter case, the Dipnoi are divided into the Arthrodira and the
Sirenoidei, the last mentioned group including _Neoceratodus, Protopterus_
and _Lepidosiren_, and their extinct allies.
{150}CHAPTER VI
EXTERNAL CHARACTERS OF CYCLOSTOMATA AND OF FISHES
EXTERNAL CHARACTERS—COLORATION—POISON GLANDS AND POISON SPINES—
PHOSPHORESCENT ORGANS.
[Illustration: FIG. 91.—_Petromyzon marinus_. A, ventral; B, lateral; and
C, dorsal, view of the head. _br.cl.1_, First branchial cleft; _buc.f_,
buccal funnel; _eye_, the eye; _mth_, mouth; _na.ap_, nasal aperture; _p_,
papillae; _pn_, pineal area; _t^1_, _t^2_, _t^3_, teeth of buccal funnel;
_t^4_, teeth on the tongue. (From Parker and Haswell, after W. K. Parker.)]
In all the Cyclostomata the body is Eel-like in shape, the head and trunk
being nearly cylindrical, and the tail somewhat flattened from side to
side. In _Petromyzon_ the head terminates in a ventrally-directed,
funnel-like cavity—the buccal funnel—in the roof of which the relatively
small mouth is situated (Fig. 91, A.). The margin of the funnel is fringed
by a series of short papillae, {151}but in the Hag-Fishes (_Myxine_ and
_Bdellostoma_), where a buccal funnel is not developed, longer
tentacle-like structures are present on each side of the mouth. On the
upper surface of the head is the single median nostril, or naso-pituitary
aperture, placed between the eyes in the Lampreys (Fig. 91, B, C), but at
the anterior margin of the head in _Myxine_ and its allies (Fig. 92). In
the living Lampreys a semi-transparent area of skin may be noticed behind
the nasal organ, which coincides with the position of the more
deeply-seated parietal eye. On each side of the body, commencing a short
distance behind the eye, is a series of small and almost circular branchial
clefts (_Petromyzon_, _Bdellostoma_). In _Myxine_, however, the clefts of
each side have a single common external aperture, situated on the ventral
side of the body and some distance behind the head (Fig. 92, A). At the
junction of the trunk with the tail is the anus, behind which is the
papilla which carries the urino-genital aperture at its extremity. There
are no paired limbs or vestiges of such organs. Median fins are represented
in the Lampreys by an anterior dorsal fin and a posterior dorsal fin, the
latter being continuous with the caudal fin which fringes the upper and
lower margins of the protocercal tail. In _Myxine_ a caudal fin only is
present, surrounding the extremity of the tail.
[Illustration: FIG. 92.—Head of _Myxine glutinosa_ (A), and of _Bdellostoma
forsteri_ (B), from beneath. _br.ap_, Left external branchial aperture;
_br.cl.1_, first branchial cleft; _mth_, mouth; _na.ap_, nasal aperture;
_oes.ct.d_, oesophageo-cutaneous duct. The smaller openings in A are those
of mucous glands. (From Parker and Haswell, after W. K. Parker.)]
{152}[Illustration: FIG. 93.—_Tilapia dolloi_. To show the external
characters of an Acanthopterygian Teleost. A, side view; B, the first
branchial arch. _a.f_, Spinose part of the anal fin; _a.f^1_, soft rays;
_c.f_, caudal fin; _d.f_, spinose portion of the dorsal fin; _d.f^1_, soft
rays; _g.f_, gill filaments; _g.r_, gill rakers; _i.l.l_, inferior lateral
line; _n_, nostril; _p.f_, pelvic fin; _p.op_, preoperculum; _pt.f_,
pectoral fin; _s.l.l_, superior lateral line; _t.s_, transverse row of
scales. (From Boulenger.)]
In Fishes the characteristic shape of the body is more or less that of a
spindle, tapering at each end and somewhat flattened from side to side;
and, as a rule, the three regions of the body—head, trunk, and tail—pass
almost imperceptibly into one another (Fig. 93, A). Nevertheless, there is
great diversity of form in different Fishes. Compare, for example, the
elongated, cylindrical shape of the Eels (which is perhaps associated with
their habit of insinuating themselves into holes and crevices, and their
undulatory, snake-like movements when swimming); the compressed, band-like
shape of the Ribbon-Fishes (Trachypteridae); the flattened bodies of those
Fishes which habitually live and move on the bottom, like the Skates and
Rays; the thin, laterally-compressed bodies, often nearly as high as long,
of the Flat-Fishes (Pleuronectidae), which always swim and rest on either
the right or left side; the almost spherical Globe-Fishes (_Tetrodon_)
which often float passively in the water; and the singular rectangular,
coffin-like Coffer-Fishes (_Ostracion_). There is also much difference in
the relative proportions of the three regions of the body in different
Fishes, as witness the enormous size and {153}grotesque appearance of the
head of the Angler-Fish (_Lophius_); the huge high trunk and abbreviated
tail of the Sun-Fish (_Orthagoriscus_); and the short high trunk and long
tail of _Notopterus_ (Fig. 334).
In its external appearance the head perhaps differs more in different
Fishes than any other part of the body. Long and flattened in the Skates
and Rays, the head becomes short and high in most Holocephali and in many
Teleosts, or is shaped like a blunt cone, as in such Dipnoi as
_Protopterus_ and _Lepidosiren_; or becomes long and pointed, as in the
North American "Gar Pike" (_Lepidosteus_); or, finally, as in the
Hammer-head Shark (_Sphyrna_), the head may be produced into great lateral
extensions, carrying the eyes at their extremities (Fig. 256, B). Apart
from its relative shape and size, the appearance of the head may be further
modified by the thinness of the investing scaleless skin, which readily
allows the surface and contour lines of the bones of the skull to be seen
through it, as in the Crossopterygii, and in such Teleosts as the Siluroid
genera _Clarias_ and _Callichthys_; or the skin, even if devoid of scales,
may be so thick that scarcely any of the bones are visible externally. The
exoskeleton, whether in the form of scales or bony plates, may extend to a
varying degree on to the surface of the head in different Teleosts, or may
even invest nearly the whole of the head. When, as is not infrequently the
case (_e.g._ many Scorpaenidae) certain of the bones of the skull are
produced into projecting spines, the head assumes a singularly formidable
appearance (Fig. 424).
The mouth differs greatly in size and position. In existing Elasmobranchs
it is generally crescentic in shape and always ventral in position, but in
certain primitive fossil members of the group, as in the Palaeozoic
_Cladoselache_, it is anterior and terminal. The Sturgeon and other living
Chondrostei have the mouth ventral. In the Dipnoi also the mouth is
ventral, but is near the extremity of the snout. As a rule, the mouth is
terminal or nearly so in the living Crossopterygii and Holostei, and in the
great majority of Teleosts, although in the latter group it is occasionally
distinctly ventral, especially when a snout is developed, and it may
sometimes look upwards by reason of the projection of the lower jaw in
front of the upper. A pronounced "beak" is sometimes formed by the forward
prolongation of both jaws, as in the Gar Pike (_Lepidosteus_), with the
result that the {154}vertical gape of the mouth is greatly increased, but
in a few Teleosts a beak may result from a forward extension of one jaw
only, the upper in the Sword-Fish (_Xiphias_) and the lower in the
"Half-Beak" (_Hemirhamphus_). A further modification is to be noted in many
Teleosts, in which, owing to the forward prolongation and inclination of
the skeletal supports of the jaws, the mouth is at the extremity of a
longer or shorter spout-like beak, and is then usually very small. This is
the case in the "Sea-Horse" (_Hippocampus_), the Pipe-Fishes
(_Syngnathus_), the "Flute-mouths" (_Fistularia_), and the Trumpet-Fish
(_Centriscus_), and especially in certain species of the African family
Mormyridae, where the pore-like mouth is at the extremity of a long,
tapering, downwardly-curved proboscis (Fig. 330). In many Teleosts the
mouth can be protruded and withdrawn at will by a sliding motion of the
bones of the upper jaw (premaxillae) on the anterior skull bones by which
they are supported. From this point of view the toothless mouth of the
Sturgeon is even more remarkable. By a forward or a backward swing of the
elements which form the upper half of the hyoid arch (hyomandibular and
symplectic) the mouth can be thrust downwards from the under side of the
head like a spout, when the Fish is feeding, and subsequently retracted. In
not a few Fishes the forepart of the head is prolonged forwards over the
mouth and jaws in the form of a rostrum or "snout"; it is, in fact, to the
growth of a snout that the ventral position of the mouth in Fishes is
generally due. This feature is more or less characteristic of most
Elasmobranchs, in which the snout forms a cut-water overhanging the mouth.
In the Holocephali the snout is short and blunt, except in _Harriotta_,
where it is pointed and unusually long. Among the Chondrostei the Sturgeon
has an exceptionally massive snout, the length and shape of which differs
in different species. In the allied _Polyodon_ the thin, flattened,
spoon-like snout is scarcely less than one-fourth the length of the body
(Fig. 289).
Simple or branched tactile filaments or "barbels" are present on different
parts of the head in many Teleostomi, sometimes at or near the chin, as in
certain Gadidae, like the Haddock and Cod, or on the under surface of the
snout, in front of the mouth, as in the Sturgeon. In the Siluridae (Fig.
356), where they are found in relation with the upper and lower jaws, and
even between the nostrils, these structures are often remarkably developed.
{155}The eyes of Fishes are usually very large. They are generally situated
on the sides of the head, but in the "Star-gazers" (_Uranoscopus_) they are
on the upper surface and close together. In the goggle-eyed
_Periophthalmus_ the eyes seem to protrude from their orbits, and in a
variety of a species of Carp, the Gold-Fish (_Cyprinus auratus_), the
protrusion is so marked that the eyes seem as if on stalks. In a few
species, which live either in caves or at very great oceanic depths, the
eyes become vestigial, and are hidden beneath the skin, or are even covered
by scales (Fig. 430).
In the Elasmobranchs and Dipnoi the olfactory organs retain their primitive
position as pit-like sacs on the ventral surface of the snout, just in
front of the mouth. In the Dipnoi (e.g. _Protopterus_) each olfactory sac
has two apertures, of which one, the external nostril, is placed on the
under surface of the snout, while the other, the internal nostril, opens
within the upper lip into the oral cavity—a feature which is unique among
Fishes. In nearly all Teleostomi, also, each sac has two nostrils, which,
however, are situated either on the upper surface or on the sides of the
fore-part of the head, and have no communication with the mouth.
Directly behind the head in Elasmobranchs, or beneath its hinder part in
all other Fishes, are placed the external apertures of the branchial
clefts. In the former group these apertures are visible externally in the
form of a series of narrow vertical slits, but in the latter they
communicate with the exterior by opening on each side into a common
branchial cavity, the outer wall of which is formed by a movable flap-like
fold with a free hinder margin and a special internal skeleton of
cartilaginous rays or of bony plates and rods, the gill-cover or operculum
(Fig. 161, B). Behind the free margin of the operculum there is a slit-like
orifice, the gill-opening or external branchial aperture, which curves from
above downward and forward toward the chin, and places the branchial cavity
in communication with the exterior. Through this aperture the water, which
has entered through the mouth, traversed the gill-clefts, and bathed the
gills, finds its exit from the body. The space on the ventral side of the
head between the two halves of the lower jaw, and between the two external
branchial apertures, is termed the "isthmus." The size of the external
branchial aperture differs greatly in different Fishes, according to the
extent to which the {156}free opercular margin fuses below with the
isthmus, or behind with the side of the head. Thus the aperture may extend
from the chin in front upward and backward to near the dorsal surface of
the head, or it may be reduced to little more than a mere pore situated on
any part of the opercular edge (e.g. _Hippocampus_); or, as in
_Symbranchus_, the reduced pores of opposite sides may coalesce in the
floor of the throat in a common median opening.
In the Elasmobranchs and in the Dipnoi the cloacal aperture is always
situated at the junction of the trunk with the tail. In the Teleostomi,
however, where the intestine has a separate external orifice or anus,
distinct from, and placed in front of, the separate or combined
urino-genital ducts, the anus may either retain its primitive position near
the union of the trunk and tail, or occupy almost any intermediate position
between this point and the throat.
Most Fishes possess both median and paired fins (Fig. 93, A). From an
evolutionary point of view the median fins have a far greater antiquity
than the paired fins. They appear before the latter in embryonic
development, and in the Cephalochordata, and such lower Craniates as the
Cyclostomata, they are the only fins which exist. The isolated median fins
of most Fishes are discontinuous remnants of a primitively continuous
structure, which, extending like a fringe along the median line of the
back, was thence continued round the end of the tail and forward along the
ventral surface as far as the cloacal or anal orifice. This primitive
condition, which, as we have seen, is characteristic of Amphioxus, is also
very general in the embryos and larvae of Fishes (Figs. 238 and 309), and
is more or less completely retained in the Dipnoi and in many adult
Teleosts, notably in those species in which the body is greatly elongated
and locomotion is effected by serpentine lateral undulations, as in the
Eels (Anguillidae), and in others which, either through their
quasi-parasitic or commensal habit (e.g. _Fierasfer acus_), or by reason of
a peculiar environment, as in certain deep-sea Fishes (Fig. 430) are
distinguished by the retention of many larval features. More generally,
however, the continuity of the fin becomes interrupted, and that portion of
it which surrounds the extremity of the tail is the first to become
separated from the rest as a caudal fin (Fig. 429). By further
interruptions the remaining dorsal portion may become divided into two or
three isolated {157}dorsal fins (Fig. 398), or even into a series of
isolated finlets; and similarly also with the ventral portion or anal fin;
or, without undergoing subdivision, both fins may become reduced in length
to an extent which differs greatly in different Fishes, and persist as
single dorsal or anal fins. But even when a median fin is reduced in length
by atrophy, or becomes subdivided by breaches in its continuity, the
externally invisible supporting radial elements frequently remain to prove
the originally greater length of the fin, or the continuity of its detached
remnants.
Like the median fins, the paired fins may also be regarded as discontinuous
remnants of an originally continuous _lateral_ fin which extended along
each side of the body from the head to the vent, and of which only the
anterior and posterior portions now remain as the pectoral and pelvic fins.
Pectoral fins are rarely absent in existing Fishes, and when present they
are always situated just behind the branchial clefts, where, as in most
Teleostomi, the outline of their supporting pectoral girdle can often be
seen. They vary greatly in form and size in different Fishes, and in the
Elasmobranchs are larger than in most others. In certain members of the
latter group, the Skates and Rays, in which the feebly-developed tail is
probably useless as a locomotor organ, the pectoral fins are exceptionally
large, forming broad triangular lobes, the broad bases of which are
continuous with the sides of the body from the anterior part of the head to
near the origin of the pelvic fins, and thus in outward form, if not in
inward structure, simulate re-acquired continuous lateral fins. Except in a
few instances, the Teleostomi have relatively small fan-shaped or
paddle-like pectoral fins, and usually only that portion of each fin which
is supported by the dermal fin-rays is visible externally. In the
Crossopterygii, however, each fin appears to consist of a central lobe
invested by scales and encircled by a peripheral fringe of fin-rays, and is
hence described as a "lobate" fin (Fig. 279). When the central lobe is much
increased in length but reduced in width the fin becomes acutely lobate. A
similar type of fin is present in the Dipnoi, but in _Protopterus_ and
_Lepidosiren_, owing to the length and narrowness of the central lobe, and
the reduction or suppression of the marginal fringe, the pectoral members
assume the condition of long tapering filaments (Fig. 304).
{158}Although as a rule smaller in size, the pelvic fins bear a general
resemblance to the pectoral fins, but in certain groups, especially in
Teleosts, they are liable to undergo extraordinary changes in position,
and, as will be seen presently, are much more prone to exhibit the effects
of adaptive modification and degeneration. They are present in all existing
Fishes, with the exception of the Crossopterygian _Calamichthys_ and some
Teleosts, and, except in the Teleostei, they invariably retain their
primitive position near the junction of the trunk with the tail, and
directly in front of the cloacal or the anal aperture; in this position
they are said to be "abdominal." In other Teleostei the fins undergo
forward displacement and come to lie directly beneath the pectorals (Fig.
415), when they are said to be "thoracic," as in the Mackerels (Scombridae)
and the Horse-Mackerels (Carangidae); or even in front of the pectoral fins
on the under surface of the throat, when they are described as "jugular,"
as in the Cod and other Gadidae (Fig. 398).
Both the median and the paired fins are supported by an internal skeleton,
consisting (i.) of a series of cartilaginous or bony, rod-like radial
elements or pterygiophores, for the support of the inner or proximal
portion of the fins, and (ii.) of a series of horny fibres, or bony dermal
fin-rays, which fulfil a like function for the outer or distal portion. The
radial elements, however, are never visible externally, even when, as in
most Elasmobranchs, they support the greater part of the fins, inasmuch as
they are invested by the fin-muscles and the skin; and in the same group,
where horny fibres complete the fin-skeleton, they too are covered by the
spinose skin, and hence offer no external evidence of their existence. In
the Teleostomi a marked reduction in the number and length of the radial
elements of the paired fins, and the insinking of those pertaining to the
median fins into the adjacent muscles of the body-wall, leaves the dermal
fin-rays, with their thin covering of transparent and usually scaleless
skin, as obvious features in the external appearance of the Fish, and
apparently as the sole support of the fins.
The dermal fin-rays of the Teleostomi exhibit an obvious distinction into
spines and soft rays (Fig. 93, A). The former are stout, rigid, and
unbranched structures, pointed at their free distal ends, which, in numbers
differing in different genera and species, support the anterior portions of
the dorsal, anal, and pelvic fins. {159}Soft rays are flexible, branched
distally, and generally exhibit a transversely-jointed structure; when
present in conjunction with spines they invariably lie behind the latter.
The presence of both kinds of fin-rays, or of soft rays only, is one of the
more obvious distinctions between the Teleostean groups of the
Acanthopterygii and the Malacopterygii, of which the Perch and the Salmon
respectively are well-known examples. Powerful spines are frequently
developed in front of the dorsal fin in many living and extinct
Elasmobranchs, and, under the general term of "ichthyodorulites,"
constitute the sole fossil remains of many extinct Devonian and
Carboniferous genera.
The caudal fin and the terminal portion of the tail exhibit interesting
modifications which are highly characteristic of particular groups of
Fishes. In the embryonic and early larval stages of most Fishes the
tapering caudal extremity retains its coincidence with the axis of the
body, and divides the caudal fin into two equal portions, a dorsal and a
ventral lobe, the two being continuous round the tip of the tail; and this
condition, which is certainly the most primitive, is termed "protocercal"
or "diphycercal" (Figs. 238 and 309). Such a symmetrical tail, as we have
seen, is retained in the Cyclostomata, and was also present in certain
extinct palaeozoic Sharks (e.g. _Pleuracanthus_), but it may be doubted if
any existing Fish has a tail which is truly and primitively diphycercal.
The Dipnoi (Fig. 304) and the Crossopterygii, including fossil
representatives of both groups, and perhaps a few Teleosts, seem to
approach this condition; but it is by no means certain that the apparent
symmetry is primitive, and has not been secondarily acquired. In other
Fishes the terminal part of the tail, including also its section of the
vertebral column, is bent upwards, and is fringed along its upper border by
the reduced dorsal lobe of the caudal fin, which, nevertheless, retains its
continuity with the ventral lobe round the tip of the tail. The latter, or
rather its hinder portion, is strongly developed, but, owing to the
prolongation of the up-tilted caudal axis beyond it, the dorsal lobe
appears longer than the ventral, and hence there is a marked want of
symmetry between the upper and lower division of the caudal fin (Fig. 253,
A). The Ostracodermi, all living and nearly all extinct Elasmobranchs, the
Acanthodei, Holocephali, some extinct Dipnoi, and amongst the Teleostomi,
{160}the living Chondrostei and certain extinct Crossopterygii, afford
examples of this unsymmetrical or heterocercal type of tail. A third type
is the "homocercal." In this type the caudal fin appears externally as if
perfectly symmetrical, the supporting fin-rays radiating from the blunt
extremity of the tail in such a way that a prolongation of the axis of the
body appears to divide the fin into equal-sized and continuous upper and
lower lobes (Fig. 343). Dissection, however, reveals the fact that the
terminal portion of the vertebral column is bent upwards as in the
heterocercal tail, and that while the dorsal lobe is almost vestigial, the
ventral lobe is enormously developed, and its supporting rays so inclined
backwards parallel to the axis of the body as to form practically the whole
of the caudal fin, with the exception of the dorsal border, which is formed
by the few remaining fin-rays of the dorsal lobe (Fig. 140). A homocercal
tail, therefore, is a disguised or masked heterocercal tail. It is
specially characteristic of Teleosts, and is closely approached in the
Holostean genera _Lepidosteus_ (Fig. 299) and _Amia_, which offer an
interesting transition from the heterocercal to the homocercal types; and,
singularly enough, even the heterocercal tail of the Palaeozoic Shark
_Cladoselache_ (Fig. 249), seems as if it had undergone some degree of
independent specialisation in the same direction. The homocercal tail
exhibits much diversity of form in different Teleosts, sometimes being
rounded or lancet-shaped, and sometimes having a deeply-forked hinder
margin. One of the Ribbon-Fishes, _Trachypterus taenia_, is singular in
having the caudal fin on the dorsal side of the tip of the tail, and
directed upwards like a fan. In some Teleosts, again, there is no
recognisable upward deflection of the terminal portion of the vertebral
axis, and the caudal fin-rays seem to be derived in equal proportions from
the dorsal and ventral lobes of the fin (Fig. 414). This apparently
diphycercal tail is probably a secondary acquisition, and may be considered
due to the atrophy of the terminal portion of the vertebral column, and the
subsequent coalescence of the dorsal and ventral lobes of the caudal fin
round the extremity of a more or less abbreviated tail. It is even possible
that in some Fishes the proper caudal fin has completely atrophied, and
that the apparent caudal fin has really been formed by a similar
modification affecting the hinder portions of the dorsal and anal fins. In
the extinct Crossopterygian genera, _Coelacanthus_, {161}_Diplurus_, and
_Undina_ (Fig. 278), there is evidence that the latter modification has
actually taken place, for the atrophying terminal part of the tail, with a
vestige of the original caudal fin, is still retained as an axial
prolongation between and even beyond the secondarily formed caudal fin. To
this secondary diphycercal tail the term "gephyrocercal" has been applied.
The apparent diphycercal tail of many Fishes, and especially of Teleosts,
is really a gephyrocercal structure. The ancestral evolution of the
different types of caudal fin is recapitulated in the embryonic histories
of their possessors. The heterocercal condition of an adult Fish is always
preceded by a transitory embryonic diphycercal stage: from the same
starting-point the homocercal condition is attained after passing through a
heterocercal stage; while the gephyrocercal may perhaps be derived by
degeneration from any one of the others.
The normal function of the fins, both median and paired, has reference to
locomotion in the form of progression, steering or balancing, but in not a
few Fishes the fins may be variously modified and adapted for quite
different purposes; and especially is this the case in the dominant group
of existing Fishes—the Teleostei. Thus, to quote a few examples, the first
dorsal fin of the Sucker-Fishes (_Remora_, _Echeneis_) forms a cephalic
sucker, by means of which the Fish attaches itself to Sharks and Turtles
(Fig. 421); or, as in the Angler-Fish (_Lophius_), its anterior rays are
much elongated, and terminate in lobes which serve as a bait to attract the
prey on which the animal feeds; again, in some of the deep-sea Fishes the
dorsal fin, like the pectoral and caudal fins in others of a similar
habitat, is produced into long trailing filaments whose use is probably
tactile. The pelagic young of many Teleosts, such as some of the
Ribbon-Fishes and the Horse-Mackerels (_Caranx_), also have certain of
their fin-rays prolonged into similar filaments. The pectoral fins are
enormously elongated and wing-like in the Flying-Fishes (_Exocoetus_), and,
after the fashion of a parachute, serve to sustain the Fish in its flying
leaps through the air. They are also similarly modified for a like purpose
in the so-called Flying-Gurnard (_Dactylopterus volitans_). The pectoral
fins may also be used for progression on land, as in the African and East
Indian Goby (_Periophthalmus_), where the fins are large and muscular and
are applied to the ground like feet, enabling {162}the Fish to hop about
the muddy or sandy flats left bare by the retreating tide, in pursuit of
the small Crustaceans on which it feeds. In other Teleosts certain of the
rays of the pectoral fin separate from the rest and from one another, and
form free tentacle-like structures the use of which is probably tactile. In
the Gurnards these organs are relatively short and stout, but in other
Fishes they may form long slender filaments twice as long as the animal,
and capable of being moved independently of the fin, as in the West African
and West Indian species of Polynemidae (_Pentanemus quinquarius_). Similar
free rays are also present in some deep-sea Scopelidae, as in _Bathypterois
dubius_, where they are nearly as long as the Fish itself (Fig. 371, B). A
familiar modification of the pelvic fins in several Teleosts is their
coalescence and more or less complete conversion into a ventrally-placed
sucker-like organ of attachment, as in the common Lump-Sucker
(_Cyclopterus_) and the Gobies (_Gobius_). In the gaudy Chilian Fish,
_Sicyases sanguineus_ (Fig. 428), the anterior part of a huge ventral
sucker is supported by the jugular pelvic fins, and the hinder part by
prolongations from the pectoral girdle. Certain Cyprinidae (e.g.
_Gastromyzon_, which frequents the rapidly-flowing mountain streams of
Borneo), have the whole ventral surface of the trunk, in conjunction with
the outwardly and horizontally directed pectoral and pelvic fins, modified
to form an efficient adhesive surface for attaching the Fish to the stones
and rocks of the river bottom[121] (Fig. 355). In the males of
Elasmobranchs, except in the Palaeozoic Shark _Cladoselache_, and of
Holocephali, the hinder portions of the pelvic fins are modified to form
copulatory organs, the claspers, mixipterygia, or pterygopodia. Lastly, it
may be mentioned that the spines, often long, pointed, and sometimes
serrated, with which the paired and median fins of many Fishes are
provided, furnish formidable offensive or defensive organs, especially when
they are associated with poison glands, and also that in by no means an
inconsiderable number of Teleosts the spines may form part of a
stridulating vocal mechanism.
In different Fishes the pectoral and pelvic fins and the {163}median fins
may, individually, all be absent through atrophy. The pectoral fins are
rarely absent: nevertheless, in certain species of Syngnathidae, and in
most Muraenidae, for example, these fins are entirely wanting. The pelvic
fins are much less constant and are often absent in entire families, as in
the Pipe-Fishes (Syngnathidae), the "Electric Eels" (Gymnotidae), and the
true Eels (Anguillidae), and in the Globe-Fishes and Porcupine-Fishes
(_Tetrodon_, _Diodon_), as well as in certain genera of families where they
are usually present, as in some of the Blennies (Blenniidae) and in the
Ophidiidae. Even when present the pelvic fins are often reduced to mere
vestiges in the shape of filaments, as in some of the Gadoids (Gadidae) and
Ribbon-Fishes, or are represented only by a pair of defensive spines, as in
some Sticklebacks (_Gastrosteus_), or even by a single spine (Balistidae).
Complete suppression of the pelvic fins, or their reduction to vestigial
remnants, seems to be of frequent occurrence in Fishes which live in the
mud, or are able to pass a longer or shorter time in soil periodically
dried during the hot season, as in some Cyprinodontidae, and in species of
such tropical Teleostean families as the Ophiocephalidae, Galaxiidae, and
Siluridae. Suppression of the dorsal fin is apparent in the Gymnotidae, and
of the anal fin in the Ribbon-Fishes. In some of the latter family, as in
the rare British visitor the Oar-Fish (_Regalecus banksii_), and in the
Sea-Horse (_Hippocampus_), where the tail becomes a prehensile organ for
coiling round seaweeds when the animal is not swimming, the otherwise
remarkably constant caudal fin is absent.
An initial stage in the degeneration of median fins is to be seen in many
of the Salmonidae and Siluridae, in which a posterior division of the
dorsal fin becomes reduced in size, loses its fin rays, acquires much fat
in its substance, and becomes an "adipose fin."
The "lateral line" is a notable feature in the external appearance of most
Fishes. Originally developed in the superficial epidermis of the skin in
the form of linear tracts of isolated and often segmentally arranged masses
of sense-cells, these organs subsequently become imbedded for protection in
the epidermic lining of either an open groove or a closed canal extending
along each side of the trunk and tail, and prolonged on to the more exposed
parts of each side of the head in the shape of a more {164}complex system
of branching grooves or of deeply-seated and externally inconspicuous
canals. The course of the lateral line can, as a rule, readily be detected
by the naked eye, and, even when not otherwise distinguishable, may be
traced by the series of simple or multiple pores through which, at
intervals, the canal communicates with the exterior (Fig. 93, A), and often
also, in the trunk and tail, by a band of coloration different to that of
the rest of the body.
COLORATION.
Contrary to popular opinion, it may be doubted if any animals, even Insects
or Birds, can vie with living Fishes in the brilliancy and changeability of
their colours. The nature of their habitat, the rapid fading of the natural
tints after death, and the fact that museum specimens, however carefully
preserved, afford but a ghostly resemblance to the colours of the living
animal, account, no doubt, for much of the prevalent ignorance of the
extraordinary extent to which colour-development may proceed in a
considerable number of Fishes. Like the generality of northern forms of
life, the Fishes of our own seas, rivers and lakes, are less conspicuous
for vivid and striking coloration than those of tropical or subtropical
climes, although such familiar Teleostean Fishes of our seas and fresh
waters as the Mackerel, the Salmon and Trout, the males of the Stickleback
and Dragonet, some of the Gurnards (Triglidae) and Wrasses (Labridae), the
Opah or King-Fish (_Lampris luna_), and many others, are notable
exceptions. Brilliancy of coloration is most conspicuous in the Teleostei:
in nearly all other Fishes the colours are more uniform, usually sober and
often sombre, with no more variety than is afforded by the presence of dark
spots or bands on a lighter ground, or _vice versâ_, or by the lighter
colour of the ventral as compared with the dorsal surface. In Teleosts all
the resources of colour-formation, pigmentation, reflection, and
iridescence through optical interference, in diverse combinations, are
employed in the production of the various tints, while the dominant ground
colour is often diversified by the presence of stripes, bands or bars,
longitudinal or transverse, or of spots of different hues, frequently
arranged in striking and intricate patterns.
The possibilities of coloration in these Fishes may be briefly illustrated
by a few examples:—
{165}In an Australian Fish (_Plectropoma richardsoni_) the prevalent ground
colour of the body is a brilliant carmine, with a tendency to yellow
beneath, and diversified on the back and sides with ultramarine spots of
almost sapphire-like intensity.[122] Certain Australian species of _Beryx_
(_B. affinis_ and _B. mülleri_)[123] have a similar ground-colour when
freshly caught, but with various opalescent tints, chiefly blue and lilac
reflections. In _Polynemus vereker_[124] the ground colour is chrome
yellow, with darker markings, the pectoral and caudal fins are bright
orange, the remaining fins being a lighter shade of the same tint, and by
contrast the long free filaments of the pectoral fins are a bright
vermilion red. The Velvet-Fish (_Holoxenus cutaneus_), also a denizen of
Australian seas, has a dominant colour of brilliant scarlet vermilion, or a
mixture of vermilion and orange. The skin has no scales and presents a
singular pilose or velvety appearance.[125] It is, however, in some of the
Pacific Trigger-Fishes (e.g. _Monacanthus_) and Coffer-Fishes (species of
_Ostracion_) that the eccentricities of coloration are perhaps most
strikingly manifest, for not only are the prevailing colours of the most
brilliant description, but the presence of differently coloured bands or
stripes, often arranged in complex patterns, adds greatly to the gorgeous
and singularly bizarre appearance of these Fishes. To quote one
illustration, the male of the Tasmanian Coffer-Fish (_Ostracion
ornatus_)[126] has the back and sides of its body grass-green and its belly
pale lemon: the caudal fin is orange-yellow, and the remaining fins a
neutral transparent tint. The sides of the trunk and head are traversed by
broad, irregular, and somewhat interrupted bands of the most brilliant
ultramarine blue, the edges of which are sharply defined by dark
chocolate-brown lines. Two or three of the blue body-bands are continued on
to the caudal fin, where they curl into characteristic loop-like patterns.
The lemon-yellow of the belly is further variegated by a reticulated
pattern in pale blue. In the female, formerly regarded as a distinct
species, the ground colour is not green but a pale pinkish-grey, or
dove-colour, with local flushes of a more decided pink, and the belly is a
pure yellow. The blue stripes of the male are represented in the female by
comparatively unbroken bands of a rich reddish-brown which, at the bases of
the pectoral and dorsal fins, {166}form an irregular spiral pattern. In
both sexes the pattern of the longitudinal bands is never precisely the
same in any two individuals. Scarcely less brilliant is the coloration of
those Teleosts, notably species of Pomacentridae and Chaetodontidae, which
frequent the coral reefs of the East Indian Archipelago and the Pacific and
feed on the coral polypes, and of many of the Wrasses (Labridae). Many
other groups, such, for example, as the Percidae, Cirrhitinae, and the
Pipe-Fishes (Syngnathidae), include species in which the coloration is
vivid and often beautiful, although less striking than is the case with the
Fishes mentioned above. As illustrating the opposite extreme in the scale
of coloration, between which and the brilliant tints just described every
conceivable gradation exists, mention may be made of the colourless
appearance of those Fishes which, like the Kentuckian Blind-Fish
(_Amblyopsis spelaea_), are denizens of subterranean rivers; and, omitting
a few species in which the coloration is almost brilliant, the prevalent
sombre tints, dark brown or black, rarely relieved by spots, bands, or
other distinctive markings, of the Fishes inhabiting the abyssal waters of
the deep sea.
The coloration of Fishes is due to the presence in the dermic portion of
the skin of (_a_) special pigment-containing cells (colour-sacs,
chromoblasts or chromatophores), and (_b_) a peculiar reflecting tissue
composed of iridocytes.[127] Chromatophores are probably branched
connective-tissue cells in which pigments of various colours are deposited.
The colouring matter present in different chromatophores is red, orange,
and yellow, all of which belong to the lipochrome group of pigments, or
black (melanin group), but by the combination or blending of
differently-coloured chromatophores other colours may be produced. Thus,
green results from the mixing of yellow and black in suitable proportions;
brown from the blending of yellow and black; and other shades or tints from
an appropriate mixture of chromatophores of various colours. As a rule the
muscles of Fishes contain but little haemoglobin, but, when visible through
the skin, the occasional presence of this substance in localised patches
may contribute a few red spots to the general coloration, as is the case in
the British Flat-Fish _Lepidorhombus megastoma_.
{167}Iridocytes consist of guanin, which, in its chemical reactions,
closely resembles the guanin obtained from guano, and therefore is to be
regarded as a further illustration of the utilisation of waste excretion
products for the production of colour in animals. In forming iridocytes the
guanin is deposited in the shape of granules, or of rounded, polygonal, or
stellate bodies, or in flattened plates. Considered as an agent in the
production of colour, the chief feature in the iridocytes is their opacity
and great reflecting power; and according to the way in which light is
reflected from them, the result may be a chalky white or a bright silvery
appearance. By interference these colour elements are also responsible for
the prismatic colours and brilliant iridescence which so many Fishes
exhibit. The optical properties of guanin has led to its use in the
manufacture of artificial pearls. "Essence d'orient," or "blanc
d'ablette,"[128] from which these pearls are made, principally in Paris, is
obtained from the scales of the Bleak (_Alburnus lucidus_), and is really
the guanin of which the iridocytes of this Cyprinoid are composed. It is
also to the presence of crystals of guanin that the beautiful metallic
lustre of the iris in many Fishes is due.[129]
[Illustration: FIG. 94.—The coloration elements in the skin of the upper
side of a freshly-killed normal Flounder (_Pleuronectes flesus_), seen by
transmitted light. The stellate black bodies are the black chromatophores;
the grey bodies of similar shape represent the yellow chromatophores; and
the small grey plates the iridocytes. (From Cunningham and MacMunn.)]
The chromatophores and iridocytes are chiefly disposed in two layers in the
skin, one outside the scales and the other on the inner surface of the
scales, between the latter and the underlying muscles; and although the two
kinds of coloration elements may be present in both layers, their relative
{168}abundance varies in different Fishes, and in different parts of the
surface of the same Fish. Where chromatophores are most abundant, usually
on the back, the reflecting tissue is relatively scanty, and _vice versâ_.
On the sides and belly of a Fish the place of the inner layer of the dorsal
surface may be taken by the "argenteum." This layer is devoid of
chromatophores, and consists of reflecting tissue in which the iridocytes
form a continuous stratum, either in the form of granules, or as a close
network of rod-like bodies or of polygonal plates in contact with one
another, instead of being less numerous and more scattered as on the back.
When iridescence is produced, it is due to the iridocytes of the outer
layer of the skin; the dead whiteness and silvery lustre, on the other
hand, have their origin in the different ways in which incident light is
reflected from the inner layer or argenteum.
To the relative abundance of chromatophores, the kind of pigment they
contain, and the manner in which they are distributed and blended, combined
with the different reflecting properties, or the iridescence, of the
iridocytes, are due the extraordinary wealth and variety of colour in
Fishes.
The part played by the different coloration elements in the production of
the characteristic colours of different Fishes may be illustrated by two
examples.[130]
In the common Whiting (_Gadus merlangus_) the back of the Fish is a dark
bluish-grey; the sides have a beautiful iridescence and silvery glitter,
while the belly is very nearly a dead white. Briefly, these appearances are
due to the fact that chromatophores (black and deep yellow) are most
abundant on the back, less numerous on the sides, and wanting altogether on
the belly; while the iridescence and silvery appearance of the sides are
due to the iridescence of the iridocytes external to the scales, combined
with the non-iridescent but highly reflective property of a layer of
iridocytes internal to the scales; and the dead white of the belly to the
different reflecting power of the argenteum, and the absence of
chromatophores in that region.
In the Mackerel (_Scomber scombrus_) the distribution of coloration
elements is different, inasmuch as they are mainly situated in the deeper
part of the skin, internal to the deciduous scales. The back is marked by
the well-known alternating wavy bands {169}of black and green; the sides
gleam with the most brilliant iridescence, changing from silver to yellow
or red gold, according to the angle at which the Fish is viewed. The black
bands of the back are produced by the crowding together of black
chromatophores and the diminished number of yellow; the green bands by an
equal blending of yellow and black. Over the dorsal surface and sides of
the Fish, where the coloured bands extend, there is also a reflecting layer
external to the chromatophores, and to this layer is due the silvery
reflection and iridescence. On the belly the disappearance of the
chromatophores and the greater thickness and opacity of the argenteum
account for the lighter colour and the diminished iridescence and silvery
glitter of this part of the skin.
Many Fishes are known to have the power of changing their colours, and in
some the change is rapid. Such changes are due to incident light reflected
from surrounding surfaces, acting through the visual organs and the nervous
system on the differently coloured chromatophores. The latter are capable
of contraction and expansion. Expansion of any particular kind of
chromatophores is accompanied by a diffusion of their pigment—black, red,
orange, or other colour as the case may be—and, according to the number and
distribution of the chromatophores affected, the prevailing tint or tints
of the whole body will be intensified, or only spots, bands, patches, or
flushes of colour will be produced. Conversely, when chromatophores
contract, they may shrink up to mere dots and bring about a diminution in
the vividness of their respective colours, or even an alteration of colour,
seeing that yellow chromatophores become orange when contracted, while
orange or red appear brown or black. Colour changes of this kind may be
artificially brought about. Experiments with Sticklebacks
(_Gastrosteus_)[131], kept in glass dishes with a bottom of black or white
tiles, have shown that the Fishes over the white tiles became partially
bleached, while others with a background of black tiles retained their
original coloration. Bleached Fishes exposed to the white tiles for a
relatively short period (three to ten days) tend to regain their original
colour when subsequently removed to a background of black tiles, but
prolonged exposure to the former conditions (five to six weeks) seems to
render the acquired light colour more or less permanent. {170}The interior
of a Minnow-can is sometimes painted white, so that the bait may assume a
lighter colour, and thus become more conspicuous in the deeper and darker
water where Perch and Pike abound. Hence the colour of a Fish may vary with
its surroundings; and, as will shortly be shown, the capacity for producing
such changes under natural conditions is of the greatest importance to
Fishes in various ways.
Change of coloration may take place through the development of new
chromatophores under the influence of new conditions, and is then
comparatively slow. Artificial illumination of the unpigmented white side
of a Flounder (_Pleuronectes flesus_), by means of a mirror, induces the
formation of chromatophores, and produces a coloration more or less closely
resembling the upper pigmented side.[132] A similar change sometimes occurs
as a natural variation, and is then usually associated with structural
deformity in other respects.
Coloration also varies with age, sex, ill-health, and even with the
emotions. Young or immature Fishes are often marked by bands, bars, or
blotches of colour (_e.g._ the Parr-marks of young Salmonidae), which, as
they disappear when the Fish approaches the adult state, are perhaps
residual traces of ancestral coloration; although, no doubt, change of
habits and surroundings are sometimes responsible for such colour changes
as are observable during growth. Conspicuous coloration is one of the most
frequent of secondary sexual characters, the colours of the male being
brighter than those of the female, particularly during the breeding season.
A diminution of colour has been noticed in the artificially-induced
pigmentation of the lower side of a Flounder when the Fish was suffering
from partial suffocation owing to the temporary failure in the supply of
fresh water, the normal colour returning when the deficiency had been
remedied. A similar pallor was caused by fright when the same Fish was
disturbed.[133] A nocturnal colour-change has been recorded in the
Tasmanian Trumpeter (_Latris hecateia_).[134] In addition to the
olive-green longitudinal bands which are always apparent, there are visible
at night five broad, transversely-arranged, blackish bands. As illustrating
the importance of vision in colour-changes, it may be mentioned that in a
specimen of this Fish, living in a tank, which had been blinded,
{171}probably by a rat or a cat, the dark bands were permanently retained.
Changes of coloration sometimes take place, which either have no
discernible relation to age, condition, or surroundings, or are brought
about by domestication; and in individuals of the same species there is
often a wide range of colour-variation, which is sometimes, but not always,
associated with particular localities. In some fresh-water Fishes a yellow
colour may replace the original tint (xanthochroism). The usually dull
greenish Tench (_Tinca vulgaris_) occasionally becomes a bright
orange-yellow. Another Cyprinoid, the common Gold Fish (_Cyprinus
auratus_), in its wild state in China is also a dull brown or green, but,
when domesticated, assumes in the first year of its life a black colour
(melanism), then a silvery hue, and finally the vivid ruddy golden colour
of the adult; occasionally, but rarely, the Fish is an albino.
The value of a particular coloration in Fishes, either as an aid to
concealment and protection from enemies, or by enabling them to secure
their prey, may now be illustrated by a few examples.
As previously shown, the colours of Fishes may be artificially varied
according to their surroundings. Changes of a similar kind occur naturally,
and when they tend to assimilate the tints of the Fish to the prevalent
hues of its surroundings, and consequently aid concealment, we have
examples of what has been termed variable protective resemblance.
Individuals of the same species vary in colour according to the opacity of
the water they live in, becoming darker in muddy or peaty water, and
brighter and lighter in shallower or clearer water. Trout caught in a
stream with a gravelly or sandy bottom are lighter in colour than those
obtained from a muddy stream, and it is well known that the same Fish
changes colour as it passes from the one background to the other.[135] In a
lake in County Monaghan, Ireland, the Trout are darker on that side which
is bounded by a bog, but are of the beautiful and sprightly variety
generally inhabiting rapid and sandy streams on the opposite side where the
bottom is gravelly; and narrow as the lake is, the two kinds of Trout
appear to confine themselves to their respective areas.[136] Trout obtained
from a {172}stream near Ivy Bridge have become much lighter since the
pollution of the water by white china clay.[137] As an illustration of the
necessity of vision to such colour-changes, it may be mentioned that blind
Fishes cannot vary their tint in this protective fashion. A blind Turbot
living upon a light sandy bottom differed from its fellows in being much
darker and more conspicuous. Dark Trout have been observed among their
light-coloured brethren in a chalk stream in Hampshire, but the former were
invariably blind, probably, as their larger size indicated, through
age.[138]
Of other forms of protective resemblance, reference may be made to the
bottom-feeding Flat-Fishes (Pleuronectidae), many of which have the upper
surface of the body coloured with various shades of brown, speckled with
black or light specks or blotches, in harmony with the prevailing tints of
the sandy banks which usually form their feeding-ground. When disturbed
these Fishes court concealment, and render themselves still less
conspicuous by partially burying themselves in the sand. Many of the Skates
and Rays, which have a white ventral surface, have the back mottled and
coloured in accordance with the colour of the sea-bottom, but in this case
it is possible that the advantage lies in enabling the Fish to secure
passing prey by concealing its own whereabouts.
Striking examples of protective coloration occur among the Pipe-Fishes and
Sea-Horses (Syngnathidae), which usually frequent groves of _Zostera_,
Fucoids, and other sea-weeds. A British species of Pipe-Fish (_Siphonostoma
typhle_),[139] which lives among the blades of the sea-grass, _Zostera_, is
olive-green in colour, and is a typical example of protective resemblance
both in colour and in the slender elongated shape of the body. Similar
protective resemblances are noticeable among the Sea-Horses, the coloration
varying with the general hue of their environment of sea-weed; but the
climax is certainly reached by the singular Australian species,
_Phyllopteryx eques_ (Fig. 388).[140] In this Fish the skin is produced
into numerous long, flattened, branched filaments, which are prolonged from
the extremities of spine-like outgrowths of the dermal skeleton, and marked
by alternate bands of brown and orange,[141] {173}thus resembling both in
shape and colour the fronds of the surrounding fucoids and other marine
algae amongst which the Fish lives.
Many of the Fishes frequenting the coral reefs of the East Indian and
Pacific areas, especially those belonging to the Teleostean families
Chaetodontidae and Pomacentridae, have a most brilliant and vivid
coloration, frequently marked by bands or stripes of different tint. So far
from rendering these Fishes unduly conspicuous, there can be little doubt
that, by harmonising with the striking and varied colours of the
anemone-like coral polypes, their coloration is distinctly protective; and
it is interesting to note that similar colour-patterns have been
independently reproduced in both families.[142] Even the reef-frequenting
Flat-Fishes (Pleuronectidae) have the usually sombre upper surface
ornamented by vivid colours and striking patterns.
Pelagic Fishes, like the Herring, Mackerel, Flying-Fish (_Exocoetus_), and
many others, often have the belly and sides silvery or white, and the back
dark green, black, or steely blue. Seen from below against the light sky,
or viewed from above against the background of the dark water, these Fishes
would seem to be practically invisible to their predatory foes, whether
Fishes or Birds, or at all events not easily detected.
Coloration may not only be protective, but also aggressive, by helping to
conceal the proximity of an animal from its prey; add to this some device
for deceiving and attracting the prey, and we have an example of "alluring"
coloration.[143]
As an example of coloration which is both aggressive and alluring, the
Angler-Fish or Fishing-Frog (_Lophius piscatorius_) of our own coasts may
be quoted. Naturally sluggish and inactive in its habits, and often using
its muscular pectoral fins for crawling about the sea-bottom, the
Angler-Fish usually hides itself in the sand or amongst sea-weeds, which it
closely resembles in general colour. Curious branched tag-like processes of
soft skin fringe the sides of the head and body, and in appearance and
colour resemble the smaller fronds of the surrounding sea-weed. So far the
coloration is simply aggressive, and helps to conceal the Fish from its
prey, but in addition the animal is provided with a special device for
luring its prey within the reach of its capacious and Frog-like mouth. The
first three spines {174}of the dorsal fin are detached from one another and
greatly elongated, and moreover extend along the middle of the dorsal
surface of the head. The first, which is the longest, terminates in lobes
or lappets of skin, and can be freely moved in every direction by the
muscles inserted into its base. By the agitation of this lure or bait
smaller Fishes, probably mistaking the disturbance for the presence of a
wriggling worm, are tempted to their fate, and soon find themselves
engulfed in the enormous mouth of the artful angler.[144] In some allied
forms (e.g. _Ceratias bispinosus_ and _Oneirodes eschrichtii_)[145] which
live in the abyssal darkness of the deep sea, use is made of the attraction
which light has to aquatic animals, and the fishing-rod spine terminates in
a phosphorescent organ, which is probably used for enticing smaller Fishes
within the reach of the jaws of these singularly modified
Angler-Fishes.[146]
It is by no means improbable that examples of "warning" coloration occur
amongst Fishes. The brilliant colours of some of the Trigger-Fishes
(_Balistes_, _Monacanthus_), Coffer-Fishes (_Ostracion_), and Globe-Fishes
(_Tetrodon_) are perhaps of this nature. They are often associated with the
presence of strong spines, defensive and often erectile, either in
connexion with the dorsal fin or on the general surface of the body, and
may therefore serve the purpose of a danger signal to such predatory foes
of these Fishes as might otherwise be tempted to attack them—to the mutual
advantage of the Fishes themselves and their would-be enemies. The British
Weever-Fish (_Trachinus_) may perhaps offer another example of warning
coloration.[147] The Fish is armed with poisonous spines on its opercula,
and, in addition, has a conspicuous black dorsal fin. When the body of the
Fish is buried in the sand, only its projecting dorsal fin remains to
indicate its whereabouts to predatory Gurnards, which might otherwise
mistake the Weever for harmless Fishes of similar size and habits. The
existence of "recognition" colours or markings peculiar to the species, to
enable individuals of the same species to recognise one another and to keep
together in shoals, has not yet been proved. It is probable that the
relatively {175}limited range of vision, even in the clearest water, would
render coloration unsuitable for this purpose. Recognition sounds are
likely to be far more effective, and there is evidence of their production
by a special vocal mechanism in many Fishes.[148]
The examples given above show how natural selection may lead to the
evolution of distinctive forms of coloration which are advantageous to the
Fish either for concealment, aggression, or protection, and in conclusion
it may be pointed out that by the same cause colour may be eliminated or
its development checked if in any way harmful to the animal; and further,
that if a particular coloration becomes useless to the Fish by reason of a
change in its habits or environment, natural selection ceasing to act where
its intervention is no longer necessary to maintain the coloration, the
latter will sooner or later tend to disappear.
The absence of pigment is sometimes protective. The surface-swimming larvae
of many Teleosts have no chromatophores, and therefore no obvious
pigmentary colours. Their bodies are so translucent that they can be seen
through, and hence are visible only with difficulty. The transparency of
the body may even be increased by the absence of the red haemoglobin of the
blood, as is the case with the pelagic _Leptocephalus_-larvae of the
Eel.[149] The iridocytes of the reflecting tissue may also disappear under
the influence of changed surroundings. The larvae of various species of
_Onus_ (Gadidae) are silvery in hue during their pelagic career, owing to
the presence of iridocytes in the skin, but on becoming mature they change
to a dull dark colour, and live under stones or in holes and crevices in
the rocks. During the change of habit the reflecting tissue (argenteum) is
lost, and the needful chromatophores are acquired.[150]
Instances of the loss of pigmentary colours, owing to the cessation of the
controlling influence of natural selection, are to be found in the absence
of chromatophores on the white under surface of the Flat-Fishes, where such
colours are useless but not necessarily harmful, and in the colourless,
cave-inhabiting Fishes, of which the Blind-Fish (_Amblyopsis_) of North
America may be taken as an example.
{176}POISON GLANDS OF FISHES.
A few Teleosts are provided with weapons of offence or defence in the shape
of poison-glands, probably derived from the epidermis, and associated with
spines on the gill-covers, or in connexion with the dorsal fin, or with
both.
[Illustration: FIG. 95.—The opercular spine of _Trachinus draco_ and its
poison-glands. _ar_, Articulation of the opercular bone with the
hyomandibular; _gl.gl_, the two poison-glands; _op.m_, opercular membrane;
_op.s_, opercular spine; _r_, outer ridge of the spine; _sh_, sheath of the
spine. (From W. Newton Parker.)]
The two British species of "Weever" (_Trachinus draco_ and _T. vipera_) are
both provided with poison-organs in connexion with a spine on the operculum
and with the five or six spiny rays of the anterior dorsal fin.[151] The
first of these spines is a structure projecting backwards from the hinder
margin of the opercular bone of the gill-cover, and is traversed along both
its upper and lower margins, from base to point, by a deep groove. Except
at its protruding naked point the spine is ensheathed in an extension of
the external skin. Along each of the grooves there extends a solid
pear-shaped mass of gland-cells, the broad base of which coincides with the
base of the spine, while the gradually tapering, narrower portion is
continued as far as the sharp point. The glands enclose no cavity, and
there is no duct, so that whatever poisonous fluid their cells secrete is
probably set free by the rupture of the cells and discharged into the
grooves, along which it passes to the point of the spine, somewhat after
the fashion of a hypodermic syringe. The origin of the gland-cells from an
in-pushing of the epidermis is indicated by the continuity of the two
structures near the point of the spine. Both in structure and in their
relation to poison-glands each of the spines of the {177}dorsal fin is
almost precisely similar to the opercular spine. There is no evidence as to
how the poison is ejected into a wound, and it can only be conjectured that
it may be caused by the pressure exerted on the gland when the spine is
forcibly thrust for some distance into the flesh. Certain it is that these
structures are capable of inflicting painful and troublesome wounds when
the Fish is incautiously handled and the skin accidentally punctured, and
no doubt they can be used with great effect as offensive organs.
A similar poison apparatus exists in certain species of Batrachidae, such
as _Thalassophryne reticulata_,[152] which is by no means uncommon at
Panama. This apparatus is formed by a spinous outgrowth from the opercular
bone and by the first two dorsal spines. Instead, however, of having two
grooves, the opercular spine resembles the fang of a venomous snake, and is
perforated by a complete canal which is only open at the base and point of
the spine. A poison-sac at the base of the spine discharges its contents
into the canal. The nature of the glands which secrete the poison has yet
to be discovered, but it is probable either that there are glands in
connexion with the poison-sac, or that the latter is lined by a glandular
epithelium. The structure of the dorsal spines is similar. In some species
of the Scorpaenoid genus _Synancia_[153] (e.g. _S. verrucosa_, from the
Indian Ocean), the terminal portions of the dorsal spines are deeply
grooved on each side, and at the origin of each groove there is a
pear-shaped bag containing a milky poison. The bag is prolonged into a duct
which, after traversing the groove, opens at the extremity of the spine.
Many Siluridae are armed with powerful and often serrated dorsal and
pectoral spines which are certainly capable of inflicting dangerous wounds,
and not a few of them possess a sac-like organ with an external opening in
the axilla of the pectoral fin. It is possible that the sac secretes a
poison for anointing the spine, but at present there is no evidence that
such is the case, or that the sac produces any poisonous secretion at
all.[154]
Among the Elasmobranchs the Eagle-Rays (_Aëtobatis_),[155] and the
Sting-Rays (_Trygon_), have barbed or serrated spines on the tail, which
inflict wounds far more severe than those caused by {178}mere mechanical
laceration; but, except the mucus secreted by the gland cells of the skin,
which may possess venomous properties, no special poison-forming glands in
connexion with the spines are at present known.
PHOSPHORESCENT ORGANS.[156]
In common with many other animals of similar habitat, phosphorescent organs
(photophores) are highly characteristic structures in many deep-sea
Teleosts belonging to widely different families (e.g. _Stomiatidae_,
_Scopelidae_, _Halosauridae_, and _Anomalopidae_). These organs probably
had their origin in local aggregations of the gland cells of the epidermis,
which had acquired the power of secreting a luminous slime. Luminous organs
vary greatly in number and in their mode of distribution in the skin.
Usually they are found on the sides and ventral surface of the body and
head, very rarely on the dorsal surface, and they often present the
appearance of brightly glistening jewels set in the skin. A very frequent
method of arrangement is in one or two longitudinal lines along the lateral
and ventral surfaces, sometimes extending continuously from the head to the
end of the tail (Fig. 371, A, and Fig. 379), but occasionally interrupted
and limited to portions of the body and tail; and in a few a distinctly
metameric disposition is obvious. On the other hand, the very numerous and
simple organs of _Opostomias_ are disposed in many transverse bands along
the sides of the Fish. In addition to these organs, which are usually
numerous, and whose arrangement is linear, specially large and often
structurally complex luminous organs are present on different parts of the
head and body. In _Opostomias micripnus_ there is a phosphorescent organ on
a median barbel depending from the chin. _Sternoptyx diaphana_ has one on
the lower jaw. The presence of one or two organs beneath the eyes (Fig. 96)
is characteristic of several species (e.g. _Opostomias micripnus_,
_Astronesthes niger_, _Pachystomias microdon_, _Scopelus benoitii_,
_Malacosteus indicus_). _Opostomias micripnus_ has a luminous organ on the
isolated and elongated first fin ray of the pectoral fin, while in certain
deep-sea Angler-Fishes (e.g. _Ceratias_) there is one on the anterior
cephalic fin-ray of the dorsal fin. The Scopelid _Ipnops {179}murrayi_[157]
(Fig. 371, C) has a singular organ, probably luminous, beneath the
transparent superficial bones of each side of the roof of the skull.
Another member of the same family (_Scopelus benoitii_) is interesting in
having a phosphorescent organ in the middle of the back, which is directed
backwards. An American genus of Batrachidae (_Porichthys_) has about 350
photophores in relation with the lateral sense-organs of each side of the
head and body.[158] The existence of luminous organs has also been noticed
in the Haddock (_Gadidae_).[159] A primitive form of photophore,
distributed in considerable numbers on the head and trunk, either in lines
or diffused over the surface, exists in eleven species of Selachii
(_Spinacidae_), of which some are known to be luminous.[160]
[Illustration: FIG. 96.—_Pachystomias microdon_, showing the two rows of
phosphorescent organs along the side of the body, and the anterior and
posterior suborbital luminous organs. (After Günther.)]
[Illustration: FIG. 97.—_Opostomias micripnus_. Median section of a simple
phosphorescent organ. _g_, Radial gland tubes. (After Lendenfeld.)]
Diversity of structure is equally marked. The essential part of each
luminous organ is always a collection of gland cells, usually disposed so
as to form the lining of a series of radially arranged gland-tubules in the
deeper part of the organ, which also contains ganglion cells, and is
supplied with nerves from contiguous spinal or cranial nerves. The simplest
form of phosphorescent organ consists of little more than these essential
elements. In the more complex organs an investing pigment-sheath,
reflecting and lens-like structures, and an iris diaphragm, either singly
or in combination, may be added. Fig. 97 represents one of the simplest
types of phosphorescent organ, which, in groups of {180}50 to 100, are
arranged in transverse bands on the sides of _Opostomias micripnus_, and
appear as small white spots on the otherwise black skin of this Fish.
Each organ has the shape of a biconvex lens, sunk to about half its
thickness in the skin. The inner half is formed of radially-arranged gland
tubes filled with small granular cells, and converging towards the centre
of the organ. Into the connective-tissue walls of the tubes extend
blood-vessels and nerves. External to the gland tubes there is a layer of
long slender cells arranged perpendicularly to the surface, and more
externally still a layer of ganglion cells. There is evidence that these
organs multiply by division. Such simple phosphorescent organs as these
differ little from the groups of epidermic gland cells, which probably
formed the evolutionary starting-point in the development of these singular
structures.
[Illustration: FIG. 98.—_Pachystomias microdon_. Section of the anterior
suborbital organ. _g_, Irregular gland tubes; _g^1_, radial gland tubes;
_i_, iris-like diaphragm; _l_, lens-like body; _p.s_, pigment sheath; _s_,
layer of light-reflecting spicules. (After Lendenfeld.)]
A much more complex type of luminous organ is to be found in the suborbital
organs of _Pachystomias microdon_, of which there are two on each side,
appearing as conspicuous white masses, one in front of the other, and
situated just below the eye. The more anterior of the two organs is
somewhat pouch-shaped in section, its walls consisting of several
concentric layers (Fig. 98). Externally there is a layer of black pigment,
within which is a stratum of irregular gland tubes. More internally still
there is a thick layer of light-reflecting spicules, probably derived from
an inverted and modified dermal scale. The axial part of the organ is
occupied by a number of radial-disposed structures, probably similar to the
gland tubes of the simple organs of _Opostomias_, and continuous with a
lens-like structure which, as it were, closes the expanded mouth of the
pouch. The superficial skin which forms the margin of the aperture
partially projects over the outer surface {181}of the lens-like body,
somewhat after the fashion of an iris-diaphragm. The organ is supplied by a
branch of the fifth cranial nerve. Between such simple and complex organs
as those above described there are various other types which are more or
less intermediate in character.
A particular type of phosphorescent organ is not necessarily restricted to
the same species; both the simplest and one or more of the more complex
types may be represented in the same Fish. Thus, _Opostomias micripnus_,
which frequents depths of over 2000 fathoms, has not only the simple organs
described above, but also others differing from the former in having an
external pigmentary sheath, which are scattered all over the body at
intervals of 1 to 3 mm. There are also larger and still more complex organs
which are disposed in two parallel rows along each side of the body; and
finally, the same species has special luminous organs on a median
chin-barbel, and also on an elongated fin-ray pertaining to the pectoral
fin.
The light emitted by phosphorescent organs is probably of use to deep-sea
Fishes in enabling them to seek and detect their prey in the sunless depths
which they frequent. The position of the organs on the sides and ventral
surface of the body, and the frequent presence of special luminous organs
in the vicinity of the mouth, render them admirably adapted to light up the
water in front of and beneath the Fish, while the existence of optical
accessories for intensifying the luminous beams, and for regulating their
distribution, combined with an abundant nervous supply, suggests that the
emission of light is under the control of the Fish, and may be varied as
the occasion requires. That these organs may also be defensive, in some
instances at all events, seems not improbable. A flash-light from the
dorsal luminous organ or "stern-chaser" of _Scopelus benoitii_ would
probably dazzle and frighten an enemy in hot pursuit of the _Scopelus_. The
use of phosphorescent organs as baits or lures for enticing prey has
already been alluded to. There is some evidence that the colour of the
emitted light differs in different Fishes; and as there is considerable
variety in the precise disposition of the organs, it seems probable that in
deep-sea Fishes recognition lights may take the place of the recognition
colours and sounds of those whose lot is cast in a sunnier habitat.
{182}CHAPTER VII
THE SKIN AND SCALES
The skin of the Cyclostomata and Fishes consists (1) of the epidermis,
formed of several layers of epidermic cells, which are constantly being
recruited by the division of the cells of the basal layer; and (2) of a
stratum of connective tissue with intermingled unstriped muscle-fibres,
blood-vessels and nerves, which constitutes the deeper layer or dermis.
From the epidermis are formed the various unicellular or multicellular
glands with which the skin is provided; and from one or both of the skin
layers originate the different calcareous structures which constitute the
hard exoskeleton.
In the Cyclostomata the epidermis is particularly rich in goblet-shaped,
mucus-secreting, gland-cells. The Myxinoids also possess numerous pockets
of so-called "thread-cells." In each of these cells the protoplasm secretes
a long spirally-coiled thread, and under the influence of appropriate
stimuli the thread is shot out and unwound to a great length. The threads
and the mucus are so abundant that one of these animals will convert a
bucket of water into a thick mass of jelly. No scales or other hard
exoskeletal structures are present in any of the Cyclostomata.
In Fishes mucus-glands are also abundant in the epidermis, and to their
activity is due the slimy mucus which lubricates the surface of the body.
They are specially numerous in the Dipnoi (e.g. _Protopterus_), where, in
addition, there are many simple multicellular glands which secrete the
"cocoon" or capsule in which the Fish is enclosed during the dry season.
From the epidermis are derived the poison-glands of some Teleosts, and also
the "glandula pterygopodia" in relation with the claspers of the male
Elasmobranchs. The glandular structures in connexion with {183}the
phosphorescent organs of the deep-sea Fishes will no doubt be traced to the
same source.
In the great majority of Fishes the skin becomes the seat of calcareous
deposit, and gives rise to such diverse exoskeletal structures as the
varied forms of spines and scales with which the surface of a Fish is
invested.[161] These structures, probably the most ancient form of
Vertebrate skeleton owing its existence to the presence of lime salts in
the tissues of the body, present highly characteristic modifications in the
different groups.
Exoskeletal structures are of two kinds: (1) those which owe their
formation to the secretory activity of cells belonging both to the
epidermis and the dermis, and (2) those which are derived solely from the
dermis. To the first belong the dermal denticles or so-called placoid
scales of most Elasmobranchs, and to the second the scales which form the
skin-skeleton of living and extinct Teleostomi and Dipnoi. With the
exception of enamel, which is always formed by the cells of the epidermis,
the hard exoskeletal tissues owe their existence to the secretion of
certain cells of the dermis (scleroblasts),[162] the inclusion of which in
a growing calcifying tissue is the cause of whatever cellular structure the
tissue may present. It will shortly be apparent that the dermic
scleroblasts are by no means uniform in their products, and that in
different Fishes they give rise to widely different hard tissues.
The dermal denticles or "shagreen" of the ordinary Sharks and Dog-Fishes
(Elasmobranchii) probably represent the most primitive form of exoskeleton.
In the development of a dermal denticle a papilla of the dermis grows up
into the overlying epidermis, pushing before it the basal layer of
epidermic cells, which forms an investment to the papilla and constitutes
the so-called "enamel organ" (Fig. 100). The papilla itself subsequently
becomes converted into dentine, leaving, however, a central pulp-cavity,
while the apex of the papilla is invested by a cap of enamel formed by the
enamel organ. Ultimately the base of the papilla widens out into a more or
less rhomboidal basal plate formed of bone. In this way there is formed a
{184}pointed, enamel-tipped spine of dentine which protrudes through the
epidermis, and projects backwards on the surface of the body, but is firmly
fixed in the skin by the basal plate with which it is continuous. The
centre of the under surface of the basal plate is perforated for the
entrance of the blood-vessels which pass to the cellular pulp in the axis
of the spine. In the adult Fish the denticles form a fairly close-set
covering to the whole body, including the head and even the surfaces of the
fins, and are larger on the dorsal than on the ventral surface (Fig. 99).
In the Rays (_Raia_) they are more sparsely scattered, and in different
parts of the body may form spines of considerable size for offensive or
defensive purposes. The spines vary greatly in shape in different members
of the group, sometimes being acutely pointed, and sometimes flattened or
depressed, and often they are furnished with smaller accessory spines
developed at their bases or from the surface of the basal plate. An
arrangement of the denticles in oblique transverse rows is observable in
some genera (e.g. _Scyllium_). In the Saw-Fishes (e.g. _Pristis_) the
denticles which fringe the lateral margins of the long flattened rostrum
are not only enormously enlarged, but are implanted in sockets and form the
teeth of the saw (Fig. 262). In the Holocephali the smooth skin is almost
entirely devoid of exoskeletal structures, but dermal denticles are present
on the frontal and anterior claspers, and in the young there may be a
double row of small denticles along the back.
[Illustration: FIG. 99.—Surface view of the dermal denticles of _Scyllium_
sp., showing their arrangement in oblique transverse rows. _b_, Basal
plate; _c_, canal which perforates the basal plate and becomes the axial
pulp-cavity of the spine; _f.b_, intersecting fibrous bands of the dermis;
_s_, spine; in the spine of one scale the dentinal tubules are shown. The
smaller denticles are those most recently formed. (After Klaatsch.)]
{185}[Illustration: FIG. 100.—Vertical section through the skin of an
embryo Shark. _C_, Dermis; _c.c.c.d_, layers of the dermis; _E_, epidermis;
_e_, enamel organ; _o_, enamel layer; _p_, papilla of the dermis. (From
Wiedersheim, after Gegenbaur.)]
In the remaining groups of Fishes, the Teleostomi and the Dipnoi, the spine
of the primitive dermal denticle is either evanescent or entirely wanting,
while the equivalent of the basal plate remains to form the unit of a scaly
armature. Evidence of this may be found in the presence of transitory
evanescent spines, provided with an enamel-cap, secreted by the basal
epidermis, on the developing rhomboidal scales, as in the young
_Lepidosteus_[163] (Fig. 101); while the entrance of blood-vessels into the
scales through perforations on their inner surfaces, as in _Polypterus_ and
_Lepidosteus_, obviously recalls the perforated base of a dermal denticle
(Fig. 99). The epidermis now ceases to take any part in the formation of
the scales, and hence enamel no longer enters into their structure. A more
regular and definite arrangement of the scales is noticeable, and whether
distinct, or articulating with one another, or overlapping like the slates
on the roof of a house, they are usually disposed in a series of successive
oblique transverse rows. In some of these Fishes the embryonic epidermic
covering of the scales becomes lost, and their outer surfaces are naked.
More frequently, as in the generality of Teleosts, and in the Dipnoi, the
reverse is the case, and the scales are more or less completely invested
both by the dermis and the epidermis. As regards their shape, size, and
minute structure there is much variation. In some Teleostomi {186}the
primitive rhomboidal shape of the dermal denticle is retained; in others a
rounded or cycloid scale supplants the earlier rhombic type. Within the
limits of the same group (_e.g._ Crossopterygii) there are examples of the
independent evolution of a cycloid from a pre-existing rhombic squamation;
and with the introduction of the cycloid type an overlapping or imbricated
disposition of the scales always takes the place of the marginal
articulation of the rhombic type.
[Illustration: FIG. 101.—Development of a scale in _Lepidosteus osseus_ ×
330. _b.p_, Basal plate, with included bone cells, at first distinct from
the spine; _e_, enamel; _e.o_, enamel organ; _ep_, epidermis, with large
gland cells; _p_, dermic papilla which forms the vestigial spine; _Scl_,
scleroblasts. (From Klaatsch.)]
As to the causes which may have determined the shape and mutual relations
of scales interesting suggestions have been made.[164] Scales bear a
segmental relation to the subjacent muscle-segments or myotomes, sometimes
being disposed in oblique transverse rows coinciding with the latter, or
the rows may be so far increased as to be multiples of the myotomes. From
mechanical considerations depending on the sigmoid shape and
interdigitating relations of the myotomes and their separating fibrous
septa or myocommata, and the attachment of the myocommata to the dermis,
the contraction of the myotomes during the lateral flexions of the trunk in
swimming has a tendency to wrinkle the skin into definitely circumscribed
rhombic areas, thus determining the shape, limits, and disposition of the
scales which are developed in those areas. The rhombic was probably the
primitive shape of scales, and is certainly characteristic of the
palaeontologically older types of scaly Fishes. Generally the rhombic
condition is associated with a peg-and-socket articulation between the
upper and lower margins of adjacent scales. But a rhombic squamation is not
without {187}disadvantages, and would certainly impose some restriction on
the lateral flexures of the body in swimming, and hence in the different
groups of Fishes it may happen that, in the more specialised forms, an
imbricated cycloid squamation supersedes a rhombic condition, and with the
change the Fish acquires greater lateral mobility. Even in the same Fish
the gradual substitution of the cycloid for the rhombic type may be
observed. In the Australian _Aetheolepis_,[165] a fossil genus related to
the European Liassic _Dapedius_, there is a gradual transition along the
sides of the body between the articulated rhombic scales of the relatively
immobile trunk and the cycloid overlapping scales of the flexible tail; and
it may be mentioned that, even where a typical rhombic squamation exists,
the peg-and-socket articulation may be wanting in the caudal region, so as
to ensure greater freedom of movement. Mechanical considerations may also
explain the overlapping of cycloid scales. From the mode of attachment of
the myocommata to the dermis, the contractions of the myotomes, through the
pull which they exert on the former, tend to deflect or depress the
scale-areas, particularly at their anterior margins.
[Illustration: FIG. 102.—_Acipenser ruthenus_. A, Side view of the trunk of
a specimen 30 cm. in length (nat. size); _d_, dorsal row of plates; _l_,
_l′_, lateral rows; between the rows of large scutes may be seen the
numerous small denticles which are represented (× 10) in B; C, one of the
large scutes (× 10). (From Hertwig.)]
In the surviving Crossopterygii, as in _Polypterus_, the scales are
rhomboidal and thick, and they only slightly overlap. They articulate with
one another by means of marginal peg-and-socket articulations (Fig. 106,
B). A thick layer of hard, glistening, enamel-like substance or "ganoin"
forms the outer layer of the scale; the inner layer consisting of bone in
which dentinal tubules as well as bone-cells are present. In the numerous
fossil members of the group the scales are either rhomboidal or cycloid.
{188}The oldest representatives of the Chondrostei, the Palaeoniscidae
(Fig. 283) possessed a complete armature of rhombic scales, but in all the
surviving members of the group the scales have undergone considerable
modification in some respects, and in others are degenerate. In the
Sturgeon (_Acipenser_)[166] the primitive rhombic squamation is retained
only on the sides of the terminal part of the tail, and there they are in
close apposition in oblique rows. The rest of the body is traversed by five
widely-separated longitudinal rows of large bony scutes, which, like the
rhombic scales, are furnished with ridges and projecting spines (Fig. 102).
Between the rows of large scales there are numerous denticle-like
structures arranged in oblique rows. Each of these consists of a basal
plate imbedded in the dermis, and of one or more projecting spines which
perforate the epidermis. All the scales have the same minute structure,
consisting mainly of bone; but the surface layer and the spines seem to be
composed of a hard laminated substance from which bone-cells are absent
(ganoin). In _Polyodon_ the scutes are wanting, but vestigial denticles are
retained.
[Illustration: FIG. 103.—Surface view of the rhombic scales of a young
_Lepidosteus_. In two scales the parts which are overlapped by adjacent
scales are shaded. _c_, Position of the central canal which perforates the
inner surface of each scale; _f.b_, intersecting fibrous bands of the
dermis; _s_, vestigial spines. (After Klaatsch.)]
Among the Holostei the scales are very different in the two surviving
members of the group. In _Lepidosteus_ (Fig. 103) the {189}scales are
rhombic, and both in arrangement and structure, as well as in their method
of articulating with one another, they closely resemble those of
_Polypterus_. In _Amia_,[167] on the contrary, the relatively thin scales
are cycloid in shape, and in their imbricated arrangement, in their
enclosure in pouches of the dermis, and in the absence of any superficial
covering of ganoin, they are very similar to the scales of the more typical
Teleosts (Fig. 295). The resemblance extends even to histological
structure, for each scale consists of an outer layer of bone, which
gradually passes into an inner fibrous stratum.
[Illustration: FIG. 104.—Diagrammatic longitudinal section through the skin
of a Teleost to show the position of the scales. _d_, Dermis; _ep_,
epidermis; _s_, scale. (After Boas.)]
In Teleosts the usually thin and flexible scales are primarily developed
from dermic papillae, but subsequently they come to lie in pockets or
pouches in the dermis. As a rule no spines are developed, and so far no
trace of an enamel organ has been detected during their development. The
scales in their dermal pouches are disposed obliquely to the surface of the
body, so that the hinder part of one scale overlaps the anterior portion of
the scale next behind it (Fig. 104). Only the free hinder part of each
scale has an epidermic investment (Fig. 105). In minute structure each
scale consists of an outer layer of bone, which, like the bone of the
endoskeleton, may either be homogeneous except for a feeble lamination, or
it may contain bone-cells arranged in successive layers, parallel to the
surface of the scale. In addition, there is an inner fibrous stratum in
which the fibrous bundles in any one plane cross those in planes above or
below them. The scales are either "cycloid," that is, they have smooth,
unbroken margins (Fig. 105), or the free margin of each scale is produced
into a series of tooth-like spines, and the scale is said to be "ctenoid"
(Fig. 106, A).
Some Teleosts, however, have scales which are neither cycloid nor ctenoid,
and in certain features seem to be intermediate between ordinary Teleostean
scales and dermal denticles. Thus, on certain parts of the body of
_Centriscus_,[168] each scale consists {190}of a rhombic basal plate,
produced into a curved, backwardly-inclined spine, the axis of which
contains a pulp-cavity opening on the inner surface of the basal plate
(Fig. 107). Some Malthidae (e.g. _Malthe_[169]) have similar scales, but
with round basal plates and solid spines (Fig. 108, B). Similar scales
(Fig. 109), sometimes rhombic in shape, with one or more spines, which may
be simple or branched, are also found in the Sclerodermi (e.g. _Balistes_,
_Monacanthus_, _Triacanthus_).[170]
[Illustration: FIG. 105.—Cycloid scale of _Salmo fario_. _a_, Anterior
portion covered by overlap of preceding scales; _b_, free portion covered
only by pigmented epidermis. (From Parker and Haswell.)
FIG. 106.—A, Ctenoid scale; B, "Ganoid" scale. (After Günther; from Parker
and Haswell.)]
[Illustration: FIG. 107.—_Centriscus scolopax_. A, Scale from the orbital
region, × 50; B, scale from the base of the pectoral fin, × 100. (From
Hertwig.)]
Amongst some of the usually scaleless Siluroid Fishes the scales assume a
very peculiar structure. In _Hypostoma_[171] (_Plecostomus_) the sides and
back of the Fish are covered by large bony plates, but on the under surface
and on the fins these are replaced by much smaller ones. Both kinds,
however, carry numerous small movable spines implanted in sockets (Fig.
110), a fact which suggests comparison with a stage in the development of
the scales of _Lepidosteus_, when the independently formed and evanescent
spines have not yet fused with the basal plates.
In other Teleosts, as in the Agonidae and some Triglidae, the body is
completely cuirassed with large keeled bony plates. The singular appearance
of many of the Plectognathi is largely {191}the result of the curious
modifications which their scales undergo. In some of the Coffer-Fishes
(_Ostracion_) these structures assume the form of polygonal bony plates
which suturally articulate with one another and enclose the trunk in a
rigid cuirass, from which the scaleless tail protrudes behind (Fig. 438);
while in some Globe-Fishes and Porcupine-Fishes (e.g. _Tetrodon_, _Diodon_)
the prolongation of the scales into strong erectile spines equally well
serves the purpose of protection (Fig. 439).
Most Teleostomi have the scales along the "lateral line" perforated by
single or multiple apertures, through which the sensory canal communicates
with the exterior.
[Illustration: FIG. 108.—A, Scale of _Antennarius hispidus_, × 100; B,
scale of a young _Malthe vespertilio_, × 100. (After Hertwig.)]
In a few Teleosts scales are entirely absent, as in most Siluridae; or they
exist only as microscopic vestiges hidden in the skin, as in Eels; or, as
in such naked forms as _Antennarius marmoratus_ and _Lepadogaster_, and in
some Siluridae, they become reduced to mere papillae of the dermis.
[Illustration: FIG. 109.—A, Scale of _Balistes capriscus_, × 20; B, scale
of _Monacanthus scopas_, × 20. (After Hertwig.)]
The concentric rings observable (Fig. 105) on the surface of many
Teleostean scales are an index to the age of the {192}Fish.[172] The
formation of these rings depends on the fact that the lines of growth on
the surface of the scale are more widely separated from one another on that
portion of the scale formed during summer, and relatively closer together
on that part which is formed during the winter; the more rapid growth in
the warmer season probably being due to favourable conditions as to food
and temperature, and the retarded growth of the colder season to the
reverse. Hence, by counting the alternating zones of close-set winter lines
and less closely approximated summer lines of growth, a reliable clue may
be gained as to the age of the Fish.
In the Dipnoi,[173] as in Teleosts, the scales are enclosed in dermal
pockets, and exhibit a regular, imbricated disposition in oblique rows
(Fig. 304, A). In shape they are nearly cycloid, or slightly oval, with the
long axis coinciding with that of the body. Structurally, also, they bear
some resemblance to Teleostean scales, although differing in details. On
the outer surface of the scales there are numerous small conical spines. No
significance, other than as an example of evolutionary convergence, can be
attached to the resemblance between the scales of Fishes so widely
separated as the Dipnoi and the Teleosts.
[Illustration: FIG. 110.—_Hypostoma commersonii_. A scale from the
periphery of the caudal fin, × 50; one of the spines (_s_) is implanted in
its socket (_s′_). (From Hertwig.)]
All known fossil Dipnoi had scales of a similar character, although
differing greatly in size in different genera. In some (e.g. _Dipterus_) a
layer of enamel-like substance invests the exposed portions of the scales.
{193}CHAPTER VIII
THE SKELETON
All Fishes possess an internal skeleton which, in order that it may be
distinguished from the more superficial scaly exoskeleton described in
Chapter VII., is termed the endoskeleton. The latter consists (i.) of an
axial part, including the vertebral column and the skull; and (ii.) of an
appendicular portion, consisting of the skeleton of the limbs and their
supporting pectoral and pelvic girdles.
THE VERTEBRAL COLUMN.[174]—The individual segments or vertebrae which,
arranged in a linear series, collectively form the vertebral column, are
highly complex structures, each being formed by a number of vertebral
elements, the sum total of which constitutes a vertebra. Perhaps the best
conception of the nature of vertebral elements is to be gleaned from the
study of such primitive Fishes as the Elasmobranchs, in which not only are
all the vertebral components present, but they are less modified by
suppression and fusion than in most other Fishes, and on this account they
afford a convenient introduction to the study of the puzzling
eccentricities of vertebral structure in other groups. Selecting any common
Dog-Fish, such as _Scyllium canicula_, and starting with an early embryonic
stage, it may be stated that the first indication of a vertebral column is
the formation of the notochord, which, invested by its chordal sheath,
extends from the tip of the tail to a point on the under surface of the
brain just behind the hypophysis or pituitary body.
{194}[Illustration: FIG. 111.—A, side view of precaudal vertebrae of
_Scyllium canicula_; B, similar view of caudal vertebrae. _b.d_,
Basi-dorsal; _c_, centrum; _h_, basi-ventral; _h.s_, haemal spine; _i.d_,
inter-dorsal; _p_, parapophysis; _r_, rib; _s.d_, supra-dorsals. The
vertical dotted lines indicate the limits of neuromeres and myotomes. The
small circles represent the exits of the dorsal and ventral roots of spinal
nerves. (After Ridewood.)]
Subsequently, a number of cartilaginous pieces are developed in connexion
with the dorsal and ventral surfaces of the notochord, which, as they form
portions of a system of dorsal and ventral arches, are termed "arcualia"
(Fig. 111). On the dorsal side there are: (i.) a series of paired
_basi-dorsal_ cartilages (neurapophyses or neural arches), the two elements
of each pair contributing to form the side walls of the neural canal in
which the spinal cord is lodged (Fig. 112, A); (ii.) a series of
_inter-dorsal_ cartilages (intercalary neural arches), regularly
alternating with the preceding, and completing the walls of the neural
canal by filling up the intervals between the basi-dorsals; and (iii.) a
series of _supra-dorsal_ elements, typically also in pairs, but in the
Dog-Fish fused to form single median cartilages. Of the latter there are
two sets—one the _supra-basi-dorsals_, or neural spines, are situated over
the basi-dorsals; and the other, _supra-inter-dorsals_, alternating with
the former, lie over the inter-dorsals, the two series forming the
keystones of the dorsal arches, and thus completing the roof of the neural
canal. On the ventral side of the notochord this arrangement is
substantially repeated by a series of ventral arcualia, which, however, are
somewhat differently arranged in the trunk and tail. Thus, in the trunk
there are: (i.) a series of _basi-ventral_ or haemal cartilages,
corresponding with the basi-dorsals above, which grow out laterally into
short processes, the _parapophyses_ or transverse processes, and terminate
in (ii.) short, slender cartilages—the costal elements or _ribs_—which may
perhaps be regarded as the ventral equivalents of supra-basi-dorsals. The
ribs project outwards into the dorsal wall of the coelom and end in the
myocommata separating the myotomes of the body-wall. In the tail the
basi-ventrals lose their ribs and, growing downwards into ventral
prolongations, they unite in pairs beneath the caudal artery and vein, and
so form a series of inverted arches {195}(_haemal arches_) enclosing a
haemal canal (Fig. 112, B). The apex of each arch is prolonged into a
median process or _haemal spine_. Although not recognisable in the
Dog-Fish, paired _inter-ventral_ cartilages, corresponding with the
inter-dorsals above, are present in some Elasmobranchs and alternate with
the basi-ventrals. In the caudal region of others (_e.g._ Skates and Rays)
ventral counterparts of the supra-interdorsals are present, and are termed
_infra-ventral_ cartilages. Much in the same way that their dorsal
equivalents enclose a neural canal, so the ventral arcualia partially
surround the viscera-containing coelom in the trunk; and in the tail, but
more completely, the vestigial coelom of that region or the haemal canal.
[Illustration: FIG. 112.—A, transverse section of a precaudal vertebra; B,
similar section of a caudal vertebra. _h.a_, Haemal arch (basi-ventrals);
_h.c_, haemal canal; _h.s_, haemal spine; _n.c_, neural canal. Other
reference letters as in Fig. 111.]
The different vertebral components are by no means of equal morphological
value. The basi-dorsals and basi-ventrals, and the inter-dorsals and
inter-ventrals, are the primary elements and the most important. The
supra-dorsals are merely cartilages segmented off from the basi-dorsals and
inter-dorsals, while the ribs and the infra-ventrals are similarly derived
from the basi-ventrals and inter-ventrals respectively. As to the vertebral
elements which collectively form a vertebra in the Dog-Fish, it would seem
from evidence afforded by the neuromeres,[175] and more especially by the
facts of development, that each complete skeletal segment or vertebra
consists of a pair of basi-dorsals with the preceding pair of
inter-dorsals, and of a pair of basi-ventrals with the next succeeding pair
of inter-ventrals. It must be emphasised, however, that, considered as a
joint or segment in a flexible back-bone, a vertebra is a physiological
unit, the morphological value of which may differ widely in different
Fishes. Hence, in other Fishes, the grouping of vertebral components to
{196}form individual vertebrae may be quite different to that which takes
place in the Dog-Fish, and may even be accompanied by their more or less
complete fusion.
In the more primitive types of vertebral column, such as are characteristic
of many fossil and not a few existing Fishes, arcualia alone are present,
and remain associated with a persistent notochord which has grown with the
growth of the animal. In the more specialised Fishes, on the contrary, the
need of an axial support for the body, which, while retaining the necessary
flexibility, must possess greater strength, has resulted in the development
of a series of solid cartilaginous, calcified or bony, discoidal joints or
segments, the _centra_, which surround and more or less completely replace
the notochord, and, while supporting, form also a bond of connexion between
the dorsal and ventral arches. Notwithstanding their superficial
resemblance, an important developmental distinction is to be noted in the
mode of formation of centra in different Fishes, which enables one kind to
be distinguished as "_chorda-centra_," and another as "_arch-centra_."[176]
Chorda-centra are centra formed by the conversion of the chordal sheath
into a series of ring-like cartilaginous segments, which subsequently, by a
process of inward thickening, become biconcave, disc-like structures, and
more or less completely replace the notochord, except in the spaces between
them. Arch-centra, on the other hand, owe their formation to the growth of
the bases of the primary arcualia round the notochord, external to the
chordal sheath, and their subsequent fusion to form annular segments,
which, later, become biconcave centra. Of Fishes which possess vertebral
centra the Elasmobranchs alone have chorda-centra; the Holostei and the
Teleostei, and very probably the Crossopterygii also, having arch-centra.
The Dipnoi and the Holocephali, and the Chondrostean Teleostomi are
acentrous—that is, they are devoid of vertebral centra and possess a
persistent notochord. Neither in their embryonic development nor in their
evolution in time are the different vertebral components synchronous in
their appearance. Developmentally, the arcualia are the first to be formed,
and of these those on the dorsal aspect of the notochord appear earlier
than their representatives on the ventral side, while the centra are the
last of all; and in a general way the palaeontological sequence agrees with
the embryological.
{197}The independent evolution of a more specialised vertebral column from
a more primitive one may often be traced within the limits of the same
group of Fishes when the more ancient genera are compared with the more
recent. In the Elasmobranchs and the Crossopterygii, for example, the
oldest known types were acentrous, while the more recent have acquired
calcified or bony centra, and altogether they have reached a more advanced
stage of vertebral evolution. Some Fishes, like the Chondrostei and the
Dipnoi, seem, however, to exhibit comparatively little advance in vertebral
structure, since both the Palaeozoic and the living representatives of
these groups agree in being acentrous.
Some of the more notable features in the structure of the vertebral column
in the Cyclostomata and Fishes will now be briefly considered.
In the Cyclostomata the acentrous vertebral column is more primitive than
in any other Craniates, and in the Lamprey it consists of a persistent
notochord, supporting a series of isolated cartilages on each side of the
spinal cord.[177] As two pairs of these cartilages are included in each
neuromere it is possible that they represent alternating basi-dorsals and
inter-dorsals. There are no ventral arcualia in the trunk and no ribs. In
the Hag-Fish (_Myxine_) the dorsal cartilages are restricted to the tail.
The description of the vertebral column of the Dog-Fish may be taken as
fairly applicable to Elasmobranchs in general, and hence only certain
notable features in some other members of the group need be referred to
here. The most primitive Elasmobranchs, the Palaeozoic genera
_Cladoselache_ and _Pleuracanthus_ were acentrous, although calcified rings
have been observed in a Permian species of the latter genus and scattered
calcifications in others. Some of the earlier Mesozoic genera (e.g.
_Hybodus_) were also devoid of centra, at least in the trunk-region. The
first indication of complete centra occurs in the Lower Lias Cestraciont,
_Palaeospinax_.[178] All the later extinct, as well as all existing forms,
have more or less well-developed centra, hardened by the deposit of lime
salts in their primitively cartilaginous substance, but never in the form
of true bone.
{198}[Illustration: FIG. 113.—Schematic transverse section through the
middle of a Cyclospondylic (A), a Tectospondylic (B), and an
Asterospondylic vertebra (C). _d_, Middle portion of the calcified double
cone; _d′_, additional concentric calcified layers; _d″_, double cone with
radiating calcified layers; _ex.m_, external elastic membrane; _h.a_,
haemal arch; _n.a_, neural arch; _n.c_, notochordal cavity. (From Zittel,
after Hasse.)]
The mode in which the lime is deposited is marked by certain peculiarities
which are characteristic of particular families[179] (Fig. 113). In some
genera, as in the extinct _Palaeospinax_ and the living _Acanthias_ and
_Scymnus_, the calcified portion of each centrum takes the form of a
cylinder constricted across the middle, like two cones joined apex to apex
(cyclospondylic). This condition is probably the most primitive, but it may
be modified in other genera by the further addition of calcic salts in two
different ways. Thus, the deposit may take place by the simple addition of
concentric layers to the original constricted cylinder (tectospondylic), as
in the Skates and Rays; or it may take the form of a series of longitudinal
plates radiating outwards from the cylinder, and giving rise to a star-like
pattern in cross-section (asterospondylic), as in _Scyllium_ and _Lamna_.
In most living Elasmobranchs (e.g. _Scyllium_), but not in such genera as
_Notidanus_, _Heterodontus_, and _Squatina_, the bases of the dorsal and
ventral arches grow round the centra and meet, or even fuse, so that the
latter become surrounded by rings of cartilage which, after a fashion,
suggest incipient arch-centra (Fig. 112, A). The caudal portion of the
vertebral column is often described as "diplospondylic," that is, there are
two centra, two pairs of basi-dorsals, two pairs of inter-dorsals, and two
pairs of basi-ventrals, {199}or in other words, two vertebrae to each
neuromere[180] (Fig. 111, B).
The Holocephali have a vertebral column essentially similar to that of
other Elasmobranchs, but of a more primitive type (Fig. 114). The notochord
is persistent and there are no centra; but ring-like calcifications, four
or five to each neuromere, occur in the chordal sheath in _Chimaera_,
although not in _Callorhynchus_. Ribs are absent. In the whip-like terminal
portion of the tail the arcualia and the notochord become replaced by a
slender continuous filament of cartilage.
[Illustration: FIG. 114.—A, transverse section of the vertebral column of
_Chimaera monstrosa_; B, lateral view, _c.r_, Calcified ring; _h.r_,
basi-ventral; _int_, inter-dorsal; _n.a_, neural arch (basi-dorsal); _nch_,
notochord; _nch.sh_, chordal sheath; _n.sp_, neural spine (supra-dorsal).
(From Parker and Haswell, after Hasse.)]
In the more obvious features of vertebral structure the Dipnoi[181] have
much in common with the Elasmobranchs, especially with certain of the
acentrous Palaeozoic representatives of that group. The notochord is
persistent, centra are wanting, and the different vertebral components
continue to retain their primitive distinctness. On the other hand, the
basi-dorsals are much better developed than the inter-dorsals, which are
either vestigial or absent. The basi-dorsals unite in pairs over the spinal
cord to form complete neural arches, and each arch supports dorsally the
legs of a Λ-shaped, gable-like element or neural spine, which probably
represents a pair of fused supra-basidorsals. Ventrally, there are
basi-ventral cartilages, fused in pairs beneath the notochord, and
supporting well-developed, bone-ensheathed ribs. {200}Inter-ventrals appear
to be absent. Each neuromere corresponds with a pair of basi-ventrals, of
basi-dorsals and of inter-dorsals. The haemal arches and spines are formed
partly by the basi-ventrals, but mainly by the ventral union of the
successive pairs of ribs. As in the Holocephali, the terminal arcualia of
the tail become fused into a straight axial cartilaginous filament,
transversely divided into segments, which replaces the notochord. Each
segment supports a variable number of dorsal and ventral gable-pieces, or
neural and haemal spines. Certain of the vertebral components, such as the
ribs, and the neural and haemal spines, are ensheathed by membrane bone.
[Illustration: FIG. 115.—Side view of the precaudal vertebrae of a Sturgeon
(_Acipenser sturio_). _a.c_, Aortic canal, formed by the median union of
ingrowths from the basi-ventrals and inter-ventrals of opposite sides;
_b.d_, basi-dorsals; _b.v_, basi-ventral; _i.d_, inter-dorsal; _i.v_,
inter-ventral; _n_, notochord; _n.c_, neural canal; _n.sp_, neural spine;
_nt.s_, cuticular sheath of the notochord; _p_, parapophysis; _r_, rib;
_s.n_, aperture for the root of a spinal nerve.]
With certain modifications in details the preceding description will also
apply to the vertebral column of the Chondrostei (Fig. 115). It will be
noted, however, that the inter-dorsals are much better developed than in
the Dipnoi, although when {201}compared with the basi-dorsals they take but
a small share in forming the walls of the neural canal. Well-developed but
somewhat fragmentary inter-ventrals are present. The haemal arches and
spines are formed by the downgrowth and ventral union of the basi-ventrals
as in the Dog-Fish, and apparently without the aid of costal elements. In
_Polyodon_ the ribs are vestigial,[182] but in _Acipenser_ they are well
developed. The neural arches and spines, and their haemal representatives
in the tail, and also the ribs, are partially ossified, or ensheathed by
bone.
[Illustration: FIG. 116.—Diagram to illustrate the grouping of vertebral
elements to form vertebrae, A, in an Elasmobranch, B, in _Amia_, and C, in
_Lepidosteus_. _B.D_, Basi-dorsals; _B.V_, basi-ventrals; _I.D_,
inter-dorsals; _I.V_, inter-ventrals; _in.v.c_, inter-vertebral cartilage
divided by a concavo-convex cleft; _p.c_, precentrum; _pt.c_, postcentrum.
The square blocks represent individual vertebrae, and the oblique lines,
the attachments of the myocommata.]
In the existing Crossopterygii, Holostei, and Teleostei, popularly known as
the "bony Fishes," the vertebral column assumes a more familiar character,
and at the same time we meet with interesting illustrations of the
different methods by which the separate component vertebral elements of the
more primitive types of "backbone" are concentrated together in groups, and
fused to form that complex physiological product, the complete bony
vertebra.[183] In most of these Fishes each vertebra is formed by the
aggregation and fusion of a pair of basi-dorsals and a pair of
basi-ventrals, and includes, in addition, a pair of inter-dorsals, which
may either be the pair in front of the basi-dorsals or the pair behind, and
also a pair of inter-ventrals, which, similarly, may be the {202}pair in
front or behind the basi-ventrals (Fig. 116). The product of this fusion is
a series of bony vertebrae, each consisting of a biconcave arch-centrum,
which includes the fused basal portions of a pair of basi-dorsals and a
pair of basi-ventrals. The distal portions of the basi-dorsals form the
neural arch, while the rib-bearing parapophyses are lateral outgrowths from
the basi-ventrals which otherwise have become merged in the centrum.
Finally, the centrum is completed by its fusion with a pair of
inter-dorsals and a pair of inter-ventrals. Supra-dorsal elements may also
be included as minor contributory factors. The supra-basi-dorsals co-ossify
with their basi-dorsals and then unite to form the ordinary unpaired neural
spine of most bony Fishes, or, as in _Amia_, they remain distinct from each
other, and are obvious as a double spine. In _Lepidosteus_ these elements
co-ossify with the neural arches and form the post-zygapophyses.
Supra-inter-dorsals have been identified in the embryo as distinct
elements, but their eventual fate is not always known. In _Lepidosteus_
they persist as distinct cartilages in the adult (Fig. 118, A).
Well-developed bony ribs are usually present. The haemal arches of the tail
are formed by the downgrowth of the parapophyses and their ribs, or by the
latter alone, and by their ventral union to form haemal spines;
consequently, each arch always includes a pair of costal elements. With
such general features in common there are certain notable variations in
some of these Fishes, to which brief reference may be made.
Little is at present known of the development of the vertebral column in
either of the only two existing genera of Crossopterygii, _Polypterus_[184]
and _Calamichthys_, and hence the precise mode of grouping of their
vertebral components to form vertebrae is unknown. The condition of the
vertebral column in the fossil forms varies greatly in different families,
but in none is it so specialised as in the surviving members of the group.
In the Devonian Holoptychidae, and even in genera so comparatively recent
as the Upper Cretaceous Coelacanth _Macropoma_, the persistence of the
notochord and the absence of centra indicate a very primitive grade of
vertebral evolution. The Devonian and Carboniferous Rhizodontidae (e.g.
_Eusthenopteron_ and _Rhizodus_), on the contrary, seem to have had
well-ossified ring-like vertebrae.
{203}In the caudal region of _Amia_ the basi-dorsals and basi-ventrals, and
the inter-dorsals and inter-ventrals, form separate arch-centra which
remain distinct; hence each vertebra is double, and there is a regular
alternation of arch-bearing "pre-centra" and arch-less "post-centra" (Fig.
117, D). In the trunk-region the pre- and post-centra have fused, and in
this region the vertebrae are single.
[Illustration: FIG. 117.—A, precaudal vertebrae of _Caturus furcatus_; B,
similar vertebrae of _Eurycormus speciosus_; C, caudal vertebrae of the
latter species; D, caudal vertebrae of _Amia calva_. _h.a_, Haemal arch;
_h.sp_, haemal spine; _hy.c_, hypo-centrum; _n.a_, neural arch; _n.sp_,
neural spine; _p_, parapophysis; _p.c_, pre-centrum; _pl.c_,
pleuro-centrum; _pt.c_, post-centrum; _r_, rib. (After Zittel.)]
A very primitive type of vertebral column occurs in some of the Jurassic
allies of _Amia_, in which certain of the vertebral components, confluent
in the adult _Amia_, retain some measure of their primitive
distinctness.[185] Thus, in the precaudal region of _Eurycormus_ (Fig. 117,
B) there is a series of alternating dorsal and ventral half-rings of bone,
which between them completely invest the persistent notochord. Each ventral
half-ring or "hypocentrum" represents a pair of fused and ossified
basi-ventrals, and possibly also a pair of included inter-ventrals, and
supports dorsally a pair of basi-dorsals, forming a neural arch, and
laterally a pair of ribs. The dorsal semi-rings, or "pleuro-centra,"
similarly represent fused and ossified pairs of inter-dorsals. In the tail,
modifications approximating to what is seen in the caudal region of _Amia_
are to be noticed (C). By the upgrowth of the ventral arch-bearing
semi-rings, and their conversion into complete rings encircling the
notochord, incipient pre-centra are formed, and by a similar modification
of the {204}down-growing, archless, dorsal half-rings, structures
comparable to post-centra are produced. In brief, _Eurycormus_, as well as
such other extinct Amioid genera as _Caturus_ (Fig. 117, A), _Callopterus_,
and _Euthynotus_, retain in the adult a stage of vertebral evolution which
is closely paralleled by transitory stages in the embryonic and young forms
of _Amia_.
[Illustration: FIG. 118.—A, two vertebrae from the trunk-region of
_Lepidosteus_; B, anterior face of a vertebra. _c.n_, Anterior convex face
of the centrum; _c.n′_, posterior concave face; _h.a_, parapophysis, with
its articular facet for a rib; _i.c_, median cartilage, representing a pair
of fused supra-interdorsals; _i.s_, radial element of the dorsal fin;
_l.l_, superior longitudinal ligament; _n.a_, neural arch. (From
Wiedersheim, after Balfour and W. N. Parker.)]
_Lepidosteus_[186] is unique amongst existing Fishes in having
opisthocoelous vertebrae; that is, the centra are convex in front and
concave behind, and therefore articulate with one another by
ball-and-socket joints (Fig. 118). This condition is due to the development
of a series of intervertebral rings of cartilage round the notochord. The
subsequent inward growth of each of these rings leads to the constriction,
and ultimately to the complete obliteration, of the notochord, much in the
same way as by the growth of ordinary centra. Later, this solid mass of
cartilage becomes transversely divided by a cleft which is convex
anteriorly and concave behind (Fig. 116, C), and of the two portions one
fuses and co-ossifies with the centrum of the vertebra in front, and the
other with the one pertaining to the vertebra behind. Reference to Fig. 116
will show that the grouping of the vertebral elements to form the
individual vertebrae is not the same as in _Amia_.
{205}In the dominant group of existing Fishes, the Teleostei, the centra
are almost invariably biconcave, although in the Eels they may be flat or
even slightly convex in front. Ribs are absent in the Syngnathidae and in
the Plectognathi. In addition to the usual articulation between the centra,
the vertebrae often articulate with one another by means of paired
processes arising from the anterior margin of each neural arch, or from the
centrum at the base of the arch (pre-zygapophyses), and meeting similar
processes which project either from the hinder margin of the arch of the
vertebra in front, or from the adjacent portion of its centrum
(post-zygapophyses). The haemal arches may have similar processes (Fig.
119). One, two, or in some Teleosts, three pairs of slender intermuscular
bones radiate outwards from the centra into the myocommata (epicentrals),
or from the neural arch (epineurals), or from the ribs (epipleurals).
[Illustration: FIG. 119.—A, side view of precaudal vertebrae of a Cod
(_Gadus morrhua_) without the ribs; B, similar view of caudal vertebrae of
the same Teleost. _c_, Centrum; _h.a_, haemal arch; _h.sp_, haemal spine;
_n.a_, neural arch; _n.sp_, neural spine; _p_, parapophysis; _p.z_,
pre-zygapophysis; _pt.z_, post-zygapophysis.]
THE RIBS.—It is doubtful if the structures termed "ribs" are homologous in
the different groups of Fishes. There appear to be two kinds,
distinguishable as dorsal and ventral ribs (Fig. 156). Dorsal ribs are
situated in the fibrous tissue separating the epiaxial from the hypaxial
muscles of the body wall, and they take no part in forming the haemal
arches of the caudal region. Ventral ribs, on the other hand, always lie
internal to the hypaxial muscles, and directly external to the peritoneal
lining of the coelom, and they usually contribute to the formation of
{206}the haemal arches. To the former belong the ribs of the Elasmobranchs,
and to the latter the ribs of the Teleostomi and Dipnoi. _Polypterus_ alone
has both kinds of ribs.
THE SKULL.
The skull is a highly complex structure, the various components of which
are as different physiologically as they are morphologically. It consists
(i.) of the _cranium_, for the enclosure and protection of the brain; (ii.)
of _sense capsules_, which fulfil a like function for the auditory, visual,
and olfactory organs; (iii.) of certain _vertebrae_ or vertebral elements
fused with the hinder part of the cranium; (iv.) of a series of _visceral
arches_; and (v.) of a series of paired or median cartilages developed in
relation with the mouth and nostrils, which may be collectively spoken of
as "_labial_" cartilages.
The cranium is formed in the embryo from two pairs of cartilaginous rods or
plates, developed in the mesoblast of the head. Of these the posterior
pair, or _parachordals_, underlie the hinder part of the brain, and are
situated one on each side of the cranial portion of the notochord. The
anterior pair or _trabeculae_ are pre-notochordal, and lie beneath the
anterior portion of the brain.[187] Between their hinder extremities, and
in front of the anterior termination of the notochord, is the pituitary
body. As development proceeds the parachordals blend with each other and
with the trabeculae, while the latter fuse in front to form a median
plate—the mesethmoid cartilage. The hinder portions of the two trabeculae
remain distinct for some time, and enclose between them the pituitary
fontanelle, but later they fuse beneath the pituitary body, leaving,
however, a pit for its reception—the pituitary fossa. Cartilaginous
capsules are formed round the cranial sense organs. The _auditory or
periotic capsules_ fuse on each side with the parachordals. The _optic
capsules_, either fibrous or cartilaginous, remain free, and do not fuse
with the adjacent trabecular region. The olfactory capsules alone are not
developed independently, but are formed as lateral {207}outgrowths from the
mesethmoid plate. Later, the parachordals and trabeculae grow upwards on
each side round the brain, and to a greater or less extent they meet and
fuse on its dorsal surface, thus enclosing the latter organ in a cranial
cavity, leaving, nevertheless, a large foramen behind (foramen magnum)
through which the brain is continuous with the spinal cord. In this
condition the primitive cartilaginous cranium, with its included
sense-capsules, has reached a stage which is permanently retained in such
Fishes as the Elasmobranchs.
The _visceral arches_ consist of a number of pairs of curved rods of
cartilage, at first simple, but subsequently segmented, and developed in
the splanchnic mesoblastic walls of the oral cavity and pharynx. Each rod
is connected with its fellow by a median cartilage in the floor of the
pharynx, so that the whole form a series of dorsally incomplete hoops
encircling the anterior portion of the alimentary canal. No doubt all the
visceral arches were originally branchial arches, and were so disposed
between the successive gill-clefts as to support their walls and the
vascular folds or gill-lamellae to which they gave rise. In Fishes most of
the arches still retain their primitive gill-supporting function, but the
first or mandibular arch has become modified to form upper and lower jaws,
although in the Sharks and Dog-Fishes it may lie in front of a gill-cleft
and still be associated with vestigial gills. The second or hyoid arch is
less removed from the condition of a branchial arch, and generally supports
either a functional or a vestigial gill, but in most Fishes it has acquired
the secondary function of forming a suspensorium for the attachment of the
jaws to the cranium.
The skull of the common Dog-Fish, _Scyllium canicula_ (Fig. 120),[188] may
be studied as a type which in the adult remains cartilaginous, and has no
secondary addition of cartilage- or membrane-bones. In this Fish the
chondrocranium, or primary cartilaginous cranium, presents the appearance
of a somewhat depressed oblong box, which has a complete roof, side-walls,
and floor, but is open in front (_anterior cranial fontanelle_) and also
behind (_foramen magnum_). The hinder, or parachordal portion of the
cranium surrounds the foramen magnum, and there forms the _occipital
region_. At the ventral margin of the foramen there are two prominences, or
_occipital condyles_, for articulation with the first vertebra, and between
them the remains of the notochord are traceable into the cranial floor.
{208}[Illustration: FIG. 120.—Side view of the skull of the common Dog-Fish
(_Scyllium canicula_). _aud.cp_, Auditory capsule; _br.a_ 1, 5, branchial
arches; _br.r_, _br.r′_, cartilaginous rays attached to the hyoid arch and
the first four branchial arches; _Cr_, cranium; _ex.br_, extra-branchial
cartilages; _hy.cn_, cerato-hyal; _hy.m_, hyomandibular; _lb_, labial
cartilages; _lg_, ligaments passing from the jaws to the cranium and to the
distal end of the hyomandibular; _lg′_, ethmo-palatine ligament; _l.j_,
lower jaw or Meckel's cartilage; _Nv_. 2, optic foramen; _Nv_. 5, foramen
for the Vth and part of the VIIth cranial nerves; _olf.cp_, olfactory
capsule; _or_, orbit; _up.j_, upper jaw or palato-quadrate cartilage. (From
Wiedersheim, after W. K. Parker.)]
In front of the occipital region two lateral bulgings indicate the
_periotic capsules_, and more anteriorly still, in the trabecular region,
the sides of the cranium are modified to form two spacious lateral
recesses, the _orbits_, each of which is bounded above and below by
supra-orbital and infra-orbital ridges respectively, behind by an outgrowth
from the periotic capsule (_post-orbital process_), and in front by a
similar projection from the hinder wall of the olfactory capsule (_lateral
ethmoidal process_). In front of the cranial cavity and the orbits may be
seen the laterally-placed dome-like _olfactory capsules_, which are open
below, where the nasal sacs communicate with the exterior. Between the two
capsules an anterior extension of the cranial floor forms a flattened
_mesethmoidal plate_, behind which is the large, membrane-closed, _anterior
cranial fontanelle_. The lateral walls of the cranium are perforated by
numerous apertures, some of which serve for the entrance or exit of
blood-vessels, and others, mostly pertaining to the inner walls of the
orbits, for the transmission of the different cranial nerves from the brain
to {209}various parts of the head. In many Elasmobranchs the roots of
certain of the anterior spinal nerves perforate the side-walls of the
occipital region, and indicate the fusion of vertebral components with the
cranium. In the cranial roof between the two periotic capsules there are
two small apertures at the bottom of a common median depression: through
each aperture the ductus endolymphaticus (_aqueductus vestibuli_) passes
from the vestibular part of the auditory organ to the exterior of the
skull.
Three cartilaginous rods, one from the roof of each olfactory capsule, and
one, the _prenasal_ or _rostral_ process, from the ethmoid cartilage,
converge and meet, or nearly meet, in front to form the rostrum or support
for the preoral or "cut-water" portion of the head.
The visceral arches are seven in number. The first or _mandibular arch_
consists on each side of an upper portion, the _palato-pterygo-quadrate_ or
_palato-quadrate_ cartilage, which passes forwards in the side-wall of the
oral cavity, along the upper margin of the mouth, its anterior or palatine
part curving inwards to a ligamentous connexion with its fellow beneath the
cranial floor. Each cartilage has an upwardly directed process
(_ethmo-palatine process_) which is connected by a suspensory
_ethmo-palatine ligament_ with the lateral wall of the cranium behind the
lateral ethmoid process. The lower or ventral half of the mandibular arch
(_Meckel's cartilage_) is similar in shape to the upper; it articulates
behind with the quadrate portion of the latter by a movable joint, and is
thence prolonged forwards and downwards in relation with the lower margin
of the mouth to a median ligamentous union with its fellow of the opposite
side. The palato-pterygo-quadrate and Meckel's cartilages together form the
primitive upper and lower jaws, and support the teeth. The _hyoid arch_
also consists of a dorsal and a ventral half on each side. The dorsal half
or _hyomandibular_ element articulates above with the periotic capsule. The
ventral portion, or _cerato-hyal_, passes downwards and is connected with
its fellow by a median copula or _basi-hyal_ cartilage situated in the
floor of the oral cavity. A series of simple cartilaginous rays (_branchial
rays_) are attached to the hinder margins of the hyomandibular and
cerato-hyal elements. The distal end of the hyomandibular is connected by
strong ligaments with the hinder portions of both the
palato-pterygo-quadrate cartilage and Meckel's cartilage; in fact, {210}the
hyomandibular is the effective suspensorium by which the upper and lower
jaws are connected with the skull, and all Fishes in which this arrangement
exists are said to be _hyostylic_.[189] Behind the hyoid arch follow five
_branchial arches_. Each of these is segmented into a dorsal or
_pharyngo-branchial_ element, followed by an _epi-_, a _cerato-_, and a
_hypo-branchial_ piece, but the later element is absent in the fifth arch.
The lateral halves of the last three arches are connected ventrally by a
large median basi-branchial cartilage, but in the first and second arches
by the median apposition of their respective hypo-branchial elements. Like
the hyomandibular and cerato-hyal segments of the hyoid arch, the epi- and
cerato-branchial elements of all the branchial arches except the fifth are
fringed along their outer convex margins by a series of _branchial rays_,
and, in addition, there are three pairs of slender, curved, cartilaginous
rods, or _extra-branchials_, in relation with the distal extremities of the
branchial rays of the second, third, and fourth branchial arches. The
function of the branchial arches, and their branchial rays, and
extra-branchial cartilages, is to support the inter-branchial septa which
separate the gill-clefts and carry the vascular gill lamellae. All the
arches lie near the inner margins of the septa, close to the hypoblastic
epithelium of the pharynx, while the outer portions of the septa are
supported by the branchial rays and the extra-branchials, the latter lying
directly beneath the external skin. The segments of the arches are movably
connected with one another by ligaments; and by the contraction of the
branchial muscles the arches may be separated or approximated so as to
enlarge or diminish the size of the intervening clefts.
The _labial cartilages_ are represented by a pair of slender rods in
relation with the outer surfaces of the palato-pterygo-quadrate cartilages,
and a similar pair in connexion with the Meckelian cartilages. There is
also a pair of small cartilages in relation with the nostrils. It is
probable that the rods which constitute the lateral elements of the rostrum
belong to the same category.
In the Cyclostomes and the Elasmobranchs the skull is entirely
cartilaginous, although it may often be superficially calcified in
Elasmobranchs, and although there may even be definitely and symmetrically
arranged calcified plates in _Pleuracanthus_, true bone is never present.
In many Fishes, and notably in the Teleostomi, {211}the embryonic
cartilaginous cranium becomes complicated by the addition of an extensive
series of investing membrane bones, formed by the ossification of the
connective tissue external to the cartilage, so that a secondary bony
cranium is formed external to the primary cranium much in the same way that
a secondary pectoral girdle is formed in connexion with the primary girdle.
Such bones probably owe their primary origin to the fusion and insinking of
exoskeletal structures (scales or dermal spines). To these investing bones
there may also be added a series of bones formed by the actual conversion
of the cranial cartilage into osseous tissue (cartilage bones), which to a
greater or less extent in different Fishes replaces the original cartilage.
The bones of the skull may conveniently be classified as follows:—(i.)
_Dermal or membrane bones_. Under this head are included—(_a_) the ordinary
investing bones of the skull. (_b_) _Tooth-bones_, that is, bones formed by
the fusion of the bases of teeth and developed in relation with the walls
of the oral cavity. Probably all tooth-bearing bones are of this nature.
(_c_) _Sensory canal bones_, that is, tubular bones developed round the
sensory canals of the head. Certain of these bones may secondarily acquire
the shape and character of investing bones while still retaining protective
relations to their sensory canals. (ii.) _Cartilage bones_.
As an easily obtainable example of a skull which has acquired a fairly
complete series of both cartilage- and membrane-bones, while retaining a
well-developed primary cranium, the skull of the Salmon (_Salmo salar_) may
be described.[190] At an early stage of development, even so late as the
second week of hatching, the primary cranium is still entirely
cartilaginous, and in this condition the Salmon's skull is comparable with
that of an adult Dog-Fish. As development proceeds the primary cranium
becomes supplemented by the addition of numerous investing dermal bones
which form the secondary cranium, and later cartilage bones appear and, to
a considerable extent, replace the original cartilage. The Salmon's skull
is interesting in this respect, that the primary cranium grows with the
growth of the Fish, so that in the adult the nasal, ethmoidal, and prenasal
regions are entirely cartilaginous, and in the hinder part of the cranium
cartilage is largely persistent between the cartilage bones.
Dealing first with the cartilage bones of the primary cranium, {212}it may
be stated that there are formed in that part of the parachordal cartilage
surrounding the foramen magnum a median _basioccipital_ below, which is
concave behind where it articulates with the centrum of the first vertebra,
a _supraoccipital_ above, and two laterally-placed _exoccipital_ bones
(Figs. 121, 122). Each periotic capsule is ossified by the formation of
five bones in the primitively cartilaginous mass, the _prootic_,
_sphenotic_, _opisthotic_, _epiotic_, and the _pterotic_. The inner walls
of the capsules have atrophied in the adult, and hence the cavities which
contain the auditory organs appear as open lateral recesses of the cranial
cavity. In front of the periotic capsules there are various bones which are
formed in the cartilage of the trabecular part of the cranium. Thus, in
front of the basi-occipital, and developed in the cartilage of the cranial
floor, there is a median Y-shaped _basisphenoid_, and, at some distance
above it on each side, an _alisphenoid_ helps to form the lateral wall of
the cranial cavity. Between the eyes the side walls of the cranium fuse to
form a vertical inter-orbital septum, and, in consequence, two
_orbito-sphenoid_ bones, which normally form the lateral cranial walls in
this region, become partially confluent in the median line and close the
cranial cavity in front. The only cartilage bones found in the massive
persistent portion of the primary cranium which forms the pre-orbital
region are the projecting _lateral ethmoids_, forming the posterior
boundaries of the recesses for the olfactory organs, and separating the
latter from the orbits.
[Illustration: FIG. 121.—Side view of the cranium of a Salmon (_Salmo
salar_). Most of the membrane bones and the jaws have been removed. The
cartilage is dotted. _al.s_, Alisphenoid; _bo_, basioccipital; _bs_,
basisphenoid; _eo_, exoccipital; _ep_, epiotic; _l.eth_, lateral ethmoid;
_ol_, olfactory capsula; _op_, opisthotic; _o.s_, orbito-sphenoid; _pr.o_,
prootic; _ps_, parasphenoid; _pt.o_, pterotic; _so_, supraoccipital;
_sp.o_, sphenotic; _t.c_, trabecular cornu; _u.l.c_, _u.l.c_^2, first and
second upper labial cartilages; _v_, vomer; II, foramen for the optic
nerve. (From W. K. Parker.)]
The roof and floor of the primary cranium is completed by {213}certain
investing dermal bones (Fig. 123, A). A pair of large _frontal_ bones form
the cranial roof, and also help to roof in the orbital cavities. Behind the
frontals, and separated from each other by the supraoccipital, there is a
pair of small _parietals_, and anterior to the frontals a median _dermal
mesethmoid_. A small _nasal_ bone overlies each olfactory recess.
Ventrally, the base of the cranium, from the basi-occipital to the prenasal
region, is strengthened by a large _parasphenoid_ behind, and a much
smaller _vomer_ in front, both of which lie in the roof of the mouth. The
vomer is a tooth-bone, and probably the parasphenoid also.
[Illustration: FIG. 122.—Vertical and longitudinal section of the cranium
of _Salmo salar_, showing the right half of the cranial cavity. Cartilage
is dotted. _f_, Frontal; _v′_, fat-containing cavity in the mesethmoid
cartilage; V, VII, IX, X, foramina for the fifth, seventh, ninth, and tenth
cranial nerves. Remaining reference-letters as in Fig. 121. (From W. K.
Parker.)]
The mandibular arch (Fig. 123, B) is more modified than that of the
Dog-Fish. The palato-pterygo-quadrate bars, or primitive upper jaw, no
longer meet in front beneath the cranial floor, but each separately
articulates in front with the lateral ethmoid of its side. Although still
partly cartilaginous each bar is largely replaced either by cartilage
bones, or by bones which begin as membrane bones or as tooth-bones and
complete their growth by invading the cartilage and becoming in part
cartilage bones. Its anterior portion is formed by a _palatine_ bone which
articulates with the lateral ethmoid, and the middle portion by a
_pterygoid_ and a _mesopterygoid_ bone, while the hinder part is ossified
above as a _metapterygoid_ and below as a _quadrate_. The latter
articulates with the lower jaw. Functionally, however, the primitive upper
jaw is now replaced by a secondary upper jaw, formed on each side by a
series of tooth-bones, situated external to the former, and meeting in
front of the prenasal region of the primary cranium (Fig. 123, A). The
series includes a dentigerous _premaxilla_ and _maxilla_, and a small
toothless, scale-like _jugal_ bone. Each half of the lower jaw (Fig. 123,
A, B) consists of a rod-like Meckel's cartilage or primary lower jaw.
{214}[Illustration: FIG. 123.—A, view of the left side of the skull of a
Salmon; B, the left half of the primary upper and lower jaws, and the hyoid
arch. The cartilage is dotted. _an_, Angular; _ar_, articular; _b.hy_,
basi-hyal; _br.r_, branchiostegal rays; _c_, cranium; _c.h_, cerato-hyal;
_c.or_, circum-orbital bones; _d_, dentary; _d.eth_, dermal mesethmoid;
_ep.h_, epihyal; _ep.o_, epiotic; _eth.p_, ethmo-palatine process; _f_,
frontal; _h.hy_, hypo-hyal; _hym_, hyomandibular; _i.op_, inter-operculum;
_j_, jugal; _mks_, Meckel's cartilage; _mpg_, mesopterygoid; _mt.pg_,
metapterygoid; _mx_, maxilla; _n_, nasal; _op_, operculum; _op′_, condyle
on the hyomandibular for the operculum; _orb_, orbit; _p_, parietal; _pa_,
palatine; _p.mx_, premaxilla; _p.op_, pre-operculum; _pt_, pterygoid;
_pt.o_, pterotic; _q_, quadrate; _so_, supra-occipital; _s.op_,
suboperculum; _sp.o_, sphenotic; _s.t_, supra-temporal (or squamosal);
_st.hy_, stylo-hyal; _sy_, symplectic; _u.l.c_, _u.l.c′_, upper labial
cartilages; _u.l.c^2_, second upper labial. (From W. K. Parker.)]
The hinder part of this is ossified to form the _articular_, which has a
deeply concave surface for articulation with the quadrate; and below this
there is a small membrane bone, the _angular_. The rest of the cartilage is
partially {215}ensheathed on its outer side by a large tooth-bone, the
dentigerous _dentary_. The hyoid arch is similar to that of the Dog-Fish,
except that its primitively cartilaginous segments are almost completely
ossified (Fig. 123, B). The large upper segment or _hyomandibular_ bone
articulates mainly with the pterotic, but partly also with the sphenotic
element of the periotic capsule; below it is connected with a slender
_symplectic_ bone, and from the cartilage connecting the two depends the
rest of the hyoid arch, consisting in succession of _stylo-hyal_,
_epi-hyal_, _cerato-hyal_, and _hypo-hyal_ bones, with a median
teeth-bearing _basi-hyal_. The palato-pterygo-quadrate bar has no direct
connexion with the skull, except anteriorly where its palatine element
articulates with the lateral ethmoid. The real suspensorium is formed by
the hyomandibular and symplectic bones, to which the hinder margins of the
quadrate and metapterygoid bones are rigidly attached by suture, hence, as
in the Dog-Fish, the skull is hyostylic. Behind the hyoid arch there are
five branchial arches, which generally resemble those of the Dog-Fish,
except that their component segments are ossified as cartilage bones.
Connected with the hyomandibular and cerato-hyal elements of the hyoid arch
there is, on each side, a series of membrane bones for the support of the
movable operculum or gill-cover. These consist of an _operculum_ above,
which articulates with a backwardly projecting process from the
hyomandibular, followed in succession below by a _sub-operculum_ and an
_inter-operculum_, the latter being connected by ligament with the angle of
the lower jaw. The series is completed by ten sabre-shaped _branchio-stegal
rays_, which are attached to the cerato-hyal and support the lower margin
of the gill-cover.
Sensory canal bones are represented in the Salmon by a ring of small bony
plates which encircle the orbit (Fig. 123, A), and by one or two small
bones situated above and on the outer side of each periotic capsule
(_squamosals_). To these may be added the _pre-operculum_ situated external
to the hinder margins of the hyomandibular and quadrate bones, firmly
clamping these bones together, and also the _post-temporals_, by which the
secondary pectoral girdle is attached to the skull. The _nasal_ bones may
also be regarded as pertaining to the same series.
In other Fishes with a more or less complete bony skull there are certain
additional cartilage- and membrane-bones which are not {216}present in the
Salmon. There is usually a median ossification of the ethmoid cartilage,
the _mesethmoid_. An _entopterygoid_ is sometimes added to the
palato-pterygo-quadrate series of bones. An ossification of the anterior
extremity of each Meckelian cartilage may form a _mento-Meckelian_ bone.
Certain additional membrane bones are sometimes developed in relation with
the lower jaw, such as _splenial_ and _coronary_ bones on the inner side,
and a _supra-angular_ bone at the angle of the jaw, above the angular
element. To these there may be added the singular series of
_infra-dentaries_, which in some fossil Crossopterygii (e.g. _Rhizodopsis_)
fringe the outer margin of the jaw beneath the true dentary (Fig. 274, A).
A system of _jugular plates_ may also form a characteristic armature for
the throat between the lateral halves of the lower jaw (Fig. 274, C).
Besides those already mentioned, additional sensory canal bones are present
in some Fishes. A transverse row of plates (_supra-temporals_) sometimes
crosses the occipital region behind the parietals. There are also other
canal-ossicles which lose their identity by fusing with certain cranial or
periotic bones. Thus, each of the pterotic and sphenotic bones often
includes a superficial dermal bone transmitting a section of a sensory
canal, which has fused with it; and as the frontal bone is often similarly
perforated, it may be taken that it also includes a canal-ossicle; and the
same can often be said of the articular and dentary bones of the lower
jaw.[191]
Having now considered the general structure of a primitive cartilaginous
type of skull, and the nature, disposition, and terminology of the various
membrane- and cartilage-bones which may be added to, or more or less
completely replace the former, reference will now be made to the more
important features in the structure of the skull in the Cyclostomata and
the Fishes.
In the Cyclostomata the skull presents a remarkable combination of
characters, in some of which it is more primitive than in any other
Craniates, while in others it has evidently attained a very high degree of
specialisation on lines peculiar to the group, but differing in the two
subdivisions.
{217}[Illustration: FIG. 124.—Skull, with branchial basket and anterior
part of the vertebral column, of _Petromyzon marinus_. _a.d.c_, Anterior
dorsal cartilage; _a.lat.c_, anterior lateral cartilage; _an.c_, annular
cartilage; _au.c_, auditory capsule; _br.b.1-9_, vertical bars of the
branchial basket; _br.cl.1-7_, external branchial clefts; _cn.c_, cornual
cartilage; _cr.r_, cranial roof; _l.c.1-4_, longitudinal bars of branchial
basket; _lg.c_, lingual cartilage; _m.v.c_, median ventral cartilage;
_n.a_, neural arches; _na.ap_, nasal aperture; _n.ch_, notochord; _Nv^2_,
foramen for optic nerve; _olf.c_, olfactory capsule; _pc.c_, cartilage
surrounding pericardial cavity; _p.d.c_, posterior dorsal cartilage;
_p.lat.c_, posterior lateral cartilage; _sb.oc.a_, subocular arch; _st.p_,
styloid process; _sty.c_, styliform cartilage; _t_, teeth. (From Parker and
Haswell, after W. K. Parker.)]
In the Lamprey[192] (Fig. 124) the paired parachordals and trabeculae
together form a trough-like chondrocranium, which has only a fibrous roof,
except where a slender _synotic_ band of cartilage extends between the two
periotic capsules. The floor is also incomplete, a large pituitary
fontanelle remaining to indicate the original separation of the trabeculae
while transmitting the hypophysial or pituitary caecum. The notochord
traverses the floor of the parachordal portion of the cranium as far as the
pituitary fontanelle, and from the sides of the synotic ring the auditory
capsules project in the shape of conspicuous lateral prominences. In front
the otherwise open end of the cranial cavity is closed by the
dorsally-placed and unpaired olfactory capsule, which is perforated behind
by two apertures for the olfactory nerves, and has only a fibrous connexion
with the cranial walls. Anteriorly to the olfactory capsule the cranial
floor is prolonged forwards over the roof of the mouth as a large
laterally-expanded plate, formed by the united anterior portions of the
trabeculae, and no doubt representing the _mesethmoid_ cartilage of the
Dog-Fish. So far the cranium presents no special difficulty, and in its
general features may be readily compared with that of an embryonic
Elasmobranch. As for the rest of the skull, it is obvious that it has been
greatly modified, partly to form and to support the skeletal framework of
the remarkable suctorial buccal funnel, and partly to form the singular
rasping lingual apparatus. Hence it is always difficult and sometimes
impossible to identify with {218}certainty the component parts as being
represented in other Craniates. On each side of the cranium, beneath the
eye, there is a characteristic V-shaped _subocular arch_. Of its two legs
the hinder one is continuous above with the periotic region of the cranium,
and the other with the anterior trabecular region, while the pointed apex
is directed obliquely downward and forward. From the hinder margin of the
posterior limb a slender _styloid process_ passes downward in the side wall
of the pharynx, and terminates below in a forwardly directed _cornual_
cartilage. A velum, fringed along its free margin with a series of
tentacles, projects forwards into the oral cavity from between the oral
apertures of the oesophagus and the branchial canal, and probably serves to
prevent the entrance of foreign particles to the gill-sacs. This valve-like
velum is supported by a _velar skeleton_, consisting of two lateral
cartilages which are prolonged into the tentacles, and extend transversely
between the inner surfaces of the two styloid processes. The apex of each
subocular arch is connected with a small and somewhat triangular cartilage
(_postero-lateral cartilage_), which is directed upward and forward, and
lies in the side wall of the oral cavity. With some degree of probability
the subocular arch may be compared to the palato-quadrate cartilage of a
skull which has become "autostylic" in order to form a rigid support for
the skeleton of the buccal funnel; the styloid processes and cornual
cartilages to the hyoid arch; while the relations of the posterior lateral
cartilages to the subocular arches suggest that they may possibly be
regarded as Meckelian cartilages which have lost their primitive function
of forming biting jaws. In the median line below, and projecting backward
for some distance beneath the branchial canal, there is a long and stout
_lingual_ cartilage, carrying a small median and a still smaller pair of
lateral cartilages at its anterior extremity, where it supports the lingual
teeth and projects into the buccal funnel beneath the mouth. In front of
the lingual cartilage, and connected by fibrous tissue with the inferior
and hinder margin of the _annular_ cartilage, there is a median T-shaped
element, the _median ventral_ cartilage. It has been conjectured that the
lingual cartilage is a free basi-hyal element, and the median ventral
cartilage the equivalent, elsewhere unknown, of the corresponding element
of the mandibular arch.[193]
{219}The remaining anterior skull elements are principally skeletal
supports for the roof and walls of the buccal funnel. The roof is supported
by an extended _anterior dorsal_ cartilage, which is overlapped behind by
the ethmoid cartilage, while the circular margin of the funnel is
strengthened by a large ring-like _annular_ cartilage. On each side of the
latter there is a slender, rod-like, _styloid_ cartilage, and above the
latter a small _anterior lateral_ cartilage. All these cartilages are
usually termed labial cartilages, and it is at least possible that they
possess representatives in the similarly named structures of the Dog-Fish
and the larvae of some of the tailless Amphibia. It must not be forgotten,
however, that the annular cartilage bears some resemblance to the ring of
cartilage which encircles the lips of the buccal cavity in Amphioxus.
The complex supporting skeleton of the gill-sacs forms a basket-like
structure. It consists on each side of nine unsegmented, irregularly
curved, and slightly branched cartilaginous rods, situated in the outer
margins of the inter-branchial septa, directly internal to the skin. The
first lies directly behind the styloid process (hyoid arch), the second and
third in front of and behind the first gill-sac, and of the remainder one
lies just behind each of the six succeeding gill-openings; above and below
each gill-aperture the rods are connected by longitudinal bars, and also in
the median ventral line by a pair of similar partially united bars. The
dorsal ends of the rods are also connected on each side by another
longitudinal bar, which runs alongside the notochord and in front blends
with the chondrocranium. The rods forming the last pair are continuous with
a cup-like cartilage, supporting the lateral and hinder walls of the
pericardium.
This singular branchial basket undoubtedly bears a superficial resemblance
to the branchial arches of Fishes, but in any comparison of the two
structures it is well to bear in mind that the branchial rods of the
Lamprey are situated along the outer edges of the inter-branchial septa,
and are therefore external to the gill-sacs and branchial arteries, and
further, that they are developed in the somatic mesoblast of the embryonic
protovertebrae, whereas true branchial arches are situated at the inner
margins of the septa, internal to the gill-clefts and branchial arteries,
and have their origin from the splanchnic layer of the mesoblast. So far as
their position is concerned, the rods agree rather with the
{220}extra-branchial cartilages of an Elasmobranch than with the more
deeply-seated branchial arches.
[Illustration: FIG. 125.—Side view of the skull of _Bdellostoma_; the
gill-apertures and their cartilages have been omitted. A, Auditory capsule;
B, B′, B″, the anterior, middle, and posterior segments of the lingual bar;
_b^1_, cartilage connecting the hyoid arch with the second branchial arch;
_br^1_, _br^2_, first and second branchial arches; _c.c_, coronal
cartilage; _Cr_, cranium; _D_, dental plate; _dt_, median dorsal tooth;
_Ex.n.c_, external part of the naso-pituitary canal; _Hp_, hypophysial
plate; _Hy_, hyoid arch; _N_, subnasal cartilage; _nc_, neural canal; _Nt_,
notochord; _OC_, olfactory capsule; _PL_, palatine portion of the
palato-quadrate cartilage _PQ_; _S_, supra-pharyngeal plate supporting the
velum; _t_, tendon of the retractor mandibuli muscle; _t^1_, _t^2_, _t^3_,
tentacular cartilages; _t^4_, cartilage supporting mouth lobe; _tr_,
trabecula; _V^1_, rod connecting _S_ with the inner surface of the hyoid
arch of its side; _V_, outer lateral rod which joins _V^1_; 1, 2, 3,
fenestrae. (Modified from Ayers and Jackson.)]
While the skull of the Myxinoid Cyclostomes[194] is constructed on the same
general lines as that of the Lamprey, it is in some respects more
primitive. It is also clear that in other features the skull has undergone
marked specialisation on lines of its own, and in some points again it
seems to deviate less from the more normal Craniate type. Of the more
obvious differences, as illustrated by the skull of _Bdellostoma_ (Figs.
125-127), it will be sufficient here to mention the following: (i.) The
more primitive condition of the chondrocranium, the roof and side walls of
the cranial cavity being entirely membranous. (ii.) The non-development of
a suctorial buccal funnel and the presence of oral tentacles, associated
with the absence of the complex system of labial cartilages and the
substitution of a special tentacular skeleton. (iii.) The special
modifications induced by the length and physiological importance of the
naso-pituitary canal and by its communication with the pharynx after
perforating the pituitary fontanelle in the cranial floor.
{221}[Illustration: FIG. 126.—View of the upper surface of the dental plate
of _Bdellostoma_. _t_, Tendon of retractor muscle. (From Ayers and
Jackson.)]
[Illustration: FIG. 127.—Dorsal view of the skull of _Bdellostoma_.
Reference letters as in Fig. 125. (After Ayers and Jackson.)]
Under this head may be included the depression of the mesethmoid or
hypophysial plate for the support of the naso-pituitary canal, the forward
prolongation and median union of the palato-quadrate cartilages of opposite
sides beneath the external portion of the canal, apparently for the support
of the latter, and the encircling of the canal by supporting annular rings
of cartilage. (iv.) The presence of two branchial arches, connected, as in
Fishes, with a median basi-branchial segment which forms the middle one of
the three divisions of the lingual apparatus. (v.) The reduction of the
complicated extra-branchial basket to small isolated cartilages in relation
with the gill-apertures and the œsophago-cutaneous duct. (vi.) The
extraordinary development of the lingual apparatus, of which it has
{222}been remarked that it "dominates the whole body, everything else
yields to it." Meckel's cartilages are represented either by the cornual
cartilages, as seems most probable, or by the dental plate (Fig. 125,
_c.c._ and D).
[Illustration: FIG. 128.—Lateral view of the skull of _Notidanus_
(_Heptanchus_) _cinereus_; _mck_, Meckel's cartilage, or primitive lower
jaw; _pal.qu_, palato-quadrate cartilage or primitive upper jaw; _pt.orb_,
post-orbital process of the cranium with which the post-orbital process of
the palato-quadrate articulates. (From Parker and Haswell, after
Gegenbaur.)]
In the generality of Elasmobranchs the skull resembles that of the Dog-Fish
in essential structure. The more important modifications within the limits
of the group relate to differences in the mode of attachment of the
primitive upper jaw to the skull, and the number of branchial arches. In
most Elasmobranchs the skull is hyostylic, as in _Scyllium_, but there are
two genera which, in different ways, are exceptions to this rule. In
_Notidanus_ the hinder part of each palato-quadrate cartilage grows upwards
into a strong post-orbital process, which articulates with the suitably
modified post-orbital process of the periotic capsule (Fig. 128); hence the
primitive upper jaw acquires a direct dorsal connexion with the cranium,
and, as the hyoid arch is now relieved from taking any part in its support,
the hyomandibular is reduced to the condition of a relatively slender rod
of cartilage. By this arrangement both the mandibular and hyoid arches have
their own separate and independent connexions with the cranium, and the
skull is said to be _amphistylic_.[195] The Port Jackson Shark
{223}(_Heterodontus_) exhibits another and quite different modification. In
this Fish the dorsal border of each palato-quadrate cartilage fits into a
deep groove along the infero-lateral surface of the cranium, and is firmly
attached thereto by ligament. Thus the first step is taken towards that
more complete fusion of the two structures which is so characteristic a
feature in the more typically _autostylic_ Fishes like the Holocephali and
the Dipnoi. Autostylism, whether incipient, as in _Heterodontus_, or
complete, is to be regarded as a secondary modification, which may be
independently acquired in widely different groups of Fishes, and is usually
associated with the need of a firm and rigid support for an exceptionally
massive dentition.[196]
[Illustration: FIG. 129.—Lateral view of skull of _Chimaera monstrosa_.
_a.s.c_, Position of anterior semicircular canal; _c.hy_, cerato-hyal;
_e.hy_, epi-hyal; _fr.cl_, frontal clasper; _h.s.c_, position of horizontal
semicircular canal; _i.o.s_, inter-orbital septum; _lb.1_, _lb.2_, _lb.3_,
labial cartilages; _Mck.C_, mandible; _Nv.2_, optic foramen; _Nv.10_, vagus
foramen; _olf.cp_, olfactory capsule; _op.r_, opercular rays; _pal.qu_,
palato-quadrate; _ph.hy_, pharyngo-hyal, _or_ hyomandibular; _p.s.c_,
position of posterior semicircular canal; _qu_, quadrate region; _r_,
rostrum. (From Parker and Haswell, after Hubrecht.)]
In the Holocephali (_e.g._ _Chimaera_[197]) the cranium retains its
primitively cartilaginous condition, and assumes a somewhat peculiar
appearance owing to the lateral compression and vertical growth of its
inter-orbital and nasal regions (Fig. 129). There is a complicated series
of labial cartilages in relation with the ventrally-placed nostrils and the
upper and lower jaws. In the males of _Chimaera_ and _Callorhynchus_, but
not in _Harriotta_, a movable cartilage is attached to the cranial roof,
and supports the frontal clasper. The skull is typically autostylic. Along
{224}the whole length of its dorsal border the palato-quadrate cartilage is
fused with the inferior lateral margin of the cranium from the periotic to
the olfactory region, thus forming a triangular plate of cartilage, the
base of which is continuous with the cranium, while the downwardly directed
apex provides an articular surface for the lower jaw. The hyoid arch is
little better developed than the succeeding branchial arches, and includes
a vestigial hyomandibular, an epi-hyal, and a cerato-hyal. As in other
autostylic skulls the hyomandibular element is attached by ligament to the
hinder margin of the palato-quadrate, instead of being directly connected
with the periotic capsule, and obviously takes no part in supporting the
jaws. Branchial rays for the support of the operculum are attached to the
cerato-hyal, and some of them have their bases fused together. The five
branchial arches resemble those of the Dog-Fish, except that they tend to
become concentrated beneath the skull.
[Illustration: FIG. 130.—Side view of the skull of a Sturgeon, with the
investing membrane bones removed. _a_, Pharyngo-branchial; _AF_, antorbital
or lateral ethmoid cartilage; _AR_, articular; _b_, epi-branchial; _c_,
cerato-branchial; _C_, notochord; _Cop_, basi-branchials; _d_,
hypo-branchial; _De_, dentary; _GK_, auditory capsule; _Hm_, hyomandibular;
_hy_, cerato-hyal; _Ih_, inter-hyal; _Md_, lower jaw; _Na_, nasal capsule;
_Ob_, neural arches; _Orb_, Orbit; _PF_, post-orbital process; _PQ_,
palato-quadrate; _Ps_, _Ps′_, _Ps″_, parasphenoid; _Psp_, neural spines;
_Qu_, quadrate; _R_, rostrum; _Ri_, ribs; _Sp.N_, foramina for spinal
nerves; _Sy_, symplectic; _WS_, vertebral column; _x_, foramen for the
vagus nerve; I-V, branchial arches; II-V, foramina for the optic and the
fifth cranial nerves. (From Parker and Haswell, after Wiedersheim.)]
The existing Chondrostei,[198] and especially the Sturgeon, are remarkable
for the persistence and continuous growth of the chondrocranium, and the
absence of true cartilage bones.
{225}[Illustration: FIG. 131.—Lateral view of the primary and secondary
upper and lower jaws of _Polyodon_. _b.br′_, First basi-branchial; _ch_,
cerato-hyal; _d_, dentary; _hy.h_, hypo-hyal; _hy.m_, hyomandibular;
_i.hy_, inter-hyal; _i.op_, inter-operculum; _lgs_, ligaments connecting
the palato-quadrate cartilage with the hyomandibular; _mk.c_, Meckel's
cartilage; _mx_, maxilla; _op_, operculum; _pa_, palatine; _pa.q_,
palato-quadrate; _ps.l_, pre-spiracular ligament; _q_, quadrate cartilage;
_sym_, symplectic. (From Bridge.)]
Numerous dermal bones invest the dorsal surface of the chondrocranium, and
only to a limited extent correspond with the less numerous membrane bones
of the Salmon. To these are added a series of circum-orbital bones and a
large parasphenoid. Undoubtedly the most striking feature in these Fishes
is the primitive character of the upper jaw. In _Polyodon_ (Fig. 131) the
palato-quadrates are wholly cartilaginous, and, as in the Dog-Fish, they
meet in front beneath the basis cranii, where the two are connected by
ligament. The secondary upper jaw is but feebly developed, and is
represented on each side by a thin splint-like maxilla in relation with the
outer surface of each palato-quadrate cartilage, which meets its fellow in
front. There are no premaxillae. The lower jaw is also very primitive.
Meckel's cartilages are persistent, and except for a mento-Meckelian bone
on each side, they are unossified, although membrane bones representing
dentary and splenial elements are present. The skull is hyostylic. The
hyoid and branchial arches are only partially ossified. Each opercular fold
is supported by an operculum and an interoperculum, and both of these
retain somewhat the shape of the cartilaginous hyoidean rays which they
have replaced. In the Sturgeon (Fig. 130) the upper jaw is greatly modified
in relation with the singular mouth of this Fish. The palato-quadrate
cartilages meet not only in front, but also along their dorsal margins,
and, with the help of the similarly opposed and somewhat fragmentary
metapterygoid {226}cartilages, they form a complete concave roof for the
protrusible spout-like mouth. Palatine, mesopterygoid, and pterygoid bones
invest, and in some measure replace these cartilages. In brief, the skull
of the Chondrostei occupies an interesting intermediate position between
the purely cartilaginous and mainly bony types. While retaining a
well-developed and unossified primary cranium, it has acquired a complete
secondary cranium of dermal bones. Equally notable is the condition of the
jaws. Unique among the Teleostomi in possessing the typical Elasmobranch
union of the palato-quadrate cartilages beneath the basis cranii, the
Chondrostei are so far specialised that they have acquired certain of the
membrane bones which constitute the secondary jaws of the more typical bony
Fishes.
As regards the general structure of the skull and the nature and
disposition of its cartilage- and membrane-bones, the remaining living
Teleostomi have much in common with the Salmon. In all the skull is
hyostylic, and, unlike the Chondrostei, each half of the primitive upper
jaw remains distinct from its fellow, and is separately articulated in
front with the lateral ethmoid of the same side by its palatine element.
The palato-quadrate cartilage is always more or less completely replaced by
bones similar to those of the Salmon, and although they often carry teeth,
as a rule they do little more than constitute a rigid buttress for the
fixation of the quadrate condyle for the lower jaw. The secondary upper jaw
is nearly always well developed, and includes a premaxilla as well as a
maxilla on each side. There are, however, certain features in each of the
minor groups which are either distinctive or highly characteristic.
In the surviving Crossopterygii (e.g. _Polypterus_[199]) the
chondro-cranium is complete in the ethmoidal and post-orbital regions,
except where it has been partially replaced by cartilage bones, but in the
inter-orbital region the continuity of the roof is interrupted by a large
fontanelle, which is only closed by the investing frontal bones (Fig. 132,
C). There is also a large basi-cranial fontanelle in the sphenethmoid,
closed, however, by the underlying parasphenoid. A large "occipital" bone
continuously ossifies in the occipital cartilage and completely surrounds
the foramen magnum.
{227}[Illustration: FIG. 132.—A, side view of the skull of _Polypterus_; B,
dorsal view, showing the chief dermal bones; C, similar view of the
chondro-cranium after the removal of the dermal bones. _An_, Angular; _Ar_,
articular; _D_, dentary; _E_, mesethmoid; _f.m_, foramen magnum; _Fr_,
frontal; _l.e_, lateral ethmoid; _Mx_, maxilla; _Na_, _Na′_, nasal and
accessory nasal bones; _occ_, occipital; _ol_, nasal aperture; _Op_,
operculum; _op.o_, opisthotic; _O.t_, _os_ terminate; _Pa_, parietal;
_Pm.x_, premaxilla; _P.t_, post-temporal; _Ptf_, post-frontal; _Qu_,
quadrate; _S.b_, _S.b′_, circum-orbital ossicles; _S.Op_, sub-operculum;
_Sp_, splenial; _sp.eth_, sphenethmoid; _sp.o_, sphenotic; _Spr_,
spiracular ossicles, between which is the spiracle; _S.t_, supra-temporals;
_Y_, cheek-plate (pre-operculum); _Y′_, _Y″_, smaller cheek-plates; _z_,
_z_, _z_, _z_, post-spiracular ossicles; _z′_, _z′_, prespiracular
ossicles. In C the cartilage is dotted. (From Traquair.)]
Prootics and pterotics are absent, and the opisthotics seem to be confluent
with their respective epiotics. The floor and side walls of the
inter-orbital section of the cranium are formed by a remarkable
"sphenethmoid" bone which occupies the position of the paired ali- and
orbito-sphenoids in other bony Fishes; and in one species, _P.
lapradei_,[200] it forms in front distinct tubular investments round the
olfactory nerves. In many respects this bone is singularly like the
sphenethmoid bone of the Frog and other tailless Amphibia. A median ethmoid
as well as lateral ethmoids are present. In addition to the ordinary dermal
bones which invest the cranial roof there is a transverse row of
supra-temporal plates crossing the cranial roof behind the paired parietals
(Fig. 132, A). Fringing the outer margins of the frontals and parietals a
row of pre- and post-spiracular ossicles extends nearly to the orbits, and
between two of them, which form a {228}valve, is the spiracular aperture
itself. There is a dentigerous splenial on the inner surface of the lower
jaw. The hyoid arch has no separate symplectic bone. An operculum and a
suboperculum are present, but no inter-operculum; and unless the hinder
part of the large cheek-plate, which is traversed by the mandibulo-hyoid
sensory canal, represents a pre-operculum, the latter is wanting.
Branchiostegal rays are absent, but there is a single pair of large jugular
plates.
Very little is certainly known about the cranial cartilage-bones in the
fossil members of the group, but the investing dermal bones, which bear a
general resemblance to those of _Polypterus_, are often somewhat more
numerous, and they form a very complete dermal armature for the entire
head. There is a very complete ring of circum-orbital bones, and very often
a ring of sclerotic plates. Two large cheek-plates are often present.
Nothing comparable to pre- and post-spiracular ossicles is known, but
squamosal and supra-temporals can often be identified. To the ordinary
bones of the lower jaw there may be added a series of infra-dentary plates,
and besides the paired principal jugular plates there may also be present a
small anterior median plate and a series of small lateral jugular plates on
each side, as in the Carboniferous _Rhizodopsis_ (Fig. 274). Most of the
superficial dermal bones, both in the living and extinct Crossopterygii,
are invested externally by a granulated or rugose layer of enamel-like
ganoin.
In the Holostei, and especially in _Amia_, the skull approximates more
closely to the normal Teleostean type as represented by the Salmon's skull.
In _Amia_[201] all the occipital cartilage-bones are present—a
basi-occipital, two exoccipitals, and a supra-occipital; and, except for
the absence of a pterotic, the periotic series of bones is also complete.
Paired ali- and orbito-sphenoids form the lateral walls of the
inter-orbital portion of the cranial cavity. Above, the complete
cartilaginous roof of the cranial cavity is invested by a shield of
suturally united and ganoin-covered dermal plates. The hyomandibular
element has a symplectic bone at its distal extremity. There is a complete
series of opercular bones, and the branchiostegal rays are numerous. A
single median jugular plate is present. The lower jaw has on each side five
dentigerous splenial bones in addition to dentary and angular bones, while
cartilage-bones are {229}represented by articular and mento-Meckelian
elements. In its essential structure the skull of _Lepidosteus_[202]
resembles that of _Amia_, but it has obviously undergone much
specialisation. In some species (_e.g._ _L. osseus_) its appearance is
greatly modified by the exceptional length and tapering shape of the beak,
due to the elongation of that part of the skull which lies between the
orbital and nasal regions; but in _L. platycephalus_ the reduced length and
greater width of the beak, combined with its somewhat flattened condition,
impart an almost Crocodilian aspect to the head. Amongst other points of
difference it may be mentioned that in _Lepidosteus_ the continuity of the
chondro-cranial roof is interrupted by a large superior fontanelle. There
is no supra-occipital, and there are no lateral ethmoids, at all events in
the usual position. The inter-orbital portion of the cranial cavity is
largely obliterated by the formation of an inter-orbital septum, consisting
of a thin vertical plate of bone, which either represents a pair of fused
orbito-sphenoids or a pair of similarly modified lateral ethmoids. In
addition to the ordinary investing dermal bones, including circum-orbitals,
squamosal, and supra-temporals, there are numerous scale-like ossicles
which take the place of the cheek-plates of _Polypterus_. The maxillae are
segmented into numerous dentigerous bones fringing the margins of the upper
jaw. The lower jaw has no mento-Meckelian bones, but there is a very
complete series of dermal elements, including dentary, coronary, splenial,
angular, and supra-angular bones in addition to an articular
cartilage-bone. One of the most remarkable features in the skull of
_Lepidosteus_ is the existence of a secondary articulation between the
metapterygoid bones and a pair of transversely elongated condyles formed on
each side by a lateral outgrowth from the parasphenoid and alisphenoid
bones. By a horizontal sliding movement of the former on the latter,
provision is made for the lateral expansion and contraction of the walls of
the oral cavity and the separation and approximation of the lateral halves
of the upper jaw.[203]
The generality of Teleosts[204] more or less closely agree with _Amia_ in
the main features of their cranial structure. There are, however, certain
minor features which are characteristic if not {230}always distinctive of
the group. As a rule, to which, nevertheless, there are notable exceptions,
there is little of the primary cartilaginous cranium in the adult, nearly
the whole of it having become absorbed or converted into cartilage-bones. A
supraoccipital is invariably present, and usually a mesethmoid and a
basisphenoid. An additional bone is added to the periotic series, viz. a
pterotic. Supra-temporal bones and jugular plates are always absent, and it
may be doubted if mento-Meckelian bones and dentigerous splenials are ever
developed in the lower jaw. Within the group itself the skull exhibits many
notable modifications, of which only a few can here be mentioned. The
shape, size, and character of the mouth and jaws, the extent to which they
can be protruded and retracted, and the nature of the dentition, are the
source of many characteristic modifications in the structure and appearance
of the fore-part of the skull, and these again largely depend upon
differences of habit and food. A protrusible mouth, or a mouth which is
projected forwards, is usually associated with a suspensorium
(hyomandibular) of considerable length, and so greatly inclined forwards as
to make a more or less acute angle with the forepart of the cranium.
The presence or absence of an inter-orbital septum is also a feature in
which considerable variation occurs. In some Teleosts there is no septum,
and the cranial cavity is prolonged forwards between the orbits, where its
lateral walls are formed by well-developed, paired ali- and orbito-sphenoid
bones, as, for example, in the Carp and other Cyprinidae. In others the
fusion of the cranial walls is accompanied by the median union of the
orbito-sphenoids, so that a partly bony and partly cartilaginous
inter-orbital septum is found, and the cranial cavity becomes largely
obliterated in this region, as in the Salmon; or the orbito-sphenoids may
be non-existent, the cartilage may undergo absorption, and the
inter-orbital septum may become reduced to a vertical fibrous sheath
extending between the frontals above and the parasphenoid below, as is the
case in the Cod (_Gadus_).
An interesting modification of certain of the bones of the primary and
secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae
are very small and edentulous, and serve no other purpose than forming
basal supports for the maxillary barbels, while the rod-like palatine bone,
losing its connexion with the pterygoid portion of the primitive upper jaw,
{231}but retaining its articulation with the lateral ethmoid, serves to
support the maxilla, and at the same time receives the insertion of the
muscles by which the barbel is moved in various directions.
In the Plectognathi the premaxillae are co-ossified with the maxillae. Many
other interesting cranial modifications occur in Teleosts, and to some of
them reference is made in subsequent chapters.
In some respects the skull of Dipnoi[205] is remarkably like that of the
Holocephali, especially in its typical autostylism; but in possessing both
cartilage- and membrane-bones it in some measure approaches the Teleostome
skull. The investing dermal bones are not always easy to identify with
those of other Fishes. In _Neoceratodus_ an anterior median membrane-bone
or dermal mesethmoid covers the ethmo-nasal region, and, on each side of
it, forming the anterior boundary of the orbit, there is situated a
pre-orbital or dermal lateral ethmoid. Behind the mesethmoid there is a
much larger posterior median bone, and on each side a singular backward
prolongation of the dermal lateral ethmoid separates it from a squamosal
element. The latter bone descends on the outer surface of the quadrate
portion of the palato-quadrate cartilage as far as the condyle for the
lower jaw. Collectively, these bones form a fairly complete investment to
the upper surface of the cranium, but the posterior median bone and the
adjacent portions of the dermal lateral ethmoid and the squamosal are
widely separated from the underlying chondrocranium by the powerful jaw
muscles, and in this respect they differ from the ordinary roofing bones of
other Fishes.
In _Protopterus_ (Fig. 133) and _Lepidosiren_ (Fig. 134) the posterior
median bone is non-existent, and its place is taken by a large
fronto-parietal, which forms the greater part of the cranial roof, internal
to the jaw muscles, and is much larger in the latter Dipnoid than in the
former. Circum-orbital bones are present only in _Neoceratodus_. A large
parasphenoid supports the cranial floor. Vomers are absent, although there
are two small vomerine teeth.
{232}[Illustration: FIG. 133.—Side view of the skull of _Protopterus_, with
the pectoral girdle and fin. _an_, Angular; _an.c_, antorbital cartilage;
_c.c_, coracoid cartilage (epi-coracoid); _c.hy_, cerato-hyal; _cl_,
clavicle; _c.r_, cranial rib; _c.sc_, coraco-scapular cartilage; _d.e_,
dermal ethmoid; _d.l.e_, dermal lateral ethmoid; _e.g.f_, external gills;
_eo_, exoccipital; _f.p_, fronto-parietal; _mk.c_, Meckel's cartilage;
_n.a_, neural arches; _ol.c_, fenestrated roof of the olfactory capsule;
_p.f_, skeleton of the pectoral fin; _p.pt_, palato-pterygoid bone; _p.q_,
palato-quadrate cartilage; _s.cl_, supra-clavicle; _sp_, splenial; _sq_,
squamosal; 1-6, the branchial arches; the segmentation of the second and
third arches is not shown. (From Wiedersheim.)]
Relatively small opercular and inter-opercular bones are present, and on
the inner surface of each may be found vestigial remains of cartilaginous
hyoidean rays. The chondrocranium is complete in _Neoceratodus_, but in the
remaining genera it has undergone considerable absorption in the
inter-orbital region, so that the roof and floor, and, in part, even the
side walls of the cranial cavity, are formed by the fronto-parietal and
parasphenoid bones. Two exoccipitals are present in all Dipnoi. There are
small labial cartilages in relation with the ventrally-placed nostrils, and
large lateral outgrowths from the ethmoid cartilage furnish the olfactory
organs with conspicuous lattice-like roofs. A pair of strong
palato-pterygoid bones fringe the lower margins of the palato-quadrate
cartilage, and meeting in front beneath the ethmoid region their symphysial
extremities support the large palatal teeth. The Meckelian cartilages are
persistent in all Dipnoi. In _Neoceratodus_ each is flanked by a dentary
and an angular externally, and internally by a splenial; but in
_Protopterus_ and _Lepidosiren_ distinct dentary bones are wanting. The
hyoid arch is best {233}developed in _Neoceratodus_,[206] and includes a
small hyomandibular cartilage, a partially bony cerato-hyal and
cartilaginous hypo-hyal and basi-hyal element. In the other genera (Fig.
133) only a cerato-hyal is retained. The branchial arches are but feebly
developed in the Dipnoi. _Neoceratodus_ has five, of which the first four
are divided into epi-branchial and cerato-branchial segments, while the
fifth is undivided. _Protopterus_ has six, but only the second and third
are segmented as in _Neoceratodus_.[207] In _Lepidosiren_ all the arches
are simple undivided rods.
In all three genera the skull conforms to the same general type of
structure, but it is much more primitive in _Neoceratodus_ than in the
other two genera.
[Illustration: FIG. 134.—Dorsal view of the skull of _Lepidosiren_. _an.c_,
Condyle on the quadrate cartilage for the lower jaw; _n.sp_, neural spine;
_op_, operculum. For other reference letters see Fig. 133. (From Bridge.)]
With reference to the fossil Dipnoi, it may be stated that, so far as they
are known, the cranial roofing bones are more numerous than in the existing
genera, and they cannot readily be compared with those of the latter, or
with the numerically reduced and more definitely arranged bones of most
Teleostomi. There is also evidence that in some fossil Dipnoi (e.g.
_Dipterus_) the chondrocranium and the mandibular suspensorium
(palato-quadrate) must have been replaced by cartilage bones to an extent
which has no parallel in any of the surviving types.[208] Jugular bones
were present in _Dipterus_ and _Phaneropleuron_.
{234}MEDIAN FINS AND APPENDICULAR SKELETON
[Illustration: FIG. 135.—The cartilaginous radialia of the first dorsal fin
of _Mustelus antarcticus_. (From Mivart.)]
THE MEDIAN FINS.—Whether existing in the form of a continuous fin, or as
discontinuous isolated fins, the median fins are provided with skeletal
supports, and also with muscles, primitively formed from intrusive clusters
of cells derived from a variable number of the neighbouring myotomes, for
their varied movements. The skeletal structures of the dorsal and anal fins
consist of a series of bony or cartilaginous, rod-like, and typically
tri-segmented radial elements or pterygiophores,[209] supporting distally a
series of dermal structures in the shape of numerous slender horny fibres
or ceratotrichia, as in the Elasmobranchii and Holocephali, or a smaller
number of bony dermal fin-rays, which are probably modified scales or
lepidotrichia,[210] as in the Teleostomi. The typical tri-segmented
character of the radialia is often retained in many existing Elasmobranchs
(Fig. 135) and in _Pleuracanthus_, in _Neoceratodus_ amongst the Dipnoi, in
the Chondrostei, in existing Holostei (Fig. 136), and to a greater or less
extent in several families of Teleosts (_e.g._ Salmonidae, Esocidae,
Cyprinidae, and some Acanthopterygii); but in the latter group the radialia
are greatly prone to reduction, and hence they are more generally
bi-segmented, and sometimes consist of a single proximal segment only (e.g.
_Gymnotus_). In all these Fishes the proximal segments are the longest and
the most persistent, and when reduction occurs it is at the expense of the
middle and distal segments.
{235}[Illustration: FIG. 136.—The tri-segmented radialia and the fin-rays
of part of the dorsal fin of _Amia calva_. _p.s_, _m.s_, and _d.s_, The
proximal, middle, and distal segments of a radial; _f.r_, fin-rays. (From
Bridge.)
FIG. 137.—The first four radialia of the dorsal fin of _Mesoprion gembra_,
showing the chain-links for the ring-like bases of the fin-rays. _r.e^1_,
_r.e^4_, First and fourth proximal radialia.]
The cause of this reduction is often, but not always, to be found in the
fact that, whenever the dermal fin-rays take the form of stout spines, as
in the anterior dorsal fin in many Acanthopterygian Teleostei, the
segmentation of their radialia would obviously detract from their value as
skeletal supports, and hence they rarely consist of more than their
proximal segments, although the radialia which in the same Fish support
soft rays may be bi-segmented or tri-segmented. The radialia are, however,
unsegmented, even slightly branched, cartilaginous rods in the
Cyclostomata; short simple rods in the Holocephali; and equally simple bony
rods in the dorsal fin of _Polypterus_, where they support the strong
spines of the numerous finlets; but they are bi-segmented in the soft-rayed
anal fin. As previously mentioned, the proportional share taken by the
radialia and the horny fibres or the dermal fin-rays in the support of the
fins differs greatly in different Fishes. In the Cyclostomata radialia are
the sole, and in Elasmobranchs the main supports, and they may extend
nearly to the free margin of the fin. In the more specialised Fishes, as in
most Teleostomi, the reverse is the case. The radialia sink into the
muscles of the body-wall and leave the strongly developed fin-rays as the
sole support of the visible portions of the fins. In not a few Fishes there
is an obvious segmental correspondence between the radialia and the
vertebral neural or haemal spines, to the extent that the former equal the
latter in number and articulate with their distal extremities, as, for
example, in the caudal region of _Pleuracanthus_ and in existing
{236}Dipnoi. In others again, as in most Teleostomi, there is no such
segmental relation, and the radialia are more numerous than the vertebrae
whenever the two are co-extensive. The exoskeletal fin-supports exhibit
similar relations to their radialia, but in inverse order. Much more
numerous than the radialia in the Elasmobranchs, Holocephali, and the
Dipnoi, the former become gradually reduced in the Teleostomi, until in the
Holostei and Teleostei they correspond in number with the supporting
radialia. Complete numerical correspondence between the neural and haemal
spines and the radialia and fin-rays is very rare, and has only been
observed in the caudal region of certain Crossopterygii (_e.g._ the
Coelacanthidae).[211]
[Illustration: FIG. 138.—The posterior dorsal fin of _Holoptychius
leptopterus_ from the old Red Sandstone of Nairnshire. Traces of dermal
fin-rays may be seen at the distal margin of the fin. (After Smith
Woodward.)]
[Illustration: FIG. 139.—A dermal fin-ray and its supporting radial or
pterygiophore in the Trout (_Salmo fario_). _D.F.R_, Dermal fin-ray; PTG.1,
PTG.2, _ptg.3_, the proximal, middle, and distal segments of which the
tri-segmented radial consists; _ptg.3_ is cartilaginous; the other two are
bony. (From Parker and Haswell.)]
In not a few Fishes the radialia of the median fins undergo modifications
which offer an interesting parallel to an early stage in the evolution of
the paired fins from primitively continuous lateral fins. The concentration
of radialia which occurs in isolated median fins often results, through
growth pressure, in the complete fusion of the proximal segments of more or
fewer of the radialia into two or three basal supports, or even into a
single basal piece. Examples of such basal fusion are frequent in the
dorsal fins of Elasmobranchs, and the same modification may also be seen in
the anal fin of _Pleuracanthus_, and especially in the {237}dorsal fin of
the Devonian Crossopterygian, _Holoptychius_[212] (Fig. 138), where several
radialia, which are free distally, have their bases united into a single
basal piece, or basipterygium. In most Teleostomi elevator and depressor
muscles arise from the radialia, and are inserted into different points on
the bases of the fin-rays, and by their contraction the latter may either
be elevated into an erect position, or folded back like a fan along the
middle line of the body, where, as in some Teleosts, there is a groove for
their reception. When fin-rays are only capable of simple elevation or
depression, the connexion between a radial element and its fin-ray is
usually by some form of a hinge-joint, the cleft base of the ray clipping
the distal segment of the radial (Fig. 139). In some Teleosts the
articulation of the two is by means of a kind of chain-link (Fig. 137). In
those Fishes in which the median fins are capable of lateral undulatory
movements the articulation is of a more mobile character.
[Illustration: FIG. 140.—Caudal end of the vertebral column of a Trout
(_Salmo fario_). CN, Centrum; _D.F.R_, dermal fin-rays; H.SP, haemal spine;
H.ZYG, haemal zygapophysis; N.SP, neural spine; N.ZYG, neural zygapophysis;
UST, the up-tilted, partly ossified, and unsegmented terminal portion of
the notochord, or urostyle. (From Parker and Haswell.)]
In the different types of caudal fin, diphycercal, heterocercal, and
homocercal, the supporting elements of the ventral lobe are formed by the
haemal spines of the terminal caudal vertebrae which are inclined
backwards, and are often greatly expanded for the purpose (Fig. 140). The
dorsal lobe may be supported either by the adjacent neural spines, or by
radialia, or by both.
THE APPENDICULAR SKELETON.[213]—It is probable that the skeleton of the
paired fins and the pectoral and pelvic girdles have been formed from the
supporting radialia of the isolated and enlarged anterior and posterior
portions of primitively continuous lateral {238}fins, by a sequence of
structural modifications in the same direction as in the median fins. The
initial stage was probably marked by the fusion of the proximal portions of
the radialia to form a basal support or basipterygium for the free distal
portions. Subsequently, it may be, a rudiment of the future limb-girdle
became segmented off from the inner extremity of the basipterygium, and by
its dorsal and ventral growth in the body-wall the lateral half of a girdle
was developed. The subsequent union of the two halves across the
mid-ventral line resulted in the evolution of the dorsally incomplete hoop
of cartilage which is the primary form of the complete limb-girdle in
Craniates. The primitive fin skeleton or "archipterygium" was formed from
the residue of the basipterygium in conjunction with the free distal
radialia which it carried. The precise structure of the archipterygium is
purely hypothetical. Possibly it was a biserial fin of the _Pleuracanthus_
or _Neoceratodus_ type, consisting of a cartilaginous segmented axis,
fringed along its anterior and posterior, or pre-axial and post-axial
margins, by a series of slender, simple, or jointed radialia (Fig. 147); or
it may have been a uniserial structure, somewhat resembling the pelvic fin
of _Pleuracanthus_, or the pectoral and pelvic fins of existing
Elasmobranchs (Figs. 250, 141), in which an axis formed by the residue of
the basipterygium or metapterygium had a fringe of radialia on its anterior
or preaxial side only. If the archipterygium was biserial then the
uniserial fin was probably derived from it by the subsequent suppression of
all the post-axial radialia; or, if uniserial, the biserial fin was evolved
by a later extension of radialia on to the post-axial margin. The evidence
of comparative anatomy is not conclusive as to the nature of the
archipterygium, and palaeontology seems to support either view with
puzzling impartiality.[214] It may be admitted that the lateral fin theory
offers the best solution of the problem of the origin of the paired fins,
but it must be borne in mind that no Fish, living or fossil, is known to
possess fins of this nature, unless the singular lateral lobes of some
Ostracodermi (_e.g._ the Coelolepidae) are kindred organs[215]; neither do
continuous lateral fins ever exist as vestiges, unless, indeed, the
bilateral series of spines, which extend between the pectoral and pelvic
{239}fins, in some of the Lower Devonian Acanthodei (e.g. _Climatius_), may
be regarded in that light.
_The Pectoral and Pelvic Girdles_.—The pectoral girdle is more primitive in
_Cladoselache_ and _Pleuracanthus_ than in any other Elasmobranch. In the
former (Fig. 145, A) it may be doubted if the girdle has passed beyond the
basipterygial stage, and although a definite girdle is present in the
latter genus (Fig. 250) its lateral halves retain their primitive
distinctness. Existing Elasmobranchs, including the Holocephali, have a
pectoral girdle in the form of a dorsally incomplete hoop of cartilage
imbedded in the muscles of the body-wall, close behind the last branchial
arch (Fig. 141). The upper or dorsal portion of each half is the scapula,
and the ventral is the coracoid. Between these two portions of the girdle,
and defining their limits, there are articular surfaces for the basal
cartilages of the pectoral fin.
[Illustration: FIG. 141.—The right half of the pectoral girdle and the fin
of an Elasmobranch (_Chiloscyllium_). _d.r_, Dermal horny fibres; _meso_,
mesopterygium; _meta_, metapterygium; _pect_, pectoral girdle; _pro_,
propterygium. (From Parker and Haswell.)]
_Cladoselache_ (Fig. 145, B) had no pelvic girdle, nor does it appear that
this primitive Elasmobranch had acquired even a basipterygium.
_Pleuracanthus_, on the contrary, had a pair of pelvic rudiments distinct
from well-developed basipterygia. In other Elasmobranchs there is a
distinct girdle, formed by the median union of primitively distinct lateral
rudiments, consisting of a simple transverse bar of cartilage, imbedded in
the ventral abdominal wall, just in front of the cloacal aperture, and
having articulated to each of its outer extremities the basal cartilage
(metapterygium) of the pelvic fin.
{240}[Illustration: FIG. 142.—The left half of the pelvic girdle and the
right pelvic fin of _Chiloscyllum_. _meta_, Metapterygium; _pelv_, pelvic
girdle. (From Parker and Haswell.)]
[Illustration: FIG. 143.—Left half of the pectoral girdle of a Trout
(_Salmo fario_), seen from the inner surface. _CL_, Clavicle (cleithrum);
COR, coracoid; _D.F.R_, dermal fin-rays; MS.COR, meso-coracoid; _P.CL_,
_P.CL′_, post-clavicles; PTG.1, proximal; _ptg.2_, distal pterygiophores;
_P.TM_, post-temporal; _S.CL_, supra-clavicle; SCP, scapula. (From Parker
and Haswell.)]
Sometimes there is a rudiment of a dorsally-directed "iliac" process at
each extremity of the girdle, but in no Fish do these processes ever
acquire a dorsal connexion with the vertebral column. In the Holocephali
the iliac processes are better developed than in any other Fishes, but
ventrally the lateral halves of the girdle are united by ligament alone. In
the Teleostomi important differences are observable in both girdles. The
primary cartilaginous pectoral girdle now consists of distinct lateral
halves which have no ventral connexion with each other. In addition, there
is developed on the outer surface of each half a series of membrane bones,
which form a secondary girdle (Fig. 143). From above downward the series
includes a supraclavicle and a cleithrum (clavicle of Teleosts) which are
always present, and to these may be added in the {241}Crossopterygii and
Chondrostei an infraclavicle or clavicle proper, while one or two
"post-clavicles" may be present in relation with the hinder margin of the
cleithrum. The infraclavicles, or in their absence the cleithra (_e.g._
Holostei and most Teleostei), usually meet in a median ventral symphysis,
so that the secondary girdle tends to acquire the characteristic hoop-like
arrangement of its parts which has been lost in the primary girdle. With
the development of a bony secondary girdle, the primary girdle (scapula and
coracoid) becomes much reduced, and, as a rule, does little more than
connect the fins with the cleithra. The secondary girdle acquires a dorsal
connexion with the skull on each side by means of the post-temporal bone,
which is attached below to the supra-clavicle and above to the periotic
capsule. In the Chondrostei and the Dipnoi the primary girdle retains its
primitive cartilaginous condition, but in the Crossopterygii, Holostei, and
in all Teleosts it is ossified as distinct scapulae and coracoids. To these
may be added in some Teleosts a mesocoracoid formed by a separate
ossification of the coracoid cartilage (Fig. 143).[216]
[Illustration: FIG. 144.—Ventral view of the pelvic girdle of
_Protopterus_. _a_, Prepubic process; _b_, lateral process for the fin;
_c_, epipubic process; _Gr_, ridge for the origin of the fin muscles; _HE_,
skeleton of the fin; _M_, myotomes; _M′_, myocommata. (From Wiedersheim.)]
With the possible exception of small paired or median cartilages inserted
between the inner extremities of the basipterygia in _Polypterus_ and a few
other Teleostomi, the pelvic girdle is absent in all the existing members
of this group, having either become completely suppressed, or remaining
unseparated from the basipterygia of the pelvic fins.[217] In the Dipnoi
(Fig. 144) there is a true pelvic girdle which has some points of
resemblance to that of certain of the caudate Amphibia. It is represented
by a median, lozenge-shape, cartilaginous {242}plate, produced in front
into a long tapering epipubic process, and on each side of this into a
forwardly inclined prepubic process. The hinder part of the plate bears two
short processes for the basal cartilages of the pelvic fins. There is no
trace, however, of iliac processes.
THE PECTORAL FINS.—The skeleton of the pectoral fins exhibits remarkable
structural variations in different Elasmobranchs. In the existing members
of the group two large basal cartilages, the propterygium and the
mesopterygium, are formed by the concentration and fusion of the proximal
portions of certain of the preaxial radialia, and they, with the
metapterygium, articulate with the pectoral girdle; hence the fin is
tribasal as well as uniserial (Figs. 141 and 146, A, B). In striking
contrast to all other Elasmobranchs the pectoral fin of _Cladoselache_
(Fig. 145, A) is far more primitive than in any other Fish. Each fin is
supported by a distal series of slender, more or less parallel, unjointed,
cartilaginous radialia, and basally by a similar series of shorter,
stouter, and less numerous cartilages, which apparently were imbedded in
the body-wall, the entire fin skeleton presenting a striking resemblance to
an isolated median fin in which the supporting radialia have concentrated
by growth pressure, and their proximal portions have been reduced in number
by partial fusion.[218] _Pleuracanthus_, on the other hand, had a biserial
fin, the preaxial and postaxial radialia supporting fan-like clusters of
horny fibres at their distal ends (Fig. 250).
[Illustration: FIG. 145.—A, Pectoral fin, and B, pelvic fin of
_Cladoselache_. (From Bashford Dean.)]
{243}[Illustration: FIG. 146.—Pectoral fins of various Fishes. A,
_Acanthias vulgaris_; B, _Raia sp_.; C, _Chimaera monstrosa_; D, _Acipenser
rhynchaeus_; E, _Amia calva_; F, _Lepidosteus platyrhynchus_; G,
_Polypterus bichir_; H, _Salmo salvelinus_. The preaxial side of each fin
is to the left and the postaxial to the right. _f.r_, Dermal fin-ray; _ms_,
mesopterygium; _mt_, metapterygium; _p_, propterygium; _r_, free radialia;
1, 5, the preaxial and postaxial basal elements in a Teleost, which may be
mesopterygial and metapterygial pieces respectively, the three remaining
basal pieces probably being intrusive metapterygial radialia directly
articulating with the pectoral girdle. In B, D, E, and F, similar intrusive
radialia are shown. (From Gegenbaur.)]
The broadly lobate pectoral fin of the existing Crossopterygii (Fig. 146,
G) is uniserial, closely resembling that of the more typical
Elasmobranchs.[219] There are three basal elements, a propterygium, a
mesopterygium, and a metapterygium, each of which supports a series of
partially ossified radialia. Little is known of the endoskeletal elements
of the broadly or acutely lobate fins of the fossil Crossopterygii, but it
seems probable that their disposition was uniserial and abbreviate in
obtusely lobate fins and biserial in acutely lobate fins. In the remaining
Teleostomi (Actinopterygii) the endoskeletal elements become {244}gradually
reduced in number and importance, their place as fin-supports being usurped
by the dermal fin-rays. In addition, more than three, usually several,
basal elements articulate directly with the pectoral girdle, and hence the
fins become multi-basal. In the Chondrostei and the Holostei a
metapterygium is always recognisable, supporting several radialia along its
preaxial border, as in _Acipenser_ (Fig. 146, D) and _Amia_ (Fig. 146, E),
or only a single one, as in _Lepidosteus_ (Fig. 146, F). The anterior part
of the fin is supported by a variable number of cartilaginous or bony
radialia, which, with the metapterygium, articulate with the limb-girdle.
In Teleosts the process of reduction reaches its maximum. Usually there is
but a single row of short, hour-glass-shaped ossicles, of which the
postaxial one may represent a vestigial metapterygium, and sometimes there
is also a distal row of small cartilages or ossicles, partially hidden in
the cleft bases of the dermal fin-rays (Fig. 146, H). In all these Fishes
the fin is a much reduced uniserial fin, in which more or fewer of the
preaxial radialia have acquired a direct secondary connexion with the
pectoral girdle. Of living Dipnoids _Neoceratodus_ has a nearly typical
biserial fin, but, as seems to be the case in all fins of this type at
present known, there is a marked absence of symmetry in the number and
disposition of the radialia on the two sides of the axis. There is also
much individual variation. No two fins are precisely alike, and the
radialia may sometimes divide. In the very acutely lobate fins of the
remaining Dipnoids it is evident that great reduction has taken place.
_Protopterus_ has lost all trace of postaxial radialia, and in
_Lepidosiren_ even the preaxial have atrophied, leaving only the long
jointed axis to represent the originally biserial fin.
[Illustration: FIG. 147.—The left pectoral fin of _Neoceratodus_. _a_, _b_,
First two segments of the axis; _FS_, preaxial horny fibres; †, †, pre- and
post-axial radialia. (After Wiedersheim.)]
{245}THE PELVIC FINS.—In the simplicity of their endoskeletal supports the
pelvic fins of _Cladoselache_ are the most primitive type of paired fins at
present known (Fig. 145, B). In general structure they resemble the
pectorals, but the radialia are fewer in number, less modified by
concentration, and exhibit little, if any, trace of basal fusion. Add to
such features as these the apparent absence of any trace of pelvic
rudiments, or of basipterygia, and it will be obvious that the pelvic fins
differ but little from the median fins of the same Fish except that they
are paired. In _Pleuracanthus_ the pelvic fins differ from the
corresponding pectorals in being uniserial instead of biserial (Fig. 250).
All other Elasmobranchs, including the Holocephali, have uniserial fins,
which consist of a large metapterygium, supporting a preaxial fringe of
segmented radialia. A propterygium is sometimes present, notably in some of
the Skates and Rays, and, like the metapterygium, it is directly connected
with the pelvic girdle.
[Illustration: FIG. 148.—Skeleton of a pelvic fin of _Polyodon folium_,
ventral view, with the anterior margin of the fin to the right; to show the
partial fusion of the proximal portions of primitively distinct radialia to
form a basipterygium. _b_, Inner or mesial extremity of the basipterygium;
_d.p_, dorsally directed, rudimentary iliac process; _n_, foramen for
nerves. (After Rautenfeld.)]
The skeleton of the pelvic fins of the Teleostomi is often extremely
degenerate. It is perhaps best developed in the Chondrostei,[220] where
each fin is supported by numerous segmented radialia, more or fewer of
which fuse towards the base of the fin, and those form a large and slightly
ossified basipterygium (Fig. 148). In the living Crossopterygii, Holostei,
and Teleostei, the pelvic fins are similar in essential structure, but are
very degenerate. The basipterygium is usually well developed and is always
bony (Fig. 149), and in many Teleosts it acquires so extensive a sutural
connexion with its fellow that, physiologically, it supplies the place of a
true pelvic girdle. At its distal end there may be a single row of small
cartilaginous or bony nodules, representing vestigial radialia, as in the
Crossopterygii, Holostei, and {246}Teleostei, but even these may be absent,
and the dermal fin-rays then articulate directly with the basipterygium.
Little is known of the skeleton of the pelvic fins in the fossil
Crossopterygii, but there is evidence of the existence of a higher grade of
structure than in their surviving allies. In _Eusthenopteron_,[221] for
example, the fin is supported by an axis of at least three bony segments,
with at least three ossified preaxial radialia; hence, it has obviously
undergone less degeneration than in _Polypterus_, where the fin-skeleton is
essentially Teleostean. In the Dipnoi the pelvic fins are similar to the
corresponding pectoral fins, but individual variation is more marked and
even the central axis may divide.[222] In the males of all existing
Elasmobranchs, including the Holocephali, certain of the more distally
situated metapterygial radialia become modified to form a supporting
skeleton for the copulatory organs, the claspers, or mixipterygia. In the
latter group the anterior claspers are also provided with cartilaginous
supports articulating with the pelvic girdle directly in front of the
pelvic fins.
[Illustration: FIG. 149.—Skeleton of the left pelvic fin of a Trout (_Salmo
fario_), seen from the dorsal surface. B.PTG, Basipterygium; _D.F.R_,
dermal fin rays; PTG, distal radialia. (From Parker and Haswell.)]
{247}CHAPTER IX
THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS
The alimentary canal is a muscular tube with an epithelial lining, formed
for the reception and the digestion of the food. It begins with a mouth,
and from thence it extends backwards through the coelom, finally
communicating with the exterior either by a cloacal or by an anal orifice.
The oral or buccal cavity into which the mouth leads is a stomodaeum, and
is lined by inpushed epidermis, while the hinder portion of the cloaca and
the anus are lined by a somewhat similar inpushing of the epidermis which
forms the proctodaeum. The rest of the alimentary canal, consisting in
succession of a pharynx, an oesophagus, a stomach, and an intestine,
constitutes the mesenteron, and is lined by endoderm. Teeth are developed
from the walls of the stomodaeum, and glands for the secretion of digestive
fluids from the endoderm of the mesenteron.
DENTITION.
In the Lampreys among the Cyclostomata teeth are developed in the form of
yellow conical structures on the inner surface of the buccal funnel, and on
the extremity of the rasping "tongue" (Fig. 91, A). Each tooth consists of
an axial papilla of the dermis, sometimes enclosing a pulp-cavity, and
invested by the epidermis, and also by a stratified horny cone which forms
the projecting hard part of the tooth. The dermal papilla with its
ectodermal investment bears a superficial resemblance to the germ of a true
calcified tooth, but no odontoblasts are formed, nor any calcic deposit,
the laminated horny teeth being formed by the gradual conversion of the
successive strata of the {248}epidermic cells into horny layers.[223] The
old teeth are vertically replaced by new teeth developed beneath the
functional teeth. With the exception of a median tooth above the oral
aperture, _Myxine_ and its allies have only lingual teeth. These are
comb-like, and they are formed by the basal fusion of primitively distinct
tooth-germs. The structure and development of the teeth of the Cyclostomes
lend no support to the view that the teeth are degenerate calcified
structures. With greater probability they represent a stage in the
evolution of teeth and dermal spines, which has been succeeded by a later
stage in which calcification superseded cornification as a method of
hardening.
[Illustration: FIG. 150.—Vertical section of developing tooth in
_Petromyzon marinus_, showing a successional tooth, which is just beginning
to cornify at its apex beneath the functional tooth. _d_, Dermis; _d.p_,
dermal papillae; _ep_, epidermis lining buccal funnel; _ep^1_, epidermis
which has formed the horny functional tooth _ht_; _ep^2_, epidermis forming
the horny cone of the successional tooth _ht^1_. (From Warren.)]
True calcified teeth first make their appearance in Fishes, where they
assume the form of modifications of exoskeletal structures.[224] The teeth
of Elasmobranchs are identical in essential structure, as well as in the
manner of their development, with the ordinary dermal spines of the skin,
and in the embryo the {249}dermal spines form a continuous series with
those which invest the jaws and eventually become teeth (Fig. 151). It is
only later, when lips become apparent, that the continuity of the teeth and
dermal spines is interrupted, and the two structures assume their
distinctive characters.
The tissues of which the teeth of Fishes are composed are (1) _dentine_,
which is a non-vascular, calcified tissue, traversed by numerous radiating,
branched, dentinal tubuli, into which extend protoplasmic prolongations
from the cells (scleroblasts) by which the dentine is secreted. Dentine
forms the greater part of the body of a tooth. (2) _vasodentine_ and (3)
_osteodentine_ are modifications of ordinary dentine, the former containing
blood-vessels ramifying in its substance but no dentinal tubules, and the
latter more closely resembling bone. (4) _enamel_, an exceptionally dense,
non-vascular, non-tubular tissue, which may or may not exhibit traces of
the prismatic structure so characteristic of this tissue in the higher
Vertebrates, forms the outer investment of the teeth.
[Illustration: FIG. 151.—Transverse section through the lower jaw of an
embryo _Scyllium_, to show the gradual transition from dermal spines (_d_,
_d_, _d_) on the outer surface of the jaw to teeth (_t_, _t_, _t_) on the
oral surface. _c_, Cartilage of the lower jaw. (From Gegenbaur.)]
As regards their fixation, the more primitive forms of teeth, such as those
of Elasmobranchs, are simply embedded in the gums, and are only connected
with the jaws by fibrous tissue; but in some of the older fossil Sharks the
fixation of the teeth is effected by the mutual articulation of the basal
plates of the teeth with one another. The Chondrostean _Polyodon_, so
{250}shark-like in many other respects, also has teeth implanted basally in
the gums, and quite free from any special connexion with the jaw-bones. In
some Teleosts with movable teeth, the latter are merely attached to the
jaws by fibrous, and often elastic, ligaments, as in the Pike (_Esox_) and
the Angler-Fish (_Lophius_). As a rule, however, the teeth are directly
ankylosed to the bones developed in relation with the jaws. Very rarely,
as, for example, in some Characinidae, are the teeth implanted in sockets.
Nearly all Fishes are polyphyodont, that is, the old teeth are constantly
replaced by new teeth as fast as they become worn down or fall out. In the
Sharks and Dog-Fishes, for example, where the teeth are arranged in rows
parallel to the axis of each jaw, the functional teeth along the upper edge
of the jaw are usually erect, while those in the rows more internally
situated point inwards towards the oral cavity; and behind these again
there are rows of developing teeth in different stages of growth, and
partially hidden beneath a projecting fold of the oral mucous membrane
(Fig. 152). As the teeth in use become lost they are successively replaced
by the inner rows, which, with the mucous membrane in which they are
embedded, move forwards to the edge of the jaw, where they become erect and
functional. The teeth of the Holocephali and of the Dipnoi are not shed,
but the loss which they sustain through wear and tear is made good by
persistent growth at their bases. In the Teleostomi the succession is less
regular, new teeth being formed between or at the bases of the old teeth.
In the case of socketed teeth the succession is usually vertical, the new
teeth being formed at the sides of the old ones; and by the absorption of
the bases of the latter, the former come to lie directly below them, and
eventually they occupy the same sockets.
[Illustration: FIG. 152.—Transverse section through the jaw of a Shark
(_Carcharias_), showing how the teeth are replaced. _c_, Cartilage of the
jaw; _t_, functional tooth; _t′_, its immediate successor; _t"_, _t"_,
still younger teeth, covered by the fold of mucous membrane, _m. m_. (From
Ridewood.)]
As might be expected from the remarkable diversity in the {251}habits and
in the food of different Fishes, the teeth exhibit an equally striking
diversity in form, size, and structure. The most primitive type of tooth
resembles an ordinary dermal spine, and is little more than a simple
pointed cone. A few Elasmobranchs and many Teleostomi possess teeth of this
kind. By the flattening of the cone parallel to the axis of the jaw, the
tooth becomes triangular, and then the margins may either remain smooth and
trenchant, or they may become complicated by the formation of marginal
serrations or of accessory basal cusps, and by such modifications the
characteristic teeth of most Elasmobranchs are formed. The simple cone may
also be modified to form crushing teeth—short, blunt, more or less
hemispherical teeth—or even transformed into a mosaic of hexagonal plates,
as in the Myliobatidae amongst Elasmobranchs. Massive, flattened,
scroll-like crushing teeth are also formed by the fusion of adjacent teeth,
or of several successional teeth, and of such composite teeth we have
examples in the Heterodontidae and in the Palaeozoic Cochliodontidae. By a
somewhat similar process of concrescence the anomalous composite teeth of
such Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish
(_Scarus_), have been evolved. The singular dental structures of the
Holocephali are probably composite teeth, and it is certain that the highly
characteristic teeth of the Dipnoi have resulted from the basal fusion of
primitively distinct simple conical denticles. The dentition is often
heterodont. In _Heterodontus_ (_Cestracion_), for example, the anterior
teeth in each jaw are pointed and prehensile, while the hinder ones are
scroll-like and crushing. Prehensile and crushing molar-like teeth are also
present in such Teleosts as many of the Sparidae, and in the Wolf-Fish
(_Anarrhichas_). The existence of sexual differences in the dentition is
illustrated in the Skates and Rays (_Raia_), where teeth which are simple
and pointed in the male become flattened and plate-like in the female. A
few Teleosts, like the Syngnathidae, Cyprinidae, and some Siluridae, are
entirely devoid of jaw-teeth.
In addition to jaw-teeth, many Teleosts possess pharyngeal or gill-teeth,
developed in connexion with the inner margins of the branchial arches, to
which they are usually firmly ankylosed (Figs. 352, 412 and 413). As a rule
"the pharyngeal dentition is inversely proportional to the extent of tooth
development {252}on the jaws."[225] Pharyngeal teeth differ greatly in size
and structure in different Teleosts, and, like the jaw-teeth, they are
capable of replacement by vertical succession. The teeth are sometimes
restricted to the inferior pharyngeal bones (cerato-branchials of the last
branchial arch), and then, as in the Carp (_Cyprinus_), they may bite
against a callous pad on the under surface of the basioccipital bone; or,
as in some of the Wrasses (_Labrus_), the inferior teeth are opposed to
superior teeth on the upper pharyngeal bones (pharyngo-branchials of more
or fewer of the branchial arches). When pharyngeal teeth are present it is
probable that they are the principal masticatory organs, the jaw-teeth
being used for seizing or holding the prey.
ALIMENTARY CANAL.
A protrusible tongue is never developed in Fishes. A rudiment of that organ
is present in the Elasmobranchs (Fig. 153) and Dipnoi, and also in the
Crossopterygii, and usually consists of an elevated area of mucous membrane
provided with free lateral edges and a forwardly projecting apex; it is
supported by the basi-hyal element of the hyoid arch. In the Crossopterygii
(e.g. _Polypterus_) the tongue contains muscle fibres, and in the Dipnoi,
where the organ is better developed than in any other Fishes, special
lingual muscles are present.
The pharynx succeeds the oral cavity, and is perforated on each side by the
branchial clefts (Figs. 153, 154). The rest of the alimentary canal differs
considerably in various Fishes in the degree of distinctness of its several
regions, and in the extent to which it is convoluted. As a rule the pharynx
is followed in succession by an oesophagus, a stomach, and an intestine
(Fig. 153), the latter terminating in a portion usually termed the
"rectum." The boundaries of these regions are not always very obvious, but
are indicated by variations in calibre, by changes in the character of the
lining epithelium, by special valves or sphincter muscles, or by the
entrance of the ducts of certain glands like the pancreas and liver.
{253}[Illustration: FIG. 153.—Dissection of a male Dog-Fish (_Scyllium_).
The left side of the body is cut away to the median plane so as to expose
the abdominal and pericardial cavities and the neural canal in their whole
length. The alimentary canal and the liver have been drawn downwards, and
the oral cavity, the pharynx, part of the intestine, and the cloaca have
been opened. The cartilaginous parts of the skeleton are dotted, and the
calcified portions of the vertebral centra are black. _abd.cav_, Abdominal
cavity; _au_, auricle; _b.br_, basi-branchial; _b.hy_, basi-hyal; _c.art_,
conus arteriosus; _cd.a_, caudal artery; _cd.st_, cardiac part of the
stomach; _cd.v_, caudal vein; _cl_, cloaca; _cn_, centrum; _cr_, cranium;
_crb_, cerebellum; _d.ao_, dorsal aorta; _dien_, thalamencephalon; _epid_,
epididymis; _fon_, fontanelle; _gul_, oesophagus; _h.a_, haemal arch;
_i.br.a^1_-_i.br.a^5_, internal gill-clefts; _int_, intestine; _kd_,
kidney; _l.j_, lower jaw; _l.lr_, left lobe of liver; _med.obl_, medulla
oblongata; _mes_, mesentery; _n.a_, neural arch; _n.cav_, neural canal;
_olf.l_, olfactory lobes; _opt.l_, optic lobes; _pan_, pancreas; _pcd.cav_,
pericardial cavity; _pct.a_, pectoral arch; _ph_, pharynx; _pin_, pineal
body; _p.n.d_, vestigial Müllerian duct; _prs_, prosencephalon; _pty_,
pituitary body; _pv.a_, pelvic arch; _pyl.st_, pyloric portion of the
stomach; _r_, rostrum; _r.lr_, right lobe of liver; _rct.gl_, rectal gland;
_sp_, spiracle; _sp.cd_, spinal cord; _spl_, spleen; _sp.s_, sperm sac;
_sp.vl_, spiral valve; _s.v_, sinus venosus; _tng_, tongue; _ts_, testis;
_u.g.s_, urino-genital sinus; _u.j_, upper jaw; _ur_, metanephric duct;
_v_, ventricle; _v.ao_, ventral aorta; _v.def_, vas deferens or mesonephric
duct; _vs.sem_, vesicula seminalis. (From Wiedersheim, after T. J.
Parker.)]
The oesophagus is occasionally separated from the stomach by a slight
constriction, but more frequently the replacement of the squamous
epithelium of the oesophagus by the columnar epithelium of the stomach and
the appearance of gastric glands in the wall of the latter cavity afford
the only distinction between the two regions. The {254}commencement of the
intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in
the form of a ring-like, inwardly projecting thickening of the
circularly-disposed muscle fibres of the terminal extremity of the stomach,
and usually also by the entrance of the distinct or united ducts of the
liver and pancreas; sometimes, as in certain Elasmobranchs and in the
Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The
rectum, or terminal portion of the intestine, is distinguished from the
rest of the gut by its straight course to the cloacal aperture or the anus,
and sometimes by an increase in calibre. In _Box vulgaris_ and a few other
Teleosts[226] a caecal diverticulum indicates the commencement of the
rectum, while in a few cases the pre-rectal portion of the intestine
communicates with the enlarged rectal segment by a much constricted
valvular orifice which is suggestive of the ileo-colic valve of the higher
Vertebrates,[227] as in the Teleosts _Amiurus catus_,[228] _Trigla
gurnardus_, and _Cyclopterus lumpus_.
The relation of the regional divisions of the intestine in Fishes to those
of other Vertebrates are somewhat difficult to determine. If we may regard
the "rectal" gland of Elasmobranchs and the intestinal caecum of certain
Teleosts as homologous with each other, and with the caecum coli of the
higher Vertebrates, then it would seem that by far the greater part of the
intestine of Fishes, including that portion in which a spiral valve may be
developed, is homologous with the pre-caecal segment of the gut or small
intestine in other Vertebrates, and that the post-caecal section, or large
intestine, of the latter is represented in Fishes only by that relatively
short portion of the gut which lies posterior to the rectal gland or its
homologue in Teleosts, the equivalent of the colon of Mammalia being, as in
Amphibia, Reptiles, and Birds, practically undifferentiated.[229]
In the Cyclostomata the alimentary canal retains much of its primitive
simplicity. It pursues a straight course from mouth to anus, and the usual
regions are very obscurely indicated. The same remarks apply also to the
Holocephali and a few Teleosts, although in these Fishes the limits of the
different regions are somewhat more clearly defined. In the Dipnoi (Fig.
155, A), a contracted sigmoid curve between the somewhat dilated stomach
and the spacious intestine is the only departure from the straight course
of the preceding groups.
{255}[Illustration: FIG. 154.—Dissection of a male Teleost (_Salmo fario_)
from the left side. _a.bl_, Air-bladder opened; _an_, anus; _au_, auricle;
_b.a_, bulbus aortae; B.HY, basi-hyal; B.OC, basioccipital; _cd.a_, caudal
artery; _cd.v_, caudal vein; CN, centrum; _crb_, cerebellum; _d.f.1_, first
dorsal fin; _D.F.R_, dermal fin-rays; _du_, duodenum or anterior segment of
the intestine; _FR_, frontal; _g.bl_, gall-bladder; _gul_, oesophagus or
gullet; H.SP, haemal spine; _int_, intestine; _kd_, kidney; _kd′_,
"head-kidney"; _lg_, tongue; _lr_, liver; N.SP, neural spine; _opt.l_,
optic lobes; _PA.SPH_, parasphenoid; _ph_, pharynx; _pn.b_, pineal body;
_pn.d_, bristle passed into ductus pneumaticus; _prsen_, prosencephalon;
_pty.b_, pituitary body; PTG, pterygiophores, or radial elements of dorsal
and ventral fins; _pv.f_, pelvic fin; _py.c_, pyloric caeca; _S.ETH_,
supra-ethmoid; S.OC, supra-occipital; _spl_, spleen; _st_, stomach; _ts_,
testis; _u.bl_, urinary bladder; _u.g.s_, urino-genital sinus and its
external aperture; _ur_, ureter or kidney-duct; _v_, ventricle; _v.ao_,
ventral aorta; _v.df_, vas deferens; _v.f_, ventral fin; _VO_, vomer. (From
Parker and Haswell.)]
{256}In the remaining Fishes the degree of convolution varies within rather
wide limits. The oesophagus is usually straight and wide, but in
_Lutodeira_, among Teleosts, it is long and even convoluted, and in the
Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"),
which extends anteriorly as far as the head, and posteriorly to the
beginning of the tail, and communicates with the oesophagus by two
apertures. The stomach may be U-shaped with the concavity directed
forwards, and consisting of a right limb passing backwards from the
oesophagus, and a left limb curving forwards to its junction with the
intestine (Fig. 153). In such instances as these the stomach and the
adjacent section of the intestine describe a characteristic siphonal curve.
In certain other Fishes (Fig. 160), the oesophageal portion of the stomach
terminates behind in a tubular or sac-like dilatation at some distance
posterior to the laterally situated pylorus, which indicates the origin of
the intestine. The intestine is straight, or nearly so, in Elasmobranchs,
Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and
very generally in Teleosts, it is more or less convoluted, notably in some
of the Mugilidae, and in the Loricariidae, where, as in _Plecostomus_, it
is disposed in numerous spiral coils like a watch-spring. The terminal
portion of the intestine or rectum either opens into a cloaca, which also
receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and
Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of
the separate or united urinogenital ducts, as is the case with all the
remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably
situated near the junction of the caudal and trunk regions, and as a rule
is median in position, rarely, as in the Dipnoi, displaced to the right or
left of the middle line; but the anus differs greatly in position,
sometimes retaining its primitive position at the hinder end of the trunk,
as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many
Teleosts, or occupying almost any position between that point and, as in
the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig.
351.)
{257}[Illustration: FIG. 155.—A, alimentary canal and liver of a female
_Protopterus_, from the left side. Part of the left wall of the stomach and
intestine, and the peritoneal investment of the spleen have been removed.
_a.p_, Abdominal pore; _b.d_, bile-duct; _b.ent_, Bursa Entiana; _cl_,
cloaca; _cl.ap_, cloacal aperture; _cl.c_, caecum cloacae; _c.m.a_,
coeliaco-mesenteric artery; _cy.d_, bile duct; _k.d_, kidney duct; _m.a_,
mesenteric arteries; _od_, oviduct; _pt.c_, post-caval vein or inferior
vena cava; _p.v_, portal vein; the other reference letters as in B. (From
Newton Parker.) B, viscera of an adult female _Lepidosteus_, ventral view.
The oesophagus, the commencement of the intestine and the rectum have been
laid open. _ab_, air-bladder; _an_, anus; _b.d_, intestinal aperture of the
bile-duct; _g.b_, gall-bladder; _gl_, oesophageal aperture of the
air-bladder; _h.d_, hepatic duct; _l_, liver; _oes_, oesophagus; _py_,
pylorus; _py.c_; pyloric caeca; _py.c′_, the four intestinal orifices of
the pyloric caeca; _r_, rectum; _s_, spleen; _sp.v_, spiral valve; _st_,
stomach. (From Balfour and Newton Parker.)]
{258}[Illustration: FIG. 156.—Transverse section of a Fish, diagrammatic.
_cn_, Centrum; _coel_, coelome; _d.a_, dorsal aorta; _d.f_, dorsal fin;
_d.m_, dorsal muscles; _d.ms_, dorsal mesentery; _f.r_, fin ray; _gon_,
gonad; _int_, intestine; _l.v_, lateral vein; _msn_, mesonephros; _msn.d_,
mesonephric duct; _n.a_, neural arch; _p_, parietal layer of the
peritoneum; _p′_, visceral layer; _p.c.v_, posterior cardinal vein; _pn.d_,
Müllerian duct; _r_, ventral rib; _r′_, dorsal rib; _sp.c_, spinal cord;
_t.p_, transverse process; _v.m_, ventral muscles; _v.ms_, ventral
mesentery. (Modified, after Parker and Haswell.)]
The whole length of the alimentary canal from the oesophagus to the rectum
is invested externally by the visceral layer of the peritoneum (Fig. 156),
which histologically consists of a stratum of connective tissue, supporting
on its free surface an epithelial stratum (coelomic epithelium). Primarily,
the investing peritoneum is continued both dorsally and ventrally into
bilaminar suspensory folds, the dorsal and ventral mesenteries (_d.ms_,
_v.ms_), which extend to the mid-dorsal or mid-ventral line of the
abdominal cavity. The two layers then separate and become continuous with
the parietal layer of the peritoneum lining the whole of the inner surface
of the body-wall. Embryologically, the two mesenteries owe their formation
to the fusion above and below the mesenteron of the contiguous walls of two
laterally situated and primitively distinct coelomic cavities. The dorsal
mesentery in the adult is occasionally complete, as in the Myxinoid
Cyclostomata and in the Elasmobranch _Hypnos subnigrum_,[230] and also in
some Dipnoi and in a few Teleosts, but much more frequently it is reduced
by absorption to anterior and posterior remnants, or to a series of
isolated bands, or even, as in the Lamprey (_Petromyzon_), to a few
filaments accompanying the intestinal blood-vessels. The ventral mesentery,
on the contrary, is rarely present, and if present is never complete. In
_Lepidosteus_[231] a ventral mesentery {259}is said to be present in
connexion with that part of the intestine which contains the spiral valve.
In _Protopterus_,[232] and also in _Neoceratodus_,[233] there is a
well-developed ventral mesentery in relation with the greater part of the
length of the intestine, although in the former Dipnoid its continuity is
interrupted by one or two vacuities, and in the latter the mesentery is
incomplete posteriorly. A ventral mesentery is also present in the
intestinal region of some of the Muraenidae among Teleosts.[234]
[Illustration: FIG. 157.—Transverse section through a portion of the wall
of the intestine, combined from the condition seen in both the higher and
the lower Vertebrata. Semi-diagrammatic. _a.c_, Epithelial cells in the
amoeboid state; _b.v_, blood-vessels; _c.m_, circular muscular layer; _g_,
one of Lieberkühn's glands in the higher Vertebrates; _i.ep_, intestinal
epithelium; _l_, leucocytes; _l′_, leucocytes in the intestinal epithelium;
_l.f_, lymph follicles; _l.m_, longitudinal muscular layer; _lym_,
lymphatic vessels; _p_, visceral layer of the peritoneum; _sm_, the
submucosa; _v_, villi of the higher Vertebrates. (From Wiedersheim.)]
Internal to its peritoneal investment the wall of the alimentary canal
consists in succession from without inwards of (1), a {260}muscular coat,
(2) the submucosa, and (3) an epithelial stratum or mucous membrane, the
first two of these layers, with the addition of the peritoneum, being
derivatives of the inner or splanchnic portion of the embryonic
mesoblast.[235]
Excluding the oesophagus, where the muscular coat is mainly composed of
striated fibres, the musculature of the alimentary canal usually consists
solely of non-striated, spindle-shaped fibres disposed in two layers, an
external stratum of longitudinally arranged fibres, and an inner stratum of
circularly disposed fibres (Fig. 157), with the addition, in the stomach,
of an oblique layer between the two. In the oesophagus the reverse
arrangement may exist, the circular layer being external and the
longitudinal internal. The muscular coat varies considerably in thickness
in different regions and in different Fishes, and in the Cyclostomata, the
Holocephali, some Teleosts, and the Dipnoi may be very feebly developed, or
even entirely absent, as in the intestine of the Hag-Fish (_Myxine_). In
the Gillaroo Trout (_Salmo stomachicus_),[236] on the contrary, the distal
section of the siphonal stomach has its musculature unusually thickened, so
as to form an incipient gizzard for the crushing of the shells of the
freshwater Molluscs on which the Fish feeds. In some of the Mullets
(Mugilidae),[237] a true gizzard is developed by the enormous thickening of
the muscular coat of the caecal stomach, the cavity of which, in
consequence, is reduced to a mere vertical fissure, and is lined by an
exceptionally thick, horny epithelium.
There are a few exceptions to the rule that the muscular fibres are of the
non-striated variety. Thus in some Teleosts, as in the Tench (_Tinca
vulgaris_), striated fibres are continued from the oesophagus into the
walls of the stomach and intestine, and there form an outer longitudinal
and an inner circular layer, situated externally to the corresponding
layers of the non-striated stratum. {261}In the Siluroid, _Amiurus_, the
striated fibres of the outer circular layer of the oesophagus are
continued, although but sparsely, into the inner circular layer of the
stomach.
The submucosa (Fig. 157) lies between the muscular layer externally and the
epithelial lining internally, and is characteristically developed in the
stomach, and even more so in the intestine. Histologically, it consists of
a framework of connective tissue, enclosing in its meshes masses of
leucocytes (lymphoid tissue), some of which are amoeboid and migratory, and
may even be found between the cells of the intestinal epithelium (including
in some instances the cloacal epithelium), probably actively participating
in the transmission of food material from the alimentary canal to the
lymphatics and blood-vessels; while other and somewhat similar, but larger,
leucocytes (phagocytes), are concerned with the elimination of waste
substances or noxious micro-organisms. In addition to the diffused lymphoid
tissue of the submucosa, special rounded or oval, and sometimes encapsuled,
masses of this tissue (lymph follicles) are common in the intestinal wall
(Fig. 157) of _Acipenser_, the Dipnoi and some Elasmobranchs, and are
perhaps the only representatives in Fishes of the solitary follicles or
"Peyer's patches" of the higher Vertebrates. A mass of lymphoid tissue
exists in the axis of the spiral valve of _Acipenser_, which has been
compared with a similarly situated structure in _Lepidosiren_.[238] In some
Elasmobranchs a large lymphoid organ is imbedded in the submucosa of the
oesophageal wall, while a local thickening of the tissue is met with in the
pyloric sphincter. _Protopterus_ is remarkable among Vertebrates for the
extraordinary development of lymphoid tissue,[239] which, apart from its
distribution in the submucosa, is abundantly present between the
longitudinal and circular muscle layers, and the peritoneal and muscular
coats of the intestine.
In addition to the lymphoid tissue the submucosa contains non-striated
muscle cells and plexuses of capillary blood-vessels, which in certain
Loaches (e.g. _Misgurnus_), where intestinal respiration occurs, extend
between the cells of the intestinal epithelium. A network of lymphatic
spaces or vessels surrounds the blood-vessels. In some Elasmobranchs the
small arteries of the submucosa of the stomach are provided with singular
sphincter muscles, which {262}occasionally encircle both the artery and the
corresponding vein.[240]
The lining epithelium differs considerably in character in different
portions of the alimentary canal. The epithelium of the mouth, pharynx, and
anterior section of the oesophagus is often squamous and is succeeded in
the hinder part of the oesophagus, and in the stomach and intestine, by a
columnar epithelium. As a rule the epithelium of the rectum is also
columnar, but in Elasmobranchs it may become squamous. Goblet cells are of
very frequent occurrence throughout the whole length of the alimentary
canal, from the mouth to the rectum inclusive, interspersed between the
superficial epithelial cells; in the same position in the intestine
migratory leucocytes have been found. The primitive ciliation of the
Vertebrate alimentary canal is retained to a greater or less extent in many
Fishes, and is sometimes, but not always, associated with a feeble
development of the musculature. In the larval form of _Petromyzon_
(_Ammocoetes_), the whole canal is ciliated except the pharynx and rectum;
but in the adult ciliation is retained only in places which gradually
become fewer as the rectum is approached. In the Myxinoids, however, cilia
are said to be absent.
In the Dipnoi (e.g. _Protopterus_) the epithelium of the stomach and
intestine is largely ciliated, but in Elasmobranchs, ciliation is usually
restricted to the posterior portion of the oesophagus and the edge of the
spiral valve. Among the more generalised Teleostomi (e.g. _Acipenser_,
_Lepidosteus_, _Amia_), the oesophagus, stomach, and intestine may be
ciliated, but to an extent which varies in different genera. The pyloric
appendages, when present, are also more or less extensively ciliated. In
Teleosts, however, the recorded instances of ciliation are relatively rare.
Nevertheless, ciliated epithelium has been found in the intestine of a few
species (e.g. _Rhombus aculeatus_ and _Syngnathus acus_), and also in the
pyloric appendages; in the stomach (e.g. _Perca_ and _Esox_), and even in
the oesophagus (e.g. _Perca_).
The mucous membrane, including the submucosa, is frequently developed into
variously arranged ingrowths projecting into the lumen of the alimentary
canal; these are generally of the nature of longitudinal or transverse
ridges, or a combination of the two, giving rise to retiform structures.
The simple longitudinal {263}folds, which are sometimes found in the
oesophagus, stomach, and rectum, often disappear on distension, and
probably merely provide for the enlargement of these cavities during the
deglutition of relatively large prey, or for the accumulation of faeces. On
the other hand, the permanent and often complicated folds of the intestinal
mucous membrane are probably related to an increase in the secretive or
absorptive area of this portion of the alimentary canal. In the stomach the
mucous membrane is usually smooth, rarely, as in the "Electric Eel"
(_Gymnotus_), reticulate. In the intestine the folds assume a highly
characteristic and often complicated disposition.[241] In the Cyclostomata
the folds are simple and longitudinally arranged. In Elasmobranchs (Fig.
158, A), obliquely transverse folds are present in addition, and, uniting
with the longitudinal ridges, bound linear depressions.
[Illustration: FIG. 158.—The intestinal mucous membrane of different
Fishes, to show the transition from simple longitudinal and transverse
folds to crypts. A, Of an Elasmobranch; B, C, and D, of various Teleosts.
(After Wiedersheim.)]
In various Teleostomi (Fig. 158, B, C, D), the union of the two series of
folds becomes more or less retiform, and the network of intersecting ridges
bounds a series of deep tubular crypts which appear to penetrate to a
considerable distance into the intestinal wall, and possibly foreshadow the
characteristic Lieberkühn's glands of Mammalia. Crypts may also be found in
the stomach, where they receive the apertures of the gastric glands, as in
_Amiurus_, but more usually they are restricted to the intestine. In the
Dipnoi (e.g. _Protopterus_) the mucous membrane of the {264}stomach,
and—excluding the Bursa Entiana where a number of oblique folds are
present—of the intestine also, is, on the contrary, perfectly smooth.
In addition to transverse and longitudinal folds the mucous membrane of the
various sections of the alimentary canal is often developed into outgrowths
which are more or less linear.[242] In the oesophagus these may be
papilliform, as in _Box_ and _Caesio_; obtuse in _Acipenser_, hard and
almost spine-like in species of _Rhombus_; or in the form of pyramidal
retroverted processes with jagged or fringed edges, as in the Spiny
Dog-Fish (_Acanthias vulgaris_). In the Basking Shark (_Selache_) similar
processes are present, which, near the stomach, become unusually long and
branched, so that the entrance to that cavity is surrounded by a series of
backwardly-directed arborescent tufts. Peculiar papillose or tag-like
processes of the mucous membrane are frequently present on the spiral valve
of Elasmobranchs, in the intestine of such Teleosts as _Balistes_, _Mugil_
and some Pleuronectidae, and also in the rectum of _Rhombus maximus_.
Of all the outgrowths from the mucous membrane of the alimentary canal the
so-called "spiral valve" of the Cyclostomata, Elasmobranchs, Holocephali,
Chondrostei, Crossopterygii, Amiidae, Lepidosteidae and Dipnoi is the most
characteristic. The first appearance of this structure was probably in the
form of a straight longitudinal fold or ridge projecting into the cavity of
the intestine, similar, perhaps, to the typhlosole of many Invertebrata.
This primitive condition is not retained in any existing Fishes, although
it may be closely approached in the larval Cyclostome (_Ammocoetes_), and
is perhaps also indicated in the straight anterior portion of the spiral
valve of _Polypterus_. Absent altogether in the Myxinoids, the valve is
represented in its simplest condition, as in certain other Cyclostomata
(e.g. _Petromyzon_), by a ridge of mucous membrane which commences
anteriorly on the dorsal side, and, after describing a partial spiral as it
passes backwards, terminates posteriorly on the ventral side, the width of
the valve not exceeding half the diameter of the intestine. This simple
type of valve is repeated in embryo Elasmobranchs, but in the adults of
these Fishes the valve becomes much more complicated, and exhibits a wide
range of structural variation. The increased complexity of the valve
{265}seems to depend on several factors, the effect of which, in different
Elasmobranchs, is best studied in a series of valves of progressively
higher differentiation.[243]
In a hypothetical simple type of valve, easily derivable from the more
primitive type of _Petromyzon_, it may be conceived that, while not
exceeding in width the semi-diameter of the intestine, the valve becomes
disposed in several complete and more or less closely approximated spiral
turns, the free edge of the valve being on the same level as its attached
margin, and leaving an open axial canal along the centre of the gut. The
nearest approach to this hypothetical type, which has been compared, not
inaptly, to _un escalier tournant sans noyau_, is perhaps to be found in
the Thresher-Shark (_Alopecias vulpes_).
The structure of the more complicated spiral valves of other Elasmobranchs
are well illustrated within the limits of the single genus _Raia_.
In one specimen of _Raia_ sp. (Fig. 159, A) the last four coils of the
valve are similar to those of the hypothetical type, but the more anterior
ones, owing to the greater width of the valve, which here exceeds the
semi-diameter of the intestine, have their free margins deflected
downwards, while that portion of the valve which forms the first half turn
is coiled inwards upon itself, so as to form a hollow cone, open dorsally,
and having its apex directed forwards. In other examples a further
modification is introduced by the increasing width of the valve, which now,
throughout its whole length, equals the semi-diameter of the intestine; and
by the formation of an axial columella by the thickened free edge of the
valve, which is traversed by a central band of unstriped muscle, as well as
by the intra-intestinal artery and vein, and takes the place of the central
canal of the preceding types. The valve is, however, still regular, and its
free margin remains on the same level as the corresponding portion of the
attached edge. In other specimens, again, additional complications are
introduced by a still further increase in the width of the valve, which now
exceeds, often considerably, the semi-diameter of the intestine, and the
consequent deflection of the free edge of the valve either forwards or
backwards (C and D). As shown in C the valve, in consequence of the
backward deflection of its free margin, presents the appearance of a nest
of {266}imperfect truncated cones with their apices directed backwards, the
successive cones adhering so closely to one another that they combine to
form a central conical chamber with a spirally disposed cavity winding
round it. In D, on the contrary, the free edge of the valve is deflected
forwards, so that, as in C, a nest of cones is formed, but the apices of
the successive cones are directed forwards instead of backwards.
Notwithstanding these variations in the structure of the valve as a whole,
the first coil or half coil nearly always resembles that described in A.
[Illustration: FIG. 159.—Examples of various types of the spiral valve in
Elasmobranchs. A, B, C, and D in specimens of _Raia_ spp.; E, in _Sphyrna
malleus_. A, B, and D represents longitudinal sections of the intestine,
the ventral portion of the valve being removed. In C successive portions of
the ventral wall of the intestine have been cut out. In E the intestine has
been opened along the mid-ventral line and its wall reflected to the right
and left; the ventral portion of each coil of the "scroll" valve has been
removed. In most of the figures the pylorus is shown in the upper part, and
the "rectal" gland in the lower. (From T. Jeffery Parker.)]
It is obvious that the structure of the valve varies considerably within
the limits of the genus, and it may be added that various intermediate
types of structure occur between A and B, {267}A and C, and A and D. The
individual variations are perhaps even more remarkable, and appear to be
quite independent of age and sex. By way of example it may be mentioned
that valves approximating to one or other of those represented by C and D
occur in different individuals of _Raia maculata_ of the same sex and
similar in size, even in young specimens not more than three inches in
length.
As regards other Elasmobranchs, the common Dog-Fish (_Scyllium
canicula_)[244] has a well-developed spiral valve disposed in twelve coils,
which structurally represents a more highly developed example of the type
D. The existence of considerable individual variation is nevertheless
indicated by the fact that in one specimen examined the valve was
intermediate between C and D, five of the eight cones projecting forwards
and three backwards. In a specimen of _Notidanus_ sp.[245] there were as
many as twenty coils, which in disposition were intermediate between B and
C, approximating, however, more nearly to B. In a specimen of the Port
Jackson Shark (_Heterodontus_)[246] the valve had eight coils, and in
structure was also intermediate between B and C, but approached more nearly
to C. Some of the Hammer-headed Sharks (e.g. _Sphyrna malleus_)[247]
possess a type of spiral valve which differs considerably from any of those
hitherto described, and is termed a "scroll" valve (Fig. 159, E). The
attached edge of the valve pursues a straight longitudinal course, or at
any rate only describes a half turn and back again in passing from the
pyloric to the cloacal extremity of the gut. In the middle of its course
the width of the valve is about equal to two-thirds of its length, but
towards either extremity it gradually diminishes until the free and
attached margins meet. The valve thus constituted is rolled upon itself
from left to right, the successive coils being comparable to a series of
cylinders placed one inside the other, and becoming gradually larger both
in length and diameter from within outwards. A similar valve is present in
some of the Carchariidae.
In the Holocephali (e.g. _Chimaera monstrosa_)[248] the valve describes
only three and a half coils, and is further remarkable in that the attached
margin, for a considerable portion of its {268}extent, does not form a
regular spiral but describes only a slightly sinuous course. Posteriorly,
the valve is more normal, and consists of about two cones with their apices
directed forwards.
In the Dipnoi the spiral valve is well developed, and in
_Neoceratodus_[249] describes nine coils, and in _Protopterus_[250] six or
seven. The structure of the valve in the latter Dipnoid resembles that of
_Scyllium canicula_, except for the smaller number of cones.
In the more generalised Teleostomi the valve is best developed in the
Sturgeon (_Acipenser_) and in _Polypterus_. In the former[251] the valve is
restricted to the posterior half of the total length of the intestine,
often extending to within an inch of the anal aperture, and describing in
its backward course about seven or eight coils. The width of the valve is
about equal to the semi-diameter of the intestine, and the thickened free
margin forms a well-marked axial columella, round which the cavity of the
gut winds, as in the type B, except that the spiral is a more open one. In
_Polypterus_ the valve begins close to the solitary pyloric caecum, and for
some distance pursues a straight longitudinal course, but eventually forms
a few spiral coils, ceasing, however, at a considerable distance from the
anus. The evidence afforded by petrified faeces or "coprolites" proves that
certain extinct Crossopterygii (e.g. _Macropoma_, _Megalichthys_), like
their living representative, _Polypterus_, possessed a spiral valve.[252]
In _Amia_ and _Lepidosteus_[253] the valve is almost vestigial, being
restricted to the terminal portion of the intestine, and is somewhat
variable as to the precise number of its coils. In _Amia_ there are nearly
four coils, extending over 3 cm., that is less than a tenth of the total
length of the intestine, but in some specimens the coils do not exceed two
and a half or three in number. _Lepidosteus_[254] has a still shorter valve
which, in specimens of 7-10 cm. in length, may not consist of more than
three and a half coils, and in much larger specimens may be reduced to less
than two coils, a variation which suggests that a reduction takes place in
the number of coils as the fish increases in age and size. The structure of
the valve in the three last-mentioned genera resembles that described in
_Acipenser_, and in none of them does {269}the width of the valve so far
exceed the semi-diameter of the intestine as, by forward or backward
deflection, to give rise to the highly characteristic cones of
Elasmobranchs and Dipnoi.
In the more specialised Teleostomi (Teleostei) the spiral valve is wholly
wanting, except perhaps as a vestigial structure in certain Clupeoids, as,
for example, _Chirocentrus_,[255] and possibly also in some
Salmonidae.[256]
From what has been said as to the structure of the spiral valve in the
different groups of Fishes, it may be concluded that the valve most nearly
retains its primitive condition in the Cyclostomata; attains its maximum
development in the Elasmobranchs, especially in the Notidanidae, and shows
no indication of degeneration in the Dipnoi. In the Holocephali and the
lower Teleostomi, on the other hand, the valve exhibits various stages of
retrogressive modification, and in the Teleosts is either absent altogether
or persists only as a vestigial structure in a very few species.
From a physiological point of view the object of the spiral valve is to
increase the absorptive inner surface of the intestine,[257] but, from what
has been said as to the structural variability of the valve, it is obvious
that its efficacy from a functional standpoint must be equally variable.
The value of the valve as an absorptive mechanism necessarily depends on
the area of absorption-surface which it provides, as well as on the degree
of resistance which it offers to the passage of food material along the
cavity of the intestine. These factors will in turn depend on the number of
coils, on the width of the valve, and on the extent to which its free
margin is deflected in forming the series of cones, but these again are
precisely the structural features which are most liable to variation. The
total absorption area in the four types of valve characteristic of the
genus _Raia_ has been calculated, and may be expressed in square
centimetres as follows:—A, 136.64; B, 143.82; C, 254.3; and D, 276.7.[258]
Hence as regards mere absorption area a spiral valve of the type D has
twice the extent of a valve of the type A, and if, in addition, account be
taken of the retardation of the food due to the increased obstruction
offered by the columella and cones in D, it is clear that the
{270}difference in physiological value between the two types must be far
more considerable than is indicated by a comparison of their relative
superficial areas alone.
The evolution of the spiral valve was probably due to the necessity of
increasing the absorptive area of an almost straight unconvoluted
intestine, a result which in other animals is often obtained by an increase
in the length and concurrent convolution of the intestine itself. Any
attempt to correlate the variations in the degree of perfection or
imperfection of the valve considered as an absorptive mechanism with any
special variations in the nature or quality of the food is, however, a very
difficult problem, and a satisfactory explanation has yet to be found. The
difficulty, moreover, is increased by the fact that the majority of Fishes
with a spiral valve are mainly carnivorous; the Elasmobranchs, in which
this structure is at the same time most highly developed and most variable,
exclusively so. On the other hand, the term "carnivorous" covers a
multiplicity of minor differences in the nature and relative digestibility
of different forms of animal food, and it is quite possible that it is with
differences of this kind that the specific or individual variations in the
development of the spiral valve are associated. The absence of the valve in
the variously nourished Teleosts, save perhaps as a vestige in one or two,
is also difficult to account for, although it is not improbable that
compensating structural modifications exist in this group. As a rule, the
intestine is much more convoluted in these Fishes, but to an extent which
varies greatly in different species, while the characteristic pyloric caeca
and the spiral valve appear to a certain extent to be developed in inverse
proportion to one another.
THE GLANDS.
The glands associated with the alimentary canal in different Fishes are (1)
the gastric glands, (2) the liver, (3) the pancreas, (4) the pyloric
appendages, and (5) the "rectal" gland.
Oral salivary glands are wanting in all Fishes, the only secretory
structures in the mouth being numerous mucus-secreting goblet cells, which
here, as elsewhere throughout the alimentary canal, are intermixed with the
ordinary epithelial cells.
THE GASTRIC GLANDS.—The Cyclostomata and Dipnoi do not possess any
specially differentiated gastric glands, and it is {271}probable that in
these Fishes the secretion of the digestive fluids is effected by the
ordinary lining epithelium of the stomach or intestine, or both. In the
remaining groups gastric glands are generally present in the form of simple
caecal structures embedded in the submucosa and opening on the surface of
the mucous membrane into the cavity of the stomach. The glands differ in
different Fishes in the character of their lining epithelium and in the
extent to which their component cells are differentiated from the
epithelium of the stomach. There does not appear, however, to be any
distinction into "central" (pepsin-forming) and "parietal" (acid-secreting)
cells, as is the case in the higher Vertebrata. Towards the pyloric end of
the stomach the true gastric glands are often replaced by mucous glands.
There are, nevertheless, not a few Teleosts in which special gastric glands
are absent, as, for example, _Syngnathus acus_, and several species of
Cyprinidae, Labridae, and Blenniidae, etc. In at least two genera
(_Gastrosteus_ and _Cobitis_), belonging to widely different families,
gastric glands are present in certain species but absent in others. As
suggested by Edinger,[259] the absence of these glands may possibly be due
to degeneration.
It may be remarked that the formation of such digestive ferments as pepsin
and trypsin, which are associated with the stomach and pancreas
respectively, in the higher Vertebrates, is not nearly so strictly
localised in Cyclostomes and Fishes. So far from peptic digestion being
limited to the stomach, it may take place in the pharynx, stomach, and
intestine of Ammocoetes, and in some Elasmobranchs (e.g. _Scyllium_), and
in such Teleosts as the Pike, Eel, and Carp, the peptic region extends from
the stomach for some distance along the intestine, while trypsin has been
obtained from the mucous membrane of the stomach, intestine and pyloric
caeca, as well as from the pancreas.[260]
Intestinal glands analogous to the glands of Lieberkühn in the higher
Vertebrates seem to be entirely wanting in Fishes, unless represented by
the sac-like or tubular crypts which are so generally present in the
Teleostomi.
THE LIVER.—Phylogenetically the oldest gland in connexion with the
Vertebrate alimentary canal, and in size by far the {272}largest, the liver
arises as a caecal outgrowth from the embryonic mesenteron, and in this
primitive stage recapitulates a condition which is retained throughout life
in Amphioxus. By the subsequent division and branching of this outgrowth
the massive compound tubular gland of the adult Fish is eventually formed.
The liver of Fishes (Figs. 153, 154) is very variable in size, shape,
colour, and degree of lobulation. Anteriorly, it is usually moulded to the
posterior face of the transverse septum between the pericardial and
abdominal portions of the coelom, and from thence extends backwards in the
abdominal cavity to a varying distance, in some Sharks as far as the
cloaca. Externally, the gland is invested by the peritoneum, which extends
on to it from the pericardial septum and forms a suspensory fold, and also
from the oesophagus and stomach. The shape of the liver usually bears some
relation to that of the body, being, for example, longest in the Eels and
broadest in the Rays. In the great majority of Fishes the liver is bilobed,
consisting of two sub-equal lateral lobes, disposed longitudinally and
confluent anteriorly for a portion of their extent. From this normal type
there are a few minor variations.[261] In _Petromyzon_, _Lepidosteus_ (Fig.
155, B), and a few Teleosts (_e.g._ the Gymnodontes, Lophobranchii, and
some Salmonidae) the liver is unilobed. In the Myxinoids and in the Dipnoi
(e.g. _Protopterus_), the organ is bilobed, but the small anterior lobe
lies immediately in front of the much larger posterior lobe, with the
gall-bladder between the two (Fig. 155, A). In some Teleosts (e.g.
_Scomber_), the liver is trilobed. A gall-bladder is invariably present in
either the larval or adult Cyclostomata, in the Chrondrostei, Holostei,
Crossopterygii and Dipnoi, and generally also in Elasmobranchs and
Teleosts. In the Elasmobranchs it is rarely entirely wanting, as in
_Sphyrna_ and _Pristis_, and in the Teleosts in some of the Gurnards
(_Trigla_). The gall-bladder and bile-duct of _Petromyzon fluviatilis_
atrophy after the metamorphosis which follows the larval _Ammocoetes_
stage, but in _Petromyzon marinus_ the duct, although usually absent, is
sometimes retained. In the Ammocoetes the epithelium lining the
gall-bladder is ciliated. In some Fishes, as, for example, in many
Elasmobranchs, the gall-bladder is more or less completely embedded in the
substance of the liver; in others, as in most Teleostomi, the organ is
quite distinct from the gland (Fig. 154).
{273}A simple arrangement of the ducts from the liver and gall-bladder is
that found in the common Dog-Fish (_Scyllium canicula_). In this
Elasmobranch a cystic duct leaves the gall-bladder, and, after receiving
several hepatic ducts from the lobes of the liver, becomes the bile-duct
and opens into the commencement of the intestine. In the Myxinoids and in
the Dipnoi (e.g. _Protopterus_), there are but two hepatic ducts, one from
each lobe of the liver; these unite and then meet the cystic duct to form
the bile-duct (Fig. 155, A). The number of hepatic ducts may, however, be
considerably increased, as, for example, in the Siluroid _Amiurus_,[262]
where 8-10 separate ducts join the cystic duct. In a few instances one of
the hepatic ducts opens directly into the intestine, independently of that
which unites with the cystic duct in forming the bile-duct. In the Dipnoi
(e.g. _Protopterus_),[263] and in some Teleostomi (e.g.
_Lepidosteus_),[264] the bile-duct receives the duct from the pancreas
before opening into the intestine.
THE PANCREAS.—In the Cyclostomes (e.g. _Petromyzon_, _Bdellostoma_,
_Myxine_) a rudimentary pancreas is apparently present, but the evidence as
to its identity is not wholly conclusive. A well-developed pancreas occurs
in Elasmobranchs, in at least one of the Dipnoi, and probably in most
Teleostomi.[265]
In Elasmobranchs the pancreas is a compact structure, uni- or bi-lobed, and
entirely distinct from the liver. In _Scyllium canicula_ (Fig. 153), the
bilobed gland lies in the angle between the distal limb of the stomach and
the adjacent portion of the intestine, and from the smaller of its two
lobes the duct issues to pass to its intestinal aperture near the
commencement of the spiral valve. In most of the Teleostomi in which its
existence has hitherto been recorded, the pancreas is a singularly diffuse
gland; and usually a considerable portion, or even the whole of it, is
embedded in the substance of the liver, its lobules accompanying the
ramifications of the hepatic artery and duct, and the portal vein. The
pancreatic duct usually opens into the intestine near the aperture of the
bile duct (e.g. _Amiurus_); sometimes the two ducts open on the apex of a
common papilla (e.g. _Acipenser_ and _Amia_), or by their union form a
common {274}duct (e.g. _Lepidosteus_). Among the Dipnoi a well-developed
pancreas is present in _Protopterus_,[266] embedded in the wall of the
stomach and intestine, internal to the peritoneal investment of these
organs, and extending even into the first fold of the spiral valve. The
gland is traversed by fine ductules which unite together and open into the
bile-duct just before the latter enters the intestine. In the remaining
Dipnoi the existence of a pancreas has yet to be ascertained.
Developmentally, the pancreas resembles the liver, and, histologically, is
very similar to that of the higher Vertebrates, consisting of terminal
glandular alveoli continuous with intermediary tubular portions, and
eventually with the finer ductules, which, by their union, form the main
efferent duct.
THE PYLORIC CAECA.—These structures are caecal outgrowths from the
intestine, and are situated close to the pyloric extremity of the stomach
and the intestinal apertures of the bile and pancreatic ducts. Wholly
wanting in the Cyclostomata and Dipnoi, and, unless represented by a pair
of caeca opening into the long, tubular, non-valvate anterior portion of
the intestine in the Greenland Shark (_Laemargus borealis_),[267] in the
Elasmobranchs also, they are very generally present in the Teleostomi,
although extremely variable both in number and arrangement in different
families. In _Amia_ there is no trace of pyloric caeca. _Polypterus_ has a
single short caecum with a thick muscular wall. In _Acipenser_, _Polyodon_,
and _Lepidosteus_, on the contrary, pyloric caeca are unusually well
developed. In _Acipenser_ the caeca are not only numerous, but are so
connected together by connective tissue and blood-vessels, and so invested
externally by the peritoneum, as to form a large, compact, gland-like mass,
communicating with the intestine by a single wide duct. In _Polyodon_ the
organ is essentially similar, but is lobed externally. In _Lepidosteus_
(Fig. 155, B, _py.c_), the caeca are also very numerous, but relatively
short, and, although united into a compact mass, open by four pit-like
orifices into the intestinal cavity. In Teleosts the caeca are subject to
extraordinary variations in number, size, and arrangement.[268] In some
families, and even in groups of higher taxonomic value, they are entirely
absent, as is the case with the {275}Siluridae, Esocidae, Cyprinodontidae,
Labridae, Plectognathi, and Lophobranchii. The "Sand-eel" (_Ammodytes_) has
but a single caecum; the Turbot (_Rhombus maximus_) two, and other
Pleuronectidae three to five; and the Perch (_Perca_), three (Fig. 160,
_py.c_).
In other Teleosts, on the contrary, these structures are much more
numerous. In _Labrus labrax_ there are about 60, in the Whiting (_Gadus
merlangus_) 120, while in the Mackerel (_Scomber scombrus_) there are no
fewer than 191. If few in number the caeca open separately into the
intestine, but when numerous, more or fewer of them may unite to form a
smaller number of efferent ducts, as in the Whiting, where four such ducts
are formed. In some instances, as in the Tunny (_Thunnus_), the union of
the caeca by connective tissue leads to the formation of a compact mass. As
regards their arrangement, the caeca may either be disposed in a whorl
round the intestine, as in the Whiting, or in a linear series, as in the
Salmon (_Salmo_) and in some of the Clupeidae.
The mucous membrane lining the anterior pyloric caeca is often developed
into a network of ridges, limiting crypt-like or tubular depressions; and
not infrequently the epithelium is ciliated.
[Illustration: FIG. 160.—The alimentary canal of a Perch (_Perca_). _an_,
Anus; _in_, intestine; _oes_, oesophagus; _py_, pylorus; _py.c_, pyloric
caeca; _st_, stomach. (After Wiedersheim.)]
The precise function of these organs, whether digestive or absorptive, is
still uncertain.[269] That they may be digestive is suggested by the
presence of certain amylolytic and proteolytic enzymes, but this obvious
conclusion is to some extent vitiated by the close proximity of these
organs to the stomach, and more especially to the intestinal orifice of the
pancreatic duct. It is by no means improbable, however, that the caeca are
both digestive and absorptive organs. An attempt has been made to show that
the pyloric caeca and the spiral valve vary inversely as regards the extent
of their development in different groups of {276}Fishes.[270] To some
extent the reciprocal variation of these structures supports this view, but
it is also evident that there are obvious objections to its unqualified
acceptance. Thus, in some Teleostomi (e.g. _Acipenser_, _Polyodon_),
exceptionally well-developed and numerous caeca and a spiral valve are both
present. _Amia_ with an almost vestigial spiral valve has no trace of
pyloric caeca, and in Teleosts the absence of a spiral valve is associated
with the complete suppression of the caeca in many large and important
groups.
THE RECTAL GLAND.—The "rectal" gland, or appendix digitiformis, is a small
organ of unknown function with complex glandular walls, and a central duct
opening dorsally into the terminal portion of the intestine.[271] The organ
is generally present in Elasmobranchs (Fig. 153, _rct.gl_), in which group
the intestinal orifice of its duct may either be close to the termination
of the spiral valve, or, as in _Chlamydoselachus_,[272] near the cloacal
outlet of the gut. An apparent representative of the gland, the "caecum
cloacae," is also present in the Dipnoi,[273] but communicates directly
with the cloaca (Fig. 155, A, _cl.c_). The "rectal" gland is perhaps
homologous with the intestinal caecum which is to be found in some Teleosts
(e.g. _Box vulgaris_), and possibly also with the "caecum" (caecum coli),
and its vermiform appendix in the higher Vertebrata.[274] The caecum
cloacae, on the contrary, is morphologically a urogenital sinus, formed as
a dilatation of the fused hinder portions of the mesonephric ducts, and
probably comparable with the sperm sacs of male Elasmobranchs, and also
with the urinary bladder of Teleostomes.[275]
{277}CHAPTER X
THE RESPIRATORY ORGANS
The principal respiratory organs consist of a series of pairs of branchial
clefts in the form of perforations in the side walls of the throat, which
place the pharynx in free communication with the exterior. The first and
most anterior of these clefts, the mandibulo-hyoid cleft or "spiracle," is
situated between the mandibular and hyoid arches; the second, the
hyo-branchial or hyoidean cleft, between the hyoid arch and the first
branchial arch; and the remaining clefts between the succeeding branchial
arches. On the anterior and posterior walls of more or fewer of the clefts
highly vascular plate-like, or variously shaped filamentous outgrowths of
their lining membrane are developed, which subserve the purpose of exposing
the blood to the influence of the oxygen-containing water, and are termed
branchial lamellae or "gills." In addition to their usual respiratory
organs, the gills, a few Fishes utilise the air-bladder either as a
functional lung or as an oxygen reservoir, and in others accessory
breathing organs of various kinds are developed.
The arrangement of the branchial clefts and the gills may be conveniently
studied first in the Elasmobranchs. Excluding the spiracles, there are
usually in this group (Fig. 161, A), five pairs of branchial clefts, but in
certain primitive members of the group the number may be larger. Thus, in
_Notidanus griseus_ (_Hexanchus_) and in _Chlamydoselachus_ there are six,
and in _Notidanus cinereus_ (_Heptanchus_), seven clefts. The pharyngeal
apertures of the clefts are relatively wide, but their external openings,
which are freely exposed on the lateral surface of the head between the eye
and the pectoral fin, are usually narrow and slit-like.
{278}[Illustration: FIG. 161.—A, Horizontal section through the head of an
Elasmobranch; B, similar section of a Teleost (diagrammatic). _b.c_,
Branchial cavity; _b.l_, branchial lamellae; _c_, coelom; _e.b.a_, external
branchial aperture; _hy.a_, hyoid arch; _hy.c_, hyo-branchial cleft; _l.s_,
interbranchial septum; _n_, nasal organ; _oes_, oesophagus; _op_,
operculum; _p.q_, palato-quadrate cartilage; _Ph_, pharynx; _sp_, spiracle;
_s.ps_, spiracular pseudobranch; 1-5, 1st to 5th branchial arches. (From
Boas, slightly altered.)]
The successive clefts are separated from one another by a series of
inter-branchial septa, each of which consists of the lining membrane of two
contiguous clefts and a median fibrous sheet; it is further strengthened on
its pharyngeal margin by a branchial arch, and more externally by the
fringe of cartilaginous rods (branchial rays) with which the outer convex
edge of each arch is provided. The anterior and posterior walls of each
septum are produced into a number of outwardly-radiating vascular plates or
folds (branchial lamellae or "gills"), which by their free edges project
into the cavity of the cleft (Fig. 161, A). Although slightly free at their
outer extremities, the lamellae do not extend so far as the external margin
of the septum to which they are attached (Fig. 164, B). Each series of
lamellae is termed a "hemibranch," and, from what has been said, it is
obvious that each inter-branchial septum and its supporting branchial arch
carry two hemibranchs, an anterior and a posterior, the two forming a
complete biserial gill or "holobranch." The hyoid arch, however, has only a
single hemibranch, viz. that pertaining to the anterior wall of the
hyo-branchial cleft, and as the fifth or last cleft has a hemibranch only
on its anterior wall, the fifth arch is {279}gill-less.[276] The spiracle
is a vestigial cleft. At an early stage of embryonic growth it differs but
little from its fellows, but subsequently degenerating it is represented in
the adult by a tubular passage between the oral cavity and the exterior,
which, however, is often complicated by the development of caecal
outgrowths.[277] The anterior wall of the spiracle often retains a rudiment
of a hemibranch in the shape of more or fewer vascular lamellae, which, as
they are supplied with arterial blood, and not with venous blood like the
ordinary gills, are said to form a mandibular or spiracular "pseudobranch."
The spiracle varies greatly in size in different families, being largest in
the Trygons and Torpedos, and very small, or even absent in the Lamnidae.
Its pseudobranch is best developed in the Notidanidae, where it has the
essential structure of a true hemibranch, and, as in other Elasmobranchs,
but to a greater extent, probably aids in the additional aeration of the
blood which is distributed to the eye and brain. The characteristic
opercular covering of the external apertures of the gill-clefts in the
Teleostomi and Dipnoi is wanting in Elasmobranchs. It is interesting to
note, however, that in _Chlamydoselachus_[278] curious frilled cutaneous
folds are developed as extensions of the outer edges of the inter-branchial
septa, as well as of the hyoid region, and, like a series of incipient
opercula, project backwards over the successive branchial clefts (Fig.
252).
While in many respects more primitive than in Elasmobranchs the branchial
system of the Cyclostomata presents certain special and peculiar features.
The branchial clefts assume the form of oval, antero-posteriorly flattened
pouches or sacs, varying, however, in number, and in their mode of
communicating with the exterior, in different genera. In the Lamprey
(_Petromyzon_) there are seven pairs of obliquely-disposed gill-sacs
opening externally by small rounded orifices, and by similar apertures, not
directly into the pharynx, but into a branchial canal (Fig. 162, _r.t_),
which underlies the oesophagus, and, while ending blindly behind the last
pair of sacs, communicating in {280}front with the oral cavity.[279] The
first of the series of gill-sacs corresponds to the hyo-branchial or
hyoidean cleft of Elasmobranchs and other Fishes. Spiracles are absent in
the adult, but in the embryo are represented by pouch-like outgrowths of
the hypoblast of the oral cavity, which subsequently undergo singular
changes.[280] Thus, the outgrowths become converted into the lateral halves
of a complete ciliated circum-oral groove, which is retained even in the
Ammocoetes stage, and recalls the ciliated peripharyngeal ring of
Ascidians. Another archaic feature is also to be noted in the continuity of
the groove with a ciliated mid-dorsal pharyngeal ridge, which has been
compared to the "dorsal lamina" of Ascidians, and to the equally
characteristic hyperbranchial groove of _Amphioxus_.[281] Ventrally also,
the lateral halves of the groove unite to form a single groove, which,
after receiving the median aperture of the thyroid rudiment,[282] is
continued backwards in the mid-ventral line of the pharyngeal wall as far
as the last branchial arch. No trace of these ciliated structures is,
however, to be met with in the adult.
[Illustration: FIG. 162.—_Petromyzon marinus_. Transverse section through
the branchial region (semi-diagrammatic). _br.m_, Branchial membrane;
_d.ao_, dorsal aorta; _d.c_, dorsal cartilage of the branchial basket;
_d.m_, dorsal muscles; _e.a_, external aperture of a gill-sac; _f.t_,
fibrous tissue enclosing neural canal; _h_, _i_, lateral longitudinal
cartilages of the branchial basket; _i.a_, internal aperture of a gill-sac;
_i.ju_, inferior jugular vein; _ju_, jugular vein (anterior cardinal);
_my_, spinal cord; _nc_, notochord; _n.ca_, neural canal; _n.p_, neural
process; _oes_, oesophagus; _p.br_, peri-branchial lymph sinus; _r.m.t_,
retractor muscle of the tongue; _r.t_, respiratory tube or branchial canal;
_s_, circum-oesophageal lymph sinus; _v.ao_, ventral aorta; _v.c_, ventral
cartilage of branchial basket; _v.m_, ventral muscles. (From T. J.
Parker.)]
The branchial lamellae are represented by a series of vascular horizontal
and parallel ridges radiating outwards along the roof, floor, and lateral
walls of each gill-sac, and invested by an {281}epithelium which is
partially ciliated. The inter-branchial septa are much thicker than in
Elasmobranchs, and include not only the walls of adjacent sacs and the
branchial muscles, but also contain cavernous peribranchial lymph-sinuses.
The cartilaginous branchial skeleton is situated wholly external to the
gill-sacs, the so-called branchial arches lying between the external
apertures of the sacs, and directly beneath the superficial skin, or, in
other words, on the outer margins of the inter-branchial septa, and not on
the inner, as is invariably the case with the branchial arches of Fishes.
[Illustration: FIG. 163—Dissection of _Myxine glutinosa_ from the left
side. _au.c_, Auditory capsule; _br.ap_, left branchial aperture; _br.b_,
rudiment of branchial basket; _br.s.1_, first gill-sac; _c.br.t_, common
branchial tube; _cn.c_, cornual cartilage; _gul_, gullet; _ht_, heart;
_lg.m_, lingual muscles; _m.v.c_, median ventral cartilage; _na.t_, nasal
tube; _nch_, notochord; _n.t_, neural tube; _oes.ct.d_,
oesophageo-cutaneous duct; _p.l.c_, posterior lateral cartilage; _sb.oc.a_,
subocular arch; _sp.c_, spinal cord; _st.p_, styloid process. (After W. K.
Parker, from Parker and Haswell's _Zoology_.)]
In the Hag-Fish (_Myxine_) (Fig. 163), there are usually six, very rarely
seven, pairs of gill-sacs, all of which open directly into the pharynx, and
not into a branchial canal as in the Lampreys. On the other hand, _Myxine_
is unique in having the outer extremities of its gill-sacs produced into a
corresponding number of tubular canals which, after a longer or shorter
course obliquely backwards and outwards, unite to form on each side a
ventrally-situated external aperture (Fig. 163). In the same genus a short
canal, or oesophageo-cutaneous duct, passes from the pharynx {282}behind
the last gill-sac of the left side, and opens externally with the common
external branchial aperture of that side.
In _Bdellostoma_ there are usually six or seven pairs of gill-sacs, but
some species have ten or even fourteen pairs.[283] They agree with those of
the Lamprey in having independent external apertures, but resemble the
corresponding organs in _Myxine_ in opening directly into the pharynx. An
oesophageo-cutaneous duct is also present.[284]
In the Holocephali there are but four branchial clefts, the fifth cleft
being closed. Spiracles are absent in the adult, although present in the
young of _Chimaera_. The branchial lamellae resemble those of
Elasmobranchs, but the inter-branchial septa are somewhat shorter, so that
the lamellae project slightly beyond their outer margins (Fig. 164, B). A
hyoidean hemibranch is present. A noteworthy feature is the development of
a cutaneous fold from the outer surface of the hyoid arch, which grows
backwards over the gill-clefts, and, uniting above and below with the
body-wall, terminates in a free posterior margin, just behind the last
gill-cleft. By the growth of this opercular fold the gills become enclosed
in a spacious branchial cavity, and the clefts communicate with the
exterior through a slit-like opening between the free margin of the fold
and the body-wall.
The reduction in the extent of the inter-branchial septa which is initiated
in the Holocephali is carried to a still further extent in the Teleostomi.
Commencing with the Chondrostei, and passing thence to the more specialised
Teleostei, the septa become gradually reduced in length, and the branchial
lamellae project freely beyond their outer margins to an increasing extent.
This modification, least marked in _Acipenser_ (Fig. 164, C) and
_Polyodon_, attains its maximum in the Teleosts (Fig. 164, D and E), where
the branchial lamellae take the form of a double series of free filaments
disposed along the convex outer margin of each branchial arch, and attached
by their bases only to the reduced and inconspicuous septa. As a general
rule each of the first four arches supports two hemibranchs,[285] forming a
{283}biserial gill or holobranch. In shape the branchial filaments are
usually somewhat triangular, and consist of an axial supporting cartilage
or bone, invested superficially by a highly vascular mucous membrane. As in
most of the preceding groups the fifth branchial arch is gill-less. All
Teleostomi possess a well-developed movable operculum, supported by a more
or less complete series of opercular bones, with or without the addition of
branchiostegal rays (Fig. 161, B). The size of the external branchial
aperture varies considerably. Usually the hinder and lower margins of the
operculum are free, and then the aperture is spacious. Not infrequently,
however, the more or less extensive fusion of the ventral and hinder edges
of the operculum with the body-wall reduces the aperture to a narrow slit,
as in the Eels and some Siluridae, or to a small upwardly directed pore, as
in the "Sea-Horse" (_Hippocampus_). In the Symbranchidae the branchial
apertures close dorsally, but fuse ventrally, leaving a single median
orifice on the under side of the throat.
[Illustration: FIG. 164.—Transverse sections of branchial arches in
different Fishes. A, Elasmobranch; B, _Chimaera_; C, _Acipenser_; D and E,
Teleosts. _b.a_, Branchial arch; _g.l_, gill-lamellae; _gr_, gill-raker;
_i.s_, inter-branchial septum. (From Boas.)]
Open spiracles are wanting in most adult Teleostomi, but are, nevertheless,
retained in the Crossopterygii (_Polypterus_), and in the Chondrostei
(_Acipenser_ and _Polyodon_). They have been observed, however, in the
embryos of some Teleosts, as in the Salmon (_Salmo_),[286] and even in the
adults of _Amia_,[287] _Lepidosteus_, {284}and a few Teleosts[288] are
represented by pouch-like recesses of the oral cavity. A few vestigial
branchial lamellae may be developed on the anterior wall of each spiracle
in _Acipenser_ and _Polyodon_, but are wanting in _Polypterus_, and, as in
Elasmobranchs, represent a mandibular or spiracular pseudobranch.
The structure usually regarded as a hyoidean hemibranch in the Teleostomi
differs greatly in its development in different members of the group. In
_Acipenser_ it is undoubtedly the hemibranch of the hyoid arch and is a
true gill, receiving venous blood from the ventral aorta and returning
arterial blood to the dorsal aorta, as in Elasmobranchs. In _Polyodon_ and
in _Polypterus_ the hemibranch is suppressed. _Lepidosteus_,[289] on the
other hand, has two series of lamellae on the inner surface of the
operculum, a dorsal and a ventral series meeting at an angle (Fig. 197).
The ventral lamellae are supplied with venous blood, the dorsal with
arterial,[290] so that while the former retain their primitive character as
a functional hyoidean hemibranch, the latter is a pseudobranch. It is
interesting to note, however, that the development of this pseudobranch and
its blood-vessels proves that it does not represent any portion of a true
hyoidean hemibranch, but is really a spiracular pseudobranch.[291] In most
other Teleostomi a degenerate hemibranch occupies a similar position. In
_Amia_[292] it is very feebly developed, and is lodged in a canal
communicating with the branchial cavity by a small aperture, and situated
directly anterior to the dorsal end of the first branchial arch. Its blood
supply is arterial, and the organ is therefore a pseudobranch. In Teleosts
the hemibranch is invariably a pseudobranch; nevertheless, its primitive
condition as a gill is indicated either by its structure or by its
embryonic history. In some genera the pseudobranch consists of short free
lamellae, as in some Pleuronectidae; or it is partly free and partly
concealed, as in some of the Horse Mackerels (_Caranx_) and in _Salmo_; or
it may be completely hidden beneath the oral epithelium, as in the Cod
(_Gadus_), where the organ is very degenerate, and is little more than a
"rete mirabile" of blood-vessels. The nature of the Teleostean pseudobranch
is not in {285}all cases quite clear. In _Salmo_ it is said that there is
no hyoidean hemibranch, and that the pseudobranch is really a persistent
spiracular pseudobranch;[293] hence it is probable that a like significance
must be attached to this singular structure in other Teleosts. The evidence
of the cranial nerves on this point is conflicting. If the pseudobranch
pertains to the spiracular cleft its nerve supply should be derived from
the nerve of that cleft—viz. the seventh or facial nerve; but if it
represents a hyoidean hemibranch, then one would expect it to be innervated
by the ninth or glossopharyngeal nerve. As a matter of fact, however, the
organ is said to be supplied by the seventh in some Teleosts, and in others
by the ninth nerve.
In the Dipnoi the branchial system is best developed in _Neoceratodus_, the
increasing importance of the lungs as respiratory organs in _Protopterus_
and _Lepidosiren_ being associated with a corresponding reduction in the
structural and functional development of the gills. There is no trace of
spiracles in the adult.
[Illustration: FIG. 165.—Transverse section through a branchial arch of
_Neoceratodus_ (semi-diagrammatic), _a.b.a_, Afferent branchial artery;
_b.a_, branchial arch; _b.f_, branchial filaments; _e.b.v_, efferent
branchial vessel; _g.r_, gill-rakers. (From Baldwin Spencer.)]
In _Neoceratodus_[294] there are five branchial clefts, including the
hyobranchial. Each of the first four branchial arches carries a pair of
hemibranchs, and, as in the Holocephali, the gill-lamellae are attached
along nearly their whole length to a well-developed interbranchial septum
(Fig. 165). A peculiarity of _Neoceratodus_, which has no counterpart in
any other Fishes, is the extension of the branchial lamellae on to the
dorsal and ventral walls of the branchial clefts, so that the hemibranchs
on opposite sides of each cleft are continuous both dorsally and ventrally
(Fig. 166). The fifth arch is gill-less. {286}In addition to the normal
gills there is also a hyoidean pseudobranch. As in other Dipnoi, an
operculum forms the outer wall of the branchial cavity, and leaves but a
narrow, slit-like external branchial aperture.
[Illustration: FIG. 166.—The second branchial cleft of _Neoceratodus_, to
show the dorsal and ventral continuity of two hemibranchs on opposite sides
of the same cleft. _b.c_, Branchial cleft; _b.f_, branchial filaments;
_g.r_, gill-rakers. (From Baldwin Spencer.)]
In _Protopterus_[295] the number of branchial arches is increased to six,
but, in consequence of the closure of the hyobranchial cleft, there are but
five open clefts. The first, second, and third arches are wholly devoid of
branchial filaments: the fourth and fifth support each a biserial gill,
while the sixth arch retains only an anterior hemibranch, which, however,
as the source of its blood supply seems to indicate, may consist of
"emigrant" gill-filaments from the posterior hemibranch of the fifth
arch.[296] Interbranchial septa are practically non-existent, the
flattened, leaf-like gill-lamellae being free except at their attached
bases, and thus repeating a characteristic Teleostean feature. A "hyoidean"
hemibranch or pseudobranch, supplied from the ventral aorta, is present,
but as the hyobranchial cleft is closed it projects into the branchial
cavity immediately in front of the cleft between the first and second
branchial arches. In _Lepidosiren_[297] the branchial arches are reduced to
five and the clefts to four, the hyobranchial and fifth clefts being
closed. There is a "hyoidean" hemibranch resembling that of _Protopterus_.
The facts furnished by the study of the numerical and structural variations
in the gill-clefts, gills, and gill-arches of different groups of Fishes
prove that atrophy of these structures takes place at opposite ends of the
series. We have examples of this anteriorly in the suppression of the
hyo-mandibular cleft and its hemibranch, and of the hyoidean hemibranch, as
the result of {287}the conversion of the mandibular and hyoid arches into
jaws, or into skeletal supports for the jaws; and posteriorly, in the
reduction which is evident when the generality of Fishes are compared with
such primitive Elasmobranchs as _Chlamydoselachus_ and _Notidanus_.
In most Fishes the concave pharyngeal margins of the branchial arches are
fringed with a double series of either cartilaginous or bony tubercles or
filaments, the "gill-rakers" (Figs. 161 and 164). The anterior row of
gill-rakers on each arch usually interdigitate with those of the posterior
row on the preceding arch, and in this way the two rows form a sieve-like
mechanism to prevent any solid particles, which may enter the pharynx with
the respiratory current of water, from passing into the gill clefts and
clogging or otherwise injuring the branchial filaments.
In a few Fishes the gill-rakers are enormously developed, and subserve a
function similar to that of the baleen plates of the Whalebone Whales in
acting as a filter for straining from the water the small pelagic organisms
on which the Fish feeds. This is notably the case in the great Basking
Shark (_Selache maxima_)[298] in which the closely-set, flattened, tapering
gill-rakers may be so long as four or five inches, and, while somewhat
resembling "whalebone" in appearance, have the histological structure of
vascular dentine. The nature of the food, which in the stomach of one
specimen examined consisted solely of an immense quantity of plankton,
including Copepods and the larvae of other Crustaceans,[299] affords clear
evidence of the great value of such a filtering mechanism to this Shark,
and, at the same time offers an explanation of the striking and significant
reduction in the size of the teeth, which, relatively to the dimensions of
the Fish, are so small as to be almost vestigial. A similar filter has been
observed in an extinct _Selache_ (_S. aurata_)[300] from the Antwerp Crag,
and also in an existing South African Shark (_Rhinodon typicus_);[301] and
in the latter, as in the Basking Shark, is associated with a marked
reduction in the importance of the dentition. The long slender gill-rakers
of the Chondrostean {288}_Polyodon_ also constitute an efficient filter,
and the same may be said of several plankton-eating Teleosts.
THE MECHANISM OF RESPIRATION.—The aeration of the blood is effected by the
rhythmical suction of water into the oral cavity, and its subsequent
expulsion through the gill-clefts, bathing the highly vascular
gill-lamellae in its course. In any single act of inspiration the mouth is
opened, and the oral cavity enlarged by the lateral expansion of its walls.
When the oral cavity is filled with water, the mouth is closed and the
expiratory process begins. By the lateral contraction of the oral walls the
water is driven outwards through the gill-clefts, and over the
gill-lamellae. During this process the branchial arches become widely
separated by the contraction of their muscles, the operculum is elevated,
and the oesophagus is closed by the contraction of its muscular wall. In
many Fishes the course of the expiratory water-current is controlled by
special valve-like folds of the oral mucous membrane, the maxillary and
mandibular "breathing-valves."[302]
The rate of "breathing" varies considerably in different Fishes, even in
allied species.[303] In the Blue Wrasse (_Labrus_), and the Rockling
(_Motella_), the number of respirations per minute is 15, in the Minnow
(_Leuciscus_), and Stickleback (_Gastrosteus_), as many as 150. A
deficiency of oxygen in the water accelerates the respiratory movements,
and the Fish appears to "pant" or breathe hurriedly. In the Lampreys, both
inspiration and expiration may take place through the external
gill-apertures by the alternate expansion and contraction of the gill-sacs,
more especially when the suctorial buccal funnel is used for the attachment
of the animal. On the other hand, the singular habits of the Myxinoids
involve a further modification of the respiratory process. In these
Cyclostomata the inspiratory current enters the external naso-pituitary
aperture and reaches the pharynx through the naso-pituitary canal, and
thence, as an expiratory stream, traverses the gill-sacs on its way
outwards. The pharynx is closed behind the last pair of gill-sacs by a
constrictor muscle, which prevents the entrance of the water into the
oesophagus, and converts the pharynx into a respiratory tube for the time
{289}being; but, when food is being swallowed, the pharyngeal constrictor
is relaxed and the internal apertures of the gill-sacs are closed by the
contraction of their own sphincter muscles.
In addition to the usual respiratory organs it is probable that in not a
few Fishes the superficial skin may share with the gills the function of
breathing. In this connexion may be mentioned the fact that in
_Periophthalmus_ the tail is used for respiration. Hickson[304] observed
that a species of this genus, frequenting the extensive sandy shores of the
Island of Celebes, often rests with its tail in the water, the head and
trunk being exposed. Under such circumstances the gills are probably of
little use, and the tail is utilised as a breathing organ, principally, as
Haddon[305] subsequently pointed out, through the agency of its extremely
vascular caudal fin.
[Illustration: FIG. 167.—Embryos of the Electric Torpedo (_Torpedo
ocellata_). A, dorsal view; B, ventral view of a slightly younger specimen.
_cl_, Cloaca; _el.o_, electric organ; _ex.b_, external gills; _p.f_,
pectoral fin; _pv.f_, pelvic fin; _sp_, spiracle; _y.s_, stalk of
yolk-sac.]
Some Fishes possess larval breathing organs; others, even when provided
with gills, either utilise the air-bladder, or develop special accessory
organs, for aquatic or, more usually, for aerial respiration.
{290}[Illustration: FIG. 168.—Head of young _Polypterus_. _ex.g_, External
gill of the left side. (From Steindachner.)]
LARVAL GILLS.—In early life many Fishes acquire larval gills, either as the
result of the precocious growth of the normal gills, or by reason of the
development of evanescent structures. In the embryos of Elasmobranchs
"external gills," in the form of long filiform processes invested by
hypoblast, are developed from the walls of all the branchial clefts,
including the spiracles, and protrude outwards for some distance through
the external apertures of the clefts (Fig. 167, B). They perhaps facilitate
respiration within the egg, as they completely disappear after hatching;
but there is also reason for believing that they aid in the absorption of
nutriment. Similar gills are present in young Holocephali. In some larval
Teleosts, as in certain genera of the Osteoglossidae and Mormyridae (e.g.
_Heterotis_ and _Gymnarchus_)[306] these structures are remarkably
developed (Fig. 239). The young of the Loach (_Misgurnus_) and of the
Salmon (_Salmo_) also have the ordinary gill-filaments prolonged externally
as filiform structures, which subsequently become reduced to their normal
size.[307] In its larval state _Polypterus_[308] has a pair of
pinnately-fringed ectodermal or cutaneous gills projecting from the lateral
surfaces of the head behind and above the external branchial apertures
(Figs. 168 and 281). Apparently as an individual peculiarity the right gill
has been retained in a specimen of _P. congicus_ so large as 22 cm. in
length, although the left one had entirely disappeared.[309] Each gill is
supplied with blood from the ventral aorta by a vessel which ascends the
{291}hyoid arch, and is apparently the representative of the artery
supplying the hyoidean hemibranch in Elasmobranchs. The efferent vessel of
each gill joins the common trunk formed by the union of the efferent
vessels of the normal gills of the same side.
The cutaneous gills of the Dipnoid _Protopterus_ may also be included in
the category of larval breathing organs. They consist of three simple
unbranched filaments on each side of the head, and, as in _Polypterus_, are
situated at the dorsal extremity of the external gill aperture (Fig. 309).
Although usually represented in the relatively young or half grown
specimens which, so far, have reached Europe, it is extremely probable that
these organs atrophy in older individuals. Similar gills are present in the
larval _Lepidosiren_ (Fig. 311), but disappear at a much earlier stage. At
no period of its development are larval gills present in
_Neoceratodus_.[310]
THE AIR-BLADDER AS A RESPIRATORY ORGAN.—In certain Fishes the air-bladder
may become subservient to the function of respiration. In _Amia_ and
_Lepidosteus_ the internally sacculated and vascular air-bladder is
obviously adapted for air-breathing, and there are not wanting
observations[311] which suggest that the organ is actually used for this
purpose after the fashion of a lung. According to Jobert,[312] this is also
the case with the sacculated air-bladder of certain Brazilian Teleosts,
viz. _Sudis gigas_, _Erythrinus taeniatus_ and _E. braziliensis_, since
these Fishes die of asphyxia when the organ is cut off from communication
with the exterior by the ligature of its ductus pneumaticus. It is in the
Dipnoi, however, that the air-bladder becomes most completely a true lung.
In _Neoceratodus_[313] the lung is probably of the greatest use to the Fish
when the rivers are low during the hot season and the water is charged with
foul gases from decomposing vegetable matter, and possibly also when the
water is filled with sediment in the rainy season. In _Protopterus_, and
more especially in _Lepidosiren_, the partial atrophy of the gills renders
it highly probable that the lungs are the principal breathing organs at all
times. Nevertheless, it must be emphasised that in all these Fishes
respiration by means of the air-bladder {292}necessarily involves a transit
of air to and from that organ through the ductus pneumaticus, and at
present nothing is known as to the method by which such inspiratory and
expiratory currents can be produced.[314]
There is also some experimental evidence for the belief that the
air-bladder of some Teleosts may be subsidiary to respiration by acting as
a reservoir for the superabundance of oxygen which is taken into the blood
through the gills, and subsequently reabsorbed into the blood when the Fish
is in water containing relatively little oxygen.[315] It is clear, however,
that the conditions under which the air-bladder can be used in this way are
by no means fully understood, for, under experiment, such Fishes died of
asphyxia even though after death the air-bladder still contained upwards of
fifty per cent of oxygen.
ACCESSORY ORGANS OF RESPIRATION.—In certain Fishes of peculiar habits, or
living under special external conditions, accessory respiratory organs are
developed.
Although in this particular instance no special organs are formed, mention
may first be made of the singular method of intestinal respiration in vogue
in some Teleosts. In one of the Loaches (_Misgurnus fossilis_),[316] air is
swallowed and passed along the alimentary canal until it is finally voided
at the anus. The mucous membrane of the intestine is extremely vascular,
and hence the blood comes into sufficiently intimate relations with the
swallowed air to admit of it exchanging carbon dioxide for oxygen.
Intestinal respiration also occurs in species of the South American
freshwater genera of Siluridae and Loricariidae, _Callichthys_, _Doras_,
_Loricaria_, and _Plecostomus_;[317] and in some cases the area of
respiratory surface is considerably increased by the development of folds
and processes of the intestinal mucous membrane.
In a few tropical Teleosts curious labyrinthiform organs are developed in
connexion with certain of the branchial arches, and serve as accessory
breathing organs. In the Indian "Climbing Perch" (_Anabas scandens_),[318]
of the family Anabantidae, the organ (Fig. 169) consists of three or more
concentrically-arranged bony {293}laminae, with wavy, crenulated margins,
attached by a common bony base to the upper extremity of the fourth
branchial arch, and enclosed in a special dorsal enlargement of the
branchial cavity. The vascular membrane which invests the laminae is
abundantly supplied with venous blood by a branch of the fourth afferent
branchial artery, the equivalent efferent vessel joining the dorsal aorta.
Essentially similar organs are found in several genera of Osphromenidae
(e.g. _Polyacanthus_, _Osphromenus_, and _Trichogaster_). A simpler form of
respiratory organ of somewhat the same type occurs in the Indian family
Ophiocephalidae.[319] In these Fishes there is, on each side, an accessory
branchial cavity, situated above that which contains the gills, but freely
communicating with it (Fig. 170). The cavity is lined by a thickened and
puckered vascular membrane, but otherwise contains no special respiratory
structures.
[Illustration: FIG. 169.—Labyrinthiform organ of _Anabas scandens_, exposed
by the removal of the greater part of the operculum. _b.a′_, First
branchial arch; _l.o_, labyrinthiform organ; _op_, operculum; _sb.c_,
supra-branchial cavity.]
[Illustration: FIG. 170.—Supra-branchial cavities of _Ophiocephalus_.
Ventral view, as seen after the removal of the ventral halves of the
branchial arches, _b.a^{1-4}_, The first four branchial arches; _o.c_, roof
of oral cavity; _oes_, oesophagus; _p.t_, pharyngeal teeth; _sb.c_; left
supra-branchial cavity; _v.f_, folds of the lining membrane of the cavity.]
In the Siluroid genera _Clarias_ and _Heterobranchus_ the accessory organ
takes the form of branched, arborescent and highly vascular structures,
developed as outgrowths from the dorsal extremities of one or two branchial
arches, and enclosed within a posterior and dorsal expansion of the proper
branchial cavity (Fig. 171).
{294}Another example of these interesting structures occurs in _Chanos
salmoneus_ and a few other Clupeidae[320] in the shape of a coiled
gill-like organ ("gill-helix"), which is supported by the dorsal segment of
the fourth branchial arch, and enclosed in a similarly curved caecal
extension of the branchial cavity. Each gill derives its blood from the
fourth afferent branchial artery, the corresponding efferent vessel joining
the fourth efferent branchial artery. A similar spirally-coiled
"gill-helix" is found also in _Heterotis ehrenbergii_,[321] amongst the
Osteoglossidae, and in several species of Characinidae.[322]
[Illustration: FIG. 171.—Accessory respiratory organ of _Clarias_, as seen
after the removal of the left operculum. _a_, Anterior arborescent organ;
_b.a^{1-4}_, the first four branchial arches and their holobranchs;
_d.b.c_, dorsal extension of the left branchial cavity; _f_, modified
gill-filaments; _op_, base of the operculum; _p_, posterior arborescent
organ.]
In other Teleosts the accessory breathing organ assumes the condition of
paired lung-like outgrowths of the branchial cavity. Thus, in one of the
Symbranchidae, the Indian "Cuchia Eel" (_Amphipnous cuchia_),[323] there is
a pair of small bladder-like sacs, with membranous and vascular walls, each
of which opens into the branchial cavity above the first gill-cleft, and is
supplied with blood by the afferent branchial artery of the gill-less first
branchial arch. An extreme modification in the same direction {295}is
presented by the Indian Siluroid _Saccobranchus_.[324] In this Fish a long
caecal diverticulum of the branchial cavity extends backwards on each side
from the dorsal region of the first branchial cleft to the tail, and in its
course is situated internally to the lateral trunk musculature, and close
to the vertebral column (Fig. 172). The walls of the caeca are vascular,
but no special respiratory structures are developed within their cavities,
which, during life, only contain air. In _S. singio_ the right caecum is
supplied with blood by an extension backwards of the dorsal portion of the
first afferent branchial artery of that side; the left, on the contrary,
being supplied by the corresponding portion of the fourth afferent artery
of the same side. In _S. fossilis_[325] both air-sacs are supplied by the
fourth afferent branchial artery. The efferent vessels join the fourth
efferent branchial artery, right or left as the case may be.
[Illustration: FIG. 172.—Air-sacs of _Saccobranchus singio_. _a.b_, The
air-bladder enclosed in its bony capsule; _a.c_, right air-sac; _a.s_, left
air-sac; _c.a_, bulbus aortae; _l.a.v_, afferent vessel of the left
air-sac; _r.a.v_, afferent vessel of the right air-sac; _r.e.v_, efferent
vessel of the right sac. (After Hyrtl, altered by Hubrecht.)]
With perhaps one or two exceptions, the accessory respiratory organs of
Fishes seem to exist for the purpose of enabling their possessors to
breathe in air. This is certainly the case with the labyrinthiform organs
of _Anabas_ and its allies, and also in such Fishes as _Amphipnous_,
_Saccobranchus_, and the Ophiocephalidae, and probably in others. Nearly
all these Fishes are tropical in geographical distribution, more or less
amphibious in their habits, and usually possess a remarkable capacity for
sustaining life out of water, under conditions which are promptly fatal to
ordinary Fishes. Thus, _Anabas scandens_ may be kept alive for days in
earthen pots without water, and when free is able to travel short distances
on land, especially in the early morning when the dew is on the ground,
while _Amphipnous_ frequents {296}marshes, lurking in holes in the grass
and about the sides of ponds. In fact, even when in the water, access to
air, which is probably swallowed and passed over their accessory breathing
organs, is indispensable to their existence. Experiments conclusively prove
that if the Fish is artificially prevented from obtaining air in this way
asphyxiation speedily ensues.[326]
In addition to breathing air through the agency of special organs evolved
for the purpose, there are many freshwater Fishes which, like those just
mentioned, periodically rise to the surface and swallow air in order to
saturate the water which bathes the gills with oxygen.[327]
{297}CHAPTER XI
THE AIR-BLADDER
[Illustration: FIG. 173.—Transverse section of the body of a Teleost, to
show the position of the air-bladder (diagrammatic). _a.b_, The
air-bladder; _c_, coelom; _d.p_, ductus pneumaticus; _k_, the kidneys;
_oes_, oesophagus; _p.p_ and _v.p_, parietal and visceral layers of the
peritoneum; _r_, rib; _v.c_, vertebral column.]
In the Crossopterygii, Chondrostei, and Holostei, in the Dipnoi, and in the
great majority of Teleosts, there is situated on the dorsal side of the
coelom, between the alimentary canal below and the kidneys and vertebral
column above, a more or less elongated sac with membranous walls, an
internal epithelial lining and gaseous contents—the air-bladder (Figs. 154
and 173). Usually developed in the embryo as a caecal outgrowth from the
dorsal surface of the oesophagus, the air-bladder grows anteriorly and
posteriorly, and may either retain throughout life its primitive connexion
with the alimentary canal by means of a longer or shorter tubular canal,
the ductus pneumaticus, or become completely separated therefrom in the
adult by the atrophy of the duct. Its walls sometimes, but rarely, contain
muscle-fibres, as in _Lepidosteus_, _Amia_, and the Dipnoi, and are always
more or less vascular, while laterally and ventrally the organ is invested
externally by the peritoneum (Fig. 173). In addition to the muscle-fibres
distributed in its walls, the bladder is often provided with powerful
extrinsic muscles, more especially in those Fishes in which it is used as
an organ for {298}sound-production. In the different groups of Fishes in
which it is present the air-bladder frequently undergoes remarkable
structural modifications and becomes adapted for various distinct
functions.
In the Cyclostomata there is no trace of an air-bladder, and, unless
represented in certain Sharks (e.g. _Mustelus_, _Galeus_, and
_Acanthias_),[328] by a small caecum embedded in the dorsal wall of the
oesophagus and communicating with its cavity, it is also absent in all
Elasmobranchs. In the Crossopterygii (e.g. _Polypterus_),[329] the
air-bladder is double, but while the right sac is long and somewhat
tubular, the left is much smaller and oval in shape (Fig. 174). Near their
anterior extremities the two sacs fuse into a single unpaired chamber,
beyond which they again project in the form of two short caeca. The median
chamber opens into the oesophagus on the ventral side by an orifice (_gl_)
bounded by prominent lips and furnished with a muscular sphincter. The
organ is devoid of internal sacculations. In the Chondrostei (e.g.
_Acipenser_) the air-bladder is oval in shape, with a smooth,
non-sacculated, inner surface, and a lining of ciliated epithelium, and it
communicates with the oesophagus by means of a relatively wide, dorsally
placed, funnel-like orifice.
[Illustration: FIG. 174.—Air-bladder of _Polypterus_. _gl_, glottis. (From
Wiedersheim.)]
In the Lepidosteidae the single air-bladder extends the whole length of the
abdominal cavity, and, as in _Polypterus_, communicates with the exterior
through a larynx-like vestibule provided with a glottis,[330] which,
however, opens dorsally into the oesophagus (Fig. 175). A strong fibrous
band runs along the median line of the inner surface of its dorsal wall,
from which extends ventrally on each side a series of transverse
fibro-muscular ridges, forming the boundaries of a double row of regularly
arranged alveoli (Fig. 176). The bottom of each alveolus {299}is still
further sacculated by finer branches of the principal fibrous bands.[331]
In the Amiidae the bladder is very large, and, except that a short median
cleft divides it in front into two short caeca, it is unpaired.
Internally, its walls are much sacculated, but the alveoli are smaller and
arranged less regularly than in _Lepidosteus_. The aperture of
communication with the oesophagus is dorsally situated.
[Illustration: FIG. 175.—Portion of the air-bladder, with the ventral wall
removed, and the glottis, of _Lepidosteus_. _a.b_, Air-bladder; _gl_,
glottis; _s_, bulging of the hinder wall of the vestibule into the cavity
of the air-bladder; _v_, cleft leading from the air-bladder into the
vestibule. (From Wiedersheim.)]
It may be mentioned that in all the preceding Teleostomi the ductus
pneumaticus is remarkably short, the connexion between the air-bladder and
the oesophagus being almost direct by means of a larger or smaller orifice,
which, except in _Acipenser_, is more anteriorly placed than in most other
Teleostomi; and further that, unlike many Teleosts, there are no special
"retia mirabilia," "red bodies," or "red glands."
[Illustration: FIG. 176.—Portion of the air-bladder of _Lepidosteus_,
opened along the mid-ventral line to show the alveoli. _av_, Alveolus;
_f.b_, medio-dorsal fibro-muscular band. (From Wiedersheim.)]
In the Dipnoi the structural resemblance of the air-bladder to a true lung,
which to some extent is indicated in _Polypterus_, _Amia_, and
_Lepidosteus_, becomes still more marked.
In _Neoceratodus_[332] the organ is not unlike that of _Lepidosteus_, and
takes the form of a spacious unpaired sac, extending from one end of the
abdominal cavity to the other. On its inner surface two fibrous bands, one
of which is dorsal and the other ventral, traverse the whole length of the
bladder, and project slightly into its cavity. {300}Between these median
ridges extend a number of transverse septa, forming the boundaries of a
series of pairs of bilaterally symmetrical oval alveoli, the walls of which
are still further sacculated by a network of finer ridges (Fig. 177). The
short ductus pneumaticus seems to be an anterior continuation of the right
half of the bladder, and opens into the oesophagus by a small glottis,
situated on the ventral side, a little to the right of the median line.
[Illustration: FIG. 177.—Interior of a portion of the air-bladder of
_Neoceratodus_. _av_, Alveolus; _f.r_, the two fibrous ridges. (From
Günther.)]
The more complicated and much more lung-like air-bladder of _Protopterus_
(Fig. 178)[333] is essentially double, consisting of an anterior unpaired
portion, and of two sac-like prolongations which extend backwards the whole
length of the coelom, gradually tapering towards the cloaca. Anteriorly,
the unpaired portion of the organ is continued into a vestibule or
pneumatic duct, which, after passing ventrally on the right side of the
oesophagus, opens into the latter by a ventrally-situated, slit-like
glottis, immediately behind the last pair of gill-clefts. The margins of
the glottis are provided with radially-arranged dilator muscles, and in
connexion with its anterior border there is an epiglottis-like
fibro-cartilaginous plate.[334] The central cavity of each lung (Figs. 178
and 179) communicates with a series of larger or smaller alveoli in the
lung-wall, and each of the latter opens in succession into smaller tubular
cavities, and then into still smaller terminal caecal sacculi. Hence, much
more than in _Neoceratodus_, the lungs approximate in structure to those of
the higher terrestrial {301}Vertebrata. Non-striated muscle cells, pigment
cells, and blood capillaries are abundantly present in the connective
tissue external to the lining epithelium of the lung-cavities.
[Illustration: FIG. 178.—A, the air-bladder of _Protopterus_, viewed from
the ventral side. Portions of the ventral walls of the pharynx and bladder
have been removed. _gl_; Glottis; _lg_, undivided portion of the lung;
_l.l_, left lung; _oes_, oesophagus; _p.a_^{1}, _p.a_^{2}, the left and
right pulmonary arteries; _ph_, pharynx; _p.v_, pulmonary vein; _r.l_,
right lung; _vb_, vestibule. (From Newton Parker.) B, portion of one lung
of _Protopterus_, opened from the dorsal side to show the alveoli. _al_;
Alveolus. (From Baldwin Spencer.)]
The air-bladder of _Lepidosiren_ closely resembles that of _Protopterus_,
and, as in the latter Dipnoid, the glottis seems to be furnished with an
epiglottis.[335]
In all the Dipnoi the air-bladder is highly vascular, but nevertheless
presents no trace of "red bodies" or "red glands."
The most striking features in the remarkably polymorphic air-bladder of
Teleosts relate to (_a_) its presence or absence; (_b_) differences in
shape and relative size; (_c_) the development of caecal outgrowths; (_d_)
the subdivision of its cavity by the formation of internal septa; (_e_) the
retention or suppression of the ductus pneumaticus, and the occasional
development of secondary ducts communicating directly with the exterior;
(_f_) the presence of "red glands" or "red bodies"; (_g_) its connexion
with the auditory organ; (_h_) its adaptation as an organ for
sound-production.
{302}(_a_) The air-bladder is by no means universally present in Teleosts.
It is absent in several entire families,[336] such as, for example, the
Flat Fishes or Pleuronectidae, the Scopelidae, and the "Lump-suckers"
(Cyclopteridae). In a few families, as in the Mackerels (Scombridae), the
"Blennies" (Blenniidae) and the Polynemidae, the organ is present in most
genera, but absent in a few, or even present or absent in different species
of the same genus. Thus, of the three British species of Mackerel, viz. the
Spanish Mackerel (_Scomber colias_), _S. pneumatophorus_, and the common
Mackerel (_S. scombrus_), an air-bladder is present in the first two, but
absent in the third.[337]
(_b_) As might be anticipated, the shape of the air-bladder is extremely
different in various Teleosts, and usually conforms to the shape of the
body, while differences in relative size are of frequent occurrence, even
in closely related species. Sometimes the organ is more or less tubular,
fusiform, ovoid, or heart-shaped; occasionally it is shaped like a
"dumb-bell," consisting of two lateral sacs connected by a median tubular
portion, as in the Siluroids _Clarias_ and _Callichthys_; or it may be
horse-shoe-shaped, as in the Silurid _Ailia_.[338] Not unfrequently a
transverse constriction divides the air-bladder into two intercommunicating
sacs, as in most of the Carp family (Cyprinidae), or three such sacs may be
formed by two constrictions (e.g. _Ophidium_). In the "Electric Eels"
(Gymnotidae) there are two sacs, connected by a slender canal, from which
the ductus pneumaticus takes its origin.[339]
[Illustration: FIG. 179.—Showing the structure of one of the larger alveoli
of the air-bladder of _Protopterus_. 1, Central cavity of the lung; 2,
alveolus; 3, tubular cavities communicating with 4, the small terminal
sacculi. (From Baldwin Spencer.)]
The air-bladder is either more or less free in the abdominal cavity, or
firmly attached to the vertebral centra and their rib-bearing processes by
fibrous extensions passing between the two structures. Not rarely the organ
extends from the abdominal {303}cavity into the tail, sometimes penetrating
for a short distance into the expanded haemal canal of the anterior caudal
vertebrae, or extending unsymmetrically along either the right or left side
of the tail. More frequently, perhaps, where the air-bladder is prolonged
into the tail, it assumes the form of two bilaterally arranged and
symmetrical caeca, which extend backwards for a variable distance internal
to the caudal muscles and in contact with the adjacent skeletal elements,
as in Notopteridae, and in some Sparidae, Carangidae, and Scombridae. The
extension of the air-bladder into the tail is often associated with a
short, laterally-compressed trunk, which, if the bladder is to attain its
normal degree of development, necessitates its prolongation into the caudal
region.
(_c_) A characteristic feature in the air-bladder of many Teleosts
belonging to widely different families is the development of a more or less
complex system of simple, or variously branched, caecal outgrowths, which,
like the internal septa, are specially characteristic of those Fishes in
which the bladder is used as a vocal organ without, however, being peculiar
to them.
In some of the Gadidae, as in the Cod (_Gadus morrhua_), the air-bladder
divides anteriorly into a pair of caecal prolongations which extend
forwards to the head, and are often curiously coiled. Somewhat similar
caeca are also present in species of Berycidae, Sparidae, Siluridae,
Clupeidae, and Notopteridae. Caecal prolongations may also be developed
from the hinder end of the bladder, and, as already mentioned, extend into
the tail; or even from both ends in the same species (e.g.
_Notopterus_).[340] In the Silurid, _Rita crucigera_,[341] a long tubular
caecum is developed from each side of the heart-shaped bladder, and thence
is prolonged backwards to the anus. In certain species of _Doras_ of the
same family (e.g. _D. maculatus_),[342] an elegant series of variously
sized branched caeca fringe each of the lateral margins of the bladder. It
is, however, in the Physoclist family of the Sciaenidae[343] that the
branching of the air-bladder attains its greatest development in extent and
variety.
{304}[Illustration: FIG. 180.—Air-bladder of _Otolithus_. (From Cuvier and
Valenciennes.)]
In _Otolithus_ (Fig. 180) two short tubular canals are given off from the
antero-lateral angles of the bladder, each subsequently dividing into two
elongated, tapering sacs, of which one is directed forwards and the other
backwards. In _Corvina lobata_ (Fig. 181) the lateral margins of the
bladder are everywhere fringed with a series of tufts of caeca, each tuft
being connected by a short common canal with the cavity of the organ. In
the "Drum" (_Pogonias chromis_) (Fig. 182) each side of the anterior third
of the air-bladder has a series of digitately branched caecal appendages,
the most posterior of which on each side are connected by a tubular canal,
also bearing branched caeca, with the corresponding postero-lateral
extremity of the bladder.
_Collichthys_[344] has a still more remarkable arrangement. In this
Sciaenoid (Fig. 183) twenty-five tubular branches are given off from each
side of the bladder, all of which soon subdivide into a dorsal and a
ventral division. These still further divide, and their branches either end
blindly or are prolonged into a series of arches to the mid-dorsal or
mid-ventral line as the case may be, where they become continuous with the
corresponding branches of the opposite side. The series of dorsal branches,
enveloped in their peritoneal investment, extend between the body of the
air-bladder and the roof of the body-cavity, while the corresponding
ventral branches, similarly invested, surround that part of the coelom
which contains the stomach, intestine, and liver.
[Illustration: FIG. 181.—Air-bladder of _Corvina lobata_. (From Cuvier and
Valenciennes.)]
(_d_) In addition to the subdivision of the cavity of the air-bladder by
the externally obvious, {305}transverse, or longitudinal constrictions
already described, or by the growth of simple or branched prolongations,
the organ is often chambered or sacculated by the development of internal
septa or partitions.
[Illustration: FIG. 182.—Air-bladder of _Pogonias chromis_. (From Cuvier
and Valenciennes.)]
In many of the Gurnards (_Trigla_)[345] the cavity of the bladder is
divided into two intercommunicating compartments by a transversely-disposed
and centrally-perforated diaphragm. The large air-bladder of some species
of _Erythrinus_[346] is subdivided internally into numerous alveoli or
sacculi. In _Notopterus_ a longitudinal septum divides the cavity of the
abdominal portion of the bladder into two lateral chambers, which, however,
freely intercommunicate anteriorly. In the great majority of the
Siluridae[347] the cavity of the organ is divided by a characteristic
T-shaped arrangement of a primary transverse and a longitudinal septum into
three communicating chambers, of which one is anterior and transversely
disposed, and two are posterior and longitudinally arranged (Fig. 222). The
posterior compartments in many genera are still further divided by the
growth of secondary transverse septa, extending outwards from the median
longitudinal septum, without, however, reaching the external lateral walls
of the chambers. In a few genera, as in certain species of
_Pangasius_,[348] additional fibrous bands and ridges passing between the
primary and secondary {306}septa give to the cavities of the lateral
compartments the appearance of being occupied by a coarse spongy network.
[Illustration: FIG. 183.—Transverse section through the abdominal region of
_Collichthys lucida_. _a.b_, Air-bladder; _d.b_ and _v.b_, the dorsal and
ventral branches of the air-bladder; _l_, liver; _m_, mesentery; _s_,
stomach; _v.c_, vertebral column. The dotted and broken lines surrounding
the bladder and its branches represent the peritoneal investment of these
parts. (From Günther.)]
(_e_) In its relations to the oesophagus and to the air-bladder the ductus
pneumaticus exhibits striking modifications in different Teleosts. With
very rare exceptions, an open ductus is wanting in the Heteromi,
Catosteomi, Acanthopterygii, Opisthomi, Pediculati, Jugulares, and the
Plectognathi, for which reason the term "Physoclisti" has often been used
as a collective name for these Fishes. On the other hand, a permanently
open ductus is generally present in the Malacopterygii, Ostariophysi,
Apodes, and the Haplomi, which, in consequence, have been designated
"Physostomi." It must be emphasised, however, that all Teleosts are
Physostomous in the embryonic condition, and whether they eventually become
Physoclistous or remain Physostomous depends entirely on the abortion or
retention of the primitive communication between the air-bladder and the
alimentary canal. When present in Teleosts, the ductus pneumaticus, with a
few exceptions (e.g. _Notopterus_), where it is both short and relatively
wide, is almost invariably much longer and narrower than in the other
orders of Teleostomi and in the Dipnoi, sometimes passing directly from the
air-bladder to the oesophagus, but not infrequently describing a sigmoid
curve, as in some Cyprinidae, or an even more tortuous course. The opening
into the alimentary canal is, with perhaps a single exception, dorsal, but
may vary from the commencement of the oesophagus to the hinder end of the
stomach. In _Erythrinus_ the oesophageal aperture is lateral. In two
instances the air-bladder has acquired secondary openings to the exterior,
and of these one occurs among the Physostomi and the other {307}in the
Physoclisti. In the Herring (_Clupea harengus_),[349] in addition to the
proper ductus, which is connected with the distal end of the caecal
stomach, a tubular canal leaves the hinder extremity of the bladder and
opens externally on the left side of the genital aperture; consequently, in
this Fish the air-bladder has a secondary and direct connexion with the
exterior in addition to the primary and indirect communication by means of
its proper duct. The Horse-Mackerel (_Caranx trachurus_)[350] is even more
peculiar. This Teleost has no true pneumatic duct, but instead a special
duct which passes from the bladder to open into the right branchial cavity
by a very minute aperture. In neither case is anything known of the mode of
origin or morphological nature of the secondarily acquired duct.
(_f_) The air-bladder differs greatly in its degree of vascularity in
various Teleosts, as well as in the extent to which its capillary
blood-vessels accumulate at special points on the inner surface to form the
so-called "red bodies" or "red glands." In some Teleosts the distribution
of capillaries is uniform or nearly so; in others, as in the Carp
(_Cyprinus carpio_) the vessels are arranged in fan-like, radiating tufts
over almost the whole extent of the inner surface; in others again, as in
the Pike (_Esox lucius_) the tufts are larger and more definitely
localised. A more extreme modification occurs in some of the Physostomi, in
which a remarkable concentration of capillaries takes place at one or more
points on the inner surface of the bladder, which project into the cavity
of the organ in the form of variously shaped blood-red masses. These "red
bodies" are essentially retia mirabilia, consisting of masses of
interlacing, tightly-packed capillaries with their afferent arteries and
efferent veins. The flattened lining epithelium of the bladder is continued
over these bodies without undergoing any special modification. In the
common Eel (_Anguilla vulgaris_) there are several of these bodies, of
which the largest are near the entrance of the pneumatic duct.
In the Physoclisti the "red bodies" seem to be replaced by true
glands,[351] which nevertheless in appearance closely resemble the former.
Some of the Gadidae, such as the Cod (_Gadus {308}morrhua_), the Haddock
(_G. aeglefinus_), and the Hake (_Merluccius vulgaris_), have a single
large "red gland" projecting into the interior of the bladder from its
dorsal or ventral wall (Fig. 184, A). The John Dory (_Zeus faber_) has five
such glands, worm-like and curved in shape, with their concavities facing a
central point between them (Fig. 184, B). In these Fishes a "rete mirabile"
of blood-vessels forms the vascular basis of the glands. The ordinary
pavement epithelium of the bladder becomes replaced by faintly granular,
columnar, and evidently glandular cells as it passes over the retia
mirabilia, and at the same time becomes invaginated into the mass of
capillaries in the form of a number of simple caecal glands (Fig. 185). So
far as is at present known, the "red glands" are only found in those
Teleosts in which the air-bladder has no ductus pneumaticus, whereas in
those Fishes which retain the ductus throughout life there are either no
special retia mirabilia, or, as in the Eel, only the so-called "red
bodies."[352]
[Illustration: FIG. 184.—Red glands, A, of the Cod (_Gadus morrhua_), and
B, of the John Dory (_Zeus faber_), seen from the interior of the
air-bladder. _bv_, Blood-vessels; _r.g_, red glands. (From Swale Vincent
and Stanley Barnes.)]
{309}(_g_) and (_h_) The structural modifications involved in the connexion
of the air-bladder with the auditory organ, and its adaptation for
sound-production, as well as its use in respiration, are considered
elsewhere.[353]
THE GASES OF THE AIR-BLADDER.—The gaseous contents of the air-bladder
consist of oxygen and nitrogen, but the relative proportions of the two
gases differ in different Fishes, and even in the same Fish, under
different conditions. Normally the proportion of oxygen is considerably
less in freshwater than in marine Fishes, and amongst the latter the
proportion of oxygen is often enormously greater, amounting in some cases
to 87 per cent., in deep-sea species as compared with their shallow water
congeners. A trace of carbon dioxide is also usually present. The gases are
derived from the blood as the latter circulates through the capillaries in
the walls of the bladder, and it is highly probable that the "red glands"
take an important part in the process; at all events, experimental research
has shown that the "secretion" or diffusion of gas into the air-bladder, as
well as the absorption of gas from the bladder into the blood, take place
most rapidly in those Fishes in which "red glands" or "red bodies" are
present.[354]
[Illustration: FIG. 185.—Vertical section through a "red gland"
(diagrammatic). _c_, Capillary blood-vessels; _g_, tubular glands. (From
Vincent and Barnes.)]
THE FUNCTIONS OF THE AIR-BLADDER.—Probably no single organ in any group of
Vertebrata is associated with the performance of a greater variety of
functions than the air-bladder of Fishes. Originally evolved, it may be, as
a glandular caecum in certain {310}Sharks, the air-bladder in the Dipnoi,
and some of the more generalised Teleostomi (e.g. _Amia_ and
_Lepidosteus_), and perhaps also in a few of the more specialised members
of the latter group (_e.g._ certain Teleosts), is to a greater or less
extent an accessory respiratory organ. In not a few Teleosts it is an organ
for sound-production, and in others again it is sometimes regarded as
having an important relation to the sense of hearing. But omitting such
subordinate functions which, as it were, have been grafted on to the
air-bladder, there can be no doubt that in the great majority of Fishes its
primary use is to act as a hydrostatic organ or "float." From this point of
view experimental investigations[355] seem to justify the following
conclusions:—
The function of the air-bladder is to render the Fish, bulk for bulk, of
the same weight as the water in which it lives. In this condition of
equilibrium, or plane of least effort, the Fish floats _in_ the water, and
therefore it is able to swim with a minimum of muscular effort. It is
obvious, however, that as a Fish rises or sinks it becomes exposed to an
increase or a diminution of hydrostatic pressure, which will necessarily
bring about the expansion or contraction of the volume of gas in the
air-bladder, and, therefore, by decreasing or increasing the specific
gravity of the animal, will tend to remove the Fish from its plane of least
effort. To counteract this, and to restore the Fish to a plane of
equilibrium at the new level, gas is either absorbed from the air-bladder,
or more gas is secreted into the bladder, as the case may be. According to
Moreau, by this process of automatic adjustment a Fish will always find,
sooner or later, a plane of least effort, whatever may be its depth in the
water; and further, this process takes place much more readily in those
Fishes which possess "red glands" or "red bodies", and with extreme
slowness in those in which these organs are absent. Nevertheless, it seems
doubtful if this process of adjustment can be of much use to a Fish in
ordinary vertical movements, inasmuch as gaseous secretion and absorption
are comparatively slow processes, the length of which in different Fishes,
and under different conditions, varies from a few hours to several days. On
the whole it seems more probable that adjustment to the varying pressures
of different depths by such means is far more likely to be useful during
such slow and gradual changes of level as are encountered {311}in the
course of diurnal, seasonal, or other periodic migrations than during the
rapid changes of level which may take place in ordinary vertical
locomotion.[356] In the generality of Fishes, and more especially in the
Physoclisti, it may be concluded that the possession of an air-bladder
restricts freedom of movement in the vertical direction, and confines
ordinary locomotion within more or less well-defined vertical limits above
or below the plane of least effort for the time being. As illustrating this
point, and as a proof of the danger incurred by a too rapid rise in the
water, the following remarks with reference to the "Kilch," a small
Salmonoid (_Coregonus_) inhabiting the Lake of Constance, and a favourite
article of food, may be quoted:[357]—The Fish "are caught in nets, and
brought to the surface of the water; they come up invariably with the belly
much distended, the air in the swimming-bladder, being relieved from the
pressure of the column of water, has expanded greatly and occasioned this
unnatural distension, which renders the Fish quite incapable of swimming.
Under these conditions the Fish is naturally unable to live for any length
of time. But the fishermen of the lake have a very simple remedy; they
prick into the air-bladder with a fine needle; the air escapes with some
force, the distension subsides, and the fishes are enabled to live under
totally changed conditions as to pressure, even in quite shallow water and
at the surface, swimming quite as freely as their companions, the natives
of the surface water. Hence the Kilch is confined to a certain depth,
because it is not capable of accommodating the tension of its
swimming-bladder to the change of pressure in the column of superincumbent
water."
It is not improbable that the Physostomi, or at any rate most of them, are
somewhat more advantageously placed in this respect. From the general
absence of "red glands" in this group, it may be inferred that whatever
capacity for gaseous secretion or absorption they possess must be exercised
with exceptional slowness, and, therefore, as a means of
pressure-adjustment may be neglected. On the other hand, they seem to
possess the compensating advantage of being able to substitute for
absorption the mechanical liberation of gas through the ductus pneumaticus.
It would seem, therefore, that the Physostomi have a distinct advantage
over the Physoclisti in that during ascent in the water they {312}can more
readily adapt themselves to the diminished pressure of a higher level by
ejecting the needful amount of gas than by relying upon the process of
gaseous absorption.[358] This conclusion is in harmony with the results of
experiment and with much that is known of the habits of these Fishes and
their greater freedom of locomotion in the vertical direction.
These briefly summarised conclusions as to the hydrostatic function of the
air-bladder must, however, be accepted only in a general sense. There are
many structural anomalies in the air-bladder of Fishes which are very
difficult to explain, or to correlate with any variations in the habits or
in the locomotor activities of its possessor.
In this connexion it may be mentioned that the presence or absence of an
air-bladder in different Fishes seems to some extent to be governed by two
causes. First, whenever the requirements of a Fish necessitate exceptional
freedom of locomotion in all directions the restrictions imposed by the
presence of an air-bladder are removed by its partial or complete
suppression; a result produced, secondly, by the assumption of a bottom
feeding or ground habit on the part of the Fish. Fishes like the Flat
Fishes or Pleuronectidae, when not in motion by the exercise of their fins,
habitually rest on the sea-bottom, and, as an air-bladder is useless under
such conditions, it has, in consequence, undergone complete atrophy. Not a
few Siluridae, and some Cyprinidae, inhabit the comparatively shallow
waters of rapidly flowing mountain torrents, and are often provided with
suckers for attachment to stones or rocks. To such Fishes as these a
hydrostatic organ is obviously useless, and it has hence become greatly
reduced in size, and in other respects approaches the condition of a
vestigial organ.
{313}CHAPTER XII
THE VASCULAR SYSTEM, THE LYMPHATICS, AND THE BLOOD-GLANDS
The Cyclostomata and Fishes possess a closed vascular system, consisting of
a heart, arteries, capillaries, and veins, the whole forming a continuous
series of blood-containing channels provided with definite limiting walls,
through which the blood is propelled in a constant direction by the
rhythmical contractions of the heart. In the course of the circulation the
blood flows from the heart through a single large trunk, the ventral aorta,
to the capillaries of the gills. From the gills the arterialised blood is
collected into a large dorsally-situated vessel, the dorsal aorta, and
thence is distributed through a system of arteries to the capillaries of
the various organs of the body. Finally, the blood is collected from the
capillaries and returned to the heart by the veins.
Although in most instances the organs of the body are supplied with
arterialised blood conveyed to them by arteries, there are nevertheless
cases in which an organ may receive venous blood by a vein in addition to
arterial blood supplied by an artery. For example, the capillaries of the
liver not only receive blood from the hepatic artery, but also venous blood
by a large vein (hepatic portal vein), formed by the union of a number of
smaller veins by which venous blood is collected from the capillaries of
the stomach, intestine, spleen, and pancreas. In this and similar
instances, where a vein formed by the union of the capillaries of an organ,
or of a series of organs, instead of uniting with other veins and
proceeding towards the heart, becomes continuous with a second set of
capillaries in some other organ, a "portal" system is said to be formed,
and in the particular example of the liver it is termed the "hepatic
portal" system. A similar, or "renal {314}portal," system also exists in
connexion with the kidneys in the majority of Fishes.
There is little doubt that, primarily, the vascular system of Vertebrate
animals consisted of a dorsal artery (dorsal aorta), running along the
median dorsal line of the alimentary canal, and a ventral or subintestinal
vein similarly related to the ventral surface of the digestive tube. The
two vessels were connected by a series of pairs of lateral branches, which
had their origins from the dorsal vessel, and, by their subdivision, formed
a capillary network in the walls of the alimentary canal. From these
networks paired veins issued and opened into the subintestinal vein. The
simplicity of this primitive arrangement was somewhat disturbed in the
region of the pharynx by the development of gill-clefts, in the walls of
which the blood circulated for respiratory purposes from the ventral to the
dorsal vessel; and also by the development of a hepatic portal circulation
in connexion with the liver. In the latter instance the subintestinal vein
entered the liver and subdivided into capillaries in the substance of that
organ, the corresponding efferent vessel, or hepatic vein, becoming
continuous with the anterior or pharyngeal section of the subintestinal
vein, or, as it is usually termed, the ventral aorta. In this low grade of
vascular system, which is perhaps most completely retained in Amphioxus,
the circulation of the blood was probably effected by the wave-like
contractions of more or fewer of the larger vessels; but subsequently a
definite chambered heart was developed at the origin of the ventral aorta.
Of Fishes in general it may be said that the primitive dorsal and ventral
vessels are present in the embryo, and for a time retain their original
relations and physiological importance. To a very unequal extent they may
also be retained in the adult, where, however, they co-exist with numerous
other vessels, which the increasing differentiation of the body has called
into existence. Thus, at a later period of embryonic life, the
subintestinal vein becomes somewhat fragmentary. Its caudal section (caudal
vein) ceases to be continuous with the precaudal portion, and the blood
collected from the muscles and other structures of the tail is conveyed to
the heart by a pair of posterior cardinal veins, which are either directly
continuous with the caudal vein, or indirectly through the intervention of
a renal portal system in the kidneys. {315}The precaudal portion of the
subintestinal vein is represented by a vein which runs forwards in the
intestinal wall, and is one of the minor affluents of the hepatic portal
vein, while its prehepatic section is represented in succession by the
hepatic vein, the heart, and the ventral aorta. Of the additional veins
which supplement these remnants of a primitively continuous subintestinal
vein, the largest and most constant are (_a_) the posterior cardinal veins
which, commencing in the kidneys and receiving the blood from those organs,
pass forwards to the heart; (_b_) a pair of anterior cardinal veins, formed
by the union of smaller veins from the head, including the brain, and
passing backwards towards the heart. At the level of the latter organ each
anterior cardinal vein joins the posterior cardinal of the same side of the
body to form a short but wide transverse vessel, the Cuvierian duct or
precaval vein, which opens into the hindermost of the cavities of the
heart, viz. the sinus venosus; (_c_) a pair of inferior jugular veins by
which the nutrient blood of the branchial apparatus is returned to the
right and left Cuvierian ducts. In addition to these principal veins there
may also be a pair of lateral veins collecting the blood from the lateral
walls of the trunk, and also opening into the Cuvierian ducts; and
subclavian and femoral veins from the pectoral and pelvic fins.
On the other hand, the primitive dorsal vessel (dorsal aorta), retains not
only its original position and relations, but also its primary function as
the main channel for the distribution of arterialised blood to all parts of
the body. The system of lateral and probably segmentally arranged vessels,
by which the dorsal and subintestinal vessels were connected in the
primitive Vertebrata, have undergone considerable modification in all
existing Fishes, but nevertheless retain much of their original disposition
and relations in the pharyngeal region of the alimentary canal, where they
are represented by the afferent and efferent vessels of the gills.
A more detailed account of the condition of the vascular system in the
Cyclostomata and Fishes will now be given.
THE VENOUS SYSTEM.—The Cyclostomata,[359] as might be expected, exhibit a
more primitive condition of the venous system {316}in certain features than
is the case in any other group. The precaudal portion of the subintestinal
vein retains much of its original importance and runs in the rudimentary
intestinal spiral valve as far as the liver, where it becomes the hepatic
portal vein. From the liver the blood is collected into a _single_ hepatic
vein, and by it is conveyed to the sinus venosus. The caudal section of the
subintestinal vein, now known as the caudal vein, bifurcates near the anus,
and its two branches become directly continuous with the right and left
posterior cardinals, without forming a renal portal system. In their
forward course to the heart the posterior cardinals are situated directly
beneath the notochord, and after receiving the blood from the kidneys and
gonads, and from the numerous pairs of segmental veins of the body-wall,
join the corresponding anterior cardinal veins, and form on each side a
short transverse Cuvierian duct which opens into the sinus venosus. There
is also a pair of inferior jugular veins which, however, unite opposite the
fifth pair of gill-sacs to form a single trunk; this vessel is continued
backwards, externally to the medio-ventral cartilage of the branchial
basket, and finally opens directly into the sinus venosus.
In Elasmobranchs (e.g. _Mustelus antarcticus_)[360] the caudal vein (Fig.
186) lies in the haemal canal of the caudal portion of the vertebral
column. On reaching the kidneys the vein divides into two renal portal
veins, which, however, are not directly continuous with the posterior
cardinal veins as in the Cyclostomata, but, on the contrary, after
receiving the posterior segmental and oviducal veins, become continuous
with the capillaries of the kidneys.
From the latter organs the blood is collected by a series of renal veins,
and by them conveyed to the posterior cardinals, and thence to the
Cuvierian ducts. In the adult, therefore, there is a well-developed renal
portal system, but it is worthy of note, nevertheless, that this system is
developed comparatively late in embryonic life, and that at an earlier
stage the caudal vein is directly continuous with the two posterior
cardinals, precisely as is the case in the Cyclostomata throughout life.
The posterior cardinal veins are situated in the dorsal wall of the coelom
(Fig. 187), beneath the vertebral column. For the hinder portion of
{317}their extent they are embedded in the kidneys (Fig. 186); and in this
region the two veins are in close relation in the median line, and here and
there freely communicate with each other. More anteriorly, they enlarge so
much that they present the appearance of cavernous sinuses. In addition to
the anterior segmental and oviducal veins, the posterior cardinals receive
the spermatic or the ovarian vein from the male or female gonad.
The precaudal section of the primitive subintestinal vein, now termed the
internal intestinal vein (Figs. 186 and 187), traverses the spiral valve as
it passes forwards to the liver, but from a physiological point of view is
now merely one of the factors of the great hepatic portal vein, the
principal tributaries of which are the veins from the stomach and
intestine, including the rectal gland, and the pancreas and spleen. On
entering the liver the hepatic portal vein divides into two principal
branches for the right and left halves of the gland. From the liver the
blood is conveyed by two hepatic veins to the sinus venosus.
[Illustration: FIG. 186.—Venous system of _Mustelus antarcticus_. _a_,
Auricle; _a.c_, anterior cardinal; _b.a_, conus arteriosus; _br.v_,
brachial vein; _c.d_, Cuvierian duct or precaval vein; _c.v_, caudal vein;
_cl.v_, cloacal vein; _f.v_, femoral vein; _h.s_, hyoidean sinus; _h.v_,
hepatic vein; _i.i.v_, internal intestinal vein; _i.j_, inferior jugular;
_k_, kidney; _l_, liver; _l.v_, lateral vein; _md.v_, mandibular vein;
_n.h.v_, nutrient hyoidean veins; _o.s_, orbital sinus; _p.c_, posterior
cardinal; _p.v_, hepatic portal vein; _rp.v_, renal portal vein; _sc.v_,
subscapular vein; _sp.v_, spermatic vein; _s.v_, sinus venosus; _v_,
ventricle; _v.a_, ventral aorta. (After T. J. Parker.)]
The lateral veins (Fig. 186) are situated in the lateral walls {318}of the
abdomen, immediately external to the peritoneum (Fig. 187). Each vein
begins near the pelvic fin, where it is connected with its fellow across
the dorsal face of the ischio-pubic cartilage, and thence runs forward
towards the pectoral fin. At its origin the lateral vein receives a femoral
vein from the pelvic fin and a cloacal vein, and also, near its anterior
end, a brachial vein from the pectoral fin, finally joining the Cuvierian
duct of its side.[361]
[Illustration: FIG. 187.—Diagrammatic transverse section of an
Elasmobranch, showing the position of the principal longitudinal
blood-vessels. _c_, Coelom; _d.a_, dorsal aorta; _d.c.v_, dorsal cutaneous
vein; _d.i.v_, dorsal intestinal vein; _i_, intestine; _i.i.v_, internal
intestinal vein; _l.c.v_, lateral cutaneous vein; _l.v_, lateral vein;
_m.v.a_, myelonic vein and artery; _p.c.v_, posterior cardinal vein;
_sp.c_, spinal cord; _sp.v_, spiral valve; _v_, vertebral centrum; _v.c.v_,
ventral cutaneous vein; _v.i.v_, ventral intestinal vein. (From T. J.
Parker.)]
The anterior cardinal vein is situated directly above the gill-arches of
its side of the head, and extends forwards from its junction behind with
the Cuvierian duct to the outer side of the auditory capsule, where it
communicates by a valvular orifice with a large sinus surrounding the
eye-muscles (orbital sinus), and ventrally, by means of a similar aperture,
with another large sinus, the hyoidean sinus, which lies on the outer face
of the corresponding hyoid arch, and is continuous ventrally with its
fellow of the opposite side. Into the orbital sinus open the anterior
facial vein from the anterior and external regions of the head, and the
anterior cerebral vein from the lateral half of the brain, and, into the
hyoidean sinus, the nutrient veins from the hyoidean hemibranch.
The inferior jugular veins are situated beneath the branchial apparatus.
Each vein begins anteriorly by communicating with {319}the hyoidean sinus
of its side, and, after receiving the nutrient veins from the holobranchs
of the first four branchial arches, opens into the corresponding Cuvierian
duct.
The venous blood from the heart itself is collected into two coronary
veins, which open into the sinus venosus.
In addition to the more important veins already described, there is also a
series of median and lateral cutaneous veins communicating at different
points with certain of the more deeply seated veins (Fig. 187).
Characteristic features in the venous system of _Mustelus_, as also of
Elasmobranchs in general, are the development of transverse connexions
between certain of the principal paired veins, and the tendency of many of
the main veins to enlarge into more or less irregularly-shaped sinuses.
In its broad outlines the venous system of the Teleostomi agrees with that
of Elasmobranchs, but is nevertheless characterised by several more or less
important modifications, while at the same time exhibiting many differences
in minor details.
A renal portal system is usually present, but is singularly variable in the
source of its tributary veins, even in closely allied forms.[362] In the
Sturgeon (_Acipenser_) and in some Teleosts, as in the Siluroid, _Amiurus
catus_, it resembles that of Elasmobranchs. In other Teleosts, on the
contrary, the renal portal system presents various grades of degeneration,
or, possibly, of imperfect evolution, as will be seen from the following
illustrations of its condition in different genera.
In _Amiurus_ the caudal vein, after giving off right and left renal portal
veins to the renal capillaries, emerges from the ventral surface of the
kidneys, and is then continued forwards between the gonads, the veins from
which it receives, as the radicle of the hepatic portal vein.
In the Eel (_Anguilla vulgaris_) the caudal vein (Fig. 188) traverses the
fused hinder portions of the kidneys, receiving several segmental veins
from the body-wall and also giving off from each side numerous renal portal
branches. More anteriorly, where the two kidneys become distinct, the
caudal vein also divides into two renal portal veins and, as each vein
extends {320}forwards along the outer border of the kidney of its side, it
receives a number of segmental veins, and, at the same time, gives off
branches to the renal capillaries. In addition, each renal portal vein is
connected with the hepatic portal vein by a series of singular arch-like
vessels into which the ovarian or spermatic veins open.
It is obvious, therefore, that in both _Amiurus_ and _Anguilla_ the
primitive direct continuity of the caudal and posterior cardinal veins has
been interrupted by the formation of a well-developed renal portal system,
and further, that the residue of the caudal venous blood finds its way to
the liver through the hepatic portal vein; hence it follows that, as in so
many of the lower air-breathing Vertebrates, the whole of the venous blood
from the tail is distributed either to the kidneys or liver in the course
of its return journey to the heart.
The Tench (_Tinca vulgaris_) exhibits the interesting anomaly of possessing
two caudal veins, a dorsal and a ventral (Fig. 189). The dorsal vein is
directly continuous with the right posterior cardinal, while the ventral
one divides into three branches, two forming right and left renal portal
veins and receiving numerous segmental veins, and the third becoming one of
the affluents of the hepatic portal vein. In this Teleost it is clear that
a portion of the caudal blood passes directly to the heart through the
right posterior cardinal without traversing either the renal portal or
hepatic portal system.
[Illustration: FIG. 188.—Renal portal circulation in the Eel (_Anguilla
vulgaris_). _c.v_, Caudal vein; _i.v_, intestinal vein; _l.p.c_, _r.p.c_,
left and right posterior cardinal veins; _p.v_, hepatic portal vein; _R_,
kidney; _r.p.v_, _r.p.v′_, renal portal veins; _sg.v_, segmental veins;
_x_, arch-like anastomoses between the renal portal and hepatic portal
veins; _y_, vein from the urinary bladder. (From Jourdain.)]
In the Cod (_Gadus morrhua_) the caudal vein divides into two branches.
The larger right vein retains its direct continuity {321}with the
corresponding posterior cardinal; the left, on the contrary, has ceased to
be continuous with the greatly reduced left posterior cardinal and forms a
renal portal vein, the distribution of which is, however, restricted to the
hinder portion of the left kidney (Fig. 190). As in _Amiurus_, a branch of
the caudal vein forms one of the tributaries of the hepatic portal vein. In
the Cod it would therefore seem that only a relatively small proportion of
the caudal blood flows through the imperfectly developed renal portal
system, the bulk of it traversing the right posterior cardinal and passing
directly to the heart, leaving, nevertheless, a modicum for transmission to
the liver. Finally, it may be mentioned that in some Teleosts the caudal
vein retains its embryonic continuity with one, usually the right,
posterior cardinal, without giving off a renal portal affluent, as in the
Perch (_Perca fluviatilis_); or, after division, with both posterior
cardinals, as in the Lump-sucker (_Cyclopterus lumpus_). In such instances
as these no portion of the caudal blood traverses the kidneys, and if a
renal portal system exists at all, the only true renal portal veins are the
adjacent segmental veins, which transmit venous blood directly to the
kidneys, instead of first uniting with renal portal branches of the caudal
vein as in the Tench and the Eel.
[Illustration: FIG. 189.—Renal portal system in the Tench (_Tinca
vulgaris_). _d.c.v_, _v.c.v_, Dorsal and ventral caudal veins; _k_, kidney;
_l.p.c_, _r.p.c_, left and right posterior cardinal veins; _p.v_, hepatic
portal vein; _r.p.v_, renal portal vein; _sg.v_, _sg.v′_, segmental veins.
(From Jourdain.)]
Whatever may be the condition of the renal portal system, all the renal
blood is eventually collected by renal veins and conveyed to the posterior
cardinals, which are often connected by one or {322}by several transverse
anastomoses (Fig. 190). In the region of the heart each posterior cardinal
joins the corresponding anterior cardinal to form a short but wide
Cuvierian duct, which finally opens into the sinus venosus.
A subintestinal vein is present in the embryo (e.g. _Lepidosteus_,
_Acipenser_, and some Teleosts),[363] but in the adult Teleostome its
precaudal section is usually absorbed, or at all events ceases to be
recognisable except, perhaps, as one of the minor tributaries of the
hepatic portal vein.[364]
The hepatic portal vein is formed as in Elasmobranchs, but in different
Teleostomi it may also receive the veins from the pyloric caeca, from a
portion of the air-bladder, the gonads, and, as previously mentioned, a
tributary from the caudal vein. There are usually two hepatic veins opening
into the sinus venosus, and generally of equal size (Fig. 190).
[Illustration: FIG. 190.—Venous system of a Teleost (diagrammatic). _A_,
Auricle; _ab.v_, vein from the air-bladder; _a.c_, anterior cardinal;
_c.d_, Cuvierian duct; _c.p.c_, transverse anastomoses between the two
posterior cardinals; _c.v_, caudal vein; _h.v_, hepatic vein; _i.j_,
inferior jugular; _k_, kidney; _l_, liver; _p.c_, left posterior cardinal;
_p.v_, hepatic portal vein; _r.p.c_, right posterior cardinal; _r.p.v_,
renal portal vein; _sc.v_, subclavian vein; _sg.v_, segmental vein; _sp.v_,
spermatic vein; _s.v_, sinus venosus.]
Most of the veins from the air-bladder join the hepatic portal {323}vein,
as already mentioned (Fig. 190), but more or fewer of them, especially
those from the dorsal wall of the organ, open into the posterior cardinals.
They may, as in _Polypterus_, even join the hepatic veins.[365]
The veins from the gonads are very variable in their destination, sometimes
joining the posterior cardinals, as in the Salmon (_Salmo salar_); or the
hepatic portal vein, as in _Amiurus_; or, as in the Perch (_Perca
fluviatilis_), forming by their union a single trunk, which communicates
directly with the left Cuvierian duct.
Representatives of the great lateral veins of Elasmobranchs appear to be
absent in the Teleostomi, the veins from the pectoral and pelvic limbs
joining the Cuvierian duct and the posterior cardinal veins respectively.
The two large anterior cardinal veins, which collect the blood from the
head and brain, occupy their usual position directly above the branchial
apparatus, and are sometimes connected by transverse anastomoses as they
pass backwards to join the Cuvierian ducts. The inferior jugular vein is
either single (e.g. _Gadus_); or paired, as in _Perca_ (Fig. 190).
In the Dipnoi the venous system is distinguished by an interesting
combination of characters, some of which are either primitive or peculiar
to the group, while others exhibit a distinct transition to the embryonic
or the adult condition of the lower air-breathing Vertebrates.
In _Neoceratodus_[366] (Fig. 191) the renal portal system is unusually
complex, the veins distributing venous blood to the kidneys being derived
from several sources, as follows: (1) from each of the two branches into
which the caudal vein divides on its exit from the haemal canal (_af.r.v_);
(2) from a common trunk (_pt.v_) which, on each side, is formed by the
union of segmental veins from certain of the post-cloacal myotomes and is
united with its fellow by a transverse anastomosis; (3) from more
anteriorly situated intercostal or segmental veins (_i.c.v_) which enter
each kidney directly; and (4) from a vein on each side corresponding to the
renal portal vein of Amphibia. The latter vein (_rp.v_) is formed by one of
the two branches of the iliac or femoral vein, and joins {324}the
corresponding vein from the caudal myotomes; from the common trunk numerous
branches enter the kidney.
[Illustration: FIG. 191.—Venous system of _Neoceratodus_. _a.ab_, Anterior
abdominal; _af.r.v_, afferent renal veins; _b.v_, brachial; _c.d_,
Cuvierian duct; _c.v_, caudal; _h.p.v_, hepatic portal; _h.v_, hepatic;
_i.c.v_, intercostal veins; _i.j_, inferior jugular; _il.v_, iliac;
_i.v.c_, inferior vena cava or postcaval; _k_, kidney; _l_, liver; _l.a_,
left auricle; _l.p.c_, left posterior cardinal; _l.v_, lateral cutaneous
vein; _pt.v_, vein from postcloacal myotomes; _p.v_, pulmonary vein;
_pv.v_, pelvic; _r.a_, right auricle; _rp.v_, renal portal; _s.c.v_,
subscapular; _s.j_, superior jugular or anterior cardinal; _t_, testis;
_v_, ventricle; _v.v_, right vertebral vein. (After Baldwin Spencer.)]
In the derivation of renal portal veins from each of the two veins into
which the caudal vein divides, _Neoceratodus_ approaches the Elasmobranchs.
On the other hand, the utilisation of ordinary segmental veins from the
caudal and pre-caudal myotomes, some of which directly enter the kidney, is
a feature which has already been remarked in some Teleosts; while the
formation of a renal portal affluent by a branch of the femoral vein is an
even more striking Amphibian characteristic.
The efferent renal veins[367] join the root of the left posterior cardinal
and the adjacent portion of the caudal vein.
Of the two great venous trunks into which the caudal vein divides, the
right is much the larger and behaves somewhat differently to the left. The
former (_i.v.c_) passes forwards in relation with the right kidney,
receiving in its course the spermatic or ovarian veins from the gonad of
its side, and then traverses the liver, finally opening into the median
portion of the sinus venosus, between the orifices of the two hepatic
veins. The left branch of the caudal vein (_l.p.c_) also passes forwards in
relation with the left {325}kidney and receives veins from the
corresponding gonad; but, instead of traversing the liver, it passes above
that organ, and finally opens into the left Cuvierian duct. The course of
the left vein, and the relations of the vessel to the caudal vein and the
left Cuvierian duct, point to the conclusion that it represents the left
posterior cardinal of other Fishes. From its continuity with the caudal
vein it is also obvious that the hinder or renal portion of the right trunk
is a remnant of the right posterior cardinal; but the more anterior section
so closely resembles the postcaval vein, or inferior vena cava of the
higher Vertebrates, in its relations to the liver, the hepatic veins, and
the sinus venosus, that its identity as such seems beyond doubt, and this
interpretation is supported by well-known observations[368] on the mode of
origin of the inferior vena cava in Amphibia, and especially the union of
the independently formed inferior vena cava with the posterior or
inter-renal portion of the embryonic right posterior cardinal vein,
combined with the atrophy of the anterior portion of the latter vein.[369]
The singular connexions and relations of these two great veins afford an
additional illustration of the significant transitional condition of the
venous system in the Dipnoi. On the other hand, the direct continuity of
the caudal vein with vessels which, wholly or in part, represent the two
posterior cardinals, is a feature alike characteristic of the adult
Cyclostome and the embryonic Elasmobranch, Teleost, and Amphibian.
As in the Cyclostomes and Elasmobranchs, the precaudal section of the
embryonic subintestinal vein is represented in the adult by an
intra-intestinal vein which traverses the spiral valve near its free edge
and is a tributary of the hepatic portal vein.
The two veins from the undivided air-bladder unite to form a single vessel,
which, instead of joining the hepatic portal or posterior cardinal veins as
in other Fishes, opens into the left auricle, like the pulmonary veins of
the Amphibia.
A further resemblance to the Amphibia is to be found in the presence of an
anterior abdominal vein. After leaving the pelvic {326}limb each femoral
vein divides into two branches; one of these forms a renal portal vein as
previously described; the other, which may rightly be termed a pelvic vein
(_pv.v_), unites with its fellow to form a median anterior abdominal vein
(_a.ab_). Pursuing its course forwards in the ventral abdominal wall, the
vein eventually reaches the heart and opens into the sinus venosus. The
direct connexion of the anterior abdominal vein with the heart is yet
another example of the retention in the adult _Neoceratodus_ of a
transitory embryonic feature in the developing Amphibian.[370]
[Illustration: FIG. 192.—Venous system of _Protopterus_. _a_, Auricle;
_a.c_, anterior cardinal; _an.v_, anastomotic veins; _c_, intestine; _f.v_,
femoral or iliac vein; _g.b_, gall-bladder; _h.p.v_, hepatic portal vein;
_i.j.v_, inferior jugular; _ov.v_, ovarian veins; _p_, pericardium;
_p.c.v_, left posterior cardinal; _p.v′_, parietal or segmental veins; _s_,
stomach; _sb.v_, subclavian. Other reference letters as in Fig. 191. (From
Newton Parker.)]
As in other Fishes, the blood from the head is conveyed to the Cuvierian
ducts by an anterior cardinal and an inferior jugular on each side. There
are no lateral veins, the blood from the pelvic fins flowing into the renal
portal system or into the anterior abdominal vein, and that from the
pectoral fin through subscapular and brachial veins into the Cuvierian
ducts. Lateral cutaneous veins are, however, present; and, as in
Elasmobranchs (e.g. _Mustelus antarcticus_), anastomose anteriorly with the
subscapular vein and behind with the caudal vein.
{327}Less is known of the venous system of _Protopterus_,[371] but it is
certain, nevertheless, that it presents a more advanced grade of evolution
than in _Neoceratodus_, and, except for the doubt as to the existence of an
anterior abdominal vein, it is essentially similar to that of a Urodele
Amphibian in which the right posterior cardinal vein has aborted.
The caudal vein (Fig. 192) divides into right and left renal portal
branches, neither of which, however, is directly continuous with the
inferior vena cava or the left posterior cardinal; on the contrary, each
renal portal vein is joined by the corresponding iliac or femoral vein, and
also by numerous segmental veins, and then distributes the whole of its
venous blood to the kidney. The radicles of the inferior vena cava and the
left posterior cardinal are formed by the renal veins from the two kidneys,
and in their forward course to the heart both veins receive in addition
genital and segmental veins. In its course through the liver the inferior
vena cava receives several hepatic veins, and finally opens into the sinus
venosus, while the left posterior cardinal vein joins the corresponding
Cuvierian duct, which also receives anterior cardinal, inferior jugular,
and subclavian veins. There is an intra-intestinal vein as in
_Neoceratodus_, but an anterior abdominal vein has yet to be discovered.
The two pulmonary veins from the double air-bladder form a single trunk
before communicating with the left auricle.
With the exception of certain doubtful details which need further
investigation, the venous system of _Lepidosiren_[372] seems to resemble
that of _Protopterus_.
THE HEART.—The heart is more anteriorly placed than in other Vertebrates,
being situated directly behind and beneath the last pair of branchial
clefts and internal to the ventral portion of the pectoral girdle. The
organ is enclosed in a pericardial cavity, which, in the adult, is
separated from the abdominal portion of the coelom by a transverse
pericardio-peritoneal septum, and in the Lamprey (_Petromyzon_) is
partially enclosed within a cartilaginous, cup-like modification of the
hinder part of the branchial basket. In the Ammocoetes-stage of the Lamprey
the pericardium is in communication behind with the general coelom, but the
connexion is lost in the adult. In Elasmobranchs the {328}two cavities are
connected by a single pericardio-peritoneal canal, or by two such canals;
and in _Chimaera_, and in the Sturgeon (_Acipenser_) and _Polyodon_, by a
single canal.
[Illustration: FIG. 193.—Diagram of the structure of the heart in different
Fishes. A, In an Elasmobranch; B, in _Amia_; and C, in a Teleost. _a_,
Auricle; _b.a_, bulbus aortae; _c.a_, conus arteriosus; _s.v_, sinus
venosus; _v_, semi-lunar valves; _v′_, auriculo-ventricular valve; _v.a_,
ventral aorta; _vt_, ventricle. (From Boas.)]
The heart consists of at least three chambers, a sinus venosus which
receives the venous blood from the body, an auricle and a ventricle, to
which is added a conus arteriosus in the Elasmobranchs, certain Teleostomi
(Crossopterygii, Chondrostei, and Holostei), and in the Dipnoi. Through
these cardiac chambers the blood is forced in the order mentioned. In the
Dipnoi the auricle is subdivided by a more or less complete interauricular
septum into a right and left auricle,[373] the former receiving the venous
blood from the sinus venosus, and the latter the aerated blood from the
lung-like air-bladder.
The sinus venosus and the auricle have very thin walls; the ventricular
walls, on the contrary, are very thick and in great measure are composed of
a sponge-like network of muscular bundles which generally encroaches
considerably on the ventricular cavity. Membranous valves, the
sinu-auricular, and the auriculo-ventricular valves, are developed at the
junctions of the sinus venosus with the auricle, and the auricle with the
ventricle respectively. The conus arteriosus is muscular and contractile,
and is interposed between the ventricle and the root of the ventral aorta.
Internally, the conus is provided with several transverse rows of
pocket-shaped or semilunar valves. In Teleosts the conus is non-muscular
and vestigial, and has but a single row of valves, corresponding to the
most anterior of the multiple rows of valves in the Elasmobranchs. In these
{329}Fishes the vestigial conus is succeeded by a non-contractile,
bulb-like dilatation, or bulbus aortae, of the root of the ventral aorta.
In only a single Teleost, viz. _Albula_, one of the Albulidae, is the
vestigial conus muscular, and at the same time provided with two rows of
valves.[374] In the Cyclostomata there is a bulbus with a single row of two
valves, but no true conus.
In the Dipnoi (e.g. _Protopterus_) the heart, like the rest of the vascular
system, exhibits certain interesting resemblances to the Amphibian heart.
In addition to a more or less complete interauricular septum separating
right and left auricles, there is a median longitudinal ridge, partly
muscular and partly fibrous, which incompletely subdivides the cavity of
the ventricle. The spirally-twisted conus arteriosus is furnished with
several transverse rows of valves, certain of which coalesce longitudinally
to form a complete septum dividing the cavity of the conus into two
distinct lateral channels: with this septum there coalesces another septum,
which cuts off the origins of the anterior two pairs from the remaining
afferent branchial arteries. The formation of these septa has the
physiological effect of subdividing the series of cardiac cavities into two
parallel channels, of which one has its origin behind in the sinus venosus
and transmits venous blood to the posterior afferent branchial vessels;
while the other, commencing with the left auricle, conveys arterial blood
to the first two pairs of afferent branchial arteries.[375] In
_Neoceratodus_, however, the longitudinal septum in the conus is
incomplete, and hence the blood which is sent to the anterior afferent
vessels is mixed.[376]
THE ARTERIAL SYSTEM.—The ventral aorta is a median artery situated beneath
the floor of the pharynx, and having its origin, behind, either directly
from the ventricle or from the conus arteriosus.
In the Cyclostomata[377] (e.g. _Petromyzon_) the ventral aorta (Fig. 194)
is continued forwards from the heart as a single vessel to the fourth pair
of gill-sacs, where it divides into right and left branches which extend as
far as the anterior walls of the first pair of gill-sacs. Eight pairs of
afferent branchial arteries arise from the ventral aorta and its two
branches, of {330}which the first and last supply the anterior walls of the
first pair of sacs and the posterior walls of the last pair respectively.
Each of the remaining afferent vessels extends into an interbranchial
septum, and supplies the gill-lamellae of the posterior wall of one sac and
those of the anterior wall of the next sac behind. The corresponding
efferent branchial vessels have a similar distribution, and unite dorsally
to form a median dorsal aorta. Beneath the base of the skull the latter
vessel divides into two branches which, after receiving the first pair of
efferent branchial vessels, pursue a divergent course forwards, but
subsequently converge and unite to form a "circulus cephalicus," as in
Teleosts. From the cephalic circle are given off on each side (1) an
"internal carotid" artery for the brain and eye; (2) an "external carotid"
for the lateral and ventral walls of the head; and (3) a large ventral
branch which supplies the lingual apparatus; while from the abdominal
portion of the dorsal aorta are derived, first, a coeliaco-mesenteric
artery for the liver and alimentary canal, and subsequently branches for
the myotomes, kidneys, and the gonad. The terminal portion of the aorta
then enters the tail and forms the caudal artery.
[Illustration: FIG. 194.—Branchial arterial system of the Lamprey
(_Petromyzon fluviatilis_). The ventral aorta and the afferent branchial
arteries are seen on the left side, and the efferent branchial and dorsal
aorta on the right (diagrammatic). _af.b.a_, _ef.b.a_, Afferent and
efferent branchial arteries; _a.o_, auditory organ; _b.c_, branchial canal;
_c.c_, cephalic circle; _cm.a_, coeliaco-mesenteric artery; _d.a_, dorsal
aorta; _e_, eye; _ex.c_, "external carotid"; _h_, heart; _i.b.s_,
interbranchial septum; _in.c_, internal carotid; _oph.a_, ophthalmic
artery; _r.v.a_, _l.v.a_, right and left ventral aorta; _v.a_, median
ventral aorta; _v.c_, "ventral carotid"; 1-7, gill-sacs. (Modified from
Vogt and Yung.)]
In Elasmobranchs[378] (e.g. _Mustelus antarcticus_) the undivided ventral
aorta gives off five pairs of afferent branchial arteries which, on each
side, ascend in succession the outer convex sides of the hyoid and first
four branchial arches (Fig. 195).
{331}[Illustration: FIG. 195.—The branchial arterial system of _Mustelus
antarcticus_. Left lateral view. The ventral aorta and afferent branchial
vessels are in solid black, the efferent arteries and their branches have
double contours. The branchial clefts have fringed borders to indicate
their hemibranchs, and the arches are in simple outline. _a.c.a_, Anterior
carotid; _a.d.a_, anterior dorsal aorta; _af.b.a_, afferent branchial
artery; _br.a_, brachial artery; _c.m.a_, coeliaco-mesenteric; _d.a_,
dorsal aorta; _E_, eye; _ep.a_, epibranchial artery; _H_, heart; _h.b.a_,
hypobranchial artery; _hy.a_, afferent pseudobranchial or hyoidean artery;
_md.a_, mandibular artery; _op.a_, ophthalmic artery; _p.c.a_, posterior
carotid; _sb.a_, subclavian; _sp_, spiracle; _v.a_, ventral aorta; 1-5, the
hyobranchial and four succeeding branchial clefts. The hypobranchial artery
is seen immediately beneath the ventral aorta. (After T. Jeffery Parker,
diagrammatic.)]
The first or most anterior of these arteries supplies the hyoidean
hemibranch, while the succeeding four supply the holobranchs of the four
branchial arches. The blood is collected from the capillaries of the
branchial lamellae by a series of efferent branchial vessels, a pair for
the two hemibranchs of each branchial arch and a single vessel for the
hyoidean hemibranch, which unite with one another in a somewhat singular
fashion. The efferent arteries from the anterior and posterior hemibranchs
of each branchial cleft unite above and below each cleft in such a way as
to form a series of complete vascular loops round the hyoidean cleft and
the three succeeding branchial clefts, which are connected by short
longitudinal trunks in each arch and also by a longitudinal commissural
vessel between their ventral extremities. As the fifth arch is gill-less,
there is no complete loop round the fifth cleft, the blood collected by the
efferent vessel of the posterior hemibranch of the fourth arch being
conveyed to the corresponding vessel of the anterior hemibranch of the same
arch by one of the short longitudinal vessels above mentioned. Dorsally,
each arterial loop is continuous with an epibranchial artery; and by the
dorsal union of the four {332}epibranchial arteries of the two sides the
dorsal aorta is formed. It may be pointed out that the anterior efferent
vessel of each arch, which is usually larger than the posterior one, is to
be regarded as the primary efferent artery of the corresponding holobranch,
and as such is directly continuous with an epibranchial artery, the
posterior efferent artery being a secondary vessel which opens not into the
primary trunk of its own branchial arch, but into that of the succeeding
arch.[379] The principal arteries which supply the various parts of the
head with blood are derived from the first efferent branchial vessel. From
the ventral end of this artery a mandibular artery is given off, which
subdivides into branches for the muscles of the lower jaw as well as into
nutrient vessels for the hyoidean hemibranch. At about the middle of its
length the same artery gives off an afferent pseudobranchial or hyoidean
artery, to the spiracular or mandibular pseudobranch. From the latter organ
the blood is collected by an anterior carotid artery which, after giving
off an ophthalmic branch to the eye, perforates the orbital wall and enters
the cranial cavity, where it is joined by an anastomotic trunk from the
posterior carotid of the opposite side; finally, the anterior carotid
divides into anterior and posterior cerebral arteries for the brain. The
third and last of the cephalic arteries is the posterior carotid; this
artery arises from the dorsal extremity of the first efferent branchial
vessel, and, on entering the orbit, gives off the anastomotic trunk
previously mentioned. The latter vessel enters the cranial cavity, and,
after crossing its fellow, joins the anterior carotid of the opposite side,
as described above. The main trunk is then continued forwards in the orbit,
and its various branches eventually supply the eye-muscles, the mandibular
adductor muscle, and some other parts of the head.
It is worthy of note that the median dorsal aorta is prolonged forwards in
front of the first pair of epibranchial arteries as a slender median vessel
(_a.d.a_), which ultimately divides into two branches, each branch uniting
with the posterior carotid of its side.
A remarkable system of arteries for the supply of nutrient blood to the
gills and heart has its origin in the following {333}manner. On each side,
the longitudinal commissural vessel, which connects the ventral ends of the
arterial loops surrounding the different gill-clefts, gives origin to a
series of pairs of short transverse vessels, and by their union these
combine to form a median longitudinal hypobranchial artery which lies
beneath the ventral aorta. From the hypobranchial artery are derived the
coronary arteries for the heart; and from the same artery, or from its
lateral connexions with the longitudinal commissural artery, and, in the
case of the hyoidean hemibranch, from the mandibular artery, are derived
the various nutrient vessels for the gills.
The arteries for the trunk, and for the pectoral and pelvic limbs, arise in
succession from the dorsal aorta. The first of the series is the subclavian
artery, which has its origin from the aorta close to the dorsal extremities
of the fourth pair of epibranchial arteries. Each subclavian artery gives
off a brachial artery to the pectoral fin, and is then continued forwards
as a lateral hypobranchial artery, which, with its fellow of the opposite
side, eventually becomes continuous with the hinder end of the median
hypobranchial artery. Behind the subclavian artery there is a median
coeliaco-mesenteric artery, the various branches of which are distributed
to the liver, stomach, and intestine. A lieno-gastric artery supplies the
pancreas and spleen, and also sends branches to the stomach. In addition,
there are also arteries for the gonads, numerous segmental arteries for the
myotomes, and renal arteries for the kidneys. Finally, the aorta gives off
a pair of iliac arteries for the pelvic fins, and then enters the haemal
canal as the caudal artery.
The more important differences in the arterial system of the Holocephali
and the Teleostomi relate to (1) the absence of the posterior efferent
branchial artery in each branchial arch; (2) modifications dependent on the
condition of the spiracular and hyoidean hemibranchs, and the mode of
origin and the course of their afferent and efferent vessels; and (3) the
source from whence the air-bladder derives its blood when that organ is
present.
(1) The branchial arterial system is somewhat more primitive than in the
generality of Elasmobranchs.[380] There are no complete vascular loops
round the gill-clefts, and the blood from the two {334}hemibranchs of each
branchial arch is conveyed to the dorsal aorta by a single efferent vessel
which corresponds to the more anterior of the two in _Mustelus
antarcticus_.[381]
(2) In _Callorhynchus_[382] among the Holocephali, where the spiracle is
absent but the hyoidean hemibranchi is still a true gill, the latter organ
is supplied with venous blood by a branch from the ventral aorta, the
corresponding efferent vessel joining the dorsal aorta (Fig. 196). In the
absence of a spiracular pseudobranch the anterior carotid may be regarded
as continuous with the hyoidean artery,[383] and as having its origin
directly from the efferent artery of the hyoidean hemibranch (Fig. 196). At
its origin the anterior carotid anastomoses with the mandibular artery.
[Illustration: FIG. 196.—Portion of the efferent branchial system of
_Callorhynchus_. _a.c_, Anterior carotid; _a.cb.a_, anterior cerebral
arteries; _d.a_, dorsal aorta; _ef.b.a_, 1-4, efferent branchial arteries;
_ef.hy_, efferent artery from the hyoidean hemibranch; _hy.c_, hyobranchial
cleft; _md.a_, mandibular artery; _my.a_, myelonal artery; _p.c_, posterior
carotid; _p.cb.a_, posterior cerebral artery. (From T. Jeffery Parker.)]
The Sturgeon more closely resembles the Elasmobranchs. The hyoidean gill is
supplied by an afferent branchial artery from the ventral aorta, and its
efferent vessel joins the corresponding trunk from the holobranch of the
first branchial arch. A hyoidean artery supplies the spiracular
pseudobranch, the efferent vessel of which contributes to the blood-supply
of the brain and the eye, and probably represents an anterior carotid.
_Lepidosteus_[384] offers a singularly interesting transition from the
{335}Elasmobranch to the Teleost. As indicated in the preceding chapter,
this Fish possesses both a hyoidean gill and a spiracular pseudobranch
(Figs. 197 and 198). The hyoidean gill is supplied by an afferent artery
direct from the ventral aorta, but the proper efferent vessel of the gill,
which primitively joined the dorsal aorta, is suppressed, and the blood is
collected into a vessel, which, like the hyoidean artery in Elasmobranchs,
becomes the afferent artery of the spiracular pseudobranch. The latter
artery unites, however, with a second hyoidean artery derived from the
efferent branchial vessel of the first branchial arch, and represents the
artery termed "hyoidean" in Teleosts. The efferent vessel from the
spiracular pseudobranch joins an internal branch from the carotid artery,
and then distributes its blood both to the eye and the brain.
[Illustration: FIG. 197.—Blood-vessels of the spiracular pseudobranch and
the hyoidean gill in _Lepidosteus_. _af.a_, _ef.a_, Afferent and efferent
vessels of the hyoidean gill; _af.ps.a_, _ef.ps.a_, afferent and efferent
vessels of the spiracular pseudobranch; _ca_, carotid (posterior); _d.a_,
dorsal aorta; _ef.b.a^{1-4}_, efferent branchial vessels; _hy.a_, hyoidean
artery; _hy.h_, hyoidean gill; _hy.ps_, spiracular pseudobranch; _v.a_,
ventral aorta. (From F. W. Müller, after Johannes Müller.)]
In Teleosts, as already mentioned in a preceding chapter, it is probable
that the hyoidean hemibranch is suppressed, the so-called hyoidean
pseudobranch being a spiracular pseudobranch. The latter is now supplied by
a "hyoidean" artery, which has its origin from the ventral end of the
efferent {336}branchial artery of the first branchial arch, the
corresponding efferent trunk forming an ophthalmic artery, and passing to
the choroid gland of the eye (Fig. 199). Both the proper afferent and
efferent arteries of the hyoidean hemibranch either disappear or, as in the
Cod (_Gadus morrhua_), the efferent artery may be represented on each side
by an anastomosis between the hyoidean artery and the cephalic circle.
Hence, the "hyoidean" artery of Teleosts corresponds to the one which has a
similar origin in _Lepidosteus_.
A brief description of the remaining efferent branchial arteries and their
derivatives in the Cod (_Gadus morrhua_) will illustrate the condition of
these structures in a well-known Teleost.
[Illustration: FIG. 198.—The branchial circulation in _Lepidosteus_
(diagrammatic). _a_, _a_, Afferent branchial arteries; _c_, carotid; _d.a_,
dorsal aorta; _e_, _e_, efferent branchial arteries; _ef.a_, efferent
vessel from the hyoidean gill which, after its union with the hyoidean
artery, becomes the afferent vessel of the spiracular pseudobranch;
_ef.a′_, efferent vessel of the spiracular pseudobranch; _hy.a_, hyoidean
artery; _hy.g_, hyoidean gill; _sp.ps_, spiracular pseudobranch; _v.a_,
ventral aorta; 1-5, the hyo-branchial and succeeding gill-clefts. (After F.
W. Müller and Ramsay Wright.)]
In this Fish the efferent branchial vessels open dorsally into right and
left suprabranchial arteries,[385] which unite behind to form a median
dorsal aorta (Fig. 199). Anteriorly, the paired suprabranchial arteries
extend towards the base of the skull as the so-called "carotid" arteries.
The two carotids enter the cranial cavity, and there unite in the median
line, as in the Cyclostomes. By the union of these arteries in front, and
of the {337}right and left suprabranchial arteries behind, the
characteristic "circulus cephalicus" of Teleosts is completed.[386] From
the anterior part of the cephalic circle are derived two internal carotid
arteries[387] for the brain, and also a pair of orbito-nasal arteries for
the eye-muscles and the nasal sacs, while more posteriorly an external
carotid has its origin from each suprabranchial artery.
[Illustration: FIG. 199.—Branchial arterial system of the Cod (_Gadus
morrhua_). Lateral view. _af.b.a_, First afferent branchial artery; _cl.a_,
coeliac artery; _d.a_, median dorsal aorta; _ef.b.a_, first efferent
branchial artery; _ex.c_, external carotid; _H_, heart; _hy.a_, hyoidean
artery; _Hy.b.a_, hypobranchial artery for the heart and pelvic fins;
_hy.ps_, spiracular pseudobranch; _in.c_, internal carotid;[387] _l.d.a_,
left suprabranchial artery; _m.a_, mesenteric artery; _on_, orbito-nasal
artery; _oph.a_, ophthalmic artery; _r.d.a_, right suprabranchial artery;
_sb.a_, subclavian artery; _v.a_, ventral aorta; 1-5, hyobranchial and
succeeding gill-clefts. (Altered from T. Jeffery Parker.)]
(3) In most Teleostomi the air-bladder is supplied with blood by branches
of the coeliac artery, with the addition of small branches arising directly
from the dorsal aorta. _Polypterus_ and _Amia_[388] are, however,
exceptional, inasmuch as the arteries for the air-bladder are derived from
the last or fourth pair of efferent branchial vessels, and in this respect,
but not in the destination {338}of the corresponding veins, the two genera
exhibit a significant resemblance to the Dipnoi.
In the Dipnoi the ventral aorta is so short that the afferent branchial
arteries arise almost directly from the conus arteriosus with their roots
in close contiguity to one another (Fig. 200).
[Illustration: FIG. 200.—Branchial arterial system of _Neoceratodus_.
Lateral view. The conus arteriosus and the afferent branchial vessels are
represented in solid black, the efferent vessels and their derivatives with
double contours. _a_, Auricle; _a.c.a_, anterior carotid; _a.cb.a_,
anterior cerebral artery; _af.b.a′_, first afferent branchial artery;
_br.a_, brachial artery; _c.a_, coronary artery; _c.ar_, conus arteriosus;
_c.m.a_, coeliaco-mesenteric; _ep.a_, epibranchial artery; _hb.a_,
hypobranchial artery; _hy.a_, hyoid artery; _hy.ar_, hyoidean arch; _l.a_,
lingual; _l.d.a_, _r.d.a_, left and right dorsal aortae; _oc.a_, occipital
artery; _oes.a_, oesophageal artery; _p_, pericardium; _p.a_, pulmonary
artery; _p.c.a_, posterior carotid; _p.cb.a_, posterior cerebral artery;
_s.v_, sinus venosus; _v_, ventricle; 1, hyobranchial cleft; 2-5, branchial
clefts. (After Baldwin Spencer, diagrammatic.)]
In _Neoceratodus_ (Fig. 200),[389] there are two efferent vessels to each
gill-bearing branchial arch, which unite above to form an epibranchial
artery, and by the successive union of the four epibranchial arteries a
short common trunk is formed on each side. Posteriorly, the two trunks
unite to form a median dorsal aorta. Immediately above the gill-clefts each
efferent vessel gives off a branch which, passing either forwards or
backwards, unites with the corresponding branch of the efferent vessel in
front or behind as the case may be. A hyoidean artery arises from the
ventral extremity of the anterior efferent artery of the first branchial
arch, and, after giving off a lingual artery, ascends the hyoid arch and
supplies the hyoidean pseudobranch. The efferent vessel of the pseudobranch
(_a.c.a_) or anterior {339}carotid artery, eventually enters the cranial
cavity and subdivides into anterior and posterior cerebral arteries for the
brain, also giving off a branch which unites with its fellow of the
opposite side directly behind the infundibulum. A posterior carotid springs
from the epibranchial of the first branchial arch and divides into
palatine, orbital, and ocular branches; and from the ventral end of the
anterior efferent vessel of the second branchial arch is derived a
hypobranchial artery for the heart and pericardium. The pulmonary arteries
for the lung-like air-bladder have their origin from the fourth pair of
epibranchial arteries.
As in so many other details of its anatomy, _Neoceratodus_ exhibits in its
arterial system abundant evidence of the wide-spreading affinities of the
group to which it belongs. In its branchial arterial system _Neoceratodus_
presents a singular combination of features which, individually, are
characteristic of Amphibia and Elasmobranchs. Special Amphibian features
may be noted in the origin of the afferent branchial arteries almost
simultaneously from the anterior end of the conus arteriosus; in the mode
of union of the epibranchial arteries to form the dorsal aortae; in the
origin of a lingual artery from the efferent vessel of the first branchial
arch; and in the derivation on either side of a pulmonary artery from the
fourth epibranchial artery. Agreement with Elasmobranchs is to be found in
the presence of two efferent branchial vessels in each branchial arch,
although the relations of these arteries are more primitive than in most
adult Elasmobranchs, inasmuch as the two efferent vessels of the _same_
arch unite to form an epibranchial artery; and also in the origin and
distribution of the anterior and posterior carotids. Lastly may be
mentioned the fact that _Neoceratodus_ agrees not only with the Amphibia
but also with those generalised Teleostomi, _Polypterus_ and _Amia_, in the
mode of origin of the great arteries for the air-bladder.
Of the two remaining Dipnoi, the arterial system of _Protopterus_[390] is
better known than that of _Lepidosiren_, but in both cases further research
is needed before a satisfactory comparison can be made with _Neoceratodus_
and other Vertebrates. It is evident, nevertheless, that both genera differ
from _Neoceratodus_ in approximating more closely to the Amphibia than to
the {340}lower Fishes, in so far as the branchial part of the arterial
system is concerned.
[Illustration: FIG. 201.—Branchial arterial system of _Protopterus_
(diagrammatic). _a_, Auricle; _a.c.a_, carotid artery; _af.b.a^{1-4}_,
afferent branchial arteries; _af_, _ef_, afferent and efferent vessels of
the hyoidean pseudobranch; _b.a^2_, second branchial arch, the vestigial
first arch being omitted; _c.a_, conus arteriosus; _e.g_, external or
cutaneous gill; _ep.a_, epibranchial artery; _hy.ar_, hyoid arch; _hy.ps_,
hyoidean pseudobranch; _l.a_, lingual artery; _l.d.a_ and _r.d.a_, right
and left dorsal aortae; _l.p.a_, left pulmonary artery; _s.v_, sinosus
venosus; _v_, ventricle; 2-6, the second branchial and succeeding clefts,
the hyobranchial cleft being closed. The vestigial first branchial arch is
not shown. The epibranchial arteries unite to form the right or left dorsal
aorta at the same point and not in succession as in the figure. (Altered
from Newton Parker.)]
In their origin from the conus the four afferent branchial arteries of
_Protopterus_ resemble those of _Neoceratodus_, but their relations to the
branchial clefts are somewhat different (Fig. 201). The first or hyoidean
cleft is closed, and the first afferent vessel lies between the second
cleft and the third, and is therefore in relation with the second branchial
arch. The remaining afferent arteries are disposed between the succeeding
clefts and are related to the corresponding arches. As the second and third
arches, like the vestigial first arch, bear no gill-lamellae, their
afferent arteries are directly continuous with the corresponding efferent
vessels, as in those Teleosts in which certain arches are gill-less, as
well as in the Tadpole-stage of the tailless Amphibia when the internal
gills begin to degenerate; and they apparently transmit arterial blood
directly to the dorsal aorta.[391] The third and fourth afferent arteries,
on the contrary, supply venous blood to the two hemibranchs which are borne
by each of the two corresponding arches, viz.: the fourth and fifth, and
from each pair of hemibranchs the blood is collected into two efferent
vessels which unite dorsally {341}to form an epibranchial artery. From the
dorsal end of the fourth afferent artery there arises a recurrent branch
which curves round the upper margin of the sixth cleft and supplies the
gill-lamellae on the posterior margin of that cleft, a fact which lends
support to the view that these lamellae are "emigrants" from the anterior
margin of the cleft; the efferent vessel from the "emigrant" lamellae joins
the fourth epibranchial artery. The blood-supply of the external or
cutaneous gills is derived from the dorsal extremities of the second,
third, and fourth afferent arteries, while the efferent vessels from these
organs join the corresponding epibranchial arteries; in this respect there
is a close resemblance between _Protopterus_ and those larval Amphibians
which possess similar cutaneous gills. All four epibranchial arteries unite
together at about the same point to form a short common trunk, the right or
left dorsal aorta, which subsequently unites with its fellow to form the
median dorsal aorta.
There is a so-called "hyoidean" artery, which, however, has its origin, not
from an anterior efferent branchial vessel as in _Neoceratodus_, but from
the first _afferent_ branchial artery. After giving off a submaxillary or
lingual artery, the "hyoidean" artery (_af_) becomes the afferent vessel
for the "opercular gill" or "hyoidean pseudobranch,"[392] and supplies the
latter with arterial blood. The efferent vessel (_ef_) from the
pseudobranch unites with the four epibranchial arteries in forming the
right or left dorsal aorta. A "carotid" artery arises from the efferent
vessel of the "hyoidean pseudobranch," and a pulmonary artery has its
origin from the root of the dorsal aorta of its side, and not from the
fourth epibranchial artery as in _Neoceratodus_.
THE BLOOD.—The blood consists of a nutritive fluid plasma in which float
red corpuscles and leucocytes. In the Cyclostomata (e.g. _Petromyzon_) the
red corpuscles are circular, but in _Myxine_ they have the usual oval
shape. In Fishes the red corpuscles are almost invariably flat, oval,
biconvex, and nucleated, and owe their colour to the presence of the
characteristic oxygen-absorbing, iron-containing pigment, haemoglobin. They
are unusually large in the Dipnoi and are only exceeded in size by those of
certain Urodele Amphibians. The leucocytes are much less numerous than the
red corpuscles, although their relative proportions are very variable, even
in the same species under different {342}conditions. They appear to be more
numerous in the Dipnoi (e.g. _Protopterus_) than in any other Vertebrates,
except under pathological conditions.[393]
THE LYMPHATIC SYSTEM.—In addition to blood-vessels, Fishes possess a
lymphatic system, consisting of smaller vessels, lymph-capillaries or
lymph-spaces, distributed in the connective tissue of different parts of
the body, and by their union ultimately forming larger lymph-vessels or
sinuses which communicate with certain of the principal veins, the whole
forming a series of channels for the collection of the blood-plasma which
has exuded from the blood-capillaries for the nutrition of the tissues, and
for its conveyance to the general venous system. The fluid in the
lymphatics, or lymph, consists of dilute blood-plasma containing leucocytes
but devoid of red corpuscles. At the points where the larger lymphatics
open into the veins, lymph-hearts may be developed. In the Eel (_Anguilla
vulgaris_) there is a lymph-heart in the tail, which communicates by a
valvular orifice with the smaller of the two caudal veins, and by its
rhythmical pulsations propels the lymph into the vein. In _Silurus_ there
are two caudal lymph-hearts. Apart from the lymphoid tissue, which is so
abundantly present in certain parts of the body, Fishes appear to be devoid
of the special "lymphatic glands" of the higher Vertebrates.
THE DUCTLESS OR BLOOD-GLANDS.—All the important blood-glands of other
Vertebrates have their representatives in Fishes. Nothing is certainly
known of the function of these organs in Fishes, but from the general
structural resemblance which they present to their equivalents in the
higher Vertebrates, it is perhaps not unreasonable to infer that they are
similar in function. If this be so, the blood-glands of Fishes are organs
for leucocyte-formation and phagocytosis, involving the destruction and
removal of effete red blood-corpuscles; in addition, they may also be
concerned with certain obscure chemical changes in the composition of the
blood, which have an important relation to general or local nutrition.
THE SPLEEN.—This lymphoid organ is the largest of all the blood-glands,
and, in the form of a compact or more or less lobulated body, is present in
all Fishes, and possibly in Cyclostomes. In position the spleen is usually
in close proximity to the stomach, to which it is attached by an extension
round it {343}of the peritoneal investment of that organ. Thus, in the
Dog-Fish (_Scyllium_), the spleen is a large reddish body attached to the
convexity of the U-shaped stomach, and, in addition, sends a long narrow
lobe between the distal limb and the valvate portion of the intestine (Fig.
153, _spl_). In the Sturgeon (_Acipenser_), the organ is also large, but is
attached to the left side of the commencement of the intestine. In the Cod
(_Gadus_) among Teleosts the spleen is much elongated and is situated on
the dorsal side of the stomach. In the Dipnoi (e.g. _Protopterus_)[394] the
organ is probably represented by a large compact lymphoid mass, closely
connected with the dorsal and lateral walls of the stomach (Fig. 154, A,
_s_).
THE THYROID GLAND.—This organ[395] usually arises in the form of a small
median evagination of the hypoblastic epithelium of the ventral wall of the
pharynx, in the region of the second visceral arch. Later it becomes
detached from the place of origin and converted into a solid spherical
body. Eventually the component cells form the limiting epithelium of a
series of follicles or vesicles embedded in a matrix of connective tissue
and blood-vessels, and the characteristic adult structure is attained.
Among the Cyclostomata the evagination is relatively large in the young
Lamprey (_Petromyzon fluviatilis_), as also is the orifice of communication
with the pharynx (Fig. 202, _th_).[396] The aperture soon becomes reduced
to a mere pore, and finally disappears. During the larval or
Ammocoetes-stage the organ consists of a median ciliated portion,
communicating with a pair of laterally placed glandular sacs, but in the
adult it is much smaller, and acquires the usual follicular structure. In
adult Elasmobranchs the thyroid is represented by a moderately large
compact organ, situated near the anterior end of the ventral aorta. In
Teleostomi the organ may be paired, or, as in the Perch (_Perca_), more
diffuse, consisting of masses of reddish lobules lying beneath the aorta,
and also scattered for a variable distance along the course of the afferent
branchial arteries.
In the Dipnoi (e.g. _Protopterus_)[397] the thyroid is small,
{344}consisting of two lateral lobes connected by a constricted median
portion, and situated beneath the epithelium of the tongue, immediately
above the hyoidean symphysis. A similar structure has been described by
Bischoff[398] in _Lepidosiren_, and was regarded by him as a salivary
gland.
As in Reptiles, Birds, and Mammals, paired or accessory thyroid bodies
("supra-pericardial organs")[399] are present in many Fishes, and appear to
be similar in structure to the median thyroid. In Elasmobranchs these
bodies originate as a pair of outgrowths from the epithelium of the pharynx
behind the last pair of branchial arches (Fig. 202, B, _a.th_).
Subsequently they become detached from the pharynx, and in the adult are
situated on the dorsal side of the pericardium, remote from the median
thyroid.
[Illustration: FIG. 202.—A, a vertical section through a just-hatched larva
of _Petromyzon_. _a.v_, Auditory vesicle; _br.c_, branchial cleft; _h_,
heart; _m_, mouth; _n_, notochord; _ol_, olfactory pit; _ph_, pharynx;
_sp_, septum or velum between the stomodaeum and the mesenteron; _sp.c_,
spinal cord; _th_, thyroid outgrowth from the floor of the pharynx. (From
Gegenbaur, after Calberla.) B, diagram illustrating the development of the
thyroid, the thymus, and the accessory thyroids, and their relations to the
branchial clefts. _a.th_, Accessory thyroids; _g.p_, gill-pouches; _Ph_,
pharynx; _t_, thymus; _th_, median thyroid. (From Hertwig, after de
Meuron.)]
According to Dohrn the median thyroid is to be regarded as the vestige of a
gill-cleft which primitively existed between the hyomandibular cartilage
and the hyoidean arch. This conclusion seems, however, to be less in
harmony with the facts of development than the view[400] that the organ is
derived from the characteristic hypobranchial groove or "endostyle" of
Ascidians {345}and Amphioxus, which has undergone a change of function from
a mucus-conveying groove to a blood-gland. On the other hand, the mode of
origin of the paired thyroids certainly favours the suggestion that they
represent a posterior pair of vestigial gill-clefts, a view which derives
some support from the fact that in _Notidanus_, where additional branchial
arches and clefts are present, the paired thyroids are absent.
THE THYMUS.—In the embryo Elasmobranch and Teleost[401] the thymus has a
multiple origin, being derived from a series of distinct epithelial
thickenings, one of which is developed at the dorsal extremity of each of
the gill-clefts except of the spiracle. These rudiments subsequently detach
themselves from the epithelial surface and sink inwards, eventually fusing
together on each side to form a single independent structure. Later, the
epithelial mass thus formed becomes invaded by connective tissue, and by
leucocytes which form lymph follicles, and the thymus gradually assumes the
structure of a lymphoid organ. From its mode of development it has been
suggested that the thymus owes its evolution to the metamorphosis and
ingrowth of branchial filaments,[402] but it is also noteworthy that each
embryonic rudiment of the organ closely resembles, both in position and
origin, one of the developing branchial tongue-bars of Amphioxus.[403] The
abundance of leucocytes which it contains has also prompted the further
suggestion that the origin of the thymus may be due to the necessity of
providing for the phagocytic protection of the gills themselves from the
ravages of harmful micro-organisms, fungoid spores, etc., as well as to aid
in the removal of such portions of the gills as may have been injured.[404]
A thymus is probably present in all Fishes, if not in the adult at all
events in the embryo, but is always relatively small in size. In
Elasmobranchs the organ lies on each side above the branchial arches and
beneath the dorsal musculature; and in Teleostomi at the dorsal extremity
of the last branchial arch, in close proximity to the mucous membrane of
the branchial cavity. In a similar position in the Dipnoi (e.g.
_Protopterus_)[405] there are, on each side, {346}two contiguous lobes of
lymphoid tissue which apparently represent a thymus.
[Illustration: FIG. 203.—Supra-renal and inter-renal bodies of Fishes. A,
of _Scyllium catulus_; B, of _Acipenser sturio_; C, of _Pagellus
centrodontus_. _d.a_, Dorsal aorta; _tr_, inter-renal body; _l.m_, lymphoid
portion of the mesonephros; _m_, mesonephros; _mtn_, metanephros; _oes_,
oesophagus; _sg.a_, segmental arteries; _sr_, supra-renal bodies; _sy.n_,
sympathetic nerves. (From Swale Vincent.)]
THE SUPRA-RENAL BODIES.—The supra-renal bodies are organs of problematic
function, which are present in the Cyclostomata, and probably in all
Fishes, and situated in close proximity to the kidneys.
In the Cyclostomata (_Petromyzon_) these bodies are represented by lobules
of cells along the posterior cardinal veins, and also by masses of peculiar
cells ("chromaffin cells") along the sides of the aorta and segmental
arteries.[406] In Elasmobranchs there are two distinct structures, the
paired supra-renals and the inter-renals (Fig. 203, A). The former are a
series of pairs of segmentally arranged bodies, situated on the successive
pairs of segmental arteries given off from the dorsal aorta. The two bodies
which form the first pair are much larger than any of the others, and were
formerly spoken of as "axillary hearts." The inter-renal is usually a thin
elongated "ochre-yellow" body, from which one or two lobes may be detached
in front, and extends for a variable distance in {347}the median line
between the two kidneys, or is unsymmetrically placed on the ventral
surface of either kidney.[407] Sometimes (e.g. in _Raia_) the inter-renals
are paired, in which case they are applied to the inner and hinder margins
of the kidneys. In the Sturgeon (_Acipenser sturio_) the "supra-renals"
appear as numerous "ochre-yellow" bodies, variable in size and distribution
(Fig. 203, B). Some of them are visible on the surface of the kidneys,
while others are scattered about in their substance, but on the whole are
more anteriorly placed than in Teleosts. In the latter group the
"supra-renals" are usually two in number (Fig. 203, C), but may be as many
as five or reduced to one. They are disposed either on the ventral or the
dorsal surface of the kidneys, generally near their hinder extremities, or
more or less deeply embedded in their substance. Besides these bodies there
are also chromaffin cells in the walls of the anterior cardinal veins.[408]
Histologically, the paired segmentally arranged bodies of Elasmobranchs
differ considerably in structure from the inter-renal bodies, the former
resembling the "medulla," while the inter-renals, as well as the so-called
supra-renals of _Acipenser_, exhibit a striking resemblance to the alveolar
"cortical" substance of the Mammalian supra-renals.[409] In Cyclostomes the
cortex is apparently represented by the lobules of cells along the
posterior cardinal veins and the medulla by the "chromaffin" cells, while
in Teleosts the cortex and the medulla have their respective counterparts
in the supra-renals and the "chromaffin" cells in the walls of the anterior
cardinal veins. It may be concluded, therefore, that Elasmobranchs,
Cyclostomes, and Teleosts possess anatomically distinct representatives of
both the "medulla" and "cortex" of Mammalia, although the Sturgeon is at
present only known to possess the equivalent of the "cortex." In Amphibia,
Reptilia, and Aves both "cortex" and "medulla" are present, and in the
varying intimacy of their relations offer a transition to the Mammalian
arrangement of a central medulla closely invested by a sheath of cortical
substance. A more or less intimate connexion exists between the paired
supra-renals of Elasmobranchs and the sympathetic nervous {348}system. The
former are usually well supplied with sympathetic nerve fibres, and contain
ganglion-cells in their substance.
The primitive origin of these organs is very obscure, and as regards their
development there is much diversity of opinion. It seems certain, however,
that the cortex and medulla of the higher Vertebrates, including their
equivalents in the Elasmobranchs, have independent origins, and the balance
of opinion seems to point to the derivation of the cortex from some portion
of the germinal coelomic epithelium, while the medulla is derived from the
embryonic nerve cells of the sympathetic ganglia.
LYMPHOID TISSUE.—In addition to certain of the ductless glands, and the
local or diffused masses of their characteristic tissue already mentioned
in connexion with the alimentary canal, lymphoid tissue is often abundantly
present in other parts of the body. There is, for example, a mass of this
tissue on the heart of the Sturgeon (_Acipenser_). The anterior enlarged
portion of the mesonephros, commonly termed the "head-kidney" of the
Teleostomi (Fig. 203, B, C), is almost entirely composed of lymphoid
tissue,[410] which has replaced, wholly or partially, the proper renal
structure; and from the presence of free red blood-corpuscles and of
crystals of oxy-haemoglobin and other derivatives of haemoglobin, it may be
inferred that the "head-kidney," in common with the more orthodox
blood-glands, performs a blood-destroying function.[411] On the other hand,
the example of the spleen, which is alike the seat of leucocyte-formation
and of blood-destruction, renders it unnecessary to reject the view that
the "head-kidney" is an organ in which leucocytes or blood-corpuscles are
formed. In but few Teleostomi is a purely lymphoid "head-kidney" entirely
wanting, as, for example, in the Sun-Fish (_Orthagoriscus mola_).[412] As
previously mentioned the Dipnoi are remarkable for the extraordinary
development of lymphoid tissue, inasmuch as it forms a thick investing mass
round the kidneys and gonads in addition to its exceptional abundance in
the walls of the alimentary canal.
The absence of ordinary lymphatic glands in Fishes is well known, and it is
at least probable that, functionally, the want of these lymphoid organs may
be compensated for by the superabundance of lymphoid tissue in other parts
of the body.[413]
{349}CHAPTER XIII
MUSCULAR SYSTEM—LOCOMOTION—SOUND-PRODUCING ORGANS—ELECTRIC ORGANS
MUSCULAR SYSTEM.—The various muscles of the body may be arranged in two
systems: (i.) the _somatic_ or _parietal_, composed of striated or
voluntary muscle-fibres; and (ii.) the _splanchnic_ or _visceral_,
consisting for the most part of unstriated or involuntary fibres. Somatic
muscles form the great lateral longitudinal muscles of the trunk and tail,
which retain the primitive embryonic metamerism to a greater extent in
Fishes than in any other Vertebrates, and are the principal muscles
associated with locomotion. The lateral muscles are composed of a series of
transverse muscle-segments or myotomes, which are >-shaped, or S-shaped,
or they even take a zigzag course from above downward. The myotomes are
disposed in pairs, and they are separated from one another by fibrous septa
or myocommata. Each myotome is divided into a dorsal or epiaxial portion,
and a ventral or hypaxial portion, by a longitudinal, horizontal, fibrous
septum extending outwards from the vertebral centra to the skin. The
muscles of the pectoral and pelvic fins are derivatives from more or fewer
of the adjacent myotomes. The splanchnic muscles include the musculature of
the walls of the alimentary canal, as well as those specialised portions of
the visceral system which are represented by the muscles of the branchial
arches and the jaws, and are composed of striated fibres.
LOCOMOTION.—A Fish and a Bird are equally remarkable for the many and
various ways in which they are adapted for locomotion in the particular
medium in which they live. In its shape the Fish is admirably adapted for
cleaving the water. Spindle-like in shape, but thicker in front than
behind, a Fish resembles {350}a double wedge, the thick part of which is
represented by the head and one of the thin edges by the free hinder margin
of the caudal fin. The body is bounded by smooth flowing contour lines,
unbroken by any sharp separation of the body regions from one another, and
with no points of resistance to its forward motion through the water. The
body being thicker in front than behind, and, as seen in transverse
section, broader above than below, it follows that its centre of gravity
will be nearer the head than the tail, and nearer the dorsal than the
ventral surface. The dorsal position of the centre of gravity necessarily
renders the equilibrium of the body unstable, and were it not for the
balancing action of the paired fins the Fish would float belly upwards, as
is always the case after death. Most Fishes are provided with a membranous
gas-containing sac, the air-bladder, the principal function of which is to
render the Fish, bulk for bulk, of the same weight as the water, so that in
this position of equilibrium, or plane of least effort, the animal can
execute its various locomotor movements with a minimum expenditure of
muscular effort—an advantage which no other animal possesses.[414] To give
stability to the body, and to steady its course when swimming, the Fish has
a dorsal and a ventral keel, formed by the anal and dorsal fins, which,
like the sliding keel of a yacht, can be raised or lowered as occasion
requires. When these fins are removed the course of the Fish becomes
zigzag, and the animal wobbles.
The organs more directly concerned with swimming are the tail and the
caudal fin, and the pectoral and pelvic fins, but the relative share which
these structures take in the actual propulsion of the Fish differs greatly.
The principal organ of locomotion in the typical Fish is the powerful
muscular tail, which, in swimming, is lashed from side to side by the
alternating contraction of the great longitudinal muscles on opposite sides
of the vertebral column.[415] In such movements the tail is first flexed or
bent, say to the right side: this stroke has been termed the non-effective
or back stroke. By a stroke in the reverse direction the tail is then
extended and straightened, that is to say, the Fish makes the forward or
effective stroke. By a rapid succession of such strokes to the right and
left sides alternately the Fish is {351}forced through the water. It is
obvious, however, that the extension or effective stroke must have a
considerable surplus of power over the flexion or non-effective stroke, and
how this result is achieved will now be briefly considered. Experiment, and
the observation of Fishes like the Sturgeon, which habitually move with
sufficient slowness to allow the phases of their swimming movements to be
followed without much difficulty, show that in swimming a Fish throws its
body into two opposite and complementary curves, a cephalic curve formed by
the anterior half of the body and a caudal curve by the tail. The double
curve enables the Fish always to present a convex, less resisting or
non-biting surface to the water during the flexion of the tail to the right
or left as the case may be, and a concave or biting surface during
extension, that is when the tail is straightening itself during the
effective stroke.
[Illustration: FIG. 204.—To illustrate the mode in which the tail of an
ordinary Fish is used in swimming. See the text for the lettering.
(Slightly altered from Pettigrew.)]
Fig. 204, which represents a Fish in two successive positions while
swimming, will serve to illustrate these conclusions. A Fish in the
position A has its body thrown into a cephalic concavity directed to the
right and a caudal concave surface facing the left. The tail is bent to the
right of the line _a b_, which corresponds to the axis of the Fish when at
rest and to the course pursued by the animal when swimming, and is in the
position which it assumes during a flexion stroke, with its convex
non-biting surface directed outwards and its concave biting surface
inwards. The tail is now ready for an extension stroke, and while this is
in progress it is clear that the concave biting surface of the tail will
meet the water, while at the conclusion of the stroke the tail will be in a
line with _a b_. At the same time the cephalic curve has so far diminished
that the long axis of the body for a momentary period will also coincide
with _a b_, and the Fish is free to advance without impediment. The tail,
{352}however, continues its movement to the left, but now as a flexion
stroke, and assumes the curvature and position indicated in B, with a
reversal in the direction of both the cephalic and caudal curves, but in
the meantime the force of the preceding extension stroke has forced the
Fish along the line _a b_ to the new position indicated by B. By a rapid
succession of alternating flexions and extensions, during which the tail
describes figure-of-8 curves, the Fish travels in an undulating forward
course with a maximum of propelling power and a minimum of "slip." In
short, the action of the tail precisely resembles the action of the
stern-oar in the operation of sculling a boat.
There are also other considerations which add to the surplus power of the
extension stroke by lessening the resistance of the water to the flexion or
non-effective stroke. During the flexion stroke the tail fin is less
expanded and its area diminished, and by the rotation of the Fish on its
long axis the surface of the tail strikes the water obliquely, and further,
the tail moves with less rapidity. On the contrary, when the extension
stroke is made these conditions are reversed. The caudal fin is expanded,
the stroke is more rapid, and by the reverse rotation of the Fish the tail
now strikes the water with its flat surface. In other words, the action of
the tail during the two strokes may be compared to the "feathering" of an
oar in rowing. Nor is this all. A Fish in motion through the water produces
a suction current behind it. The current offers but little resistance to
the flexion stroke, inasmuch as the direction of the two coincide, but
during the extension stroke the tail meets the full force of the current,
and consequently its grip and propelling power are greatly enhanced. There
is a striking analogy between the movements of a Fish's tail in swimming
and the action of the screw of a steamer, but as a propelling organ the
former is far superior to the latter. As we have seen, the tail of a living
Fish can so adjust its shape and surface that it alternately eludes and
grips the water in accordance with the needs of particular strokes.
The curves into which the body of a Fish is thrown when swimming are never
less than two, but in long-bodied Fishes, such as the Eels, the number may
be increased, and in every case the curves occur in pairs and are
complementary to one another.
{353}Many Fishes can jump out of the water, either in pursuit of insect
food, like the Trout, or to enable them to escape the pursuit of their
foes, like the Flying-Fish (_Exocoetus_), by means of a single forcible
stroke of the tail, when the Fish is in a nearly vertical position close to
the surface of the water. It is thus that the Salmon executes its
remarkable leaps over weirs or up salmon-ladders when ascending rivers for
spawning.
The tail is also used for steering. If kept bent to one side when the Fish
is moving the tail acts like a rudder, and the course of the Fish is
deflected to that side; or the direction may be altered by single strokes
of the tail to the right or left, according to the course which the Fish
desires to pursue.
In the majority of Fishes the paired fins are probably of little use for
propulsion, and their action in this as in other functions is not always
clear. In the Sharks and Dog-Fishes as well as in some Teleosts their
planes are nearly horizontal when the fins are extended from the body; in
others they are more oblique, so that the surfaces of the fins look upwards
and backwards, and downwards and forwards; and in others again their
surfaces are so nearly vertical that their strokes will be backwards and
forwards. The pectoral fins also vary in their position on the sides of the
body, being much more dorsal in some Fishes than in others. The paired fins
may act as lateral keels in steadying the course of the Fish especially
when the fins are extended and their planes are horizontal. They certainly
seem to act as balancers in keeping the Fish on an even keel, and in
counteracting the tendency of the Fish to turn belly upwards—a result which
is attained by a slight upward and downward movement of the fins, and
particularly of the pectoral fins. A Fish deprived of its pectoral members
sinks downwards at the head and assumes an oblique position in the water.
Removal of both the pectoral and pelvic fins of one side causes the Fish to
roll over to that side; and if the fins are removed from both sides the
animal turns belly upwards like a dead Fish. The pectoral fins may also be
used for steering: a backward stroke of one fin while the other is kept
folded back against the body will wheel the Fish round to the opposite
side. From the ventral position of its mouth a Shark is forced to turn over
to one side in order to seize its prey, and this movement of rotation is
probably produced by the down strokes of the pectoral fin of one side. In
{354}some Fishes it would seem that the pectoral fins may assist locomotion
by acting as paddles. The 15-spined Stickleback (_Gastrosteus spinosus_)
frequently progresses by their aid alone; and, as their action can be
reversed at pleasure, it is not unusual to see this Fish move backwards.
The fins appear to be rotated or twisted in spiral movements like the tail
when used for swimming, or like the wings of Insects in flying.
It has been mentioned that the function of the median fins (dorsal and
anal) is to give stability to the Fish by acting as dorsal and ventral
keels. This is certainly the case in the generality of Fishes.
Nevertheless, there are exceptional instances in which one, or even both,
of these fins are important swimming organs, acting either as a substitute
for a tail which has become adapted for other uses, or as supplementary to
that organ. Thus, in some of the Syngnathidae (Pipe-Fishes and Sea-Horses)
the small size or absence of the caudal fin, and its use as a prehensile
organ, renders the tail of little or no value as a propelling organ: hence
it is that these Fishes swim by a lateral undulating movement of the dorsal
fin. To enable them to do this the supporting skeleton presents certain
interesting modifications. In the majority of Teleosts the arrangement of
the fin-muscles, and the nature of the articulation between the dermal
fin-rays and their basal radial supports, which is generally some form of a
hinge-joint, are such as to limit the motion of the rays to simple
elevation or depression in the vertical plane, and no lateral motion of the
fin is possible. But in the Syngnathidae, as in the Pipe-Fish
(_Siphonostoma typhle_), there is an exceptionally mobile articulation
between the dermal fin-rays and the distal radial nodules which their cleft
bases embrace and the bony proximal or basal radials, so that the fin can
be flexed or bent to the right or to the left. In addition to this, by a
change in the insertion of their tendons, the muscles corresponding to the
ordinary elevator and depressor muscles of the fin-rays in other Fishes are
capable of producing extensive lateral movements of the fin, or, by
contracting in orderly sequence, of bringing about the characteristic
undulating motion of the fin. A similar mechanism exists in many
Plectognathi (_e.g._ species of _Balistes_, _Monacanthus_, _Diodon_,
_Tetrodon_ and _Orthagoriscus_)[416] in connexion with both the dorsal and
anal fins, but in these Fishes the {355}action of the median fins in
swimming must be regarded as supplementary to that of the tail.
Swimming is by no means the only form of locomotion in vogue amongst
Fishes. A few, like the Angler-Fishes (_Lophius_), habitually use the
pectoral fins for crawling about the sea-bottom. The East Indian Goby,
_Periophthalmus_, uses its pectoral fins, which are bent at an angle like
an elbow-joint, for hopping over sandy flats left bare by the retreating
tide. The Flying-Fish (_Exocoetus_), when projected from the water by a
stroke of its powerful tail, expands its large pectoral fins, and, using
them after the fashion of a parachute, floats through the air for
considerable distances before returning to its natural medium. The "Flying
Gurnards" (_Dactylopterus_) are also capable of short aerial excursions in
a similar fashion. Nor is tree-climbing beyond the province of a Fish, if
credit be given to the assertion that the Indian "Climbing-Perch" (_Anabas
scandens_) uses its opercular spines for ascending trees. Many freshwater
Fishes are known to migrate across land from one pool or river to another,
usually during the night. Eels do so by a serpentine or wriggling motion of
their long bodies, but in others the pectoral fins seem to be the principal
organs used for the purpose, aided, it may be, by a perverted use of the
tail.
SOUND-PRODUCING ORGANS.—Contrary to popular belief sound-producing or vocal
organs are by no means uncommon in Fishes, especially in certain families
of Teleosts. It is not always easy, however, to discriminate between
involuntary, abnormal, or accidental sounds, and those due to the action of
special vocal organs. There are, moreover, some Fishes which observations
have shown to utter highly characteristic sounds, although the precise
nature of the sound-producing mechanism is at present unknown; while other
Fishes appear to possess organs which, on anatomical grounds, are perhaps
vocal in function, although nothing is known of the nature of the sounds
they emit. Here those organs only will be considered which, either with
certainty or with some degree of probability, may be regarded as vocal
structures. For most of our knowledge of these interesting structures we
are indebted to the researches of Sörensen and Dufossé.[417]
{356}[Illustration: FIG. 205.—Stridulating apparatus of _Callomystax
gagata_. _is^1_, The first interspinous bone, the lower part of which forms
the double file and fits into the interval between the cleft neural spines
_ns^4_ and _ns^5_; _is^2_, _is^3_, second and third interspinous bones;
_ns^3_, _ns^4_, _ns^5_, neural spines of the third, fourth, and fifth
vertebrae; _s^1_, _s^2_, spine-like rays of the dorsal fin; _so_,
supra-occipital. (After Haddon.)]
(_a_) _Stridulation_.—Stridulation as a method of sound-production has been
recorded in many Teleosts, and one of the most interesting examples occurs
in the singular Indian Siluroid, (_Callomystax gagata_).[418] In this Fish
(Fig. 205) the first five vertebrae are rigidly connected with one another
and with the skull, mainly through the union of the neural spines of the
third, fourth, and fifth vertebrae, and their articulation with the
supra-occipital bone. The united spines together form a high,
laterally-compressed lamina of bone, the hinder portion of which is
vertically cleft into two thin plates separated by an interval sufficiently
wide to receive the first interspinous bone of the dorsal fin. The inner
surface of each of the two plates is traversed by a series of about thirty
parallel, close-set, vertical ridges, while the first interspinous bone is
similarly ridged on both its faces like a double file. Lastly, it may be
mentioned that owing to the width of the intervertebral ligament between
them the fifth and sixth vertebral centra are articulated by a joint of
unusual mobility. The action of the mechanism is simple. By the vertical
movements of the sixth and succeeding trunk vertebrae, with the
interspinous bones which they support, on the rigid structure formed by the
head and first five vertebrae, the file-like first interspinous bone moves
backwards and forwards, and, by scraping against the ridges on the inner
surfaces of the cleft neural spines, gives rise to a harsh grating noise,
which is particularly unpleasant when artificially produced. The lateral
movements of the trunk in ordinary locomotion do not affect the mechanism:
it is only when the trunk is alternately flexed and extended in the
vertical plane that the mechanism comes into play and a noise is
{357}produced. In the Bull-head (_Cottus scorpius_) the preoperculum is
modified for stridulation, and in _Dactylopterus_ the hyomandibular bone;
in other Fishes, as in some Siluroids (_e.g._ species of _Doras_),
stridulation takes place between a basal process from the great spine of
the pectoral fin and the wall of a socket in the cleithrum into which the
process is received, or between the small first spine of the dorsal fin and
a roof-like process at the upper extremity of the first interspinous bone;
also, in a somewhat similar fashion in the anterior dorsal fin of such
widely different Fishes as certain Trigger-Fishes (Sclerodermi) pertaining
to the genera _Balistes_, _Monacanthus_, and _Triacanthus_, _Acanthurus
chirurgus_ (Acanthuridae), the Boar-Fish (_Capros aper_), _Centriscus
scolopax_ (Centriscidae), and the Three-spined Stickleback (_Gastrosteus
aculeatus_); and even between the spinose ray of the pelvic fin and the
basipterygium in _Triacanthus_, _Capros_, and _Gastrosteus_.
In the "Drumming" Trigger-Fish (_Balistes aculeatus_),[419] which frequents
the coral-reefs off the Island of Mauritius, stridulation takes place
between the postclavicles and a longitudinally grooved area on the inner
surface of each cleithrum. Both the cleithra and postclavicles are in
intimate relation with the air-bladder, and the sound produced by friction
is apparently strengthened by the transference of the vibrations to the
walls and gaseous contents of that organ. The passage of the
sound-vibrations to the surrounding medium is facilitated by the fact that
for a portion of their extent the lateral walls of the air-bladder are in
contact with the superficial skin, which visibly shares in the vibratory
movement of the bladder when the characteristic drumming sounds of
_Balistes_ are being emitted.
Stridulating sounds may also be produced by the friction of the upper and
lower pharyngeal teeth, as in a species of Mackerel (_Scomber brachyurus_).
By the grating of its teeth the Sun-Fish (_Orthagoriscus mola_) is said to
emit sounds similar to those produced by the grinding of the teeth in Pigs
and Ruminants; and Moseley[420] has remarked of a species of _Balistes_
that the "living Fish when held in the hand makes a curious metallic
clicking noise by grating its teeth."
(_b_) _Breathing sounds_.—Characteristic breathing or murmuring sounds, or
"bruits de souffle" as Dufossé terms them, are {358}produced by a few
Teleosts, among which may be mentioned the Eels, certain Cyprinidae, as,
for example, the Carp (_Cyprinus carpio_), several species of Loaches (e.g.
_Misgurnus fossilis_ and _Cobitis taenia_), and the European Siluroid,
_Silurus glanis_. According to Dufossé these sounds originate in some cases
from the expulsion of gas from the air-bladder through the ductus
pneumaticus and mouth, and in others, as in _Misgurnus fossilis_, they are
produced by the rapid ejection through the anus of bubbles of air
previously taken in at the mouth.
(_c_) _Sounds produced through the agency of muscles connected with the
air-bladder_.—In addition to its usual function as a hydrostatic organ or
"float" the air-bladder is often modified in various ways in different
Teleosts, and adapted for use as a sound-producing organ.
[Illustration: FIG. 206.—The air-bladder and elastic-spring-mechanism in
_Auchenipterus nodosus_. A, Cavity of the bladder exposed by the removal of
its ventral wall: _a.c_, anterior chamber; _cl_, clavicle; _c.tr_,
crescentic process of the tripus; _l.c_, left lateral chamber; _l.s_,
longitudinal septum separating the two lateral chambers; _oes_, oesophagus;
_p.s_, pectoral spine; _t.s_, the narrow transverse septum which partially
separates the anterior from the two lateral chambers. B, Ventral view of
the anterior vertebrae, to show the elastic springs: _es_, the oval bony
plates in which the elastic springs terminate; _r^1_, first rib; _t.p^5_,
transverse process of the fifth vertebra; _v^1_, first vertebral centrum;
_cl_, _oes_, and _ps_, as in A. (From Bridge and Haddon.)]
In the South American Siluroid, _Auchenipterus nodosus_, the transverse
processes of the fourth vertebra are bent downwards and backwards, and at
the same time become converted into flexible and highly elastic springs
(Fig. 206, B). Their distal extremities expand into oval bony plates which
are imbedded in the anterior wall of the air-bladder, and often cause the
latter to bulge inwards (Fig. 206, A). From the occipital region of the
skull arise two powerful muscles which pass backwards to {359}their
insertion into the anterior faces of the two springs. By the contraction of
these muscles the springs, and consequently also the front wall of the
bladder, are drawn forwards; but directly the muscles relax, the elasticity
of the springs causes them to move backwards to their former position,
carrying with them the wall of the air-bladder. Hence it follows that the
rapid alternating contraction and relaxation of the muscles will impart a
vibratory movement to the anterior wall of the bladder and to the gaseous
contents of that organ, with the result that a sound is produced. As a
rule, those Fishes in which an elastic-spring-mechanism is present have the
air-bladder subdivided by internal septa into a series of chambers freely
communicating with one another; and no doubt the intensity of the sound is
greatly increased by the vibratory movements of the gases across the free
edges of the septa, and from one chamber to another. The elastic-spring
type of vocal organ is apparently restricted to the Siluridae,[421] and
besides occurring in _Auchenipterus_ is found also in the South American
genera _Doras_, _Oxydoras_, _Rhinodoras_, and _Euanemus_; in the African
genera _Synodontis_ and _Malopterurus_; and in at least four species of the
Indian genus _Pangasius_.[422] There are also a few Teleosts in which the
air-bladder is provided with special muscles, but, instead of being
connected with elastic springs, the muscles extend from the skull, and are
inserted directly into the wall of the bladder (Fig. 207); or, without
being in any way attached to the skeleton, the muscles simply invest some
portion of the surface of the air-bladder. In other Fishes the air-bladder,
without possessing special muscles of its own, may, nevertheless, be
partially invested by tendinous, or partly muscular and partly tendinous,
extensions from the muscles of the body-wall (Fig. 208), or may be
intimately related to certain muscles connected with the pectoral girdle.
{360}[Illustration: FIG. 207.—Ventral view of the air-bladder and its
extrinsic muscle in _Platystoma_. _a.b_, Air-bladder; _a.l.c_, left
antero-lateral caecum of the bladder; _b.o_, basioccipital; _b.w_,
body-wall in contact with the lateral wall of the bladder; _c^1_, centrum
of the first vertebra; _cl_, clavicle; _d.p_, ductus pneumaticus; _m^1_ and
_m^2_, extrinsic muscles of the bladder; _pt.i_, post-temporal. (From
Bridge and Haddon.)]
[Illustration: FIG. 208.—Air-bladder and its muscles in _Micropogon
undulatus_. _a.b_, Air-bladder; _l.b_, right lateral caecum; _m_, _m_,
musculo-tendinous extensions from the muscles of the body-wall, which
partially invest the surface of the air-bladder. (From Sörensen.)]
Whatever the precise relation of the air-bladder to its muscles it is
probable that the physiological effect is in most cases the same. By the
rapid alternating contraction and relaxation of the muscles, some part of
the wall of the bladder becomes alternately compressed and relaxed in such
a way as to initiate a series of vibratory movements in the gases of that
organ, and so produce definite sounds. In not a few of the Fishes the
cavity of the bladder is subdivided by external constrictions or by
internal septa, or is complicated by the development of lateral, tubular,
caecal branches; and hence the vibratory movements of the gases will be
greatly strengthened by their passage across the edges of the septa, or the
apertures of the caeca, and the intensity of the resultant sounds also
increased. It will be readily understood that the nature and quality of the
sounds emitted by different Fishes will necessarily vary with the shape of
the air-bladder, the number and arrangement of the internal septa and the
caeca, and the strength and disposition of the contracting muscles. In a
few Teleosts (Triglidae and Zeidae) sounds are said to be produced by the
rapid vibration of an annular, or centrally-perforated, muscular diaphragm,
which stretches across the cavity of the air-bladder.[423] Nevertheless, it
must be strongly emphasised that, while in some Fishes the air-bladder and
its muscles {361}undoubtedly constitute a vocal organ, there are many
others in which the bladder can only be inferred to be sound-producing from
its general agreement in anatomical structure with the same organ in Fishes
where its vocal function has been clearly proved.
By one or other of these various methods the air-bladder is either known to
be sound-producing, or is believed with good reason to be such, in the
following Teleosts,[424] and many others:—Certain species of the
South-American genera of Siluridae, _Pimelodus_, _Sorubim_, _Platystoma_,
_Piratinga_, _Centromochlus_, and _Trachelyopterus_; species of the
South-American family Characinidae; _Amblyopsis spelaea_, the blind Fish
from the Mammoth Cave of Kentucky (Amblyopsidae); among the Syngnathidae,
the short-snouted Sea-Horse (_Hippocampus brevirostris_) of the British
Coasts; certain Sclerodermi, such as the Trigger-Fishes, _Batistes vetula_,
_Triacanthus brevirostris_, _T. biaculeatus_, and _Monacanthus pardalis_,
and also some "Coffer Fishes" (_e.g._ species of _Ostracion_); some
Gymnodontes (species of _Diodon_ and _Tetrodon_); a few Serranidae (_e.g._
species of _Therapon_ and _Pristipoma_); species of _Holacanthus_
(Chaetodontidae) and in _Holocentrum sogho_ (Berycidae); such Sciaenidae as
the "Drum" (_Pogonias chromis_), the "Maigre" (_Sciaena aquila_), which has
sometimes been taken in British waters, _Umbrina cirrhosa_, _Otolithus
regalis_, and _Micropogon undulatus_, and, with more or less probability,
many other species of the same family; one species of Zeidae, the John Dory
(_Zeus faber_); _Batrachus tau_ among the Batrachidae; several species of
Gurnards (Triglidae) belonging to the genera _Prionotus_ and _Trigla_; the
so-called Flying Gurnard, _Dactylopterus volitans_ (Dactylopteridae); the
Indian species _Ophiocephalus marulius_ and _O. gachua_ (Ophiocephalidae);
amongst the Gadidae, the Cod (_Gadus morrhua_) and the Haddock (_G.
aeglefinus_); in such Zoarcidae as the blind Fish (_Lucifuga subterranea_)
from the subterranean waters of the caves of Cuba, and also in some
Ophidiidae (_e.g._ species of _Ophidium_).
In Fishes other than Teleosts, instances of normal sound-production by
special vocal structures are rare. No recorded instances are known in the
Cyclostomes or the Elasmobranchs,[425] {362}but there is evidence that
sounds are emitted by _Polypterus_ among the Crossopterygii, and by the
Dipnoids _Neoceratodus_,[426] _Protopterus_, and _Lepidosiren_, although it
is not certainly known how they are produced, or that they may not be the
accidental concomitants of the inspiratory or expiratory action of the
lungs in breathing.
As to the nature of the sounds produced by the air-bladder and its muscles
in different Teleosts, a few examples may be given.
The sound produced by the elastic-spring-apparatus of a recently caught
_Doras maculatus_, has been described as a "deep growling tone," which may
be distinctly heard at a distance of 100 feet when the Fish is out of the
water. Under like conditions the air-bladder and its muscles, in a species
of _Platystoma_, emit a similar sound. On the other hand, the sound
produced by the elastic springs of the Electric Siluroid (_Malopterurus
electricus_) has been compared to the hissing of a cat. The Sea-Horse
(_Hippocampus brevirostris_) utters a monotonous sound analogous to that of
a tambour, which is characteristic of both sexes, but is more intense and
frequent in the breeding season. The "Coffer Fish" (_Ostracion trigonus_)
emits a growling sound, as also does the "Globe Fish" (_Tetrodon
honckenii_) when taken out of the water.[427] The air-bladder and its
muscles in the "Drum" (_Pogonias chromis_), constitute the most powerful
sound-producing organ yet found in any Fish. The sounds emitted by the
"Drum" are better expressed by the word drumming than by any other, and
have frequently been heard by persons in vessels lying at anchor on the
coasts of the United States, where these Fishes abound.[428] The "Drum"
begins its drumming noise in the spawning season in April, but is rarely
heard afterwards. The "Maigre" (_Sciaena aquila_), whose musical
performances are perhaps responsible for the Homeric fable of the song of
the Sirens, is remarkable among Fishes for the variety of its sounds, which
have been compared to bellowing, purring, buzzing, and whistling.[429] The
sound is often so intense that it may be heard when the Fish is at a depth
of 18 metres, and the {363}ear of the observer two metres above the water;
and it has been recorded that by listening for these sounds, shoals of
Maigres have been successfully netted. They rarely emit sounds when
isolated; but in shoals, during the breeding season, they do not cease to
make sounds with a vigour and a persistency which apparently must soon wear
out their strength. One of the Indian Horse-Mackerels (_Caranx hippos_)
grunts like a young Pig when captured, and the sound is repeated whenever
it is moved, as long as vitality remains. A West Indian species of the same
family (_Argyriosus vomer_) has been observed to produce a like sound,
while an Egyptian _Caranx_ (_C. rhonchus_) is known to the Arabs as the
"Chakoura" or "Snorter."[430] The sounds produced by the different British
Gurnards, such as the Grey Gurnard (_Trigla gurnardus_), the Piper (_T.
lyra_), the Elleck or Cuckoo Gurnard (_T. cuculus_), and the Tub-Fish (_T.
hirundo_), have been compared to snoring, a sonorous and prolonged
grunting, crooning (whence, perhaps, the term "crooner," by which the Grey
Gurnard is known in Ireland), and croaking. The John Dory (_Zeus
faber_)[431] also utters sounds analogous to those of the Gurnards. Among
the Dipnoi _Lepidosiren_ is said to make a growling sound, and
_Neoceratodus_ a grunting noise which may be heard at night for some
distance.
Whatever the nature of the vocal mechanism, it is highly probable that the
sounds produced by Fishes travel to considerable distances in the water,
inasmuch as the latter medium is a far better conductor of sound than air,
and, moreover, the transmission of sound-vibrations from the air-bladder to
the water is facilitated in many Fishes by the fact that, for a portion of
its extent on each side the bladder is in direct contact with the
superficial skin behind the pectoral girdle.
From the by no means exhaustive list of examples given above, it is obvious
that in some form or other vocal organs are present in a considerable
number of Fishes, both freshwater and marine, belonging to widely different
groups; and further, that even in the same species (e.g. _Doras maculatus_
and other Siluridae), both stridulation and the action of extrinsic muscles
on the air-bladder may be utilised as a means of sound-production. Certain
Teleostean families like the Siluridae, the Sciaenidae, and the Triglidae,
seem to be distinguished above all others by the {364}prevalence of some
form of vocal organ. According to Sörensen, the first mentioned of the
three families includes no less than 68 species, which utilise the
air-bladder alone as a sound-producing organ. Nevertheless, there still
remain many Teleostean families, rich in genera and species, and with an
almost world-wide geographical distribution, in which such organs have not
yet been found.
The advantages which Fishes derive from the possession of sound-producing
organs are sufficiently obvious.
A characteristic feature in the reproduction of most Fishes is the general
absence of any process of conjugation between the sexes, the eggs being
fertilised in the water after their extrusion from the body of the female,
and, consequently, any device which will facilitate the formation of shoals
during the breeding season must be of great advantage to the species by
largely increasing the chances that the ova will be fertilised, and thus
secure the more successful propagation of the race. Hence it may be
concluded that the vocal organs of Fishes are a means to this end, and that
the sounds they produce are in fact recognition-sounds which enable Fishes
of the same species to congregate together at periods when reproductive
activity is greatest. This view is in harmony with much that is known of
the habits of these Fishes, especially with the fact that particular sounds
are often characteristic of particular species, and that the sounds are
produced most frequently and with greater intensity during the breeding
season than at any other time. While useful to all Fishes that possess
them, vocal organs are, no doubt, specially serviceable to those Fishes
which, from the nature of their habitat, can make but little use of their
eyes; and this fact may perhaps explain the prevalence of such organs in
the Siluridae, which are frequently bottom- or ground-feeding Fishes, and
often live in muddy waters.
The sounds emitted by Fishes may also, in some instances at least, be
warning sounds. Many of the sound-producing Fishes are provided with
exceptionally strong spines either in connexion with the median and paired
fins, as in many Siluridae, or on the general surface of the body, as in
_Diodon hystrix_. Such spines are very effective weapons for offensive or
defensive purposes, and are capable of inflicting very severe wounds. The
natural enemies of these Fishes learn by experience or instinct to
{365}associate particular sounds with the possession of dangerous spines,
and warned by the sounds, they refrain from attacking the owner of the
spines, to the mutual advantage of both.
[Illustration: FIG. 209.—An Electric Ray (_Torpedo_) dissected to show its
electric organs. On the left the nerves supplying the organ are dissected
out. The prismatic areas on the surface of the organ indicate the vertical
columns of electric plates, of which there may be 500,000 in each organ.
The dorsal surface of the brain is exposed. _br_, Gills; _f_, spiracle;
_o_, eye; _o.e_, electric organs; _t_, mucus canals; _tr_, tri-geminal
nerve; _tr′_, its electric branch; _v_, vagus; _I_, fore-brain; _II_,
mid-brain; _III_, cerebellum; _IV_, electric lobe of the medulla oblongata.
(From Parker and Haswell, after Gegenbaur.)]
ELECTRIC ORGANS.—Electric organs capable of generating more or less
powerful electric discharges are present in certain Fishes, both marine and
freshwater. They occur in a few Elasmobranchs (species of _Raia_,
_Torpedo_, and _Hypnos_), in such Teleosts as the African Silurid
_Malopterurus_ the "Electric Eel" (_Gymnotus_), and in species of
Mormyridae (e.g. _Mormyrus_). With one exception electric organs are
composed of metamorphosed muscular fibres, and their nerve-endings or motor
end-plates. The species of _Raia_ have two small electric organs, one on
each side of the terminal portion of the tail.[432] In _Gymnotus_[433] the
{366}organs are much larger, and extend the whole length of the tail, which
is fully four-fifths of the total length of the Fish. The Mormyridae also
have their feeble electric organs in the caudal region. In all these Fishes
the electric organs are modified portions of the caudal muscles. In the
Torpedo, however, these organs are two large oval masses, one on each side
of the head, between the gills and the cephalic prolongation of the
pectoral fin (Fig. 209). _Malopterurus_[434] is exceptional in possessing
an electric organ derived from the epidermis and not from the muscular
system. In this Fish the organ envelops nearly the whole body like a
mantle, between the skin and the subjacent muscles of the trunk and tail.
An electric organ is composed of an immense number of "electric plates"
(modified motor end-plates), abundantly supplied with nerves on one of
their surfaces, and disposed in a series of vertical (_Torpedo_) or
longitudinal (_Gymnotus_) columns, separated by septa of connective tissue.
In the active state of the organ in the Torpedo[435] the ventral surfaces
of the plates, on which the nerves are distributed, become negative to the
dorsal, and "the effect in all the plates of a column when summed up is,
therefore, such that the dorsal end of a column becomes positive to the
ventral end."[436] Hence the current in the form of a succession of shocks
passes from the ventral to the dorsal surface of the head. In _Gymnotus_,
where the columns are longitudinally arranged, it is the anterior and
posterior surfaces which become oppositely electrified, and the current
passes from the tail to the head. The shock imparted by an electric
discharge is most powerful in _Gymnotus_,[437] _Malopterurus_, and
_Torpedo_, in the order named, and relatively weak in the remaining genera.
The strength of the shock increases with the number of electric plates
included in the circuit. Thus in _Gymnotus_ the maximum shock is given when
the body of the Fish is so curved that the head and the tail are in contact
with different points on the surface of some other Fish. The discharge may
be reflex or voluntary. Repeated discharges induce fatigue and weaken the
shocks. Electric organs are powerful offensive or defensive structures,
enabling the Fish to repel the attacks of enemies, or to stun or kill their
prey.
{367}CHAPTER XIV
NERVOUS SYSTEM AND ORGANS OF SPECIAL SENSE
The nervous system consists of the brain and the spinal cord, and of the
cranial and spinal nerves. The rudiment of the future brain and spinal cord
first appears in the embryos of some Cyclostomes (e.g. _Bdellostoma_), of
Elasmobranchs, and of Chondrostei (e.g. _Acipenser_), and of _Neoceratodus_
among the Dipnoi, in the form of a tubular medullary canal pinched off from
the epiblast of the dorsal surface of the body. By a somewhat different
method, but with the same final result, a medullary canal is formed in
other Cyclostomes (e.g. _Petromyzon_), in the Holostei and Teleostei, and
in _Lepidosiren_,[438] from a solid ingrowing keel of epiblast which
subsequently becomes tubular. Later, the medullary canal in the head
enlarges, and becomes divided by two transverse constrictions into three
vesicles, the primary fore-, mid-, and hind-brain, leaving the rest of the
canal to form the spinal cord.
THE SPINAL CORD.—This portion of the medullary canal retains a simpler and
more uniform cylindrical structure. Its walls thicken and their component
cells become converted into nerve cells and nerve fibres, but a remnant of
the original cavity remains in the adult as a minute axial canal, with a
ciliated epithelial lining, the central canal of the spinal cord or
myelocoele. In most Fishes the spinal cord extends the whole length of the
body, but in some Teleosts, especially in certain Plectognathi, it is
remarkably short. In a Sun-Fish (_Orthagoriscus_), 2½ metres long, and
weighing about a ton and a half, the cord was only 15 mm. in length, or
shorter than the brain.
THE BRAIN.—At an early stage in its embryonic history the {368}brain
consists of three simple vesicles, the _fore_-, the _mid_-, and the
_hind-brain_, the first of which lies in front of the anterior end of the
notochord and is therefore pre-chordal in position. As development proceeds
the walls of the vesicles undergo local thickenings, or they give rise to
hollow paired or median outgrowths, and by one or other of these methods
the different parts of the complex adult brain are evolved, while the
original cavities of the vesicles or of their outgrowths persist as a
continuous system of epithelium-lined spaces or "ventricles."[439] The
fore-brain is remarkable for the number and importance of the parts to
which it gives rise. First, it bulges out in front into a hollow vesicle,
the _prosencephalon_, leaving the rest of the fore-brain as the
_thalamencephalon_ or _diencephalon_ (Fig. 210). The cavity of the
prosencephalon is the _prosocoele_, and a pair of thickenings in its floor
form two basal ganglia or _corpora striata_. In many Fishes the
prosencephalon retains this simple vesicular condition, in which case the
roof or _pallium_ is usually epithelial and non-nervous; but in others two
hollow lobes grow out from it in front and give rise to two _cerebral
hemispheres_ or _parencephala_.[440] Both contain extensions of the
prosocoele, the _paracoeles_ or _lateral ventricles_, from the floor of
which the corpora striata now project. The prolongation of the pallium
forming the roof of the lateral ventricles either remains partially
epithelial, or it may acquire a wholly nervous structure and thicken to an
extent which differs greatly in different Fishes. With the formation of the
hemispheres the prosencephalon and its prosocoele become of secondary
importance, and may cease to be recognisable as distinct from the
thalamencephalon and its ventricle. The lateral ventricles then appear to
communicate directly with the third ventricle by two apertures, the
_foramina of Munro_. The forward growth of the brain is completed by the
development of two hollow lobes, the _olfactory lobes_ or _rhinencephala_,
each of which contains a ventricle or _rhinocoele_ communicating behind
with the prosocoele, or, if hemispheres are present, with the corresponding
lateral ventricle.
{369}[Illustration: FIG. 210.—Diagram of the general structure of the brain
in Craniates. A, vertical longitudinal section; B, dorsal view showing the
brain cavities on the right side. _c_, Cerebellum; _c.c_, central canal of
the spinal cord; _c.h_, cerebral hemispheres; _c.s_, corpus striatum;
_F.B_, fore-brain; _f.m_, foramen of Munro; _H.B_, hind-brain; _in_,
infundibulum; _l.v_, lateral ventricle; _m_, mesocoele; _M.B_, mid-brain;
_m.o_, medulla oblongata; _o.l_, olfactory lobe; _op.l_, optic lobe;
_op.t_, optic thalamus; _p_, paraphysis; _pc_, prosocoele; _pn.o_, pineal
organ; _p.o_, parietal organ; _pr_, prosencephalon; _pt_, pituitary body;
_rh_, rhinocoele; _sp.c_, spinal cord; _s.v_, saccus vasculosus; _th_,
thalamencephalon; iii, iv, third and fourth ventricles. (After Parker and
Haswell.)]
Scarcely less complicated, and perhaps even more interesting from a
morphological standpoint, are the structures arising out of the
thalamencephalon. By thickenings of its lateral walls two large ganglia,
the _optic thalami_, are formed, and on the inner or dorsal aspect of each
of these a _ganglion habenulae_ is developed. From the sides of the
thalamencephalon the primary _optic vesicles_ are derived, which later
become transformed into the retinal parts of the paired eyes and the optic
nerves. Besides the optic vesicles there is a second pair of embryonic
outgrowths which arise from the roof of the thalamencephalon. These
outgrowths form stalked vesicles and represent a pair of degenerate visual
{370}organs. Usually they become so displaced that the left one lies in
front of the right, and they appear as if median. The subsequent fate of
the vesicles differs greatly in different Craniates. Both persist in the
Lamprey, the right vesicle to some extent retaining its primitive visual
function as a _parietal eye_ and directly overlying the left or pineal
vesicle. In Elasmobranchs the two unite to form a glandular organ, the
so-called _pineal body_ of the adult, and in Teleosts the left vesicle
disappears, leaving the right as a pineal body.[441] There is also an
embryonic median outgrowth from the roof of the prosencephalon, the
_paraphysis_, which soon disappears and whose significance is not known. A
median hollow downgrowth from the floor of the thalamencephalon forms the
_infundibulum_, which becomes attached to a caecal diverticulum from the
roof of the mouth. With rare exceptions the diverticulum loses all
connexion with the mouth, and, as the _pituitary body_ or _hypophysis_, it
appears as an appendage to the extremity of the infundibulum. In the
Crossopterygii the connexion is retained even in the adult by means of a
slender canal extending from the pituitary body and opening into the oral
cavity. Laterally, the base of the infundibulum grows out into a pair of
rounded lobes, the _lobi inferiores_, and distally into a thin-walled
glandular sac, the _saccus vasculosus_, which lies just behind the
pituitary body. The cavity of the thalamencephalon persists as the _third
ventricle_ or _diacoele_. The parts of the brain developed from the
mid-brain and the hind-brain are much less complicated, and, except for
variations in size, they present a fairly uniform character in most Fishes.
In the mid-brain the roof bulges out into a pair of _optic lobes_, and by
the growth of lateral thickenings in its floor two thick strands of
longitudinally disposed nerve fibres, the _crura cerebri_, are formed. The
cavity of the mid-brain remains as the _mesocoele_, and from it an
extension may be prolonged into each optic lobe.
From the hind-brain are formed the _cerebellum_ or _epencephalon_ and the
_medulla oblongata_ or _metencephalon_, the former as a dorsal bulging, the
latter as a ventral thickening. Except where the cerebellum is developed
the dorsal wall remains epithelial, and forms the roof of the persistent
cavity of the {371}hind-brain, the _fourth ventricle_ or _metacoele_, which
retains its primitive continuity with the central canal of the spinal cord.
Lateral lobe-like outgrowths from the dorsal columns of the medulla are
conspicuous structures in some Fishes, and are known as _corpora
restiformia_. The paired portions of the brain are connected across the
middle line by a series of _transverse commissures_. The more important
modifications of the brain in Cyclostomes and Fishes will now be briefly
dealt with.
[Illustration: FIG. 211.—Dorsal (A) and ventral (B) views of the brain of
_Petromyzon marinus_. _ch.pl.1_, Anterior choroid plexus forming the roof
of the prosencephalon and thalamencephalon; _ch.pl.2_, aperture in the roof
of the mid-brain exposed by the removal of the middle choroid plexus;
_ch.pl.3_, the fourth ventricle exposed by the removal of the posterior
plexus; _cr.crb_, crura cerebri; _crb_, cerebellum; _crb.h_, cerebral
hemispheres; _dien_, thalamencephalon; _inf_, infundibulum; _l.gn.hb_, left
ganglion habenulae; _med.obl_, medulla oblongata; _nv.1_, olfactory;
_nv.2_, optic; _nv.3_, oculomotor; _nv.5_, trigeminal; and _nv.8_, auditory
nerves; _olf.l_, olfactory lobes; _opt.l_, optic lobes; _pn_, pineal organ;
_r.gn.hb_, right ganglion habenulae. (From Parker and Haswell, after
Ahlborn.)]
{372}[Illustration: FIG. 212.—Dorsal view of the brain of _Myxine_. _c.r_,
Corpora restiformia; _m.o_, medulla oblongata; _n.p_, naso-pituitary canal;
_ol.o_, olfactory organ enclosed in its fenestrated cartilaginous capsule;
_op.l_, optic lobes; _pr_, prosencephalon; _s_, _s_, dorsal roots of spinal
nerves; _sp.c_, spinal cord; _th_, thalamencephalon. (From Wiedersheim,
after Retzius.)]
In the Cyclostome _Petromyzon_ there is a small prosencephalon with an
undivided prosocoele, and on each side of it a small cerebral hemisphere
which appears as a mere appendage to the much larger olfactory lobe (Fig.
211). The prosocoele divides in front into two outwardly directed branches,
and of the two diverticula into which each branch divides one extends as a
lateral ventricle into the hemisphere of its side, and the other as a
rhinocoele into the corresponding olfactory lobe. The ganglia habenulae are
unusually large, the right one being larger than the left. The optic lobes
are large, but not obviously double. So small is the cerebellum that it
seems to be little more than a narrow transverse band crossing the
fore-part of the fourth ventricle. The roof of the brain is largely
epithelial, especially in the prosencephalon, the thalamencephalon, and the
hind-brain. Over these epithelial areas the pia mater is unusually vascular
and forms a series of "choroid plexuses." The ventricular system is
complete and continuous. By contrast with the Lamprey the brain of
_Myxine_[442] is very primitive, more so perhaps than in any other Craniate
(Fig. 212). In a dorsal view the brain is divided into four pairs of
laterally expanded and longitudinally compressed lobes by a median
longitudinal fissure and three transverse fissures. The two anterior lobes
are little more than the thickened anterior wall of the thalamencephalon,
although, judging from their histological structure, they represent a very
imperfectly differentiated prosencephalon and olfactory lobes. The second
and largest pair constitute the thalamencephalon. The last two pairs of
lobes represent a transversely divided pair of optic lobes, or "corpora
quadrigemina." There is a large medulla oblongata with a pair of corpora
restiformia, but the {373}cerebellum is entirely absent. The ventricles are
subject to some individual variation. Third and fourth ventricles are
generally recognisable, either as isolated cavities or connected by a
remnant of the mesocoele. In the feeble development of the prosencephalon,
in the striking preponderance of the mid-brain over the rest of the brain,
and in the absence of a cerebellum, _Myxine_ is unique amongst Craniates.
[Illustration: FIG. 213.—The brain of a Dog-Fish (_Scyllium canicula_). A,
dorsal view; B, ventral view. The choroid plexuses covering the roof of the
third and fourth ventricles have been removed. _b.o_, Olfactory lobe; _ep_,
origin of the stalk of the pineal body; _f.b_ (in A), prosencephalon; _f.b_
(in B), cerebral hemispheres; _fr_, fourth ventricle; _h.b_, cerebellum;
_h.p_, pituitary body; _i.f_, lobi inferiores; _m.b_, optic lobes; _m.d_,
medulla oblongata; _sc_, saccus vasculosus; _th_, thalamencephalon; _t.o_
(i) olfactory peduncle; i.-x. cranial nerves. (From Wiedersheim.)]
In Elasmobranchs among Fishes the brain attains a much higher grade of
structure. In _Scyllium_ (Fig. 213) there is a large prosencephalon, and
directly in front of it a pair of imperfectly differentiated cerebral
hemispheres, while from its antero-lateral regions the large olfactory
lobes arise. The prosocoele divides in front into four diverticula, of
which the two {374}inner ones extend into the hemispheres as lateral
ventricles, and the two outer as rhinocoeles into the olfactory lobes (Fig.
214). In connexion with the infundibulum there is a pair of sacci
vasculosi, consisting mainly of gland-tubules, opening into the
infundibular cavity.[443] The cerebellum is exceptionally large, but it
does not form a "valvula cerebelli." Large ear-like corpora restiformia are
present. The third and fourth ventricles alone retain an epithelial roof in
relation with choroid plexuses.
In all essentials the brain of the Holocephali is a repetition of the
Elasmobranch type, more especially of the elongated form seen in
_Notidanus_. Indications of a higher grade of structure are, however, to be
seen in the reduction of the prosencephalon which, with its prosocoele, is
now scarcely distinguishable from the thalamencephalon and its ventricle;
and in the more complete differentiation of the cerebral hemispheres from
one another and from the rest of the brain. Large frilled corpora
restiformia are conspicuous structures on each side of the medulla
oblongata. Besides the usual intra-cranial pituitary body, there is also a
separate extra-cranial portion lodged in a pit on the ventral surface of
the basis cranii: in the embryo the two are continuous.
[Illustration: FIG. 214.—Horizontal longitudinal section of brain of
_Chiloscyllium_, to show the ventricles; semi-diagrammatic. _cer_, Origin
of cerebellar ventricle or epicoele; _dia_, third ventricle; _iter_,
mesocoele; _meta_, fourth ventricle; _opt_, optocoele, or cavity of an
optic lobe; _para_, lateral ventricles; _pros_, prosocoele; _rh_,
rhinocoele. (From Parker and Haswell.)]
In the Teleostomi the brain is distinctly of a more primitive type than in
any other Fishes (Fig. 215).[444] The most striking feature is the absence
of cerebral hemispheres, the evolution of the primary fore-brain proceeding
no farther than the formation of an undivided prosencephalon with a
non-nervous roof, and a prosocoele which forms a continuous cavity with the
third ventricle, or at the most is only separated from it by an infolding
of the epithelial roof or velum transversum.
{375}[Illustration: FIG. 215.—A, dorsal view of the brain of a Trout
(_Salmo fario_); B, a vertical longitudinal section. _c.il_, Commissura
interlobularis; _g.h_, ganglion habenulae; _h.c_, habenular commissure;
_i.c_, inferior commissure; _l.i_, lobus inferior; _myc_, myelocoele;
_p.c_, posterior commissure; _v.o_, valvula cerebelli; _v.t_, velum
transversum; _ii_., optic nerve; _v.iii_., _v.iv_., third and fourth
ventricles; _v_, _vii_, _viii_, _ix_, _x_, fifth, seventh, eighth, ninth,
and tenth cranial nerves; remaining reference letters as in Fig. 210. (A,
From Wiedersheim; B, after Haller.)]
Amongst other diagnostic characters may be mentioned the predominance of
the mid-brain over the other divisions, the anterior extension of the large
cerebellum into the mesocoele as a "valvula cerebelli," {376}and the
absence of corpora restiformia. This type of brain is most strongly marked
in the Teleostei, but in other Teleostomes some, like _Acipenser_,[445] are
typically Teleostean in this respect (Fig. 216), while others, such as
_Lepidosteus_, have small cerebral hemispheres with lateral ventricles as
well as a prosencephalon.
[Illustration: FIG. 216.—Vertical longitudinal section of the brain of a
Sturgeon (_Acipenser ruthenus_.) _c.p_, Posterior commissure; _c.r_,
cranial roof; _mc_, mesocoele; _op.ch_, optic chiasma; _p.ch.p_, posterior
choroid plexus; _v.c_, valvula cerebelli; _v.t_, velum transversum;
_v.iii_, _v.iv_., the third and fourth ventricles; other lettering as in
Fig. 210. (From Goronowitsch.)]
The most obvious feature in the brain of the Dipnoi is the great
development of the cerebral hemispheres. In this respect these Fishes
approach the Amphibia, but in other features of brain-structure they
present points of agreement with most other groups of Fishes without being
closely related to any one of them. In _Protopterus_[446] (Fig. 217) the
hemispheres are quite distinct except behind, and the walls of their
spacious lateral ventricles are entirely nervous. Olfactory lobes are
sessile on their anterior extremities, and behind and below they enlarge
into ventral lobes which probably represent the hippocampal lobes of the
higher Vertebrates. A vesicular pineal body at the end of a slender stalk
overlies a singular conical projection from the roof of the
thalamencephalon or "pineal pillow."
{377}[Illustration: FIG. 217.—Dorsal (A), ventral (B), and lateral (C)
views of the brain of _Protopterus annectens_. _C_, Cerebellum; _C.H_,
cerebral hemisphere; _D.S.E_, branches of the sinus endolymphaticus; _In_,
infundibulum; _L.I_,, lobi inferiores; _M.O_, medulla oblongata; _O.L_,
olfactory lobe; _Op.L_, optic lobe; _P_, pituitary body; _P.B_, "pineal
pillow"; _S.E_, sinus endolymphaticus; _Sp.c_, spinal cord; _Sp.n_, spinal
nerve; _Vel_, velum transversum; _Z_, pineal body; IV.V., fourth ventricle;
ii., iii., iv., v., vi., vii., viii.1, viii.2, viii.3.4, ix., and x., roots
of the cranial nerves. (From Burckhardt.)]
The optic lobes form a single oval body, and, as in _Petromyzon_ and the
Amphibia, the cerebellum is very small. A posterior choroid plexus covers
the roof of the fourth ventricle, and an anterior plexus in connexion with
the roof of the thalamencephalon projects downwards into the third
ventricle, and is also prolonged forwards into each lateral ventricle. In
_Neoceratodus_[447] the brain is certainly more primitive and distinctly
less Amphibian. As compared with _Protopterus_ the olfactory lobes and the
cerebellum are larger, and the optic lobes are paired. The smaller
hemispheres are {378}non-nervous dorsally and medianly, the roof and inner
wall of each being formed by an extension of the thick, glandular choroid
plexus which forms the roof of the thalamencephalon.
THE SPINAL NERVES.—The spinal nerves of Cyclostomes (e.g. _Petromyzon_)
consist of a series of dorsal nerves arising on each side from the dorsal
surface of the spinal cord, and of a similar double series arising from the
ventral surface, the dorsal nerves regularly alternating with the ventral
nerves. Each myotome is supplied by a dorsal and a ventral nerve which pass
separately to their peripheral distribution in the skin and muscles. In
Fishes, as in the higher Vertebrates, each dorsal nerve, now termed a
dorsal root, enlarges into a ganglion and then unites, either before or
directly after issuing from the neural canal, with the next ventral nerve
or ventral root in front to form a main spinal nerve. At the same time the
spinal nerves of opposite sides tend to form pairs in the same transverse
plane. After the union of the two roots the spinal nerve divides into three
typical branches: a dorsal nerve (ramus dorsalis), and a ventral nerve
(ramus ventralis), both of which include somatic sensory or afferent
fibres, and somatic motor or efferent fibres, for the innervation of the
skin and muscles of the dorsal and lateral portions of a myotome; and a
visceral branch (ramus visceralis), composed of afferent and efferent
visceral fibres, which supplies the adjacent viscera (alimentary canal and
its glands and blood-vessels), and helps to form the sympathetic nervous
system.[448] The somatic afferent and the visceral afferent fibres enter
the spinal cord by the dorsal roots, the somatic efferent leaving the cord
through the ventral roots, although the visceral efferent fibres traverse
both roots. In the region of the paired fins more or fewer of the rami
ventrales unite to form a plexus, the brachial or the pelvic plexus, from
which the nerves to the fins take their origin.
THE CRANIAL NERVES.—It is usual to describe the cranial nerves of
Cyclostomes and Fishes as consisting of ten serially disposed pairs, viz.:
the _olfactory_ (i.), _optic_ (ii.), _oculomotor_ (iii.), _trochlear_
(iv.), _trigeminal_ (v.), _abducens_ (vi.), _facial_ (vii.), _auditory_
(viii.), _glossopharyngeal_ (ix.), and the _vagus_ (x.) Like the spinal
nerves, the cranial nerves collectively include somatic sensory (general
cutaneous) and motor fibres, and also visceral sensory {379}and motor
fibres, all of which have their own special centres in the brain, but the
proportions of these nerve components differ greatly in different nerves.
Certain preoral nerves (iii., iv., and vi.) are exclusively somatic motor;
others (i. and ii.) are special sensory nerves for the olfactory and visual
organs; but most of the other cranial nerves include several components,
and are therefore "mixed" nerves. Besides these components some cranial
nerves include also a quasi-independent system of nerve-fibres, which
converge from certain cutaneous sense-organs to an independent centre in
the medulla oblongata, the _tuber acusticum_,[449] and is probably derived
from the general cutaneous system of nerve components. Such nerve fibres,
including also the auditory nerve, which has its origin from the same
centre, constitute the _lateralis system_. Perhaps the most striking
feature in the postoral cranial nerves is the predominance of the
visceralis or sympathetic system over the somatic. Omitting the lateralis
fibres and a relatively few somatic sensory fibres, visceral fibres,
sensory and motor, are the principal components of all these nerves,
including v. but excluding viii. The reason for this is to be found in the
fact that splanchnic or visceral muscles in relation with the jaws and
branchial arches have usurped the place of somatic muscles in the muscular
system of the head. For developmental and other reasons the olfactory and
optic nerves stand in a category of their own, and the same may be said of
the third, fourth, and sixth nerves, which innervate the muscles of the
eyeball. The remaining nerves, all of which have their origin in the
medulla oblongata, possess certain features in common, and as they are
related to the gill-clefts in such a way that each forks over a cleft, they
may be conveniently distinguished as "_branchial_" or "_branchiomeric
nerves_." A typical branchial nerve consists of (1) a _principal ganglion_
near the origin of the nerve from the brain; (2) a _main trunk_ which gives
off (3) a somatic sensory branch or _dorsal nerve_ to the skin; (4) a
_palatine nerve_ (visceral sensory) to the oral or pharyngeal mucous
membrane; (5) an _epibranchial ganglion_ which is associated with a
transitory embryonic _epibranchial sensory organ_ at the dorsal border of a
branchial cleft; (6) a _pre-branchial_ nerve (visceral sensory), skirting
the anterior margin of a cleft in its ventral course; and (7) a
_post-branchial_ branch (visceral motor) similarly related to the hinder
margin.
{380}[Illustration: FIG. 218.—Diagram showing the principal branches of the
cranial nerves in a Fish, _mk.c_, Meckel's cartilage; _ol.o_, olfactory
organ; _p.q_, palato-quadrate; _s_, spiracle; i-v, branchial clefts; i, ii,
iii, iv, vi, the first, second, third, fourth, and sixth cranial nerves.
The remaining nerves are differently shaded. _Black_.—The trigeminal nerve:
_g.g_, Gasserian ganglion; _md_, mandibularis; _mx_, maxillaris; _op.p_,
ophthalmicus profundus. _Oblique shading_.—The lateralis system and its
centre (_t.a_), the tuber acusticum: _bucc_.vii, buccalis branch of vii;
_md.ex_, external mandibular branches of vii; _l.n._x, lateralis nerve,
with its supra-temporal branch (_s.t_), and its commissural connexion (_c_)
with _op.s._vii, the ophthalmicus superficialis of vii; viii, auditory
nerve. _Dotted_.—The facialis proper, including _c.t_, chorda tympani;
_gn.g_, geniculate ganglion; _hy_, hyomandibularis, with its motor branches
_m_, _m_, _m_; _p.n._vii., palatine. _Dark grey_.—The glossopharyngeal
(ix), with its pre- and post-branchial branches and its palatine nerve,
_p.n._ix; anastomosing with the palatine branch of vii (Jacobson's
anastomosis). _White_.—The vagus: x^{1-4}, the branchial nerves,
ganglionated and forking over clefts ii-v; _v.n._x, visceral nerve; _oc_,
occipito-spinal nerves; _d.r_ and _v.r_, the dorsal and ventral roots of
the first two spinal nerves. (Slightly modified after Wiedersheim.)]
{381}The first six cranial nerves resemble those of the higher Craniates in
their mode of origin from the brain, in the physiological nature of their
component fibres, and in their peripheral distribution, and therefore they
need not be specially referred to here. The principal branches of the fifth
or _trigeminal_ nerve are shown in Fig. 218. Comparing this nerve with a
typical branchial nerve it would seem that the _profundus_ and
_superficialis ophthalmic nerves_ are dorsal nerves; the _maxillaris_ and
_mandibularis_, pre- and post-branchial branches, respectively, in relation
with the modified gill-cleft which forms the mouth, while the branch to the
oral surface represents a _palatine_ nerve. The most important of the
distinctive features in the cranial nerves of Fishes are to be found in the
relations of nerves vii., ix., and x. to branchial clefts, and in the
lateralis system of nerve components and its association with the lateral
line sensory organs. The seventh or _facial_ nerve is an exceptionally
interesting nerve. Besides the usual components of a typical branchial
nerve certain of its so-called branches are wholly or largely derived from
the lateralis system. For this reason the nerve may be said to consist of
two portions, the _facial proper_, or those fibres which constitute the
facial nerve in air-breathing Craniates, and the _lateralis branches_ which
solely innervate lateral line sense-organs, and are therefore peculiar to
aquatic forms. The facial proper has a ganglion (_the facial or geniculate
ganglion_) on its root, and on entering the orbit after traversing the
cranial wall it gives off a _palatine_ nerve. Just over the spiracle a
_pre-branchial_ nerve, the representative of the _chorda tympani_ of
Mammals, leaves the main trunk, and passes ventrally in relation with the
anterior wall of the spiracle to its ultimate distribution in the walls of
the mouth cavity. The main trunk, now called the _ramus hyomandibularis_,
then pursues a ventral course behind the spiracle as a post-branchial
nerve, and certain of its mainly motor branches which pass downwards in
connexion with the hyoid arch supply the muscles of that arch, and, if an
operculum is present, the opercular muscles as well. The lateralis portion
of the facial includes the following principal branches, {382}each of which
may have a ganglion on its root: (1) an _ophthalmicus superficialis_; (2) a
_buccalis nerve_ with its _ramus oticus_; and (3) _external mandibular_
nerves which course in the ramus hyomandibularis. The addition of the great
_lateralis_ nerve, which is usually described as the lateral branch of the
tenth nerve, and of the eighth or _auditory_ nerve which supplies the
auditory organ, completes the enumeration of the main factors of the
lateralis system. The ninth or _glosso-pharyngeal_ nerve, perhaps the most
typical of all the branchial nerves, has pre- and post-branchial branches
which enclose the hyo-branchial cleft. Its palatine nerve usually extends
forwards and anastomoses with the corresponding branch of the seventh, thus
forming a connexion (_Jacobson's anastomosis_) between the two cranial
nerves. In some Elasmobranchs and Teleosts fibres derived from the dorsal
branch of the ninth nerve innervate a few sense-organs of the lateral
sensory canal of the head, and hence that nerve sometimes contains
lateralis fibres. The tenth or _vagus_ is a compound nerve. Besides the
great lateralis nerve generally associated with it, the _vagus_ includes as
many typical branchial nerves as there are branchial clefts behind the
hyo-branchial cleft, and in Elasmobranchs and in _Chimaera_ these nerves
have independent origins from the medulla oblongata. Each nerve has the
typical structure, a ganglionated trunk which forks over a gill-cleft into
the usual pre- and post-branchial branches, and palatine branches to the
pharyngeal walls. In the Dipnoi the lateralis nerve is connected with the
superficial ophthalmic branch of the seventh nerve by a commisural nerve
which curves across the outer face of the auditory capsule. A somewhat
similar anastomosis is also present in _Petromyzon_. The _vagus_ also
includes a large _ramus intestinalis_, which in Elasmobranchs, at all
events, has a distinct ganglionated root. The nerve forms characteristic
plexuses on the oesophagus and stomach, and in Cyclostomes its branches may
extend nearly the whole length of the intestine. In Ganoids and Teleosts
there is an interesting nerve known as the "_lateralis accessorius_." It is
a compound nerve, and owes its formation to the union of somatic sensory
fibres derived in succession from dorsal branches of the v., vii., ix., and
x. nerves, and also from the corresponding branches of a variable number of
spinal nerves. The finer branches of the nerve are distributed to the skin
of one or more of the fins, or {383}even, as in _Gadus_, to all the fins,
especially to the numerous "end-buds" which are present on those organs. In
many Fishes a variable number of the anterior spinal nerves
(_spino-occipital_) perforate the occipital region of the skull. They
probably represent the ventral roots only of the ordinary spinal nerves of
this region.
SENSE-ORGANS
THE CUTANEOUS SENSE-ORGANS.—These organs, the most remarkable and certainly
the most characteristic of the sense-organs of Cyclostomes and Fishes, are
bud-like groups of epidermic cells in relation with the ends of sensory
nerve fibres. Each consists of a central core of sensory cells, provided
with terminal cuticular sensory hairs, and surrounded by a zone of
supporting and mucus-secreting cells which leave the hairs exposed at the
apex of the bud. Two kinds of these organs can be distinguished, which
differ in their innervation and in their position in the skin. Of the two,
the so-called _end-buds_ are the more primitive. They occupy a superficial
position in the epidermis, and their sense-cells are as long as the
supporting cells. They are present in Cyclostomes and Elasmobranchs, and
especially in Teleosts, where they are irregularly distributed over the
surface of the body, on the fins, lips, and barbels, and also in the
epithelium of the mouth and pharynx. In the Dipnoi they are limited to the
oral cavity, and in the higher Craniates they become taste-buds.[450] Their
somatic sensory nerves[451] are derived from the vii., ix., and x. cranial
nerves, and the lateralis accessorius. In the second type, usually called
"_nerve-eminences_," the sensory cells are shorter than the supporting
cells, and they are always innervated by the lateralis system. When first
developed in the embryo they are quite superficial, like end-buds, but
later the epidermis in which they lie sinks inwards so as to line a series
of pits, closed sacs, tubes, open grooves, or closed canals. _Pit-organs_,
so abundant on the head and trunk of Teleosts (Fig. 220), are simple
epidermic pits with insunken nerve-eminences, disposed in groups or in
{384}lines (accessory lateral lines) or irregularly distributed. The
"_Spalt-papillen_" of Elasmobranchs are pit-organs in which the orifice of
the pit is reduced to a slit. The more deeply-seated _Savi's vesicles_ on
the ventral surface of the Torpedo, and the _nerve-sacs_ of Ganoids, are
similar organs converted into closed sacs and pinched off from the rest of
the epidermis. _Lorenzini's ampullae_ or mucus canals, which are found in
definitely located groups on the lateral and upper surfaces of the head in
Elasmobranchs, may perhaps be compared to pit-organs prolonged inwards to
form subcutaneous tubes, each of which terminates in a radially-septate,
chambered dilatation or ampulla, containing groups of sensory cells.
Besides the more diffusely scattered sense-organs there are others which
become disposed in definite lines along the sides of the body and on the
head, and, enclosed in grooves or closed canals, constitute the highly
characteristic _lateral line system_ of Cyclostomes and Fishes.[452] The
auditory organ must also be included as a specialised portion of this
system. Both organs are innervated by the lateralis system, and both arise
from a common rudiment in the embryonic epidermis in the position of the
future auditory organ. This rudiment grows backwards along the side of the
body in the form of a cord of cells differentiated from the epidermis, and
also forwards, where it soon divides into the rudiments of future
supra-orbital and infra-orbital canals. Sense-organs are differentiated at
intervals along the line of the cord; and in the body, but not on the head,
they frequently exhibit a segmental disposition. Each sensory organ then
sinks down into a short epidermic groove, which by the subsequent meeting
of its lips becomes a canal detached from the epidermis. The short canals
then become continuous, leaving, however, an externally opening primary
pore between every two consecutive canals, and the result is a continuous
canal having sense-organs imbedded in its epidermic lining and connected
with the exterior by pores at intervals (Fig. 219).[453] The enclosure of
the canals in the scales of the lateral line of the trunk or in special
drain-pipe ossicles on the head, and the dichotomous subdivision of the
primary pores into groups of surface-pores, complete the evolution of the
system in its more advanced condition.
{385}[Illustration: FIG. 219.—Vertical longitudinal section through the
lateral canal of _Amia calva_. _l.n_, Lateralis nerve with its branches,
_n_, _n_, to the sensory organs, _s.o_, _s.o_; _p_, _p_, _p_, external
pores; _s.c_, sensory canal; _s_, _s_, scales of the lateral line. (From
Wiedersheim, after Allis.)]
Typically, the lateral line system consists of certain canals or grooves,
usually but not invariably continuous, and defined by their innervation,
(i.) a _lateral canal_ extending along the side of the body and the hinder
part of the head, and having its sensory organs supplied by the great
lateralis nerve (Fig. 220); (ii.) a _supra-orbital canal_ passing forwards
over the eye and innervated by the superficial ophthalmic branch of the
facial nerve; (iii.) an _infra-orbital canal_ supplied by the buccalis and
otic branches of the same nerve; and (iv.) a _hyo-mandibular_ or
_operculo-mandibular canal_, situated on the outer side of the hyoid
region, and thence prolonged downward and forward in relation with the
lower jaw, and innervated by the external mandibular branches of the facial
nerve. The hyo-mandibular canal is sometimes distinct from the other
canals, as in Elasmobranchs and some Teleosts (Fig. 220); and in certain
North American Siluroids the same may be said of the supra-orbital. But, as
a rule, the infra-orbital is continuous behind both with the lateral and
the supra-orbital canals, while the hyo-mandibular canal joins the
infra-orbital, or, exceptionally, {386}the supra-orbital canal. Transverse
commissural canals often connect the lateral and supra-orbital canals of
opposite sides across the dorsal surface of the head, and the corresponding
infra-orbital and hyo-mandibular canals may also be continuous at the
extremity of the snout or at the mandibular symphysis.
[Illustration: FIG. 220.—Sensory canals of the left side of the head of
_Gadus virens_. _e_, Eye; _i.o_, infra-orbital canal (dotted); _l.c_,
lateral canal (oblique shading); _n_, nasal apertures; _op_, operculum;
_op.m_, operculo-mandibular canal (longitudinal shading); _p.o_,
pit-organs; _s.o_, supra-orbital canal (cross-hatched); _s.o.c_,
supra-orbital commissure; _s.t_, supra-temporal branch; _t.t_, tubuli by
which the canals communicate with the exterior. (From Cole.)]
Throughout their extent the canals communicate with the exterior by pores,
or short canals terminating in pores, or by branched canals ending in
groups of pores. In Cyclostomes[454] the lateral line system is represented
by pit-organs disposed as in Fishes, and innervated by a true lateralis
nerve. Some Elasmobranchs have the lateral canal of the trunk represented
by an open groove protected by marginal denticles. _Chimaera_ is more
primitive still in this respect, for on the head as well as on the body the
sensory organs are in open grooves. Amongst Fishes these organs are most
primitive in the Dipnoi, where they retain their superficial position in
the epidermis. In Teleostomes the lateral canals perforate the scales of
the lateral line, and at intervals they open externally by simple or
multiple pores which perforate the scales. On the head they are more or
less completely enclosed in special ossicles which either remain distinct
or fuse with certain of the adjacent dermal or cartilage bones of the
skull. The use of the lateral line organs is not certainly known. They
occur only in Fishes and Amphibia, and as blind Fishes are able to avoid
obstacles with the greatest ease when swimming, it is possible that these
organs enable their possessors {387}to appreciate undulatory movements in
water in the shape of reflex waves from contiguous surfaces or
objects.[455] Their great antiquity is shown by their existence in most of
the Heterostraci, and in the Antiarchi and Arthrodira, although they have
not yet been discovered in the Osteostraci.
THE AUDITORY ORGANS.—In its more typical condition each auditory organ
consists of a membranous sac or vestibule, partially constricted into an
upper portion or utriculus and a lower or sacculus (Fig. 221, A). Three
semicircular canals are connected with the utriculus, of which two are
vertical and at right angles to one another, and the third is horizontal.
One end of each canal is dilated into an ampulla. A slender tube, the
ductus endolymphaticus, leaves the sacculus, and ends in a sac-like
swelling, the sinus endolymphaticus, which apparently represents a portion
of the embryonic epidermic involution from which the auditory organ is
formed. A smaller sac-like outgrowth from the sacculus, the _lagena_,
corresponds to the cochlea of the higher Vertebrates. The epidermic lining
of this system of cavities is differentiated into patches or ridges of
sense-cells (maculae or cristae), separated by supporting cells and
innervated by the terminal branches of the auditory nerve. There is a
crista acustica in each ampulla; and maculae acusticae are present in the
utriculus, sacculus, and lagena. A fluid, the _endolymph_, fills all the
cavities, and a similar fluid or _perilymph_ occupies the spaces in the
periotic capsule in which the various chambers are lodged. Among the more
notable deviations from this type of auditory organ the Cyclostome
_Myxine_, apparently, has but a single semicircular canal with an ampulla
at each end, and the vestibule is a simple sac (Fig. 221, B). _Petromyzon_
has two canals, but lacks the horizontal canal. In Elasmobranchs, including
_Chimaera_ (C), the ductus endolymphaticus retains its primitive connexion
with the exterior by means of a pore on the dorsal surface of the head. In
the Dipnoi (e.g. _Protopterus_) the paired endolymphatic sinuses divide
into a number of caecal branches containing otoliths, which meet and
interlace over the fourth ventricle (Fig. 217).[456] Otoliths, either in
the form of fine, {388}mucus-connected, calcareous particles, as in
Elasmobranchs, or as massive solid concretions in Teleosts, are present in
relation with the sensory areas of the utriculus, sacculus, and lagena.
[Illustration: FIG. 221.—Auditory organs of Fishes. A, of a typical Fish;
B, of _Myxine_; C, of _Chimaera_; and D, of _Perca_. _a.c_, Anterior canal;
_am_', _am_", _am_'", ampullae; _am.n_, nerves to ampullae; _c_,
semicircular canal (in _Myxine_); _d.e_, ductus endo-lymphaticus; _h.c._
horizontal canal; _l_, lagena; _mc_, macula acustica; _m.s_, macula
acustica of the sacculus; _n_, nerves to ampullae; _o_, external aperture
of the ductus endo-lymphaticus; _p.c_, posterior canal; _s_, sacculus;
_s.e_, sinus endo-lymphaticus; _sk_, superficial skin; _s.s_, sinus
superior; _u_, utriculus; viii, auditory nerve. (From Wiedersheim, after
Retzius.)]
In a few marine and in a large number of freshwater Teleosts the auditory
organ enters into a more or less intimate connexion with the air-bladder by
one of three different methods.
{389}[Illustration: FIG. 222.—Cavity of the air-bladder of a Siluroid
(_Macrones nemurus_) exposed by the removal of its ventral wall. _a.c_,
Anterior chamber; _b.o_, basioccipital; _b.w_, body wall, here reduced to
the external skin; _cl_, clavicle; _l.c_, lateral chamber; _l.s_,
longitudinal septum; _pt_, post-temporal; _tr.a_, anterior portion of the
tripus; _tr.c_, crescentic portion of the tripus; _t.s_, transverse septum;
_t.s_', shorter transverse septum. (From Bridge and Haddon.)]
The first and simplest is by the apposition of the extremities of a pair of
caecal tubular prolongations from the air-bladder to the outer surfaces of
the fibrous membranes which close a pair of vacuities in the outer bony
walls of the periotic capsules, the inner surfaces being bathed by the
perilymph surrounding the auditory organs. This method is characteristic of
certain Serranidae, Berycidae, Sparidae, Gadidae, and Notopteridae,[457]
and probably in the Hyodontidae. In the second method, of which several
Clupeidae (_e.g._ Herring, Pilchard, etc.) furnish examples, the periotic
vacuities are open instead of closed, and the sac-like ends of the tubular
extensions from the air-bladder are in actual contact with protruding
outgrowths from the utriculus.[458] The third method, by far the most
elaborate, is by the intervention of a series of movably connected
"Weberian" ossicles, of which the most posterior on each side (the tripus)
is inserted into the dorsal wall of the air-bladder (Fig. 223), while the
anterior one (scaphium) forms the outer wall of a median backward
prolongation (sinus impar) of the perilymph-containing spaces surrounding
the two auditory organs. This in turn encloses a similar median
prolongation (sinus endolymphaticus) from the two sub-cerebrally united
endolymphatic ducts (Fig. 223).[459] This complex mechanism is present in
the Cyprinidae, Siluridae, Characinidae, and Gymnotidae; and hence the term
"Ostariophysi"[460] as a collective name for these families.[461] {390}The
physiological _raison d'être_ of the connexion between the air-bladder and
the auditory organ cannot yet be regarded as satisfactorily determined. It
is possible, as Weber thought, that it may be an auxiliary to the function
of hearing by transmitting to the ear sound-waves impinging on the surface
of the body and affecting the gases in the air-bladder.[462] On the other
hand, it may be urged with perhaps greater probability that the connexion
exists for the purpose of conveying to the ear stimuli due to the varying
degrees of distension of the air-bladder, such as, it may be presumed, are
naturally brought about by the variations of hydrostatic pressure which a
Fish encounters in the course of its ascent or descent in the water.[463]
Whether regarded as an accessory to hearing, or as a means of regulating
the distension of the air-bladder, the physiological value of the connexion
must be considerable, and on this point it is at least significant that the
Weberian mechanism is characteristic of the dominant families of freshwater
Teleosts at the present day.[464]
[Illustration: FIG. 223.—Diagram to show the Weberian ossicles and their
relations to the ear and the air-bladder. _at_, Atrium, an extension of the
sinus impar; _a.v.c_, anterior vertical canal; _b.w_, bony wall of the
periotic capsule; _d.e_, the medianly-united endolymphatic ducts; _h.c_,
horizontal canal; _in_, intercalarium, a third ossicle imbedded in the
ligament (_i.lg_) connecting the scaphium with the tripus; _n_, bony
nodules on the sides of the complex vertebral centrum; _p.v.c_, posterior
vertical canal; _s_, sacculus; _sc_, scaphium; _s.e_, sinus
endolymphaticus; _s.i_, sinus impar; _tr.a_, _tr.c_, the anterior and
crescentic parts of the tripus; _ut_, utriculus. The radial lines represent
the fibres of the dorsal wall of the air-bladder. (From Bridge and
Haddon.)]
THE OLFACTORY ORGANS.—These organs are essentially a pair of pit-like
inpushings of the skin of the ventral side of the head in front of the
mouth, with their lining epidermis differentiated into sensory cells
separated by supporting cells, and connected with the olfactory lobes of
the brain by olfactory nerves.
{391}[Illustration: FIG. 224.—Two stages in the development of the
olfactory organ and the pituitary involution in _Petromyzon_. A is the
earlier, B a much later stage. _br_, Brain; _in_, infundibulum; _l.lp_,
lower lip; _ms_, mesenteron; _n_, notochord; _ol.o_, olfactory organ; _pn_,
pineal body; _pt.s_, pituitary sac; _st_, stomodaeum; _u.lp_, upper lip.
(From Parker and Haswell, after Dohrn.)]
The Cyclostomata are unique amongst Craniates in the apparently unpaired
condition of the olfactory organ, and in its remarkable relation to the
pituitary involution. In the embryo Lamprey the median and ventral
olfactory pit is carried inwards with the pituitary invagination, so that
the former appears as a dorsal outgrowth from the latter, and the two have
a common external opening, the naso-pituitary aperture (Fig. 224). Later
the extraordinary forward growth of the upper lip to form the roof of the
buccal funnel has the effect of shifting the naso-pituitary involution and
its aperture to a final position on the dorsal side of the head. It is due
to this dorsal displacement that, as we shall see, the pituitary caecum
reaches the ventral surface of the brain by perforating the basis cranii
from above, instead of from below as in all other Craniates. The pituitary
body is pinched off from the dorsal side of the naso-pituitary involution.
In the adult Lamprey the olfactory organ appears as a round sac divided by
a median septum into two lateral chambers (Fig. 225), the lining epithelium
of which is raised into prominent ridges. Behind the sac the pituitary
involution is prolonged backwards beneath the brain, and, after traversing
the basi-cranial fontanelle, it widens out into a spacious cul-de-sac and
terminates on the dorsal side of the pharynx, beneath the anterior end of
the notochord. In _Myxine_ the pituitary involution ends by opening into
the pharynx.
{392}[Illustration: FIG. 225.—Side view of the brain of _Petromyzon_, with
the olfactory organ and the pituitary caecum in section. _cblm_,
Cerebellum; _crb.h_, cerebral hemisphere; _dien_, thalamencephalon; _f_,
fold in the nasal tube; _gl_, nasal glands; _inf_, infundibulum; _l.gn.hb_,
left ganglion habenulae; _med.obl_, medulla oblongata; _na.ap_,
naso-pituitary aperture; _n.ch_, notochord; _Nv^1-nv^{10}_, cranial nerves;
_Nv^{12}_, first ventral spinal nerve; _olf.cp_, olfactory capsule;
_olf.l_, olfactory lobe; _olf.m.m_, olfactory mucous membrane; _opt.l_,
optic lobe; _pn_, pineal body; _pn′_, inferior pineal body; _pn.e_,
parietal eye; _pty.b_, pituitary body; _pty.p_, pituitary cul-de-sac; _sp_,
median septum of the olfactory sac; _sp^1_, first dorsal spinal nerve.
(From Parker and Haswell, after Ahlborn and Kaenische.)]
The apparently monorhinal condition of the Cyclostomes is probably a
secondary acquisition. At the earliest embryonic stage at which any trace
of an olfactory organ is apparent, there is a median thickening of the
epidermis, possibly a vestige of some older sensory organ comparable, it
may be, to the so-called olfactory organ of Amphioxus; on each side of it
there is a lateral thickening, the rudiments of the paired organs.[465] The
three thickenings, or "plakodes," then sink inwards to form an olfactory
pit. The partial subdivision of the adult organ by a vertical septum, and
the presence of two olfactory nerves, point to the same conclusion.[466]
All Fishes possess olfactory organs which are obviously paired. In
Elasmobranchs and Dipnoi they retain their primitive ventral position. Many
Sharks and Dog-Fishes possess an oro-nasal groove leading from each
olfactory organ to the corresponding angle of the mouth. The Dipnoi proceed
a stage farther, and, by the conversion of the grooves into short canals,
the olfactory pits communicate with the mouth by true internal nostrils, as
in the higher {393}Vertebrates. In the adults of existing Teleostomi the
orifice of each organ is usually divided into two by the growth of a fold
of skin across it, and the two apertures rotate outwards and upwards on to
the lateral or the upper surface of the snout. Of the two nostrils the
posterior one probably corresponds to an external nostril, and the anterior
one to the internal nostril. Occasionally each olfactory organ has only a
single orifice. In the Crossopterygii and in some Teleostei the nostrils
become tubular. The lining epithelium of the olfactory pits is usually
produced into ridges, disposed longitudinally or transversely, or in the
form of radii from a central point in the roof. Many Teleosts have each
olfactory organ prolonged backwards into one or two sacs, the nasal sacs,
which are either simple reservoirs, or glandular and mucus-secreting. In a
species of Chinese Sole (_Cynoglossus semilaevis_) the two sacs, one from
each olfactory organ, unite over the roof of the mouth in a common median
sac, and in one unique specimen the latter communicated with the mouth by a
large naso-pharyngeal aperture.[467]
THE EYES.—In essential structure the eyes of Cyclostomes and Fishes
resemble those of the higher Craniates. As a rule, in Fishes they are
relatively larger, however, and the lens is globular and the cornea
somewhat flatter. Ciliary processes and ciliary muscles are absent. As the
eyes are nearly always lateral in position it is probable that monocular
vision is the rule. In Teleosts and in _Amia_ a "choroid gland," consisting
of a mass of capillary blood-vessels, surrounds the optic nerve externally
to the retina, and derives its blood from the efferent artery of the
pseudobranch (Fig. 226). In most Teleostomi, but not in Cyclostomes,
Elasmobranchs, and Dipnoi, there is a singular prolongation of the choroid
coat, known as the "processus falciformis," which extends across the
vitreous humour to the inner face of the lens, where it ends in an
expansion, the "campanula Halleri" (Fig. 226). Accommodation to vision at
different distances is not effected by alterations in the convexity of the
lens, but by a change in its position with regard to the retina, apparently
brought about by the contraction of a special retractor muscle.[468] Some
oceanic pelagic Teleosts are remarkable for their curious telescopic eyes
in the shape of short protruding {394}cylinders, each terminating in a
strongly convex cornea (Fig. 227).[469] The eyes are directed either
upwards or forwards, and, as their long axes are parallel in either
position, it is probable that these Fishes are capable of binocular vision.
In the young of certain Teleosts occurring in the Antarctic and Indian
Oceans the large eyes are situated at the extremities of extraordinary long
stalks extending from the sides of the head.
[Illustration: FIG. 226.—Vertical section of the eye of _Salmo fario_
(semi-diagrammatic). _arg_, Argentea; _ch_, choroid; _ch.gld_, choroid
gland; _cn_, cornea; _cp.hal_, campanula Halleri; _ir_, iris; _l_, lens;
_opt.nv_, optic nerve; _pg_, pigmentary layer; _pr.fl_, processus
falciformis; _rt_, retina; _scl_, sclerotic. (From Parker and Haswell.)]
In the quasi-parasitic Cyclostome, _Myxine_, and in many Teleosts belonging
to widely different families, which live at great depths in the sea or
inhabit subterranean waters, the eyes suffer from disuse and degenerate in
structure. The influence of a deep-sea habitat on the eyes of Fishes is
somewhat varied. The eyes are often small. A few abyssal Fishes are totally
blind, and no external trace of eyes can be seen (Fig. 430). In such Fishes
compensation is often afforded by an extraordinary development of tactile
organs in the form of long barbels, or of trailing filaments derived from
the median or the paired fins (Fig. 371, B). Many deep-sea forms possess
eyes of the normal size, or even exceptionally large eyes, probably because
either they occasionally migrate towards the surface, or else they possess
phosphorescent organs and are able to see by the aid of the light they
themselves emit. A blind Siluroid (_Amiurus nigrilabris_) frequents the
cave streams of Pennsylvania, and many members of the same family which
live in muddy waters have very small or even minute eyes. One of the Gobies
(_Typhlogobius_),[470] which buries itself in the sand, or is found under
stones in the holes of a burrowing Crab on the coast of California, is also
{395}blind. Amongst other blind Fishes _Amblyopsis_ and _Typhlichthys_
(Amblyopsidae)[471] and _Lucifuga_ (Zoarcidae) may be mentioned, the first
two inhabiting the cave streams of North America, while the third has a
similar habitat in Cuba. When the eyes degenerate they dwindle in size and
recede from the surface. The lens and the iris wholly or partially
disappear, and although it is generally recognisable the retina loses
certain of its characteristic layers, or the latter are but imperfectly
formed. In _Myxine_ even the eye-muscles are absent.
The eyelids of Fishes are little more than marginal folds of skin, capable
of little if any movement, and leave the eyes largely uncovered. Some
Sharks have a third eyelid or "membrana nictitans" at the anterior corner
of the eye. Lachrymal glands are unknown.
[Illustration: FIG. 227.—The telescopic eyes of _Opisthoproctus soleatus_,
Vaill. (A), and of a species of a new family of Teleosts from the Indian
Ocean (B). Nat. size. (From Chun.)]
THE PARIETAL EYE.—It is only in the Cyclostomes that this structure can
have any claim to be regarded as a visual organ. In the Lamprey (Fig. 228)
the parietal eye is a slightly flattened vesicle lying directly over the
pineal vesicle, and connected by a slender stalk or nerve with the right
ganglion habenulae. The dorsal or more external half of the vesicle is
bi-convex, and forms the "pellucida," while the inner half or retina is
said to consist of supporting cells with interspersed deeply pigmented
sense-cells and ganglion cells.[472] The external skin over the parietal
eye is partially transparent in the living animal.
In many of the oldest known Fishes, such as the Ostracodermi, the
Antiarchi, and the Crossopterygian Osteolepida, there are indications of
the existence either of one or of two median sense-organs on the upper
surface of the skull, in the shape of one or two foramina, or hollow
protuberances, or pit-like grooves or {396}depressions, but, as a rule,
when one of them is present the other is absent. It is probable that both
these structures were associated with sensory organs, of which one may have
been a parietal eye and the other a pineal eye. Some Teleosts (_e.g._ many
deep-sea Scopelidae) have a transparent, convex, cornea-like prominence on
the upper surface of the head which may be related to one of these singular
organs.[473]
[Illustration: FIG. 228.—Vertical section through the parietal eye and the
pineal vesicle of _Petromyzon marinus_. _c.t_, Connective tissue; _p_,
pellucida; _p.o_, pineal organ; _pt.o_, parietal eye; _r_, retina; iii _v_,
third ventricle. (From Wiedersheim, after Studnička.)]
{397}CHAPTER XV
THE KIDNEYS AND THE REPRODUCTIVE ORGANS—BREEDING
The kidneys and the reproductive organs are so intimately connected that it
is necessary to deal with them together. Both organs are specialised
portions of the coelom and its epithelial lining. The KIDNEYS are
essentially a series of tubular and at first segmentally-disposed
outgrowths from the coelom (urocoeles) which acquire a connexion with the
exterior, while the gonads have their origin from local modifications of
the coelomic epithelium. At a very early embryonic stage each lateral half
of the coelom presents three well-marked divisions: (1) a series of dorsal
portions ("myocoeles"), the cavities of the myotomes or muscle-segments;
(2) a longitudinally continuous unsegmented portion extending round the
alimentary canal, the "ventral coelom"; and (3) a series of intermediate
tubular portions or "nephrotomes," each of which leads from a myocoele to
the ventral coelom (Fig. 229, A). The essential components of the kidneys,
the urocoeles or renal tubules, are derived from the nephrotomes. In its
typical condition each kidney consists of three portions, which, in
accordance with their embryological and evolutionary sequence, are termed
the "pronephros," the "mesonephros," and the "metanephros." The pronephros,
the larval or provisional kidney, is formed from a limited number of the
nephrotomes immediately behind the head. From each nephrotome a hollow
tubular outgrowth is formed, which grows towards the lateral surface of the
body, and then unites with its fellows of the same side to form a main
longitudinal duct—the "archinephric" or "pronephric duct" (Fig. 229, A,
Fig. 230, A). This duct grows backwards until it opens into the
cloaca.[474]
{398}[Illustration: FIG. 229.—Diagrammatic transverse sections through an
embryo Craniate to show the mode of development of the pronephros (A) and
of the mesonephros (B). The right side of each figure shows an earlier
stage than the left. In B (left side) the connexion of a vas efferens with
a mesonephric tubule, and the division of the archinephric duct into
Müllerian and mesonephric ducts are shown, _a_, Aorta; _a.c_, alimentary
canal; _a.d_, archinephric duct; _g_, glomus; _gl_, glomerulus; _i.n_,
inner nephrostome; _mb_, Malpighian body; _md_, Müllerian duct; _mnd_,
mesonephric duct; _mnt_, mesonephric tubule; _myc_, myocoele; _myt_,
myotome; _n_, notochord; _np_, nephrotome; _nt_, nephrostome; _o.n_, outer
nephrostome; _pn.t_, pronephric tubule; _s.c_, spinal cord; _t_, testes;
_v.c_, ventral coelom; _v.ef_, vas efferens. (After Kingsley and Semon.)]
At the same time the nephrotomes lose their connexion with the myocoeles,
although they still retain their "nephrostomes" or apertures through which
they communicate with the ventral coelom. When fully developed the
pronephros consists of a few tubules, more or less convoluted, opening at
their inner extremities into the coelom by means of their ciliated
nephrostomes, and at their outer ends communicating with the exterior
through the archinephric duct. In relation with the pronephros a branch
from the dorsal aorta forms a tuft of capillary blood-vessels or "glomus,"
opposite the nephrostomes, which projects into the ventral coelom on each
side. Later, a second series of much more numerous tubules is formed behind
the pronephros, which constitute the mesonephros. In forming mesonephric
tubules the nephrotomes become disconnected from the myotomes and their
myocoeles, and curving outwards they {399}come to open into the
archinephric duct, although they do not in any way contribute to its
formation (Fig. 229, B). Segmentally-arranged twigs from the dorsal aorta
end in tufts of capillaries or glomeruli, each of which projects into a
small sac-like enlargement of a mesonephric tubule, pushing before it the
wall of the sac. In this way a double-walled "Malpighian body," containing
a "glomerulus," is formed in connexion with each tubule. Subsequently, the
mesonephric tubules increase in number by budding. New nephrostomes and
Malpighian bodies are developed on the secondary branches, and the original
segmental arrangement of the tubules becomes obscured. With the growth of
new tubules, and the formation of blood-vessels and of connective and
lymphoid tissues between them, each mesonephros finally assumes the
condition of a compact gland imbedded in the dorsal wall of the coelom,
with its ventral surface invested by the peritoneum. A "metanephros," which
in the higher Vertebrates replaces the mesonephros as the functional
kidney, is perhaps not represented in Fishes.
A more or less well-developed pronephros is present in the embryos or
larvae of the Cyclostomes and of all Fishes, but as a rule it completely
disappears at an early period and is replaced by the mesonephros. It is
retained throughout life, however, in the Myxinoid Cyclostomes (Fig. 230,
B), and has its persistent nephrostomes opening into the pericardial
cavity.[475] In a few Teleosts the pronephros is also persistent, as in
_Fierasfer_ and _Dactylopterus_, and in others the organ may not completely
disappear until the approach of sexual maturity. But with these exceptions
the mesonephros is the sole functional kidney in the adults of the
Cyclostomes and of all Fishes. As regards the nature of the duct by which
the excretion of the mesonephros is conveyed outwards, there are notable
differences in different Craniates. The Cyclostomes and the Teleostomi
retain that part of the archinephric duct into which the mesonephric
tubules open, and which remains after the atrophy of the pronephros (Fig.
230, B, E, F). In Elasmobranchs, and probably also in the Dipnoi, a special
mesonephric duct is developed in a way which will be described later (Fig.
230, C, D).
{400}[Illustration: FIG. 230.—Showing the principal modifications of the
kidneys and reproductive organs in Cyclostomes and Fishes. A, The
pronephros and its duct in the embryo; B, the kidneys and genital pores in
_Petromyzon_, the vestigial pronephros represented as in _Myxine_; C and D,
the urinogenital organs of a male and female Elasmobranch; E, of a male or
female Teleost, or a male _Lepidosteus_; F, of a female _Polypterus_,
_Acipenser_, _Amia_, or _Osmerus_. _a_, Anus; _a.d_, archinephric duct;
_c_, cloaca; _c.a_, the coelomic aperture of the Müllerian duct; _c.p_,
cutaneous pit; _g_, gonad; _gd_, gonoduct; _g.p_, genital pore; _i_,
intestine; _m_, Malpighian body; _m.d_, Müllerian duct; _mn_, mesonephros;
_mn^1_, vestigial mesonephros; _mn^2_, excretory portion of the mesonephros
("metanephros"); _mn^3_, genital portion of mesonephros; _mn.d_,
mesonephric duct; _mtn.d_, metanephric duct; _n_, nephrostome; _ov_, ovary;
_p.a_, abdominal pore; _p.f_, peritoneal funnel; _pn_, pronephros; _pn′_,
vestigial pronephros; _s.g_, shell gland; _s.s_, sperm sac; _t_, testis;
_ug.s_, _u.s_, urinogenital sinus; _v.ef_, vasa efferentia; _v.s_, vesicula
seminalis.]
{401}In the males of Elasmobranchs some of the hinder mesonephric tubules
unite to form a single main duct opening into the terminal part of the
mesonephric duct, and these tubules and their separate duct are sometimes
regarded as a metanephros and a metanephric duct. The mesonephric
nephrostomes are persistent throughout life in a few Elasmobranchs (_e.g._
Notidanidae, Heterodontidae, Rhinidae, and some Scylliidae), and also in
_Amia_:[476] in all other Fishes as well as in the Cyclostomes they become
closed in early life.
[Illustration: FIG. 231.—Diagrammatic horizontal section through the
abdominal pores and cloaca of an Elasmobranch. _a.p_, Abdominal pore; _c_,
coelom; _cl_, cloaca; _cl.p_, cloacal papilla; _c.p_, cloacal pit; _od_,
oviducal apertures in the female; _r_, rectum; _u.s_, cloacal aperture of
the urinary sinus (female), or the urogenital sinus (male). In some
Elasmobranchs the abdominal pore opens at the base of the cloacal papilla,
as shown at _a.p^1_. (Modified from Bles.)]
In many Fishes the hinder extremity of the coelom communicates directly
with the exterior through "abdominal pores," of which there is usually a
pair, rarely a single pore, situated close to the cloacal or the anal
aperture.[477] Elasmobranchs usually have a pair, often at the extremities
of a pair of cloacal papillae (Fig. 231), but they are absent in some
families (_e.g._ Heterodontidae and Rhinidae); and in some Scylliidae (e.g.
_Scyllium canicula_) they are very variable, being either present or absent
on both sides, or an open pore is present on one side only. Pores are
present and paired in the Crossopterygii, the Chondrostei, and the
Holostei. Amongst the Dipnoi _Neoceratodus_ has a pair of pores.
_Protopterus_ sometimes has two pores opening into the cloaca, but as a
rule the two become confluent and have a single external aperture. In
_Lepidosiren_ pores are wanting. Abdominal pores are rarely present in
Teleostei. They exist, however, in the Mormyridae (_Gymnarchus_ and several
species of _Mormyrus_), and also in the {402}Salmonidae,[478] where they
are as singularly variable in different species and individuals as in the
Elasmobranch Scylliidae. The use of abdominal pores is not certainly known,
unless the coelom of those Fishes which possess them continues to retain
some measure of its primitive excretory function, and the pores act as
excretory ducts. That the nephrostomes are excretory organs has been shown
by experiment, and it is worthy of note that there exists a reciprocal
relation between these structures and abdominal pores, to the extent that
while there are a few Fishes (_e.g._ certain Elasmobranchs and _Amia_) in
which both coexist, there are many others in which the presence of
nephrostomes is correlated with the absence of pores and _vice versâ_.
The male and female GONADS, testes and ovaries, are derived from the
coelomic epithelium near the inner or median aspect of the nephrotomes
(Fig. 229, B). Here the epithelium remains columnar, and soon projects into
the ventral coelom as a continuous longitudinal ridge. It is probable that
at first the modified epithelium is segmented as a series of "gonotomes,"
but if so, the latter must soon coalesce into a continuous ridge. Some of
the epithelial cells enlarge to form the primitive sex-cells. In the
development of an ovary, portions of the epithelium sink inwards, carrying
with them the primitive ova. Certain of the cells form the epithelial walls
of a number of ovisacs, each of which encloses an ovum. As the ovisacs
increase in number and size the germinal ridges project more and more into
the coelom until, as ripe ovaries, they become suspended from its dorsal
wall by a double peritoneal fold, the "mesovarium" (Fig. 156). The testes
develop in a similar fashion except that the primitive sex-cells, which
later give rise to spermatozoa, form the lining of a number of simple or
ampulla-like tubules, the seminiferous tubules, and the suspensory fold is
termed the "mesorchium."
The Cyclostomes have gonads in the shape of unpaired organs extending
nearly the whole length of the coelom, but in all Fishes the organs are
primarily paired, although by fusion, or by the absorption of one gonad,
the ovaries or the testes sometimes appear as if single. The ovaries may
either be naked, as in Elasmobranchs, Dipnoi, Crossopterygii, and
Chondrostei, and in _Amia_ amongst the Holostei; or, as in _Lepidosteus_
and most Teleosts, they become enclosed in coelomic sacs. The {403}former,
or "gymnoarian," condition is primitive; the latter, or "cystoarian," is
secondary, and is brought about by the growth of two peritoneal folds round
the ovary and the union of their margins. Into these coelomic sacs the
egg-bearing or real ovarian tissue projects either in the form of processes
or of transversely- or longitudinally-arranged plates or folds (Fig. 232,
B). The testes are composed of seminal ampullae, as in Elasmobranchs, or of
radially-arranged and sometimes plexiform tubules opening into the
gonoduct, as in nearly all other Fishes (Fig. 232, A).
[Illustration: FIG. 232.—Diagrams to show the structure of the testes (A)
and of the ovaries (B) in a Herring. (From Cunningham.)]
In the Cyclostomes (e.g. _Petromyzon_) the eggs and spermatozoa are
discharged from the gonads into the coelom, whence they reach the exterior
through a pair of "genital pores" leading from the hinder end of the coelom
into a urinogenital sinus formed by the united extremities of the two
archinephric ducts.[479] _Myxine_ has, however, but a single median pore,
opening into an integumentary cloaca, which also receives the rectal and
urinary orifices. _Bdellostoma_ has two such pores communicating with a
similar cloaca.[480]
{404}The nature and homologies of the genital ducts in the different groups
of Fishes are amongst the most puzzling of the many problems which vex the
soul of the Vertebrate morphologist, and although there is a fairly general
agreement on some points, there are others of great importance of which it
may be said _quot homines, tot sententiae_.
Broadly speaking, there are two types of genital ducts in Fishes: (1) those
which are obviously derived from some part of the kidney system; and (2)
those which are special ducts and appear to have no connexion with
kidney-ducts.
The Elasmobranchs offer a typical example of gonoducts of the first kind.
At an early embryonic period in both sexes each archinephric duct becomes
longitudinally split into two ducts, of which one continues to receive the
openings of the mesonephric tubules and remains as a mesonephric duct (Fig.
229, B).[481] The other, which has no connexion with the mesonephros, opens
anteriorly into the coelom by means of the united nephrostomes of the
pronephros, and is known as the "Müllerian duct" (Fig. 230, C and D). In
the adult male the Müllerian ducts are useless vestiges, but in the female
they persist and act as oviducts, receiving the eggs set free from the
ovarian ovisacs through their coelomic apertures, and thence conveying them
to the cloaca. In the male, certain of the anterior mesonephric tubules
become connected with the testicular ampullae by means of a network of
slender tubules, the "vasa efferentia" or testicular network, and through
the latter the spermatozoa pass from the testes to the mesonephric duct
(Fig. 230, C). Consequently, the mesonephric duct conveys both spermatozoa
and the kidney excretion to the cloaca. It is obvious, therefore, that
both the male and female gonoducts are derived from kidney-ducts.
The Teleostei afford an equally typical illustration of the second type.
Each female gonoduct (oviduct) is formed by a backward growth of the same
two peritoneal folds which enclose the ovary; these are converted into a
"peritoneal tube" or canal by the union of their margins. The male
gonoducts are also formed in continuity with the testes, that is, as
backward prolongations from the latter. Each duct, male or {405}female,
seems to be a duct _sui generis_ and to have no connexion whatever with the
kidney system (Fig. 230, E). In the Salmonidae, Anguillidae, Galaxiidae,
Hyodontidae, Notopteridae, and Osteoglossidae, and also in _Misgurnus_, the
oviducts lose their continuity with ovaries and degenerate to an extent
which differs greatly in different families. Thus in some Salmonidae, as in
the Smelt (_Osmerus eperlanus_),[482] the oviducts end anteriorly in wide
funnel-like coelomic apertures after the fashion of Müllerian ducts, and do
not embrace the ovaries: hence the ovaries are naked and not cystoarian,
and their ducts are not peritoneal tubes but "peritoneal funnels" (Fig.
230, F). In other Salmonidae and in the Anguillidae the oviducts appear to
have so far degenerated that they are represented either by a pair of very
short funnels or by a pair of genital pores, which, as in the Salmon, have
a common external aperture behind the anus and in front of the single
orifice of the united archinephric ducts (Fig. 233, A). In all such
instances the eggs are set free from the ovaries into the coelom, from
whence they escape through the peritoneal funnels or genital pores. In the
Eels the male gonoducts also degenerate, and, losing all connexion with the
testes, they become reduced to genital pores as in the female.
The Holocephali and probably the Dipnoi conform to the Elasmobranch type in
the nature of their male and female gonoducts. In the Crossopterygii[483]
each testis has its own proper duct, which has no connexion with the kidney
system and apparently belongs to the Teleostean type, while the oviduct,
which is almost certainly not a Müllerian duct, is probably a peritoneal
funnel. On the other hand, the Chondrostei and the Holostei are in the
interesting transitional condition of possessing male ducts of the
Elasmobranch type and female ducts of the Teleostean type, the latter being
either ducts directly continuous with the ovaries, as in _Lepidosteus_, or
of the nature of peritoneal funnels, as in _Acipenser_, _Polyodon_, and
_Amia_ (Fig. 230, E and F).
How far the distinction between the two types of gonoduct holds good in the
case of the male is not quite clear, and it has recently been argued that
the Dipnoi offer a connecting link between the two.[484]
{406}[Illustration: FIG. 233.—Diagram to show the kidneys and gonoducts of
a female Salmon (A), and of a male _Protopterus_ (B). _md_^1 and _md_^2,
Anterior and posterior vestiges of the Müllerian duct; _t.t_, tubular
posterior portion of the testis (_t_). Other reference letters as in Fig.
230. (B, after Graham Kerr.)]
In _Protopterus_ each testis is divided into an anterior sperm-producing
part and a posterior tubular portion which has lost the capacity of
producing sex-cells. The testicular network is greatly reduced, and forms
but a limited connexion between the tubular portion of the testes and the
mesonephric duct (Fig. 233, B). If it be supposed that the testicular
network became still further reduced so that the connexion between the
testes and the kidney-duct took place directly through a single channel
instead of through several, the result would be a gonoduct essentially
similar to the male duct of an ordinary Teleost. Should this view prove to
be correct, it will follow that the male gonoducts of _all_ Fishes are
differently-modified examples of the Elasmobranch type. But there will
still remain the female gonoducts of Ganoids and Teleosts, which must be
regarded as distinct from Müllerian ducts unless it can be shown that their
different methods of development are not necessarily fatal to their
homology with Müllerian ducts, or that both types of gonoduct can be
derived from some intermediate type. Assuming that some Fishes do possess
male or female ducts which have not been derived from the kidney system,
but have been independently acquired, there is still the question, which of
the two types is the more primitive, or, in other words, has the
Elasmobranch type superseded the Teleostean, or _vice {407}versa?_ To this
question no decisive answer can at present be given.
The terminal relations of the kidney-ducts and the gonoducts, and the
presence of accessory or of vestigial organs in connexion with them, will
now be briefly dealt with. In the males of the Elasmobranchs the
mesonephric ducts which, as already pointed out, act both as kidney-ducts
and gonoducts, dilate posteriorly to form a pair of vesiculae seminales,
and then unite to form a urinogenital sinus, opening into the cloaca at the
extremity of a median papilla (Fig. 230, C). The sinus also receives ducts
from the hinder part of the mesonephros, either separately, as in the
female, or by a common duct on each side—the so-called metanephric duct—as
in the male. Two tubular caecal outgrowths from the sinus form two sperm
sacs. Only the anterior portions of the Müllerian ducts with their coelomic
apertures are retained in the adult. In the female the mesonephric ducts
are purely excretory, but otherwise they are similar, and the oviducts
(Müllerian ducts) open into the cloaca separately or by a common orifice
(Fig. 230, D). A glandular dilatation of each oviduct forms the oviducal or
shell gland by which the horny egg-cases are secreted. In the males of the
Holocephali the gonoducts open into a urinogenital sinus with an external
orifice distinct from and behind the anus; but the female has separate
apertures for the rectum, the conjoined oviducts, and the united
mesonephric ducts. Both sexes have complete Müllerian ducts communicating
with the coelom in front, and behind with the exterior. The Dipnoi of both
sexes essentially resemble the Elasmobranchs in the general relations of
their ducts, but the Müllerian ducts of the male exhibit marked differences
in the three genera.[485] In _Neoceratodus_ the ducts are as complete as
their functional representatives in the female. _Protopterus_ retains
anterior vestiges and the coelomic apertures, and also vestiges of the
hinder portions which unite and end blindly in the urinogenital papilla,
but the middle sections of the two ducts are suppressed (Fig. 233, B). In
the Teleostomi there is a general similarity in the terminal relations of
the gonoducts and kidney-ducts. In the Ganoids the archinephric ducts unite
and then expand into a urinary sinus or bladder, and the gonoducts of the
female, or of both sexes in _Lepidosteus_, open either into the
archinephric {408}ducts or into the common sinus, and therefore both ducts
communicate with the exterior by a urinogenital orifice behind the anus.
Peritoneal funnels, similar to the functional oviducts of the female, are
present in the males of the Chrondrostei and of _Amia_. In Teleosts the
terminal connexions of the ducts tend to become less intimate. The
archinephric ducts often dilate into a urinary bladder either before or
after their union, and the common duct joins the united gonoducts to form a
short urinogenital sinus which opens externally, or the confluent gonoducts
have an independent genital orifice between the anus and the urinary
aperture. Not rarely the genital or the urinogenital orifice is prolonged
into a tubular papilla, which in the male acts as an intromittent organ,
or, as in the females of the Cyprinoid _Rhodeus amarus_, the long oviducal
tube serves the purpose of an ovipositor. The males and females of the
Siluroid _Plotosus_ have a remarkable vascular and glandular arborescent
appendage just behind the urinogenital papilla, the use of which is
unknown.[486]
The EGGS of different Fishes[487] exhibit considerable diversity in size
and shape as well as in the nature of their external coverings and their
mode of deposition.[488] The size of the eggs largely depends on the
quantity of food-yolk stored up in their substance for the nutrition of the
embryo: hence the eggs of Elasmobranchs, which resemble Fowls' eggs in the
superabundance of their yolk, are by far the largest. Teleostomi have much
smaller eggs. The largest Teleostean ova are those which are heavy and sink
(demersal ova); the smallest, those which are buoyant and float (pelagic
ova). Of the former, the eggs of _Gymnarchus_ are about 10 mm. in diameter;
those of the Salmon about 5 mm.; and those of some species of _Arius_, 5 to
10 mm. The eggs of the Wolf-Fish (_Anarrhichas lupus_) are about 6 mm.
Smaller demersal ova are those of the Lump-sucker (_Cyclopterus_) and
_Heterotis_, which are 2.6 and 2.5 mm. respectively. Pelagic {409}eggs are
very small, those of the Plaice, which are exceptionally large, varying
from 1.65 to 1.95 mm.
[Illustration: FIG. 234.—Different types of egg-segmentation in Fishes. A,
a typical telolecithal egg. Holoblastic and unequal segmentation in _Amia_
(B) and in _Lepidosteus_ (C). D, the meroblastic segmentation of a Teleost.
_a.p_, Animal pole; _e.m_, egg-membrane; _ma_, macromeres; _mi_,
micromeres; _n_, nucleus; _o.g_, oil globule; _p_, protoplasm; _v.p_,
vegetative pole; _y_, yolk. (From Ziegler: A, after Hertwig; B, after
Whitman and Eycleshymer; C, after Eycleshymer.)]
An egg-cell consists of living protoplasm and a nucleus, a variable
quantity of non-living food-yolk, and of certain enveloping and protective
egg-membranes. The ova of Fishes differ principally in the amount and
disposition of the food-yolk, in the character of the egg-membranes, and in
the presence or absence of special perforations in the egg-membranes for
the entrance of spermatozoa into the eggs. In the small ova of some of the
lower Chordata (_e.g._ Amphioxus), where the very small quantity of
food-yolk is uniformly distributed, and its presence affects all parts of
the egg alike, the process of segmentation which follows fertilisation
results in the transformation of the entire egg into a mass of
approximately equal-sized cells or blastomeres (Fig. 82). {410}The eggs are
therefore described as "alecithal," and the segmentation as being
"holoblastic" and "equal." On the other hand, all Fishes possess
"telolecithal" eggs, that is, ova in which the food-yolk is more or less
abundant, and tends to accumulate at one pole of the egg ("vegetative
pole"), while the opposite or "animal pole" consists of protoplasm,
comparatively free from yolk granules and containing the nucleus (Fig. 234,
A). The term telolecithal is, however, a somewhat comprehensive one, and
covers important variations in the relations of the inert food-yolk and the
living protoplasm in different Fishes, which greatly modify the process of
segmentation. Thus there are some Fishes in which the amount of food-yolk
at the vegetative pole is sufficient to retard segmentation in that part of
the egg without actually preventing it, and consequently segmentation
begins in the animal pole, and takes place more rapidly there than it does
when it extends into the vegetative pole. Hence it follows that although
the entire egg is segmented the blastomeres are of unequal size, the animal
pole giving rise to a large number of small cells or micromeres, and the
vegetative pole to a smaller number of much larger cells or macromeres. The
segmentation of such an egg is said to be holoblastic but unequal (Fig.
234, B and C). This type of egg is characteristic of the Chondrostei, the
Holostei, and the Dipnoi. In other Fishes, like the Elasmobranchs and the
Teleostei, the food-yolk so greatly preponderates that it entirely prevents
segmentation in the vegetative part of the egg, and segmentation is
restricted to the small mass of protoplasm (germinal disc) at the animal
pole, in which the nucleus is situated (Fig. 234, D). Eggs undergoing
partial segmentation in this way are termed "meroblastic." No hard and fast
line can be drawn between the two types, and in the Chondrostei and
Holostei an interesting transition between the holoblastic and meroblastic
ova may be observed. The egg-membranes are formed either by the egg itself
or by the epithelium of the ovarian ovisacs, and, as will shortly be seen,
the character of the outer egg-membrane greatly influences the mode of
deposition of the eggs and their location afterwards. In Elasmobranchs the
egg is enclosed in a stout horny egg-shell, secreted by the oviducal shell
gland.[489] In many Fishes, as in the Chondrostei, Holostei, and Teleostei,
the egg-membranes {411}are perforated at the animal pole of the egg by a
small aperture or "micropyle," which is only large enough to admit of the
entrance of a single spermatozoon at a time (Fig. 235). Generally, there is
only a single micropyle, but, according to Salensky, the Sturgeon (_A.
sturio_) has from 3 to 9, and the Sterlet (_A. ruthenus_) from 5 to 13.
An important distinction may be made between the ova of different
Teleostomi as regards their location after extrusion from the female. From
this point of view two types of ova can be distinguished, _demersal_ and
_pelagic_ ova. Demersal eggs are characterised by their larger size and
greater weight, so that they always sink after extrusion; and by their
opacity. They may either have an outer egg-membrane which is viscid and
adhesive, so that the eggs readily adhere to one another or to foreign
objects, or the membrane is smooth and non-adhesive. The Salmonidae, for
example, produce non-adhesive demersal eggs, which remain separate after
being deposited on the gravelly bed of a stream. Most freshwater and many
marine shore Fishes have adhesive demersal eggs, which are deposited at the
bottom of the water, generally adhering to one another in larger or smaller
clumps, masses, or sheets, and attached to rocks, stones, or empty shells,
like the eggs of many shore Fishes, or to aquatic plants after the fashion
of the eggs of the Carp, Perch, and Pike, or even to branching zoophytes,
as is the case with the eggs of the Sea-snail (_Liparis_). In some adhesive
eggs the external egg-membrane forms threads for their attachment. The eggs
of the Gar-Fish (_Belone_), and those of the Saury Pike (_Scombresox_) and
of the Flying Fishes (_Exocoetus_), have viscid threads developed from
opposite points on the surface, which are either attached to foreign
objects or they become entangled with those of other eggs of the same
species. The oval eggs of some of the Gobies have a bunch of fibres at one
pole which serves to attach them. In the Smelt (_Osmerus eperlanus_) a
portion of the outer egg-membrane breaks away from the rest and becomes
turned back, inside out, but remains attached to the egg at one point. By
means of this membrane the egg is attached to rocks or stones. Pelagic eggs
are distinguished by their lightness and buoyancy, so that they always
float near the surface of the water, and by their smaller size and
remarkable transparency (Fig. 235). A conspicuous feature in many of them
is the presence of a single {412}large oil globule on the surface of the
yolk, and not infrequently the yolk becomes partially or completely broken
up into small masses. Pelagic eggs are always non-adhesive and free, and
they invariably belong to marine Fishes. Amongst the British food Fishes
which produce pelagic ova may be mentioned the Gadidae (_e.g._ Cod,
Whiting, Hake, Ling), the Pleuronectidae (_e.g._ Turbot, Brill, Sole,
Plaice), Scombridae (_e.g._ Mackerel), Triglidae (_e.g._ the Gurnards),
Percidae (_e.g._ the Bass), and Clupeidae like the Pilchard and Sprat, but
not the Herring, whose adhesive demersal eggs are deposited in clumps on
shingly banks in the sea at varying distances from the shore.
[Illustration: FIG. 235.—Diagrams of the pelagic ova of a Cod or a Plaice
(A) and of a Ling (_Molva_). _G_, Germinal disc; _M_, micropyle; _O.G_, oil
globule; _Y_, yolk. (From Cunningham.)]
The eggs of Elasmobranchs are deposited singly or in pairs at considerable
intervals, and the period of egg-laying is prolonged over a considerable
part of the year. In most other Fishes, as in Teleosts, the period of
spawning is limited to a few months, usually in the spring and summer in
temperate latitudes; and in the case of a single Fish it may last only a
few days or weeks, but the number of eggs produced is often enormous. Thus,
in a Ling 61 inches long and weighing 54 pounds the ovaries contained
28,361,000 eggs. A Turbot, 17 pounds in weight, had 9,161,000 eggs; and a
Cod of 21½ pounds 6,652,000. The least prolific of the British food Fishes
is the Herring, in which the number of ovarian eggs varied from 21,000 to
47,000 in four specimens examined.[490] The extraordinary fecundity of many
Fishes seems to bear no relation to the relative abundance of the Fishes
themselves, but rather it is to be associated with certain
{413}disadvantages attendant on the sexual relations of Fishes, involving a
considerable waste of the sex-cells, while in many Fishes it no doubt helps
to compensate for any subsequent mortality among the larvae, which may
result from an uncertain and precarious food supply and from the attacks of
enemies. Whenever internal fertilisation is the rule, or when, as in
nest-building and marsupial Fishes, the propinquity of the sexes in the
breeding season ensures the fertilisation of a larger proportion of the
eggs and the protection of the young, the number of eggs produced is small.
The male sex-cells or spermatozoa are essentially similar to those of other
Vertebrates, although in different Fishes they may vary in such details as
length, and the shape and size of the head, which may be rod-like and wavy,
elliptical or globular.
As a rule, in Fishes females are more numerous than males, and generally
they are larger, but to both statements there are notable exceptions. The
relations of the sexes in the breeding season are usually very promiscuous,
especially in those Teleosts which discharge their sex-cells while swimming
together in shoals. A female may, however, consort with several males
(_polyandry_), or a male with several females (_polygamy_); or, as in some
of the nest-building Fishes (e.g. _Gastrosteus_), there are not wanting
examples of the pairing of one male with one female (_monogamy_).
Fishes often migrate at the commencement of the breeding-season to
localities most suitable for the deposition of the eggs. Many marine
species seek banks or shallower water near the shore, and some, like the
Salmon and the Sturgeon, are _anadromous_, and ascend rivers for long
distances to deposit their spawn.
In all Fishes except the Elasmobranchs and a few Teleosts the fertilisation
of the eggs takes place in the water after their extrusion, the male
depositing its seminal fluid over the eggs or in their neighbourhood. The
waste of the sex-cells is often, no doubt, very considerable, especially
when the eggs are adhesive and fixed, and the seminal fluid is liable to
drift at the mercy of tides and currents. With pelagic ova the waste is
perhaps not so great, inasmuch as the eggs as well as the spermatozoa would
probably drift at the same rate and in the same direction. Liability to
waste must also be greatly diminished in many Fishes by their habit of
living in shoals, or of congregating {414}together in the breeding season,
in which they are sometimes aided by their power of emitting characteristic
sounds, and in the case of nest-building Fishes by the still more intimate
relations of the sexes. Even when the liability to waste is very great,
compensation may be afforded by exceptional fecundity. The copulation of
the sexes and the internal fertilisation of the eggs occur only in
Elasmobranchs and some Teleosts. The copulatory organs of Elasmobranchs are
the so-called "claspers" with which the males are provided. Some form of
copulation is probably the rule in the viviparous Teleosts, where the eggs
are fertilised in the oviducts, or even while they are still in the
ovaries, and the young are born alive. As mentioned above, an intromittent
organ is often formed by the prolongation of the genital or the
urinogenital orifice into a papilla, or a longer or shorter tube.[491] Some
Cyprinodontidae[492] (e.g. _Anableps_) have the anterior part of the anal
fin modified in the male to form an intromittent organ along which the
urinogenital canal runs (Fig. 374). In the females the genital aperture is
covered by a special scale, which is free on one side and not on the other.
"The male organ in some individuals is turned to the right, in others to
the left, and in some females the opening beneath the special scale is to
the right, in others to the left. Copulation thus takes place sideways, a
left-sided male pairing with a right-sided female, and _vice versa_."[493]
The anal fin also forms an intromittent organ in the "Half-beak"
(_Hemirhamphus_). In a genus (_Girardinus_) of the same family the anal fin
is modified to form an apparatus for holding the female during sexual
congress.[494] The singular method of fertilisation practised by the males
and females of _Callichthys paleatus_ is referred to elsewhere.[495]
With the exception of the pelagic _Antennarius_, which builds its nest in
the Sargasso weed in mid-ocean, nest-building and parental solicitude for
the young are confined to freshwater Fishes and to marine forms with
demersal ova. Pelagic ova must necessarily be beyond the scope of parental
care. As a rule it is the male which acts as guardian nurse, the female
troubling herself but little about the fate of her eggs or her offspring.
{415}[Illustration: FIG. 236.—The Butter-Fish (_Pholis gunnellus_) coiling
round a mass of eggs. (From Cunningham, after Holt.)]
Perhaps the more primitive form of parental foresight is exhibited by
those Fishes which, like the females of the Salmonidae, make a furrow in
the gravelly bottom of a running stream for the reception of the eggs, and
then cover them over with a layer of gravel, or like the Siluroid _Arius
australis_, of the Burnett river in Queensland, which deposits its eggs in
circular excavations in the sandy bed of the river and covers them with
layers of large stones. But in neither case does it appear that either the
male or the female takes any further interest in the eggs or in the young
when hatched. Without actual nest-building, or even the preparation of a
place for their reception, the eggs may be protected in various ways by the
male. The common British Gunnel or Butter-Fish (_Pholis gunnellus_) rolls
its eggs into a rounded mass by coiling its body round them, the male and
female taking possession of them alternately. The little clumps of eggs are
then deposited in holes made by the boring Mollusc, _Pholas_. Some British
Blennies attach their eggs in a single layer to the sides of cavities in
rocks, or between stones, where they are watched over by the male parent.
The eggs of the Lump-Sucker (_Cyclopterus lumpus_) are attached in masses
to rocks or to piles and guarded by the male, who aerates them by keeping
up a flow of water over the spawn through the action of his pectoral fins.
When hatched, the young fry cling to the body of their watchful parent by
their suckers. A more decided approach to nest-building is exhibited by the
Sand Goby (_Gobius minutus_). In this species the male scoops out the sand
from beneath an empty shell, generally that of a Pecten, and the female
deposits her adhesive eggs on the under surface of the shell.
{416}[Illustration: FIG. 237.—Showing the embryos of _Rhodeus amarus_ in
the gill-cavities of _Unio_. _e_, Embryos; _g_, inter-lamellar cavities;
_i.l.j_, an inter-lamellar junction. (From Olt.)]
The male remains on guard, and by the movements of its pectoral fins
promotes the aeration of this rude form of nest. References to some of the
more striking examples of true nest-building in Fishes will be found in the
systematic part of this volume, especially in those chapters treating of
the Dipnoi and Amiidae, and such Teleosts as the Mormyridae,
Osteoglossidae, Siluridae, Gastrosteidae, Centrarchidae, Osphromenidae,
Labridae, and Antennariidae. Other illustrations of parental care are to be
found in the development of marsupial pouches or grooves for the reception
of the eggs in the males of the Syngnathidae (Fig. 387) and the females of
the Solenostomidae, and the use of the oral cavity for a similar purpose in
the males, rarely in the females, of some Siluridae, and the males or
females, according to the species, of the Cichlidae. The singular method by
which the female _Aspredo_ safeguards both her eggs and her progeny is
referred to on p. 596. The Cyprinoid, _Rhodeus amarus_ (the "Bitterling" of
Central Europe), is unique in the means which it adopts to {417}secure the
same result.[496] By means of its long ovipositor the female Fish deposits
its eggs in the mantle cavity of a _Unio_, or of an Anodon. Here they are
fertilised by spermatozoa carried in through the inhalent siphon of the
Mollusc with the inspiratory water current, and they complete their
development in the gill-cavities (Fig. 237).[497]
The time which elapses between the fertilisation of the egg and the
hatching out of the young Fish varies greatly in different Teleosts. The
eggs of some Clupeidae hatch in a very short time, two to three days in the
Anchovy, and three to four days in the Sprat. In most of the British marine
food Fishes the period rarely exceeds twelve to fourteen days. The larger
demersal eggs with much food-yolk are longer in hatching; in the Salmon the
time ranging from thirty-five to one hundred and forty-eight days. A low
temperature lengthens the time. The eggs of the Herring which hatched in
eight to nine days at a temperature of 52° to 58° F. took forty-seven days
in water at 32° F.
The extent to which the development of the embryo proceeds while it is
still enclosed in the egg-membranes, and consequently the condition of the
embryo when hatched, depends largely but not exclusively on the quantity of
food-yolk which is present in the egg and available for the nutrition of
the embryo during its earlier stages. Embryos hatched from pelagic ova are
very small and imperfectly developed. The mouth is usually not yet formed.
The median fins, which later become isolated, are continuous, and the
caudal fin is diphycercal, although it subsequently becomes homocercal
after passing through a heterocercal stage. The blood is colourless, and
even the gill-clefts may at first be lacking. In this condition the
newly-hatched Fish is nourished at the expense of the residual food-yolk,
which is enclosed in a yolk-sac projecting from the ventral surface of the
body (Fig. 238). As the yolk is gradually used up the mouth is formed, and
the young Fish feeds on the minute organisms of various kinds living in the
sea, and by degrees the form, proportions, and structure of the more mature
Fish are acquired. In the case of the larger demersal eggs the young are
not only longer in hatching, but when hatched they are larger and more
advanced in development. The young of many Fishes are {418}provided with
larval or provisional organs, and they may be so unlike the adult in other
respects that their subsequent development assumes the form of a more or
less striking metamorphosis. As examples of larval organs, mention may be
made of the adhesive or cement organs of the larval Chondrostei and
Holostei, and of the Dipnoi (e.g. _Protopterus_ and _Lepidosiren_), and
also of a Teleost, probably the Mormyrid (_Hyperopisus bebe_, Lacép);[498]
the cutaneous gills of the Crossopterygii and some Dipnoi; the so-called
external gills of such Teleosts as _Cobitis_, _Gymnarchus_ (Fig. 239), and
_Heterotis_, which are singularly like those of Elasmobranchs; and the
defensive spines which are developed on the scales or scutes of the trunk,
and the dermal bones of the skull, in the young of some Plectognathi. The
most striking metamorphosis to be found in Fishes occurs in the Flat-Fishes
and in the Eels, an account of which will be found in other parts of this
volume (pp. 685, 602).
[Illustration: FIG. 238.—Newly-hatched embryo Teleost from a pelagic egg.
_A_, Auditory organ; _E_, eye; _FM_, continuous median fin; _Ht_, heart;
_I_, intestine; _N_, nostril; _Yk_, yolk-sac. (From Cunningham.)]
The only examples of viviparous Fishes occur in certain families of
Elasmobranchs,[499] and in five families of Teleosts, viz. the Blenniidae,
the Cyprinodontidae, the Scorpaenidae, the Comephoridae, and the
Embiotocidae.[500] In the Teleosts mentioned the eggs are fertilised while
they are still either in the ovarian ovisacs or in the cavity of the ovary,
and their development may take place in either position. In such
Cyprinodonts as _Gambusia_ and _Anableps_ the embryos are developed in the
ovisacs, but as a rule both fertilisation and development occur in the
ovarian cavity. During a prolonged gestation the young are nourished partly
by the {419}food-yolk present in the eggs, and partly by a nutritive
secretion derived from the ovarian walls or from the epithelial wall of the
ovisacs as the case may be. In _Anableps_ the secretion of the walls of the
ovisacs is absorbed by papillae developed on the surface of the yolk-sac of
the embryo along the course of its blood-vessels. The eggs of the
Embiotocidae have little food-yolk, and the embryos are mainly nourished by
the secretion of the ovarian walls, which is swallowed by the embryo and
absorbed by villi on the inner surface of the intestine. The number of
young produced varies considerably. In the Embiotocidae the ovarian cavity
contains 40 to 50 young. The viviparous Scorpaenid, _Sebastes norvegicus_
of Northern Europe, produces, it has been estimated, about 1000 young,
while the Blenny (_Zoarces viviparus_), the only other European viviparous
Teleost, produces from 20 to 300 or more, according to the size of the
female. In the Blenny the eggs are hatched in about twenty days, but the
young are not born until about four months after fertilisation, when they
are about an inch and a half long, and in every outward respect similar to
the adult Fish.
[Illustration: FIG. 239.—Young _Gymnarchus niloticus_, with its large
yolk-sac (_y.s_) and its long external gills (_e.g_). (From Budgett.)]
Besides the distinction between the sexes resulting from the different
nature of their gonads and sex-cells, the males and females are often
distinguished by secondary sexual characters {420}("sexual
dimorphism"[501]). As mentioned above, females are usually larger as well
as more numerous than the males, although in one or both respects the
reverse may be the case. Secondary sexual characters are best marked in
Teleosts, where they are generally related to the special rôle which each
sex takes in the deposition and fertilisation of the eggs, and the nurture
and protection of the young, of which examples have already been given. To
a more limited extent they may be associated with the struggle of the males
for the females, and in at least a few Teleosts the exuberant coloration of
the males in the breeding season suggests that instances of courtship and
sexual selection are not altogether wanting.[502]
Although the vast majority of Fishes are dioecious, instances of functional
hermaphroditism are not unknown in a few Teleosts.[503] Species of the
Percoid genus _Serranus_ (e.g. _S. cabrilla_, _S. hepatus_, and _S.
scriba_) are invariably hermaphrodite and self-fertilising. _Chrysophrys
auratus_ is an example of successive hermaphroditism, the male and female
sex-cells ripening alternately. As an occasional variation hermaphroditism
has been recorded in several other Teleosts, including amongst others such
well-known Fishes as the Cod, the Mackerel, and the Herring. The relations
of the gonads in hermaphrodites is subject to much variation. In the Cod,
for example, the testes may be double, each being continuous with the
hinder end of the ovary of its side, or there may be only a single testis
confluent with the anterior or the posterior portion, or with some other
part of the surface, of either the right or left ovary. In other Teleosts
individuals occasionally present themselves with a testis and an ovary on
opposite sides.
{421}CHAPTER XVI
CYCLOSTOMATA (SYSTEMATIC)
CLASS I. CYCLOSTOMATA
The Cyclostomata, or, as they are sometimes called, the Marsipobranchii,
from the pouch-like, or rather sac-like, shape of their branchial clefts,
are divided into two orders, the first comprising the "Hag-Fishes" or
"Borers," and the second the Lampreys.
ORDER I. MYXINOIDES.
The Hag-Fishes are probably the most primitive of all existing Craniates.
The mouth is nearly terminal, and there is no buccal funnel. The
naso-pituitary involution communicates behind with the oral cavity and
functions as a channel for the in-streaming water-current to the gills.
Four pairs of short tentacles, supported by a special tentacular skeleton,
are present in relation with the mouth and the terminally-placed
naso-pituitary orifice. The gill-sacs open directly into the pharynx. The
branchial basket is but feebly developed, and at the most it is only
represented by small isolated cartilages in relation with the external
branchial apertures. The lingual apparatus is remarkably developed. Besides
the lingual teeth there is only a single dorsal tooth in the roof of the
mouth. The dorsal arcualia are restricted to the tail, or they extend for a
short distance only into the trunk. A spiral valve is absent. There is a
row of mucus-secreting sacs along each side of the body. The brain has no
obvious cerebral hemispheres, nor a cerebellum. Only one semicircular canal
is present in the auditory organ. The eyes are degenerate, and the usual
eye-muscles with the cranial nerves {422}supplying them have atrophied. The
embryonic pronephros is retained in the adult. The eggs are large;
segmentation is meroblastic; and development is direct, without a larval
metamorphosis. Two families can be distinguished.
FAM. 1. MYXINIDAE.—Gill-sacs not exceeding six pairs, with a common
external aperture on each side of the body.
[Illustration: FIG. 240.—_Myxine glutinosa_. A, lateral view; B, view of
the ventral surface of the head, showing the mouth and tentacles. _l.l.p_,
Lateral pore-like apertures of the mucus-sacs; _v_, anus.]
The family includes a single genus, _Myxine_, of which the common Hag (_M.
glutinosa_) from the North Atlantic is the best known species (Figs. 92, A,
and 240). This Hag-Fish occurs off the coasts of Northern Europe, including
the British Isles, as well as on the Atlantic sea-board of North
America,[504] southwards to Cape Cod. Other species are found off the
coasts of Chili and Japan. _Myxine_ is quasi-parasitic in its habits,
boring its way into the bodies of large Fishes. By means of its rasping
"tongue" it devours all the soft parts of its prey, leaving little more
than a mere shell of skin and bones. The Fishes usually attacked are the
Cod and other Gadoids, but the Sturgeon is not immune, and the presence of
a Hag in the abdominal cavity of a Shark (_Lamna cornubica_) has been
recorded. _Myxine_ has the reputation of being very destructive to Fishes
caught on lines, and it is said that whole "catches" have been destroyed by
its depredations, so that North Sea fishermen have been forced to change
their fishing-ground. To what extent the Hags attack Fishes which are
living and free is somewhat uncertain, but the little evidence obtainable
seems to point to the conclusion that, as a rule, they only prey on Fishes
when the latter are hooked or netted, or injured or dead. When not seeking
food the Hag lives {423}in the mud of the sea-bottom at depths ranging to
nearly 350 fathoms. They are able to swim very rapidly in an undulatory
eel-like fashion. _M. glutinosa_ may grow to a length of nearly two feet.
The Hag has been described as a protandrous hermaphrodite, that is, it is
first a male and then a female, the gonad of the young first producing
spermatozoa, and at a later period becoming an ovary and giving rise to
eggs. This view has hitherto met with general acceptance, but it has
recently been urged with some force that the presence of the two kinds of
sex-cells in a young animal is no proof of functional hermaphroditism,
since it is not uncommon "to find immature eggs in the testis of many
Vertebrates (Teleosts, _Petromyzon_, Amphibia), where the assumption of
hermaphroditism, to say nothing of its protandric form, is entirely
unwarranted."[505] _Myxine_ produces eggs similar to those of
_Bdellostoma_. Nothing is known of its breeding habits, or of its
embryology.
FAM. 2. BDELLOSTOMATIDAE.—Gill-sacs 6-14 pairs, all with separate external
orifices. _Bdellostoma_ (Fig. 92, B) is found on the Pacific sea-board of
both North and South America, at the Cape of Good Hope, and on the coasts
of New Zealand. The numerical variation of the gill-sacs in different
species, and in different individuals of the same species, and even on
opposite sides of the same individual, is very remarkable. Out of 354
examples of the Californian species (_B. stouti_) examined by Dr.
Ayres,[506] 101 had 11 gill-sacs on each side; 26 had 11 on one side and 12
on the other; 208 had 12 on each side; 11 had 12 on one side and 13 on the
other; and 8 had 13 on each side. Occasional specimens may have 14
gill-sacs on each side. The variations are apparently quite independent of
size, age, or sex; and when the gill-sacs are asymmetrically developed, the
additional sac may be either on the right side or on the left. In the
Chilian species there are 10 gill-sacs on each side, but in the species
from the Cape of Good Hope the number is reduced to 6 or 7. _Bdellostoma_
closely resembles _Myxine_ in its habits and mode of feeding. The
Californian species attaches itself to the gills or to the isthmus of large
Fishes, and then rapidly bores its way into the body, devouring the viscera
and muscles but leaving the skin intact. It usually attacks large
{424}Flounders and species of _Sebastodes_, and it is especially
destructive to Fishes taken in gill-nets. At Monterey every net in the
summer contains the empty shells of eviscerated Fishes, and when these are
taken out of the water the Hag scrambles out with great alacrity. Large
fishes of even 30 pounds weight are often captured without either flesh or
viscera, and it cannot be supposed that they entered the net in this
condition.[507] The species lives on the sea-bottom most abundantly at a
depth of 10-20 fathoms, but becomes rarer as the water deepens or becomes
shallower.
[Illustration: FIG. 241.—A, Cluster of the eggs of _Bdellostoma stouti_,
connected by the interlocking of their anchor-shaped filaments; B, the
animal pole of an egg, showing the polar "anchors" and the opercular ring.
(From Bashford Dean.)]
The eggs of the Californian _Bdellostoma_ are large, varying in size from
14.3-29 mm. in length, and from 6.8-10.5 mm. in width, and each egg is
enclosed in a horny egg-case secreted by the epithelium of its ovarian
ovisac[508] (Fig. 241). At each pole of the egg-case there is a tuft of
numerous horny filaments which end in 2- 3- or 4-hooked, anchor-like
extremities. In the centre of the tuft of filaments at the animal pole of
the egg the egg-case is perforated by a micropyle, and a little below this
{425}point the case is encircled by an opercular groove, which enables the
polar portion to be thrown off like a cap at the time of hatching, so as to
allow the young _Bdellostoma_ to make its escape. The large size of the
egg, which almost completely fills the cavity of the egg-case, is due to
the fact that it consists mainly of food yolk, the germinal protoplasm
containing the nucleus forming only a small hillock near the inner
extremity of the micropyle. _Bdellostoma_ spawns during the greater part of
the year, but chiefly in the early summer, and probably about 20 eggs are
deposited at one time, generally on a shelly or rocky bottom. After
deposition the eggs become connected together in long chains or clusters by
the interlocking of their polar hooks. Fertilisation takes place after
extrusion, and the segmentation is meroblastic and discoidal, much as in
Teleosts. The embryo completes its development within the egg, and when
hatched it is a miniature of the adult.
[Illustration: FIG. 242.—Embryo of _Bdellostoma stouti_ near the time of
hatching. (From Bashford Dean.)]
ORDER II. PETROMYZONTES.
In the Lampreys there is a large suctorial buccal funnel leading behind and
above into the mouth, which is supported by special cartilages, and
furnished with a marginal fringe of small cirri. Numerous horny teeth are
present on the inner surface of the funnel as well as on the tongue. The
naso-pituitary involution forms a caecum and does not communicate with the
mouth. The gill-sacs, seven in number, open externally by separate
orifices, but internally they open into a median branchial canal, situated
below the oesophagus and opening into the mouth in front. There is a
well-developed branchial basket. Dorsal arcualia are present throughout the
precaudal as well as in the caudal region. A rudimentary spiral valve is
present. The brain consists of parts usually present in other Craniates,
including cerebral hemispheres and a cerebellum. The auditory organ
{426}has two semicircular canals, and the eyes are not degenerate. The
pronephros is suppressed in the adult. The eggs are small; the segmentation
is holoblastic; and there is a larval metamorphosis. There is but one
family.
FAM. 1. PETROMYZONTIDAE.—The family has a nearly world-wide distribution.
Most Lampreys are marine, although to a greater extent in some species than
in others, but all of them seem to ascend rivers for spawning. The genus
_Petromyzon_ is characteristic of the northern hemisphere, where it is
represented by various species on the coasts and in the rivers of Europe,
West Africa, Japan, and North America. Three species, widely distributed in
Europe, occur in the British Isles, viz.:—the Sea-Lamprey (_Petromyzon
marinus_), which may reach or even exceed three feet in length, and is also
found on the west coast of Africa and on the Atlantic coast of North
America; the "Lampern" or fresh-water Lamprey (_P. fluviatilis_), about 18
inches long; and the Sand-Pride, Sand-Piper, or lesser freshwater Lamprey
(_P. planeri_), usually less than a foot in length. _Ichthyomyzon_,
_Bathymyzon_, _Entersphenus_, and _Lampetra_ are also northern forms,
collectively distributed along the Atlantic and Pacific coasts and in the
rivers and great lakes of North America.[509] Other Lampreys occur only in
the southern hemisphere. _Geotria_ is common in the rivers of Chili,
Australia, and New Zealand; and another genus, _Mordacia_, has a parallel
distribution, being found on the coasts of Chili and Tasmania. A new genus
and species from Chili has been recently described under the name of
_Macrophthalmia chilensis_.[510] This Lamprey, which is only 107 mm. in
length, has remarkably large eyes (2.5 mm. in diameter), vertically
compressed gill-clefts, and a simple dentition resembling that of _Myxine_.
All Lampreys are carnivorous. They feed by attaching themselves to the
bodies of Fishes by their suctoral buccal funnels, and then rasping off the
flesh with their lingual teeth. While thus engaged they are carried about
by their victims. Salmon have been captured in the Rhone with the marine
Lamprey attached to them. The Lamprey usually keeps near the bottom, either
swimming with a graceful serpentine movement, or attached to stones by the
buccal funnel.
{427}[Illustration: FIG. 243.—Spawning of the Brook-Lamprey (_P. wilderi_).
On the right side of the figure a male is attached to the head of a female.
(From Bashford Dean and F. B. Sumner.)]
In the spring the Sea-Lamprey ascends the rivers to spawn, and, after
depositing its eggs in furrows which it excavates in the river-bottom, it
returns to the sea. The river-Lampreys spawn in the smaller streams and
brooks. The North American Brook-Lamprey, _Petromyzon_ (_Lampetra_)
_wilderi_, which is found in the neighbourhood of New York, deposits its
eggs on the gravelly bottom of a brook, in a small gravel-filled hole lying
between a number of large rounded stones[511] (Fig. 243). In the vicinity
of the "nest" some ten to twelve Lampreys congregate, the males, however,
being much more numerous (five to one) than the females.
{428}[Illustration: FIG. 244.—Head of the Ammocoetes of _P. fluviatilis_.
A, ventral view; B, side view. _br.1_, First branchial aperture; _eye_,
eye; _l.l_, lower lip; _na.ap_, naso-pituitary aperture; _u.l_, upper lip.
(From Parker and Haswell, after W. K. Parker.)]
Much energy is spent by both sexes in moving stones by lifting them with
the buccal funnel, but it is not always clear that this is done to
circumscribe the nest, or to remove impeding obstacles. Eventually, a male
attaches himself to the back of the head of a female, who at the same time
is holding fast to a stone. The male then rotates its body so that the
urino-genital papilla is brought near the genital orifice of the female,
and the simultaneous extrusion of eggs and spermatozoa at once follows.
Owing to the small amount of food-yolk which they contain the eggs of the
Lamprey (e.g. _P. planeri_) are small, measuring about 1.1-1.2 mm. in
length, and from 0.9-1.0 mm. in width. There is a micropyle at the animal
pole of the egg, but the characteristic horny egg-case and the polar hooks
of the Myxinoids are both wanting. The embryo hatches out as a larva known
as the "Ammocoetes." At this stage of its development the larva lacks
several of the most striking features which characterise the adult, and it
is highly probable that the Ammocoetes represents a stage in the evolution
of Vertebrates in some respects intermediate between _Amphioxus_ and a very
primitive Craniate. The mouth of Ammocoetes is bounded laterally and in
front by a curious hood-like upper lip, and behind by a short transverse
lower lip (Fig. 244). The eyes are deeply seated and rudimentary, and as
visual organs they are useless, but the parietal eye is well developed. As
in the adult, there are seven pairs of gill-sacs, but they open internally
into a pharynx, directly continuous behind with the rest of the alimentary
canal, and there is no dorsal oesophagus. Like the skull, the branchial
basket is still very rudimentary. The dorsal and caudal fins are
{429}continuous. A gall-bladder is present, and also a bile duct opening
into the gut. In its mode of life, and especially in the manner in which it
obtains its food, Ammocoetes presents a most remarkable resemblance to
_Amphioxus_ and the Ascidians. In the median line of the pharyngeal floor
there is an open groove, the hypopharyngeal groove or endostyle, and a
tract of ciliated cells along the dorsal wall represents a hyperpharyngeal
groove: connecting the two in front there is a peripharyngeal ciliated
groove.[512] The Ammocoetes feeds on small food particles carried through
the mouth into the pharynx by currents of water produced by ciliary action.
The food becomes entangled in strings of mucus probably secreted by the
cells lining the endostylar groove. The mucus is then swept upwards in the
pharyngeal groove, and finally wafted backwards to the stomach and
intestine by the cilia of the hyperpharyngeal band. The skin exhibits the
remarkable peculiarity of containing a peptic ferment capable of digesting
proteids in a .2 per cent solution of hydrochloric acid. As the larva lives
buried in the mud, the epidermic secretion probably helps to keep the skin
free from bacteria, microscopic spores, and fungoid, or other parasitic
growths.[513] The young Lamprey lives as an Ammocoetes from 3-4 years, and
then in the course of a few weeks in the winter it undergoes a
metamorphosis, losing its larval characters and acquiring the structure and
habits of the adult. During this period the buccal funnel is completed and
teeth are developed. The eyes approach the surface and become functional.
The continuity of the median fins becomes interrupted. The endostylar
groove becomes transformed into a thyroid gland, the gall-bladder
disappears, and the bile duct becomes obliterated and changed into a mass
of small follicles. The skull and branchial basket complete their
development. At the same time the pharynx loses its connection with the
rest of the alimentary canal and remains as the branchial canal. The
so-called oesophagus of the adult is apparently a new formation which grows
forwards and acquires a connection with the mouth. It is probable that it
represents a hyperpharyngeal groove constricted off from the dorsal wall of
the pharynx.
Both the marine Lamprey and the "Lampern" are captured {430}for food,
either by nets or wicker traps. Formerly the Lampern was taken in enormous
numbers in several British and Irish rivers, especially in the Severn from
February to May, and in the Thames during May and June, but for various
reasons the supply has much diminished in recent years. The Lampern makes
excellent bait for Cod and Turbot, and for this purpose large numbers used
to be taken in the Trent and Thames for despatch to Grimsby and other
fishing ports.[514]
{431}CHAPTER XVII
ELASMOBRANCHII: GENERAL CHARACTERS—PLEUROPTERYGII—ICHTHYOTOMI—ACANTHODEI—
PLAGIOSTOMI—SELACHII—BATOIDEI—HOLOCEPHALI
CLASS II. PISCES.
SUB-CLASS I. ELASMOBRANCHII.
In both the ancient and the modern Sharks, Dog-Fishes, and Rays, the
exoskeleton takes the form of a more or less uniform investment of dermal
denticles or "shagreen." The endoskeleton is wholly cartilaginous or
partially calcified, and there are neither cartilage- nor membrane-bones.
The vertebral column is acentrous or chordacentrous, generally with
alternating basi- and inter-dorsal elements, and terminating in a
heterocercal tail. The skull is usually hyostylic, very rarely amphistylic
or autostylic, and the lateral halves of the primary upper jaw
(palato-quadrate cartilages) usually meet in a highly characteristic median
symphysis beneath the base of the skull. Branchial arches and clefts are
five to seven in number, and the clefts are separated by complete
interbranchial septa, which, as a rule, are continuous externally with the
skin. An operculum is developed only in the Holocephali. A pelvic girdle is
present. With rare exceptions the pectoral fin is uniserial. The pelvic fin
is invariably uniserial. The exoskeletal supports of all the fins consist
of ceratotrichia, and, when present, the fin-spines are invested by enamel.
Claspers are generally present in the males.
In the surviving members of the group the nostrils retain their primitive
ventral position. There is a conus arteriosus with several rows of valves.
A spiracle, often furnished with a spiracular pseudobranch, is generally
present, and, as a rule, {432}there is a hyoidean hemibranch supplied with
venous blood from the ventral aorta. The gill-filaments are attached
throughout their length to the interbranchial septa. There is an optic
chiasma. An air-bladder is not developed. The intestine has a spiral valve,
and there is a cloaca. The gonoducts in both sexes are derived from the
kidney system. The ova are large, few in number, and enclosed in horny
egg-cases, and they are fertilised before extrusion. The segmentation is
meroblastic, and the embryo is furnished with long external gills.
The Elasmobranchs are for the most part active predaceous Fishes, living at
different depths in the sea, from the surface to nearly a thousand fathoms,
and ranging from mid-ocean to the shallower waters round the coasts in
almost every part of the world. Although typically marine, they sometimes
ascend rivers beyond the reach of tides, and a few are permanent
inhabitants of fresh water. They are most abundant in tropical and
subtropical areas, where they also attain their greatest size, and are
numerous in temperate regions, but there are some species which are
typically Arctic. None of them are small, and some of the Sharks are the
largest of living Fishes. All are carnivorous, but so diversified is their
food that in different species it may range from other Fishes of no mean
size to Molluscs, Crustaceans and other Invertebrates, or even to plankton.
In their breeding habits the Sharks and Dog-Fishes present many interesting
features. Unlike the generality of Fishes, the eggs are fertilised
internally as a sequel to the copulation of the sexes. For this purpose the
males are furnished with special intromittent organs, the myxopterygia or
so-called claspers, which are developed as modifications of the hinder
portions of the pelvic fins.[515] Each clasper is supported by an internal
skeleton, consisting of several cartilages derived from the radialia of the
fins, and is traversed along its inner aspect by a groove. When sexual
congress takes place the claspers are thrust through the cloaca of the
female into the oviducal orifices, and in some instances it is probable
that they are retained in this position by hook-like denticles developed at
their free extremities. The seminal fluid then flows along these conduits
into the oviducts, in the upper portions of which it meets and impregnates
the eggs. After fertilisation the egg is enclosed in a dark brown horny
egg-case, secreted by the oviducal gland.
{433}[Illustration: FIG. 245.—Egg-case of _Heterodontus_ (_Cestracion_)
_galeatus_. (From Parker and Haswell, after Waite.)]
As a rule each egg-case has but a single egg, but in _Rhinobatus_ and
_Trygonorhina_ (Batoidei), both of which are viviparous, each case contains
three to four eggs. Generally the egg-cases are somewhat quadrangular in
shape, with the four angles, two at each end, prolonged either into short
horns, or into long tapering tendrils (Fig. 246). The oval egg-cases of the
Heterodontidae are remarkable not only for their size, but also for the
presence of a broad spiral lamina winding round the exterior of the case
from one pole to the other (Fig. 245). The majority of the Sharks,
Dog-Fishes, and Rays are viviparous, that is, the young are born alive; the
rest, like the Scylliidae (_e.g._ the common British Dog-Fishes, _Scyllium
canicula_ and _S. catulus_), the Heterodontidae, and the Raiidae are
oviparous, that is, the young are hatched out after the extrusion of the
eggs. In the oviparous species the eggs are extruded either singly or in
pairs, and generally deposited on the sea-bottom. When, however, the
egg-cases are provided with tendrils, as, for example, in the two British
Dog-Fishes just mentioned, these organs act as anchoring filaments. When
extruding an egg, the female swims round and round some piece of upright
seaweed, and the curling tendrils become entwined round it in such a way
that the egg becomes securely attached thereto (Fig. 246).[516] The embryos
are long in developing, and in _Scyllium_ it may be several months after
fertilisation (200 to 275 days) before they are hatched, the young Fish
finally escaping through a rupture in the egg-case.
{434}[Illustration: FIG. 246.—Egg of the Spotted Dog-Fish (_Scyllium
canicula_), showing its mode of attachment after extrusion. (From Hertwig,
after Kopsch.)]
In the oviparous species the nutritive food-yolk stored up, first in the
egg and subsequently in the yolk-sac (Fig. 248), suffices for the
nourishment of the embryo until the period of hatching, but in viviparous
forms, whose embryonic development is completed within special uterine
dilatations of the oviducts, additional means of nutrition are provided for
the young. Such Elasmobranchs as _Spinax_, _Acanthias_, _Centrina_,
_Scymnus_, _Trygon_, _Torpedo_, and _Myliobatis_ have long filaments (villi
or trophonemata) developed from the inner surface of the uterus, which
secrete a nutritive fluid, and this fluid is either absorbed by the
blood-vessels of the embryonic yolk-sac, or it is taken up by the embryo in
some more direct manner. In some of the Trygonidae and Myliobatidae of the
Indian Ocean it seems probable that the secretion is taken into the
alimentary canal of the embryo either through the mouth or through the open
spiracles.[517] One species, _Pteroplatea micrura_, has its long and highly
vascular and glandular trophonemata gathered into two bundles, which are
thrust through the huge spiracles into the pharynx of the embryos, of which
there may be from one to three, and the nutritive secretion is apparently
digested in the alimentary canal of the embryo and absorbed by the foetal
blood-vessels (Fig. 247). A few Sharks, like most species of _Mustelus_,
develop a placenta when the food-yolk in the yolk-sac of the embryo is
nearly used up. Folds or projections from the highly vascular wall of the
yolk-sac interlock with similar vascular folds of the lining membrane of
the uterus, and a diffusion of nutrient material takes place from the
maternal blood in the uterine blood-vessels to the foetal blood in the
{435}vessels of the yolk-sac.[518] Each embryo has its own placenta, and in
_Mustelus antarcticus_ the uterine portion of the oviduct is divided by
septa into several chambers, each containing a single embryo.[519] It is
worthy of note that in the viviparous species a distinct but very thin,
delicate egg-case is formed, occasionally even with the rudiments of
tendrils, which may either be retained or thrown off in the uterus. The
Greenland Shark (_Laemargus borealis_) is unique amongst Elasmobranchs. Its
eggs are small and unprotected by egg-cases, and their fertilisation is
said to be effected in the water after deposition, as in the generality of
Fishes.
Fossil remains of Elasmobranchs in the shape of fin-spines
(ichthyodorulites) and dermal denticles, associated with various
Ostracodermi (Coelolepidae, Pteraspidae, and Cephalaspidae), are amongst
the earliest undoubted indications of Vertebrate life. They first appear in
the Upper Ludlow Bone Bed and in Silurian rocks in other parts of Europe,
and in North America; and in greater or less abundance the group is
represented in almost every subsequent geological period. It cannot be said
that the group shows signs of decadence, for Elasmobranchs still survive in
apparently undiminished numbers and variety in the marine fauna of the
present day.
[Illustration: FIG. 247.—Embryo of an Indian Sting Ray (_Pteroplatea
micrura_) as seen when the uterus is laid open. _t. t_, Two bundles of
trophonemata inserted into the spiracles, _sp_, _sp_. (From Wood-Mason and
Alcock.)]
The Elasmobranchs are certainly a very primitive race of Fishes. Their
earliest representatives of whose structure we have any precise knowledge
(e.g. _Cladoselache_ and _Pleuracanthus_) are in many respects the most
archaic of known gnathostomatous {436}Craniates, and from such types as
these, amongst others, we may reasonably look for the ancestors of all or
most of the remaining groups of Fishes. It has been well said of
_Pleuracanthus_ that "it is a form of Fish which might with little
modification become either a Selachian, Dipnoan, or Crossopterygian,"[520]
while the condition of the primary upper jaw in the Chondrostean _Polyodon_
suggests that even the more primitive Actinopterygii had an Elasmobranch
origin. The important researches of Dr. Traquair render it also highly
probable that the ancient Ostracodermi may claim kinship through their
Coelolepid ancestors with some primitive type of Elasmobranch; and within
the limits of the group there is ample evidence that differentiation has
taken place on many divergent lines, of which we have notable examples in
such specialised offshoots as the Acanthodei and the Holocephali, to say
nothing of several highly specialised families which became extinct at
successive periods in the history of the group.
[Illustration: FIG. 248.—An embryo Shark, with its yolk-sac (_y.s_); _sp_,
spiracle.]
ORDER I. PLEUROPTERYGII.
The only certain representative of this group is the Palaeozoic form
_Cladoselache_, probably the most primitive Elasmobranch at present known
(Fig. 249). Elongated and somewhat cylindrical in shape,
_Cladoselache_[521] has a terminal mouth, five or possibly seven pairs of
branchial clefts, and a pair of olfactory organs, lateral in position near
the extremity of the snout. Wide-based, triangular pectoral and pelvic
fins, a low anterior and a posterior dorsal fin, devoid of spines, and a
heterocercal caudal fin with homocercal tendencies, are present, but no
anal fin has yet been detected.
{437}[Illustration: FIG. 249.—Restoration of _Cladoselache fyleri_. Lateral
and ventral views. (From Parker and Haswell, after Dean.)]
The exoskeleton consists of minute lozenge-shaped denticles, which invest
the body and extend on to the surfaces of the fins, and there is also a
circumorbital ring of several concentric rows of small square plates. A
lateral line, in the form of a groove between two rows of denticles,
extends along each side of the body. The notochord is persistent. Calcified
neural and haemal arches (basidorsals and basiventrals) have been observed
in the caudal region, where they correspond numerically with the remains of
the myotomes, but interdorsal or intercalary arcualia seem to be absent.
The upper and lower jaws, similar in size and shape, are apparently
supported by a hyomandibular cartilage; hence the skull is hyostylic. The
endoskeletal supports of the pectoral, and especially those of the pelvic
fins, exhibit a more primitive disposition than in any other Fishes. They
extend nearly to the distal margins of the fins, where they seem to
interdigitate with the proximal ends of feebly-developed ceratotrichia
(Fig. 145). The extension of the fins in the horizontal plane, the gradual
shading off of their broad bases into the sides of the body, and the
resemblance between their radialia and those supporting median fins, are
very suggestive of the origin of the paired fins from continuous lateral
fin-folds. Claspers are absent. The dentition is well developed, and
several rows of teeth seem to be functional at the same time. {438}Each
tooth consists of a broad base, supporting a long pointed central cusp and
a variable number of similarly shaped but much shorter lateral cusps. The
teeth in the various transverse rows from without inwards are closely
wedged together by the interlocking or overlapping of their bases.
FAM. 1. CLADOSELACHIDAE.—Several species of _Cladoselache_, varying from 2
to 5 feet in length, have been found in the Cleveland Shale (Upper
Devonian) of Ohio. Isolated teeth similar to those of _Cladoselache_ occur
in the Lower Carboniferous of Europe, India, and North America, and have
been referred to various species of the genus _Cladodus_, but with one
exception nothing more is known of the structure of these Fishes, and
consequently their relationship to _Cladoselache_ is doubtful. _C.
neilsoni_,[522] from the Lower Carboniferous (Calciferous Sandstones) of
Kilbride in Scotland, has a very different type of pectoral fin, which
appears to be distinctly uniserial, but intermediate in structure between
the biserial fin of _Pleuracanthus_ and that of the modern sharks. There
are several other genera from the Devonian and Lower Carboniferous whose
claims to inclusion in this group rest on no better foundation.
ORDER II. ICHTHYOTOMI.
While more specialised than the Pleuropterygii the Fishes included in this
group represent an extremely generalised type of Elasmobranch, which, as
already indicated, may easily have been the ancestor of more than one group
of Fishes. In the typical genus _Pleuracanthus_[523] (Fig. 250)[524] the
body is elongate, but slightly depressed, with a terminal mouth, and a
tapering diphycercal tail fringed above and below by a continuous caudal
fin. A long dorsal fin, two small anal fins, and well-developed paired fins
with contracted bases, are present. The head is armed with a prominent,
serrated, dorsal spine, but it is doubtful if dermal denticles (shagreen)
are present. The vertebral column {439}is acentrous, and the persistent
notochord supports a series of basidorsal cartilages, which alternate with
small interdorsals, a series of basiventrals supporting small ribs, and in
the caudal region well-developed haemal arches. The dorsal fin is supported
by slender, tri-segmented radialia, which appear to be twice as numerous as
the neural arches in the trunk; but in the dorsal lobe of the caudal fin
the two structures agree in number. Ventrally-prolonged haemal spines are
the sole endoskeletal supports of the inferior lobe of the caudal. The
coraco-scapular cartilages of opposite sides remain distinct, and each
supports a biserial fin. The pelvic girdle is represented by a pair of
small cartilages supporting the basipterygia. The pelvic fins are
uniserial, with post-axial skeletal supports for claspers in the males.
Both the median and the paired fins are provided with marginal
ceratotrichia. The skull is probably amphistylic. Five, possibly six or
seven, branchial arches, bearing clusters of minute denticles, are present.
Circumorbital plates are wanting. All the endoskeletal structures are
partially calcified. The teeth are tricuspid, each with two long divergent
lateral cusps and a minute median cusp; the broad bases of the teeth
overlap and articulate with one another by means of facets.
[Illustration: FIG. 250.—Restoration of _Pleuracanthus ducheni_. A',
Ventral fin; B, basal cartilages of the paired fins; D, ceratotrichia; DS,
head-spine; HA, haemal arches; _HM_, hyomandibular; IC, interdorsal
cartilages; MC, Meckel's cartilage; N, notochord; NA, neural arch; P,
supposed pelvic cartilage; the triangular cartilage behind it is the
basipterygium; PQ, palato-quadrate; _R_, radialia of the paired fins; _R′_,
rib; _RB_, radialia of the dorsal fin; _SG_, shoulder-girdle. (From Parker
and Haswell, after Dean.)]
{440}FAM. 1. PLEURACANTHIDAE.—The single family included in the group
ranges from the Lower Carboniferous to the Lower Permian. Within these
limits the family is widely distributed in different formations in Great
Britain, Continental Europe, New South Wales (Lower Hawkesbury Formation),
and North America. _Pleuracanthus_, of which complete skeletons and skulls
have been found, is the best known genus.
ORDER III. ACANTHODEI.
The Fishes comprising the Acanthodei[525] may be regarded as a highly
specialised and terminal offshoot from some primitive race of early
Elasmobranchs. The Elasmobranch kinship of the Acanthodei is indicated by
their exoskeleton of shagreen tubercles; the completely heterocercal tail;
the absence of an operculum, the external gill-clefts apparently being
exposed; the position of the lateral line of the trunk between two rows of
shagreen denticles; the nature of the powerful spines in connexion with the
dorsal and anal, and the pectoral and pelvic fins; and the formation of the
hard parts of the skeleton, not by ossification involving the presence of
bone-cells, but by the calcification of cartilage, or of more superficial
membranous or fibrous tracts. On the other hand, it may be noted that the
Acanthodei appear to have undergone much specialisation on lines in some
respects parallel to those which have marked the evolution of the
Teleostomi, but by methods which are simply an exaggeration of features
normally characteristic of Elasmobranchs. Perhaps the most striking
illustration of this is to be seen in the development of a species of
secondary skull by an extension of a process of calcification as
distinguished from ossification. Hence the presence of
membrane-calcifications in relation with the upper and lower jaws, whose
development is proportional to the size of the teeth they support, and of
smaller investing plates of the cranial roof. Similar exoskeletal
calcifications, when most completely developed (e.g. _Diplacanthus_), form
a dorsally incomplete arch, apparently corresponding to a secondary
pectoral girdle for the support of the stout pectoral spines, in which
elements {441}analogous to clavicles or cleithra and infra-clavicles can be
recognised. Each pectoral spine forms the preaxial margin of the fin, and
behind it there is a series of ceratotrichia. Nothing is known of the
endoskeletal supports, but having regard to the nature and proportions of
the pectoral spines it may be inferred that the exoskeletal elements of the
fins predominate over the former to an extent which is only paralleled
elsewhere in the Teleostei.
Apparently the notochord is persistent, and there are long and slender
neural and haemal arches, but no ribs. The dermal denticles are uniform in
size, and so small as to give a granular appearance to the skin. In
structure they are thick, with a flat, enamelled, often sculptured,
external surface, quadrate or rhombic in shape, and fitting closely
together. Teeth are either absent or very minute, but sometimes (e.g.
_Acanthodopsis_ and _Ischnacanthus_) they are few in number and large,
conical in shape, occasionally with minute cusps between the larger teeth.
Claspers are absent. The Acanthodei are small Fishes, most of them being
less than .3 m. in length, and ranging from the Upper Silurian to the Lower
Permian inclusive. Two families are recognised.
FAM. 1. DIPLACANTHIDAE.—Two dorsal fins are present. Usually there is a row
of lateral spines extending along each side of the body between the
pectoral and pelvic fins. Exclusively Upper Silurian and Devonian.
The genera _Diplacanthus_, _Climatius_, _Parexus_, _Euthacanthus_, and
_Ischnacanthus_ are all found in the Lower Old Red Sandstone of Scotland.
_Climatius_ and _Diplacanthus_ are also represented in the Devonian of
Canada.
[Illustration: FIG. 251.—Restoration of _Acanthodes wardi_. Carboniferous
of England and Scotland. (From Smith Woodward.)]
FAM. 2. ACANTHODIDAE.—A single dorsal fin; lateral spines vestigial or
absent. Lower Devonian to the Lower Permian.
{442}The widely-distributed genus _Acanthodes_ (Fig. 251) is represented in
the Lower Old Red of Scotland, the Devonian of Siberia and Canada, the
Carboniferous of England and Scotland, and the Lower Permian of France,
Germany, and Bohemia. _Acanthodopsis_ (Coal Measures), and _Mesacanthus_
and _Cheiracanthus_ (Lower Old Red) are the remaining genera.
ORDER IV. PLAGIOSTOMI.
Head prolonged in front of the ventrally-situated mouth as a more or less
prominent preoral rostrum, vertebral column consisting of alternating basi-
and inter-dorsal cartilages, generally supported by more or less
well-developed chorda-centra. Pectoral and pelvic fins uniserial. Pelvic
girdle and claspers present. Except in two families the branchial arches
and clefts are invariably five in number. An operculum is not
developed.[526]
SUB-ORDER 1. SELACHII.
Body elongate or fusiform, shading imperceptibly into a powerful swimming
tail. Pectoral fins of moderate size, with contracted bases; not confluent
with the sides of the head. Branchial clefts lateral in position. Vertebral
centra generally asterospondylic or cyclospondylic.
This sub-order includes such typical Elasmobranchs as the modern Sharks and
Dog-Fishes as well as numerous fossil representatives ranging from the
Carboniferous, and probably from still earlier periods, to the present day.
FAM. 1. NOTIDANIDAE.—Body moderately elongate, the spineless dorsal fin
opposite the anal. Mouth ventral; nostrils ventral, near the extremity of
the snout, without oro-nasal grooves. Branchial arches and clefts six or
seven. Interbranchial septa devoid of marginal frills. Notochord persistent
and continuous, partially constricted by simple chorda-centra, each
consisting of two distinct rings, without either concentric or radial
lamellae, except {443}in one species (_Notidanus cinereus_), which exhibits
a feeble asterospondylism in the caudal vertebrae. Skull amphistylic. Teeth
unlike in the two jaws; those in the upper jaw usually with a large central
cusp and smaller lateral cusps; those in the lower jaw comb-like, each
consisting of numerous graduated pointed cusps inclining in the same
direction, and supported on a long basal plate.
The very few species included in this family are widely distributed in the
tropical and subtropical regions of the Atlantic and Pacific Oceans.
_Notidanus_ (_Heptanchus_) _cinereus_, which has seven branchial arches and
clefts, inhabits the Mediterranean and Atlantic. _N_. (_Hexanchus_)
_griseus_, with six branchial arches and clefts, has a similar
distribution, but besides being an occasional visitant to the British
coasts, it is not uncommon at Cuba in the West Indies. It is said to grow
to a length of 26 feet.
Fossil remains of _Notidanus_, principally teeth, occur in the Middle and
Upper Jurassic, in the Cretaceous, and in the Eocene and Pliocene of
England and the Continent.
FAM. 2. CHLAMYDOSELACHIDAE (Frilled Sharks).—Body much elongate. Median
fins as in _Notidanus_. Mouth nearly terminal. Nostrils lateral, nearly
terminal, and without oro-nasal grooves. Branchial arches and clefts six.
The outer margins of the interbranchial septa are produced into overlapping
cutaneous frills, the first of which is developed from the hyoid arch and
overlaps the hyobranchial cleft, like a rudimentary operculum. Vertebral
column as in the preceding family, but in the hinder part of the trunk the
notochord is unconstricted and uniform in diameter, centra being absent.
Skull hyostylic. Lateral line an open groove. Teeth alike in both jaws,
each consisting of a broad basal plate supporting three slender curved
cusps, separated by a pair of much smaller cusps.
The only living species known is _Chlamydoselachus anguineus_ (Fig.
252),[527] which occurs in the Pacific near Japan, in deep water off
Madeira, and also off the Azores and the coast of Norway. It reaches a
length of 4 to 5 feet. Teeth from the Pliocene deposits of Tuscany have
been referred to an extinct species, _C. lawleyi_.
{444}Scarcely anything is known of the habits of the Notidanidae and the
Chlamydoselachidae. It is evident that they are closely-related forms, and
from the unusual number of their gill-clefts and branchial arches, and the
condition of the vertebral column, it is also obvious that they are the
most archaic of modern Selachians.
[Illustration: FIG. 252.—_Chlamydoselachus anguineus_. (From Günther.)]
FAM. 3. HETERODONTIDAE (Bullhead Sharks).—Head large and high, with a blunt
snout projecting but little in front of the small and almost terminal
mouth, and with prominent supraorbital crests. Trunk thick-set and somewhat
trihedral, covered with fine shagreen. Nostrils ventral but nearly
terminal, with oro-nasal grooves. Spiracles small, beneath the eyes. Two
dorsal fins, each with a spine in front, the first opposite the interval
between the pectorals and pelvics, the second in front of the anal.
Vertebral centra asterospondylic when fully developed. Palato-quadrate
cartilages with an extensive articulation with the sides of the preorbital
regions of the cranium, the normal suspensoria of a hyostylic skull
(hyomandibular cartilages) taking little share in their support. Dentition
similar in both jaws. Teeth at the symphyses numerous, small, and conical,
furnished with three to five cusps in the young; those behind broad and
pad-like, arranged in oblique rows, the teeth forming the two middle rows
being much larger than those in the front or behind. Living species,
oviparous. Egg-cases large, with an external spiral lamina (Fig. 245).
About four species belonging to one genus, _Heterodontus_ (= _Cestracion_)
(Fig. 253), or possibly to two, represent this dwindling family. All are
inhabitants of the Pacific Ocean (Japan, Amboyna, Australia, the Galapagos,
and the Californian coast of North America). Little is known of their
habits. They feed {445}principally on Molluscs, the shells of which are
crushed by their massive grinding teeth. The different species vary in size
from 2 to 5 feet.
The Heterodontidae were the most characteristic and abundant Sharks of the
Mesozoic period. Amongst extinct genera _Hybodus_ ranges from the Middle
Trias to the Lower Cretaceous (Wealden); an allied genus, _Acrodus_, from
the Middle Trias to the Upper Cretaceous (Gault). _Palaeospinax_ occurs in
the Lias and possibly in the Upper Trias. _Synechodus_ is a Cretaceous
genus, and _Asteracanthus_, which has large hooked spines on the head, is
characteristic of the Middle and Upper Jurassic. An even greater antiquity
may be claimed for the Heterodontidae if, as is not improbable, such
Palaeozoic Sharks as _Orodus_, _Sphenacanthus_, _Tristychius_
(Carboniferous), and _Wodnika_ (Permian) belong to this family. Many
ichthyodorulites are probably the spines of various extinct Heterodontidae.
[Illustration: FIG. 253.—Port Jackson Shark (_Heterodontus philippi_). A,
lateral view; B, mouth and nostrils. _d_, Clasper. (From a specimen in the
Cambridge University Museum.)]
FAM. 4. COCHLIODONTIDAE.[528]—This Palaeozoic family includes a number of
Sharks probably related to the Heterodontidae, but of which little is known
except their dentition. The teeth are in some respects similar to those of
_Heterodontus_, except that those which appear to correspond to one or both
of the middle rows of the latter genus tend to fuse and form a few large,
convex, and often scroll-like plates. The typical Cochliodonts are
exclusively Carboniferous (Europe and North America). _Psephodus_,
_Pleuroplax_, _Deltodus_, _Poecilodus_, _Cochliodus_, _Deltoptychius_,
_Helodus_, and _Menaspis_ (Permian) are characteristic genera.
{446}Probably some ichthyodorulites described under various generic names
belong to this family.
FAM. 5. PSAMMODONTIDAE.—Teeth large, flat or slightly arched, oblong or
quadrate, and arranged in one, two, or more longitudinal rows. Only the
teeth are known, and from differences in their shape, size, and surface
markings, the genera _Psammodus_, _Archeobatis_, and _Copodus_ have been
recognised. The family is confined to the Lower Carboniferous of Great
Britain and Ireland, Russia, Belgium, and North America.
FAM. 6. PETALODONTIDAE.—Teeth transversely elongated, with a blunt or a
sharply-ridged crown, separated from a single or multiple root by a
constricted neck, and disposed in transverse and longitudinal pavement-like
rows; exoskeleton of smooth, oval, rounded or quadrate shagreen denticles.
Only the teeth, and in some genera the dermal denticles, are known, except
in _Janessa_, which has a Ray-shaped body, with large pectoral fins
prolonged towards the head. The family is mainly confined to the
Carboniferous formations of Great Britain, Europe, and North America.
_Petalodus_, _Janessa_ (also represented in the Permian), _Glossodus_,
_Polyrhizodus_, and _Callopristodus_ are characteristic genera.
FAM. 7. SCYLLIIDAE (Dog-Fishes).—Dorsal fins two in number, small, and
without spines, the first above or behind the pelvic fins, the second
usually behind the anal. Tail not bent upwards or but slightly so, without
lateral keels. Spiracles present. Nictitating membranes absent. Vertebrae
asterospondylic. Teeth small, each with a median cusp, and one to four
small cusps on each side. Oviparous. Egg-cases (Fig. 246) large, quadrate,
with long twining tendrils at the angles for attachment.
The genus _Scyllium_ includes the true Dog-Fishes (Fig. 254). The species
are coast Fishes of small or moderate size, and are widely distributed in
temperate and tropical seas, at depths not as a rule exceeding 400 fathoms.
Two species, _S. canicula_ and _S. catulus_, are common on the British
coasts, living near the bottom and feeding on Crustaceans and Molluscs. An
allied form, _Pristiurus_, is also common in European and British waters.
_Chiloscyllium_ is a widely-distributed genus ranging from the Cape of Good
Hope through the Indian Ocean to the coasts of Australia, China, and Japan.
_Stegostoma tigrinum_ of the {447}Indian Ocean attains a length of 10 to 15
feet, and is remarkable for its handsome coloration of dark bands on a
yellow ground, which has suggested the name of Tiger- or Zebra-Shark. The
pelagic genus _Ginglymostoma_ has the terminal portion of the tail bent
upwards, and grows to a length of 6 to 12 feet. It is represented by
species in the Indian Ocean and the tropical parts of the Atlantic (West
Indies and the west coast of Mexico). _Crossorhinus_ includes species of
large size, some of which are 10 feet long. They are ground-sharks,
frequenting the coasts of Australia and Japan, which lie on the bottom
watching for their prey, and in accordance with this habit their coloration
closely resembles that of their surroundings.[529] A large North Atlantic
Shark (_Pseudotriakis microdon_), of which only two specimens are known,
one taken on the Portuguese coast, and the other, 10 feet in length, off
Long Island, on the Atlantic coast of North America, has the general
characters of the Scylliidae, except that the first dorsal fin is opposite
the interval between the pectoral and pelvic fins. Some Scylliidae live at
great depths, _Scyllium_ (_Scylliorhinus_) _profundorum_ having been
obtained from a depth of 816 fathoms in the North Atlantic.[530]
[Illustration: FIG. 254.—A female Dog-Fish (_Scyllium canescens_), from the
south-western coast of South America. (From Günther.)]
Most of the fossil Scylliidae belong to existing genera. The earliest known
representatives of the family occur in the Upper Jurassic (Lithographic
Stone of Bavaria), where the extinct genus _Palaeoscyllium_, a near ally of
the existing _Scyllium_, and _Pristiurus_, are found, nearly complete.
_Scyllium_ itself ranges from the Cretaceous through the different Tertiary
formations. A species of _Chiloscyllium_ has been recorded from the Miocene
Tertiaries, and detached teeth of _Ginglymostoma_ from the Eocene of
Belgium and North America. An extinct genus (_Mesiteia_), which is found in
the Upper Chalk of Mount Lebanon and the Upper Eocene of Monte Bolca, is
remarkable for the enclosure of {448}its lateral sensory canals in a series
of incomplete calcified rings, as in the Holocephali.
FAM. 8. CARCHARIIDAE.—Sharks with two dorsal fins, the first in front of
the pelvic fins and the second opposite the anal fin, both devoid of
spines. Tail without lateral keels. Preoral rostrum elongated. Mouth
crescentic. Eyes with nictitating membranes. Spiracles small or absent.
Vertebrae asterospondylic. Teeth usually consisting of a single triangular
cusp, with smooth, trenchant, or serrated margins, rarely with basal cusps;
generally with an axial cavity when fully developed. Viviparous. The family
comprises about twenty genera, and approximately sixty species; found in
all seas, often in mid-ocean. Amongst the more important genera may be
mentioned _Carcharias_ (_Carcharhinus_), _Galeocerdo_, _Triakis_,
_Thalassorhinus_, _Galeus_, _Mustelus_ and _Scylliogaleus_.
[Illustration: FIG. 255.—The Blue Shark (_Carcharias glaucus_). (From
Müller and Henle.)]
Species of _Carcharias_ are found in nearly all tropical and subtropical
seas. The genus is a somewhat comprehensive one, and groups of its species
have been distinguished as sub-genera under the names of _Prionodon_,
_Hypoprion_, _Scoliodon_, _Aprionodon_,[531] etc. One of the most widely
distributed of the thirty to forty species is the Blue Shark, _C_.
(_Prionodon_) _glaucus_ (Fig. 255), of the Atlantic and Pacific Oceans,
which may grow to a length of 25 feet, although the young forms not
infrequently captured in British waters do not exceed 6 to 8 feet. It is a
slender, swift, pelagic Shark, of a slaty-blue colour above and white
underneath, and a voracious hunter of other Fishes. _C. nicaraguensis_, a
Shark about 7 feet long, is confined to Lake Nicaragua and its outlet the
Rio San Juan, and is one of the very rare strictly freshwater Sharks.
_Galeocerdo_ is a large Shark found in temperate and tropical waters, but
one species, _G. arcticus_, is confined to Arctic seas. The variegated _G.
tigrinus_, or West Indian Tiger-Shark, is said {449}to reach a length of 15
to 20 feet. The genus _Galeus_ includes the small Sharks commonly known as
"Topes," which are common in nearly all tropical and temperate seas. The
British species, _G. canis_, which ranges from 4 to 6 feet in length, is a
bottom-feeding Fish, preying on Molluscs, Crustacea, Star-Fish, and small
Fishes. The various species of _Mustelus_, or "Hounds," resemble the Topes
in their habits and distribution. Living principally on Molluscs and
Crustaceans, the dentition has lost the trenchant, unicuspidate type
characteristic of most other Carchariidae, and is adapted for crushing and
grinding, the teeth being flat, without cusps, and arranged in
pavement-like rows. Two species, _M. vulgaris_ and _M. laevis_, are
abundant on the coasts of Europe and the British Isles. _Scylliogaleus_,
which combines the general characters of _Mustelus_ with nostrils similar
to those of a _Scyllium_, is known only from a single specimen from the
coast of Natal.[532]
The Carchariidae are comparatively modern Sharks. No undoubted remains are
known earlier than the Eocene, in which, as in the succeeding Miocene and
Pliocene deposits, they are represented principally by their characteristic
teeth. The extinct fossil genera are few in number, and so far as their
dentition is concerned they differ but little from their living allies.
FAM. 9. SPHYRNIDAE (Hammer-head Sharks).—In their general characters the
Hammer-head Sharks agree with the Carchariidae. They are distinguished,
however, by the remarkable shape of the head, which is prolonged into two
conspicuous lateral lobes, supported internally by corresponding
cartilaginous outgrowths from the post-orbital and the lateral ethmoidal or
nasal regions of the skull, with the eyes at their distal extremities, and
the nostrils in relation with their anterior margins. One genus and five
species.
The Sphyrnidae are denizens of nearly all tropical and subtropical seas.
_Sphyrna_ (_Zygaena_) _tudes_ occurs in the Mediterranean, and _S. zygaena_
is a very rare visitant to the British coasts. A specimen over 13 feet in
length was captured at Ilfracombe in 1865, and other examples have been
taken off Banffshire, at Newlyn in Cornwall, at Yarmouth, and in Carmarthen
Bay.[533] The shape of the head differs in different species, and in young
{450}forms the peculiarities of the adult are less marked. In the Bonnet
Shark (_S. tiburo_) (Fig. 256, A), the head is crescentic or kidney-shaped,
with prominent postero-lateral angles, and between this type of head and
the more pronounced "hammer" of _S. zygaena_ (Fig. 256, B) an almost
perfect gradation is supplied by other species. The Hammer-heads are
voracious Sharks, usually living in deep water, and they may grow to a
length of 15 feet. As many as thirty-seven embryos have been taken from the
oviducts of a female nearly 11 feet in length.[534]
[Illustration: FIG. 256.—Ventral view of the head and trunk (A) of a young
Bonnet Shark (_Sphyrna tiburo_), and (B) of a young male Hammer-head (_S.
zygaena_). _c_, Clasper; _cl_, cloacal aperture; _e_, eye; _n_, nostril;
_n′_, nasal groove.]
Teeth assigned with more or less probability to _Sphyrna_ are found in the
Miocene of Europe and North America.
FAM. 10. LAMNIDAE (Porbeagle Sharks).—Large, stout-bodied Sharks with two
dorsal fins, the first just behind the pectoral fins, the second, which is
small, opposite the small anal fin; both {451}without spines. Tail with a
prominent lateral keel on each side. Nictitating membranes absent.
Spiracles minute or wanting. Branchial clefts very wide. No oro-nasal
grooves. Vertebrae asterospondylic. When fully developed the teeth are
solid.
In the genus _Lamna_, which includes the Porbeagle Sharks, the teeth are
large, each consisting of a long narrow central cusp, usually with smaller
cusps at the base. The common Porbeagle (_L. cornubica_), a fierce pelagic
Shark, which may reach a length of 10 feet, frequents the North Atlantic
and the North Pacific (Fig. 257). It has often been captured off the coasts
of Great Britain and Ireland in Mackerel or Salmon nets, or by lines laid
for food Fishes. An allied genus, _Isurus_, is represented by species on
the Atlantic coast of North America, in the Mediterranean and the
neighbouring parts of the Atlantic, and also in Asiatic seas. _Carcharodon
rondeletii_[535] is a pelagic Shark with large, triangular, finely-serrated
teeth, without basal cusps, and is found in all tropical and subtropical
seas from the Mediterranean to Australia and New Zealand. It is one of the
largest and most formidable of Sharks, and it is said to grow to a length
of 40 feet. Nothing is known of its breeding habits. _Odontaspis_, which
has minute pore-like spiracles, but no lateral caudal keels, is a Shark of
moderate size, chiefly inhabiting the Atlantic, but found also in the
Mediterranean and the Southern Pacific. Its teeth are long and awl-like,
with small basal cusps.
[Illustration: FIG. 257.—The Common Porbeagle (_Lamna cornubica_). (From
Parker and Haswell, after Bashford Dean.)]
The Thresher or Fox Shark (_Alopecias vulpes_) is remarkable for the
extraordinary length of the upper lobe of the caudal fin, {452}which is as
long as the rest of the body (Fig. 258). Its teeth are of moderate size,
triangular in shape, and without serrations. The "Thresher" has a wide
distribution, being abundant in the Atlantic and Pacific Oceans, besides
being the commonest of the larger Sharks frequenting the British coasts. It
grows to a length of 15 feet, of which the tail forms at least one-half.
Quite inoffensive to man, the Thresher feeds on the shoals of smaller
Teleosts, such as Pilchards, Herrings, and Sprats. When feeding it swims in
gradually diminishing circles round the shoal, splashing the water with its
long tail, and keeping its victims so crowded together that they become an
easy prey. A remarkable Lamnoid Shark (_Mitsukurina owstoni_),[536] which
has the snout produced into a "long, flat, flexible, leaf-like blade,"
somewhat resembling that of _Polyodon_, but narrower and more pointed, and
has protractile jaws and large spiracles, is found in deep water near
Yokohama, and may prove to be generically identical with the Cretaceous
Shark _Scapanorhynchus_.[537]
[Illustration: FIG. 258.—The Thresher Shark (_Alopecias vulpes_). (From
Jordan and Evermann.)]
Lamnoid Sharks are not certainly known to have existed until the Upper
Cretaceous formations, in which, as well as in different Tertiary deposits,
teeth indistinguishable from those of the existing genera _Lamna_,
_Odontaspis_, and _Carcharodon_ are found. The interesting genus
_Carcharodon_ has one extinct species in the Cretaceous and several others
distributed in Tertiary formations in nearly every part of the world. The
teeth of some of the Tertiary species measure 5 inches along the margin and
4 inches across the base, and it is evident that they belonged to Sharks so
gigantic as completely to dwarf the existing species. That these giant
Lamnidae have only recently {453}become extinct is proved by the fact that
similar teeth have been dredged from the bottom of the Pacific. Teeth and
detached vertebrae from various Tertiary deposits have been referred to
species of _Alopecias_. Entire Fishes, with an elongated rostrum and an
extensive anal fin, from the Cretaceous of Mount Lebanon, have been
assigned to an extinct genus, _Scapanorhynchus_.
FAM. 11. CETORHINIDAE (Basking Sharks).—Two dorsal fins, without spines,
the anterior midway between the pectoral and pelvic fins. Tail without
lateral keels. Nictitating membranes absent. Spiracles small, situated just
above the angles of the mouth. Branchial clefts wide and of great vertical
extent, extending from the dorsal to the ventral surface. Teeth small, very
numerous, conical in shape, without serrations. Claspers of the male
provided with horn-like denticles.
[Illustration: FIG. 259.—The Basking Shark (_Cetorhinus_ (_Selache_)
_maximus_). (From Goode and Bean.)]
The single species included in this family, the Basking Shark,
(_Cetorhinus_ (_Selache_) _maximus_), is one of the largest of living
Fishes, reaching a length of 40 feet (Fig. 259). It is a pelagic Shark,
inhabiting the Arctic seas, but wandering as far south on opposite sides of
the Atlantic as the Mediterranean, the coasts of Portugal and Virginia, and
in the Pacific to the Californian coast. Although generally described as a
northern form, _Cetorhinus_ is known to occur in Australian waters.[538] It
is fairly common off the coasts of Scotland, and it has been seen or
captured at various points on the western coast of Ireland, and {454}the
eastern and southern coasts of England. The Fish is gregarious in its
habits, often swimming in shoals near the surface. The name "Basking Shark"
has been suggested by its habit of lying motionless on the surface in warm
or calm weather, as if basking in the sun, with its dorsal fin protruding
from the water. Unless attacked, this Shark is quiet and inoffensive. It
derives its food-supply from small pelagic Fishes, and also from marine
Invertebrates, which are strained from the water by the fringes of long,
slender gill-rakers with which the branchial arches are provided. At one
time harpooned and caught off the Irish, Scotch, and Norwegian coasts for
the sake of the oil obtained from its liver, the Fish is now of little
economic importance. Nothing is known of its mode of reproduction.
Extinct species of _Cetorhinus_ have been founded on detached vertebrae and
isolated teeth from deposits of Pliocene age in Belgium and Italy, and
possibly from still earlier Tertiary formations. Dermal spines similar to
those found on the claspers of the males in the existing species occur in
the Antwerp Crag, and in the Red Crag of Suffolk.
FAM. 12. RHINODONTIDAE.—Two dorsal fins, without spines, the anterior a
little in front of the pelvic fins, the second opposite the anal. Tail with
lateral keels and a pit at its root. Spiracles small. Nictitating membranes
absent. Mouth and nostrils nearly terminal. Teeth very minute, numerous,
and conical in shape.
One genus, _Rhinodon_, with one or two species, is known. These Sharks are
very widely distributed, specimens having been seen or captured in the
neighbourhood of Ceylon, at the Seychelles, the Cape of Good Hope, Callao
on the Peruvian coast, in the Gulf of California, and off the coast of
Florida. _Rhinodon_ is probably the largest known Shark. It is stated to
exceed 50 feet in length, but to be quite harmless. Scarcely anything is
known of its habits, but the small size of the teeth, and the length of the
gill-rakers, which resemble those of the Basking Shark, suggest a similar
kind of food.
FAM. 13. SPINACIDAE.—Two dorsal fins, the first in advance of the pelvic
fins. Anal fin absent. Nictitating membrane absent. Spiracles rather large.
Vertebrae cyclospondylic. Teeth variously modified in different genera.
{455}The more typical representatives of this family are the Spiny
Dog-Fishes, which are distinguished by the presence of a strong spine in
front of each dorsal fin. They are more abundant in temperate regions than
in the intervening tropics. The more important genera are _Acanthias_,
_Centrina_, _Centrophorus_, _Spinax_, and _Centroscyllium_. _Acanthias
vulgaris_, the Picked or Piked Dog-Fish, is a gregarious, voracious Shark,
about 3 to 4 feet in length, and is frequently seen in huge shoals all
round the British coasts, especially during the summer months. It is very
destructive to food Fishes, and its ravages result in serious loss to
fishermen. _Acanthias_ is viviparous. _Centrina salviani_ is a much smaller
Shark, which frequents the Mediterranean and the Bay of Biscay; on rare
occasions it has been taken off the southern coast of England.
_Centrophorus_ occurs in deep water in the Mediterranean and adjacent
portions of the Atlantic, and off the coasts of Japan. _Centroscyllium_ is
found on opposite sides of the North Atlantic (Greenland and
Massachusetts), and in the opposite hemisphere at the Falkland Isles. A
deep-water form, _Paracentroscyllium_, has been obtained in the Bay of
Bengal at depths from 285 to 405 fathoms.[539]
[Illustration: FIG. 260.—The Greenland Shark (_Laemargus borealis_). (From
Goode and Bean.)]
Three remaining genera (_Scymnus_, _Laemargus_, and _Echinorhinus_) differ
from the preceding in the absence of dorsal spines.
_Scymnus lichia_ is common in the Mediterranean and the neighbouring parts
of the Atlantic. The Greenland Shark (_Laemargus borealis_) (Fig. 260) is
an inhabitant of the Arctic regions, wandering as far southwards on
opposite sides of the Atlantic as the French coast and Cape Cod. It is a
huge, clumsy shark, reaching a length of 26 feet. Numerous instances are
recorded of its capture off the coasts of Great Britain, especially in
northern waters. The Greenland Shark is said to be a determined foe to
{456}the Right Whale, which it attacks, biting pieces out of its body.
_Scymnus_ is viviparous, _Laemargus_ oviparous, and the latter is unique
among Sharks in producing eggs devoid of a horny shell, which are deposited
on the sea-bottom. _Echinorhinus_ has dermal denticles in the form of
relatively large rounded tubercles, each surmounted by a tuft of fine
spines. One species only is known, _E. spinosus_, a large Shark attaining a
length of 10 feet, and frequenting deep water off the Atlantic coasts of
Europe and Africa from the North Sea to the Cape of Good Hope. A single
specimen has been taken at Cape Cod on the eastern coast of the United
States, and another off Dunedin, New Zealand. The capture of thirty
examples in British waters since 1828 has been recorded,[540] the largest a
female 9 feet in length.
Most of the existing genera of Spinacidae are represented by teeth or
detached spines in the later Tertiary deposits, but none are certainly
known to occur earlier than the Pliocene.
[Illustration: FIG. 261.—The Angel-Shark (_Rhina squatina_). A, dorsal
view; B, view of the mouth and nasal barbels. _p.f_, Pectoral fin; _pv.f_,
pelvic fin; _sp_, spiracle.]
FAM. 14. RHINIDAE (Angel-Sharks).—Ray-like Sharks with a flattened head and
body, and nearly terminal mouth and nostrils. Pectoral fins very large,
horizontally expanded, but constricted at the base and not adherent to the
sides of the head or trunk. Two dorsal fins, both small, without spines,
and situated on the tail behind the pelvic fins. Anal fin absent. Spiracles
large {457}and crescentic. Vertebrae tectospondylic. Teeth conical and
pointed. A single species only is known.
_Rhina squatina_, the Angel-Shark or Monk-Fish (Fig. 261), is intermediate
between the ordinary Sharks and the Skates and Rays, both in external
appearance and internal structure, but is more Ray-like than Shark-like in
its habits. Within the temperate and tropical regions of both hemispheres
it is almost cosmopolitan in its distribution, frequenting the coasts of
Europe, including the British Isles, the Atlantic and Pacific coasts of
North America, and the shores of South Australia and Japan. The Angel-Shark
is viviparous, producing about twenty young at a time. Not rarely it grows
to a length of 5 feet.
The family ranges from the Upper Jurassic to the present time. Species of
_Rhina_ are represented by more or less complete skeletons in the
Lithographic Stone of Bavaria, and in the Upper Cretaceous of Westphalia
and Mount Lebanon, and by teeth and vertebrae in the English Chalk, as well
as in different European Tertiary formations.
FAM. 15. PRISTIOPHORIDAE.—Prenasal portion of the head and cranium produced
into a long flattened rostrum, furnished with a pair of long tentacles on
its under surface, and, as in Saw-Fishes, with a series of large,
tooth-like, dermal denticles, of equal or unequal size, along each of its
lateral margins. Two dorsal fins, without spines, the first in front of the
pelvics. No anal fin. Pectoral fins large, distinct from the head and
trunk, with a contracted base. Spiracles large and crescentic. Teeth small,
with a conical cusp and a broad base.
These singular Sharks closely resemble the true Saw-Fishes (Pristidae), but
they differ in the lateral position of their gill-clefts, the presence of
rostral tentacles, and their smaller size. The few species known belong to
the genus _Pristiophorus_, and are confined to the Australian and Japanese
seas.
_Pristiophorus_ is represented in the Upper Cretaceous of Mount Lebanon,
and in the Miocene deposits.
SUB-ORDER 2. BATOIDEI.
Body generally discoidal or rhombic in shape, the axial portion being
formed by the flattened head and trunk, and the lateral portions by the
enormously expanded pectoral fins, which {458}are usually confluent with
the sides of the head. Tail slender, sharply marked off from the trunk, to
which it usually appears as a mere appendage. Dorsal fins, when present, on
the tail. Anal fin absent. Branchial clefts ventral in position. Spiracles
large, usually crescentic. Vertebrae tectospondylic.
For the most part the Batoidei are sluggish ground-Fishes, slowly moving
over the sea-bottom by the gentle undulatory vibrations of the margins of
their huge pectoral fins, the tail being of little use in locomotion. They
feed principally on Crustacea, Molluscs, and the smaller Teleosts. As with
other Fishes of similar habits, the coloration of the dorsal surface
harmonises with that of the sea-bottom, while the ventral surface is either
deficient in pigment or white. The majority of them are coast Fishes,
rarely descending to a greater depth than 500 fathoms, but some are
pelagic. The Batoidei are a relatively modern race, first appearing towards
the middle of the Mesozoic period, and evidently representing an assemblage
of specialised Elasmobranchs adapted for a bottom-living existence. As
remarked by Smith Woodward, the three families, Rhinobatidae, Raiidae, and
Trygonidae, are not so clearly differentiated before the close of the
Cretaceous period as they subsequently become.[541]
The first two families, the Pristidae and the Rhinobatidae, are interesting
connecting-links between such Selachii as the Rhinidae and the
Pristiophoridae and the more specialised Batoidei like the Skates, Rays,
and Trygons. While they agree with the latter in the ventral position of
the gill-clefts, the absence of an anal fin, and the caudal position of the
dorsal fins, the body still retains an elongated and somewhat Shark-like
shape, and shades off imperceptibly into a powerful swimming tail, and in
the Pristidae at all events the pectoral fins are of moderate size and free
from any fusion with the sides of the head. It must be admitted that the
institution of the two sub-orders introduces a somewhat arbitrary
distinction between certain families of Plagiostomes which has little to
recommend it except custom and some measure of convenience. The two series
of Fishes shade almost imperceptibly into one another, and the importance
of the ventral position of the gill-clefts has probably been overestimated.
Primitively, the gill-clefts are lateral, and lie wholly in front of the
pectoral fins, a position which is retained in many {459}Selachii. In
others, however, the hinder gill-clefts tend to extend backwards above the
base of the pectoral fins, while in some the clefts assume a more ventral
position, and extend beneath the pectoral fin; hence, even within the
limits of the Selachii the position of the gill-clefts varies to the extent
that these structures may be lateral, or they may tend to become either
dorsal or ventral.[542] On the score of convenience the customary usage is
adopted here.
FAM. 1. PRISTIDAE (True Saw-Fishes).—Although somewhat depressed, the body
is still elongate and Shark-like, with a well-developed tail terminating in
a heterocercal caudal fin. Dorsal fins large, the first opposite the pelvic
fins. Head and skull prolonged into a long flattened rostrum, the lateral
margins of which are armed with a series of strong tooth-like denticles,
firmly implanted in sockets in the calcified rostral cartilage. No rostral
tentacles. Teeth in the jaws minute and obtuse. One genus and about four or
five species are known, all inhabitants of tropical and subtropical seas.
[Illustration: FIG. 262.—The Saw-Fish (_Pristis antiquorum_). (From
Cuvier.)]
Some of the true Saw-Fishes attain a considerable size, 10 to 20 feet or
even longer, and "saws" 6 feet long and a foot in width across the base are
not uncommon. By means of powerful lateral strokes of its saw the Fish is
capable of lacerating the bodies of other animals and tearing off pieces of
flesh, which it then devours. Indian species are known to ascend rivers
beyond tidal influence, and an American species, ranging northwards to the
West Indies and the Gulf of Mexico, where it is abundant, enters the lower
Mississippi. _P. antiquorum_ occurs in the Mediterranean and the Atlantic,
but does not extend so far northward as the British coasts.
The earliest known representative of the family is the {460}extinct genus
_Sclerorhynchus_ from the Upper Chalk of Mount Lebanon, in which the
smaller size and more superficial position of the rostral "teeth," and the
absence of sockets in the rostral cartilage, prove that the "teeth"
approximate more to ordinary dermal spines in this genus than in any of the
more recent Saw-Fishes. An extinct genus _Propristis_, from the Upper
Eocene of Egypt, with non-socketed teeth, and species of the existing genus
_Pristis_ from the English Middle Eocene, are also known.
FAM. 2. RHINOBATIDAE.—Owing to the increased expansion of the pectoral fins
and the forward growth of their anterior cutaneous portions along the sides
of the head, as well as backwards along the trunk, the body now assumes a
sub-rhombic shape, and approximates to the disc of the more typical
Batoidei, but the tail with its dorsal and caudal fins is still strongly
developed, and blends imperceptibly with the trunk in front. Teeth very
obtuse. No electric organs. About five genera and twenty species are known,
distributed in most tropical and subtropical seas.
[Illustration: FIG. 263.—_Rhinobatus granulatus_. (From Müller and Henle.)]
The cosmopolitan _Rhinobatus_ is represented by species from the
Mediterranean, the Red Sea, the west coast of Africa, the Indian Ocean,
Australia and China, as well as from the Atlantic and Pacific coasts of
America, and the Galapagos. _Rhynchobatus_ ranges from the Red Sea through
the Indian Ocean to China, _Zapteryx_ occurs at San Diego and Panama, and
_Platyrhinoidis_ on the Californian coast. _Trygonorhina_ is an Australian
genus.
The family dates from the Upper Jurassic. _Rhinobatus_ is {461}represented
by complete skeletons in the Lithographic Stone of Bavaria, the Upper
Cretaceous of Mount Lebanon, and the Upper Eocene of Monte Bolca.
_Trygonorhina_ occurs in the Eocene.
FAM. 3. RAIIDAE (Skates or Rays).—The endoskeletally supported portions of
the large pectoral fins extend along the lateral margins of the trunk and
head from the pelvic fins to the snout, and are confluent therewith,
forming the lateral portions of a large rhombic disc. The tail is slender,
and sharply marked off from the trunk. Usually two small dorsal fins on the
tail. Caudal fin small or absent. No serrated spine on the tail. Caudal
electric organs are often present. Larger or smaller denticles or spines
are generally present on the skin. Oviparous. Egg-cases four-horned,
without tendrils. Four genera and from thirty to forty species. Found in
all temperate seas, a few ranging into deep water.
[Illustration: FIG. 264.—_Raia murrayi_, from Kerguelen Island. A, male; B,
female. (From Günther.)]
The great majority of the species belong to the genus _Raia_ (Fig. 264),
which chiefly inhabits temperate seas, but is more abundant in the northern
than in the southern hemisphere, and approaches nearer to the Arctic and
Antarctic regions than any other Batoidei. The colour of the upper surface
of the body is closely assimilated to that of the sandy or gravelly bottom
on {462}which they live, and thus concealed, small Fishes, Crustaceans, and
other organisms are lured unsuspectingly within the reach of the
comparatively inactive and sluggish Ray. From the ventral position of the
mouth the Ray cannot at once seize its prey, but the Fish darts over its
victim and covers it with its body, and then readily devours it. The sexes
are usually distinguished by secondary sexual characters, which take the
form of differences in size and coloration, in the dentition, and also in
the presence and position of patches or rows of specially modified dermal
spines on the dorsal surface (Fig. 264). Some of the larger species reach a
great size, the disc measuring 7 to 8 feet in width. A few species range
into deep water. _R. mamillidens_, a uniformly jet-black species, has been
obtained from a depth of 597 fathoms in the Bay of Bengal,[543] and _R.
abyssicola_ from 1588 fathoms off Queen Charlotte Islands, British
Columbia.[544] The following are British species: the Thornback (_R.
clavata_); the Spotted Ray (_R. maculata_); the Painted Ray (_R.
microcellata_); the Starry Ray (_R. radiata_); the Cuckoo or Sandy Ray (_R.
circularis_); the Skate (_R. batis_); the Flapper Skate (_R.
macrorhynchus_); the White Skate (_R. alba_); the Long-nosed Skate (_R.
oxyrhynchus_); and the Shagreen Ray (_R. fullonica_).[545] Most of the
species are of some economic value as food Fishes. _Psammobatis_, with a
circular disc, frequents the southern coasts of South America, and
_Platyrhina_ the coasts of India, China, and Japan.
The family ranges from the Upper Cretaceous, in which, as well as in
different Tertiary deposits, it is represented by species of _Raia_. An
extinct genus, _Cyclobatis_, with a circular or oval disc, occurs in the
Upper Cretaceous of Mount Lebanon.
FAM. 4. TAMIOBATIDAE.—The systematic position of the only representative of
this family, _Tamiobatis vetustus_,[546] from the Devonian or Lower
Carboniferous of Kentucky, is very uncertain, but in some respects this
unique type seems to be intermediate between the modern Sharks and the
Rays.
FAM. 5. TORPEDINIDAE (Electric Rays).—A disc is formed as in the Raiidae,
but it is sub-circular in shape rather than rhombic, and in the nature of
its endoskeletal supports it is in some respects unique. Its semicircular
anterior margin is supported {463}in the centre by a branched prenasal
rostrum, and laterally by the curiously branched preorbital cartilages,
each of which radiates outwards and forwards from a common basal
articulation with the lateral ethmoid regions of the skull. Tail relatively
short and thick, with two dorsal fins, a caudal fin, and two lateral
longitudinal folds. Skin smooth, without denticles. Mouth transverse and
ventral. A characteristic quadrangular naso-frontal lobe, with a free
hinder margin, which forms the anterior lip, is enclosed by the two nasal
organs and the oro-nasal grooves leading from them to the corresponding
angles of the mouth. A pair of large electric organs between the pectoral
fins and the head. Seven genera and about fifteen species. Inhabitants of
most warm seas.
[Illustration: FIG. 265.—The Electric Ray (_Torpedo ocellata_). Dorsal (A)
and ventral (B) views. _p.f_, Pectoral fin; _pv.f_, pelvic fin; _sp_,
spiracle.]
The well-known genus _Torpedo_ (Fig. 265) is represented by species in the
Mediterranean (_T. marmorata_, _T. narce_, _T. hebetans_), the Red Sea, and
the Atlantic and Pacific Oceans. _T. hebetans_ {464}has been taken at
several places in British waters. An American _Torpedo_ (_Tetronarce_) is
represented by species on the Atlantic and Pacific coasts. _Narcine_ is a
very widely distributed genus, species having been recorded from the East
Indies, Tasmania, China, Japan, South Africa, and the Atlantic coasts of
North and South America. _Discopyge_ is an eastern Pacific genus (Peru and
Panama). _Hypnos_ frequents the Australian seas.
The family seems to be exclusively Tertiary, and its earliest fossil
representatives are from the Upper Eocene of Monte Bolca.
FAM. 6. TRYGONIDAE (Sting- or Whip-tailed Rays).—Disc sub-rhombic, broader
than long. Pectoral fins confluent with the sides of the head, their
preaxial endoskeletal radialia meeting in front of the skull along the
lateral margins of a slender prenasal rostral cartilage. Tail usually
whip-like, terminating in a small caudal fin, and generally armed with a
sharp, serrated spine, which takes the place of a dorsal fin. Skin smooth
or spinose. A rectangular naso-frontal flap in front of the mouth. About
ten genera and fifty species. Found in nearly all tropical and subtropical
seas.
Of the more important genera, _Trygon_ (_Dasyatis_) is represented by
numerous species in the tropical parts of the Atlantic and Pacific Oceans,
including the Pacific coasts of North and South America. Two species occur
in the Mediterranean, and one of them (_T. pastinaca_), ranges from the
coasts of Norway and the British Isles through the Atlantic and Indian
Oceans to Japan. _Urogymnus_ frequents the Red Sea and the Indian Ocean.
_Urolophus_ includes a few species of small size, distributed along the
Atlantic and Pacific coasts of Central and North America, and in Australian
seas. _Pteroplatea_ comprises rather large species, and is almost
cosmopolitan in its distribution, being represented by species on the
Atlantic and Pacific coasts of North and South America, in the
Mediterranean and the Red Sea, the Indian Ocean, the Malay Archipelago, and
on the coasts of China and Japan. The caudal spines, which may be 8 to 9
inches long in some of the larger species, are capable of inflicting very
severe wounds, the danger of which is greatly increased by the apparently
poisonous cutaneous mucus introduced into the wound. As the spines become
lost they are replaced by others developed from behind. Some Trygonidae
live in fresh waters. _Trygon_ (_Dasyatis_) _sabina_ frequents the streams
and estuaries of Florida {465}as well as on the adjacent coasts, and
specimens have been obtained from Lake Munroe at some distance from salt
water.[547] _Ellipesurus_ and _Paratrygon_ are freshwater genera, found in
Colombia, Venezuela, and Guiana.
Fossil remains of undoubted Trygonidae appear to be confined to the
Tertiary period.
FAM. 7. MYLIOBATIDAE (Eagle-Rays).—Disc much broader than long, and rhombic
in shape. The huge pectoral fins are not continued to the extremity of the
snout, but cease on the sides of the head, and reappear in front of the
snout as a pair of distinct folds, the so-called cephalic fins. The head
projects above the level of the disc, and consequently the eyes and
spiracles are lateral in position. Tail long, slender, and whip-like, with
a single dorsal fin near the root, and usually one or two serrated spines
behind the fin. A rectangular naso-frontal fold is present. The dentition
consists of flat, hexagonal, pavement-like crushing teeth arranged from
before backward in arched rows in both jaws, and there is either a single
median row of large teeth, with (e.g. _Myliobatis_) or without (e.g.
_Aëtobatis_) the addition of several rows of much smaller teeth on each
side, or there are numerous rows, the teeth then decreasing in size from
the middle line laterally (e.g. _Rhinoptera_). Skin smooth. Sexes similar.
Five genera and about twenty-seven species are known; all inhabitants of
tropical and subtropical seas.
_Myliobatis_ is represented in the Mediterranean by two species, and one of
them, the almost cosmopolitan _M. aquila_ (Fig. 266), has been taken at
various points on the eastern and southern coasts of England. _Aëtobatis_
is also widely distributed in tropical seas, but is unknown in European
waters. _Rhinoptera_ has one species in the Mediterranean, while others
have been recorded from Brazil, the Atlantic and Pacific coasts of North
America, and the East Indies. The two tropical genera _Dicerobatis_ and
_Ceratoptera_ have the cephalic fins prolonged anteriorly into a pair of
horn-like appendages, which are said to be used in conveying food to the
mouth. The teeth are small, flat or tubercular, and are arranged in
numerous rows. In _Ceratoptera_ they are wanting in the upper jaw. The
Eagle-Rays feed principally on Molluscs, the shells of which they crush
with their large grinding-teeth. Some of them attain {466}an enormous size,
and are among the largest of Fishes. _Ceratoptera vampyrus_ of the West
Indies, for example, grows to a width of 20 feet, and an embryo extracted
from the oviduct of a gravid female 15 feet wide, and from 3 to 4 feet in
thickness, measured 5 feet across the disc and weighed twenty pounds.[548]
This Fish is much dreaded by the divers engaged in the pearl fisheries near
Panama, whom it is said to devour after enveloping them with its vast
wings.[549]
[Illustration: FIG. 266.—The Eagle-Ray, _Myliobatis aquila_.]
The family is exclusively Tertiary, and with the exception of an extinct
genus, _Promyliobatis_, from the Eocene of Monte Bolca, all the fossil
species belong to the existing genera _Myliobatis_, _Rhinoptera_, and
_Aëtobatis_.
ORDER V. HOLOCEPHALI
The propriety of including the Holocephali in the sub-class Elasmobranchii
is scarcely open to doubt. Like the Acanthodei {467}they seem to represent
a divergent and specialised offshoot from some primitive Elasmobranch type,
and while retaining most of the essentially distinctive features of their
ancestors, they have acquired, perhaps independently, certain characters
distinctive of the Teleostomi, combined with others peculiar to themselves.
In the few surviving genera agreement with the Elasmobranchs is to be seen
in the wholly cartilaginous condition of the endoskeleton and the complete
absence of cartilage- and membrane-bones. The vertebral column is acentrous
and ribless, and the notochord is persistent; the dorsal arcualia include
supradorsals and regularly alternating basi- and inter-dorsals. The limbs
and limb-girdles are essentially Elasmobranch. Dermal denticles are
present, either locally, or, as in some of the fossil types, in the form of
a general investment. The brain and the reproductive organs agree more
closely with the corresponding structures in the Elasmobranchs than with
those of any other Fishes, and the agreement extends to the large size of
the eggs and their enclosure in horny egg-cases. In both groups the
nostrils are connected with the mouth by oro-nasal grooves; the hyoidean
hemibranch is a true gill, and there is no air-bladder. The Holocephali
also agree with the Elasmobranchs in retaining such primitive features as
an intestinal spiral valve and a conus arteriosus. On the other hand,
indications of specialisation in the Teleostome direction are to be noticed
in the tendency to the concentration of the branchial arches towards and
beneath the skull; the reduction of the interbranchial septa to the extent
that they are no longer continuous with the skin, and the gill-filaments
project beyond their outer margins; the presence of an operculum; the
suppression of the spiracles; and the absence of a cloaca, the rectum
opening externally by an anus in front of the urino-genital apertures.
Among the more notable features evolved within the limits of the group
mention may be made of the autostylic condition of the skull, probably an
adaptive modification induced by the large size of the crushing dental
plates which have taken the place of ordinary teeth; and the singular
development of anterior and frontal "claspers."
The group is one of great antiquity. Apart from the isolated spines or
"ichthyodorulites" common in Devonian and Carboniferous strata, some of
which are probably the frontal or the {468}fin-spines of ancient
Holocephali, dental plates, closely resembling those of modern Chimaeroids
and referred to the Ptychodontidae, are probably the earliest indications
of the existence of the group. The Holocephali become more abundant in the
Mesozoic period, but of the four families usually recognised, only one, the
Chimaeridae, has survived.
FAM. 1. PTYCTODONTIDAE.—This Palaeozoic family is known only by the dental
plates, of which there is a single pair in each jaw, meeting at the
symphysis. _Ptyctodus_[550] and _Rhynchodus_ occur in the Devonian of
either Russia or Germany, and in North America, and _Palaeomylus_ only in
the Devonian of North America.
FAM. 2. SQUALORAIIDAE.—General shape of the body similar to the existing
_Harriotta_. There is a long, depressed, preoral rostrum, and in the male
the head carries a long slender frontal spine. Conical denticles are
sparsely present on the head and body. No dorsal fin-spine. Dental plates
similar to those of the living Chimaeroids, but thinner, the tritoral areas
being less well defined. The only genus is _Squaloraia_ from the English
Lias, of which nearly complete skeletons are known.[551]
FAM. 3. MYRIACANTHIDAE.[552]—Body elongate, but less depressed. A dorsal
fin-spine is present, and in the males a frontal spine. The dentition
consists of a median incisor-like tooth at the symphysis of the lower jaw,
in addition to dental plates similar to those of _Squaloraia_. There is a
symmetrical series of tuberculated dermal plates on the lateral surfaces of
the head, which probably represent groups of fused denticles. One species
(_Myriacanthus granulatus_) has its rostrum terminating in a cutaneous
flap, as in _Callorhynchus_. _Myriacanthus_, from the Lower Lias of Lyme
Regis, and _Chimaeropsis_, from the Lithographic Stone of Bavaria, are the
only two genera.
FAM. 4. CHIMAERIDAE.—Body elongate and shark-like in form, but the head is
compressed and the mouth is small. Pectoral and pelvic fins large,
especially the former, which are somewhat ventrally placed. Two dorsal
fins, the anterior over the pectorals, with a stout spine in front; and a
small anal fin. Dermal denticles restricted to the claspers, and to
localised areas {469}on the dorsal surface in young forms. Dental plates
large and thick, including a single pair in the lower jaw and two pairs,
vomerine and palatine teeth, above, which combine trenchant edges with
well-marked grinding areas. Three genera are known.
[Illustration: FIG. 267.—_Chimaera monstrosa_ (male). _m_, Mouth; _n.p_,
frontal clasper; _op_, operculum.]
In _Chimaera_ (Fig. 267) the mouth and nostrils are ventral, posterior to a
bluntly conical snout. Head surmounted in the males by a club-shaped
appendage armed with a pad of recurved denticles, the frontal clasper;
there is also an anterior clasper armed with similar denticles and
retractile into a shallow glandular pouch in front of each pelvic fin, in
addition to the ordinary clasper behind the fin. The caudal fin consists of
nearly equal-sized dorsal and ventral lobes, between which the slightly
up-tilted caudal axis is prolonged as a long tapering filament: hence the
tail appears to be nearly diphycercal. _C. monstrosa_ occurs off the coasts
of Europe from Norway to Portugal, including the Mediterranean, and also in
the neighbourhood of the Azores, as far south as the Cape of Good Hope, and
eastwards off the coast of Japan. It is the largest of the living species,
reaching a length of 3 feet. _C. affinis_ was first taken off the coast of
Portugal, and subsequently on the North American side of the Atlantic, at
depths ranging from 200 to 1200 fathoms. _C_. (_Hydrolagus_) _colliei_ is
restricted to the North Pacific, and is especially plentiful off
South-eastern Alaska, and about the wharves at Esquimalt. Unlike most other
Chimaeroids this species swims at the surface, and there is no evidence
that it is a deep-sea form. In its breeding habits, and in the mode in
which its eggs are fertilised, _Chimaera_ probably resembles the oviparous
Sharks and Dog-Fishes.
{470}[Illustration: FIG. 268.—Egg-case of a species of _Chimaera_. _a_,
Transverse section across the case at _x_, showing the lateral valvular
slits; _b_, similar section across _x′_, showing the vertical ridges. (From
Günther.)]
[Illustration: FIG. 269.—_Callorhynchus antarcticus_. Male. A, lateral
view; B, front view of the mouth; C, front view of nasal process. _a.c_,
Anterior clasper; _b.a_, external branchial aperture; _f.c_, frontal
clasper; _p.c_, posterior clasper. (From a specimen in the Cambridge
Museum.)]
The eggs appear to be deposited on the sea-bottom in deep water, but they
are very rarely obtained. An egg-case dredged up off the south-west coast
of Ireland, at a depth of 315 fathoms, and about 6½ inches in length, is
shown in Fig. 268.[553] It consisted of a broad, somewhat oval, flattened
portion which contained the egg, and terminated at one end in a truncated
margin, while at the other it was produced into a long tapering styliform
process, traversed by dorsal, ventral, and lateral ridges. The cavity of
the egg-case was open in front, and also along each side, where linear,
slit-like valvular apertures freely admitted sea-water into the central
cavity. A similar egg-case from Japan, measuring 9 inches in length, had
its surface traversed by longitudinal and {471}transverse ridges, and no
doubt belonged to a Japanese _Chimaera_.[554] In neither egg-case was there
any trace of tendrils. The eggs probably lie on the sea-bottom, or, when
the cases have styliform prolongations, it is possible that they are
implanted in the ooze.
_Callorhynchus_ (Fig. 269) is distinguished by a singular prolongation of
the rostrum, which terminates in a downwardly-directed cutaneous flap,
evidently from its abundant nerve-supply an important tactile organ. A
frontal clasper is present in the male. The prolonged caudal axis is
up-tilted, and the tail is more distinctly heterocercal than in _Chimaera_.
The only species, _C. antarcticus_, is confined to the Antarctic basin and
the South Pacific. The egg-cases of _Callorhynchus_ differ considerably
from those of _Chimaera_, and so large are they that one may measure 25 cm.
in length, or nearly as long as the abdominal cavity of the Fish. Each case
is ovoid in shape, surrounded by a wide flat margin which is covered on one
side with yellow hair-like fibres, thus giving to the case a protective
resemblance to a mass of seaweed (Fig. 270). In the central part of the
case there is a pear-shaped cavity in which the egg or the embryo is
contained. From one end of this cavity a passage, guarded by a valve, leads
to the exterior, and provides for the escape of the young. While in the
egg-case the nearly ripe embryo has long external gills, and its body is
nearly sessile on a large and singularly lobed yolk-sac.
[Illustration: FIG. 270.—Egg-case of _Callorhynchus antarcticus_, laid open
to show the embryo and its lobed yolk-sac (_y.s_); _s_, dorsal spine.
(Cambridge Museum.)]
{472}[Illustration: FIG. 271.—_Harriotta raleighana_. A, lateral view; B,
ventral view of a male. (From Goode and Bean.)]
The third genus, _Harriotta_ (Fig. 271),[555] is remarkable for its
elongated, tapering, and depressed rostrum, and for the large size and
wing-like appearance of the pectoral fins. There is no frontal clasper, and
the ordinary claspers in the young male examined {473}were very small and
simple. The caudal filament, which is longer in older specimens than in the
younger, and is not developed at all in the youngest examples at present
known (Fig. 272, A), is not uptilted, although the lower lobe of the caudal
fin is much larger than the upper. Young forms have a double row of stout
spine-like denticles in front of the second dorsal fin, and also in the
interval between the latter and the upper caudal lobe. Similar denticles
are also present on the upper surface of the head between the orbits (Fig.
272). _H. raleighana_ is found in the North Atlantic. Individuals varying
in length from 4 to 25 inches have been taken at depths ranging from 707 to
1081 fathoms. A species of _Harriotta_ has also been recorded as occurring
in Japanese waters.[556]
[Illustration: FIG. 272.—Young example of _Harriotta raleighana_, 4 inches
in length. A, side view; B, dorsal view. (From Goode and Bean.)]
With the probable exception of _Chimaera colliei_ the surviving Holocephali
are denizens of deep water; hence their comparative rarity and our almost
complete ignorance of their habits. Young forms of _C. monstrosa_, 1½ to 5
inches in length, have been dredged in the Färoe Channel at depths from 505
to 555 fathoms;[557] and the youngest specimen of _Harriotta_ was obtained
from 991 fathoms. Egg-cases are rarely obtained, and then only from
considerable depths. It is therefore reasonable to {474}infer that these
Fishes breed in deep water. As might be expected, little is known of the
embryology of any of the Holocephali, but that little adds further proof of
the Elasmobranch relationship of the group. The segmentation of the egg of
_Chimaera_ and the overgrowth of the yolk by a circular blastoderm are
essentially as in Elasmobranchs. The early embryos are said to be
shark-like, and to possess both spiracles and "external gills," and the
primary upper jaw is less completely confluent with the skull than in the
adult. It is also said that the palatine dental plates are represented at
an early stage by series of small, more or less conical elements, which,
outwardly at least, resemble the rudiments of the grinding teeth of the
Cestraciont Sharks.[558]
The Chimaeridae first appear in the Lower Oolites, and attain their maximum
development in the Cretaceous and the Eocene.[559] _Ganodus_ is an Oolitic
genus. _Ischyodus_ ranges from the Lower Oolites to the Lower Cretaceous.
_Edaphodon_ is Cretaceous and Eocene, extending, however, into the Miocene,
and _Elasmodus_ ranges from the Upper Cretaceous into the Eocene. Teeth of
the existing genus _Callorhynchus_ occur in the Cretaceous of New Zealand,
and of _Chimaera_ in the Upper Tertiary of Europe and Java. The fossil
Holocephali afford little evidence of the origin of the group from more
typical or more primitive Elasmobranchs. So far as their structure is
known, they all possess the essentially distinctive features of their
modern representatives, and offer little evidence of transitional forms.
The surviving Chimaeroids seem to have acquired a more specialised
dentition, but in other respects they are either more primitive, or
possibly somewhat degenerate.
{475}CHAPTER XVIII
TELEOSTOMI: GENERAL CHARACTERS—CROSSOPTERYGII—CHONDROSTEI—HOLOSTEI
SUB-CLASS II. TELEOSTOMI.
In this group of Fishes the primary upper and lower jaws (palato-quadrate
and Meckelian cartilages) are supplemented by the addition of certain
tooth-bearing membrane bones which form secondary jaws corresponding to the
functional jaws of the higher Craniates.[560] The chondrocranium and the
primary jaws are usually more or less completely ossified by cartilage
bones, and there is always a secondary cranium of dermal bones, of which
paired parietals and frontals above, and a median vomer and a parasphenoid
below, are amongst the most constant. The skull is hyostylic. An operculum
covering the gill-clefts and supported by a special opercular skeleton is a
constant feature. The vertebral column is often acentrous, and when centra
are present they are invariably arch-centra. There is a well-developed
secondary pectoral girdle, connected dorsally with the hinder part of the
skull. As a rule the pelvic girdle is absent altogether, and when present
it is rarely more than a rudiment or a vestige. The endoskeletal supports
of the paired fins are uniserial. The dermal fin-rays of the paired and
median fins are probably modified scales or lepidotrichia. In the median
fins the fin-rays are at first more numerous than their supporting radials,
but in the more specialised Teleostomes they ultimately equal them in
number. The body is usually invested by an exoskeleton of articulated
rhombic or imbricated cycloid scales. Claspers are unknown. In the
surviving members of the group there is usually an {476}air-bladder. The
gill-filaments project freely beyond the outer edges of the greatly reduced
interbranchial septa. The external opening of each nasal sac is usually
divided into two distinct apertures, and there is no oro-nasal groove
leading from the sac to the mouth. The brain has no proper cerebral
hemispheres, but retains an undivided prosencephalon with a non-nervous
roof. A cloaca is not developed, the rectum opening externally by an anus
in front of, and distinct from, the separate or united urino-genital
apertures. The ova are small and numerous, and the segmentation is either
holoblastic and unequal, or meroblastic. Besides a large number of fossil
forms the group includes the vast majority of living Fishes.
The Teleostomi include four "Orders," the CROSSOPTERYGII, the CHONDROSTEI,
the HOLOSTEI, and the TELEOSTEI. Of these the Crossopterygii occupy a
remarkably central position. Remotely connected with the Elasmobranchs on
the one hand, and more intimately related to the Holostei and Teleostei on
the other, they also probably represent the ancestral stock from which the
Stegocephalan Amphibia and the Dipneusti have had their origin. Of the
three remaining groups, often collectively spoken of as "Actinopterygii,"
the Chondrostei are the oldest and most primitive. Like the Crossopterygii,
they are not without evidence of a remote kinship with the Elasmobranchs,
but in a broad general sense they also represent the initial stages in a
sequence of structural modifications, of which the Teleostei, the dominant
Fishes of the present day, are the final outcome.
ORDER I. CROSSOPTERYGII.
Pectoral fins obtusely lobate and probably uniserial, or acutely lobate and
probably biserial. Pelvic fins abdominal in position, uniserial,
non-lobate, or obtusely lobate. Scales rhombic or cycloid, and, like the
dermal cranial bones, they are generally invested by a layer of enamel-like
ganoin. Tail heterocercal, or apparently diphycercal or gephyrocercal.
Vertebral column acentrous, or with ring-like centra, or even with complete
bony amphicoelous centra. Lower jaw with dentigerous splenials. As a rule,
the opercular series includes an operculum and a suboperculum.
Branchiostegal rays absent, their place being taken by a remarkable
armature of jugular plates (Fig. 274). Secondary pectoral girdle
{477}complete, including a pair of infra-clavicles. With rare exceptions
the fin-rays of the median fins retain their numerical preponderance over
the supporting radials. The group is divisible into two "sub-orders," the
OSTEOLEPIDA and the CLADISTIA.[561]
SUB-ORDER 1. OSTEOLEPIDA.
The obtusely or acutely lobate pectoral fins articulate with the pectoral
girdle by a single basal endoskeletal element. Nostrils on the ventral
surface of the snout. Two dorsal fins and an anal fin. Dermal bones of the
ethmoid region often fused with one another and with the premaxillae in
front and the frontals behind to form a continuous rostral shield.
Infra-dentary bones may be present. A series of lateral jugular plates
often present in addition to the pair of principal plates. The Osteolepida
first make their appearance in the Old Red Sandstone and Devonian
formations, where they become abundant. They are also well represented in
the Carboniferous, but only one family survived to the Mesozoic period,
finally becoming extinct in the Upper Cretaceous. The following are the
more important families:—
[Illustration: FIG. 273.—Restoration of _Osteolepis macrolepidota_. Old Red
Sandstone. (From Traquair.)]
FAM. 1. OSTEOLEPIDAE.—Scales rhombic and thickly enamelled. Pectoral and
pelvic fins obtusely lobate. Tail heterocercal. Teeth simple, not
complicated by surface infoldings except quite at the base.
Genera:—_Osteolepis_ (Fig. 273), _Thursius_, _Diplopterus_ (Middle Old Red
Sandstone, Scotland), _Glyptopomus_ (Upper Old Red Sandstone, Scotland),
{478}_Megalichthys_ (Carboniferous and Lower Permian of Europe and North
America).
[Illustration: FIG. 274.—Skull of a Rhizodont (_Rhizodopsis sauroides_),
Lower Carboniferous. A, lateral view; B, the dorsal surface; and C, ventral
view. _an_, Angular; _d_, dentary; _f_, frontal; _i.d_, infra-dentary; _j_,
principal jugular plates; _l.j_, lateral jugulars; _m_, mandible; _m.j_,
median jugular; _mx_, maxilla; _o_, orbit; _op_, operculum; _p_, parietal;
_p.f_, post-frontal; _p.mx_, premaxilla; _p.op_, preoperculum; _so_,
suborbital; _s.op_, suboperculum; _sq_, squamosal; _st_, supra-temporal;
_x_, _x′_, cheek plates. (After Traquair.)]
FAM. 2. RHIZODONTIDAE.—Scales cycloid and overlapping. Paired fins obtusely
lobate. Tail heterocercal, sometimes apparently gephyrocercal. Teeth with
the external enamelled layer of dentine infolded towards the axis in the
form of radially arranged folds. In some genera ring-like vertebral centra
have been recognised and also a preoperculum. Genera:—_Rhizodus_, Lower
Carboniferous of Scotland and Northumberland; _Tristichopterus_[562] (Fig.
275), Old Red Sandstone of Scotland; _Eusthenopteron_[563] (Fig. 276),
Upper Devonian of Scaumenac {479}Bay, Canada; _Gyroptychius_, Old Red
Sandstone, Scotland; _Rhizodopsis_[564] (Fig. 274), Carboniferous of
England, Scotland, Silesia, and North America; _Strepsodus_, Carboniferous
of Great Britain, Ireland, and North America.
[Illustration: FIG. 275.—Restoration of _Tristichopterus alatus_. Old Red
Sandstone. _cl_, Clavicle. Remaining reference letters as in Fig. 274.
(After Traquair.)]
[Illustration: FIG. 276.—Restoration of _Eusthenopteron foordi_. Upper
Devonian of Scaumenac Bay, Canada. The scales have been omitted in the
hinder part of the body to show the vertebral column and the radials of the
median fins. _cl_, Clavicle; _i.cl_, infra-clavicle; _s.cl_,
supra-clavicle; for other reference letters see Fig. 274. × about ¼. (After
Whiteaves.)]
[Illustration: FIG. 277.—Restoration of _Holoptychius flemingi_. × ⅛. (From
Traquair.)]
FAM. 3. HOLOPTYCHIDAE (Dendrodontidae).—Scales cycloid. Pectoral fins
acutely lobate; pelvic fins short and somewhat obtusely lobate. Tail
heterocercal. Teeth similar to those of the Rhizodontidae but more
specialised, the enamelled dentine infoldings being much more complicated,
presenting a radiating {480}arborescent appearance in transverse sections.
Vertebral column acentrous. Genera:—_Holoptychius_[565] (Fig. 277), Old Red
Sandstone of Scotland; Devonian of Belgium, Russia, North America, and East
Greenland. _Glyptolepis_ has a similar range.
[Illustration: FIG. 278.—Restoration of _Undina gulo_. Lower Lias of
Dorset. Scales and supra-clavicle omitted. The ossified air-bladder is
shown beneath the anterior part of the vertebral column. The facial bones
in front of the orbit are unknown, and the cheek-plates are supposed to be
arranged as in other Coelacanths. × about ⅐. (From Smith Woodward.)]
FAM. 4. COELACANTHIDAE.[566]—Scales cycloid. Paired fins obtusely lobate.
Tail symmetrical but apparently gephyrocercal, usually with a protruding
axial vestige of the disappearing terminal part of the tail and of the
proper caudal fin. Radialia of the functional caudal lobes agree in number
with the contiguous neural and haemal arches and dermal fin-rays, the
diagnostic feature of Smith Woodward's Actinistia. Proximal radials of the
dorsal and anal fins fused into a single, internally-forked basipterygium
in each fin. Teeth simple. Vertebral column acentrous. The skull presents
several interesting features. The hyomandibular and the palato-quadrate
bar, for example, are fused on each side into a continuous triangular bone,
articulating with the cranium above and with the lower jaw below. The
opercular skeleton is reduced to an operculum and two jugular plates. A
very singular feature in these Fishes is the ossification of the walls of
the air-bladder (Fig. 278), a structural modification which has no parallel
in Fishes, except in certain Teleosts (Siluridae and Cyprinidae)[567]
{481}in which the organ becomes encapsuled by bone owing to the partial
ossification of its walls.
From their first appearance in the Lower Carboniferous the Coelacanthidae
range, practically unchanged, through the intervening formations to the
Upper Cretaceous. _Coelacanthus_ itself occurs in the Carboniferous and
Permian of England, Scotland, and Germany, and in the Carboniferous of
North America. _Undina_[568] (Fig. 278) is a Jurassic genus. _Diplurus_ is
found in the Trias of North America, and _Macropoma_ is a well-known form
from the Middle and Upper Cretaceous beds of England, and other parts of
Europe.
SUB-ORDER 2. CLADISTIA.
Pectoral fins uniserial and abbreviate, with three basal endoskeletal
elements. Nostrils on the upper surface of the snout. Entire skeleton well
ossified. Notochord replaced by bony, amphicoelous vertebral centra. Bones
of the ethmoid region not fused to form a rostral shield. Infra-dentary
bones absent. Jugular plates reduced to a single pair of large plates. As
this group includes the only Crossopterygii which have survived to the
present day, it is noteworthy that they retain certain primitive features
indicative of their remote origin. The spiracles are persistent; the
intestine has a spiral valve; and the conus arteriosus is furnished with
several rows of valves. Amongst other characters of contrary significance,
the air-bladder is double; its oesophageal aperture is ventral; and its
afferent arteries are pulmonary arteries derived from a posterior aortic
arch.
FAM. 5. POLYPTERIDAE.[569]—Pectoral fins obtusely lobate. Pelvic fins
non-lobate. Scales rhombic and thickly enamelled. Dorsal fin in the form of
a series of isolated finlets, each consisting of a stout spine-like[570]
fulcral scale supporting a single soft ray, or a fringe of several rays,
along its hinder margin. Tail symmetrical, apparently gephyrocercal. Teeth
simple. Nostrils tubular.
The only representatives of the sub-order and the sole {482}surviving
family of Crossopterygii, the Polypteridae, are restricted to the Nile and
to the river basins of tropical Africa which drain into the Atlantic (Fig.
280). Only two genera are known, _Polypterus_ and _Calamichthys_, neither
of which has yet been discovered in any geological deposits, ancient or
recent.
In _Polypterus_ each of the spines of the dorsal fin supports several soft
rays. Pelvic fins and a suboperculum are present. Ten species are known, of
which six pertain to the Congo and its tributaries.[571] _P. bichir_ is
said to attain a length of four feet.
Until recently little was known of the habits of _Polypterus_, but the
observations of Budgett[572] on the widely distributed _P. senegalus_ and
those of Harrington[573] on _P. bichir_, have brought to light many
interesting facts about these most interesting Fishes.
[Illustration: FIG. 279.—_Polypterus senegalus_. From a specimen in the
Cambridge University Museum. The arrow points to the position of the left
spiracle. × ⅓.]
_P. bichir_ haunts the deeper holes and depressions of the muddy bed of the
Nile, although it is "not essentially a bottom-liver or a mud-fish." It is
most active at night when in search of food, and then it may readily be
taken by trawl lines. The lobate pectoral fins are used for progression,
but their primary function is to act as balancers, and they exhibit the
characteristic trembling movements so often seen in the balancing fins of
Teleosts. _Polypterus_ does not readily live out of water, rarely longer
than three to four hours, and then only when covered with damp grass or
weeds. _P. bichir_ is said to feed on small Teleosts, which it swallows
whole, and to these there may be added in other species, Batrachians and
Crustaceans. The observations of Budgett show that in captivity
_Polypterus_ often remains motionless for a long time at the bottom of the
water, the anterior part of the body resting upon the tips of the
{483}pectoral fins. According to the same observer, the air-bladder is an
accessory respiratory organ, supplementary to the gills, rather than a
hydrostatic organ.
[Illustration: FIG. 280.—Map showing the distribution of the Polypteridae.]
In _P. bichir_ the eggs ripen from June to September, inclusive, and, as in
most other Nile Fishes, the breeding season is during or just after the
period of inundation. _P. senegalus_ and _P. lapradei_ spawn during the
rainy season in the months of July, August, and September, but nothing is
certainly known as to the place or mode of deposition of the eggs. During
the breeding season _Polypterus_ is unusually active and excitable, and at
this period the anal fin of the male becomes greatly thickened and
enlarged, and has its surface thrown into deep folds between the successive
fin-rays.[574] The use of the modified fin is not known. During his stay at
McCarthy Island, about 160 miles up the River Gambia, Budgett[575] was
fortunate in securing a larva of _P. senegalus_, 1 to 1¼ inches in length,
or only about one-third the length of any larval _Polypterus_ previously
known (Fig. 281). The larva is described as a most beautiful object,
"marked with black stripes on a golden ground, with a conspicuous golden
stripe on each side above the eye, across the spiracle, and along the
dorsal surface of the external gill." The pinnate external or cutaneous
gills were relatively of much greater size than in the considerably more
advanced stage figured elsewhere,[576] and reached half-way to the tail.
The dorsal fin is not divided into finlets, and behind it is continuous
with the caudal, while the anal fin is scarcely distinct from the
{484}lower lobe of the caudal. The fin-rays which support the ventral
portion of the caudal fin are more numerous and longer than those in
relation with the dorsal lobe, and hence at this stage the tail is really
heterocercal.
[Illustration: FIG. 281.—Larva of _Polypterus senegalus_. × 4. Showing its
characteristic attitude when resting on the bottom of an aquarium, and the
large size of the cutaneous gills. (From Budgett.)]
In the genus _Calamichthys_ the body is greatly elongate and Eel-like in
shape. Pelvic fins are absent, and normally there is no suboperculum. The
dorsal finlets are more isolated than in _Polypterus_, and each spine
supports but a single soft ray. Only a single species is known, _C.
calabaricus_[577] (Fig. 282).
[Illustration: FIG. 282.—_Calamichthys calabaricus_. × ⅔. (From a specimen
in the Cambridge University Museum.)]
_Calamichthys_ has a more restricted distribution than _Polypterus_, and is
confined to certain rivers of West Africa. First obtained at Creek Town on
the Old Calabar river, it is now known to occur in the delta of the Niger,
on the coast of Cameroon, and as far south as the river Chiloango,
frequenting the smaller muddy rivers opening into the estuaries.[578] It is
a {485}very agile Fish, swimming like a snake, and subsisting on insects
and crustaceans. The anal fin is enlarged in the male, and the young are
provided with cutaneous gills. _Calamichthys_ may attain a length of nearly
40 cm.
* * * * *
In the remaining Teleostomi (ACTINOPTERYGII) the paired fins are invariably
non-lobate, with abbreviate, multibasal endoskeletal supports. Fin-rays are
the main support of both the median and paired fins. Jugular plates are
usually replaced by branchiostegal rays, but both may co-exist. The
Actinopterygii are the successors of the Crossopterygii in palaeontological
sequence, and when the latter began to decline in Carboniferous and Permian
times, the former, mainly represented by the earlier Chondrostei, had
already become the dominant Fishes of the period.
ORDER II. CHONDROSTEI (ACIPENSEROIDEI).
In these Fishes, the oldest and the most primitive of the Actinopterygii,
the fin-rays of the median fins still continue to retain their primitive
numerical superiority over the radials, and the tail is heterocercal. There
is a single dorsal and an anal fin, which, like the upper lobe of the
caudal fin, are generally provided with fulcra. Pelvic fins abdominal.
Squamation typically rhombic and ganoid. Vertebral column acentrous. So far
as is known the chondrocranium is but little ossified, and the cranial
bones are mainly dermal. The secondary pectoral girdle still includes a
pair of infra-clavicles.
The Chondrostei are first represented in the Lower Devonian by the solitary
Palaeoniscid genus _Cheirolepis_, a contemporary of the earliest
Crossopterygii. They occur throughout the Mesozoic period, except in the
Cretaceous, and also in the Eocene, and while steadily diminishing in
number and variety they gradually approximate to their degenerate and in
some respects highly specialised descendants, the Sturgeons and
Paddle-Fishes of the existing Fish fauna. Of the seven families included in
the group the Palaeoniscidae are the oldest and the most generalised. The
Platysomidae are a specialised offshoot from the Palaeoniscidae, and, if
they are rightly to be considered as Chondrostei, perhaps the same may be
said of the problematic Belonorhynchidae. On {486}the other hand, there are
certain features in the Catopteridae which indicate an approach to Fishes
of an altogether more modern type. Finally, the Chondrosteidae represent a
stage in a career of degeneration, the climax of which is reached by the
modern Polyodontidae and Acipenseridae.
[Illustration: FIG. 283.—_Palaeoniscus macropomus_. Restoration, nearly
one-half nat. size. (From Traquair.)]
[Illustration: FIG. 284.—Outline restoration of the skull and secondary
pectoral girdle of _Palaeoniscus macropomus_. _an_, Angular; _br.r_,
branchio-stegal rays; _cl_, clavicle (cleithrum); _d_, dentary; _d.ect_,
dermal lateral ethmoid; _f_, frontal; _i.cl_, infra-clavicle; _i.op_,
suboperculum; _mx_, maxilla; _n_, nostril; _op_, operculum; _or_, orbit;
_p_, parietal; _p.f_, pectoral fin; _p.mx_, premaxilla; _p.op_,
preoperculum; _p.t_, post-temporal; _s.cl_, supra-clavicle; _s.o_,
circumorbitals; _sq_, squamosal; the single median bone overlying the short
rostrum is probably a dermal mesethmoid, and the one intercalated between
the squamosal and post-temporal a supra-temporal. The dotted lines indicate
sensory canals. (From Traquair.)]
FAM. 1. PALAEONISCIDAE.[579]—Fishes with fusiform bodies, short dorsal and
anal fins, and usually with a complete investment of articulating rhombic,
rarely cycloid, ganoid scales (Fig. 283). Fulcra generally present at the
bases of the median fins, and especially along the dorsal border of the
upper caudal lobe. Ribs are not known to be present. Skull invested by a
very complete series of paired dermal bones, which in number and
disposition conform to the normal Teleostome type (Fig. 284). The secondary
upper jaw includes both premaxillae and large maxillae; and, as a rule,
both the dentary and splenial bones {487}of the lower jaw are dentigerous.
Except for the absence of an interoperculum, the opercular series of bones
is complete, including numerous branchiostegal rays. There is a single
small median jugular plate.
The Palaeoniscidae are remarkable both for their individual and specific
abundance and for their extensive range in time. Represented only by
_Cheirolepis_ in the Middle Old Red Sandstone and Devonian, the family
attained its maximum development in the later Palaeozoic rocks
(Carboniferous and Lower Permian), became rare in the Mesozoic, finally
dwindling away at the close of the Jurassic period. Their geographical
distribution in the past is hardly less remarkable. In various geological
formations they have been found in Great Britain and Ireland, in widely
remote parts of continental Europe, and in North America, South Africa, and
Australia. _Cheirolepis_, _Amblypterus_, _Canobius_, _Phanerosteon_,
_Elonichthys_, _Cryphiolepis_, _Palaeoniscus_, and _Trissolepis_ are
Palaeozoic genera. _Gyrolepis_, _Urolepis_, _Coccolepis_, _Oxygnathus_, and
_Centrolepis_ are characteristic Mesozoic forms.
[Illustration: FIG. 285.—Restoration of _Eurynotus crenatus_. _in.cl_,
Infra-clavicle; _l.l_, lateral line; _orb_, orbit; other reference letters
as in Fig. 284. (From Traquair.)]
FAM. 2. PLATYSOMIDAE.[580]—More or less deep-bodied Fishes, with elongated
dorsal and anal fins, a high head, short jaws, usually armed with bluntly
conical tritoral teeth, and a complete investment of high, narrow, rhombic
scales. They agree with the Palaeoniscidae in their osteology and in most
other essential {488}features, and they flourished in large numbers during
the Carboniferous and Permian periods. _Platysomus_ ranges from the Lower
Carboniferous to the Upper Permian in Great Britain and continental Europe,
and also occurs in the Carboniferous of North America. _Eurynotus_ (Fig.
285), and the singularly deep-bodied _Cheirodus_ (Fig. 286), in which
pelvic fins are unknown, are British Carboniferous genera.
[Illustration: FIG. 286.—Restoration of _Cheirodus granulosus_. _d.ect_,
Dermal lateral ethmoid; _d.eth_, dermal mesethmoid; _d.sp_, either a dermal
sphenotic or a post-orbital bone; _l.l_, lateral line; _orb_, orbit. The
pectoral fin is indicated in dotted outline. Other reference letters as in
Fig. 284. (From Traquair.)]
FAM. 3. BELONORHYNCHIDAE.—The systematic position of these Triassic forms
is very doubtful, and it is by no means clear that they are Chondrostei at
all.
FAM. 4. CATOPTERIDAE.—It is very probable that this widely-distributed
Triassic family is an offshoot from the Palaeoniscidae. It agrees with the
latter in the general character of the head and pectoral girdle and in the
rhombic squamation, but differs from its progenitors and approaches the
more modern Holostei in the semi-heterocercal condition of the tail, and in
the approximate numerical agreement between the fin-rays and radialia of
the dorsal and anal fins.[581]
{489}FAM. 5. CHONDROSTEIDAE.—This family affords an interesting annectant
link between the Palaeoniscidae and their degenerate living representatives
the Polyodontidae and Acipenseridae. They agree with the latter in the
general shape of the body, the growth of a preoral rostrum, and in the
relatively small size of their ventrally-placed and probably protrusible
mouth (Fig. 287). The skin is entirely scaleless, except on the upper lobe
of the caudal fin, where, as in _Polyodon_ and _Acipenser_, the primitive
rhombic squamation and a series of fulcra are retained.
[Illustration: FIG. 287.—Restoration of the skeleton of _Chondrosteus
acipenseroides_. _a.f_, Anal fin; _c.h_, cerato-hyal; _e_, eye; _h.a_,
haemal arches; _hym_, hyomandibular; _j_, jugal; _n.a_, neural arches;
_n.c_, notochord; _n.s_, neural spines; _pc.f_, pectoral fin; _p.f_, pelvic
fin; _s.o_, suborbital; _s.op_, suboperculum; other reference letters as in
Fig. 284. (After Smith Woodward.)]
On the other hand, their relationship to the Palaeoniscidae is indicated by
the general disposition of the dermal bones of the cranial roof, and the
presence of a transverse row of supra-temporals and of an extensive series
of branchiostegal rays (Fig. 288). The family is represented by
_Chondrosteus_[582] from the Lower Lias of Dorset and Leicestershire, and
_Gyrosteus_ from the Upper Lias of Yorkshire. From an evolutionary point of
view it is significant that the Chondrosteidae do not make their appearance
until the Palaeoniscidae are approaching extinction.
The two remaining families, the Polyodontidae and the Acipenseridae, agree
in presenting a remarkable leaven of characters otherwise distinctive of
the typical Elasmobranch, associated with certain primitive features which
they have doubtless inherited from some remote ancestral stock common both
to existing Elasmobranchs and to the other primary groups of Fishes, and
also with others obviously due to degeneration.
{490}[Illustration: FIG. 288.—Lateral view of a restored skull and pectoral
girdle of _Chondrosteus acipenseroides_. _a_, Angular; _br_, branchiostegal
rays; _c.h_, cerato-hyal; _h.m_, hyomandibular; _j_, jugal; _p.f_,
post-frontal; _s.op_, suboperculum; _s.t_, supra-temporal; other reference
letters as in Fig. 284. (After Traquair.)]
The most interesting illustration of the first point is to be found in the
condition of the primitive upper jaw which, especially in the
Polyodontidae, is typically Elasmobranch in the median union of the
palato-quadrate bars beneath the basis cranii, but Teleostome in the
presence of a secondary upper jaw formed by two maxillae. Both families
also agree in possessing an acentrous vertebral column which, if it does so
far resemble that of Teleostomes in being potentially arco-centrous,
nevertheless has a better developed series of distinct inter-dorsal and
inter-ventral cartilages, regularly alternating with only partially bony
basi-dorsals and basi-ventrals, than is to be met with in any other adult
Fishes except Elasmobranchs. Primitive features are apparent in the
presence of spiracles, sometimes associated with pseudobranchs; the
presence in one family (Acipenseridae) of a hyoidean hemibranch supplied
with blood directly from the ventral aorta, and the existence of a
multi-valvular conus arteriosus and an intestinal spiral valve. Finally,
the massive growth of the chondrocranium wholly devoid of cartilage bones,
except in so far as they may be represented by splint-like membrane bones,
the fragmentation of the investing dermal bones, the degeneration of the
opercular skeleton and the loss of branchiostegal rays, and the almost
complete disappearance of the primitive rhombic squamation, are probably to
be regarded {491}as the outcome of a long-continued career of degeneration
from some remote Palaeoniscid ancestor.
FAM. 6. POLYODONTIDAE.—The Polyodontidae are more generalised, and in some
features decidedly more Selachioid than the Acipenseridae. Body fusiform
and apparently scaleless, but the primitive squamation is still represented
by isolated vestigial scales imbedded in the otherwise soft skin, and by a
continuous series of rhombic scales on the upper caudal lobe, which also
has a dorsal fringe of large fulcra.[583] Rostrum exceptionally long,
spatulate or somewhat conical, with a rigid axis and thinner and more
flexible margins. Barbels absent. Mouth wide, not spout-like. Pectoral fins
devoid of spines. Two pairs of membrane-closed vacuities separate the
paired dermal bones of the cranial roof (possibly parietals and frontals)
from the more laterally-placed post-temporals and squamosals, and there are
no median plates posterior to the orbits, nor any representatives of
supra-temporals. A feeble suboperculum is retained in addition to a small
rayed operculum. Hyoidean hemibranch completely suppressed. Two genera only
are known, each with a single species.
[Illustration: FIG. 289.—_Polyodon folium_. _a_, Anus; _f_, fulcra; _n_,
nostrils; _op_, operculum; _sc_, rhombic scales on the upper caudal lobe;
_sp_, left spiracle.]
The Paddle-Fish or Spoon-Bill, _Polyodon folium_ (Fig. 289) inhabits the
rivers of the Southern States of North America, the Mississippi, Ohio, and
Missouri, and their numerous tributary rivers and streams. A Fish of
sluggish habits, _Polyodon_ feeds chiefly on mud and the minute organisms
it contains, the exceptionally long gill-rakers probably forming an
efficient filter to prevent the food particles escaping through the
gill-clefts with the expiratory water current. The singular rostrum is
apparently used for stirring up the mud when feeding, but in view of the
muddy waters the Fish frequents, and the very small size of {492}the eyes,
its value as a tactile organ must not be overlooked. _Polyodon_ may attain
a length of 5 to 6 feet. The time of spawning varies, according to
locality, from March to June. Nothing is known of the development of
_Polyodon_. Young less than 6 to 8 inches in length are unknown, and
specimens of this size are very rarely seen. The jaws are furnished with
minute teeth until the Fish is about half-grown, when they become
edentalous. Caviare is made from the eggs, and the centres at which this
industry is carried on are chiefly situated along the course of the
Mississippi. The second species, _Psephurus gladius_, inhabits the
Yang-tse-Kiang and Hoangho rivers of China, and differs from _Polyodon_ in
the conical shape of its rostrum and the smaller number and larger size of
its fulcra. _Psephurus_ is stated to reach a length of 20 feet. The family
is represented in the Eocene of Wyoming by the genus _Crossopholis_, which
is note-worthy for the retention of trunk scales in the form of small,
somewhat quadrate denticulated discs, arranged in oblique rows.
FAM. 7. ACIPENSERIDAE.—In the Sturgeon family the body is elongate,
cylindrical, and somewhat bulky. Rostrum well developed and often massive,
with a transverse row of simple or branched preoral barbels on its ventral
surface. Mouth small and remarkably protrusible. Jaws devoid of teeth
except in the larvae. As in the preceding family, the primitive rhombic
squamation is confined to the upper lobe of the tail, which, like the
dorsal and anal fins, is furnished with fulcra. Elsewhere the scales are
represented by five longitudinal rows of large bony scutes and by
intervening small scattered ossifications. The anterior dermal ray of the
pectoral fin is stout and spine-like. The dermal bones of the cranial roof
suturally articulate with one another to form a continuous shield,
uninterrupted by lateral vacuities. A median dermal bone in the occipital
region transmits the occipital sensory canal. The opercular series is
represented only by an opercular bone.
The family includes but two genera, _Acipenser_ (Fig. 290) and
_Scaphirhynchus_, and about twenty species, confined to the seas,
estuaries, and rivers of the temperate and north temperate regions of the
northern hemisphere. _Acipenser_ includes the more typical Sturgeons, and
is distinguished by the presence of spiracles, and by the fact that the
longitudinal rows of scutes remain distinct to the base of the caudal fin.
{493}[Illustration: FIG. 290.—The Sterlet (_Acipenser ruthenus_). _o_,
Barbels; _c.f_, caudal fin; _d.f_, dorsal fin; _pct.f_, pectoral fin;
_pv.f_, pelvic fin; _sc_, scutes; _v.f_, ventral or anal fin. (From Parker
and Haswell, after Cuvier.)]
There are probably about fifteen species, but the exact number is
uncertain. Sturgeons are abundant in the Black Sea, the Sea of Azov, the
Caspian, and their tributary rivers, notably the Danube, Don, Dnieper,
Ural, and Volga. They are also present in the rivers and on the coasts of
Northern Europe and of China. Five species occur in North America, on the
Atlantic and Pacific coasts, and in the rivers of these regions as well as
in the Great Lakes.[584] One or two species are almost exclusively
fresh-water, but most Sturgeons are migratory Fishes, living in the sea,
but ascending rivers for spawning. Their food consists of worms, molluscs,
the smaller Fishes and aquatic plants; and in feeding the mouth is
protruded downwards in the form of a cylindrical, spout-like structure and
thrust into the mud. The only species certainly known to frequent the
British coasts is the common Sturgeon (_A. sturio_), which is also found in
the Black Sea and the Mediterranean, and is abundant on the Atlantic coast
of North America from Maine to South Carolina. The species occurs all round
our coasts, more plentifully, perhaps, on the northern and eastern shores.
In the spring and summer the Fish ascends the rivers, often to a
considerable distance. Its presence has been recorded in the Severn, near
Shrewsbury; in the Trent at Nottingham, and also, but not in recent years,
in the Thames above London Bridge.[585] In this country the species is a
"Royal Fish," and by an unrepealed Act of Edward II. it is enacted that
"the King shall have the wreck of the sea throughout the realm, Whales and
Great Sturgeons, except in certain places privileged by the King."[586] If
not so large as some of its Russian relatives, _A. sturio_ often attains a
great size. Even on {494}our own coasts the capture of individuals 8 to 10
feet in length has been recorded. The great Russian Sturgeon (_A. huso_),
which is common in the Black Sea, the Sea of Azov and the Caspian, and in
the rivers flowing into them, is the largest of all the Sturgeons,
individuals weighing 2760 and 3200 pounds having been captured. The Sterlet
(_A. ruthenus_), similarly distributed and often ascending the Danube to
Vienna, is much smaller, rarely exceeding a length of three feet.
[Illustration: FIG. 291.—Larval _Acipenser ruthenus_. _a_, Anus; _b_,
barbels; _e_, eye; _g_, gills; _m_, mouth, with teeth; _ol.o_, olfactory
organ; _op_, operculum; _pt.f_, pectoral fin; _sp_, spiracle. × 10. (From
Kitchen Parker.)]
In Europe _A. sturio_ spawns about July, but in North America (Delaware
river) during May. Small in size, the eggs are produced in enormous
numbers, a single female, it is said, producing about 3,000,000 in one
season. They are invested by a gelatinous sheath, so that they readily
stick to one another or to other objects, and, when deposited, they adhere
in streaks or sheet-like masses to the bed of the river. The young are
hatched very early, about the third or fourth day in _A. sturio_, and in
the Sterlet between the ninth and twelfth, the length of the larva then
varying from 7 to 10 mm. When they are a few days old the larvae closely
resemble those of existing Holostei except that the small opercular folds
leave the gills freely exposed (Fig. 291). A shallow pigmented groove in
front of the mouth apparently represents the sucker of the young _Amia_ and
_Lepidosteus_. Although toothless in the adult, both the Sturgeon and
Sterlet possess vestigial rudimentary, uncalcified, larval teeth, which in
shape resemble the teeth of a Dog-Fish, consisting of a broad base and a
sharp spine.
The Sturgeon is a Fish of considerable economic importance. The flesh is an
article of food, and from the ovaries of certain Russian and American
species thousands of hundredweights of {495}caviare are prepared annually.
Large quantities of isinglass are obtained from the air-bladders, in the
United States and in Russia. The organ is split open and washed; the inner
lining is then stripped off and the bladder dried as rough isinglass.
The second genus, _Scaphirhynchus_, which includes the Shovel-nosed
Sturgeons, differs from _Acipenser_ in the long, flattened, and almost
spatulate shape of the rostrum, the suppression of the spiracles, and the
union of the longitudinal rows of scutes beneath the dorsal fin to form a
scaly armature completely investing the tail. The distribution of the genus
affords an interesting parallel to that of the Polyodontidae. Of the four
species, one (_S. platyrhynchus_) is common in the Mississippi valley and
in the rivers of the Western and Southern States of North America, while
the remaining species, also exclusively fresh-water, frequent the rivers of
Tartary.
The Acipenseridæ are not known to occur earlier than the Tertiary. Scutes,
pectoral spines and fragmentary bones, indistinguishable from the
corresponding parts of existing species, have been recorded from the London
Clay of the Isle of Sheppey (Lower Eocene), and from later Eocene deposits
in the Isle of Wight and Hampshire; and also from the Pliocene of England
(Red Crag of Suffolk) and Virginia.
ORDER III. HOLOSTEI (LEPIDOSTEOIDEI).
The Holostei include a large and somewhat heterogeneous assemblage of
Fishes, most of which are now extinct. As a group they are by no means easy
to define or delimit. Widely separated from the Chondrostei, there is
little evidence of the existence of connecting links between the two
groups, although in some respects the Catopteridae may be regarded as
transitional. On the other side, however, the Holostei shade off almost
imperceptibly into the Malacopterygian Teleostei. In different fossil and
recent Holostei there may be traced the gradual acquisition of the more
special Teleostean characters and the elimination of the more archaic
features of their remote Teleostome ancestors; and in a general sense this
may be taken as the key to the more salient attributes of the group. It is
not suggested that all the families of Holostei are on the direct lines of
Teleostean descent. Some families, like the Eugnathidae and {496}Amiidae,
may possibly occupy this position, but others, such as the Pycnodonts, for
example, seem to be highly specialised and terminal offshoots which have
left no descendants. Of the more generalised features which different
Holostei retain, mention may be made of the prevalence of rhombic scales
which, like the dermal cranial bones, are generally invested by a variously
ornamented coat of ganoin; the presence of fulcra, cheek-plates, post- or
sub-orbital ossicles, and of a complex lower jaw, which includes
dentigerous splenials; and the abdominal position of the pelvic fins. On
the other hand, indication of advancing specialisation in the Teleostean
direction are to be noted in the numerical agreement between the dermal
fin-rays of the median fins and their supporting radialia, and in the
character of the vertebral column. Some Holostei, especially the earlier
forms, are acentrous, but between this primitive condition and the
possession of well-ossified centra, associated with equally bony arcualia,
almost every gradation is to be found. The chondrocranium is more or less
completely replaced by cartilage bones corresponding to those generally
present in Teleosts, while the palato-pterygoid cartilages, likewise
modified by the growth of cartilage bones, separately articulate with the
lateral ethmoid regions instead of meeting in a ventral symphysis beneath
the basis cranii. With rare exceptions (_e.g._ certain Pycnodonts) the
opercular skeleton is complete, and includes branchiostegal rays; and
although a single gular plate is often present, it may be absent in entire
families. Like so many other structures, the tail is in a transitional
state: really heterocercal, but incipiently homocercal, it may be described
as semi-heterocercal. Infra-clavicular plates no longer form part of the
secondary pectoral girdle, their place being taken by cleithra which, as in
most Teleosts, meet in a ventral symphysis.
Indications of transition are not wanting in the squamation in certain
families, and may be seen in the partial or complete replacement of the
rhombic type by thin, imbricated, cycloid scales. Lastly, the soft parts of
the two surviving genera are not without features of similar significance.
A multivalvular conus arteriosus, it is true, is still retained, but the
spiral valve is vestigial, the spiracles are closed, and in the female of
one genus (_Lepidosteus_) the gonoducts are peritoneal tubes, continuous,
as in most Teleosts, with the investments of the ovaries.
{497}The Fishes here included in the Holostei constitute the Protospondyli
and Aetheospondyli of Smith Woodward.[587] In the former group vertebral
centra are either entirely absent, or, if present, their components in the
form of alternating hypo- and pleuro-centra invariably remain distinct in
the tail. The latter group has been instituted for the provisional
reception of two highly specialised families of uncertain relationships,
which differ from the Protospondyli in their higher grade of vertebral
structure, the centra always being complete without any indication of
distinct hypo- and pleuro-centra.
The Holostei first appear in the Permian, where they are represented by a
single genus (_Acentrophorus_). During the Mesozoic period they were
abundant in the Trias, reaching their maximum development and becoming the
dominant Fishes of the period in the Jurassic. In the Cretaceous they began
to decline, and in the Tertiaries became reduced to the two families which
at the present day are the sole survivors of the group.
Of the six families of Protospondyli the Semionotidae are the oldest and
most generalised, and the Macrosemiidae a closely allied group. The
Pycnodontidae are a highly specialised and terminal offshoot. The
Eugnathidae obviously lead to the Amiidae, and from the same stock it is
probable that the Pachycormidae have been derived. The relations of the
Aspidorhynchidae and Lepidosteidae (Aetheospondyli) are extremely doubtful.
That the two families are allied seems probable, but beyond the possibility
of a remote connection with the Protospondyli there is no clue to their
ancestry.
[Illustration: FIG. 292.—Restoration of _Lepidotus minor_. Upper Jurassic,
Dorset. × ⅕. (After Smith Woodward.)]
FAM. 1. SEMIONOTIDAE.—Small-mouthed, fusiform or {498}deep-bodied
Holosteans with rhombic scales, rarely, as in _Aetheolepis_, cycloid in the
caudal region. All the fins possess fulcra. Teeth more or less conical,
with a tendency to become tritoral in certain genera. Jugular plate present
or absent. _Acentrophorus_ (Upper Permian); _Semionotus_ (Trias of England,
Germany, S. Africa, and N. America); _Lepidotus_ (Fig. 292) (Trias of
Germany, Jurassic of Europe and India, Cretaceous of Brazil); the
deep-bodied _Dapedius_ (Lias of Dorset, Fig. 293), and _Aetheolepis_
(Jurassic of New South Wales) are characteristic genera.
[Illustration: FIG. 293.—Restoration of _Dapedius politus_. Lower Jurassic,
Dorset. × ¼. (After Smith Woodward.)]
FAM. 2. MACROSEMIIDAE.—Closely related to the Semionotidae, but with a more
extended dorsal fin. _Macrosemius_ (Upper Jurassic of England, Germany,
France); _Notagogus_ (Upper Jurassic of Naples, Bavaria, France);
_Petalopteryx_ (Upper Cretaceous of Syria).
FAM. 3. PYCNODONTIDAE.—Highly specialised deep-bodied Fishes, with a small
mouth and characteristic grinding or tritoral teeth. Scales rhombic. Fulcra
absent. Dorsal and anal fins long. There is no jugular plate. The family
ranges from the Lower Lias to the Lower Eocene, inclusive. _Mesodon_,
_Mesturus_, _Gyrodus_, and _Microdon_ are Jurassic genera. _Coccodus_ and
_Xenopholis_ occur in the Upper Cretaceous of Syria (Mount Lebanon), and
_Pycnodus_ in various European Eocene formations.
FAM. 4. EUGNATHIDAE.—Large-mouthed, elongate fusiform {499}predaceous
Fishes, with pointed teeth, rhombic scales, short dorsal and anal fins, a
single jugular plate and prominent fulcra. The vertebral centra are
represented by distinct hypo- and pleuro-centra, which may form complete
alternating rings in the tail.
The family first appears in the Trias and ranges throughout the Jurassic
period. _Eugnathus_ (Jurassic) and _Eurycormus_ (Upper Jurassic). _Caturus_
(Fig. 294) has a more extensive range, occurring in the Upper Trias of the
Tyrol and in the Upper Jurassic of England and Bavaria. _Caturus_ and
_Eurycormus_, with their relatively thin, imbricated, cycloid scales, which
have lost the peg-and-socket articulation, form connecting links between
the more typical _Eugnathus_ and the Amiidae.
[Illustration: FIG. 294.—Restoration of _Caturus furcatus_, omitting the
squamation. × 1/11. Upper Jurassic of Bavaria. (From Smith Woodward.)]
FAM. 5. AMIIDAE.—Body fusiform and somewhat compressed. Scales uniformly
thin, cycloid, and imbricated. Single dorsal fin long and low. Anal fin
short. Tail nearly homocercal, with a rounded hinder margin. Fulcra absent
from all the fins. Moderately large conical teeth are present on the
premaxillae, maxillae, palatines and dentaries, and smaller teeth on the
vomers, pterygoids, splenials and parasphenoid. Pre- and post-centra fused
in the trunk, forming complete bony amphicoelous centra, but distinct in
the tail. A single large jugular plate is present. In the solitary living
species the air-bladder is cellular, and its afferent arteries are derived
from a posterior aortic arch. Pyloric caeca absent. Two peculiar comb-like
structures are present on the throat.
The Bow-Fin (_Amia calva_), the sole existing representative of the family,
is abundant in the rivers and lakes of Central and Southern North America,
including the great lakes Huron and {500}Erie. It is a voracious,
carnivorous Fish, preying upon other Fish as well as upon fresh-water
Crustaceans and Insects, very tenacious of life, and of no economic value.
The male is smaller than the female, about 18 inches in length, and is
distinguished by the presence of a round black spot, encircled by a margin
of orange, at the base of the caudal fin (Fig. 295). The female may exceed
24 inches.
[Illustration: FIG. 295.—The Bow-Fin (_Amia calva_). (From a specimen in
the Cambridge University Museum.) × ⅕.]
_Amia_ frequently rises to the surface, especially when the water is foul,
and takes in large mouthfuls of air, and it is probable that the air is
subsequently passed into the spacious cellular air-bladder which acts as a
lung. The breeding season, during which the coloration of the Fish is more
brilliant than at other times, lasts from the beginning of May to June, but
it may begin and end somewhat earlier if the temperature be favourable. The
Fish makes its way from the deeper water, where it has remained sluggish
during the winter, to the spawning ground. This is usually at the swampy
end of a lake where there is an abundance of aquatic herbage intersected by
channels of clear water. There the Fish is said to circle round until the
soft weeds and rootlets are bent and crushed aside, so as to leave an area
having the appearance of a crude form of nest,[588] in which the eggs are
deposited. They may be found in enormous numbers adhering to the leaves and
rootlets of the weedy home. After oviposition the male remains on guard
until the young are hatched out, when they appear to leave the nest in a
body, still under the protection of their watchful parent. At all events a
little later the male has been observed to be accompanied by a swarm of
young fry, which he keeps together by circling round them. The development
of the eggs is remarkably rapid. From the first cleavage of the egg to the
hatching of the embryo the whole {501}process may be completed within from
4 to 8 days. When hatched the larvae are about 5 to 6 mm. long. They
possess a large yolk sac and a preoral sucker for attachment. The pectoral
fins are conspicuous structures before there is any trace of the pelvic
fins.
[Illustration: FIG. 296.—_Amia_ and its nest. (From Bashford Dean.)]
The Amiidae, represented by _Megalurus_,[589] first appear in the Upper
Jurassic of Dorset, France, and Bavaria. In the Cretaceous period the
family is represented by _Amiopsis_. Species of _Amia_ occur in the Eocene
of Europe and North America. In the former area the genus became extinct at
the close of the Lower Miocene period, but in the latter _Amia calva_ still
survives.
FAM. 6. PACHYCORMIDAE.—Large-mouthed, predaceous Amioid Fishes with a more
or less prominent snout and a short dorsal fin. Scales rhombic but thin,
rounded behind, and overlapping, sometimes absent. A single large jugular
plate.
In the earlier forms (_e.g._ _Pachycormus_, Lias) the snout is but slightly
produced, but in _Hypsocormus_ (Upper Jurassic), and {502}especially in
_Protosphyraena_ (Cambridge Upper Greensand and the Cretaceous of Europe
and North America), it becomes greatly elongated and associated with an
exceptionally strong dentition.
[Illustration: FIG. 297.—Restoration of _Hypsocormus insignis_, omitting
the squamation. Upper Jurassic of Bavaria. × ⅛. (From Smith Woodward.)]
FAM. 7. ASPIDORHYNCHIDAE.—Long-bodied Fishes, with a pointed preoral
rostrum, sharp teeth, and deep rhombic scales. Fins small, the dorsal and
anal being remote from the pelvic fins. Fulcra vestigial or absent. Jugular
plates not known.
Two genera only are known. _Aspidorhynchus_ is a Jurassic form.
_Belonostomus_ is Upper Jurassic and Cretaceous. Species of the latter
genus have a very wide distribution (Europe, North and South America, and
Australia).
[Illustration: FIG. 298.—Restoration of _Aspidorhynchus acutirostris_.
Upper Jurassic of Bavaria. × 1/11. (From Smith Woodward.)]
FAM. 8. LEPIDOSTEIDAE.—Body elongate, with a relatively short caudal
region. Tail semi-heterocercal. Scales rhombic, thick, ganoin-coated and
articulated, not vertically elongated on the sides of the body. Dorsal and
anal fins short and remote from the pelvic fins. Median fins with fulcra.
Both the upper and lower jaws more or less elongated, forming a broad and
depressed or a long tapering beak, near the anterior end of which the
nostrils are placed. Eyes small. Vertebral centra well {503}ossified,
opisthocoelous and fused with the neural arches. The metapterygoid bones
have a secondary articulation with the skull.[590] Maxillae segmented into
numerous pieces. Jugular plates absent. Branchiostegal rays reduced to
three on each side. Teeth numerous, slender, and of unequal size. In the
larger teeth the dentine is intricately folded. Pyloric caeca branched and
compacted together into a gland-like mass. Air-bladder cellular, but its
blood is not derived from a posterior aortic arch.
The only known genus is _Lepidosteus_, the existing species of which
frequent the fresh waters of North America.[591] The common or Long-nosed
Gar-Pike (_L. osseus_), remarkable for its long and slender beak, is
generally abundant in the rivers and lakes of the United States from
Vermont to the Rio Grande, and it may reach a length of five feet. The
"Short-nosed Gar" (_L. platystomus_, Günther) has a much shorter and
broader beak, and a similar distribution (Fig. 299). The "Great" or
"Alligator Gar" (_L. viridis_, Günther) has a more southerly habitat,
frequenting the rivers of the Southern States, Northern Mexico, and Cuba.
It is by far the largest species, sometimes reaching a length of 8 to 10
feet.
[Illustration: FIG. 299.—Short-nosed Gar Pike (_Lepidosteus platystomus_.)
× ⅛. (From Bashford Dean, after Goode.)]
_Lepidosteus_ is a voracious Fish, preying upon smaller Fishes, and, except
in the breeding season, it frequents the deeper parts of the rivers or
lakes. The Fish is constantly in the habit of rising to the surface and
emitting bubbles of gas, either through the mouth or by the branchial
clefts, and it is probable that this gas is air which has been previously
swallowed at the surface and passed into the air-bladder. About May
_Lepidosteus_ resorts in large numbers to shallower water, where the
temperature is {504}higher, for the purpose of spawning, each female being
attended by from one to four males.[592] During brief recurring periods of
excitement, accompanied by convulsive lashing movements, the eggs and sperm
are emitted. The eggs are extremely sticky, and adhere tenaciously to the
rocks and stones on which they are deposited. In a few days the embryos
hatch out, and at this stage the larva has a huge mouth surmounted by a
terminal preoral disc, fringed with a row of marginal wart-like suckers
(Fig. 300). The yolk sac is so large as greatly to hamper the movements of
the larva; hence, by means of its suckers, the young _Lepidosteus_ attaches
itself to surrounding objects, and remains almost entirely motionless for
some little time after hatching. Later, about a fortnight after escaping
from the egg, the yolk becomes completely absorbed, the suckers degenerate
and eventually disappear, and the larva, freed from its load of nutritive
reserve, assumes a more active life. After the absorption of the yolk the
larvae of Mosquitos appears to form the exclusive diet of the young
_Lepidosteus_ for some time, but very soon young Fishes are readily
devoured.[593]
[Illustration: FIG. 300.—Larval _Lepidosteus osseus_, 11 mm. long. _a_,
Anus; _a.f_, _c.f_, _d.f_, developing anal, caudal, and dorsal fins; _m_,
mouth; _ol_, olfactory organ; _op_, operculum; _pt.f_, pectoral fin; _s_,
sucker. (From Balfour and N. Parker.)]
_Lepidosteus_ seems to have been abundant in Europe during the Eocene and
Miocene periods, but became extinct before the Pliocene. In North America,
also, the genus dates from the Eocene, and still survives.
ORDER IV. TELEOSTEI.
[For the account of this Order, see pp. 541 f.]
{505}CHAPTER XIX
DIPNEUSTI
SUB-CLASS III. DIPNEUSTI (DIPNOI).
These singularly interesting Fishes are distinguished by their more or less
acutely lobate paired fins and their overlapping cycloid scales, and by the
fact that the bony dermal fin-rays of the median fins are much more
numerous than their supporting radialia. Tail heterocercal or apparently
diphycercal. Nostrils inferior. Vertebral column acentrous. The radialia of
the median fins articulate with the contiguous neural or haemal spines and
agree with them numerically. Skull autostylic. Premaxillae and maxillae
absent, but a secondary lower jaw is represented by certain dermal bones of
which tooth-bearing splenials are the most important, the dentary bones
being absent altogether, or, if present, toothless and small. The cranial
dermal bones include median as well as paired lateral plates, but their
relations to those of other Fishes are very obscure. Two opercular bones
are always present, but branchiostegal rays are unknown. One of the most
important diagnostic features is the dentition. All Dipneusti agree in
possessing large tritoral dental plates supported by the palato-pterygoid
and splenial bones. The secondary pectoral girdle includes only cleithra
and infraclavicles (clavicles). There is a pelvic girdle. Claspers absent.
Of the four families of Dipneusti, two, the Ctenodontidae and the
Uronemidae, are exclusively Palaeozoic. The third, the Ceratodontidae, is
Mesozoic, and still survives. The fourth, the Lepidosirenidae, is known
only by two existing genera.
{506}[Illustration: FIG. 301.—Restoration of _Dipterus valenciennesi_. × ⅕.
(From Traquair.)]
[Illustration: FIG. 302.—Outline restoration of _Phaneropleuron andersoni_.
Upper Devonian. (From Dollo, after Traquair.)]
[Illustration: FIG. 303.—Dental plates of _Dipterus valenciennesi_, nat.
size. A, Upper jaw; B, lower jaw. _n_, Position of the nostrils; _p.p_,
palatine dental plates; _p.pt_, palato-pterygoid bones; _sp_, splenial
teeth. (From Smith Woodward, after Traquair.)]
FAM. 1. CTENODONTIDAE.—Body fusiform. Tail heterocercal or apparently
diphycercal. Excluding the anal fin, which is always distinct, the
remaining median fins are either distinct or continuous. Dental plates
traversed by radiating transverse ridges terminating in rows of conical
denticles (ctenodont dentition, Fig. 303). Vomerine teeth not known.
Cranial bones numerous and small, and, like the squamation, with or without
an investment of ganoin. Jugular plates present or absent.[594] The oldest
genus is _Dipterus_[595] from the Old Red Sandstone of Scotland, where it
is contemporaneous with the earliest Crossopterygii and also with the
oldest known Actinopterygii (_Cheirolepis_). _Phaneropleuron_ (Old Red
Sandstone of Scotland, Upper Devonian of Canada, Fig. 302), _Scaumenacia_
(Upper Devonian of Canada), _Ctenodus_ (Carboniferous of Great Britain and
North {507}America), and _Sagenodus_ (Carboniferous of Great Britain and
Lower Permian of Bohemia) belong to the same family.
FAM. 2. URONEMIDAE.—Body fusiform. Dentition non-ctenodont, consisting of
patches of distinct rounded denticles with a row of basally-confluent
conical denticles along the outer margin of each. Scales thin. All the
median fins are continuous. Tail apparently diphycercal. Cranial dermal
bones as in _Dipterus_. _Uronemus_[596] (Lower Carboniferous of Scotland),
and perhaps _Conchopoma_[597] (Lower Permian of Prussia), are the only
known genera.
The two remaining families possess certain features which cannot be
affirmed to have existed in their extinct allies. Thus, both agree in
exhibiting those striking and, so far as Fishes are concerned, unique
modifications of the air-bladder and vascular system, and the olfactory
organs,[598] which are more or less closely associated with air-breathing
habits and indicate a marked convergence towards the Amphibia. Side by side
with such indications of advancing specialisation in certain directions,
ample evidence of a remote ancestry is to be seen in such primitive
features as the presence of a spiral valve and a multi-valvular conus
arteriosus, and in the short and simple alimentary canal. Of other points
of agreement mention may be made of the absence of jugular plates, the
presence of vomerine teeth, the continuity of all the median fins, and the
apparently diphycercal but probably gephyrocercal character of the tail.
FAM. 3. CERATODONTIDAE.—Body elongated and compressed. Scales large, thin,
non-ganoid, and partially enclosed in dermal pouches. Paired fins biserial.
Chondrocranium complete. Dermal bones wholly devoid of ganoin, reduced in
number but increased in size. Circumorbital bones present. Dental plates
oval, crescentic or triangular, traversed by several radiating enamelled
ridges, terminating in smooth or feebly denticulated biting margins. Lower
jaw with a small toothless dentary on each side. The hyoid arch includes a
small hyomandibular and a hypo-hyal in addition to a cerato-hyal. Branchial
arches five in number and bisegmented. The gills exhibit little evidence of
{508}degeneration. Hyo-branchial cleft open, and associated with a
pseudobranch. The first four branchial arches carry holobranchs.
Air-bladder single. Young not provided with cutaneous gills. Two genera
only are known, the Mesozoic _Ceratodus_ and the still living
_Neoceratodus_. The former genus includes numerous species, for the most
part known only by their dental plates, and has a remarkably wide
distribution in different geological formations. Species occur in the Trias
of England, Germany, India, South Africa (Upper Karoo strata), and also,
but more rarely, in certain Jurassic deposits in England and in
Colorado.[599] _Neoceratodus_ is represented by a solitary species, _N.
forsteri_[600] (Fig. 304, A), which is now restricted to the Burnett and
Mary rivers in Queensland. A somewhat wider distribution of the species in
recent times is indicated by the presence of teeth in the later Tertiary
(alluvial) deposits of Darling Downs, near the borders of New South Wales.
The _Neoceratodus_[601] of the Burnett frequents the comparatively stagnant
pools or water-holes which alternate with shallow runs and are usually full
of water all the year round. In these pools, filled with a rich growth of
aquatic vegetation, and often the favourite haunt of the Platypus
(_Ornithorhynchus_), the Fish is fairly abundant. Inactive and sluggish in
its habits, usually lying motionless on the bottom, the Fish is easily
captured by the natives with hand-nets or baited hooks. _Neoceratodus_
lives on fresh-water Crustaceans, worms, and molluscs, and to obtain them
it crops the luxuriant vegetation of the water-holes much in the same way
that a Polychaet or a Holothurian swallows sand for the sake of the
included nutrient particles. Apparently the air-bladder is a functional
lung at all times, acting in conjunction with the gills. At irregular
intervals the Fish rises to the surface and protrudes its snout in order to
empty its lung and take in fresh air. While doing so the animal makes a
peculiar grunting noise, "spouting" as the local fishermen call it, which
may be heard at night for some distance, and is probably caused by the
forcible expulsion of air through the mouth. Useful as the lung is as a
breathing organ under normal conditions, there can be little doubt that its
value as such is much greater whenever gill-breathing becomes difficult or
impossible.
{509}[Illustration: FIG. 304.—A, _Neoceratodus forsteri_, Queensland; B,
_Protopterus annectens_, Gambia. C, _Lepidosiren paradoxa_, Paraguay. The
lozenge-shaped markings on the surface of B do not represent scales but
areas of the skin outlined by pigment cells. In a fresh specimen the scales
are as completely invisible as in C. (A, from Günther; B and C, from
Lankester.)]
{510}[Illustration: FIG. 305.—A young _Neoceratodus_ four weeks after
hatching. _c_, Cloacal aperture; _l.l_, lateral line; _m_, mouth; _op_,
operculum; _p.f_, pectoral fin. (From Semon.)]
This seems to be the case during the hot season, when the water becomes
foul from the presence of decomposing animal or vegetable matter. Semon
records a striking illustration of this in the case of a partially dried-up
water-hole, in which the water had become so foul that it was full of dead
fishes of various kinds. Fatal as these conditions were to ordinary Fishes,
_Neoceratodus_ not only survived but seemed to be quite healthy and fresh.
Such observations are of exceptional interest. Not only do they afford a
clue to the conditions of life which, in the course of time, probably led
to lung-breathing in _Neoceratodus_, but they also suggest the possibility
that a similar environment has been conducive to the evolution of
air-breathing Vertebrates from gill-breathing and Fish-like progenitors. In
spite of its pulmonary respiration, _Neoceratodus_ more closely resembles
the typical Fishes in its habits than any other Dipneusti. It lives all the
year round in the water. There is no evidence that it ever becomes dried up
in the mud, or passes into a summer sleep in a cocoon, and the
well-developed condition of its gills suggest that these organs play a more
important rôle in breathing than in either _Protopterus_ or _Lepidosiren_.
The Fish is not known to leave the water, and the paired fins, useful no
doubt as paddles, are quite incapable of supporting the bulky body on terra
firma. In fact, when _Neoceratodus_ is taken out of its natural element it
seems to be more helpless than most other Fishes, and, in spite of its
capacity for lung-breathing, soon dies unless kept moist by artificial
means. Spawning takes place from April to November, principally in
September and October. The eggs, invested by a jelly-like coat, secreted by
the oviducal walls, are deposited {511}not in a nest, but singly amongst
aquatic vegetation, and, as they are not adherent, it is probable that they
finally rest on the mud. The early developmental stages exhibit a general
resemblance to those of Amphibia. There is no larval metamorphosis, and at
no period does the young _Neoceratodus_ (Fig. 305) possess cutaneous gills
or a cement organ. The tail is apparently diphycercal from the first, and
the pelvic limbs do not appear until about six weeks after the pectoral
members. It is interesting to note that the dental plates are first
represented by lines or patches of separate denticles (non-ctenodont),
which subsequently fuse basally (ctenodont) before the adult condition is
reached.[602] _Neoceratodus_ is stated to grow to a length of 5 to 6 feet.
FAM. 4. LEPIDOSIRENIDAE.—Body elongate, cylindrical and more or less
Eel-like, with small cycloid scales completely enclosed in the skin. Paired
fins so acutely lobate as to present the appearance of tapering cylindrical
filaments, equally devoid of scales and fin-rays. In a general way the
cranial dermal bones correspond with those of _Neoceratodus_, but the place
of the posterior median bone is taken by a large, gable-like
fronto-parietal bone, situated internal to the head muscles, and in direct
relation with the chondrocranium, which is largely aborted in the
interorbital region. Circumorbital bones absent. Opercular bones much
reduced. Lower jaw without dentary plates. Palatine and splenial dental
plates with three non-denticulate, trenchant ridges. Hyoid arch consists of
cerato-hyals only. Hyoidean cleft closed. Certain of the anterior branchial
arches devoid of branchial filaments; when present the latter are leaf-like
and free. Air-bladder a double lung. There is a larval metamorphosis, and
the young possess cutaneous gills. The family includes two genera,
_Protopterus_ and _Lepidosiren_. In the former genus the paired fins are
either uniserial or they consist of axial mesomeres only; there are six
branchial arches and five clefts; and the larval gills are usually retained
as vestiges throughout life. In _Lepidosiren_ the paired fins are reduced
to the segmented axis, without pre- or post-axial radials. There are five
branchial arches and four clefts, and the cutaneous gills disappear soon
after the larval metamorphosis.
{512}[Illustration: FIG. 306.—Map showing the distribution of the surviving
Dipneusti.]
_Protopterus_ has a wide distribution over the middle portion of the great
African continent, ranging from the river Senegal and the White Nile on the
north to the Congo basin, Lake Tanganyika, and the Zambesi on the south.
Three species are known, _P. annectens_ (Fig. 304), _P. aethiopicus_, and
_P. dolloi_. _Protopterus_[603] is usually found in marshes in the vicinity
of rivers. Voracious in its habits the Fish is mainly carnivorous,
subsisting principally on Frogs, worms, insects, and crustaceans. It is by
no means averse to preying upon its own kind, and if several of these
Fishes are confined in the same aquarium they are apt to give free vent to
their cannibal instincts by biting off the tails or limbs of their fellows.
The missing parts are soon regenerated, but the new members are usually
somewhat abnormal, the tail, for instance, never regaining its original
length, while a new pectoral limb may be bifid or even trifid.[604] The
tail is the principal organ of locomotion, and by its means the Fish is
capable of remarkably quick, agile movements. When slowly moving over the
bottom of an aquarium the paired limbs are observed to move to and fro on
opposite sides alternately in a somewhat bipedal fashion. The limbs are
useless for swimming, although it is possible that they may be helpful in
creeping over the bottom, or in balancing, or as tactile organs.
_Protopterus_ is said to breathe by its lungs as well as by its gills, and
to rise to the surface at short intervals to take in fresh air.
{513}[Illustration: FIG. 307.—Diagram of a torpid _Protopterus_, _in situ_.
_c_, Cocoon; _e_, earth; _f_, funnel leading to the mouth of the Fish; _l_,
lid; _m_, mouth; _m.b_, mouth of the burrow; _t_, tail. (From Newton
Parker.)]
In the dry seasons the marshes in which _Protopterus_ lives become dried
up, and to meet this adverse change in its surroundings the Fish
hibernates, or passes into a summer sleep, until the next rainy season
brings about conditions more favourable to active life. Preparatory to this
summer sleep, and before the ground becomes too hard, the Fish makes its
way into the mud to a depth of about 18 inches, and there coils itself up
in a flask-like enlargement (Fig. 307) at the bottom of the burrow, which
is lined by a capsule of hardened mucus secreted by the glands of the
skin.[605] The mouth of the flask is closed by the capsular wall or lid,
which is perforated by a small aperture. The margins of this aperture are
pushed inwards, so as to form a tubular funnel for insertion between the
lips of the Fish. While encapsuled in its cocoon the Fish is surrounded by
a soft slimy mucus, no doubt for the purpose of keeping the skin moist, and
its lungs are the sole breathing organs, the air passing from the open
mouth of the burrow through the hole in the lid directly to the mouth of
the animal. The nutrition of the dormant Fish is effected by the absorption
of the fat stored up about the kidneys and gonads, somewhat after a fashion
not unknown in the fat-bodies of Insects and the hibernating glands of
Rodents. Even portions of the caudal muscles undergo fatty degeneration,
and thus, in a way which recalls the mode of nutrition of the Salmon during
the breeding season, and of the Tadpole during its metamorphosis, a further
store of nutritive material becomes available for the sustenance of the
Fish during {514}its long summer nap. It is highly probable that the
exceptionally numerous leucocytes act as carriers in the work of
transporting the fatty particles to the different organs and tissues of the
body. The length of the summer sleep naturally varies with the duration of
the dry season, and probably it lasts on an average nearly half the year
(August to December). The cocoons, imbedded in an outward casing of
hardened mud, have often been brought to Europe, and when placed in water
of suitable temperature the long torpid _Protopterus_ escapes from its
prison in a perfectly healthy condition, and resumes its partly branchial
and partly pulmonary mode of breathing. The negroes of the West Coast of
Africa are very partial to these Fishes, which they dig out of the dried
marshes and preserve in their clumps of mud for food. With the advent of
the rainy season, when the marshes become flooded, the _Protopterus_
emerges from its cocoon, and returning to its former active life, soon
enters upon the task of reproducing its kind. The important observations of
Budgett[606] have thrown much light on the curious breeding habits and
development of these Fishes. The Fish makes a nest near the edge of a
swamp. The nest is simply a hole of irregular shape, about a foot in depth,
filled with water and surrounded by long grass (Fig. 308). There is no
lining to the nest, and the eggs are deposited on the bare mud. Until the
eggs are hatched, which occurs about the eighth day, and while the larvae
are in the nest, the male remains on guard, and is apt to bite severely an
incautious intruder. Probably with the view of aerating the eggs the water
is continually lashed about by the tail of the guardian parent. The male
has no trace of the peculiar vascular filaments which adorn the pelvic
limbs of the male _Lepidosiren_ during the breeding season. The early
developmental stages are similar in their main outlines to those of
_Neoceratodus_, but the young are very different. When the young
_Protopterus_ (Fig. 309) is hatched it is provided with a crescentic
glandular sucker or cement-organ, situated on the under side of the head
behind the mouth, by means of which the larva attaches itself to the sides
of the nest, or of the vessel in which it is confined, much in the same way
as the young _Lepidosteus_, and probably for the same reason. It may be
remarked that the sucker agrees in structure, position, and function with
that found in Amphibian tadpoles, but it differs both in position and
structure from its preoral analogue in the young of _Acipenser_, _Amia_,
and _Lepidosteus_.
{515}[Illustration: FIG. 308.—Nest of the _Protopterus_ of the Gambia.
(From Budgett.)]
[Illustration: FIG. 309.—Larval _Protopterus_ on the seventeenth day. _c_,
Cement organ; _c.g_, cutaneous gills; _op_, operculum; _p.l_, pectoral
limb; _pv.l_, pelvic limb; _y.s_, yolk-sac. (From Budgett.)]
A month-old larva has much the aspect of a larval Newt. It has four pairs
of vascular plumose cutaneous gills (Fig. 309), which are retained as
vestiges for a long time or even throughout life, and two pairs of
synchronously-developed limbs. As an interesting instance of a nocturnal
and protective change of colour, it may be mentioned that the dark
chromatophores of the skin of the larva expand in the day-time and the
young Fish becomes darker in colour, and therefore less conspicuous when
seen against a background of black mud or soil. At night the contraction of
the colour-sacs renders the larva more transparent and probably less easily
{516}visible than if opaque. The commencement of pulmonary respiration is
coincident with the degeneration of the cutaneous gills, which takes place
about seven weeks after the deposition of the eggs, and about a month after
the larvae leave the nest. _Protopterus_ is said to attain a length of six
feet.
_Lepidosiren paradoxa_,[607] probably the only species of the genus, is
confined to South America. It occurs along the course of the main Amazon
river, entering some of its larger affluents, such as the Ucayale, the
Madeira, the Rio Negro, and the Tapajóz, and also in the Chaco Boreal to
the west of the Upper Paraguay river. The home of the _Lepidosiren_ (or
"Lolach," as the natives call the Fish) of the Chaco country is to be found
in the wide-spreading marshes and swamps, which for a great part of the
year are almost choked by a luxuriant growth of their own peculiar
vegetation and covered by a floating carpet of surface weeds, with here and
there deeper and clearer water and slow-flowing streams. In the dry season
the water gradually shrinks and the swamps eventually become dried up. Of
sluggish habits, the Fish wriggles slowly about at the bottom of the swamp
like an Eel, using its hind limbs in an irregular bipedal fashion as it
wends its way through the dense network of subaqueous plants. _Lepidosiren_
is not exclusively carnivorous. The large fresh-water snail, _Ampullaria_,
which lives in the swamps in enormous numbers, seems to be its favourite
food; but masses of confervoid Algae are also eaten, and in its earlier
stages it is probable that the Fish is more herbivorous than carnivorous.
The Jacare (_Caiman sclerops_) feeds on _Lepidosiren_, and this fact, and
probably also the cannibal habits of the Fish itself, may explain the
capture of specimens with mutilated tails and regenerated, branched,
pectoral limbs. Like other living Dipneusti, _Lepidosiren_ rises to the
surface to breathe. The intervals are, however, very variable, and no doubt
depend on the relative purity or impurity of the water. Both expiration and
inspiration are said to take place through the mouth. The snout is
protruded on the surface, and the creature expires. After being withdrawn
for a moment the head is again projected, and inspiration takes place
through the partially open lips.
{517}[Illustration: FIG. 310.—Pelvic limb of the male _Lepidosiren_ during
the breeding season. (From Graham Kerr.)]
When the Fish finally sinks a few bubbles of surplus air escape through the
gill-clefts. A nocturnal and protective change of colour, similar to that
described in _Protopterus_, has been observed, and although most strikingly
manifest in the larvae, it also occurs in individuals of older growth. The
flesh is much esteemed as food by the Indians, who wade into the swamps and
transfix the Fishes with spears. During the rainy season the _Lepidosiren_
eats voraciously, and a reserve of fat is stored up in the tissues. Like
its African relative, the Fish ceases to feed on the approach of the dry
season, and eventually hibernates at the dilated extremity of a deep
tubular burrow, the entrance to which is plugged by a small lump of clay
perforated by several round holes. On the rising of the water at the next
rainy season the _Lepidosiren_ pushes out the plug and soon emerges from
its burrow.[608] The breeding season begins soon after the escape of the
Fish. The eggs are deposited in nests in the form of underground burrows
excavated in the black peaty soil at the bottom of the swamp, with an
entrance about 4-5 inches wide. At a depth of about a foot the burrow takes
a horizontal course, its total length varying from 2-5 feet. After the eggs
are laid the male remains to guard them. During the breeding season the
pelvic limbs of the male enlarge and become covered by a rich growth of
highly vascular, blood-red filaments 2-3 inches in length[609] (Fig. 310).
The use of these curious structures is uncertain, but it is not improbable
that they act as accessory gills to enable the male to guard the eggs in
the nest without being forced to resort to the surface to breathe air. The
development is essentially similar to that of _Protopterus_. The larva
(Fig. 311) has four pairs of cutaneous gills in relation with the first,
second, third, and fourth branchial arches, inclusive, the first three
pairs being the homologues of the cutaneous gills of the tailed Amphibia;
and also a cement-organ {518}which disappears shortly before the larval
metamorphosis. At that period the circulation in the cutaneous gills
becomes sluggish, and very soon these organs completely atrophy. About the
same time the hyo-branchial cleft closes up, as in _Protopterus_. The young
_Lepidosiren_ soon begins to breathe air and to become more active and
lively in its habits.[610] The adult may attain a length of four feet.
[Illustration: FIG. 311.—Larval _Lepidosiren_ thirty days after hatching.
_c_, Cement organ; _c.g_, cutaneous gills; _p.l_, pectoral limb; _pv.l_,
pelvic limb. (From Graham Kerr.)]
The relations of the different genera of Dipneusti to one another has been
discussed by Dollo in a remarkably suggestive paper.[611] Until the
publication of this treatise it was generally believed that the modern
Dipneusti, _Neoceratodus_, _Protopterus_, and _Lepidosiren_, especially the
first mentioned, were the most primitive and the more nearly related to the
ancestral stock, while the older types, such as _Dipterus_, were regarded
in the light of highly specialised offshoots. The continuity of the median
fins, the apparently diphycercal character of the tail, and the wholly
cartilaginous condition of the chondrocranium in the modern Dipneusti, were
contrasted with the divided median fins, the heterocercal tail, and the
more extensively ossified chondrocranium of the Palaeozoic forms, and the
belief seemed inevitable. Dollo has shown, however, that there is good
reason for the view that the evolution of the group has taken place in
exactly the opposite direction; that, in fact, the older Dipneusti are the
more archaic, and that their modern representatives have been derived from
them by a sequence of retrogressive changes; or, in other words, the latter
have much the same relation to the former as the degenerate Sturgeons and
Paddle-Fishes to their Palaeozoic ancestors, the Palaeoniscidae. Taking
_Dipterus_, the {519}most ancient of all the known Dipneusti, as a
starting-point, it is possible to select a series of genera which
illustrate the evolution of the group both in structure and in
palaeontological sequence.[612] The series is as follows:—_Dipterus_,
_Scaumenacia_, _Phaneropleuron_, _Uronemus_, _Ceratodus_ (_Neoceratodus_),
_Protopterus_ and _Lepidosiren_. Briefly, the more important structural
modifications observable in the transition from the older to the recent
genera are (_a_) the gradual union of isolated median fins to form a
continuous fin[613]; (_b_) the substitution of a gephyrocercal tail for a
heterocercal[613]; (_c_) the degeneration of the squamation, the thick
ganoid scales of the earlier types being replaced by thin, non-ganoid
scales; (_d_) a reduction in the number of cranial dermal bones and the
loss of their original ganoid investment; (_e_) the suppression of the
jugular plates; and (_f_) a reduction in the size of the opercular bones.
In the last two genera of the series, in which specialisation in some
respects and degeneration in others have reached their maximum, the body no
longer retains the fusiform and more typically Fish-like shape of the older
genera, but, in accordance with Eel-like habits and mode of progression,
has become more or less Eel-like in form.[614] The paired fins are almost
vestigial, while the scales, so deeply insunken in the skin as to be
externally invisible, suggest that the modern Dipneusti are approximating
to a final scaleless as well as to an ultimately limbless condition. As to
the origin of the Dipneusti as a group, it seems reasonable to look for
their ancestors in the early Devonian Crossopterygii with acutely lobate
fins, or, with greater probability, to some still more primitive
Crossopterygian with simple, non-rhizodont teeth, capable by fusion of
giving rise to massive tritoral plates, and involving as a consequence the
substitution of an autostylic for an originally hyostylic skull, and the
suppression of the secondary upper jaw. In fact, when our knowledge of the
development of the surviving Dipneusti and Crossopterygii is more complete,
it is not improbable that the inclusion of the two series of Fishes in
subordinate divisions of the Teleostomi will prove to be amply justified.
The relations of the Dipneusti to the Amphibia are {520}somewhat deceptive,
and it seems improbable that the former group stands in the direct line of
Amphibian descent. In most of their structural features not directly or
remotely associated with air-breathing the Dipneusti are true Fishes, and
the striking resemblances which they present to the Amphibians in the
vascular system and lungs seem to be rather the outcome of physiological
convergence, associated with adaptive and parallel modifications in
structure, and due to the influence of a similar environment, than
indicative of direct ancestral relations. With more reason it may be
inferred that both the Dipneusti and the Amphibia have been derived from
some primitive Crossopterygian ancestor with Elasmobranch tendencies, and
subsequently became modified in certain respects on parallel lines.
{521}CHAPTER XX
APPENDIX TO THE FISHES: PALAEOSPONDYLIDAE—OSTRACODERMI—HETEROSTRACI—
OSTEOSTRACI—ANASPIDA—ANTIARCHI—ARTHRODIRA.
In this chapter it is proposed to treat of certain fossil "Fishes" which,
from our ignorance of much that is essential to a proper estimate of their
true relationships, cannot at present be referred to any of the recognised
primary groups of Fishes.
I. PALAEOSPONDYLIDAE.
The interesting little fossil, _Palaeospondylus gunni_,[615] discovered in
the Lower Old Red Sandstone of Caithness, and first described by Traquair,
represents the calcified endoskeleton of an elongated fish-like organism
about an inch, or not exceeding two inches, in length. The vertebral column
consists of a series of broad, calcified ring-like centra, destitute of
ribs, but possessing neural arches and spines, and in the caudal region
haemal arches and spines in addition. The skull, of which only the ventral
surface is known, has a complete basis cranii, laterally expanded behind by
periotic capsules, and in front by what seem to be bulging olfactory
capsules. Anteriorly, the skull terminates in a ring of calcified cirri.
Behind the skull there are two singular post-occipital plates, one on each
side of the anterior section of the vertebral column. The tail was
apparently furnished with a fringing caudal fin, supported dorsally by the
long forked, neural spines, and below by the much shorter haemal spines.
There is no trace {522}of limbs, limb-girdles, jaws, or branchial arches,
nor any evidence of the existence of scales, denticles, or other
exoskeletal structures.
_Palaeospondylus_ has been regarded as a Cyclostome, a view which derives
its principal support from the resemblance of the cirri-encircled orifice
at the anterior end of the skull to an unpaired nasal or naso-pituitary
aperture, and perhaps some measure of credence from such purely negative
evidence as the apparent want of limbs, and of any structures comparable to
jaws. But even if it be admitted that there is some reason for this view,
it is certain that _Palaeospondylus_ obtained a far higher grade of
specialisation in certain respects than any of the existing Cyclostomata;
the presence of calcified vertebral centra and neural arches is conclusive
on this point.[616] _Palaeospondylus_ has also been compared with a larval
Arthrodiran and with a larval Dipnoid.[617]
[Illustration: FIG. 312.—Restoration of _Palaeospondylus_. The figure shows
the ventral surface of the skull and a lateral view of the vertebral
column. _c_, Calcified cirri; _p.a_, auditory capsule; _t.p_, nasal capsule
(?); _x_, post-occipital plate. (From Parker and Haswell, after Traquair.)]
II. OSTRACODERMI.
The Palaeozoic fish-like forms, which, more as a matter of convenience than
as an expression of real kinship, it has been customary to include in this
group, are amongst the earliest Craniates of which we have any precise
knowledge. {523}Of the three subordinate groups or "Orders" into which they
have usually been divided hitherto, two, the Heterostraci and the
Osteostraci, may, with some show of reason, be considered as related forms,
and although they are characterised by much specialisation on independent
lines, there is yet some evidence of connecting links between the two. The
organisms comprising the third group, the Antiarchi, stand upon a very
different footing, and at present it cannot be said that they are in any
way related to either the Heterostraci or the Osteostraci, or indeed to any
other Craniates whatsoever. The association of the Ostracodermi with the
Cyclostomata, a view which has received more influential support than it
deserves, is based on the presumed absence of jaws and paired fins. The
absence of jaws, which, if present, were almost certainly cartilaginous,
has yet to be proved, and even in the latter group it is by no means
certain that they do not possess structures which, morphologically if not
functionally, are veritable jaws. Nor is it quite certain that the lateral
lobes of some Ostracodermi are neither pectoral flaps nor lateral
fin-folds, to say nothing of the lateral appendages of the Antiarchi. And
to these objections there is the further difficulty that there is
absolutely no evidence that the Ostracodermi are monorhinal in the sense in
which this term is applied to the Cyclostomata.[618] On these grounds it
would seem more in accordance with our present knowledge to regard the
Ostracodermi as an independent group whose exact position in the system has
yet to be determined, including, however, besides the generally accepted
orders Heterostraci and Osteostraci, the recently founded provisional order
Anaspida, but excluding the Antiarchi as a separate and distinct section;
rather than to crystallise in a definite system of classification views
which are either purely conjectural or wholly unjustifiable. Even with this
limitation the Ostracodermi are by no means easy to define, especially if
we include those remarkable shark-like forms from the Upper Silurian rocks
of the south of Scotland which have been so admirably described in the
recent classical memoirs of Dr. Traquair. As a rule, the head and the
anterior part of the body are laterally expanded, and more or less sharply
defined from the rest of the body by prominent postero-lateral angles. The
exoskeleton, which exhibits an extraordinary variety of {524}structure in
the different families, ranges from a uniform covering of dermal denticles
to a condition in which the denticles fuse to form anteriorly a highly
characteristic tessellated or continuous dorsal shield, while posteriorly
they become replaced by a nearly typical rhombic squamation. The tail is
heterocercal. Paired fins of the ordinary piscine type are absent. In some
Ostracodermi it seems probable that the gill-clefts opened into a common
branchial chamber on each side, with a single external aperture, but in
others they may have been ventral. The endoskeleton, jaws, dentition, and
the nostrils are unknown.
ORDER I. HETEROSTRACI.
The exoskeletal structures consist of dentine, or of a tissue resembling
it, never of true bone. The orbits are marginal or lateral in position.
With the exception of a caudal fin there are no median fins.
[Illustration: FIG. 313.—Restored outline of _Lanarkia spinosa_, in the
position in which it occurs as a fossil, the head being flattened and the
tail twisted round so as to appear in profile. On each side a much enlarged
dermal denticle is shown. (From Traquair.)]
FAM. 1. COELOLEPIDAE.[619]—Head and anterior portion of the body flattened
and expanded, with prominent lappet-like postero-lateral lobes, which may
represent continuous lateral fin-folds or a very primitive type of pectoral
fin. Nothing is known of the mouth, but it must have been ventral, nor of
the position of the orbits. Branchial apertures unknown, but transverse
markings on each side of the anterior part of _Thelodus pagei_ may be
indications of a branchial apparatus. The exoskeleton consists of a uniform
covering of hollow pointed spines, devoid of a basal plate and open below
(_Lanarkia_); or of minute shagreen-like tubercles (_Thelodus_). The
tubercles or spines consist of dentine coated by ganoin. Of the only two
known genera, _Thelodus_ is a characteristic Upper Silurian genus {525}from
the Ludlow and Downtonian Beds of Lanarkshire. Detached scales are also
known in the Upper Silurian of England. One species (_Th. pagei_) occurs in
the Lower Old Red Sandstone of Forfarshire, and another (_Th. tulensis_) in
the Upper Devonian of Russia. _Lanarkia_ has only been found in the
Downtonian Beds. None of the Coelolepidae exceed fourteen to fifteen inches
in length.
[Illustration: FIG. 314.—Restored outline of the dorsal surface of
_Drepanaspis gemündenensis_. The tail appears in profile. _m.d_, Median
dorsal plate; _p.l_, postero-lateral plate; _r_, rostral plates. (From
Traquair.)]
FAM. 2. DREPANASPIDAE.—This family[620] affords an interesting transition
to the more highly specialised and carapaced Pteraspidae. The head and
anterior part of the trunk now form a broad oblong shield, rounded in front
and abruptly marked off from the tail by conspicuous rounded angles. The
exoskeleton is no longer uniform. In the caudal region the scattered spines
or shagreen tubercles of the Coelolepids have become transformed into
tuberculated quadrangular scales, which are further differentiated along
the dorsal and ventral margins into ridge scales or fulcra; and from a
similar source by a process of basal fusion a series of larger or smaller
dermal plates are formed as components of large dorsal or ventral shields.
The dorsal shield (Fig. 314) is formed by a large central plate; the
postero-lateral portions by two narrow falciform plates; and the anterior
margin by a series of smaller rostral plates. Between the larger plates the
shield is completed by numerous {526}small polygonal plates. All the plates
are superficially ornamented by small stellate tubercles. The ventral
armature (Fig. 315) is similar to the dorsal. A large mental plate forms
the hinder margin of the transverse slit-like mouth, the anterior limit of
which is defined by the rostral plates already mentioned. Laterally may be
seen a pair of small plates (_x_), each perforated by a small aperture, and
probably indicating the position of some kind of sense-organ. Posteriorly
there is a large median ventral plate, in relation with a pair of anterior
and a pair of posterior ventral plates. The areas between the larger plates
are filled in by numerous small polygonal plates. It is possible that there
is a single external branchial aperture on each side, near the
postero-lateral angle of the shield and behind the posterior ventro-lateral
plate. The sole representative of the family is _Drepanaspis
gemündenensis_, from the Lower Devonian of Gemünden in Rhenish Prussia.
Large examples of this fossil must have exceeded two feet in length.
[Illustration: FIG. 315.—Ventral surface of _Drepanaspis_ (tail in
profile). _a.v.l_, Anterior ventro-lateral plates; _e.l_, external lateral;
_m_, mental plate; _m.v_, mid-ventral; _p.l_, postero-lateral; _p.v.l_,
posterior ventro-lateral; _r_, rostral; _x_, orbit or sensory plate. The
mouth and the supposed cloacal aperture are indicated in black. (From
Traquair.)]
FAM. 3. PSAMMOSTEIDAE.—To this family are referred certain dermal plates
occurring, in a more or less fragmentary condition, in the Old Red
Sandstone and Devonian formations of Great Britain and Russia. In their
size and shape, and in their stellate tubercles, these have been compared
to the dorsal, postero-lateral, and ventral plates of {527}_Drepanaspis_.
That _Psammosteus_ is closely allied to _Drepanaspis_ seems certain, but
for the present the two genera may be retained in separate families.
FAM. 4. PTERASPIDAE.[621]—Until the recent inclusion of the three preceding
families, the Pteraspidae were the only representatives of the
Heterostraci. In the best known genus, _Pteraspis_, there is a marked
reduction in the number of the component plates of the carapace, and only
seven can now be distinguished (Fig. 316): (_a_) a large posterior dorsal
plate, supporting behind a stout spine; (_b_) a conical rostral plate,
covering the preorbital part of the head; (_c_) a pair of small marginal
orbital plates, each with a small aperture, probably for the eye; (_d_) a
pair of posterior lateral or cornual plates, each of which is perforated by
a large oblique foramen, conjecturally an external branchial aperture; and
(_e_) a large ventral plate. There is probably, also, a small median
"parietal," or "pineal," plate, with a pit on its inner surface, situated
between the rostral and posterior dorsal plates. Externally the plates are
sculptured into fine ridges, which in their minute structure and their
crenated free margins are suggestive of linear series of fused denticles.
The tail appears to have been invested by imbricated rhombic scales.
_Pteraspis_ (Lower Old Red Sandstone of Scotland and England, and the Lower
Devonian of Galicia); _Cyathaspis_ (Upper Silurian and Lower Old Red
Sandstone), known only by its dorsal and ventral shields; and _Holaspis_
(Lower Old Red Sandstone of Monmouthshire, and the Upper Silurian of
Pennsylvania), are the only genera.
[Illustration: FIG. 316.—Restored outline of _Pteraspis rostrata_, seen
from the side. The scales on the hinder part of the tail are omitted. (From
Parker and Haswell, after Smith Woodward.)]
ORDER II. OSTEOSTRACI.
While agreeing with the more specialised Heterostraci in the division of
the body into an anterior carapaced portion and a free {528}hinder part
invested by a rhombic squamation, the Osteostraci are distinguished by the
presence of bone as a histological component of the dermal hard parts; by
the position of the orbits, which, instead of being marginal in position,
are close together on the dorsal aspect of the carapace; and by the
possession of a median dorsal fin.
FAM. 1. ATELEASPIDAE.[622]—The general shape of the body is much the same
as in the Coelolepidae, but the exoskeleton consists of numerous polygonal
tuberculated plates in front of the postero-lateral lobes, and of
sculptured rhombic scales behind. A pair of crescentic markings, placed
close together about the middle of the dorsal surface of the head, probably
indicate the outer margins of orbital recesses (Fig. 317). The only species
at present known (_Ateleaspis tessellata_) occurs in the Downtonian beds.
[Illustration: FIG. 317.—Outline sketch of _Ateleaspis tessellata_. The
crescentic markings indicate the position of the supposed orbits. (From
Traquair.)]
[Illustration: FIG. 318.—_Cephalaspis murchisoni_. Upper Silurian and Lower
Old Red Sandstone, _op_, Operculum(?). (From Smith Woodward.)]
FAM. 2. CEPHALASPIDAE.[623]—In this family the dorsal shield is rounded in
front, strongly arched above, with its postero-lateral angles produced into
highly characteristic cornua (Fig. 318). The shield consists of a single
piece, but as the outer surface is ornamented by small tubercles arranged
in polygonal areas, it is probable that it has been formed by the basal
fusion of {529}numerous primitively distinct polygonal plates (Fig. 319,
A). Between the orbits there is a separately calcified but fixed plate,
which bears a hollow prominence, probably for the reception of a parietal
organ. In some genera certain of the anterior dorsal and ventral scales of
the trunk fuse into a continuous plate. Internally to the postero-lateral
cornua the middle layer of the shield is prolonged backwards into a pair of
singular flap-like lobes, which have been variously interpreted as
corresponding to the lateral lobes of the Coelolepidae, to pectoral fins,
or to opercula. The scales of the trunk and tail are rhombic and
imbricated; on the sides of the body they are remarkably high and narrow.
[Illustration: FIG. 319.—The dorsal shield of _Cephalaspis lyelli_ (A), and
an outline sketch of the dorsal shield of _Eukeraspis pustulifera_ (B).
_c_, Postero-lateral cornu; _d_, posterior angle; _i.p_, interorbital
prominence; _o_, orbit; _o.p_, orbital prominence; _p.s_, posterior spine;
_p.v_, postorbital valley. (From Lankester.)]
The best known genus is _Cephalaspis_. The earliest remains are found in
the Ludlow Tilestones. The genus is also represented in the Ledbury Passage
Beds, the Lower Old Red Sandstone of Scotland, and the Upper and Lower
Devonian of Canada. Most of the species are of small size, but _C.
magnifica_,[624] from the Caithness Flagstones, the largest of all the
Cephalaspids, has a shield 8½ inches long, and 12 inches across the widest
part. _Auchenaspis_ occurs in the Ludlow Tilestones and the Ledbury Passage
Beds, and also in the Upper Silurian of the Isle of Oesel in the
{530}Baltic. Another genus, _Didymaspis_, has been found in the Lower Old
Red Sandstone of Ledbury.
FAM. 3. TREMATASPIDAE.—The interorbital plate is free, and hence it is
often lost in the fossils. Several species of _Tremataspis_ occur in the
Upper Silurian of the Isle of Oesel.
As regards the origin and mutual relationships of the different families
comprising the Heterostraci, it has been urged with great force by Dr.
Traquair[625] that they constitute a natural sequence of forms, beginning
with organisms whose Elasmobranch ancestry is extremely probable, and
leading to highly-specialised types, which, considered by themselves,
possess little to justify any conclusions whatever as to their origin or
kinship. The Coelolepidae form the starting-point, and in the light of
their exoskeleton of dermal denticles, their derivation from some primitive
Elasmobranch prototype seems a reasonable inference.[626] From the
Coelolepids the path of specialisation through the Drepanaspidae and
Psammosteidae to the Pteraspidae is marked (i.) by the basal concrescence
of isolated denticles to form, first, numerous small polygonal plates, and
then larger and less numerous plates, as the constituent elements of a
characteristic dorsal shield, leaving, however, the denticles of the rest
of the body to become converted into a rhombic squamation; (ii.) by
modifications in the "lateral fin-lobes," which may become enclosed in the
developing dermal armour (_e.g._ _Drepanaspis_), or cease to be
recognisable (_e.g._ _Pteraspis_). The affinities of the Osteostraci are
very obscure, and their inclusion with the Heterostraci in the same group
(Ostracodermi) has hitherto rested mainly on such negative evidence as the
supposed absence of paired limbs, jaws, and teeth; in fact, it has been
affirmed that "there is absolutely no reason for regarding _Cephalaspis_ as
allied to _Pteraspis_ beyond that the two genera occur in the same
rocks."[627] It is possible, however, that in _Ateleaspis_ we have an
annectent form, which in some measure combines the structural peculiarities
of the two groups. That this singular genus belongs to the Osteostraci is
proved by the presence of bone lacunae in its dermal hard parts, a
conclusion which is strengthened by the apparently dorsal position of the
orbits and the presence of a dorsal fin. On the {531}other hand, its close
resemblance to the Coelolepids in the general contour of its
laterally-lobed body, and the probability that its mosaic and tuberculated
head-shield has been formed by the concrescence of Coelolepid denticles, is
at least significant of a relationship to the more primitive Heterostraci.
Little can be conjectured as to the habits of these ancient "Fishes." The
form and regional proportions of the body, which in some respects often
remind one of organisms so diverse as a King Crab, or a Loricaroid Teleost
(such as _Liposarcus_), are strongly suggestive of a grovelling,
bottom-feeding, sluggish habit of life, in sharp contrast to the more
active and predaceous Fishes whose appearance is coincident with the
extinction of the Ostracodermi at the close of the Devonian period. Habits
such as these may well be associated with much structural degeneration,
even, it may be, with the loss of paired fins, and hence it is not
altogether improbable that the Ostracodermi are outcasts from the
Elasmobranchs, a degenerate race which has sought safety in a sequestered
life and a coat of mail.
ORDER III. ANASPIDA.
This group has been instituted by Traquair[628] for the provisional
reception of two remarkable genera, which, owing to the absence of precise
knowledge of the histology of their exoskeletal structures, cannot at
present be referred either to the Heterostraci or the Osteostraci, and for
which, as their discoverer remarks, no place can be found in the system
unless they are admitted to the Ostracodermi.
FAM. 1. BIRKENIIDAE.—Body fusiform and fish-like. Head bluntly rounded,
without a cranial shield. Caudal fin bilobate and heterocercal A median row
of scales with recurved spines arranged along the ventral surface. Orbits,
jaws, teeth, paired fins, and endoskeleton unknown.
In _Birkenia_ (Fig. 320) the body is invested by longitudinal rows of
narrow scales arranged in oblique transverse rows, which are replaced on
the head by much smaller, peculiarly disposed, spindle-shaped scutes. On
the side of the hinder part of the head there is an oblique row of small
apertures, possibly branchial. A small remote dorsal fin, invested by the
trunk scales, is present. {532}_Birkenia elegans_, the only species known,
does not exceed 3½ inches in length. Less is known about the second genus,
_Lasanius_, of which there are two species. Except for the mid-ventral
series of spiny scutes, and a row of slender, parallel, rod-like
structures, the body appears to have been naked (Fig. 321). The two genera
belong to the remarkable series of fossil Fishes from the Silurian rocks of
Lanarkshire. Rare in the Ludlow series, _Birkenia_ is by far the most
common of the Fishes of the over-lying Downtonian Beds. _Lasanius_ is
confined to the latter horizon. _Euphanerops_, from the Upper Devonian of
Canada, is probably related to this family, but lateral branchial apertures
are not known.[629]
[Illustration: FIG. 320.—Restored outline of _Birkenia elegans_ Traq.,
one-half larger than natural size. _b.a_, Branchial aperture; _d_, dorsal
fin. (From Traquair.)]
[Illustration: FIG. 321.—Restored outline of _Lasanius problematicus_,
enlarged, _r_, Post-cephalic rods; _r′_, row of small spine-like scutes;
_v.s_, mid-ventral spine-like scales. (From Traquair.)]
III. ANTIARCHI.
The organisms comprising this group[630] resemble the Ostracodermi in
possessing a well-developed carapace of bony plates and a heterocercal
tail, as well as in many of the purely negative features which are
characteristic of the latter group.
{533}[Illustration: FIG. 322.—Restored outline of _Pterichthys milleri_.
The upper figure represents a dorsal view, and the lower a lateral view.
The dotted lines indicate the course of the lateral line system. _a.d.l_,
Antero-dorso-lateral; _ag_, angular; _a.m.d_, anterior median dorsal;
_a.v.l_, anterior ventro-lateral; _e.l_, extra-lateral or operculum; _l_,
lateral; _l.occ_, lateral occipital; _m_, median or interorbital plate;
_m.occ_, median occipital; _o_, orbit; _p.d.l_, posterior dorso-lateral;
_p.m_, pre-median; _p.m.d_, posterior median dorsal; _pt.m_, post-median;
_p.v.l_, posterior ventro-lateral. ——, Plates investing the limbs: _c_,
central; _d.a_, dorsal anconeal; _d.ar_, dorsal articular; _e.m_, external
marginal; _i.m_, internal marginal; _m.m_, marginals; _t_, terminal. (From
Traquair.)]
The remarkable dorsal shield is divided into a small cephalic portion and a
much larger hinder part investing the greater part of the trunk, both of
which are strongly arched above and flattened ventrally, with a movable
articulation between the two. The cephalic shield is formed by numerous
symmetrically-disposed tuberculated plates, suturally connected with one
another, and, like the other exoskeletal structures, containing bone
lacunae (Fig. 322).[631] The orbits are close together, near the middle of
the dorsal surface, and between them there is a small median interorbital
plate, with a deep pit on its inner surface, possibly for a parietal organ.
A small lateral plate (_e.l._), evidently free behind, suggests the
presence of an operculum. Nothing is certainly known about the jaws or the
nostrils. The mouth is situated just behind the anterior margin of the
cephalic shield on the ventral surface, and in front of it there are two
plates, {534}which in _Bothriolepis canadensis_ have their oral margins
fringed by small "denticles"; it is possible that these plates represent
the components of a secondary upper jaw. The dorsal armature of the trunk
is shown in Fig. 322. Ventrally it is completed by a pair of anterior
ventro-lateral plates and a pair of posterior ventro-lateral plates with a
small median plate between the two pairs. Articulating with the anterior
ventro-lateral plates by means of a complex hinge joint there is a pair of
pectoral appendages of a kind entirely without parallel in any other
vertebrated animals. Each appendage is completely encased by numerous
suturally connected plates, and about the middle of its length there is a
second movable joint. The appendages are hollow, and their cavities
probably contained the muscles by which the limbs were moved, and the
blood-vessels and nerves for their nutrition and innervation. A lateral
line system of the normal type is present in _Pterichthys_, consisting of a
lateral groove along the side of the trunk, and of supra-orbital and
infra-orbital grooves, and post-temporal and infra-orbital commissures, on
the head. The free portion of the body and the tail are invested by
imbricated and finely tuberculated scales, which form fulcra in front of
and behind the small dorsal fin. There are no pelvic fins. The caudal fin
is heterocercal.
FAM. 1. ASTEROLEPIDAE.—The best known genera are _Pterichthys_ from the
Lower Old Red Sandstone of Scotland and the Devonian of Eifel, and
_Bothriolepis_, a more widely distributed genus which occurs in the Upper
Old Red of Scotland and Shropshire, and in the Upper Devonian of Russia and
Canada. Two other genera, _Asterolepis_ and _Microbrachius_, are also found
in the Old Red Sandstone of Scotland.[632]
Beyond an uncertain and shadowy relationship to the Ostracodermi, and
perhaps some points of resemblance to the Arthrodira, the Antiarchi stand
alone among Craniates. Nothing is known of their origin; no intermediate
forms link them to any other groups, and the high specialisation they have
attained is sufficient to negative any idea that they can "be credited with
any share in the evolution of the Fishes of more recent periods."
{535}IV. ARTHRODIRA
This group has been instituted for the reception of a number of remarkable
armoured Fishes of uncertain relationships which flourished in Europe
during the Devonian and Old Red Sandstone periods, and in North America
from the Devonian to the Lower Carboniferous. The head (_e.g._ in
_Coccosteus_)[633] is invested dorsally by a series of median and lateral
symmetrically-disposed tuberculated plates (Fig. 323). Two of the lateral
plates are notched for the orbits, and between them there is an
interorbital plate which either has a pit on its inner surface or is
perforated by an open funnel, as in _Dinichthys_, possibly for a parietal
or a pineal organ. Some of the bones present some analogy, to say the
least, to certain of the dermal bones of a typical Teleostome, apparently
representing such elements as paired parietals and frontals, a dermal
mesethmoid, and toothless premaxillae and maxillae (Fig. 324, A). As in the
Antiarchi, the anterior portion of the trunk is also armoured, above by a
dorsal shield, formed by median and lateral plates, and below by a
similarly constructed ventral shield (Fig. 324, B). A huge joint connects
the head and trunk shields: hence the term Arthrodira or "joint-neck." The
rest of the body is naked.
[Illustration: FIG. 323.—Restoration of _Coccosteus decipiens_. Old Red
Sandstone of Scotland. × ¼. _A_, Articulation of the cephalic and trunk
shields; _DB_ and _DR_, radials of the dorsal fin; _H_, haemal arches and
spines; _MC_, sensory canals; _N_, neural arches and spines; _NT_,
notochord; _U_, median plate; _VB_, basipterygium; _VR_, radialia of the
pelvic fin. (From Parker and Haswell, after Bashford Dean and Smith
Woodward.)]
Pectoral fins are unknown, but pelvic fins, each supported by a stout basal
plate or basipterygium, and with traces of radials, are present. There is a
small dorsal fin. Little is known of the primary cranium, but in the trunk
and tail it is evident {536}that there are well-developed and partially
calcified neural and haemal arches associated with a persistent notochord.
It is possible that the skull is autostylic. Gill-arches are not known. A
pair of plates (Fig. 324, A, _j_) at the postero-lateral angles of the
cephalic shield may perhaps be opercula. The teeth are conical. Those in
the upper jaw are supported by two pairs of plates, probably vomers and
palatines. In the lower jaw there are two series of teeth, one in front
near the symphysis, and the other behind, supported by a single bone in
each ramus. There is a well-developed lateral line system, indicated by
surface markings on the head and trunk shields.
[Illustration: FIG. 324.—Dorsal view of the cephalic and trunk shields of
_Coccosteus_ (A); and a view of the ventral part of the trunk armour (B).
_a.d.l_, Anterior dorso-lateral; _a.l_, antero-lateral; _a.m.v_, anterior
median ventral; _a.v.l_, anterior ventro-lateral; _c_, central; _e.o_,
external occipital; _i.l_, internal lateral; _j_, jugal; _m_, marginal;
_m.d_, median dorsal; _m.e_, dermal mesethmoid; _m.o_, median occipital;
_m.v_, median ventral; _mx_, maxilla; _n_, nasal aperture; _o_, orbit; _p_,
pineal plate; _p.d.l_, posterior dorso-lateral; _p.mx_, premaxilla; _p.o_,
preorbital; _pt.o_, post-orbital; _p.v.l_, posterior ventro-lateral. (From
Traquair.)]
FAM. 1. COCCOSTEIDAE.—_Coccosteus_ occurs in the Devonian of Europe and
North America, and includes species of relatively small size, not exceeding
half a metre in length. _C. decipiens_, the best known species, is a
characteristic fossil in the Old Red Sandstone of Scotland.
_Phlyctaenaspis_[634] is found in the Lower Devonian of Canada, England,
and Poland. A larger Arthrodiran, with slender toothless jaws,
_Homosteus_,[635] is met with in the Lower Old Red Sandstone of the North
of Scotland, and in the Devonian of Germany and Russia. The Old World
Arthrodira must yield, however, to those of the New World for variety in
size and shape, and in the character of their dentition.[636] {537}Some of
the North American genera (_e.g. Dinichthys_) probably attained a length of
ten feet, or even, as in _Titanichthys_, a much greater size. Some are
fusiform in shape, but _Mylostoma_ is flattened and Ray-like, and, judging
from the dentition, their food and habits must have been equally varied.
_Mylostoma_ has tritoral plates not unlike those of _Neoceratodus_ or
_Chimaera_. In others the teeth are single, and conical or pointed; in
_Titanichthys_ the front teeth in both jaws are beak-shaped structures. It
is highly probable that _Titanichthys_, _Mylostoma_, and perhaps other
genera, are types of distinct families.
The Arthrodira have been regarded as armoured Dipneusti, a view which is
mainly based on their supposed autostylism and the nature of the dentition.
But this autostylism has yet to be verified, and, if proved, the
possibility that it may be a secondary feature, associated with the
evolution of a peculiar dentition, must not be forgotten. Much more may be
said for their claim to be regarded as a highly specialised race of
primitive Teleostomi. Besides a well-developed lower jaw, bones comparable
to the elements of a secondary upper jaw are known, and in a general way
the disposition of the cranial roofing bones, and the arrangement of the
endoskeletal elements of the pelvic fins, tend to conform to the normal
Teleostome type. In fact, Dr. Traquair has expressed the opinion that the
Arthrodira are Teleostomi and Actinopterygii.[637]
TELEOSTEI
(SYSTEMATIC PART)
BY
G. A. BOULENGER, F.R.S., V.P.Z.S.
Of the British Museum (Natural History)
{541}CHAPTER XXI
TELEOSTEI: GENERAL CHARACTERS—MALACOPTERYGII—OSTARIOPHYSI
ORDER IV. TELEOSTEI.[638]
As stated above (p. 495), the Holostean Ganoids pass very gradually into
the Teleosteans, the lower groups of which appear to have been directly
derived from them. The precise definition of the Order Teleostei, as
compared with the Ganoid Order Holostei, is a matter of some difficulty.
The most important character appears to be the presence of an ossified
supraoccipital bone.[639] Remnants of primitive characters, such as ganoid
scales, fulcra, rudiments of a splenial bone, spiral valve to the
intestine, multivalvular conus arteriosus, are still found in some lower
Teleosteans, but no longer in that combination which serves to define the
preceding order. Although _Albula_ is exceptional among all Teleosteans in
having two transverse series of valves to the bulbus arteriosus instead of
one, no Ganoid has fewer than three.
The first remains of Teleosteans appear scantily in the Upper Trias, and it
is not before we reach the Upper Cretaceous that they assume preponderance
over other Teleostomes; whilst in the Upper Eocene they have already
attained a development and variety of types comparable to their present
condition. Out of some 12,000 well-established species of Fishes known to
exist at {542}the present day, about 11,500 belong to this order. The
classification of such an array of forms is, of course, a matter of great
difficulty, and gives scope for much difference of opinion among those who
have attempted to grapple with the subject. It is now recognised that the
study of the skeleton affords the safest guide to a natural arrangement of
the families and higher divisions. Much has been done in this line by Cope,
Gill, Sagemehl, A. S. Woodward, and Jordan and his pupils; but the
osteology of many important types still remains unknown. For some years a
large number of skeletons have been prepared in the British Museum with the
object of settling open questions, and this material has enabled me to draw
up a scheme of classification which, whatever its defects, and however
provisional, I feel sure is on the whole an improvement on those hitherto
proposed, and especially on that generally in use in this country. The
latter was, to a great extent, based on physiological principles; the
present aims at being phylogenetic. In its preparation I have derived great
benefit from the labours of the authors quoted above, but have endeavoured
in every instance to verify their statements on a larger osteological
material than appears to have been available to them. I have also had the
advantage of the criticism, on many points, of my young colleague, Mr. C.
Tate Regan, who has himself endeavoured to settle some important questions
of classification.[640]
The Order Teleostei is divided into thirteen sub-orders, the probable
relations of which are expressed in the following diagram:—
11. Opisthomi. 13. Plectognathi. 12. Pediculati. –––+
| | | |
+––––––––––––––––––+––––––––––––––––––––+ |
| |
9. Anacanthini. 10. Acanthopterygii. 8. Percesoces. |
| | | |
+––––––––––––––––––+––––––––––––––––––––+ |––Teleostei.
| |
7. Catosteomi. 5. Haplomi. 6. Heteromi. |
| | | |
+––––––––––––––––––+––––––––––––––––––––+ |
| | |
| 4. Apodes. |
| 3. Symbranchii. |
| |
1. Malacopterygii. 2. Ostariophysi. |
| | –––+
+––––––––––––––––––––––+
|
Ganoidei Holostei.
{543}In the classification of Günther, which has been generally in use in
this country for the last thirty years, the Teleosts were divided into six
principal groups, of ordinal rank: I. Acanthopterygii; II. Acanthopterygii
Pharyngognathi; III. Anacanthini; IV. Physostomi; V. Lophobranchii; VI.
Plectognathi. Group I. corresponds to Sub-Order 6 (part), 7 (part), 8
(part), 10 (part), 11 and 12 of the present work; Group II. to Sub-Order 10
(part); Group III. to Sub-Order 9 and 10 (part); Group IV. to Sub-Order 1,
2, 3, 4, 5, 6 (part), and 8 (part); Group V. to Sub-Order 7 (part); and
Group VI. to Sub-Order 13.
SUB-ORDER 1. MALACOPTERYGII.
Air-bladder, if present, communicating with the digestive tract by a duct.
Opercle well developed. Pectoral arch suspended from the skull;
mesocoracoid arch present.[641] Fins without spines, the ventrals
abdominal, if present. Anterior vertebrae distinct, without Weberian
ossicles.
This sub-order, which corresponds to the Isospondyli and Scyphophori of
Cope and to a part of the Isospondyli of A. S. Woodward, embraces the most
generalised of the Teleosts, and is intimately connected with the Ganoids
by the fossil forms which are placed at the base of the series of families.
The physostomous condition of the air-bladder, the connexion of the
pectoral arch with the skull, the presence of the mesocoracoid arch, the
backward position of the many-rayed ventral fins, the normal condition of
the anterior vertebrae, the absence of true spines to the fins, and the
separation of the supraoccipital bone from the frontals by the parietals,
are primitive characters which among the Teleosts occur combined in some
families of this suborder only. The mesocoracoid arch is retained by the
Ostariophysi, which differ in the remarkably modified condition of the
anterior vertebrae, but it disappears in all other Teleosts, which
gradually acquire a more forward position of the ventral fins and a
reduction in the number of their rays, develop spines in the vertical fins,
and lose the communication of the air-bladder with the outside.
The Malacopterygii may be divided into twenty-one families, the characters
of which are contrasted in the following synopsis:—
I. Fins fringed with fulcra, or scales coated with ganoin; {544}
notochord
usually continuous through the vertebrae.
Vertebral centra not more than rings; fins with fulcra; scales
rhombic, united by peg-and-socket joints 1. _Pholidophoridae_.†
Vertebral centra not more than rings; fins with fulcra; scales
cycloid 2. _Archaeomaenidae_.†
Vertebral centra complete or with minute perforation; fins with
fulcra; scales cycloid 3. _Oligopleuridae_.†
Vertebral centra nearly complete, but with perforation; no fulcra;
scales cycloid 4. _Leptolepididae_.†
II. Fins without fulcra; scales without ganoin; vertebral centra
complete.
A. Supraoccipital separated from the frontals by the parietals or
upper surface of skull; no adipose fin.
1. Ventral fins with 10 to 16 rays.
An intergular bone; parasphenoid narrow 5. _Elopidae_.
No intergular bone; parasphenoid very broad 6. _Albulidae_.
2. Ventrals with not more than 7 rays.
a. Supratemporal very large, plate-like.
Praemaxillary single, its posterior extremity free from the
maxillary; symplectic absent; basis cranii simple
7. _Mormyridae_.
Praemaxillary paired, its posterior extremity firmly attached
to the maxillary; symplectic present; basis cranii double
8. _Hyodontidae_.
b. Supratemporal small; maxillary firmly attached to posterior
extremity of praemaxillary.
Praemaxillary paired; a large hole on each side of the skull,
between the post-frontal and the squamosal; basis cranii
double; suboperculum absent; ribs sessile
9. _Notopteridae_.
Praemaxillary paired; basis cranii simple; suboperculum reduced;
ribs inserted on parapophyses 10. _Osteoglossidae_.
Praemaxillary single; basis cranii simple; suboperculum and
interoperculum absent; ribs inserted on parapophyses
11. _Pantodontidae_.
c. Supratemporal small; maxillary movable; ribs sessile; ventral
fins below the pectorals 12. _Ctenothrissidae_.†
B. Supraoccipital separating parietals, or adipose fin present.
1. Interoperculum enormous; symplectic absent; basis cranii simple
13. _Phractolaemidae_.
2. Interoperculum normal; symplectic present; basis cranii double.
a. Teeth in sockets; maxillary firmly attached to praemaxillary.
Symplectic exposed 14. _Saurodontidae_.†
b. Teeth not in sockets.
Symplectic concealed between the quadrate and the hyomandibular
15. _Chirocentridae_.
Postclavicle on outer side of clavicle; no adipose fin
16. _Clupeidae_.
Postclavicle on inner side of clavicle; an adipose dorsal fin
17. _Salmonidae_.
Postclavicle absent; no adipose fin 18. _Alepocephalidae_.
3. Interoperculum normal; basis cranii simple. {545}
Maxillary large, toothed; praecaudal vertebrae without well-marked
parapophyses; scales cycloid or absent; adipose dorsal fin
present or absent 19. _Stomiatidae_.
Mouth small, toothless; vertebrae with strong parapophyses; head
and body covered with spiny scales 20. _Gonorhynchidae_.
Mouth small, toothless; no symplectic; head and body naked
21. _Cromeriidae_.
† This sign indicates that the group is represented by fossil forms only.
FAM. 1. PHOLIDOPHORIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter large and loosely attached;
teeth small and conical. Parietal bones separating the supraoccipital from
the frontals; opercular bones well developed. Vertebral centra never
advanced beyond the annular stage; ribs delicate; no fused or expanded
haemal arches at the base of the tail; no epipleurals or epineurals.
Fin-fulcra present, but usually small; dorsal and anal fins small, the
former above or behind the ventral fins, which are small or may be absent.
Scales ganoid, rhombic, those on the flanks united by peg-and-socket
joints.
This family, which appears to me to be related to the Dapediidae, is
provisionally placed here by A. S. Woodward on account of its resemblance
to the Leptolepididae, but it is not yet quite clear that the mandible was
destitute of splenial and coronoid elements, while the bones at the base of
the pectoral fin have not hitherto been observed. The principal genera are
_Pholidophorus_, ranging from the Upper Trias to the Purbeck;
_Thoracopterus_, from the Upper Trias; and _Pleuropholis_, from the Upper
Jurassic. The species of _Pholidophorus_ are very numerous in the Jurassic
period, and Woodward has observed that the scales of the later species are
more elaborately ornamented than those of earlier date.
FAM. 2. ARCHAEOMAENIDAE.—Distinguished from the preceding by the thin,
cycloid scales. Conspicuous obtuse ridge-scales are present along the
dorsal and ventral lines. _Archaeomenes_, from the Jurassic (?) of New
South Wales.
FAM. 3. OLIGOPLEURIDAE.—Characters as in Pholidophoridae, but scales
cycloid and vertebrae completely or nearly completely ossified.
_Oligopleurus_, from the Jurassic of England and France; _Oenoscopus_, from
the Jurassic and Cretaceous of France, Germany, and Italy; and
_Spathiurus_, from the Cretaceous of Mount Lebanon and Dalmatia.
{546}FAM. 4. LEPTOLEPIDIDAE.—Praemaxillaries very small; maxillaries large,
loosely attached; teeth small and conical. Parietal bones separating the
supraoccipital from the frontals; opercular bones well developed. Vertebral
centra well ossified, but always pierced by the notochord; ribs delicate;
epipleurals present; no fused or expanded haemal arches at the base of the
caudal fin. Dorsal and anal fins small, the former above or behind the
ventrals. Ventrals with 5 to 10 rays. Scales thin, cycloid and deeply
imbricate, usually coated with ganoin in their exposed portion.
[Illustration: FIG. 325.—_Leptolepis dubius_. (Restoration of skeleton by
A. S. Woodward.)]
_Leptolepis_, with numerous species, from the Jurassic and Cretaceous of
Europe and New South Wales; _Vidalia_, Jurassic of France; _Aethalion_,
Jurassic of Bavaria; _Thrissops_, Jurassic and Cretaceous of Europe; and
_Lycoptera_, Jurassic of Asia.
FAM. 5. ELOPIDAE.—Margin of the upper jaw formed by the praemaxillaries and
the maxillaries, the latter the more developed, and movably articulated
above the former to the ethmoid. Parietal bones in contact behind the
frontals; opercular bones well developed. Basis cranii double. A bony
intergular or sublingual plate. Jaws, palatines, pterygoids, vomer,
parasphenoid, glossohyal, and pharyngeals toothed. Ribs mostly sessile,
inserted very low down, behind parapophyses; epineurals similar to the
ribs, but directed upwards. Pectorals low down, folding like the ventrals.
Post-temporal forked, the upper branch attached to the epiotic, the lower
to the opisthotic; post-clavicle small; scapular foramen in the scapula;
pterygials well developed, three in contact with coracoid. Ventrals with 10
to 16 rays. Branchiostegal rays very numerous (over 20). Air-bladder large.
{547}This family is abundantly represented in Cretaceous times by the
genera _Osmeroides_ and _Elopopsis_, and from the Lower Eocene to the
present day by _Elops_ and _Megalops_. _Elops saurus_ is a handsome
elongate silvery Fish, found in all the warm and tropical seas; the young
are ribbon-shaped like those of _Albula_. A second species, _E. lacerta_,
is from the West Coast of Africa, entering rivers. _Megalops_,
distinguished by larger scales, the absence of pseudobranchiae, and the
curious prolongation of the last ray of the dorsal fin, includes the
well-known Tarpon _M. atlanticus_, and the Indian _M. cyprinoides_. The
Tarpon occurs from the south-eastern coasts of North America and the West
Indies to Brazil, and reaches a length of 6 feet and a weight of 110 lbs.
It often leaps out of the water, after the manner of Grey Mullets, and its
chase when hooked affords good sport, the landing of so active a giant
being attended with great difficulties. Its remarkably large scales, over
two inches in diameter, are much prized for fancy work in the Florida
curiosity shops.
[Illustration: FIG. 326.—Tarpon, _Megalops atlanticus_, much reduced.
(After Goode.)]
FAM. 6. ALBULIDAE.—Margin of the upper jaw formed by the praemaxillaries
and the maxillaries, the latter movably articulated above the former to the
ethmoid. Parietal bones separating the supraoccipital from the frontals;
suboperculum large; interoperculum small and entirely or nearly entirely
hidden below the praeoperculum. Basis cranii double. Praemaxillaries,
mandible, vomer, palatines, pterygoid, parasphenoid, glossohyal, and
pharyngeals toothed. Ribs sessile, inserted behind and somewhat below small
parapophyses, which are absent or merely indicated on the anterior
vertebrae, and gradually increase in size towards the caudal region; these
parapophyses, as well as the neural and haemal arches, are autogenous
bones; epineurals, no epipleurals. {548}Pectorals low down, folding like
the ventrals. Post-temporal trifid, the upper branch attached to the
epiotic, the median to the squamosal, the lower to the opisthotic;
post-clavicle large (formed of three pieces); scapular foramen between
scapula and clavicle; pterygials well developed, two in contact with
coracoid. Ventrals with 10 to 14 rays. Branchiostegal rays 6 to 14.
Air-bladder large, not communicating with the ear.
Elongate fusiform Fishes, covered with large silvery scales forming regular
longitudinal series; head naked; mouth rather small, with thick lips;
gill-openings wide. Dorsal fin originating in front of ventrals; anal
short; caudal well developed, forked.
[Illustration: FIG. 327.—_Albula conorhynchus_. ¼ nat. size.]
The type of this family, _Albula_ or _Butirinus_, is remarkable among
Teleosts in possessing a rudiment of a conus arteriosus to the heart,
provided with two rows of valvules.[642] Its single species inhabits all
the warm and tropical seas. Prof. Gilbert has ascertained that the young
pass through a metamorphosis, analogous to that of the Eels. The
"Leptocephalid" described as _Esunculus_ by Kaup is probably a larval
_Albula_.
[Illustration: FIG. 328.—Larva of _Albula conorhynchus_. (After Gilbert.)]
The deep-sea Japanese _Pterothrissus_ (_Bathythrissa_) must be placed in
this family; its skeleton is very similar to that of _Albula_. It differs
in the elongate dorsal fin, in the presence of small teeth on the maxillary
bone, and in the small number of branchiostegal rays (6 instead of 12 to
14).
{549}_Albula_ is represented in the Eocene (London Clay and Bruxellian);
and the Cretaceous _Istieus_ and _Anogmius_ are believed to be possibly
related to _Pterothrissus_. Four Cretaceous types (_Plethodus_,
_Thryptodus_, _Syntegmodus_, and _Ancylostylus_) are referred with doubt to
the Albulidae.
FAM. 7. MORMYRIDAE.—Margin of the upper jaw formed by the single
praemaxillary and the maxillaries, the latter articulated above the former
to the ethmoid. Parietal bones separating the supraoccipital from the
frontals; a large hole on each side of the skull, between the squamosal,
the epiotic, and the opisthotic, covered by a large, thin, bony plate (the
supratemporal), which may extend over a part of the parietal; symplectic
absent; suboperculum small and hidden under the operculum, or absent;
interoperculum well developed. Basis cranii simple. No pharyngeal teeth.
Opercular bones hidden under the skin; gill-clefts narrow. Anterior ribs
sessile; epineurals, no epipleurals. Pectorals directed upwards. Ventrals
with 6 or 7 rays. Air-bladder communicating with the ear.
[Illustration: FIG. 329.—_Mormyrus caballus_. ⅕ nat. size.]
Curious-looking Fishes, very variable in the form of the head and body and
in the extent of the fins. Mouth often very small; teeth in jaws usually
few; teeth usually present on the parasphenoid, working against a similar
patch on the glossohyal; eye covered over by skin, sometimes very
indistinct; scales small, cycloid; branchiostegal rays 4 to 8. The dorsal
and anal fins may be nearly equally developed (_Genyomyrus_,
_Gnathonemus_); or the former (_Mormyrus_) or the latter (_Hyperopisus_)
are several times the longer. _Gymnarchus_, Eel-shaped, apodal, and
deprived of the caudal fin, lacks the anal fin, the dorsal extending over
the whole extent of the body. Some species of _Mormyrops_ show how a form
{550}like _Gymnarchus_ may have been evolved out of a more typically-formed
Fish. Nothing is more striking than the variation in shape of the snout
within one and the same genus, and the names given to some of the species
(_ovis_, _caballus_, _elephas_, _tamandua_, _numenius_, _ibis_) are
suggestive of resemblances with the heads of various animals.
[Illustration: FIG. 330.—Head of _Gnathonemus curvirostris_.]
[Illustration: FIG. 331.—Head of _Gnathonemus numenius_.]
The Mormyrids are highly remarkable for the enormous development of the
brain, the weight of which equals 1/52 to 1/82 of the total, a thing
unparalleled among lower Vertebrates; and for the problematic organ which
surmounts it; also as being among the few Fishes in which an electric organ
has been discovered. The organ, situated on each side of the caudal region,
is derived from the muscular system and is of feeble power, as ascertained
by Babuchin and by Fritsch; it was long considered as "pseudo-electric."
The natural affinities of this family appear to be with the Albulidae, and
there is nothing to justify the term "Nilhechte" (Nile-pike) which has been
bestowed on them by German {551}authors. Ninety-three species are known
from the fresh waters of Africa north of the Tropic of Capricorn, and may
be referred to two sub-families and ten genera[643]:—
(i.) MORMYRINAE, with teeth on the parasphenoid and tongue, with ventral,
anal, and caudal fins, and a simple air-bladder; vertebrae 37 to 64;
peculiar (Gemmingerian) linear bones, without known homologues, along each
side of the tail, above and beneath the electric organ; scapular foramen in
the scapula, or between the scapula and the coracoid. _Mormyrops,
Petrocephalus, Isichthys, Marcusenius, Stomatorhinus, Myomyrus,
Gnathonemus, Genyomyrus, Mormyrus_.
(ii.) GYMNARCHINAE, without teeth on the parasphenoid and tongue, without
ventral, anal, or caudal fins, and with a cellular air-bladder; vertebrae
about 120; Gemmingerian bones absent; scapular foramen in the coracoid.
_Gymnarchus_.
Fossil Mormyrids are unknown.
Venerated by the ancient Egyptians, the Mormyrs of the Nile are frequently
represented on hieroglyphics and mural paintings as well as in bronze
models. Very little is known of the habits of these Fishes. Prof. G.
Fritsch, of Berlin, during his stay in Egypt for the purpose of
experimenting on electric Fishes, observed that they perish very rapidly
when removed from the river, and he had the greatest difficulty in keeping
some alive in an aquarium for two or three days. The species with
comparatively large mouths (_Mormyrops_, _Gymnarchus_) feed principally on
fishes and crustaceans, the others on tiny animals and vegetable and more
or less decomposed matter. Delhez, on the Congo, found that many are
attracted to the borders of the river in the neighbourhood of human
dwellings, where they feed on the refuse thrown into the water. It is
probable that the species with a rostrum use it to procure small prey
hidden between stones or buried in the mud, and that the fleshy mental
appendage with which many are provided is a tactile organ compensating the
imperfection of the vision in the search for food. A small Mormyrid from
the Congo (_Stomatorhinus microps_) has the eyes so much reduced and the
skin so feebly pigmented as to convey the impression of a cave Fish. Until
quite recently, absolutely nothing was known {552}of the breeding habits
and development in this important family. To the late J. S. Budgett we owe
some very interesting observations made in the Gambia on _Gymnarchus
niloticus_.[644] The Fish makes a floating nest, emerging on three sides,
over which the male keeps a fierce watch; the recently-hatched larvae are
remarkable for the enormous size of the yolk-sac, which hangs down, acting
as a sort of anchor, and for the presence of long external branchial
filaments, as in Selachian embryos. The Fish propels itself through the
water entirely by the action of its dorsal fin, forwards and backwards with
equal facility; when swimming rapidly backwards, it may be seen to use the
end of its tail as a feeler to guide the way. Budgett has also identified,
with some doubt, the eggs of _Hyperopisus bebe_, out of which emerged
embryos not unlike those of some tailless Batrachians, which hung suspended
to rootlets of grass in swamps by means of threads of viscid mucus secreted
from glands on the top of the head.
[Illustration: FIG. 332.—_Gymnarchus niloticus_. ¼ nat. size.]
FAM. 8. HYODONTIDAE.—Margin of the upper jaw formed by the praemaxillaries
and the maxillaries, the latter the more developed and firmly united to the
end of the former. Parietal bones separating the supraoccipital from the
frontals; a large hole on each side of the skull, between the parietal, the
squamosal, and the epiotic (paroccipital), closed by a large, thin, bony
plate (the supratemporal), which extends over the greater part of the
parietal; suboperculum and interoperculum small, the latter partly hidden
below the praeoperculum. Basis cranii double. Jaws, palatines, pterygoids,
vomer, parasphenoid, and glossohyal toothed; no pharyngeal teeth. Ribs
sessile, inserted above and behind well-developed parapophyses; epineurals,
no epipleurals. {553}Pectorals low down, folding like the ventrals.
Post-temporal forked; the upper branch attached to the epiotic, the lower
to the squamosal; no post-clavicle; coracoids forming together a ventral
keel; scapular foramen between scapula and clavicle; pterygials well
developed, three in contact with coracoid. Ventrals with 7 rays.
Branchiostegal rays in moderate number (8 to 10). Air-bladder communicating
with the ear. No oviducts, the eggs falling into the abdominal cavity
before exclusion.
[Illustration: FIG. 333.—Upper (A) and posterior (B) views of skull and
pectoral arch of _Hyodon alosoides_ (the supratemporal removed on the left
side). _bo_, Basioccipital; _cl_, clavicle; _cor_, coracoid; _eo_,
exoccipital; _eot_, epiotic; _eth_, ethmoid; _f_, frontal; _m_, maxilla;
_mcor_, mesocoracoid; _n_, nasal; _oo_, opisthotic; _p_, parietal; _pcl_,
postclavicle; _pm_, praemaxilla; _por_, praeorbital; _ptf_, postfrontal;
_ptr_, pterygials; _ptte_, post-temporal; _scl_, supraclavicle; _so_,
supraoccipital; _sor_, suborbital; _sq_, squamosal; _ste_, supratemporal.]
Elongate, compressed, silvery Fishes, covered with moderate-sized cycloid
scales; head naked; mouth large, with strong dentition; gill-openings wide;
dorsal fin short, posterior to the ventrals; anal rather elongate; caudal
well developed, forked.
A single genus (_Hyodon_) with three species (Moon-Eyes of the Americans),
all reaching the length of about a foot, inhabiting the fresh waters of
North America east of the Rocky Mountains.
{554}FAM. 9. NOTOPTERIDAE. The Fishes which form this family may be
regarded as an eccentric modification of a type very similar to the
preceding, with which they agree in most osteological features as well as
in the dentition, in the connexion between the air-bladder and the ear, and
in the absence of oviducts. They differ in the absence of the lid-like
supratemporal, the hole which it covers in _Hyodon_ being here bordered by
the post-frontal and the squamosal (fused with the opisthotic), sometimes
also by the epiotic, in the absence of the suboperculum, in the absence or
incomplete ossification of the upper branch of the post-temporal (the lower
articulating with the opisthotic), and in the presence of accessory bones
(named adpleurals) attached to or fused with the distal extremity of the
anterior ribs, which they prolong to the mid-ventral line, where they are
embraced by dermal ossifications forming a doubly serrated crest.
[Illustration: FIG. 334.—_Notopterus afer_, skeleton, with outline of soft
parts. ⅔ nat. size.]
The bones of the head are cavernous, the mouth is large; the anterior
nostril is produced into a tentacle. The body is very strongly compressed,
with very short precaudal region, attenuate behind; the ventral fins are
much reduced or absent; the dorsal is short or absent, {555}and the anal is
much elongate and confluent with the caudal, which may be regarded as
aborted. The scapular foramen is entirely in the scapula. The air-bladder
is very large, with several divisions, forked in front and behind, and
prolonged along each side of the caudal region; its extraordinary condition
has been described by Bridge.[645]
These Fishes live in marshes and lakes, fresh-water or brackish, and feed
on worms and insects. Nothing is known of their breeding habits and
development.
Two genera: _Notopterus_, with a dorsal fin and 6 to 9 branchiostegal
rays—three species from India, Burma, and the Malay region, and one from
West Africa; _Xenomystus_, without dorsal fin and with only 3
branchiostegal rays, the unique species inhabiting the White Nile and West
Africa. Remains of _Notopterus_ have been found in the marl slates
(Tertiary) of Padang, Sumatra. The largest species, the Indian _N.
chitala_, attains 4 feet in length; its flesh is said to be uncommonly rich
and well flavoured, but a strong prejudice exists against it, owing to the
Fish being supposed to live on human carcases.
FAM. 10. OSTEOGLOSSIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter the more developed and
firmly attached to the end of the former. Parietal bones separating the
supraoccipital from the frontals; suboperculum much reduced, and entirely
or partially concealed under the praeoperculum. Basis cranii simple. Teeth
in jaws, and on pterygoid and hyoid bones; no pharyngeal teeth. Head
scaleless, the thin skin confluent with the bones; body covered with large
bony scales, composed of pieces like mosaic. Ribs inserted on the strong
parapophyses; epineurals, no epipleurals. Pectoral fins low down.
Post-temporal forked, the upper branch attached to the epiotic, the lower
to the squamosal; post-clavicle present; scapular foramen in scapula;
pterygials well developed, only one in contact with coracoid. Dorsal and
anal fins originating behind the ventrals; latter with 5 or 6 rays. No
oviducts, the eggs falling into the abdominal cavity before exclusion (at
least in _Heterotis_, as observed by Budgett).
This family is represented at the present day by five species, referred to
four genera; thus characterised:—
{556}[Illustration: FIG. 335.—Principal forms of Osteoglossids. A,
_Dapedoglossus testis_ (restoration); B, _Scleropages leichardti_; C,
_Osteoglossum bicirrhosum_; D, _Arapaima gigas_; E, _Heterotis niloticus_.
All much reduced.]
{557}_Scleropages_.—Mouth large; vomer, palatines, pterygoids, and
glossohyal toothed; mandibular barbels; branchiostegal rays 15 to 17; body
compressed, with trenchant abdomen; coracoids forming a ventral keel;
dorsal fin short; ventral fins nearly equally distant from end of snout and
caudal fin; vertebrae 29 to 31 + 30; air-bladder not cellular. One species
from the northern parts of Australia, and one from Sumatra, Banka, and
Borneo.
_Osteoglossum_.—Mouth large; vomer, palatines, pterygoids, and glossohyal
toothed; mandibular barbels; branchiostegal rays 10; body compressed, with
trenchant abdomen; coracoids forming a ventral keel; dorsal fin long;
ventral fins nearly twice as far from the caudal as from the end of the
snout; vertebrae 28 + 59; air-bladder not cellular.—South America (Guianas,
Brazil).
_Arapaima_.—Mouth rather large; vomer, palatines, pterygoids, and
glossohyal toothed; branchiostegal rays 16; belly rounded; dorsal fin
rather long; ventral fins equidistant from head and caudal fin; vertebrae
36 to 38 + 41 to 42; air-bladder cellular.—South America (Guianas, Brazil).
_Heterotis_.—Mouth moderate; branchiostegal rays 7; belly rounded; dorsal
fin rather long; ventral fins nearer end of snout than caudal fin;
vertebrae 27 + 42 to 43; air-bladder cellular; fourth branchial arch with
an accessory breathing-organ. Africa (Nile, Senegal, Gambia, Niger).
_Dapedoglossus_, from the Eocene of Wyoming, appears to be nearest to
_Scleropages_, and _Brychaetus_, from the Eocene (London Clay) of Sheppey,
Kent, to _Arapaima_, so far as the state of preservation of these fossils
enables us to form an opinion.
[Illustration: FIG. 336.—Distribution of the Osteoglossids.]
Dr. Günther has directed attention to the remarkable {558}coincidence of
the geographical distribution of this family and the Dipneusti, although,
however, the latter are not known to be represented in the Malay
Archipelago. "Not only," he adds, "are the corresponding species found
within the same region, but also in the same river systems; and although
such a connexion may and must be partly due to a similarity of habit, yet
the identity of this singular distribution is so striking that it can only
be accounted for by assuming that the Osteoglossidae are one of the
earliest Teleosteous types which have been contemporaries of and have
accompanied the present Dipnoi since or even before the beginning of the
Tertiary epoch."
The Queensland species of _Scleropages_ (_S. leichardti_) is known to the
settlers by the name of Barramunda, which has also been applied to
_Neoceratodus_. _Arapaima gigas_ is one of the largest fresh-water Fishes
known, exceeding a length of 15 feet and a weight of 400 pounds. Its flesh
is much valued. Sir R. Schomburgh has observed that the mother protects the
young, who, for some time after their birth, always swim in front of her. A
similar observation has been made in the Gambia on _Heterotis niloticus_ by
the late J. S. Budgett, who states that the Fish builds enormous nests in
swamps, in about two feet of water; the walls of the nest are made of the
stems of the grasses removed by the Fish from the centre; the floor is the
swamp-bottom, and is made perfectly smooth and bare. The nest appears to be
used for at most four or five days; the newly-hatched larvae are provided
with long external gill-filaments of a blood-red colour.[646]
FAM. 11. PANTODONTIDAE.—The little West African Fish described by Peters as
_Pantodon buchholzi_ is the unique representative of a family closely
related to the Osteoglossidae, but distinguished by the very small, single
praemaxillary and the absence of suboperculum and interoperculum. The
pectoral fins are very large and are remarkable for the fleshy process to
which the inner ray is adnate; the ventrals, formed of 7 rays, some of
which are simple and prolonged into filaments, are placed more forward than
in any other type of this sub-order, the Ctenothrissidae excepted, viz.
immediately behind the pectorals. Teeth in the jaws and on the vomer,
palatines, pterygoids, parasphenoid, {559}glossohyal, and pharyngeal bones.
Mesocoracoid arch slender, strongly curved, and meeting its fellow on the
median line; coracoids forming a ventral keel. Vertebrae few (16 + 14).
[Illustration: FIG. 337.—_Pantodon buchholzi_, natural size.]
Observed by M. de Brazza to be a freshwater Flying-Fish.
[Illustration: FIG. 338.—_Ctenothrissa vexillifer_ (restored by A. S.
Woodward).]
FAM. 12. CTENOTHRISSIDAE.—A curious type characterised by small
praemaxillaries, large maxillaries, with feeble dentition, {560}the
parietals in contact on the median line, vertebral centra without
transverse processes, a moderately large dorsal with simple anterior rays,
and large ventrals advanced far forwards and formed of 8 rays. Its
affinities are still obscure, but the condition of the jaws decides its
allocation to the suborder Malacopterygii, whilst in the position of the
ventrals it is most nearly approached by the Pantodontidae. Small Fishes
known only by two genera, of the Cretaceous period (England and Mount
Lebanon), one with ctenoid scales (_Ctenothrissa_), the other with cycloid
scales (_Aulolepis_).
FAM. 13. PHRACTOLAEMIDAE.—Mouth edentulous, projectile, bordered by the
very slender praemaxillaries and maxillaries. Supraoccipital in contact
with the frontals, widely separating the small parietals; operculum and
suboperculum well developed; praeoperculum small; interoperculum enormous,
covering the gular region and overlapping its fellow; symplectic absent.
Basis cranii single. No pharyngeal teeth. Only 3 slender branchiostegal
rays. Ribs stout, sessile, nearly completely encircling the body; slender
epineurals; no epipleurals; caudal region very short. Supratemporal small,
simple, fixed to the parietal and squamosal; no postclavicle; scapular
foramen in the scapula. Pectoral fin inserted low down, folding like the
ventrals; latter with 6 rays.
[Illustration: FIG. 339.—_Phractolaemus ansorgii_. ⅔ nat. size.]
The remarkable little Fish, _Phractolaemus ansorgii_, discovered by Dr. W.
J. Ansorge in the Niger Delta in 1900, and which has since also been found
in the Congo, stands quite apart among the Malacopterygians, its nearest
allies being apparently the Osteoglossidae. The body is elongate and
subcylindrical, covered with large striated scales; the head is small, the
skull strongly ossified, covered with thin skin; the mouth small,
proboscidiform, {561}capable of being thrust forwards, when at rest folded
over and received into a depression on the upper surface of the head; the
narial orifice is single, and preceded by a barbel; the gill-openings are
narrow, restricted to the sides. The ventral fins are inserted far back,
the dorsal and anal are short. The air-bladder is very large, and the
intestine extremely long and much convoluted. Vertebrae 26 + 8.
FAM. 14. SAURODONTIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter the more developed and
firmly united to the former; these bones, as well as the mandible, with
teeth implanted in deep sockets; palate toothless. Supraoccipital
separating the small parietals; opercular bones well developed; symplectic
present, exposed. Basis cranii double. Ribs sessile, very low down on the
centra; no parapophyses; neural arches not fused with the centra. Pectorals
inserted very low down; postclavicle apparently absent. Caudal fin deeply
forked, without fused hypurals.
This family, comprising several Cretaceous genera, may be regarded as
ancestral to the _Chirocentridae_, with or near which it has been placed by
Cope and various later authors. The normal position of the symplectic,
however, entitles its members to rank as a separate family, and the
autogenous neural arch, as well as the distinctness of the bones supporting
the caudal fin, are also indicative of a greater generalisation. The
restoration of _Ichthyodectes_ as given by Loomis, shows a general form
similar to an ordinary Herring, but it does not appear to be reliable.
The members of the Saurodontidae have been referred to two groups: (_a_)
with praedentary (praesymphysial) bone, _Saurocephalus_, _Saurodon_; (_b_)
without praedentary, _Chirocentrites_, _Portheus_, _Ichthyodectes_,
_Spathodactylus_, _Cladocyclus_. These Fishes are from the Chalk of Europe
and North America, and some among them attain a very large size, perhaps
not less than two metres in length.
FAM. 15. CHIROCENTRIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter the more developed and
firmly united to the former; these bones, as well as the mandible, with
large teeth not implanted in true sockets; minute teeth on the palatines,
pterygoids, and hyoid bones, Supraoccipital in contact with the frontals,
separating the small parietals; opercular bones well developed; symplectic
hidden {562}between the inner surface of the quadrate and a descending
process of the hyomandibular. Basis cranii double. Ribs very slender,
sessile, very low down on the centra; no parapophyses; epipleurals and
epineurals. Pectorals inserted very low down. Post-temporal forked;
postclavicle absent; a thin bony lamina, similar to the postclavicle, above
the pectoral fin, attached to the scapula; scapular foramen in scapula;
coracoids in contact with each other, forming a keel. Ventrals very small,
with 7 rays. Brachiostegal rays 8. Air-bladder large, not communicating
with the ear, incompletely divided into cells. Mucous membrane of the
intestine forming a spiral fold.
The body is very elongate and strongly compressed, covered with thin,
deciduous scales; the vertebrae number 75. The dorsal fin is short and
opposite to the anal, which is long.
[Illustration: FIG. 340.—Side view of skull and pectoral arch of
_Chirocentrus dorab_.]
_Chirocentrus dorab_, the only representative of this family, inhabits the
Indian Ocean and the seas of China and Japan.
FAM. 16. CLUPEIDAE.—Margin of the upper jaw formed by the praemaxillaries
and the maxillaries. Supraoccipital separating the small parietals;
opercular bones well developed. Basis cranii double. Ribs mostly sessile,
inserted behind parapophyses; intermuscular bones (epineurals, epipleurals,
adpleurals) usually numerous. Post-temporal forked, the upper branch
attached to the epiotic, the lower to the opisthotic; post-clavicle applied
to outer side of clavicle. Ventrals with 6 to 11 rays. Air-bladder large,
communicating with the ear.
Four sub-families:—
(i.) THRISSOPATRINAE.—Mouth large; praemaxillaries very small; maxillaries
large, with rather narrow supplemental bone, firmly attached to
praemaxillaries; branchiostegals about 30; abdomen compressed to an edge,
without serration; no lateral line. _Thrissopater_, from the Gault of
Folkestone.
{563}(ii.) ENGRAULINAE.—Mouth moderate or large; praemaxillaries very
small; maxillaries large, with narrow supplemental bones, more or less
firmly attached to praemaxillaries; branchiostegals 6 to 19; abdomen
rounded or more or less compressed, with or without serration; no lateral
line. Recent genera: _Dussumieria_, _Etrumeus_, _Engraulis_,
_Cetengraulis_, _Heterothrissa_, _Coilia_. Fossil: _Spaniodon_, Upper
Cretaceous.
(iii.) CLUPEINAE.—Mouth small or moderate; maxillaries freely movable
behind the praemaxillaries, usually with large supplemental bones;
branchiostegals 5 to 10; abdomen usually serrated; lateral line usually
absent. Recent genera: _Clupea_, _Hyperlophus_ (_Diplomystus_),
_Opisthonema_, _Brevoortia_, _Pellonula_, _Clupeichthys_, _Odaxothrissa_,
_Pellona_, _Chirocentrodon_, _Pristigaster_, _Raconda_, _Chatoessus_.
Fossil: _Pseudoberyx_, _Histiothrissa_, _Scombroclupea_, _Leptichthys_,
Upper Cretaceous.
[Illustration: FIG. 341.—Showing the wide range of variation, within the
family, of the bones (_pm_, praemaxillary, _m_, maxillary) forming the
upper border of the mouth. A, _Dussumieria_; B, _Coilia_; C, _Pellona_; D,
_Chatoessus_; E, _Chanos_. In these semi-diagrammatic figures the orbit is
represented of the same size in all, as affording the best term of
comparison in judging of the relative development of the bones of the upper
jaw.]
(iv.) CHANINAE.—Mouth small, toothless; maxillaries firmly attached to
praemaxillaries; branchiostegals 4, very broad; abdomen rounded or
flattened; lateral line distinct. _Chanos_, recent; _Chanoides_, Upper
Eocene; _Prochanos_, Cretaceous.
{564}Heralded by the genus _Thrissopater_,[647] which may be regarded as a
connecting type between the Elopidae and the Clupeidae, this family is
largely represented in Cretaceous times, more abundantly still in the
Eocene and Miocene, where _Clupea_ and _Engraulis_ occur in numerous
species; _Hyperlophus_, distinguished from _Clupea_ by the presence of a
dorsal serrated ridge similar to the ventral, occurs in the Upper
Cretaceous of Syria, Southern Europe, and South America, in the Eocene of
North America and Europe, and is represented at the present day on the West
Coast of South America and on the coast and in the rivers of New South
Wales. About 200 Clupeids are known to live at the present day, mostly
marine species, but a few are confined to fresh-waters; none may be termed
deep-sea forms; some, like the Allis Shad (_Clupea alosa_) and Twait Shad
(_C. finta_), are anadromous, ascending rivers to spawn. The range of the
family is almost cosmopolitan. Several species are remarkable for the
extreme abundance of individuals, as for example the Herring (_Clupea
harengus_), the Pilchard or Sardine (_C. pilchardus_), and the Anchovy
(_Engraulis encrasicholus_). The Herring inhabits the northern parts of the
Atlantic and the seas north of Asia. As Dr. Günther first showed, the
so-called "Whitebait" consists chiefly of the fry of Herrings, which, like
those of the Sprat (_C. sprattus_), have a predilection for brackish water.
The Anchovy and the Pilchard, on the other hand, seldom if ever enter
estuaries. The eggs of the Herring, contrary to those of most British
marine food-fishes, are heavy and adhesive, sticking firmly to stones or
fixed objects on the sea bottom, whilst those of the Sprat and Pilchard
float on the surface. The larvae are long, slender, and transparent. The
Sardine, which affords so valuable a fishery on the West Coast of France,
is the immature state of the Pilchard, which grows to a length of 10 to 14
inches. Its movements are not yet well understood, and its scarcity during
certain years in the waters where it usually swarms has caused periodical
crises in an important industry. Ripe Pilchards are mostly found at a
considerable distance from the coasts. The Anchovy is especially abundant
in the {565}Mediterranean, but it is also regularly fished in Holland,
especially in the Zuydersee, where it breeds, as well as in the
Mediterranean; it makes only temporary appearances, and has not been
observed to spawn, in the English Channel, although eggs have recently been
obtained off the coast of North Lancashire.[648]
The imperfectly known Cretaceous Crossognathidae (_Crossognathus_ and
_Scyllaemus_), referred by some authors to the Percesoces, should probably
be placed with or near the Clupeidae.
FAM. 17. SALMONIDAE.—Margin of the upper jaw formed by the praemaxillaries
and the maxillaries. Supraoccipital in contact with the frontals, but
frequently overlapped by the parietals, which may meet in a sagittal
suture; opercular bones all well developed. Basis cranii double. Ribs
sessile, parapophyses very short or absent; epineurals, sometimes also
epipleurals, present. Post-temporal forked, the upper branch attached to
the epiotic, the lower to the opisthotic; postclavicle, as usual, applied
to inner side of clavicle. A small adipose dorsal fin. Air-bladder usually
present, large. Oviducts rudimentary or absent, the ova falling into the
cavity of the abdomen before exclusion.
Marine and fresh-water Fishes, mostly from the temperate and Arctic zones
of the northern hemisphere: one genus (_Retropinna_) on the coasts and in
the rivers of New Zealand; a few deep-sea forms (_Argentina_, _Microstoma_,
_Nansenia_, _Bathylagus_) occur in the Arctic Ocean, the North Atlantic
Ocean, the Mediterranean, and the Antarctic Ocean, down to 2000 fathoms.
Apparently of comparatively recent age, no remains older than Miocene
(_Osmerus_, _Thaumaturus_, _Prothymallus_) being certainly referable to
this family. The recent genera may be grouped as follows:—
_A_. Air-bladder present.
_a_. Branchiostegal rays 8 to 20; ventral rays 9 to 13; stomach
siphonal; pyloric appendages more or less numerous (17 to 200). Breed
in fresh water. _Salmo_, _Brachymystax_, _Stenodus_, _Coregonus_,
_Phylogephyra_, _Thymallus_.
_b_. Branchiostegal rays 6; ventral rays 11 to 14; stomach caecal;
pyloric appendages in moderate numbers (12 to 20). _Argentina_.
{566}_c_. Branchiostegal rays 6 to 10; ventral rays 6 to 8; stomach
caecal; pyloric appendages few (2 to 11) or rather numerous. _Osmerus,
Thaleichthys, Mallotus, Plecoglossus, Hypomesus_.
_d_. Branchiostegal rays 3 or 4; ventral rays 8 to 10; stomach caecal;
pyloric appendages absent. _Microstoma, Nansenia, Bathylagus_.
B. Air-bladder absent; branchiostegal rays 3 to 6; ventral rays 6 or 7;
stomach siphonal; pyloric appendages absent. _Retropinna, Salanx_.
Only about 80 species can, at present, be regarded as valid.
[Illustration: FIG. 342.—Distribution of Salmonidae (deep-sea forms not
included).]
The beauty, gameness, and great economical value of the Salmonids have
caused more attention to be bestowed on them than probably upon any other
group of fishes. As Professor Smitt tells us, a Swedish proverb says "A
dear child has many names," and this applies well to our Salmon and Trout,
the species of which have been unduly multiplied by some writers. The genus
_Salmo_, characterised by a large mouth and powerful dentition, is divided
into three sections: _Oncorhynchus_, Quinnat Salmon, of the North Pacific,
ascending rivers in North America and Asia, with 12 to 17 developed rays in
the anal; _Salmo_, Salmon and Trout, with 8 to 12 rays in the anal, and
teeth not only on the head of the vomer but also along its shaft, at least
in the young, {567}represented in the seas and freshwaters of Europe, Asia,
and North America, extending southwards to North-West Africa, Asia Minor,
Northern Persia, the Hindu Kush, the head of the Gulf of California, and
the Rio Grande; _Salvelinus_, Charr, with 8 to 10 rays in the anal, and
teeth on the raised head of the vomer only, of the lakes of Northern and
Central Europe and the rivers of the northern parts of Asia and North
America as far north as 82° 34´, sometimes descending to the sea.
[Illustration: FIG. 343.—Trout (_Salmo trutta_). × ⅓. (After
Valenciennes.)]
The changes in form and colour which these fishes undergo when passing from
fresh water into the sea or when artificially transported from one place to
another are very great, and this plasticity, together with the connecting
links which render the naming of not a few specimens impossible, have
caused most recent students of the genus _Salmo_, in Europe at least, to
reduce many of the so-called species to the rank of local varieties, and
even our common Brown Trout or Brook Trout (_S. fario_) is now generally
regarded as not specifically separable from the anadromous Sea Trout (_S.
trutta_). The anadromous true Salmon (_S. salar_) may be distinguished by
its somewhat larger scales, there being only 11 or 12 in a transverse
series running from the posterior border of the adipose fin forwards to the
lateral line, Trout having 13 to 16. The Charr of the lakes of Wales, the
North of England, Scotland, and Ireland are also regarded as mere varieties
of the common Northern migratory Charr (_S. alpinus_), of which the "Omble
Chevalier" of the Swiss lakes and the "Saeblings" of the Alpine lakes of
Germany and Austria are likewise varieties. An allied species (_S.
fontinalis_) has been introduced into England from North America, as well
as a true Trout (_S. irideus_). The large size of the eggs, their lack of
{568}adhesiveness, and the fact that the ova fall into the abdominal
cavity, out of which they may easily be squeezed, renders artificial
impregnation particularly easy, and the species of _Salmo_ have always
occupied the first place in the annals of fish-culture. Fertilised eggs are
transported in ice, the development being simply suspended for several
weeks, and several forms of British and American Salmonidae have thus been
introduced into New Zealand and Tasmania, where some have thoroughly
established themselves.
The White-Fish, _Coregonus_, are more numerous in species than _Salmo_, and
as a rule more readily defined. They are easily recognised by their large
silvery scales and their smaller mouth without or with minute teeth. Some,
like the Houting (_C. oxyrhynchus_) of Northern Europe, occur in the sea,
entering rivers to spawn, whilst others, like the Sik, Weiss, Felchen, or
Lavaret (_C. lavaretus_), are confined to lakes. British species are the
Gwyniad (_C. clupeoides_), of Loch Lomond, Haweswater, Ullswater, and Bala,
the Vendace (_C. vandesius_), of Loch Maben, and the Pollan (_C. pollan_)
of Lough Neagh in Ireland.
[Illustration: FIG. 344.—Capelin (_Mallotus villosus_.) ½ nat. size. (After
Valenciennes.)]
The Grayling (_Thymallus vulgaris_ or _vexillifer_), with its high dorsal
fin formed of about 20 rays, one of the handsomest British fishes, inhabits
the rivers and lakes of Northern and Central Europe, and is represented by
a few allied species in Asia and North America. It derives its name from
having the odour of thyme.
The Smelt (_Osmerus eperlanus_) breeds in salt water, and although it often
enters rivers, it does not ascend beyond tidal influence. The Capelin
(_Mallotus villosus_), of the coasts of Arctic America and North-eastern
Asia, deposits its eggs in the sand along the shores in incredible numbers,
the beach becoming a {569}quivering mass of eggs and sand. _Plecoglossus_,
from Japan and Formosa, is highly remarkable for its lamellar, comb-like,
lateral teeth. The Siel-Smelts (_Argentina_) are deep-sea Salmonids of
which examples have occasionally been taken off the coasts of Scotland and
Ireland; large numbers have been brought from Norway to English markets.
_Bathylagus_ is still better adapted for life at great depths (down to 1700
fathoms), the eyes being of enormous size. As Dr. Günther has observed,
"these fishes must be entirely dependent for vision on the phosphorescent
light which is produced by other abyssal creatures. Not being fish of prey
themselves, or only to a slight degree, they would be attracted by the
light issuing from the Pediculates and Stomiatids of the deep, and thus
form an easy prey to these fishes."
Secondary sexual characters are very strongly developed in many Salmonids.
In adult males of Salmon, Trout, and Quinnat the snout becomes greatly
distorted, both jaws being hooked and the base of the teeth more or less
enlarged; in the latter species a fleshy hump is developed before the
dorsal fin, and the scales of the back become embedded in the flesh.
Pearl-like excrescences appear on the scales of many of the White-Fish
during the breeding season, being more prominent in males than in females,
and _Mallotus villosus_ is so called from the villous bands formed by the
scales of mature males, the scales above the lateral line and along each
side of the belly becoming elongate-lanceolate, densely imbricated and
produced into free, projecting points.[649]
The Pachyrhizodontidae, with the Cretaceous genus _Pachyrhizodus_, are
placed by some authors with the Salmonidae, but the remains at present
known are too fragmentary to afford a correct idea of their exact
systematic position. There seems to be less justification for placing them
among the Elopidae.
FAM. 18. ALEPOCEPHALIDAE.—Deep-sea Fishes similar in general structure to
the Clupeidae and Salmonidae, but destitute of a postclavicle and of an
adipose dorsal fin,[650] the rayed fin being situated far back on the body,
in the caudal region, and opposed {570}or slightly anterior to the anal.
The skeleton of _Alepocephalus_[651] is remarkable for its feeble
ossification. Epipleurals and epineurals are present, and the bilateral
division of the neural arch remains perfectly distinct throughout the
praecaudal region, both halves being very loosely apposed. The air-bladder
is absent. Ventrals are absent in _Platytroctes_, and the snout is much
produced in _Aulostomatomorpha_.
Eleven genera are distinguished:—A, with scales: _Alepocephalus_,
_Conocara_, _Bathytroctes_, _Leptochilichthys_, _Narcetes_, _Platytroctes_,
_Aulostomatomorpha_. B, without scales:—_Xenodermichthys_, _Aleposomus_,
_Leptoderma_, _Anomalopterus_.
Represented by about 35 species in nearly all the seas; as usual with
deep-sea forms, individuals of the same species have been obtained from
stations very remote from one another.
[Illustration: FIG. 345.—_Malacosteus indicus_. (After Günther.)]
FAM. 19. STOMIATIDAE.—I would unite under this name the Stomiatidae and
Sternoptychidae of Günther, an assemblage of aberrant deep-sea Fishes which
agree in having the maxillary bone more developed than the praemaxillary,
and beset with teeth, a character which differentiates them at once from
all other deep-sea forms of this sub-order, as well as from the Scopelidae
among the Haplomi. The ventral fins are usually inserted very far back, and
the number of their rays varies from 5 to 8. Contrary to what occurs in
other groups of fishes, the pectoral fins have a tendency to reduction, and
actually disappear in some genera, whilst the ventrals remain well
developed; whenever the pectoral fins are fully developed, as in
_Maurolicus_, _Chauliodus_, _Astronesthes_, and _Photichthys_, the
mesocoracoid arch is present.[652] The form of the body varies exceedingly,
even within the smaller groups into which this family has been divided; it
may be excessively short and compressed, or excessively elongate, {571}but
the mouth and eyes are always large, these fish being essentially
predatory; the dentition is often very powerful, and may extend to the
palate or be confined to the jaws. The body is naked or scaly; luminous
spots (photophores) are more or less developed.[653] The development and
position of the vertical fin is highly variable within this group, and the
several families which have been founded upon this character have no more
taxonomic importance than in the better-understood groups _Characinidae_
and _Siluridae_. All authors, besides, have been compelled to admit that
the presence or absence of an adipose dorsal fin has no high significance
in this case, a view which is further strengthened by Dr. Gilchrist's
discovery, off the Cape of Good Hope, of a deep-sea Fish agreeing in every
respect with _Astronesthes_, but for the presence of a small adipose fin,
absolutely similar to the dorsal, but situated on the ventral side,
immediately in front of the anus. Two species with similar ventral adipose
fins have just been discovered by Dr. Brauer and referred to
_Astronesthes_. I am therefore unable to adopt the elaborate arrangement in
favour with the modern American school.
[Illustration: FIG. 346.—_Sternoptyx diaphana_. (After Günther.)]
The genera may be arranged in five sub-families:—
I. Anal not exactly opposed to the rayed dorsal, or much longer than the
latter; no hyoid barbel.
_A_. Rayed dorsal far forward, between pectorals and ventrals;
pectorals well developed (CHAULIODONTINAE). _Chauliodus_.
_B_. Rayed dorsal above or behind the ventrals; pectorals well
developed.
1. Body more or less elongate; ventrals well developed
(GONOSTOMATINAE).
_a_. A hyoid barbel. _Astronesthes_.
_b_. No barbel. _Bathylychnus_, _Gonostoma_, _Cyclothone_,
_Triplophos_, _Photichthys_, _Bathylaco_, _Diplophos_,
_Maurolicus_, _Ichthyococcus_.
2. Body short and deep; ventrals rudimentary or absent
(STENOPTYCHINAE). _Argyropelecus_, _Sternoptyx_, _Polyipnus_.
II. Dorsal and anal opposed to each other and very far back on the caudal
region; pectorals often reduced or absent; hyoid barbel often present.
{572}(STOMIATINAE). _Stomias, Macrostomias, Echiostoma, Opostomias,
Pachystomias, Photonectes, Malacosteus, Thaumatostomias, Photostomias_.
This family, comprising about 55 species, has a world-wide distribution,
but most of the known forms have been obtained from the Atlantic; some of
the species occur both in the Atlantic and the Indo-Pacific. _Chauliodus_,
_Astronesthes_, and _Stomias_ are among the fishes with the most formidable
dentition.
FAM. 20. GONORHYNCHIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter articulated above the
former to the ethmoid. Supraoccipital in contact with the frontals, widely
separating the small parietals; opercular bones well developed; symplectic
present. Basis cranii simple. Mouth small and toothless, inferior,
surrounded by thick, fringed lips. Four branchiostegal rays. Head and body
entirely covered with small spiny scales. Praecaudal vertebrae with strong
parapophyses, to the extremity of which slender ribs and epipleurals are
attached. No postclavicle. Pectoral fins inserted low down, folding like
the ventrals; latter with 10 rays.
[Illustration: FIG. 347.—_Gonorhynchus greyi_. ⅓ nat. size. (After
Valenciennes.)]
The single existing species, _Gonorhynchus greyi_, is characterised by an
elongate, cylindrical body, a pointed projecting snout bearing a single
barbel, short dorsal and anal fins, the former opposed to the ventrals, and
the gill-membranes broadly attached to the isthmus. Teeth are present on
the pterygoid and hyoid bones. No suborbital arch. Vertebrae, 45 + 20.
Air-bladder absent. Its distribution is a very wide one, the species being
on record from the coasts of the Cape of Good Hope, Australia, New Zealand,
and Japan.
The genus _Notogoneus_, from the freshwater Eocene beds of France and North
America, has been referred to this family by Cope, and has been shown by A.
S. Woodward to be closely related to _Gonorhynchus_, differing only in the
absence of teeth on the palate and tongue, and in the more forward position
of the dorsal fin. The genus _Charitosomus_, with several species {573}from
the Upper Cretaceous of Westphalia and Mount Lebanon, has also been
included in this family, but the precise shape and character of the scales
have not yet been ascertained.
FAM. 21. CROMERIIDAE.—Margin of the upper jaw formed by the praemaxillaries
and the maxillaries. Supraoccipital large and widely separating the very
small parietals; opercular bones well developed; symplectic absent. Basis
cranii simple. Mouth small and toothless, inferior; gill-opening narrow.
Three branchiostegal rays. Body naked. Praecaudal vertebrae with
parapophyses; ribs and epipleurals slender. No postclavicle. Pectoral fin
inserted low down, folding like the ventrals.
A single genus, _Cromeria_, recently discovered in the White Nile. In its
elongate, naked body and the posterior position of the dorsal fin, it
resembles the Galaxiidae, to which it was at first referred. But this
allocation has proved to be incorrect, now that the osteological structure
of the minute Fish (only about 30 mm. long) has been worked out by
Swinnerton.[654] The vertebrae number 42 to 45 (28-30 + 14-15). A long,
slender air-bladder is present.
SUB-ORDER 2. OSTARIOPHYSI.
Air-bladder, if well developed, communicating with the digestive tract by a
duct. Pectoral arch suspended from the skull; mesocoracoid arch present.
Fins without spines, or dorsal and pectoral with a single spine formed by
the co-ossification of the segments of an articulated ray. The anterior
four vertebrae strongly modified, often co-ossified and bearing a chain of
small bones (so-called Weberian ossicles) connecting the air-bladder with
the ear.
This is one of the most natural groups of the Class Pisces, although its
members are so diversified in outward appearance as to have been widely
separated in the systems of older authors. It is to Sagemehl[655] that is
due the credit of having first grouped, under the above name, the
Characines, the Carps, the Cat-Fishes, and the Gymnotids, the relations of
which had been realised, to a certain extent, by Cope. But it was not until
the homology throughout the group of the ossicula auditus, first described
by E. H. Weber in 1820, had been demonstrated by Sagemehl that the
justification for the course here followed appeared in its full
{574}strength, as such an agreement in the structure of so complicated and
specialised an apparatus can only be the result of a community of descent
of the families which are possessed of it. It is invariably the anterior
four vertebrae that take part in the support of the Weberian apparatus. The
first vertebra is much reduced; its upper arch is absent and replaced by
the ossicles termed _claustrum_ and _scaphium_[656] (the former being
perhaps nothing but the modified neural arch), which fill in the space
between the exoccipital and the neural arch of the second vertebra; the
principal piece of the apparatus, the _tripus_, variable in form, is
related to the third vertebra, of which it is regarded as a modified rib; a
fibrous ligament extends from the anterior extremity of the _tripus_ to the
_scaphium_, and in this ligament is inserted the fourth piece, the
_intercalarium_. The various forms of this sub-order also show a complete
agreement in the spinal nerves which pass through these ossicles. The
parietal bones either separate the frontals from the supraoccipital or are
fused with the latter.
This sub-order is divided into six families. The Characinids are the most
generalised, and the others are probably derived from them in the manner
expressed by the following diagram:—
_Loricariidae_ _Aspredinidae_
| |
+––––––––––––+––––––––––+
|
_Cyprinidae_ _Siluridae_ _Gymnotidae_
| | |
+–––––––––––+–––––––––––+ |
| |
+–––––––––––––––––––+–––––––––––––––––+
|
_Characinidae_.
SYNOPSIS OF THE FAMILIES
I. Parietal bones distinct from the supraoccipital; symplectic present;
ribs mostly sessile, all or the greater number of the praecaudal
vertebrae without parapophyses.
Mouth not protractile, usually toothed; pharyngeal bones normal; body
scaly; an adipose dorsal fin often present 1. _Characinidae_.
Mouth not protractile, usually toothed; pharyngeal bones normal; body
Eel-shaped, naked or scaly; vent under the head or on the throat
2. _Gymnotidae_.
Mouth usually more or less protractile, toothless; lower {575}
pharyngeal bones large, falciform; body naked or scaly; no
adipose dorsal fin 3. _Cyprinidae_.
II. Parietal bones usually fused with the supraoccipital; symplectic
absent; body naked or with bony scutes; mouth usually toothed, with
barbels; adipose fin often present.
Ribs attached to strong parapophyses; operculum well developed
4. _Siluridae_.
Ribs sessile; parapophyses absent; operculum more or less developed;
mouth inferior 5. _Loricariidae_.
Ribs sessile; strong parapophyses to the vertebrae; operculum absent
6. _Aspredinidae_.
FAM. 1. CHARACINIDAE.—Mouth non-protractile, usually bordered by the
praemaxillaries and the maxillaries, rarely by the praemaxillaries only;
jaws usually toothed. Parietal bones united in a sagittal suture or
separated by a fontanelle; opercular bones well developed; symplectic
present. Pharyngeal bones normal, with small teeth. Ribs mostly sessile; no
parapophyses in the thoracic region; epipleurals and epineurals, mostly
free floating. Pectoral fins inserted very low down, folding like the
ventrals. Body covered with scales. An adipose dorsal fin often present.
This is a very generalised type, although perhaps not directly derived from
the bony Ganoids, as believed by Sagemehl. The species number about 500,
and are confined to the freshwaters of Africa and Central and South
America. The classification of the family is still in an unsatisfactory
state, but the division into the following groups (hardly deserving the
rank of sub-families), although quite provisional, appears preferable to
the highly artificial arrangement hitherto adopted:—
I. No adipose fin.
A. ERYTHRININAE.—Carnivorous; teeth strong; maxillary large;
gill-openings wide; scales cycloid. American: _Macrodon_, _Erythrinus_,
_Lebiasina_, _Pyrrhulina_, _Corynopoma_.
II. Adipose fin usually present.
B. HYDROCYONINAE.—Entirely or partially carnivorous; teeth strong;
maxillary well developed; scales cycloid; lateral line usually nearer
ventral than dorsal outline (sometimes only on the tail). African:
_Sarcodaces_, _Hydrocyon_, _Bryconaethiops_, _Alestes_, _Micralestes_,
_Petersius_. American: _Acestrorhynchus_, _Boulengerella_,
_Acestrorhamphus_, _Crenuchus_, _Chalceus_, _Brycon_, _Bryconops_,
_Bryconodon_, _Creagrutus_, _Chalcinus_, _Brachychalcinus_,
_Pseudocorynopoma_, _Stichonodon_, _Gastropelecus_, _Tetragonopterus_,
_Scissor_, _Chirodon_, _Piabucina_, _Iguanodectes_, _Aphiocharax_,
_Salminus_, _Oligosarcus_, _Agoniates_, _Paragoniates_,
_Leptagoniates_, _Anacyrtus_.
{576}C. SERRASALMONINAE.—Carnivorous; teeth strong; belly serrated;
scales cycloid. American: _Serrasalmo_, _Myletes_, _Myleus_,
_Metynnis_, _Catoprion_.
D. ICHTHYOBORINAE.—Carnivorous; teeth strong; maxillary very small;
upper jaw movable; scales ciliated. African: _Eugnathichthys_,
_Paraphago_, _Mesoborus_, _Phago_, _Ichthyoborus_, _Neoborus_.
E. XIPHOSTOMINAE.—Carnivorous; teeth very small; maxillary rather
small; scales ciliated. American: _Xiphostoma_.
F. ANOSTOMINAE.—Herbivorous, entirely or partially; teeth well
developed in both jaws; maxillary very small; gill-openings narrow;
scales cycloid. American: _Anostomus_, _Leporinus_, _Characidium_,
_Chorimycterus_, _Nanostomus_, _Nanognathus_.
G. HEMIODONTINAE.—Partially herbivorous; dentition imperfect; maxillary
well developed; scales cycloid. American: _Hemiodus_, _Caenotropis_,
_Saccodon_, _Parodon_.
H. DISTICHODONTINAE.—Entirely or partially herbivorous; teeth small but
well developed; maxillary well developed; scales ciliated. African:
_Nannaethiops_, _Neolebias_, _Distichodus_, _Nannocharax_,
_Xenocharax_.
I. CITHARININAE.—Herbivorous; teeth minute or absent; maxillary small;
scales cycloid or ciliated. African: _Citharinus_, _Citharidium_.
American: _Prochilodus_, _Curimatus_.
[Illustration: FIG. 348.—Distribution of the Characinidae.]
The genera in the above sub-families are mostly founded on the dentition
and the extent of the praemaxillary and maxillary bones, which are
astonishingly varied, as may be seen from the annexed figures showing the
open mouths of a few of the most remarkable types. As I have already
pointed out, the character often given as diagnostic of this family, viz.
the maxillary forming part of the oral border, is not absolutely constant;
this bone is often much reduced, and it is entirely excluded from the mouth
in _Ichthyoborus_ and _Neoborus_. The branchiostegal rays {577}number 3 to
5 only. The fins never bear pungent spines, and the ventrals have 6 to 13
rays. Barbels are absent. In most of the herbivorous forms the brain-case
is produced forward to the nasal capsule, whilst in most of the carnivorous
forms they are separated by an interorbital septum; but there are
exceptions to this correlation, and as otherwise closely related genera may
differ in this respect, I have not been able to make use of the character
in defining sub-families.
[Illustration: FIG. 349.—Open mouths of Characinidae. (After Müller and
Troschel.) A, _Macrodon trahira_; B, _Piabucina argentina_; C, _Brycon
falcatus_; D, _Chalceus angulatus_; E, _Serrasalmo rhombeus_; F,
_Distichodus niloticus_.]
{578}The air-bladder is divided into an anterior and a posterior part by a
constriction; the posterior part is the longer, and its anterior portion is
cellular in _Erythrinus_ and _Lebiasina_. Pyloric appendages to the
stomach, which are constantly absent in the Cyprinids, are more or less
numerous. An accessory respiratory organ in a diverticulum above the fourth
branchial arch has been observed in _Xenocharax_ and _Citharinus_.
[Illustration: FIG. 350.—_Hydrocyon goliath_, from the Congo. ⅒ nat. size.]
The appearance and habits of the genera which compose this family vary
greatly. Some resemble the Cyprinids and are mainly vegetarians, whilst
others recall Salmonids and Pike. Among the most formidable are
_Hydrocyon_, the Dogs of the Water, or Kelb-el-Bahr of the Arabs, with
their powerful jaws with shark-like teeth, visible when the mouth is
closed, and which grow to the size of the Salmon. The five known species
inhabit the Nile and the rivers and lakes of tropical Africa. No less
ferocious are the "Piranha" or "Cariba" (_Serrasalmo_) of South America,
whose bite has been compared to the cut of a razor. They abound in some
rivers and are much dreaded by people having to enter the water, as they
fiercely bite off big pieces of flesh as with a pair of scissors, and the
smell of blood is said to attract them by thousands; they show a great
tenacity of life and can remain for hours out of the water. _Serrasalmo
niger_ has been observed by Schomburgk to produce a grunting noise in the
water. _Salminus orbignianus_, of the Plate River, "Dorado" of the
Spaniards, which reaches a length of 3 feet, has the predacious habits of
the Pike, and follows other fishes moving in shoals; its flesh is much
valued, although very full of bones, like that of all Characinids.
{579}As an example of phytophagous types may be mentioned the Moon-Fish of
the Nile (_Citharinus geoffroyi_), with its feeble dentition, deep
compressed body, and falciform dorsal fin; it is often represented on the
monuments of the ancient Egyptians.[657]
FAM. 2. GYMNOTIDAE.—Mouth non-protractile, bordered by the praemaxillaries
and the maxillaries, the latter sometimes much reduced; jaws usually
toothed. Parietal bones united in a sagittal suture, or separated by a
fontanelle; opercular bones well developed; symplectic present. Pharyngeal
bones normal, with small teeth. Anterior ribs sessile, the posterior
inserted on transverse processes; epipleurals and epineurals. Body much
elongate, Eel-like, naked or scaly; dorsal fin absent or reduced to an
adipose strip; anal very long; caudal rudimentary or absent;[658] ventrals
absent. Vent under the head or at a very short distance behind the throat.
Gill-openings narrow.
In spite of their external appearance, these fishes have nothing to do with
the Eels; they are strongly modified, degraded Characinids, as first
pointed out by Reinhardt. The few genera and species (about 30) are
confined to the fresh waters of Central and South America. No fossils are
known. Eight genera may be distinguished:—
A. A cranial fontanelle; maxillary bone larger than the praemaxillary;
anterior nostril on the upper surface of the head; vent below the head;
body scaly: _Sternopygus_, _Eigenmannia_, _Sternarchus_,
_Rhamphosternarchus_, _Rhamphichthys_, _Steatogenys_.
B. No cranial fontanelle; maxillary bone very small; anterior nostril on
the upper lip; vent on the throat.
_a_. Body scaly: _Carapus_.
_b_. Body naked; an electric organ: _Gymnotus_.
The mouth is small or very small, and the modifications of the snout in the
genera _Sternarchus_ and _Rhamphichthys_ recall those noticed among the
Mormyridae. The air-bladder is divided into an anterior and a posterior
part, united by a slender duct.[659] The vertebrae vary in number from 70
(_Sternopygus_) to 240 (_Gymnotus_). _Gymnotus_ is unique in this sub-order
in having as many as 8 pterygials (actinosts) to the pectoral fin, as in
_Anguilla_.
{580}[Illustration: FIG. 351.—Outlines of heads, showing shape of snout and
position of vent (_v_). A, _Sternarchus albifrons_; B, _Sternarchus
macrostoma_; C, _Rhamphosternarchus curvirostris_; D, _Rhamphosternarchus
tamandua_.]
The best known member of this family is the so-called Electric Eel
(_Gymnotus electricus_), of the Orinoco, Amazons, and intermediate
river-systems. It grows to a length of 8 feet and the thickness of a man's
thigh, and is much feared for the electric shocks it is able to discharge.
The "Tremblador," as it is called by the Spanish-speaking inhabitants of
the Orinoco district, is found only in marshes and in comparatively shallow
parts of rivers, to the great annoyance of travellers who have to ford at
such points, beasts of burden being frequently knocked down by the electric
shock. Specimens have often been exhibited alive in this country; two
brought to London in the year 1842, neither of them weighing more than one
pound, had by 1848 reached the weights of 40 and 50 pounds respectively.
About four-fifths of the length of the fish is occupied by the tail, which
contains the electric organ; this is formed by modified muscular tissue,
and consists of two huge masses, longitudinal bands or columns, of cells
filled with a jelly-like substance, occupying the whole of the caudal
region below the vertebral column and separated by a narrow median septum;
a smaller body, of similar structure, extends along each side at the base
of the anal fin. The whole apparatus is supplied with a great number of
nerves branching from the spinal nerves. The electrical apparatus is
exercised by the will of the fish, even to {581}a distance, but this
faculty is exhausted by continuous employment, and is recovered during
repose. Although apparently not exempt from exaggeration and fable,
Humboldt's account in _Observations de Zoologie_, p. 497, is recommended
for further information on the habits and modes of capture of
_Gymnotus_.[660]
FAM. 3. CYPRINIDAE.—Mouth usually more or less protractile, toothless,
bordered by the praemaxillaries and the maxillaries, or, more frequently by
the praemaxillaries only. Parietal bones united in a sagittal suture, or
separated by a fontanelle; opercular bones well developed; symplectic
present. Lower pharyngeal bones falciform, subparallel to the branchial
arches, provided with teeth arranged in one, two, or three series, and
often remarkably specialised. Ribs mostly sessile; no parapophyses in the
thoracic region; epipleurals and epineurals, mostly free, floating.
Pectoral fins inserted very low down, folding like the ventrals. Body naked
or scaly. No adipose dorsal fin.
The brain-case is produced forward to the nasal capsule. The branchiostegal
rays are reduced to 3; the branchiostegal membrane is usually more or less
extensively grown to the isthmus. The suborbital branch of the sensory
canals is usually produced on the operculum, as in the Characinidae. The
ventral rays number 7 to 12, rarely 5 or 6. Pyloric appendages to the
stomach are absent.
Freshwater fishes feeding on vegetable substances or small animals, and
dispersed over the whole world with the exception of South America,
Madagascar, Papuasia, and Australasia. The species are exceedingly
numerous, about 1300 being known, referable to four sub-families, as
proposed by Sagemehl.
(i.) CATOSTOMINAE.—Margin of upper jaw formed in the middle by the small
praemaxillaries and on the sides by the maxillaries, which are hidden in
thick fleshy lips; no barbels; pharyngeal teeth in a single row, very
numerous, comb-like; air-bladder large, divided into two or three parts by
transverse constrictions, not surrounded by a bony capsule. Mostly from
North America; two species from China and one from Eastern Siberia. Fossil
in the Lower Tertiary of North America.
Principal genera:—_Sclerognathus_, _Carpiodes_, _Catostomus_, _Moxostoma_.
{582}(ii.) CYPRININAE.—Maxillaries not bordering the mouth; barbels absent,
or one or two pairs; pharyngeal teeth in one to three rows, in small
number, often very large, and working against a sclerous plate attached to
a ventral process of the basi-occipital, which extends under the anterior
vertebrae. Air-bladder usually large and divided into an anterior and a
posterior part, rarely tripartite, not surrounded by a bony capsule. The
great bulk of the family, represented in every part of its range. Remains
of several of the existing genera have been found in Oligocene and later
beds of Europe, Sumatra, and North America.
Principal genera:—_Cyprinus_, _Catla_, _Catlocarpio_, _Osteochilus_,
_Labeo_, _Discognathus_, _Psilorhynchus_, _Capoëta_, _Barbus_, _Gobio_,
_Pseudogobio_, _Saurogobio_, _Rhinogobio_, _Oreinus_, _Schizothorax_,
_Ptychobarbus_, _Gymnocypris_, _Diptychus_, _Aulopyge_, _Ceratichthys_,
_Pimephales_, _Campostoma_, _Cochlognathus_, _Exoglossum_, _Meda_,
_Lepidomeda_, _Rhinichthys_, _Rohteichthys_, _Leptobarbus_, _Rasbora_,
_Luciosoma_, _Nuria_, _Amblypharyngodon_, _Cyprinion_, _Semiplotus_,
_Xenocypris_, _Leuciscus_, _Tinca_, _Leucosomus_, _Chondrostoma_,
_Achilognathus_, _Rhodeus_, _Danio_, _Pteropsarion_, _Hypophthalmichthys_,
_Abramis_, _Nematabramis_, _Aspius_, _Leucaspius_, _Alburnus_, _Barilius_,
_Bola_, _Neobola_, _Chelaethiops_, _Chela_, _Culter_, _Pelecus_,
_Parapelecus_, _Cachius_, _Opsariichthys_, _Scombrocypris_,
_Squaliobarbus_, _Luciobrama_.
(iii.) COBITIDINAE.—Maxillaries not bordering the mouth; barbels three to
six pairs; pharyngeal teeth in one row, in moderate number. Anterior part
of the air-bladder divided into a right and left chamber separated by a
constriction, and enclosed in a bony capsule, the posterior part free, or
absent. Loaches, characterised externally by a low, elongate body, without
or with minute scales. Europe, Asia, Abyssinia. Miocene of Oeningen.
Principal genera:—_Botia_, _Lefua_, _Diplophysa_, _Nemachilus_,
_Misgurnus_, _Cobitis_, _Lepidocephalichthys_, _Acanthophthalmus_,
_Eucirrhichthys_, _Apua_.
(iv.) HOMALOPTERINAE.—Maxillaries not bordering the mouth, which is
inferior; barbels three or four pairs; pharyngeal teeth in one row, in
moderate number. Air-bladder rudimentary, divided into two lateral halves,
encased in a bony capsule. Mountain forms with depressed head and
horizontally expanded paired fins. China, India, Further India, Malay
Peninsula and Archipelago.
Genera:—_Homaloptera_, _Helgia_, _Glaniopsis_, _Gastromyzon_.
The recently described _Gyrinochilus_, from Borneo, resembling
{583}_Homaloptera_ in habit, with two gill-clefts on each side, an upper
and a lower, a tadpole-like mouth without barbels, and a small, free
air-bladder, should probably be regarded as the type of a fifth sub-family.
Many of the genera of the Cyprininae are partly founded on the shape and
the disposition of the pharyngeal teeth, which, adapted to various
requirements, may be conical, hooked, spoon-shaped, molariform, etc., etc.
The importance attached to the disposition of these teeth in one, two, or
three series for the definition of genera, has been rather
exaggerated.[661]
[Illustration: FIG. 352.—Lower pharyngeals of _Barbus tropidolepis_.]
[Illustration: FIG. 353.—_Labeo falcifer_, from the Congo, showing nuptial
tubercles on the snout. ¼ nat. size.]
The Cyprinids constitute the majority of the freshwater fishes in Europe,
Asia, and North America; they are comparatively few (about 100 species) in
Africa, where they coexist with the Characinids. Some, like the Carp
(_Cyprinus carpio_) and the Tench (_Tinca vulgaris_), are sluggish, except
during the breeding season, when they show great excitement and indulge in
leaps out of the water; others, like the Bleak (_Alburnus lucidus_) are
constantly on the move in large shoals near the surface; whilst others
again, like the M‘Biriki of Lake Tanganyika (_Barbus tropidolepis_), behave
after the manner of Salmon and Trout, {584}travelling long distances,
against rapids and over waterfalls, to reach their breeding places at the
heads of rivers. During the breeding season, the males of many species
assume a more brilliant livery, or develop pearl-like or spiny excrescences
on various parts of the head, or also on the body and fins.[662] Cyprinids
are oviparous, with the exception of a small Barbel from Natal, discovered
and described by Prof. Max Weber as _Barbus viviparus_.
A most striking instance of symbiosis is offered by a little Carp-like fish
of Central Europe, the "Bitterling" (_Rhodeus amarus_). The genital papilla
of the female acquires a great development during the breeding season,
becoming produced into a tube nearly as long as the fish itself; by means
of this ovipositor the comparatively few and remarkably large eggs,
measuring 3 millimetres in diameter—the fish being only 60 to 80
millimetres long—are introduced through the gaping valves, between the
branchial laminae of pond mussels (_Unio_ and _Anodonta_) where, after
being inseminated, they undergo their development, the fry leaving their
host about a month later, having attained a length of 10 or 11
millimetres.[663] The mollusc reciprocates by throwing off its embryos on
the parent fish, in the skin of which they remain encysted for some time,
the period of reproduction of the fish and mussel coinciding.
Some members of this family grow to a very large size,—4 to 6 feet; such is
the case with the Carp, a native of Asia, introduced into England towards
the beginning of the seventeenth century; the Catla (_Catla buchanani_) of
India, Burma, and Siam; the Mahaseer (_Barbus mosal_) of the mountain
streams of Asia, the scales of which may be as large as the palm of a hand;
and _Hypophthalmichthys molitrix_ of China and Manchuria, remarkable for
the low position of the eyes, the fusion of the gill-rakers into thin
plates of spongious appearance, which must act as a most efficient sifting
apparatus, and the presence of an involuted problematic superbranchial
organ to each branchial arch.[664]
Among well-known aberrations produced by artificial selection may be
mentioned the "Leather Carp," a race in which the scales are either lost or
much reduced in number, and enlarged {585}along the lateral line and the
back, and the Gold-Fish, a variety of _Cyprinus carassius_, remarkable for
its golden or bright red colour, or its perfect albinism, as well as its
monstrous form the Telescope Fish, with enormously projecting eyes, and
enlarged, horizontally spread caudal fin.[665] This family has also yielded
numerous more or less well-established examples of hybridism, congeneric
and digeneric, originally described as distinct species, the produce of
which is believed to be in some cases fertile for at least one generation.
The crystalline silvery colouring matter of various Cyprinids is said to
have been employed from time immemorial for ornamental purposes by the
Chinese. The well-known and important industry of "Essence Orientale" and
artificial pearls, carried on in France and Germany with the scales of the
Bleak, was not introduced before the middle of the seventeenth century.
[Illustration: FIG. 354.—Pond Loach (_Misgurnus fossilis_), with lower view
of mouth. ⅓ natural size.]
The Loaches, Cobitidinae, which form a very natural sub-family, are small
fishes, few species growing to a foot in length, mostly living in small
streams and ponds. Many delight in the mud at the bottom, in which they
move like Eels. In some cases, the branchial respiration appears to be
insufficient, and the intestinal tract acts as an accessory breathing
organ. The air-bladder, which is partially encased in a bony capsule, may
be so reduced as to lose its hydrostatic functions and becomes transformed
into a sensory organ, its outer exposed surface being connected with the
skin by a meatus between the bands of muscle, and conveying the
thermo-barometrical impressions to the auditory nerves; hence the name of
"Wetterfisch," by which Loaches are known in some parts of Germany.
The Homalopterinae are more or less perfectly adapted to life in rapid
streams, the most remarkable in this respect being {586}_Gastromyzon_ of
North Borneo, in which the pectoral and ventral fins are much expanded to
form, with the belly, a sucker by which the fish adhere to the stones of
mountain torrents, showing a remarkable analogy to _Exostoma_ among the
Silurids.[666]
[Illustration: FIG. 355.—_Gastromyzon borneensis_, ventral view, natural
size.]
FAM. 4. SILURIDAE.—Mouth non-protractile, bordered by the praemaxillaries
and the maxillaries, or by the praemaxillaries only, the maxillaries being
often rudimentary and supporting the base of a barbel; jaws usually
toothed. Parietal bones usually confluent with the supraoccipital, forming
a single large plate (parieto-occipital); symplectic and suboperculum
absent. Pharyngeal bones normal, with small teeth. Ribs attached to the
lower surface of long parapophyses; epipleurals absent. Pectoral fins
inserted very low down, folding like the ventrals, often armed, like the
dorsal, with a strong bony spine. Body naked or with bony plates. An
adipose dorsal fin often present. One to four pairs of barbels.
The skull and the opercular apparatus show a reduction in the number of
elements as compared with the Characinids and Cyprinids, such as the
absence of the metapterygoid, the often rudimentary, rod-like condition of
the palatine, and the fusion of the parietals with the supraoccipital.[667]
The scapular arch is solidly united to the skull and is often very massive,
and the occiput may be connected with the base of the dorsal fin by a
buckler formed by the expansion of the first and second inter-neural bones.
The pterygials or supports of the pectoral rays are large and reduced to
two or three.[668] Teeth are rarely present {587}on the maxillary bones
(_Diplomystes_, _Eutropiichthys_), being usually confined to the
praemaxillaries and dentaries; they often occur on the palate. The
branchiostegal rays vary from 4 to 17. The lips are sometimes much
developed, and may form a sucking disk, as in _Euchilichthys_ and
_Exostoma_. As in the Cyprinids, the pungent spines which may arm the fins
have nothing in common with the true spines of Acanthopterygians; they
result from the co-ossification, with age, of successive articles; but,
contrary to the condition in the Cyprinids, the axis of the spine is
single, not double. The ventral rays vary from 6 to 16, 6 being the most
frequent number. Some of the exterior vertebrae may be solidly fused
together, and also with the occipito-nuchal buckler. Prof. Ramsay
Wright[669] has shown, by a study of the development, that the complex
which follows the first vertebra, which is more or less rudimentary, if
distinct, represents the fusion of the 2nd, 3rd, and 4th vertebrae, without
even nerve-foramina denoting its compound origin; the first, strongly
developed, transverse process represents that of the 4th vertebra. The
air-bladder is usually large and trilocular, but additional septa may
greatly complicate its structure, and external diverticula may also
exist;[670] it may be more or less reduced and entirely or partially
enclosed in a bilateral bony capsule formed by the transverse processes of
the vertebrae, and sometimes (_Nematogenys_) ankylosed to the skull. In a
few genera, like _Cetopsis_, the air-bladder seems to be altogether absent:
it is reduced to two small oval sacs encased in the large compound anterior
vertebra. As in Loaches, the air-bladder is often in immediate contact with
the skin behind the shoulder-girdle. The intestinal tract may be simple and
short (carnivorous forms) or extremely long and convoluted (_Callichthys_);
as in Cyprinids, pyloric appendages are absent.
Cat-Fishes, as Silurids are usually called, are a large family embracing
some 1000 species, spread over the fresh waters of all parts of the world,
but mostly from between the tropics.[671] Only a few are marine
(_Plotosus_, _Arius_, _Galeichthys_).
This family may be divided into eight subfamilies
{588}(i.) CLARIINAE.—Dorsal and anal fins very long, extending to the
caudal; gill-membranes free, or narrowly united to the isthmus.
Asiatic-African genera: _Clarias_, _Heterobranchus_, _Plotosus_.
Asiatic-Australian: _Copidoglanis_. Asiatic: _Cranoglanis_. African:
_Clariallabes_, _Allabenchelys_, _Gymnallabes_, _Channalabes_. Australian:
_Cnidoglanis_.
(ii.) SILURINAE.—Dorsal fin very short or absent, anal very long;
gill-membranes free. Europaeo-Asiatic: _Silurus_. Asiatic: _Silurodon_,
_Silurichthys_, _Saccobranchus_, _Wallago_, _Belodontichthys_,
_Cryptopterus_, _Callichrous_, _Hemisilurus_, _Ailiichthys_, _Ailia_,
_Schilbichthys_, _Laïs_, _Pseudeutropius_, _Pangasius_, _Osteogeniosus_,
_Helicophagus_, _Silondia_. African: _Eutropius_, _Schilbe_, _Siluranodon_,
_Physailia_, _Parailia_. Australian: _Eumeda_, _Neosilurus_.
(iii.) BAGRINAE.—Dorsal fin short, followed by a more or less elongate
adipose fin; anal short or moderate; gill-membranes free. Asia, Africa,
America, Australia: _Arius_. Asia and America: _Amiurus_. Asiatic:
_Macrones_, _Pseudobagrus_, _Liocassis_, _Bagroides_, _Bagrichthys_,
_Rita_, _Acrochordonichthys_, _Acysis_, _Olyra_, _Hemipimelodus_. African:
_Bagrus_, _Clarotes_, _Chrysichthys_, _Gephyroglanis_, _Auchenoglanis_,
_Notoglanidium_, _Anoplopterus_, _Galeichthys_. American: _Diplomystes_,
_Paradiplomystes_, _Aelurichthys_, _Genidens_, _Noturus_, _Callophysus_,
_Pimelodus_, _Pimelodina_, _Nanoglanis_, _Heptapterus_, _Nematogenys_,
_Pariolius_, _Pirinampus_, _Conorhynchus_, _Notoglanis_, _Callophysus_,
_Sorubim_, _Piramutana_, _Bagropsis_, _Sciades_. Australian: _Nedystoma_,
_Pachyula_.
(iv.) DORADINAE.—A short-rayed dorsal fin and an adipose, the latter
sometimes replaced by a second rayed dorsal; anal short or moderate;
gill-clefts more or less widely interrupted below. African: _Synodontis_,
_Chiloglanis_, _Atopochilus_, _Euchilichthys_, _Mochocus_, _Doumea_,
_Phractura_, _Paraphractura_, _Andersonia_, _Trachyglanis_, _Belonoglanis_.
Asiatic: _Bagarius_, _Glyptosternum_, _Gagata_, _Pseudecheneis_,
_Exostoma_, _Sisor_, _Breitensteinia_, _Sosia_, _Chaca_. South American:
_Doras_, _Oxydoras_, _Leptodoras_, _Physopyxis_, _Glanidium_,
_Centromochlus_, _Wertheimeria_, _Cetopsis_.
(v.) MALOPTERURINAE.—No rayed dorsal fin, an adipose; anal short;
gill-clefts interrupted below. African: _Malopterurus_.
(vi.) CALLICHTHYINAE.—Dorsal, anal, and adipose fins short; body completely
cuirassed; praemaxillaries much reduced, the border of the upper jaw formed
mainly by the maxillaries. South American: _Callichthys_, _Corydoras_.
{589}(vii.) HYPOPHTHALMINAE.—Dorsal fin short, behind the ventrals, anal
long; gill-clefts wide or interrupted below. South American: _Ageniosus_,
_Trachelyopterus_, _Auchenipterus_, _Epapterus_, _Tetranematichthys_,
_Hypophthalmus_, _Helogenes_.
(viii.) TRICHOMYCTERINAE.—Dorsal fin short, far back, behind the ventrals;
no adipose fin; anal short; operculum and interoperculum armed with
erectile spines. South American: _Trichomycterus_, _Eremophilus_,
_Stegophilus_, _Vandellia_, _Acanthopoma_.
Our knowledge of the distribution in time of the Silurids is still very
scanty, and throws no light on the derivation of the group. _Arius_, and
two genera apparently related to it, _Rhineaster_ and _Bucklandium_, have
left remains in the Eocene of Europe and North America, and traces of
various recent genera have been found in later Tertiary deposits in Europe,
Asia, and North and South America.
The habits of the Silurids are extremely diversified, and the shape of the
body varies accordingly. The body may be very short and the head enormous
and excessively depressed, for instance in the Indo-Burmese _Chaca
lophioides_, which, as its name implies, resembles the Fishing-Frog or
Angler; stout and _Cottus_-like in some South American _Pimelodus_;
Loach-like in _Trichomycterus_ and _Stegophilus_; more or less Eel-shaped
in _Clarias_ and its allies, etc.; the extreme of slenderness obtains in
the African _Channalabes_, the body being excessively elongate (over 100
vertebrae), the ventral fins absent, and the pectorals rudimentary or
absent. Among other remarkable forms may be mentioned the Indian _Sisor_,
which resembles _Aspredo_, and in which the upper caudal ray is much
thickened and greatly prolonged; _Pseudecheneis_, living in rapids of the
Himalayas and Khasia hills, provided with a transversely plaited ventral
disk between the pectoral fins; the African _Phractura_ and _Andersonia_,
resembling _Loricaria_; and the likewise African _Belonoglanis_, comparable
to a Needle-Fish. The spines which so frequently arm the dorsal and
pectoral fins may be barbed or serrated, and constitute formidable
defensive weapons; in the South American _Ageniosus valenciennesi_, the
maxillary bone is transformed into a strong, barbed, erectile spine,
replacing the barbel. Stings of even the smaller Cat-Fish are at least as
painful as that of a bee, and this is probably due to some poisonous
property of the dermal secretion of the Fish.
{590}[Illustration: FIG. 356.—Harmout, _Clarias anguillaris_ (after
Valenciennes). ¼ nat. size.]
Cope believed an orifice at or above the axil of the pectoral fin in
_Noturus_ to be the opening of the duct of a poison-gland; "from it may
frequently be drawn a solid gelatinous style ending in a tripod, each limb
of which is dichotomously divided into short branches of regular length." I
think this condition of things has nothing to do with a poison-organ, and
is merely a repetition of what is observed in Loaches and in the Characinid
_Xenocharax_, where I have found a gelatinous substance filling the short
duct by which the membrane of the air-bladder is placed in communication
with the skin and the sensory organ of the lateral line. Most Silurids can
live in very foul water, taking in air from the surface, and spend a
comparatively long time out of the water, without being possessed of any
special apparatus for atmospheric respiration. A few genera, however, are
provided with an accessory breathing organ: in _Clarias_, _Heterobranchus_,
and allies, there is a dendritic superbranchial organ, in _Saccobranchus_ a
long air-sac, extending from the first branchial cleft along the side of
the body, as described above, p. 295; and these Fish can live for days on
land. _Clarias lazera_ has been observed, in Senegambia, to spend several
months of the dry season in burrows, from which it emerges at night to
crawl about in search of food. Many Silurids, but especially _Doras_ and
_Synodontis_, are known to produce sounds in and out of the water by means
of a special mechanism of the air-bladder and the processes of the
vertebrae above it, combined with the movements of the pectoral spine
grinding in the glenoid cavity.[672] In South America, _Doras_ has been
observed to move rapidly on land, projecting itself forward on the pectoral
spines by the elastic spring of the tail, travelling long journeys over
land, from one drying pond to another, spending whole nights on the way;
these migrations sometimes take place {591}in numerous bands, baskets of
the small Fish being filled by the Indians who come across them.[673] The
African _Synodontis_ are much in the habit of floating or swimming
leisurely on the surface with the belly in the air, as was well known to
the ancient Egyptians, who have frequently depicted the Fish in this
anomalous position. A curious fact in connexion with this habit is that _S.
membranaceus_ and _S. batensoda_, in which it has most frequently been
observed, show an inversion of the ordinary mode of coloration, the lower
parts being dark brown or black and the upper pale silvery grey. The
electric Cat-Fish (_Malopterurus electricus_), is also a native of Africa,
occurring all over the tropical parts of that continent and also in the
Lower Nile, growing to a length of three feet. Its flesh is more esteemed
than that of other Silurids. It avoids light and is slow in its movements.
The electrical apparatus differs absolutely from that of all other Fishes,
being derived from the integument, belonging to the glandular system, and
surrounding the whole body with a thick coat of grease or gelatinous
substance; the apparatus is governed by a single nerve on each side
proceeding from a huge ganglionic cell at the anterior extremity of the
spinal cord.[674] The shocks given by _Madopterurus_ are very powerful, and
the Fish is called "Raad" by the Arabs, a name which means "thunder." Kept
in an aquarium with other Fishes, even of the same species, the "Raad" soon
kills its companions.
[Illustration: FIG. 357.—_Synodontis decorus_, from the Congo. ⅓ nat.
size.]
{592}[Illustration: FIG. 358.—_Callichthys littoralis_, from South America.
⅔ nat. size.]
In this family the eggs and young are usually looked after by the parents.
Aristotle observed that the male of the European _Silurus glanis_ watches
over and defends the eggs. In one of the commonest North American
Cat-Fishes, _Amiurus nebulosus_, a species which has been largely
introduced into some parts of Europe of late, now thriving in many ponds
and more or less polluted streams of the Continent, the eggs are deposited
near the banks of weedy ponds and rivers without currents, in concealed
places beneath logs, stumps, or even in pails or other receptacles, failing
which both parents join in excavating a sort of nest in the mud, a work
often requiring two or three days of incessant labour. The male watches
over the eggs, and later leads the young in great schools near the shore,
seemingly caring for them as the hen for her chickens.[675] The _Doras_ and
the _Callichthys_ of South America, according to Hancock[676] and
Vipan,[677] build regular nests of grass or leaves, sometimes placed in a
hole scooped out in the bank, in which they cover their eggs and defend
them, male and female sharing in this parental duty. In the likewise South
American _Corydoras_ (_Callichthys paleatus_), as observed by
Carbonnier,[678] a lengthy courtship takes place, followed by an embrace,
during which the female receives the seminal fluid in a sort of pouch
formed by the folded membranes of her ventral fins; immediately after, five
or six eggs are produced and received in the pouch, to be afterwards
carefully placed in a secluded spot. This operation is repeated many times,
until the total number of eggs, about 250, have been deposited. In
{593}accordance with these pairing habits, the pectoral spines of the male,
which are used in amplexation, are longer and stronger than those of the
female. These Fish are monogamous, and both parents remain by the side of
the nest, furiously attacking any assailant. Dr. R. Semon[679] has made
observations in Queensland on the habits of _Arius australis_, which builds
nests in the sandy bed of the Burnett River. These nests consist of
circular basin-like excavations, about 20 inches in diameter, at the bottom
of which the eggs are laid, and covered over by several layers of large
stones. A still more efficient protection is afforded their progeny by the
marine and estuarine species of _Arius_,[680] _Galeichthys_,[681] and
_Osteogeniosus_,[682] the male, more rarely the female, carrying the eggs
in the mouth and pharynx; these eggs, few in number, are remarkably large,
measuring as much as 17 or 18 millimetres in diameter in _Arius
commersonii_, a Fish of three or four feet in length. According to
Babuchin, _Malopterurus_ also is said by the Nile fishermen to shelter its
fry in the mouth.
Some of the Silurids attain to a very large size. Among these is the type
of the family, _Silurus glanis_, the "Wels" of the Germans, its only
European representative, which occurs over a great part of Europe, but is
absent from the British Isles, France, the Spanish Peninsula, and Italy. It
is most abundant in the Danube basin, where it sometimes reaches a length
of 10 feet or more and a weight of 400 lbs. It is the largest strictly
fresh-water Fish of Europe. Among the smallest species, we have to mention
the "Candiru" of Brazil, _Vandellia cirrhosa_, 60 millimetres in length and
3 or 4 in diameter, which is believed to enter and ascend the urethra of
people bathing, being attracted by the urine; the Fish, having once made
its way into the urethra, cannot be pulled out again, owing to the erectile
spines which arm its gill-covers. The natives of some parts of the Amazons
are in great dread of this Fish, and protect themselves when entering the
water by wearing a sheath formed of a small, minutely-perforated
cocoanut-shell suspended from a belt of palm-fibres.[683] According {594}to
Reinhardt[684] the allied _Stegophilus insidiosus_, a small colourless
Fish, 30 to 40 mm. long, from Brazil and Argentine, lives parasitically in
the gill-cavity of large Cat-Fishes (_Platystoma_). Dr. F. Silvestri has
noticed that it sucks the blood in the gills of _Platystoma coruscans_, a
Silurid growing to a length of 6 feet.
[Illustration: FIG. 359.—Upper view of heads of _Chaetostomus cirrhosus_,
male and female. (Nat. size.)]
FAM. 5. LORICARIIDAE.—Distinguished from the preceding by the sessile ribs
and the absence of the transverse processes in the praecaudal vertebrae,
which have bifid neural spines. The air-bladder is always much reduced, and
enclosed in a right and a left bony capsule formed by the skull and the
anterior vertebrae. Gill-openings narrow clefts. The mouth is inferior,
with more or less developed circular lips and feeble dentition; it is used
as a sucker, by which the Fish fixes itself to any hard object with such
strength that it cannot be pulled off without great difficulty. The teeth
are usually slender and bicuspid. The food consists of very small prey and
more or less putrefied organic substances, the intestine being usually
extremely elongate and much convoluted. The habits of these Fish are very
little known, but the fact that the males of many species have the pectoral
fins much stronger than the females renders it probable that they pair like
{595}_Callichthys_. There are other sexual differences in many species of
_Plecostomus_, _Chaetostomus_, and _Loricaria_, as the presence of dermal
tentacles on the snout (see Fig. 359), or of hair-like bristles on various
parts of the head and fins in the males, which are usually of larger size.
About 200 species are known, all from the tropical and subtropical parts of
Central and South America. The largest species (_Chaetostomus gigas_)
measures 2½ feet; many are of very small size.[685] The genera may be
referred to two sub-families:—
(i.) ARGINAE.—Body naked; ribs strong. _Arges_, _Stygogenes_,
_Astroblepus_.
(ii.) LORICARIINAE.—Body cuirassed by bony plates; ribs very slender.
_Plecostomus_, _Liposarcus_, _Chaetostomus_, _Cochliodon_,
_Pterygoplichthys_, _Rhinelepis_, _Acanthicus_, _Otocinclus_,
_Hypoptopoma_, _Loricaria_, _Acestra_.
[Illustration: FIG. 360.—_Acestra gladius_, from the Jurua River, with
upper and lower views of head and trunk. (⅔ nat. size.)]
The "Prenadillas" of the Andes, _Arges_ and _Stygogenes_, were believed to
live in subterranean waters within the bowels of active volcanoes, and to
be ejected with streams of mud and water during eruptions, a story that has
been repeated by Humboldt. The fact is that they live in small torrents at
great altitudes (up to 10,700 feet), and are swept down during periods of
disturbance caused by the eruption of the volcano.[686] The members of the
sub-family Loricariinae vary much in the shape of the body, which may be
short and stout, or more or less slender, the extreme in the latter respect
being attained by the species of the genus _Acestra_.
{596}FAM. 6. ASPREDINIDAE.—This family is also closely related to the
Siluridae. The ribs are sessile as in the Loricariidae, but inserted very
low down on the centra, which higher up bear strong transverse processes.
The opercular bone is entirely absent. The gill-opening is reduced to a
foramen in front of the pectoral fin. The head is extremely depressed and
the mouth terminal; the tail is very slender; the body is naked. The
air-bladder is large and free, the intestinal canal short. Four genera from
South America: _Aspredo_, _Bunocephalus_, _Bunocephalichthys_,
_Dysichthys_. Species 18. _Aspredo_, of the Guianas, the largest form,
reaching to about a foot in length, is remarkable for the manner in which
the female carries her eggs. The skin of the lower parts assumes a spongy
condition about the breeding season, and the eggs, after being deposited,
become attached to the lower surface of the head, belly, and paired fins,
forming a single layer; each egg becomes connected with the skin of the
mother by a cup-shaped, pedunculate base, supplied with blood-vessels and
coated with a layer of epithelium, the formation of which is still
unexplained.[687]
{597}CHAPTER XXII
TELEOSTEI (_CONTINUED_): SYMBRANCHII—APODES—HAPLOMI—HETEROMI—CATOSTEOMI—
PERCESOCES—ANACANTHINI
SUB-ORDER 3. SYMBRANCHII
Eel-shaped Fishes without paired fins, with the pectoral arch free or
suspended from the skull, and with the anterior vertebrae distinct, without
Weberian ossicles. Gill-openings confluent into a single, ventral slit.
Air-bladder absent.
The structure of the skull conforms to that of typical Malacopterygians.
The praemaxillary and maxillary are well developed, the latter placed
behind the former, and forming but a very small part of the oral border;
the symplectic is present; the parietals form a long sagittal suture, and
separate the frontals from the supraoccipital. The vertebrae are very
numerous, the praecaudal bearing very strong parapophyses, to which short,
slender ribs are attached. The skin is naked (Symbranchidae) or covered
with minute scales (Amphipnoidae), and the vertical fins are rudimentary,
reduced to mere dermal folds.
Like the Apodes, which they resemble in general appearance, these Fishes
are no doubt derived from some low type with abdominal ventral fins, but
whether from the Malacopterygii or the Haplomi we have as yet no data from
which to conclude. Only two families are known.
FAM. 1. SYMBRANCHIDAE.—Post-temporal well developed, forked, attached to
the skull. Inhabitants of the fresh or brackish waters of South-Eastern
Asia, Tropical America, Australia, and Tasmania. Three genera are known:
_Symbranchus_, with two species from India and the Malay region, and one
from Central and South America; _Monopterus_, with a single species
{598}from China, Japan, and the Malay region; and _Chilobranchus_, with two
species from Australia and Tasmania. Although the South American
_Symbranchus_ has been observed to live in marshes which periodically dry
up, the Fish burying itself in the mud like a _Lepidosiren_, the branchiae
are fully developed on the four branchial arches. In _Monopterus_, of
similar habits, the branchial laminae are rudimentary, and on three arches
only. No accessory breathing organ is known to exist.
[Illustration: FIG. 361.—_Monopterus javanensis_. Lower view of head,
showing gill-opening (_go_): lower view of middle praecaudal vertebrae: and
side view of skull and pectoral arch. _ar_, Articular; _br_, branchiostegal
rays; _bra_, branchial arches; _cl_, clavicle; _d_, dentary; _eot_,
epiotic; _eth_, ethmoid; _f_, frontal; _hm_, hyomandibular; _iop_,
interoperculum; _m_, maxilla; _mpt_, metapterygoid; _n_, nasal; _op_,
operculum; _p_, parietal; _pm_, praemaxilla; _pop_, praeoperculum; _ppt_,
pterygopalatine; _ptte_, post-temporal; _q_, quadrate; _scl_,
supra-clavicle; _so_, supra-occipital; _sop_, suboperculum; _sq_,
squamosal; _sy_, symplectic.]
FAM. 2. AMPHIPNOIDAE.—Post-temporal absent, the shoulder-girdle free from
the skull. The Cuchia, _Amphipnous cuchia_, the sole representative of this
family, an inhabitant of the fresh and brackish waters of India and Burma,
growing to two feet in length, is remarkable for the presence of a
respiratory air-sac on each side of the neck behind the head, communicating
with the gill-cavity. Of the three branchial arches the second alone
possesses gill-filaments; the third supports, in their place, a thick and
semi-transparent tissue; the principal organs of respiration are two small
bladders, resembling the posterior portions of the lungs of snakes, which
the animal has the power {599}of filling with air immediately derived from
the atmosphere. Although covered over by the common integuments, these
bladders present externally, when inflated, two protuberances of a round
shape. Of the whole volume of blood contained in the branchial artery,
one-third passes through the gills and respiratory bladders, whilst the
other two-thirds are conveyed directly from the heart to the aorta without
being exposed to the action of the air.[688] This amphibious Fish, when in
the water, constantly rises to the surface for the purpose of respiration,
and it is often found lying in the grassy sides of ponds after the manner
of Snakes.
SUB-ORDER 4. APODES.
Air-bladder, if present, communicating with the digestive tract by a duct.
Praemaxillaries absent; the maxillaries, if present, separated on the
median line by the coalesced ethmoid and vomer. Pectoral arch, if present,
not connected with and remote from the skull; mesocoracoid arch absent.
Fins without spines, the ventrals absent. Anterior vertebrae distinct,
without Weberian ossicles.
The Apodes or Eels are elongate, serpentiform Fishes with naked skin, or
with minute scales imbedded in the skin, the opercular bones small and
completely hidden under the integument; narrow or minute gill-openings; the
vertical fins, if present, confluent behind or separated by the projecting
tip of the tail. The pterygo-palatine arch is often reduced or absent, and
there is no distinct symplectic; the supraoccipital bone is small,
separated from the frontals by the parietals, which meet on the middle
line. The vertebrae are very numerous (up to 225), and the praecaudals bear
strong parapophyses, to which short, slender ribs may be attached;
epineurals are sometimes present. The five families into which this
sub-order is divided show remarkable degrees of simplification of the
skull, through reduction or loss of either the maxillary or the
pterygo-palatine arches.
There has been much difference of opinion in the determination of the bones
of the upper jaw in these Fishes. Cuvier regarded the lateral bones of the
upper jaw as praemaxillaries, Owen and Richardson as palatines (at least in
Muraenas), whilst Peters {600}and most recent authors have identified them
throughout the order as maxillaries.[689] The conclusion I have come to
from the examination of numerous skulls belonging to various genera, is
that the praemaxillaries have disappeared in all, whilst the maxillaries
have persisted in the true Eels (Anguillidae) and disappeared in the
Muraenidae, their place being taken by the fused palato-ectopterygoids,
which may even join the mandibular suspensorium. The vestigial bone,
regarded by Jacoby as the pterygoid in _Muraena helena_, may be identified
as the meta-pterygoid, and therefore does not disprove the homology, here
suggested, of the other elements of the palate.
FAM. 1. ANGUILLIDAE.—Maxillaries present, separated on the median line by
the ethmo-vomer; palato-pterygoid bone present, connected with the
hyomandibular and quadrate; gill-clefts separate, opening into the pharynx
by wide slits; tongue present; vent far removed from the head.
[Illustration: FIG. 362.—Skull and pectoral arch of _Conger vulgaris_, side
view. _Ar_, Articular; _br_, branchiostegal rays; _ch_, ceratohyal; _cl_,
clavicle; _cor_, coracoid; _d_, dentary: _eot_, epiotic; _eth_, ethmoid;
_f_, frontal; _hm_, hyomandibular; _iop_, interoperculum; _m_, maxilla;
_n_, nasal; _op_, operculum; _p_, parietal; _pop_, praeoperculum; _por_,
praeorbital; _ppt_, pterygo-palatine; _ps_, parasphenoid; _ptf_,
post-frontal; _ptr_, pterygials; _q_, quadrate; _sc_, scapula; _scl_,
supra-clavicle; _so_, supra-occipital; _sop_, suboperculum; _sq_,
squamosal; _uh_, urohyal; _v_, vomer.]
Spread over all the seas of the temperate and tropical zones, often
descending to the greatest depths, a few entering fresh waters. Many are
known to undergo very striking metamorphoses, the pellucid, strongly
compressed larvae (_Leptocephalus_) having long been a puzzle to
naturalists.
{601}Nearly 150 recent species are known, of which some 50 are deep-sea
forms, occurring down to 2500 fathoms. Scanty fossil remains, referable to
recent genera or scarcely different from them, are known from the Eocene of
Europe. The Cretaceous genus _Urenchelys_, from England and the Lebanon, is
interesting as representing a more generalised type, the hindmost vertebrae
bearing a pair of expanded hypural bones, showing the diphycercal Eels to
have been derived from Fishes with a normal caudal fin.
The genera are numerous. The following are the principal:—_Anguilla_,
_Simenchelys_, _Ilyophis_, _Conger_, _Coloconger_, _Congromuraena_,
_Uroconger_, _Heteroconger_, _Muraenesox_, _Nettastoma_, _Nettophichthys_,
_Saurenchelys_, _Nettenchelys_, _Myrus_, _Myrophis_, _Derichthys_,
_Chilorhinus_, _Muraenichthys_, _Liuranus_, _Ophichthys_, _Moringua_.
In the first four genera, small, more or less lineal rudimentary scales are
embedded in the skin, arranged in small groups, which are placed obliquely
at right angles to one another, forming a curious pattern; but these scales
are so small that they escape the notice of the superficial observer, hence
Eels have been improperly included among the Fishes forbidden as food by
the Mosaic prescriptions. In the other genera, including the exclusively
marine Conger of our coasts, scales are really absent.
The Common Eel (_Anguilla vulgaris_) has a very wide distribution, being
found over the greater part of Europe, North Africa, Temperate Asia, and
perhaps also North America east of the Rocky Mountains, Mexico, and the
West Indies. Its record from Australia and New Zealand is probably due to
the imperfection of our knowledge of the specific characters. It is not
found in the Black Sea nor in the rivers flowing into it, owing, no doubt,
to the sulphurous nature of the bottom of the sea, to which, as we now
know, these Fish would have to resort for breeding.
The mode of propagation of the Eel long remained a mystery, from the fact
that individuals found in fresh water never show ripe genital glands. The
idea had been entertained of their being hermaphrodite, and internal
parasites had also given rise to the belief in their viviparous nature. The
genital glands of the female were first investigated by Rathke in 1838, but
it was not until 1874 that those of the male were discovered by Syrski, and
shortly after fully described by L. Jacoby, who, in his final contribution
to the subject, concluded that Eels need salt water for the development of
their organs of generation, and that this development takes place, not near
the coast, but further out in deep water.
{602}[Illustration: FIG. 363.—Larva of Common Eel, _Leptocephalus
brevirostris_ of Kaup. (After Kaup.)]
As a rule it is not until the fifth or sixth year that the Eels go to the
sea for the purpose of propagation, which takes place at great depths—at
least 200 fathoms. Males have been observed to precede the females. The
breeding season over, the Eels do not return to fresh waters, but are
believed to die soon after. The eggs were discovered by Raffaele in 1888 in
the Gulf of Naples, and shortly after Grassi and Calandruccio finally
settled the question of the breeding and development of the Fish from
observations made in the Mediterranean. Their conclusions are thus summed
up:—"The Common Eel matures in the depths of the sea, where it acquires
larger eyes than are ever observed in individuals which have not yet
migrated to deep water. The abysses of the sea are its spawning places; its
eggs float in the sea water. In developing from the egg, it undergoes a
metamorphosis, it passes through a larval form denominated _Leptocephalus
brevirostris_." What length of time the development requires is not yet
fully established, since the Leptocephali are rarely found at the surface,
most of the specimens studied by Grassi and Calandruccio having been
obtained from the stomach of the Sun-Fish (_Orthagoriscus mola_) in the
Straits of Messina; but it is believed that the young Eels or "elvers,"
which ascend our rivers in such prodigious numbers in spring and summer
("Eel-Fares") are already one year old. Some individuals apparently spend
their whole life in fresh waters, but they are barren.[690] A specimen was
kept in confinement in the family of the French naturalist Desmarest for
upwards of 40 years, growing to a length of 4½ feet, being already of large
size at the time of {603}its capture. Eels are extremely voracious, and
endowed with an extraordinary tenacity of life; they can live for many
hours out of the water, and are often met with at night creeping through
the grass of meadows from one pond or stream to another.
One of the most remarkable among the deep-sea Eels is the Snub-nosed Eel
(_Simenchelys parasiticus_), which has been found in great numbers off
Newfoundland and the Azores, at depths of 200 to 900 fathoms. The maxillary
and mandibular bones are very short and massive, provided with large obtuse
teeth; the head is short and bulldog-like in aspect, the mouth small and
bordered by a thick circular lip. Some specimens have been observed to
burrow in the muscles of living Halibut and other large Fishes, after the
manner of _Myxine_.
FAM. 2. NEMICHTHYIDAE.—Distinguished from the preceding by the position of
the vent, which is close to, or at no great distance from, the
gill-openings. The rays of the vertical fins are connected by thin membrane
instead of being imbedded in thick skin, as in most Eels; in some of the
genera the jaws are excessively prolonged, needle-like, sometimes recurved.
Deep-sea Eels of small size, represented in the Atlantic, Pacific, and
Indian Oceans by about 10 species, referred to 6 genera: _Dysomma_,
_Dysommatopsis_, _Nemichthys_, _Spinivomer_, _Serrivomer_, _Gavialiceps_.
FAM. 3. SYNAPHOBRANCHIDAE.—Maxillaries narrowly separated on the median
line, their extremity strongly attached by ligament to the mandible;
pterygo-palatine arch absent. Gill-openings externally confluent into a
single ventral slit. Deep-sea Fishes, resembling the true Eels in the
general form and in the presence of linear scales placed at right angles,
but differing in the absence of the pterygo-palatine arch, as in the
Saccopharyngidae. Eight species of _Synaphobranchus_ are known, from the
Atlantic, Pacific, and Indian Oceans, at depths of 200 to 2000 fathoms.
FAM. 4. SACCOPHARYNGIDAE.—Maxillaries narrowly separated on the median
line, extremely elongate; mouth enormous; pterygo-palatine arch absent;
hyomandibular arch slender and movably articulated to the cranium, the two
bones (hyomandibular and quadrate) of which it is composed being capable of
being swung in all directions; branchial arches far behind the skull; no
branchiostegal rays or pharyngeal bones.
Extraordinary-looking deep-sea Fishes allied to the Eels, of which they
appear to be a further degraded type, the muscles {604}being feebly
developed and the skeleton imperfectly calcified. The mouth, furnished with
rather long but feeble, or even minute teeth, and the pharynx and stomach
are capable of great distension, these Fish being able to get outside a
prey very much larger than themselves; the eyes are situated far forward on
the head; the tail is extremely slender and elongate. Four genera are
known, each with a single species, from the Atlantic: _Saccopharynx_,
_Eurypharynx_, _Macropharynx_, and _Gastrostomus_. The depths at which they
have been obtained vary between 389 and 1467 fathoms, but three out of the
four known specimens of _Saccopharynx_ were brought to the surface by
having swallowed a Fish too large for the capacity of the stomach. The
length of the largest specimen is about 6 feet, of which the tail
constitutes nearly three-fourths.
[Illustration: FIG. 364.—_Saccopharynx ampullaceus_, ⅓ nat. size. (After
Günther.)]
[Illustration: FIG. 365.—Skull of _Thyrsoidea meleagris_, side view, _ar_,
Articular; _d_, dentary; _eot_, epiotic; _eth_, ethmoid; _f_, frontal;
_hm_, hyomandibular; _iop_, interoperculum; _n_, nasal; _op_, operculum;
_os_, orbitosphenoid; _p_, parietal; _pop_, praeoperculum; _por_,
praeorbital; _ppt_, pterygo-palatine; _ps_, parasphenoid; _ptf_,
post-frontal; _q_, quadrate; _so_, supraoccipital; _sop_, suboperculum;
_sor_, suborbitals; _sq_, squamosal; _v_, vomer.]
FAM. 5. MURAENIDAE.—Maxillaries absent, replaced by the palato-pterygoid,
the mouth bordered by the latter and the {605}ethmo-vomer; palato-pterygoid
bone separated from hyomandibular arch; branchial openings into the pharynx
narrow slits; no tongue.
The body is naked, pectoral fins are usually absent, and the gill-cleft is
a small round opening. The opercular bones are much reduced in size, and
the pectoral arch may be totally absent.
Voracious marine Fishes, inhabiting tropical and subtropical waters, being
especially abundant about coral reefs. Some 120 species are known, many
reaching a very large size, and being also remarkable for their variegated
coloration. The genera are mostly founded on the dentition, which shows
much diversity; the following are the principal:—_Myroconger_,
_Enchelycore_, _Muraena_, _Thyrsoidea_, _Lycodontis_, _Pythonichthys_,
_Echidna_, _Channomuraena_. The Muraena of the ancient Romans, _Muraena
helena_, of the Mediterranean, Eastern Atlantic, and neighbouring parts of
the Indian Ocean, occurring exceptionally as far north as the English
coast, grows to 4 feet, and its flesh was more esteemed than that of any
other Fish, large numbers being reared in specially constructed reservoirs
near the sea, and fed on the corpses of slaves. _Channomuraena vittata_,
from the coast of Cuba, is known to attain a length of 8 feet, and
_Thyrsoidea macrura_, from the Indian Ocean and the Malay Archipelago, to
upwards of 10 feet.
SUB-ORDER 5. HAPLOMI.
Air-bladder, if present, communicating with the digestive tract by a duct.
Opercle well developed. Pectoral arch suspended from the skull; no
mesocoracoid arch. Fins usually without, rarely with a few spines; ventrals
abdominal, if present. Anterior vertebrae distinct, without Weberian
ossicles.
The absence of the mesocoracoid arch distinguishes the Haplomi from the
Malacopterygii, with which they are united by various authors. They lead to
the Percesoces through the Cyprinodontids, and to the Lower
Acanthopterygians, such as the Berycidae, through the Scopelids,
Stephanoberycids, and Percopsids, as is evidenced by the structure of the
mouth and the forward position, in some of the genera, of the ventral fins,
which, however, are never attached to the pectoral girdle. Most of the
forms which are here included inhabit either fresh water or the deep sea.
{606}SYNOPSIS OF THE FAMILIES.
I. Parietals separating the frontals from the supraoccipital; post-
temporal simple; praecaudal vertebrae with autogenous parapophyses.
Margin of the upper jaw formed by the praemaxillaries and the
maxillaries; basis cranii simple; no adipose dorsal fin
1. _Galaxiidae_.
Margin of the upper jaw formed by the praemaxillaries only; basis
cranii double; adipose dorsal fin present 2. _Haplochitonidae_.
II. Frontals in contact with the supraoccipital.
A. Praecaudal vertebrae without parapophyses.
1. Margin of the upper jaw formed by the praemaxillaries and the
maxillaries.
Body without or with minute scales, usually with rows of scutes;
adipose dorsal fin usually present 3. _Enchodontidae_.†
Body scaly; post-temporal forked; no adipose dorsal fin; ventrals
with 6 to 11 rays 4. _Esocidae_.
Body scaly; post-temporal incompletely ossified; pectoral fin
without pterygials; no adipose dorsal fin; ventrals with
3 rays only 5. _Dalliidae_.
2. Maxillaries excluded from the oral border.
a. Adipose dorsal fin usually present; ventral fin with 7 to
10 rays.
Post-temporal forked; dorsal fin formed of articulated rays
6. _Scopelidae_.
Post-temporal simple; dorsal fin very long, formed of slender,
non-articulated, simple or bifid rays 7. _Alepidosauridae_.
b. No adipose dorsal fin; head and mouth enormous, dentition
feeble; body naked; ventral fins, if present, with 5 rays
8. _Cetomimidae_.
B. Praecaudal vertebrae with well-developed parapophyses; maxillaries
excluded from the oral border.
1. Dorsal and anal fins without spines; scales cycloid, or with
erect spines; no adipose fin.
Mouth not protractile; ventral fins far forward, with 7 to
17 rays 9. _Chirothricidae_.†
Mouth not protractile; ventral fins remote from the pectorals,
with 9 rays 10. _Kneriidae_.
Mouth protractile; ventral fins, if present, with 5 to 7 rays
11. _Cyprinodontidae_.
Mouth scarcely protractile; ventral fins rudimentary or absent;
vent close to the gills 12. _Amblyopsidae_.
Mouth slightly protractile; ventral fins with 5 or 6 rays
13. _Stephanoberycidae_.
2. Dorsal and anal fins with true spines; scales ctenoid; an
adipose dorsal; ventral fins with 9 rays 14. _Percopsidae_.
† Fossil only.
{607}FAM. 1. GALAXIIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter behind the former, and
toothless. Parietals in contact with each other, and separating the
frontals from the supraoccipital; opercular bones all well developed. Basis
cranii simple. Ribs inserted on strong, autogenous parapophyses;
epipleurals and epineurals. Post-temporal simple, attached to the epiotic;
post-clavicle present. Body naked. Vertical fins far back; no adipose
dorsal fin. Pectoral fins inserted very low down. Ventrals, if present,
with seven rays. Air-bladder present. Ova falling into the cavity of the
abdomen before exclusion.
[Illustration: FIG. 366.—_Galaxias brevipinnis_, from New Zealand, ½
natural size.]
[Illustration: FIG. 367.—Distribution of the Galaxiidae.]
The genus _Galaxias_ has an interesting distribution, the species of which
it is made up occurring in the fresh waters of the southern hemisphere,
viz. 8 in New Zealand and neighbouring islands, 7 in New South Wales, 3 or
4 in South Australia, 1 in West Australia, 2 in Tasmania, 7 in South
America, from Chili southwards, and 1 at the Cape of Good Hope. One species
(_G. attenuatus_) is even believed to be identical in New Zealand,
Tasmania, South Australia, the Falkland Islands, and South America. This
conclusion is probably correct from the fact, which may account for the
distribution of the whole genus, {608}that it is not confined to fresh
waters, but occurs also in the sea. Specimens were observed by Mr. Rupert
Vallentin in the Falkland Islands, where the Fish is known to the
inhabitants as "Smelts," in shoals in the shallow water along the shore;
and, according to Mr. F. E. Clarke, the same species, in New Zealand,
periodically descends to the sea, where it spawns, from January to March,
and returns from March to May. A marine species has recently been
discovered at the Chatham Islands. In New Zealand, the _Galaxias_ were
called "Trout" by the settlers before the introduction of Salmonids, whilst
the fry of _G. attenuatus_ are eaten as "Whitebait." The largest species
reach the length of a foot. _Neochanna_, from New Zealand, differs from
Galaxias in the absence of ventral fins; it has been found in burrows,
which it excavates at a distance from water.
FAM. 2. HAPLOCHITONIDAE.—Small fresh-water Trout-like Fishes, agreeing in
most respects with the Galaxiidae, to which they are unquestionably closely
related, differing only in the greater extent of the praemaxillaries, which
exclude the maxillary from the oral border, in the double basis cranii (the
prootics uniting under the brain, leaving a canal between them and the
parasphenoid), in the shorter parapophyses, which, like the neural arches
of the praecaudal vertebrae, are autogenous, and in the presence of a small
adipose dorsal fin, opposed to the anal.
Two genera: _Haplochiton_, naked, with a single species from Chili, the
southern extremity of South America, and the Falkland Islands, and
_Prototroctes_, covered with small scales, of which one species inhabits
Queensland, another South Australia, and a third New Zealand. In
_Haplochiton_, the urogenital orifice of both sexes is produced into a
cylindrical tube, which lies concealed in a groove in front of the anal
fin.
FAM. 3. ENCHODONTIDAE.—Margin of the upper jaw formed by the
praemaxillaries and the maxillaries, the latter sometimes toothed like the
former. Frontals in contact with the supraoccipital; basis cranii simple.
Ribs sessile; praecaudal vertebrae without transverse processes. Rayed
dorsal fin never much extended; sometimes an adipose fin behind it. Scales
delicate or absent, but occasional longitudinal series of scutes occur, the
dorsal series, when present, being unpaired.
Cretaceous Fishes allied to, and apparently more generalised than, the
Esocidae and Scopelidae. Numerous remains from {609}Europe and North
America, referred to 7 genera: _Enchodus_, _Eurypholis_, _Palaeolycus_,
_Halec_, _Cimolichthys_, _Prionolepis_.
FAM. 4. ESOCIDAE.—Margin of the upper jaw formed by the praemaxillaries and
maxillaries, the latter behind the former, and toothless. Supraoccipital in
contact with the frontals, separating the small parietals; opercular bones
all well developed; basis cranii simple. No parapophyses, except to the
hindermost praecaudal vertebrae; epipleurals and epineurals. Post-temporal
forked, the upper branch attached to the epiotic, the lower to the
exoccipital; post-clavicle present. Vertical fins far back; no adipose
dorsal fin. Pectoral fins inserted very low down; ventrals with 6 to 11
rays. Air-bladder present.
As in the Haplochitonidae, the neural and haemal arches are bones distinct
from the centra, and although parapophyses are not developed, the ribs are
not inserted on the centra, but on distinct bases wedged into the
latter.[691] Teeth are present on the vomer, palatine, and pharyngeal
bones.
[Illustration: FIG. 368.—Skeleton of _Esox lucius_. (After Jordan and
Evermann.)]
A small family of carnivorous freshwater Fishes, including the Pike
(_Esox_), of predaceous habits, unsurpassed in greediness and voracity, and
the small and insignificant-looking _Umbra_, distinguished by the more
anterior position of the dorsal fin, the larger scales, and the moderately
large gape, with feeble villiform teeth. The range of the Esocidae is
restricted to the cold and temperate parts of the northern hemisphere.
Besides the well-known _Esox lucius_ of Europe, Northern Asia, and the
northern parts of North America, growing to a length of 4 feet, and the
Maskinongy (_E. nobilior_) of north-eastern North America, reaching twice
that length, the first genus comprises three smaller species from the
Eastern United States. Remains of _Esox_ have been found in {610}various
freshwater deposits in Europe as far back as the Oligocene. _E. lepidotus_,
of which very perfect specimens have been found in the Upper Miocene beds
of Oeningen in Baden, differs from the living species in its much larger
scales and in the greater approximation of the ventral and anal fins, two
characters in which it approaches _Umbra_. Only two species of the latter
are known: _U. crameri_ ("Hundsfisch"), from the stagnant waters of
Austria-Hungary, and _U. limi_ ("Mud-Fish"), living in swamps and ditches
in Canada and the north-eastern United States, often remaining imbedded in
the mud of prairie sloughs and bog-holes.
[Illustration: FIG. 369.—Distribution of the Esocidae.]
FAM. 5. DALLIIDAE.—The genus _Dallia_, with a single species inhabiting the
streams and ponds of Alaska and Siberia, is related to _Umbra_, but differs
in the very thin and papery skeleton, with the post-temporal imperfectly
ossified and the pectoral fin without pterygials or actinosts. The dorsal
fin is far back and opposite to the anal, as in the Pike. The ventral fins
are composed of three rays only, and the pectorals, which have a somewhat
fleshy base, have as many as 36. The scales are extremely small, and partly
imbedded in the skin. The Black-Fish, _D. pectoralis_, abounds in Sphagnum
ponds, feeding on plants and worms, and forming the chief food of the
natives of some parts of Northern Alaska, where, with the exception of the
Salmonids, it is the only freshwater Fish. Turner, its discoverer, says its
vitality is {611}extraordinary: Black-Fishes will remain frozen in baskets
for weeks, and when thawed are as lively as ever, one having been swallowed
in a congealed condition by a dog, thawed out by the heat of the stomach,
and vomited up alive.
[Illustration: FIG. 370.—_Dallia pectoralis_, ½ natural size. (After L. M.
Turner.)]
FAM. 6. SCOPELIDAE.—Praemaxillaries much elongate, and completely excluding
the maxillaries from the oral border. Supraoccipital in contact with the
frontals, sometimes partly covered by the parietals; opercular bones all
well developed. Basis cranii simple. Ribs sessile; no parapophyses on the
praecaudal vertebrae; epipleurals and epineurals. Post-temporal forked, the
upper branch in contact with the epiotic or the supraoccipital, the lower
with the opisthotic; post-clavicle present. An adipose dorsal fin often
occurs; luminous spots often present on head and body. Ventral fins with 7
to 10 rays. Air-bladder sometimes absent.
A large family (over 100 known living species), mostly of pelagic and
deep-sea Fishes. A great number of fossil types have been described.
Recent genera:
A. Without photophores: _Saurus_, _Saurida_, _Bathysaurus_, _Harpodon_,
_Scopelarchus_, _Aulopus_, _Odontostomus_, _Omosudis_, _Sudis_,
_Paralepis_, _Bathypterois_, _Benthosaurus_, _Chlorophthalmus_, _Ipnops_.
B. With photophores: _Scopelus_, _Dasyscopelus_, _Neoscopelus_,
_Scopelengys_, _Nannobrachium_, _Scopelosaurus_.
Fossil genera:
A. Cretaceous: _Sardinioides_, _Acrognathus_, _Leptosomus_, _Sardinius_,
_Dactylopogon_, _Nematonotus_, _Microcoelia_, _Opisthopteryx_, _Apateodus_,
_Rhinellus_. B. Eocene, Oligocene, and Miocene: _Omiodon_, _Scopeloides_,
_Parascopelus_, _Anapterus_.
{612}[Illustration: FIG. 371.—A, _Scopelus crocodilus_ (after Goode and
Bean). B, _Bathypterois dubius_ (after Collett). C, _Ipnops murrayi_, with
dorsal view of head (after Goode and Bean).]
{613}The members of this Family vary much in form, and among them are to be
found some of the most curious adaptations to bathybial existence. One of
the best known is _Harpodon nehereus_, which, when newly taken, is
brilliantly phosphorescent all over the body; in a salted and dry condition
it is the "Bombay-duck," a delicacy eaten with curries, and exported in
large quantities from the west coast of India. It is not known to occur at
any great depth, and is not even restricted to the sea, being very abundant
in the rivers and estuaries of Bengal and Burma; whilst an allied species,
_H. squamosus_, is found in the Indian Ocean at depths of 120 to 300
fathoms. In _Bathypterois_, the eyes are very small; some of the rays of
the paired fins being excessively prolonged, acting as tactile organs, and
compensating the reduction in the eyes. Sir John Murray has observed about
_B. longipes_: "When taken from the trawl [from 2650 fathoms] they were
always dead, and the long pectoral rays were erected like an arch over the
head, requiring considerable pressure to make them lie along the side of
the body; when erected they resembled Pennatulids like _Umbellula_." In
_Ipnops_, which resembles in general form the large-eyed _Chlorophthalmus
gracilis_, the upper surface of the broad spatulate snout is occupied by a
luminous organ longitudinally divided into two symmetrical halves, and the
eyes are absent, unless, as first supposed, this extraordinary organ be a
modification of them; but Professor Moseley's examination seems to have
proved beyond doubt that it is a special form of phosphorescent organ, the
object of which would be to attract other creatures to the wide gape of a
Fish which, living in the abysses of the sea and deprived of organs of
sight and touch, would have great difficulty in procuring its food.
_Odontostomus_, with a very large eye which can be turned upwards and
sidewards, and enormous compressed curved teeth, barbed at the tip and
depressible backwards, is one of the few Scopelids in which scales are
completely absent.
The numerous species (about 50) of _Scopelus_ and their allies are
moderate-sized or small pelagic and deep-sea forms found in nearly all the
seas, some coming to the surface at night, whilst others are confined to
great depths; they are remarkable for the series of phosphorescent spots
(photophores) on the body, and in some species also on the head, where they
may form large patches on the snout. The arrangement of these photophores
is a very {614}definite one, and it has been used for the division of these
Fishes into genera or sub-genera.[692] The ventral fins have a more forward
position than in most other members of the Family.
FAM. 7. ALEPIDOSAURIDAE.—Characters as in the preceding, but supratemporal
simple, attached to the opisthotic, and dorsal fin very long, formed of
slender, non-articulated, simple or bifid rays, extending along nearly the
whole length of the back, followed by a small adipose fin. The air-bladder
is absent and the body scaleless. The skeleton is feebly ossified; the
dentition is very powerful, some of the teeth on the palate and mandible
being very strongly enlarged. 4 or 5 species are known, from considerable
depths in the Atlantic and Pacific Oceans, referable to one genus,
_Alepidosaurus_ or _Plagyodus_. _A. ferox_, from the Atlantic, reaches a
length of 4 feet.
[Illustration: FIG. 372.—_Alepidosaurus ferox_, ⅑ nat. size. (After Goode
and Bean.)]
FAM. 8. CETOMIMIDAE.—The affinities of the recently discovered genera
_Rondeletia_ and _Cetomimus_, deep-sea Fishes from the North Atlantic, at
depths of 1000 to 1600 fathoms, are still uncertain, as the skeleton could
not be examined; they are probably most nearly related to the Scopelidae.
The head is enormous, with very wide gape, that of _Cetomimus_ being
suggestive of that of a Right Whale; the teeth are small and coarsely
granular; the gill-openings are very wide; the body is more or less
compressed and scaleless; the dorsal and anal fins are opposed to each
other; no adipose dorsal fin. In _Rondeletia_, the eyes are moderately
large, and ventral fins, with 5 rays, are present; in _Cetomimus_, the eyes
are very small, and ventral fins are absent.
{615}FAM. 9. CHIROTHRICIDAE.—Praemaxillaries delicate and styliform,
completely excluding the maxillaries from the upper border of the mouth;
jaws with feeble dentition or toothless; opercular apparatus complete.
Praecaudal vertebrae with robust parapophyses, to which ribs are attached.
Ventral fins far forwards.
These Fishes, of which three fossil genera are known from the Cretaceous of
Germany and Syria, appear to be related to the Scopelidae, from which the
strong parapophyses distinguish them. _Chirothrix_ is remarkable for its
excessively enlarged ventral fins with about 17 rays; these fins were taken
for the pectorals by the early describers. In _Telepholis_ and
_Exocoetoides_, the ventral fins are smaller than the pectorals, and formed
of 7 or 8 rays only; the dorsal region, in the former, is protected by a
covering of small, thin, rounded or polygonal dermal scutes, each bearing a
median tubercle.
[Illustration: FIG. 373.—_Chirothrix libanicus_, restored by A. S.
Woodward.]
FAM. 10. KNERIIDAE.—Margin of the upper jaw formed by the praemaxillaries;
mouth toothless, not protractile. Parietals separated by the
supraoccipital. Pharyngeal bones toothless. Praecaudal vertebrae with
parapophyses. Body covered with small scales. Ventrals with 9 rays. No
adipose dorsal fin. Air-bladder present.
{616}The genus _Kneria_ comprises two species from the fresh waters of
tropical Africa, one from Angola, the other from East Africa.[693] Small
Loach-like Fishes, two to four inches long, with the upper jaw projecting
beyond the mouth, which is inferior and transverse; no barbels;
gill-membranes entirely grown to the isthmus, the gill-opening being a
rather narrow vertical slit; dorsal and anal fins short, the former
opposite, or nearly opposite, to the ventrals; the snout of the male(?) of
_K. angolensis_ is described as beset with small spine-like excrescences;
the intestinal tract makes several convolutions.
FAM. 11. CYPRINODONTIDAE.—Mouth protractile, the maxillaries excluded from
the oral border; teeth in the jaws and on the pharyngeal bones;
pterygo-palatine arch weak or rudimentary; opercular bones all well
developed. Basis cranii simple. Praecaudal vertebrae with strong
parapophyses, bearing the ribs; epipleurals inserted on the ribs.
Post-temporal forked. Ventrals, if present, with 5 to 7 rays. No adipose
dorsal fin. Air-bladder sometimes absent.
From a physiological point of view, this Family may be divided into
carnivorous forms, with short digestive tract, and phytophagous or
limnophagous ones, in which the intestine forms numerous coils. To the
first division belong the living genera _Cyprinodon_, _Characodon_,
_Tellia_, _Haplochilus_, _Fundulus_, _Rivulus_, _Cynolebias_, _Orestias_,
_Empetrichthys_, _Jenynsia_, _Pseudoxiphophorus_, _Belonesox_, _Gambusia_,
_Anableps_, among existing forms, and the fossil genera _Prolebias_
(Oligocene and Miocene) and _Pachylebias_ (Miocene); to the second, the
living genera _Poecilia_, _Mollienesia_, _Platypoecilus_, and _Girardinus_.
_Procatopus_, a near ally of _Haplochilus_, recently discovered in South
Cameroon, is remarkable for having the ventral fins inserted far forward,
below the pectoral fins.
These are small or very small Fishes,[694] only a few reaching a length of
about a foot, confined to fresh or brackish waters, recognisable externally
by the flat head with protractile mouth, the usually large scales, and the
absence of a well-developed lateral {617}line. The teeth vary much in
shape: cardiform, villiform, or compressed, and bi- or tri-cuspid; the
palate is either toothless, or teeth are present on the vomer. About 200
species are known, mostly from the American continent, only about 30 being
known from other parts of the world, viz. Southern Europe, Southern Asia
and Japan, and Africa. In many species the sexes are dissimilar, the female
being larger and less brilliantly coloured, with smaller fins; the anal fin
of the male may be modified into an intromittent organ by means of which
internal fertilisation takes place, the ova developing in a sort of uterus,
which the young leave in a more or less advanced stage of growth. The most
curious of the Cyprinodontids is the genus _Anableps_, of Central and South
America, surface-swimming Fishes, the strongly projecting eyes of which are
divided by a horizontal band of the conjunctiva into an upper part adapted
for vision in the air, and a lower for vision in the water, and the pupil
is divided into two parts by a constriction; the larger species grows to
the length of a foot.
[Illustration: FIG. 374.—_Anableps tetrophthalmus_, male, ½ nat. size.]
[Illustration: FIG. 375.—Distribution of the Cyprinodontidae.]
{618}FAM. 12. AMBLYOPSIDAE.—Mouth scarcely protractile, the maxillaries
excluded from the oral border; teeth small, in jaws and palate, and on the
pharyngeal bones. Praecaudal vertebrae with very strong parapophyses,
bearing the ribs on their upper surface; epipleurals inserted on the ribs.
Ventral fins rudimentary or absent. Vent jugular, close to the gill-clefts.
Air-bladder present.
[Illustration: FIG. 376.—A, _Chologaster cornutus_, and B, _Amblyopsis
spelaea_, nat. size. (After Jordan.)]
Small ovoviviparous Fishes, closely related to, and evidently derived from,
the Cyprinodontids, measuring from 1 to 5 inches, inhabiting ditches and
small streams, or confined to subterranean waters of limestone caves, in
the United States east of the Rocky Mountains. Six species, referable to
three genera, are known. In _Chologaster_, the eyes are well developed and
the body is coloured. _C. cornutus_ inhabits the lowland streams and swamps
of the South Atlantic States, from Virginia to Florida; _C. agassizii_ is
found in the underground streams of Kentucky and Tennessee; and _C.
papilliferus_ occurs under stones in the springs of south-western Illinois.
_Amblyopsis_ and _Typhlichthys_, which are evidently derived from the
former, or from forms closely related to it, have the eyes rudimentary and
more or less concealed under the skin, and the body is colourless.
_Amblyopsis spelaea_ is widely distributed in the caves east of the
Mississippi, {619}both north and south of the Ohio River; it is common in
the River Styx of the Mammoth Cave. _Typhlichthys subterraneus_ is found
with the latter species in the caves east of the Mississippi, but is
confined to the south side of the Ohio River, whilst _T_. (_Troglichthys_)
_rosae_ is found in the caves west of the Mississippi River. Of _Amblyopsis
spelaea_, the late Professor Cope has observed: "If these Amblyopses be not
alarmed, they come to the surface to feed, and swim in full sight, like
white aquatic ghosts. They are then easily taken by the hand or net, if
perfect silence is preserved, for they are unconscious of the presence of
an enemy except through the medium of hearing; this sense, however, is
evidently very acute, for at any noise they turn suddenly downwards, and
hide beneath stones, etc., on the bottom." Dr. Garman thinks, on the
contrary, that such a sense can hardly be developed in recesses where we
are accustomed to think any sounds other than those made by the rippling or
dripping water are almost unknown, and that it is through the sense of
touch, and not through hearing, that the Fish is disturbed. In fact, the
head is provided with a great number of tactile papillae, arranged in
transverse ridges, provided with nervous filaments, which evidently
compensate the loss of the visual organ.[695]
FAM. 13. STEPHANOBERYCIDAE.—This Family has hitherto been placed near the
Berycidae, among the Acanthopterygii, but there are no spinous rays in the
dorsal and anal fins; and the ventrals, formed of one simple and four or
five branched rays, are abdominal. The genus _Stephanoberyx_, with two
species from the Atlantic, at depths of 535 to 2949 fathoms, is
characterised by a large, thick, cavernous head, with thin bony
spine-bearing ridges, a large mouth bordered by the protractile
praemaxillaries, behind which are the large maxillaries, a short dorsal and
a short anal, opposed to each other behind the ventrals, and the body
covered with feebly imbricated scales, each bearing in the centre one or
several erect spines. The largest specimen measures 6 inches.
_Malacosarcus_, a small Fish from the Pacific, at depths of 2350 and 2425
fathoms, is very closely allied to _Stephanoberyx_, but its scales are very
thin and cycloid. The striking resemblance which the head {620}bears to
that of the Berycid _Melamphaes_ may be merely a case of convergence, and
it must be borne in mind that this appearance is approached by some species
of _Scopelus_, with which both _Malacosarcus_ and _Melamphaes_ were
originally confounded. The praecaudal vertebrae are provided with
parapophyses. I have ascertained on a specimen of _Stephanoberyx monae_
that the air-bladder is connected with the dorsal side of the stomach by a
short and comparatively wide duct.
[Illustration: FIG. 377.—_Stephanoberyx gillii_, nat. size. (After Goode
and Bean.)]
FAM. 14. PERCOPSIDAE.—Margin of the upper jaw formed by the
praemaxillaries; mouth small, not protractile, toothed; palate toothless.
Supraoccipital in contact with the frontals, separating the small
parietals. Basis cranii simple. Most of the praecaudal vertebrae with
parapophyses, on the upper surface of which the ribs are inserted; no
epipleurals. Post-temporal forked; post-clavicle present; scapular foramen
in the scapula, on which three hour-glass-shaped pterygials are inserted, a
fourth being inserted on the coracoid. Dorsal fin with two true spines;
anal with one or two; ventrals far forward, with 9 rays; pectorals inserted
rather high. A small adipose dorsal fin. Body covered with strongly ctenoid
scales. Air-bladder present (with open duct).
This is a most interesting group of Fishes, from the resemblance which they
bear to the Perches, and they have therefore been raised to the rank of a
sub-order, Salmopercae, by Jordan and Evermann, who regard them as "archaic
fishes, relics of some earlier fauna, and apparently derived directly from
the extinct transitional forms through which the Haplomi and Acanthopteri
have descended from allies of the Isospondyli [Malacopterygii]." On the
other hand, an analysis of their characters shows them to belong to the
Haplomi, of which they {621}may be regarded as highly specialised members,
having evolved in the direction of the Acanthopterygii.
Only two genera are known, each with a single species: _Percopsis_, from
the rivers and streams of Canada and the north-eastern United States, and
_Columbia_, more recently discovered in the sandy or weedy lagoons along
the Columbia River. These Fishes are of small size, not exceeding 6 inches
in length. Their eggs are unusually large.
[Illustration: FIG. 378.—_Columbia transmontana_, natural size. (After
Eigenmann.)]
SUB-ORDER 6. HETEROMI.
Air-bladder without open duct. Opercle well developed; parietal bones
separating the frontals from the supraoccipital. Pectoral arch suspended
from the supraoccipital or the epiotic, the post-temporal small and simple
or replaced by a ligament; no mesocoracoid. Ventral fins abdominal, if
present.
The Halosauridae and Notacanthidae are deep-sea Fishes of obscure
affinities. In the abdominal position of the many-rayed ventral fins and in
the absence of the mesocoracoid arch they agree with the Haplomi; but if,
as the investigations of Günther lead us to believe,[696] there is really
no open communication between the air-bladder and the digestive tract, they
{622}should be removed from this physostomous sub-order. The two families
have many characters in common, such as the attachment and structure of the
pectoral arch, which is devoid of a post-clavicle, the position of the
pectoral fins high up the sides, the strong parapophyses inserted very low
down on the centra of the vertebrae, the extent of the parietal bones,
which meet in a sagittal suture and separate the frontals from the
supraoccipital. The recent discovery of a third family, the Lipogenyidae,
which, in the structure of the dorsal fin, is exactly intermediate between
the two others, has lessened the gap between the Lyomeri (Halosauridae) and
Heteromi (Notacanthidae) of Gill, which I have proposed to unite in a
suborder under the latter name.
These Fishes are no doubt derived from forms in which a separate caudal fin
existed; such a type must have been near the Dercetidae, as defined by A.
S. Woodward, which may provisionally be placed here.
An imperfectly known Fish from the Chalk of Mount Lebanon, _Pronotacanthus
sahelalmae_, appears to bear some affinity to _Notacanthus_, and has been
placed in the same family; but its characters are not sufficiently defined
to refer it without doubt to this division.
There is a fifth family which may enter this sub-order: the Fierasferidae,
the structure of which has been exquisitely described and figured by
Emery.[697] Hitherto placed with or near the Ophidiidae, they differ widely
from them, as well as from all Acanthopterygians, in the conformation of
the skull, the supraoccipital being separated from the frontals by the
parietals, which form a long median suture. This is a feature which has
only been observed in Fishes with abdominal ventral fins, and although the
total absence of those fins in _Fierasfer_ deprive us of an important
criterion in deciding on its affinities, I am inclined to regard this
family as derived from an "abdominal" type. The conformation of the
pectoral arch has much in common with that of the Halosaurs, and,
notwithstanding the interpretation that has been given to the bones at the
back of {623}the cranium in the latter type, the same may be said, in a
general way, of the skull.
As pointed out by Emery, the very anterior position of the vent in the
Fierasferidae is directly related to the curious mode of life of these
Fishes, and the analogous condition obtained in various families, such as
the Gymnotidae, Nemichthyidae, Amblyopsidae, shows it to be a character of
relatively small systematic importance.
SYNOPSIS OF THE FAMILIES.
A. Vent posterior.
_a_. A distinct caudal fin; ordinary scales small or wanting, but
enlarged scutes along the side 1. _Dercetidae_.†
_b_. Tail tapering to a point; scales cycloid.
No spines; dorsal fin short, anal very long 2. _Halosauridae_.
Fins with spines, dorsal short, anal long 3. _Lipogenyidae_.
Dorsal fin formed of a series of spines, anal long, formed partly
of spines and partly of soft rays 4. _Notacanthidae_.
B. Vent immediately behind the gill-opening; no caudal fin; scales
absent 5. _Fierasferidae_.
FAM. 1. DERCETIDAE.—Body much elongate; ordinary scales small or wanting,
but two or more continuous series of enlarged scutes along each side; mouth
large, praemaxillaries apparently forming the greater part of the upper
border of the mouth, which is toothed; opercular apparatus complete. Dorsal
fin more or less extended, without spines; anal short, caudal separate;
ventrals with not less than 7 or 8 rays.
_Dercetis_, _Leptotrachelus_, _Leptecodon_, _Pelargorhynchus_, and
_Stratodus_, from the Upper Cretaceous of Europe, Syria, and North America.
FAM. 2. HALOSAURIDAE.—Body elongate, covered with cycloid scales, the tail
tapering to a point, without caudal fin; head with scales; mouth moderate,
bordered by the praemaxillaries and the maxillaries, both toothed;
suborbitals large; praeopercle rudimentary. Dorsal fin short, formed of
soft rays, above or a little behind the ventrals, which are rather far
back, and formed of 9 or 10 rays; anal very long, without spines, extending
to the end of the tail. Ovaries transversely laminated, the ova falling
into the abdominal cavity. Some 10 living species are known, referred to
three genera, inhabiting the Atlantic, Pacific, and Indian Oceans, at
depths of 500 to 1400 fathoms.
{624}In _Halosaurus_ the scales of the lateral line, which runs near the
lower profile, are scarcely enlarged, and are destitute of luminous organs.
_Halosaurichthys_ differs in the union of the ventral fins with each other,
as in _Notacanthus_. In _Halosauropsis_ the scales of the lateral line are
strongly enlarged and pouch-like, and bear photophores.
This family is one of great antiquity, being represented in the Upper
Cretaceous of Westphalia by _Echidnocephalus_, which, as shown by A. S.
Woodward, appears to have been closely related to _Halosaurus_.
[Illustration: FIG. 379.—_Halosauropsis macrochir_, ⅓ nat. size. (After
Günther.)]
FAM. 3. LIPOGENYIDAE.—Similar to the preceding in shape and in the position
of the dorsal fin, but with a toothless, roundish, inferior, suctorial
mouth, and with the short dorsal and the long anal formed partly of spines
and partly of soft rays. Head and body covered with minute scales; lateral
line nearer the dorsal than the ventral profile. Ventrals with 3 spines and
7 soft rays. A single species, _Lipogenys gillii_, from the North Atlantic,
865 fathoms.
[Illustration: FIG. 380.—_Lipogenys gillii_. (After Goode and Bean.)]
FAM. 4. NOTACANTHIDAE.—Body elongate, covered with very small cycloid
scales, the tail tapering to a point, without caudal fin; head scaly; mouth
small, inferior, bordered by the praemaxillaries only; jaws toothed; no
suborbitals; praeoperculum small; post-temporal replaced by ligament.
Dorsal fin formed of a series of short disconnected spines; anal very long,
formed partly of spines and partly of soft rays, extending to the end of
the tail. Ventrals with 1 to 5 spines and 7 to 10 soft rays.
{625}Two genera: _Notacanthus_, with the ventrals connate or confluent and
with 6 to 12 dorsal spines; and _Polyacanthonotus_, with the ventrals
separated and 27 to 38 dorsal spines. Nine species, from the Mediterranean,
the Atlantic, and the Pacific, at depths of 400 to 1875 fathoms.
[Illustration: FIG. 381.—_Notacanthus bonapartii_, ½ nat. size. (After
Vaillant.)]
FAM. 5. FIERASFERIDAE.—Body elongate or extremely attenuate, naked, the
tail tapering to a point or truncate, without distinct caudal fin; mouth
small, inferior, bordered by the praemaxillaries; jaws toothed; no
suborbitals; praeoperculum well developed. Dorsal and anal fins very long,
extending to the end of the tail, and formed entirely of soft rays. Ventral
fins absent. Vent situated immediately behind the gill-opening. Air-bladder
with a muscular apparatus for dilatation of its anterior part.
A single genus, _Fierasfer_, with about 10 species, distributed over nearly
all the warm and tropical seas, rarely found as far north as the west coast
of Ireland. _Encheliophis_, without pectoral fins, is the larval form of
_Fierasfer_.
_Fierasfer_ spends the greater part of its existence in the interior of
Holothurians and other Echinoderms as well as in bivalve Mollusca. It has
been observed to enter Holothurians by the posterior or anal aperture,
either head first or tail foremost, in the latter case availing itself of
the suction which takes place alternately with the expulsion of water by
that orifice; it remains near the anus, from which it projects its head in
search of food outside its host. It is neither a true parasite nor a
commensal or mutualist, in the sense given to these terms by Van Beneden,
but simply a lodger, "inquilino," as Emery puts it. Semper, however,
regards _Encheliophis vermicularis_ as a true parasite, feeding on the
viscera of the Holothurian in which it lives. Putnam has examined eight
specimens of a _Fierasfer_ from the Bay of Panama, which were obtained
alive from pearl oysters, and also one beautifully {626}enclosed in a
pearly covering deposited upon it by the oyster; a similar specimen is
preserved in the British Museum.
[Illustration: FIG. 382.—_Fierasfer acus_, penetrating into Holothurians, ⅖
nat. size. (After Emery.)]
SUB-ORDER 7. CATOSTEOMI.
Air-bladder, if present, without open duct. Parietal bones, if present,
separated by the supraoccipital. Pectoral arch suspended from the skull; no
mesocoracoid arch; coracoid usually very large. Ventral fins, if present,
abdominal, or pelvis attached to the coracoid bones.
The mouth is small and bordered by the praemaxillaries or by the
praemaxillaries and a small portion of the maxillaries. The air-bladder is
present, except in the Solenostomidae and Pegasidae.
Following the suggestions of Kner and Steindachner and Cope to their
logical conclusion, A. S. Woodward, in his valuable catalogue of the Fossil
Fishes in the British Museum, has united the Lophobranchs of Cuvier with
the Hemibranchs of Cope, a course which seems fully justified, and has
received {627}further support from the recent investigations of
Swinnerton,[698] who has proposed to unite the two groups under the new
name of Thoracostei. The name Phthinobranchii has also been suggested by O.
P. Hay for the same association. The structure of the Lophobranchs
(Solenostomidae and Syngnathidae) shows that these fishes are only
extremely specialised forms of the group of which the Sticklebacks are the
well-known type, and the character of the "tufted" gills alone is surely
not of sufficiently great importance to warrant the retention of the
Lophobranchii as a division equivalent to the sub-orders adopted in the
present classification. Besides, as recently pointed out by A. Huot,[699]
there is no fundamental difference, but only one of degree, between the
so-called tufted gill and the normal type; each "tuft" corresponds to one
branchial lamella, and at a certain stage of development the disposition of
the branchial lamella is the same in a _Syngnathus_ and in an ordinary
Teleostean. I have recently attempted to show[700] that the Lamprididae are
related to the Hemibranchii, although sufficiently distinct to warrant the
establishment of a division, named Selenichthyes.[701]
SYNOPSIS OF THE FAMILIES.
I. Praeoperculum and symplectic distinct; branchial apparatus fully
developed; gills pectinated; mouth terminal, toothless; post-temporal
forked, free; pelvic bones connected with scapular arch; ventral fins
with 15 to 17 rays; ribs long, sessile; fins without spines
(SELENICHTHYES) 1. _Lamprididae_.
II. Praeoperculum and symplectic distinct, latter much elongate;
branchial apparatus more or less reduced; gills pectinate;
post-temporal simple, immovable; mouth terminal (HEMIBRANCHII).
A. Mouth toothed.
1. Pelvic bones usually connected with scapular arch; spinous dorsal
represented by isolated spines.
Snout conical or but slightly tubiform; ventral fins with
1 spine and 1 or 2 soft rays; ribs slender, free; anterior
vertebrae not enlarged 2. _Gastrosteidae_.
Snout tubiform; ventral fins with 1 spine and 4 soft rays; {628}
ribs flattened, fused with the lateral bony shields;
anterior vertebrae not enlarged 3. _Aulorhynchidae_.
Snout tubiform; ribs slender, free; first vertebra enlarged
4. _Protosyngnathidae_.
2. Pelvic bones not connected with scapular arch; ventrals without
spine, with 5 or 6 rays; snout tubiform; first vertebra very
elongate, formed by the fusion of several.
Isolated dorsal spines; body scaly 5. _Aulostomatidae_.
No dorsal spines; body naked 6. _Fistulariidae_.
B. Mouth toothless; snout tubiform; two short dorsal fins, the first
with a few spines; ventral fins with 3 to 5 rays; anterior
vertebrae elongate.
Body covered with bony shields and small rough scales
7. _Centriscidae_.
Body completely cuirassed by bony shields which are fused with the
endoskeleton 8. _Amphisilidae_.
III. Praeoperculum absent; symplectic much elongate; branchial
apparatus more or less reduced; gill-lamellae reduced in number and
enlarged, forming rounded lobes; post-temporal simple, immovably
attached to the skull; mouth toothless, at the end of a tubiform
snout; body covered with bony plates (LOPHOBRANCHII).
Two dorsal fins; ventral fins present, with 7 rays; gill-openings
wide; exoskeleton of large star-like plates 9. _Solenostomidae_.
A single dorsal fin; no ventral fins; gill-openings very small;
exoskeleton in the form of rings 10. _Syngnathidae_.
IV. Praeoperculum and symplectic absent; gills pectinated; mouth
inferior, toothless; body entirely covered with bony plates; ventral
fin with 2 or 3 rays (HYPOSTOMIDES) 11. _Pegasidae_.
FAM. 1. LAMPRIDIDAE.—Body short and deep, with minute scales. Snout short;
mouth toothless, bordered by the praemaxillaries and, to a small extent, by
the maxillaries; opercular bones well developed. Gills four, pectinated;
branchial apparatus fully developed. Post-temporal bone forked. Vertebrae
very numerous (21 + 25), without transverse processes; ribs strong, long.
Fins without spines; dorsal and anal elongate. Pectoral fins with very
short pterygials folding downwards against the body. Pelvic bones connected
with the coracoids, which are very large, and do not form a suture at their
ventral extremity. Ventral fins with 15 to 17 rays.
The Opah or King-Fish (_Lampris luna_), the sole representative of this
family, is remarkable for its large size (growing to a length of four feet)
and its vivid colours. Its flesh is rich, and intermediate between that of
the Salmon and that of the Tunny. It is a pelagic fish of wide
distribution, known from the North {629}Atlantic and Mediterranean and from
distant points in the Pacific; specimens are occasionally captured on our
coasts. It feeds on other fish, but little is known of its habits and
nothing of its development.
The affinities of the _Lamprididae_ are very doubtful. _Lampris_ has
usually been placed with the Acanthopterygians, a view which is still
upheld by Gill.[702] I now agree with this high authority in regarding the
bone which I took for an infraclavicle as a much developed coracoid, and
the bone termed by me the coracoid as a pterygial. But it has also been
shown, by Starks, that such a thing as an infraclavicle does not exist in
the Stickleback, the bone so-called being only a part of the coracoid; and
as in most of the Sticklebacks the pelvic bones join the latter, the
resemblance between them and _Lampris_ remains. As I have previously
pointed out, the absence of spines in the fins, and the position of the
ventral fins, together with the great number of rays in the latter, which
is only met with in the lower Teleosteans, are characters which necessitate
the removal of _Lampris_ from the Acanthopterygians, and I cannot find a
better place for them than near the Gastrosteidae.
The whole question of the arrangement of the Physoclists with abdominal
ventrals (Catosteomi and Percesoces) is, I feel, much in need of revision,
and it may be found advisable to break up this group into a greater number
of sub-orders, in which case the Selenichthyes would stand by themselves;
the Hemibranchii and Lophobranchii would be united under the former name,
as proposed by Woodward, or under that of Thoracostei (Swinnerton) or
Phthinobranchii (Hay). The position in the system of the Pegasidae is still
somewhat doubtful. This family is regarded by some authors as related to
the mail-cheeked Acanthopterygians.
FAM. 2. GASTROSTEIDAE.—Body more or less elongate, naked or protected by
bony shields, tapering to a slender caudal peduncle. Head moderate, with
short or elongate and tubiform snout; mouth small, terminal, toothed;
opercular bones well developed; suborbitals in contact with praeoperculum,
protecting the cheek. Gills four, pectinated. Praecaudal vertebrae with
strong transverse processes and slender, free ribs. Spinous dorsal
represented by isolated spines. Pectoral fins with short {630}pterygials.
Pelvic bones usually connected with scapular arch. Ventral fins with one
spine and one or two soft rays.
[Illustration: FIG. 383.—Pectoral arch (left side) of _Gastrosteus
aculeatus_. (After Parker.) _cl_, Clavicle; _cor_, coracoid; _pt_,
pterygials; _sc_, scapula; _scl_, supra-clavicle.]
[Illustration: FIG. 384.—_Gastrosteus aculeatus_. × 1. (After Goode.)]
Four genera: _Gastrosteus_, _Apeltes_, _Eucalia_, _Spinachia_.
The little Three-spined and Two-spined Sticklebacks (_Gastrosteus
aculeatus_ and _G. pungitius_), which include many varieties that have been
regarded as distinct species, are among the best known of our British
Fishes. They are remarkable for the perfect indifference with which they
can be transported from fresh into salt water, and _vice versa_, and for
the elaborate nests which the males build in fresh or brackish water, and
over which they watch with the greatest vigilance after the female has
deposited her relatively large eggs.[703] These nests are made of weeds and
twigs fastened together by threads secreted by the kidneys of the male. The
{631}larger fifteen-spined Stickleback (_Spinachia vulgaris_) is entirely
marine; its nests are to be found on our coasts in sheltered rock-pools,
and they are made chiefly of sea-weeds and Hydrozoa. Sticklebacks are
short-lived, and are believed to breed only once.
The Gastrosteidae are restricted to the northern hemisphere, being more
abundant in the higher latitudes, extending to Iceland, Greenland, and
Bering Straits; the southernmost points of their distribution are Algeria
in the Old World, and Lower California in the New.[704]
A very large number of species have been described, but probably only about
a dozen deserve to stand.
[Illustration: FIG. 385.—Distribution of the Gastrosteidae.]
FAM. 3. AULORHYNCHIDAE.—The genera _Aulorhynchus_ and _Auliscus_, each with
one species from the Northern Pacific, much resemble _Spinachia_ in outward
form and in the equal size of the anterior vertebrae, but the snout is
still more produced, tubiform, and the ventral fins are formed of one spine
and four soft rays. The difference which justifies their separation as a
distinct family resides in the disposition of the ribs, which are flattened
and ankylosed to the lateral bony shields.
FAM. 4. PROTOSYNGNATHIDAE.—This family appears to be intermediate between
the Gastrosteidae and the Aulostomatidae, agreeing with the former in
possessing slender, free ribs, with the latter in having the first
vertebrae elongate, though to a {632}less degree than in _Aulostoma_. Its
only representative is _Protosyngnathus sumatrensis_, from a Tertiary
freshwater formation in Sumatra, which has been referred, without adequate
grounds, to _Aulorhynchus_ or _Auliscops_.[705]
FAM. 5. AULOSTOMATIDAE.—Allied to the Aulorhynchidae, differing in the
ventral fins devoid of spines, formed of 5 or 6 rays, widely removed from
the pectoral arch, the very elongate, saddle-shaped anterior vertebra
formed by the fusion of several, the large supratemporals produced backward
over the anterior vertebra, the very elongate pterygials of the pectoral
fin, and the compressed body covered with small ctenoid scales. Ribs are
rudimentary or absent. No suborbitals. The snout is long, tubiform; the
small terminal mouth bears bands of minute teeth, and the lower jaw has a
small barbel at the symphysis. A single genus, _Aulostoma_, with two
species from the Atlantic coasts of tropical America, and two from the
Eocene and Miocene of Europe. _A. coloratum_ grows to a length of 26
inches.
FAM. 6. FISTULARIIDAE.—Body greatly elongate, naked. First vertebra much
elongate, formed by the fusion of several; strong transverse processes to
the ribs in front and behind, those of two vertebrae suturally united; ribs
rudimentary or absent. Supratemporal much produced posteriorly, extending
over the anterior vertebrae; suborbitals absent; snout forming a long tube,
which terminates in a narrow mouth with minute teeth. Spinous dorsal
entirely absent. Pterygials of pectoral fin very elongate. Ventral fins
very small, with 6 soft rays, inserted far behind the pectoral girdle.
The Flute-mouths, _Fistularia_, which Dr. Günther describes as "gigantic
marine Sticklebacks living near the shore, from which they are frequently
driven into the open sea," are represented by three species, from the
tropical and subtropical parts of the Atlantic and Indo-Pacific. The middle
rays of the forked caudal fin are produced into a long filament. The
largest species, _F. tabaccaria_, reaches a length of 6 feet. The same
genus is represented by two species in the Upper Eocene and Oligocene of
Europe, and _Urosphen_, from the Upper Eocene, is regarded as allied to it.
{633}FAM. 7. CENTRISCIDAE.—Body moderately elongate, partially enclosed in
a bony armour, which is distinct from the endoskeleton. Anterior vertebrae
elongate, with strong parapophyses ankylosed to the exoskeleton; no ribs.
Suborbitals absent; snout forming a long tube, with small, terminal,
toothless mouth. Two dorsal fins, the anterior with a very strong spine.
Pterygials of pectoral fin very small. Ventral fins small, with 4 or 5
rays, the pelvic bones in contact with the postclavicles.
_Centriscus_, with five species in the Atlantic and Pacific Oceans,
represents this family at the present day. _C. scolopax_ has occasionally
been found on the English coast. Isolated spines from the Pliocene of
Tuscany have been referred to the same genus. _Rhamphosus_, from the Eocene
of Monte Bolca, is believed to have been allied to _Centriscus_.
FAM. 8. AMPHISILIDAE.—Near the preceding, but body extremely compressed and
completely enclosed in a thin bony armour which is fused with the
endoskeleton; the caudal region, much abbreviated, is free and relegated to
the ventral surface, the body terminating in the two dorsals, of which the
first bears a strong spine. The ventral fins are far back, very small,
formed of 3 or 4 rays.
_Amphisile_ is represented by three or four recent species in the Indian
and Pacific Oceans, and two are known from Upper Eocene and Oligocene beds
in Europe. Dr. Arthur Willey has observed these fishes in the Southern
Pacific. _A. strigata_ "lives in small shoals of about half-a-dozen
individuals, and swims about with rapidity in a vertical position, cleaving
the water with its razor-shaped body."
FAM. 9. SOLENOSTOMIDAE.—Body moderately elongate, with large star-like
ossifications. Anterior vertebrae elongate, without transverse processes;
no ribs. Snout much produced, tubiform; mouth, small, terminal, toothless;
no praeoperculum; symplectic elongate; gill-opening wide; gill-lamellae
small rounded lobes. Two short dorsal fins, the rays of the anterior not
articulated, flexible spines. Pterygials of pectoral fin very small.
Ventral fins large, with 7 rays, behind the pectoral arch. No air-bladder.
The unique genus, _Solenostomus_, with three or four species from the
Indian and Pacific Oceans, may be regarded as in many respects intermediate
between the _Centriscidae_ and the _Syngnathidae_. In the female the inner
side of the ventral fins coalesces {634}with the integuments of the body,
forming a large pouch for the reception of the eggs.
_Solenorhynchus_, from the Upper Eocene of North Italy, probably belongs to
this family, but its form is much more elongate, and the exoskeleton is in
regular rings.
FAM. 10. SYNGNATHIDAE.—Body more or less elongate, protected by an
exoskeleton forming rings. Anterior vertebrae not elongate; parapophyses
strong, ankylosed to the exoskeleton; no ribs. Snout much produced,
tubiform; mouth small, terminal, toothless; no praeoperculum; symplectic
elongate; gill-cleft reduced to a very small opening near the upper
posterior angle of the gill-cover; gill-lamellae small rounded lobes. A
single dorsal fin. Pectoral fins, if present, with very small pterygials;
ventrals absent. Caudal fin often absent; tail sometimes prehensile.
[Illustration: FIG. 386.—_Syngnathus pelagicus_.]
This family embraces about 175 marine species, and is represented over the
greater part of the world. Principal genera: _Siphonostoma_, _Syngnathus_,
_Penetopteryx_, _Ichthyocampus_, _Nannocampus_, _Osphyolax_, _Urocampus_,
_Doryichthys_, _Coelonotus_, _Stigmatophorus_, _Nerophis_, _Protocampus_,
_Gastrotoceus_, _Solenognathus_, _Hippocampus_, _Acentronura_,
_Phyllopteryx_.
Remains are found in the Upper Eocene and Miocene beds of Europe, and have
been referred to _Siphonostoma_ and _Syngnathus_, and to the extinct genus
_Calamostoma_. It is probable that _Pseudosyngnathus_, from the Upper
Eocene of Monte Bolca, is the type of a distinct family.
The best known members of this family are the Needle-Fish or Pipe-Fish
(_Siphonostoma_ and _Syngnathus_) and Sea-Horse (_Hippocampus_) of our
coasts. The latter, like _Amphisile_, swims with the body in a vertical
position. In most species the male takes charge of the eggs, in a pouch
under the tail (_Siphonostoma_, {635}_Syngnathus_, _Penetopteryx_,
_Nannocampus_, _Stigmatophorus_, _Hippocampus_), in a groove under the tail
(_Phyllopteryx_), or in a groove on the abdomen (_Doryichthys_,
_Coelonotus_, _Nerophis_, _Gastrotoceus_).
[Illustration: FIG. 387.—_Hippocampus guttulatus_. Male, showing
brood-pouch (_mp_). _a_, Anus; _b.a_, branchial aperture.]
[Illustration: FIG. 388.—_Phyllopteryx eques_. ½ nat. size.]
An Australian species of _Syngnathus_ has been described by E. P. Ramsay
under the name of _S. intestinalis_, from its living inside Holothurians,
in the manner of _Fierasfer_, and G. Lunel has observed a _Doryichthys_ to
offer a similar instance of inquilinism.
One of the most remarkable types of Syngnathids is _Phyllopteryx_, from
Australia. The spines and knobs of the head and body are furnished with
dermal appendages, which closely imitate the fucus among which they live.
FAM. 11. PEGASIDAE.—Body short or moderately elongate, encased in an
exoskeleton forming rings. Anterior vertebrae not elongate; no ribs. Snout
produced beyond the mouth, which is small, inferior, and toothless; no
praeoperculum, no symplectic; gill-opening very small; gills pectinated. A
single dorsal fin. Pectoral fins large, horizontal; ventrals reduced to one
or two filamentous rays, behind the scapular arch. Air-bladder absent.
{636}Five or six species, referable to two genera, _Pegasus_ and
_Parapegasus_, make up this family. They are very small fishes, inhabiting
the coasts of China, Japan, Arabia, the Malay Archipelago, and Australia.
_Pegasus_ is remarkable among all fishes in having the five anterior rays
of the pectoral fin transformed into strong spines.
SUB-ORDER 8. PERCESOCES.
Air-bladder, if present, without open duct. Parietal bones separated by the
supraoccipitaL Pectoral arch suspended from the skull; no mesocoracoid
arch. Ventral fins, if present, abdominal, or at least with the pelvic
bones not solidly attached to the clavicular arch.
This group connects the Haplomi with the Acanthopterygii, the
Scombresocidae being somewhat related to the Cyprinodonts,[706] whilst the
Anabantidae show distinct affinity to the Osphromenidae in the following
sub-order. Other families, previously included among the Scombriform
Acanthopterygians, are placed here on the assumption that the loose
attachment of the pelvic bones to the clavicles is a primitive character,
and not the result of degeneration, such as occurs in some cases among true
Acanthopterygians. Although this sub-order is perhaps only an artificial
association, it must be borne in mind that, notwithstanding the very wide
divergence which exists between the first and last families, and however
dissimilar their members may appear to be at first sight, a gradual passage
may be traced connecting the most aberrant types.
SYNOPSIS OF THE FAMILIES.
I. Ventral fins, if present, inserted far behind the pectorals; no spines
to the fins.
Ribs attached to the extremity of much-developed parapophyses; lower
pharyngeal bones completely united; pectoral fins inserted very
high up 1. _Scombresocidae_.
Ribs mostly sessile; lower pharyngeal bones distinct; pectoral {637}
fins nearer the ventral than the dorsal line 2. _Ammodytidae_.
II. Ventral fins, if present, more or less approximated to the pectorals.
A. Two well-developed dorsal fins, the anterior small and formed,
at least in part, of spinous rays.
1. Ribs attached to strong parapophyses.
Pelvic bones free or connected with the clavicles by ligament;
pectoral fins inserted high up 3. _Atherinidae_.
Pelvic bones suspended from the postclavicles; pectoral fins
inserted very high up; teeth very feeble or absent
4. _Mugilidae_.
Pelvic bones suspended from the postclavicles; pectoral fins
low down, with detached lower rays 5. _Polynemidae_.
Pelvic bones connected with the clavicles by ligament; pectoral
fins nearer the ventral than the dorsal line; dentition
powerful, cardiform; scales minute or absent
6. _Chiasmodontidae_.
2. Anterior ribs sessile; pelvic bones not connected with the
scapular arch; pectoral fins nearer the ventral than the
dorsal line 7. _Sphyraenidae_.
B. Spinous dorsal, if present, connected with the soft.
1. Anterior vertebrae without parapophyses; scales on head, if
present, small.
Oesophagus with lateral sacs which are beset with papillae
internally; spinous dorsal long; scales rhomboidal, in
oblique transverse series; pelvic bones free
8. _Tetragonuridae_.
Oesophagus with lateral sacs which are beset with toothed
papillae internally; spinous dorsal, if distinct, shorter
than the soft dorsal; scales moderate or small, cycloid,
often deciduous 9. _Stromateidae_.
No sacs in the oesophagus; fins without spines; scales very
small or absent 10. _Icosteidae_.
2. All, or all but the anterior two vertebrae with parapophyses;
scales on head large; a superbranchial cavity.
No spines to the fins 11. _Ophiocephalidae_.
Strong spines to the dorsal, anal, and ventral fins
12. _Anabantidae_.
FAM. 1. SCOMBRESOCIDAE.—Maxillary entering the border of the upper jaw;
dentition moderately strong or feeble. Lower pharyngeal bones united.
Praecaudal vertebrae with strong parapophyses supporting the ribs. Body
covered with cycloid scales. Pectoral fins inserted very high up; ventral
fins widely separated from the pectorals, without spines, with 6 rays.
Dorsal fin opposed to the anal, and likewise formed entirely of soft rays.
Air-bladder generally present, sometimes cellular.
The shape of the head and body vary greatly, and the pectoral fin may reach
an extraordinary wing-like development. The dorsal fin may be followed by a
series of finlets, as in many of the Scombridae. Most of the
Scombresocidae, of which about 200 species are known, are marine; some are
carnivorous, {638}others (_Hemirhamphus_) mainly herbivorous, feeding on
green algae. Nearly all are in the habit of making great leaps out of the
water, this tendency culminating in the Flying-Fish (_Exocoetus_), which
skip or sail through the air in a manner the explanation of which has given
rise to much controversy. According to the latest evidence[707] the sole
source of motive power is the action of the strong tail while in the water;
no force is acquired while the fish is in the air. The pectorals are not
used as wings but as parachutes. There is every passage between the small
pectoral fin of a Saurie (_Scombresox_) or a _Hemirhamphus_ and the
swallow-like wings of the most developed _Exocoetus_. The genus
_Hemiexocoetus_ is a very remarkable connecting form. The Gar-Pike
(_Belone_), of which one species is common on our coasts, have both jaws
produced into a long slender beak; the bones are green. In _Hemirhamphus_
the lower jaw only is prolonged; some of the species, living in fresh
water, are viviparous, the anal fin being modified into a copulatory organ,
as in many Cyprinodonts.
[Illustration: FIG. 389.—Gar-Pike (_Belone annulata_), × ⅓. (After Cuvier
and Valenciennes.)]
Scombresocidae occur in all the tropical and temperate seas. _Belone_,
_Scombresox_, and _Hemirhamphus_ are found in Upper Eocene and Miocene beds
of Europe, and, as stated above, _Protaulopsis_ should perhaps be referred
to this family.
{639}FAM. 2. AMMODYTIDAE.—Maxillary excluded from the border of the upper
jaw; mouth protractile; dentition feeble or absent. Lower pharyngeal bones
separate. Praecaudal vertebrae without parapophyses. Body covered with very
small cycloid scales. Pectoral fins nearer the ventral than the dorsal
line; ventral fins, if present, widely separated from the pectorals,
without spine, with 6 rays. Dorsal and anal fins more or less elongate,
formed of soft rays. Air-bladder absent.
The existing genera, _Ammodytes_, with 8 species, from the temperate coasts
of the northern hemisphere, and _Hypoptychus_, from northern Japan, with a
single species, are deprived of ventral fins, and their exact relations
remained obscure until the structure of the Oligocene _Cobitopsis_ revealed
their affinity to the _Scombresocidae_, or at least their pertinence to the
present suborder. The Greater Sand-Eel or Launce (_Ammodytes lanceolatus_)
and the Lesser Sand-Eel (_A. tobianus_) are common on our coasts, and are
remarkable for the manner in which, by means of their sharp-pointed snout,
they bury themselves with great rapidity in the sand, darting in and out
like arrows.
[Illustration: FIG. 390.—_Cobitopsis acuta_. (Restoration by A. S.
Woodward.)]
FAM. 3. ATHERINIDAE.—Maxillary excluded from the border of the upper jaw;
dentition more or less developed. Body covered with cycloid or ctenoid
scales. Ribs attached to strong parapophyses. Pectoral fins inserted high
up; ventral fins more or less approximated to the pectorals, with one spine
and five soft rays; pelvic bones connected with the clavicular symphysis by
a ligament. Two well-separated dorsal fins, the anterior small and formed,
at least in part, of spinous rays. Air-bladder present.
Carnivorous Fishes, mostly marine and of small size, much valued as food,
and distributed along the coasts of most tropical and temperate seas; some
inhabit fresh waters. A silvery lateral band, or "stole," is usually
present. About 65 species are known, referred to 14 genera: _Atherina_,
_Iso_, _Chirostoma_, _Thyrina_, _Atherinella_, _Labidesthes_,
_Atherinopsis_, _Atherinops_, _Telmatherina_, _Neatherina_, _Pseudomugil_,
_Rhombatractus_, _Aida_, _Melanotaenia_.
{640}Represented in the Upper Eocene of Europe by several species of
_Atherina_ and by the extinct genus _Rhamphognathus_.
FAM. 4. MUGILIDAE.—Maxillary excluded from the border of the upper jaw;
dentition feeble or absent. Body covered with cycloid scales. Ribs attached
to the extremity of strong parapophyses. Pectoral fins inserted high up;
ventral fins more or less approximated to the pectorals, with one spine and
five soft rays; pelvic bones suspended from the post-clavicles. Two
well-separated dorsal fins, the anterior formed of a small number of
spines. Air-bladder present.
These Fishes are closely related to the preceding, of which they are a
further specialisation, the pharyngeal bones having a complicated
structure, much reducing the oesophageal opening, and the vertebrae being
reduced in number (24 to 26 instead of 32 to 60). They feed on organic
matter contained in mud, and inhabit the fresh waters and coasts of the
temperate and tropical regions. The species number about 100. Principal
genera: _Mugil_, _Myxus_, _Anostomus_, _Joturus_. Grey Mullets (_Mugil_)
are represented on our coasts by three species, valued as food, one of
which (_M. capito_) has a remarkably wide range, occurring from Scandinavia
to the Cape of Good Hope. Remains referred to the same genus occur in the
Miocene and Oligocene.
[Illustration: FIG. 391.—Shoulder-girdle and pelvis of _Polynemus
quadrifilis_. _cl_, Clavicle; _cor_, coracoid; _pec_, pectoral rays; _pel_,
pelvis; _pt_, pterygials; _ptcl_, post-clavicle; _pte_, post-temporal;
_sc_, scapula; _scl_, supra-clavicle; _ven_, ventral rays.]
FAM. 5. POLYNEMIDAE.—Maxillary excluded from the border of the upper jaw;
dentition feeble. Body covered with ctenoid {641}scales. Ribs attached to
the extremity of strong parapophyses. Pectoral fin inserted low down, with
a lower portion consisting of free rays; the upper portion, or fin proper,
attached to the scapula, the lower to a fenestrate bone which appears to be
formed by coalesced pterygials (Fig. 391, _pt_.). Ventral fin more or less
approximated to the pectoral, with one spine and five soft rays; pelvic
bones suspended from the post-clavicles. Two well-separated dorsal fins,
the anterior formed of a small number of spines. Air-bladder, if present,
very large.
The vertebrae number 24 (10 + 14).
[Illustration: FIG. 392.—_Polynemus quadrifilis_, × ¼. (After Cuvier and
Valenciennes.)]
Three closely allied genera: _Polynemus_, _Pentanemus_, and _Galeoides_,
with about 25 species, from the shores of tropical seas, often entering
rivers. Some attain a length of 4 feet, and are valued as food or for the
isinglass yielded by their air-bladder. The free pectoral filaments are
organs of touch, and can be moved independently of the fins.
FAM. 6. CHIASMODONTIDAE.—The deep-sea genera, _Chiasmodon_,
_Pseudoscopelus_, and _Champsodon_, which have been placed either with the
Gadidae, the Trachinidae, or the Berycidae, may be referred to the
Percesoces, as the pelvic fins have only a ligamentous connexion with the
pectoral arch. Unfortunately, the skeleton has only been examined in
_Champsodon_; it is remarkably similar to that of the Atherinidae. As in
_Atherinichthys_, the posterior extremity of the air-bladder is protected
by a bony sheath formed by the expanded ring-like haemal processes of the
anterior caudal vertebrae. Vertebrae 32 (16 + 16). The {642}scales are
absent or very small and spinulose, the mouth large, with cardiform teeth;
spinous dorsal short, soft dorsal and anal elongate. _Chiasmodon_ and
_Pseudoscopelus_ have a complicated system of sensory organs on the body,
which in the latter suggest the photophores of Scopelids. _Champsodon
vorax_ is a fish of extreme voracity, swallowing prey much larger than
itself. Only four species of this family are known.
FAM. 7. SPHYRAENIDAE.—Maxillary excluded from the border of the upper jaw;
dentition very strong. Body covered with cycloid scales. Anterior ribs
sessile, the rest inserted on parapophyses. Pectoral fin nearer the ventral
than the dorsal outline; ventral fin more or less approximated to the
pectoral, with 1 spine and 5 soft rays; pelvis not connected with the
pectoral arch. Two well-separated dorsal fins, the anterior formed of a
small number of spines. Air-bladder large. Vertebrae 24.
Carnivorous Pike-like Marine Fishes from the tropical and sub-tropical
seas, often found at the mouths of rivers. The "Barracudas" form a single
genus, _Sphyraena_, with about 20 species, the largest of which grow to 8
feet and are dangerous to people bathing; many are valued as food, but some
are reported to be poisonous, at least at certain seasons. Remains of
several species are known from the Eocene and later periods in Europe and
North America.
FAM. 8. TETRAGONURIDAE.—Maxillary excluded from the border of the upper
jaw; dentition feeble. Oesophagus with lateral sacs which are beset with
papillae internally; a series of gill-raker-like knobs below the
pseudobranchiae. Body covered with rhomboidal, striated scales in oblique
transverse series, those of every single transverse series coherent. Ribs
mostly sessile. Pectoral fin nearer the ventral than the dorsal outline.
Ventral fin with 1 spine and 5 soft rays, near the pectoral, but pelvis
free from the pectoral arch. A long continuous dorsal fin, its anterior
portion formed of numerous short spines. Air-bladder absent. Vertebrae 58.
This family includes a single, rather rare fish, _Tetragonurus cuvieri_,
from the Mediterranean and neighbouring parts of the Atlantic and the South
Pacific. It is said to descend to great depths at certain seasons, and to
feed on Medusae; its flesh is poisonous. Young specimens have been observed
by Emery to live in the respiratory cavity of large Salpae.
{643}FAM. 9. STROMATEIDAE.—Although including a number of forms very unlike
_Tetragonurus_ in external appearance, there is no doubt that this family,
hitherto placed near the Scombridae, is very closely allied to the
preceding, agreeing with it in the presence of lateral oesophageal sacs
bearing internally papillae (which are besides beset with setiform teeth),
and, in most genera, in the presence of a series of knobs, more or less
similar to gill-rakers, below the pseudobranchiae. The pelvic bones are
sometimes free from the pectoral arch, as in the Tetragonuridae, sometimes
more closely attached, but only by ligament, and movable. The principal
difference resides in the scales, which are always cycloid and usually very
small and more or less deciduous, and in the spinous dorsal being shorter
than the soft, or even quite rudimentary. The ventrals are sometimes
absent. The air-bladder is present or absent. The number of vertebrae
varies from 24 to 46.
Marine Fishes, pelagic or deep-sea, feeding on Crustaceans, Medusae, or the
fry of other fish. About 45 species are known, referable to 10 genera:
_Nomeus_, _Cubiceps_, _Psenes_, _Seriolella_, _Psenopsis_, _Centrolophus_,
_Lirus_, _Stromateus_, _Peprilus_, and _Stromateoides_. Many of the species
have a wide distribution, but are rare in collections.[708] The Black-Fish
(_Centrolophus niger_) and its close ally _C. britannicus_, and the
Rudder-Fish (_Lirus perciformis_ and _L. medusophagus_), have occurred, at
rare intervals, on the British coasts. The Stromateidae were represented by
several species in the Cretaceous (_Platycormus_ and _Homosoma_).
The widely distributed _Nomeus gronovii_, so remarkable for its enormous
ventral fins, folding in a ventral groove, has been observed in New South
Wales to be only found on the coast when the Siphonophores called
"Portuguese Men-of-War" or _Physalia_ are driven ashore, the fish swimming
beneath them, as the young _Caranx_ are in the habit of doing under
Medusae. As observed by Waite,[709] the benefit of such a partnership must
primarily be with the fish, for it is a voluntary agent, whereas the
_Physalia_ has no power of locomotion. "If the fish secures safety from its
enemies by entering the area embraced by the deadly tentacles of the
_Physalia_, which attain a length of 10 to {644}12 feet, it must be immune
to their influence: a remarkable condition, considering that small fish
have often been seen in their stomachs and entangled in their tentacles."
This observer adds: "It is probable that, in addition to protection, the
fish derives its food from association with the _Physalia_, much as does
the Remora in accompanying a shark. The _Physalia_ doubtless paralyses many
more animals than it can consume—the residue falling to the lot of the
fishes, which may be present to the number of ten."
FAM. 10. ICOSTEIDAE.—The so-called "Rag-Fishes," in which the skeleton is
quite soft and cartilaginous, are aberrant deep-sea forms evidently related
to the Stromateidae; they lack the oesophageal teeth and the processes of
the last gill-arch, but _Icosteus_ at least has the gill-raker-like knobs
below the pseudobranchiae. The pelvis is widely separated from the
clavicles. Spines are absent in the fins, and the body is naked or covered
with small cycloid scales. Vertebrae in large number (up to 70).
_Icosteus_, _Icichthys_, and _Acrotus_, each with a single species, from
the Pacific coast of North America.
[Illustration: FIG. 393.—_Icosteus enigmaticus_, × ⅓. (After Goode and
Bean.)]
FAM. 11. OPHIOCEPHALIDAE.—Maxillary excluded from the border of the upper
jaw. Head and body covered with cycloid scales. Anterior ribs sessile, the
remainder inserted on the parapophyses. Pectoral fin low down; ventral fin,
if present, near the pectoral, with 6 soft rays; pelvic bones connected
with the clavicular symphysis by ligament. Dorsal and anal fins long,
without spines. Air-bladder present, much elongate.
These Fishes are provided with an accessory superbranchial cavity, and are
able to breathe atmospheric air. All are {645}inhabitants of fresh waters
and are carnivorous. Only two genera are known: _Ophiocephalus_, with about
25 species from Eastern Asia and 3 from Tropical Africa, and _Channa_,
distinguished by the absence of ventral fins, with 3 species from Ceylon
and China.
[Illustration: FIG. 394.—Distribution of the Ophiocephalidae.]
[Illustration: FIG. 395.—Distribution of the Anabantidae.]
FAM. 12. ANABANTIDAE.—Differ from the preceding, to which they are closely
related, in having part of the dorsal and anal fins and the outer ventral
ray spinous, and a shorter, Perch-like body covered with ctenoid scales.
The accessory superbranchial organ is still more developed, with thin bony
laminae, which are more or less folded and covered with a mucous membrane.
These Fishes can live a long time out of water, and the name _Anabas
scandens_, or Climbing Perch, recalls the fact that its first observers in
India ascribed to it the habit of climbing up low trees by means of the
spines with which its {646}gill-covers and ventral fins are armed. This
species, which attains a length of 8 inches, is found in estuaries and
fresh waters of India, Ceylon, Burma, and the Malay Peninsula and
Archipelago; 3 other species occur in the Malay Archipelago, and 11 in
Africa.
SUB-ORDER 9. ANACANTHINI.
Air-bladder without open duct. Parietal bones separated by the
supraoccipital; prootic and exoccipital separated by the enlarged
opisthotic. Pectoral arch suspended from the skull; no mesocoracoid arch.
Ventral fins below or in front of the pectorals, the pelvic bones posterior
to the clavicular symphysis and only loosely attached to it by ligament.
Fins without spines; caudal, if present, without expanded hypural,
perfectly symmetrical, and supported by the neural and haemal spines of the
posterior vertebrae and by basal bones similar to those supporting the
dorsal and anal rays. This type of caudal fin must be regarded, as I have
pointed out,[710] as secondary, the Gadidae being, no doubt, derived from
Fishes like the Macruridae, in which the homocercal fin had been lost. The
scapular foramen or fenestra is nearly always between the scapular and
coracoid bones, as in the Trachinidae and several allied families, not in
the coracoid, as in the other Acanthopterygians. The first two vertebrae
have no epipleurals.
Mr. C. Tate Regan,[711] who has recently given a good definition of the
Anacanthini, divides them into three families.
[Illustration: FIG. 396.—Skeleton of caudal fin of _Gadus virens_.]
{647}FAM. 1. MACRURIDAE.—Mouth more or less inferior, protractile; teeth
small, none on palate. Anterior vertebrae without transverse processes,
with the ribs sessile, the rest with strong transverse processes supporting
the ribs, which themselves bear epipleurals. Gill-membranes free from
isthmus or narrowly attached; 6 or 7 branchiostegal rays; gills 3½ or 4;
pseudobranchiae rudimentary or absent. Ventral fins below the pectorals,
with 7 to 12 rays. Body short, tail elongate and tapering to a point,
without caudal fin. A short anterior dorsal, with a single simple ray, and
a long dorsal and anal meeting together at the end of the tail, formed
entirely of articulated rays—the two dorsals sometimes continuous
(_Lyconus_).
Deep-sea Fishes with very large eyes and small or rather large mouth,
usually covered with rough spiny scales; a mental barbel is present, except
in _Lyconus_, and the muciferous cavities of the skull are strongly
developed, the bones being remarkably thin. About 120 species are known,
some of which have a wide distribution. Macrurids have been found in all
the seas where deep-sea dredging has been practised—the greatest depth at
which they have been obtained being 2650 fathoms. Principal genera:
_Macrurus_, _Gadomus_ (with perforate scapula) _Coryphaenoides_,
_Hymenocephalus_, _Malacocephalus_, _Lionurus_, _Trachyrhynchus_,
_Steindachneria_, _Bathygadus_, _Lyconus_, _Macruronus_. A larval form of
this family has received the name of _Krohnius_; it is remarkable for the
filamentous prolongation of the ventral rays, which recalls the larval
_Trachypterus_.
[Illustration: FIG. 397.—_Macrurus carminatus_, × ⅓. (After Goode.)]
FAM. 2. GADIDAE.—Mouth moderate or large, more or less protractile.
Anterior vertebrae without transverse processes, with the ribs sessile, the
rest with strong transverse processes, {648}usually supporting ribs,[712]
which themselves bear epipleurals. Gill-membranes free from isthmus or
narrowly attached; 6 to 8 branchiostegal rays; gills 4, a slit behind the
fourth; no pseudobranchiae. Ventral fins jugular, with 1 to 9 soft rays.
Body more or less elongate, covered with small cycloid scales. Dorsal and
anal fins elongate, formed of articulated rays, sometimes divided into two
or three distinct portions. Caudal fin more or less distinct, supported by
the unmodified or but slightly modified neural and haemal spines of the
last vertebrae, which are perfectly symmetrical (diphycercal or isocercal
type).
A mental barbel is often present, as in the Macruridae, and the suture
between the frontal bones has disappeared in most of the members of this
very natural family. About 120 species are distinguished, mostly marine,
many being adapted to life at great depths. All are carnivorous. They
inhabit chiefly the northern seas, but many abyssal forms occur between the
tropics and in the southern parts of the Atlantic and Pacific. Principal
genera: _Gadus_, _Merluccius_, _Holargyreus_, _Lotella_, _Physiculus_,
_Phycis_, _Haloporphyrus_, _Tripterophycis_, _Lota_, _Molva_, _Onus_,
_Bregmaceros_, _Antimora_, _Raniceps_, _Brosmius_.
[Illustration: FIG. 398.—Cod (_Gadus morrhua_), × ⅕. (After Goode.)]
Several species, referred to _Gadus_ and _Brosmius_, have been described
from the Miocene. _Nemopteryx_, which is allied to _Gadus_, is from the
Oligocene.
The fishes of this family are among the most important from an economic
point of view. It will suffice to allude merely by name to the following
among the European forms:—The Cod-Fish (_Gadus morrhua_), the largest
species, reaching a length of {649}4 feet and a weight of 100 lbs., the
Haddock (_G. aeglefinus_), the Whiting (_G. merlangus_), the Bib or Pout
(_G. luscus_), the Pollack (_G. pollachius_), the Coal-Fish (_G. virens_),
the Hake (_Merluccius vulgaris_), and the Ling (_Molva vulgaris_). Species
of _Merluccius_ occur also on the coasts of Chili and of New Zealand. The
Rocklings (_Onus_ or _Motella_) are of small size; several species are of
common occurrence in our tide-pools. The Burbot (_Lota vulgaris_) is a
freshwater fish, exceeding a length of 3 feet, of excellent quality, but
unfortunately local and rare in this country.
FAM. 3. MURAENOLEPIDIDAE.—Closely related to the Gadidae, from which they
differ in not having a separate caudal fin, in the gill-openings being
narrow and below the base of the pectorals, in the increased number (ten)
of the pectoral pterygials,[713] and in the peculiar scales, similar to
those of the Anguillidae. Ventrals with 5 rays. A mental barbel.
A single genus, _Muraenolepis_, from the coast of Kerguelen Island.
{650}CHAPTER XXIII
TELEOSTEI (_CONTINUED_): ACANTHOPTERYGII—OPISTHOMI—PEDICULATI—PLECTOGNATHI
SUB-ORDER 10. ACANTHOPTERYGII.
Air-bladder usually without open duct. Opercle well developed;
supraoccipital in contact with the frontals. Pectoral arch suspended from
the skull; no mesocoracoid. Ventral fins thoracic or jugular, more or less
firmly attached to the clavicular arch. Gill-opening usually large, in
front of the base of the pectoral fin.
The character from which this sub-order, the most comprehensive of the
whole class, derives its name, viz. the presence of non-articulated, more
or less pungent, rays in the dorsal and anal fins, is by no means
universal, exceptions to the rule being numerous. The mouth is usually
bordered by the premaxillaries to the exclusion of the maxillaries, and if
these should, by exception, enter the oral edge, they are always toothless.
The ventral fins are sometimes inserted at some distance behind the base of
the pectorals (_Haplodactylidae_, _Platycephalidae_), in which case,
however, this is due to the elongation of the pelvic bones, which are
solidly attached to the clavicular arch. The sub-order is broken up into
divisions, which follow in somewhat arbitrary order, the natural affinities
being opposed to a linear arrangement; the annexed diagram is intended to
remedy this defect.
SYNOPSIS OF THE DIVISIONS.
I. No suborbital stay, or process extending from the suborbital bones
towards the praeoperculum; basis cranii double in the symmetrical
forms. Primary shoulder-girdle composed of a perforate scapula {651}
and a coracoid; of the four or five pterygials, or basal bones
of the pectoral fins, only one or two are in contact with the
coracoid; ventral fins thoracic.
Rays of the caudal fin not strongly forked at the base; hypural
usually with a basal spine or knob-like process on each side;
epipleural bones usually inserted on the parapophyses or on the
ribs; dorsal fin usually with strong spines; caudal peduncle
rarely much constricted I. PERCIFORMES.
Rays of the caudal fin strongly forked at the base, embracing a
considerable portion of the hypural, which always bears a basal
spine; epipleural bones usually inserted on the centra or on the
parapophyses, rarely on the ribs; dorsal spines feeble or detached;
caudal peduncle much constricted; scales usually very small or
absent II. SCOMBRIFORMES.
[Illustration: FIG. 399.—Diagram showing probable relationship of the
various groups to one another.]
Rays of the caudal fin not strongly forked at the base; ventral {652}
fins with one spine and six to eight soft rays, _or_ cranium
asymmetrical III. ZEORHOMBI.
II. No suborbital stay; basis cranii double; scapula absent, the
pterygials inserted on the coracoid; ventral fins thoracic
IV. KURTIFORMES.
III. No suborbital stay; basis cranii simple; scapula and coracoid
more or less reduced, sometimes vestigial; pterygials large, only
one or two in contact with the coracoid; ventral fins thoracic
V. GOBIIFORMES.
IV. No suborbital stay; basis cranii simple; a perforate scapula;
three pterygials in contact with the coracoid; ventral fins
thoracic; a suctorial laminated disk on the upper surface of
the head VI. DISCOCEPHALI.
V. A suborbital stay, the second suborbital bone more or less produced
on the cheek or joining the praeoperculum; ventrals thoracic
VII. SCLEROPAREI.
VI. No suborbital stay; ventrals usually jugular or mental; if thoracic,
structure of the pectoral arch differing from that ascribed to the
first five divisions of this Synopsis.
Pectoral fin with vertical or subvertical base; anal fin usually
elongate, rarely small VIII. JUGULARES.
Pectoral fin with horizontal or sub-horizontal base; body exceedingly
compressed; dorsal fin with all the rays simple; anal fin absent
or very small IX. TAENIOSOMI.
DIVISION I.—PERCIFORMES.
No bony stay for the praeoperculum. Basis cranii double. Spinous dorsal
usually well developed. None of the epipleural bones attached to the centra
of the vertebrae in the praecaudal region. Pectoral arch with
well-developed scapula and coracoid, the former pierced by a foramen or
fenestra; pterygials longer than broad, more or less regularly
hour-glass-shaped, four or five in number, one or two of which are in
contact with the coracoid. Ventral fins thoracic.
This large group, consisting chiefly of marine forms, has members in all
parts of the world, with the exception of the Arctic and Antarctic regions,
and was already represented by numerous Berycidae and a few Serranidae and
Scorpididae in the Upper Cretaceous. The division into families is a task
of considerable difficulty, and the necessities of a linear arrangement
result in the breaking up of some natural sequences. Thus it appears highly
probable that the Scorpididae, themselves derived, together with the
Serranidae, from the Berycidae, lead to the Carangidae in the division
Scombriformes, whilst a nearly perfect passage can be traced between the
Acanthuridae of this division and the Balistidae among the Plectognaths.
{653}SYNOPSIS OF THE FAMILIES.
I. Gills four, a slit behind the fourth.
A. Two nostrils on each side.
1. Ventrals with 1 spine and 6 to 13 soft rays. 1. _Berycidae_.
2. Ventrals with not more than 5 soft rays.
a. Lower pharyngeal bones not completely united, showing at least
a median suture.
α. Gill-membranes nearly always free from isthmus.
* Ventrals little if at all behind the pectorals.
† Third vertebra without transverse processes _or_ with
sessile ribs.
§ A more or less developed subocular shelf, or inner lamina
of the suborbitals supporting the eye-ball, sometimes
reduced to a mere process of the second suborbital.
‖ Ribs inserted on the transverse processes, when these
are developed.
Body covered with very large bony scales; ventrals
with a very strong spine and 2 or 3 very short
soft rays 2. _Monocentridae_.
Dorsal very short, with few graduated, adnate spines,
anal very long 3. _Pempheridae_.
Spinous dorsal usually well developed, soft dorsal
usually not much more developed than the anal;
palate usually toothed 11. _Serranidae_.
Dorsal and anal fins elongate and formed mostly of
articulated soft rays, the spines feeble and few
12. _Pseudochromididae_.
Dorsal and anal fins much elongate, without distinct
spines; body band-like 13. _Cepolidae_.
Teeth in the jaws fused to form a beak
14. _Hoplognathidae_.
Soft dorsal and anal much elongate; a separate
spinous dorsal 15. _Sillaginidae_.
Soft dorsal much longer than the anal; a separate
spinous dorsal 16. _Sciaenidae_.
‖‖ Ribs mostly sessile, behind the parapophyses; body
deep; mouth moderately large and protractile.
Post-temporal forked, distinct from skull
25. _Scorpididae_.
Post-temporal completely ankylosed to the skull;
mouth very protractile 26. _Caproidae_.
§§ No subocular shelf.
‖ Ribs mostly sessile, behind the parapophyses;
anal spines 3 to 14.
Teeth conical; palate toothed; mouth feebly
protractile 4. _Centrarchidae_.
Teeth incisor-like; fins densely scaled
5. _Cyphosidae_.
Teeth conical; palate toothless 6. _Lobotidae_.
Maxillary very slender; mouth very protractile
7. _Toxotidae_.
No entopterygoid; mouth very protractile
8. _Nandidae_.
‖‖ Ribs inserted on the transverse processes when {654}
these are developed; not more than 3 anal spines.
Mouth not or but feebly protractile; spinous dorsal
usually longer than the soft; anal with 1 or 2
spines 9. _Percidae_.
Mouth moderately protractile; palate toothed; spinous
dorsal not longer than the soft; anal with 2 or 3
spines 10. _Acropomatidae_.
Mouth very protractile; palate toothless; praemaxillary
with an upwardly directed lateral process
17. _Gerridae_.
Mouth moderately protractile; palate toothed; anal
longer than soft dorsal; body scaly
18. _Lactariidae_.
Mouth moderately protractile; palate toothless; anal
much longer than soft dorsal; body naked
19. _Trichodontidae_.
†† Transverse processes developed on the third vertebra and
bearing the ribs; palate usually toothless.
No subocular shelf; teeth small 22. _Pristipomatidae_.
A subocular shelf; teeth often large, either cutting
in front or molar-like on the sides 23. _Sparidae_.
A subocular shelf; teeth very small or absent; a pair
of barbels on the throat 24. _Mullidae_.
** Ventrals rather far behind the base of the pectorals;
lower pectoral rays unbranched, often thickened; no
subocular shelf.
Anal fin nearly as long as the soft dorsal
20. _Latrididae_.
Anal fin much shorter than the soft dorsal
21. _Haplodactylidae_.
β. Gill-membranes attached to the isthmus.
* Scales well developed; vertebrae 24 or more.
A subocular shelf; mouth small; palate toothless
27. _Chaetodontidae_.
No subocular shelf; mouth small; palate toothless
28. _Drepanidae_.
Subocular shelf more or less developed; a superbranchial
respiratory organ 31. _Osphromenidae_.
** Scales minute; mouth small; vertebrae 22 or 23.
Post-temporal not distinctly forked; vertebrae with strong
transverse processes; ventrals with 1 spine and 2 to 5
soft rays 29. _Acanthuridae_.
Post-temporal forked; vertebrae without transverse
processes; ventrals with 2 spines and 3 soft rays
between them 30. _Teuthididae_.
b. Lower pharyngeals completely united into one bone, without
median suture 32. _Embiotocidae_.
B. A single nostril on each side; lower pharyngeal bones more or
less completely united, but with persistent suture; no subocular
shelf; palate toothless 33. _Cichlidae_.
II. Gills three and a half; lower pharyngeals completely united into
one bone, without median suture; palate toothless.
A single nostril on each side; teeth conical or incisor-like;
a subocular shelf 34. _Pomacentridae_.
Two nostrils on each side; anterior teeth usually strong and
canine-like; teeth on pharyngeal bones conical or tubercular;
no subocular shelf 35. _Labridae_.
Two nostrils on each side; anterior teeth more or less coalesced
into a beak; teeth on pharyngeal bones flat, tessellated; no
subocular shelf 36. _Scaridae_.
{655}FAM. 1. BERYCIDAE.—One or several of the suborbitals emitting an
internal lamina supporting the eye; entopterygoid present. Anterior
vertebrae without transverse processes; all or most of the ribs inserted on
the transverse processes where these are developed. Two nostrils on each
side. Gill-membranes free from isthmus; 4 to 10 branchiostegal rays; gills
4, a slit behind the fourth; pseudobranchiae. Lower pharyngeal bones
separate. Ventral fins with 1 spine and 6 to 13 soft rays.
[Illustration: FIG. 400.—_Beryx splendens_, ⅓ nat. size. (After Goode and
Bean.)]
This family is remarkable for the retention of two archaic characters: the
large number of rays to the ventral fins and the duct between the
air-bladder and the digestive tract; the latter character is, however, not
universal, and has only been found in two genera (_Beryx_ and
_Holocentrum_). The scaling of the body varies greatly, and so does the
development of the spines in the vertical fins. Several genera
(_Melamphaes_, _Anoplogaster_, _Trachichthys_, etc.) have the head studded
with large muciferous cavities which are covered with a thin skin. The vent
is usually situated far behind the ventral fin, but in _Paratrachichthys_,
a genus closely allied to _Trachichthys_, it occupies a more anterior
position, between the ventrals, whilst in _Aphredoderus_ it shifts still
further with age, opening on the throat in the adult.
The Berycidae were abundantly represented in Cretaceous deposits by _Beryx_
and other genera more or less closely related to living forms, and they
appear to have been the precursors of other Perciform Fishes. About 70
species, referred to 13 genera, are known to live at the present day,
mostly at great depths, in {656}the seas nearly all over the world. But one
freshwater form is known, _Aphredoderus sayanus_, the little Pirate Perch
of North America, growing to 5 inches in length. The largest marine forms
(_Beryx_ and _Gephyroberyx_) measure from 1 to 2 feet.
Recent genera: _Beryx_, _Polymixia_, _Aphredoderus_, _Melamphaes_,
_Plectromus_, _Scopelogadus_, _Anoplogaster_, _Caulolepis_, _Trachichthys_,
_Paratrachichthys_, _Gephyroberyx_, _Myripristis_, _Holocentrum_.
[Illustration: FIG. 401.—_Hoplopteryx lewesiensis_. (Restored by A. S.
Woodward.)]
Fossil genera: _Sphenocephalus_, _Acrogaster_, _Pycnosterinx_,
_Hoplopteryx_, from the Upper Cretaceous. _Asineops_, from the Eocene of
North America, is supposed to be allied to _Aphredoderus_. _Beryx_ is
represented by several species in the Upper Cretaceous, and _Holocentrum_
occurs in the Eocene and Miocene.
FAM. 2. MONOCENTRIDAE.—The single genus _Monocentris_, with two species,
one from the seas of Japan, China, and India, and one from the South
Pacific, is very nearly related to the Berycidae, but differs in the
absence of ribs on the anterior six vertebrae, in the very large bony
scales, forming together a coat of mail, and in the structure of the
ventral fin, which is reduced to a strong spine and two or three very short
soft rays. The spines of the dorsal fin are very strong and isolated.
FAM. 3. PEMPHERIDAE.—The resemblance which the fishes united under this
family bear to _Beryx_ is very striking, and applies to the skeleton as
well as to the external characters. But the ventral fins are formed of one
spine and five soft rays, as in most Acanthopterygians. _Bathyclupea_
agrees with _Beryx_ in {657}being possessed of an open duct to the
air-bladder. About twelve species are known, referable to four genera:
_Pempheris_, _Parapriacanthus_, _Neopempheris_, from the Indian, Pacific,
and tropical Atlantic Oceans, and the deep-sea _Bathyclupea_, from the
Indian and Caribbean Seas, at depths of 145 to 419 fathoms.
[Illustration: FIG. 402.—_Pempheris muelleri_. (After Jordan and
Evermann.)]
FAM. 4. CENTRARCHIDAE.—No subocular lamina of the suborbitals, or subocular
shelf; entopterygoid present; palate toothed; teeth conical. Praecaudal
vertebrae with transverse processes from the third or fourth to the last;
ribs mostly sessile, behind the transverse processes. Two nostrils on each
side. Gill-membranes free from isthmus; 5 to 7 branchiostegal rays; gills
4, a slit behind the fourth; pseudobranchiae more or less developed, often
rudimentary or absent. Lower pharyngeal bones separate. Soft portion of
dorsal fin not more developed than the anal. Carnivorous freshwater fishes,
some entering brackish water. Many are known to build nests. Mostly
inhabitants of North America, the best known being the Sun-Fishes
(_Lepomis_), and Black Bass (_Micropterus_), several species of which have
recently been introduced into continental Europe. Principal genera:
_Pomoxys_, _Centrarchus_, _Ambloplites_, _Chaenobryttus_, _Micropterus_,
_Lepomis_, _Elassoma_, _Kuhlia_. Thirty-two species are known.
FAM. 5. CYPHOSIDAE.—Herbivorous fishes, agreeing in their essential
osteological characters with the preceding, differing in the incisor-like
outer teeth and densely-scaled fins. Some 14 species are known, from the
Pacific and Indian Oceans, referable to 4 genera: _Cyphosus_
(_Pimelepterus_), _Hermosilla_, _Sectator_, _Medialuna_.
{658}FAM. 6. LOBOTIDAE.—As in Centrarchidae, but transverse processes of
vertebrae very short, and palate toothless. Two genera: _Lobotes_, with two
species from the warm parts of the Indian and Pacific Oceans, the
Mediterranean, and the Atlantic coast of America, and _Datnioides_, with
two species from the estuaries of the Ganges and the rivers of Burma, Siam,
and the Malay Peninsula and Archipelago.
FAM. 7. TOXOTIDAE.—No subocular shelf; entopterygoid present; palate
toothed; mouth very protractile; maxillary very slender. Ribs sessile,
behind parapophyses which commence from the third vertebra. Two nostrils on
each side. Gill-membranes free from isthmus; 7 branchiostegal rays; gills
4, a slit behind the fourth; pseudobranchiae present. Lower pharyngeal
bones separate. Ventral fins with 1 spine and 5 soft rays.
A single genus, _Toxotes_, with 5 species from the fresh waters and coasts
of the East Indies, N. Australia, Polynesia, and New Zealand. _Toxotes
jaculator_ derives its name from its habit of capturing insects flying near
the surface of the water by shooting drops of water at them, a habit which
it continues in captivity.
FAM. 8. NANDIDAE.—No subocular shelf; no entopterygoid; palate toothed;
mouth very protractile. Praecaudal vertebrae with parapophyses from the 7th
or 8th; ribs mostly sessile, behind the parapophyses. Two nostrils on each
side. Gill-membranes free from isthmus; 6 branchiostegal rays; gills 4, a
slit behind the fourth; pseudobranchiae absent. Lower pharyngeal bones
separate. Soft portion of dorsal fin not more developed than the anal.
Ventral fins with 1 spine and 5 soft rays.
Small carnivorous freshwater fishes, of which 14 species are known,
referable to 6 genera: _Nandus_, _Catopra_, and _Badis_ from South-Eastern
Asia, _Polycentropsis_ from West Africa, _Polycentrus_ and _Monocirrus_
from South America.
FAM. 9. PERCIDAE.—No subocular lamina of the suborbitals; entopterygoid
present. Anterior vertebrae without transverse processes; all or most of
the ribs inserted on the transverse processes when these are developed. Two
nostrils on each side. Mouth not or but feebly protractile. Gill-membranes
free from the isthmus; 6 to 8 branchiostegal rays; gills 4, a slit behind
the fourth; pseudobranchiae more or less developed, often rudimentary,
rarely absent. Lower pharyngeal bones separate. Soft portion of dorsal fin
not very much more developed than {659}the anal; latter with 1 or 2 spines
only. Ventrals with 1 spine and 5 soft rays.
Embrace about 90 species from the freshwaters of the Northern Hemisphere,
referable to 12 genera: _Perca_, _Lucioperca_, _Percina_, _Etheostoma_,
_Boleosoma_, _Ulocentra_, _Diplesium_, _Ammocrypta_, _Crystallaria_,
_Aspro_, _Percarina_, _Acerina_. The British representatives of this family
are the Perch (_Perca fluviatilis_) and the Pope (_Acerina cernua_). The
largest forms are the Pike-Perches or Sander (_Lucioperca_) of Eastern
Europe, Western Asia, and North America, which reach a length of 4 feet and
are highly valued for the table. The American Darters (_Etheostoma_ and
allies), on the other hand, are among the smallest fishes, but many are
remarkable for their brilliant coloration.
FAM. 10. ACROPOMATIDAE.—An ill-defined group of marine fishes, some
deep-sea, placed here provisionally as annectant between the Percidae and
the Serranidae (Pomatominae), differing from the latter in the absence of a
subocular shelf. Spinous dorsal short.
About 28 species, mostly from the Pacific Ocean, distributed in 9 genera:
_Propoma_, _Xenichthys_, _Xenocys_, _Synagrops_, _Malacichthys_,
_Acropoma_, _Melanostoma_, _Epigonus_ (_Telescops_), _Dinolestes_.
FAM. 11. SERRANIDAE.—Second suborbital with an internal lamina supporting
the globe of the eye; entopterygoid present; palate usually toothed.
Anterior vertebrae without transverse processes; all or most of the ribs
inserted on the transverse processes where these are developed. Two
nostrils on each side. Gill-membranes free from isthmus; 6 or 7
branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae
usually present. Lower pharyngeal bones usually separate. Soft portion of
dorsal usually not much more developed than the anal. Ventral fins with 1
spine and 5 (rarely 4) soft rays.
One of the largest families of fishes. The principal genera may be grouped
as follows:—
SERRANINAE.—_Percichthys, Percilia, Lateolabrax, Niphon, Morone,
Percalates, Ctenolates, Macquaria, Siniperca, Coreoperca, Acanthistius,
Trachypoma, Centrogenys, Polyprion, Oligorus, Stereolepis, Dinoperca,
Liopropoma, Aulacocephalus, Plectropoma, Epinephelus, Cromileptes,
Paranthias, Serranus, Centropristes, Chelidoperca, Gilbertia, Caesioperca,
Caprodon, Anthias, Callanthias, Pseudoplesiops, Plesiops, Trachinops_.
{660}GRAMMISTINAE.—_Grammistes, Rhypticus_. PRIACANTHINAE.—_Priacanthus,
Pseudopriacanthus_. CENTROPOMINAE.—_Lates, Psammoperca, Centropomus_.
POMATOMINAE.—_Pomatomus, Scombrops_. AMBASSINAE.—_Ambassis_.
CHILODIPTERINAE.—_Chilodipterus, Apogon_. LUTJANINAE.—_Lutjanus,
Glaucosoma, Therapon, Hoplopagrus, Etelis, Aprion, Aphareus, Odontonectes_.
CIRRHITINAE.—_Cirrhites, Cirrhitichthys_. PENTACEROTINAE.—_Pentaceros,
Pentaceropsis, Histiopterus_.
The number of recent species amounts to about 550, the great majority of
which are marine.
[Illustration: FIG. 403.—Sea Perch (_Serranus cabrilla_). × ⅓. (After
Cuvier and Valenciennes.)]
The earliest fossil form is _Prolates_, from the Upper Cretaceous of
France. _Morone_, _Serranus_, _Percichthys_, _Anthias_, and _Apogon_ are
represented in Eocene and later strata.
The range of the family is almost cosmopolitan; few of the Marine Perches
descend to any great depth. Some of the species of _Stereolepis_ and
_Epinephelus_ grow to a length of 6 to 10 feet. Several species of
_Serranus_ (_S. cabrilla_, _S. scriba_, _S. hepatus_), inhabiting the
Mediterranean and neighbouring parts of the Atlantic, and some _Lutjanus_
are normally hermaphrodite. Some _Chilodipterus_ and _Apogon_ are
remarkable for their nursing habits, the male sheltering the eggs in his
mouth.
The curious genera _Anomalops_ and _Photoblepharon_, of each of which a
single species is known from the Malay Archipelago and the South Pacific,
have been made the types of a family, ANOMALOPIDAE, the systematic position
of which remains uncertain since the osteological characters have not been
examined. {661}They are remarkable for the movable flap below the eye,
bearing a luminous organ, the nature of which has recently been
investigated by Max Weber.[714]
FAM. 12. PSEUDOCHROMIDIDAE.—Closely allied to the Serranidae, and connected
with them through _Plesiops_ and its allies. Dorsal and anal fins elongate
and formed mostly of articulated soft rays, the spines being feeble and
few.
A. With two lateral lines: _Pseudochromis_, _Cichlops_.
B. With a single lateral line: _Opisthognathus_, _Latilus_, _Caulolatilus_,
_Lopholatilus_, _Malacanthus_, _Bathymaster_, _Rathbunella_.
[Illustration: FIG. 404.—Tile-Fish (_Lopholatilus chamaeleonticeps_). × ¼.
(After Goode and Bean.)]
Marine, mostly of small size, inhabiting the Atlantic, Indian, and Pacific
Oceans. About 30 species. One of the largest and best-known members of this
family is the Tile-Fish (_Lopholatilus chamaeleonticeps_), living upon the
bottom of what is known as the Gulf Stream slope, off the coast of New
England, where it was first observed in 1879. Here the water is normally
comparatively warm, coming as it does from the superheated region of the
Gulf of Mexico. During a series of unusually severe gales in 1882, this
mass of water was pushed aside, as it were, and replaced by colder water.
As a result, millions and millions of these fishes were killed, and their
dead bodies literally covered the surface of the sea for hundreds of square
miles. It was feared that the Tile-Fish was exterminated; this was not so,
however, and the fish has reappeared in tolerable abundance within the last
few years.
FAM. 13. CEPOLIDAE.—Agree in essential characters with the preceding, but
body band-like with very numerous vertebrae {662}(15 + 54), and very
elongate dorsal and anal fins formed of soft rays, of which all except the
first three dorsal and the first anal are articulated and branched.
Although these fishes have hitherto been placed near the Blenniidae, the
Gobiidae, or the Trachypteridae, they are nothing but extremely elongate
Perches, and they stand in the same relation to the Serranidae as the
Trichiuridae to the Carangidae and Scombridae. They hardly deserve to rank
as a family distinct from the Pseudochromididae.
[Illustration: FIG. 405.—_Cepola rubescens_. × ½. (After Cuvier and
Valenciennes.)]
Two genera, _Cepola_ and _Acanthocepola_, with 10 species, from the
Mediterranean and North-Eastern Atlantic, the Indian Ocean, and the Western
Pacific. The Band-Fish (_Cepola rubescens_), which is common in the
Mediterranean, is sometimes found on the British coasts; it grows to a foot
and a half in length, and is remarkable for its bright red colour.
FAM. 14. HOPLOGNATHIDAE.—Characters of Serranidae, but teeth fused to form
a beak as in _Tetrodon_; palate toothless.
_Hoplognathus_, with 4 species, from the Pacific Ocean.
FAM. 15. SILLAGINIDAE.—As in Serranidae, but soft dorsal and anal much
elongate, as in Pseudochromididae, from which the Sillaginidae differ in
the separate spinous dorsal. Palate toothed. Connecting the Serranidae and
the Sciaenidae.
Small Marine Fishes from the Indian and Pacific Oceans, ascending rivers. A
single genus, _Sillago_, with about 10 species.
{663}FAM. 16. SCIAENIDAE.—Also closely related to the Serranidae. Dorsal
fin with a short spinous and a long soft portion; anal much shorter than
the latter. Palate usually toothless.
A large family of about 150 species, mostly marine. Principal genera:
_Arripis, Sciaena, Corvina, Otolithus, Ancylodon, Nebris, Larimus,
Pogonias, Haplonotus, Umbrina, Eques_.
Many of these fishes reach a large size, and the flesh of nearly all is
esteemed. The Meagre (_Sciaena aquila_) is sometimes taken on our coast.
The Drum (_Pogonias chromis_) so called from the sounds which it produces,
in common with many other Sciaenids, is remarkable for having the lower
pharyngeal bones united, as is also the case in the North American
freshwater genus _Haplonotus_. The air-bladder is usually large and
complicated, provided with more or less numerous appendages.
FAM. 17. GERRIDAE.—Agree in the character of the vertebral column with the
Serranidae, but differ in the absence of a subocular shelf; the very
protractile mouth usually descends when protruded and the praemaxillary
emits an upward lateral process; palate toothless; lower pharyngeal bones
usually large and more or less completely coalesced.
About 60 species of carnivorous, mostly small, fishes, from the tropical
seas, referable to 3 genera: _Gerres, Equula, Gazza_.
FAM. 18. LACTARIIDAE.—Intermediate between Serranidae and Trichodontidae.
No subocular shelf; palate toothed; branchiostegal rays 7; scales small,
cycloid, deciduous; spinous dorsal short; anal longer than the soft dorsal;
scapula with two foramina.
_Lactarius delicatulus_, from the coasts of Southern Asia.
FAM. 19. TRICHODONTIDAE.—Agree in the character of the vertebral column
with the Serranidae, but have no subocular shelf; body naked, and anal much
longer than the soft dorsal; palate toothless; only 5 branchiostegal rays.
Two genera, each with a single species, from the North Pacific, _Trichodon_
and _Arctoscopus_, bearing some resemblance to the Trachinidae, with which
they have usually been associated.
FAM. 20. LATRIDIDAE.—Marine Fishes intermediate between the Serranidae and
the Haplodactylidae, agreeing with the former in the extent of the anal
fin, which is nearly as long as the soft dorsal, and with the latter in the
absence of a subocular shelf and the posterior position of the ventrals. A
single genus, _Latris_, with 3 or 4 species, from the coasts of Australia
and New Zealand.
{664}FAM. 21. HAPLODACTYLIDAE.—No subocular shelf; entopterygoid present;
palate usually toothless. Vertebrae with transverse processes from the
third or fourth; all the ribs attached to the transverse processes when
these are present; anterior epipleurals strong. Two nostrils on each side.
Gill-membranes free from the isthmus; 5 or 6 branchiostegal rays; gills 4,
a slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones
separate. Soft portion of the dorsal fin much more developed than the anal.
Ventral fins with 1 spine and 5 soft rays, inserted far back behind the
pectorals, the lower rays of which are simple and more or less thickened.
This family embraces the genera _Haplodactylus, Chilodactylus, Chironemus_,
and _Threpterius_, with some 30 species from the seas of the Southern
Hemisphere and Japan. They feed chiefly on crustaceans, molluscs, and other
invertebrates living among sea-weed.
FAM. 22. PRISTIPOMATIDAE.—No subocular shelf; entopterygoid present; palate
toothless. Vertebrae with transverse processes from the third; all the ribs
attached to the transverse processes. Two nostrils on each side.
Gill-membranes free from isthmus; 5 to 7 branchiostegal rays; gills 4, a
slit behind the fourth; pseudobranchiae present. Lower pharyngeal bones
separate. Ventral fins with 1 spine and 5 soft rays.
_Pristipoma, Haemulon, Diagramma_, and _Pentapus_, distributed over all the
tropical and subtropical seas, a few entering fresh waters. About 130
species are known.
FAM. 23. SPARIDAE.—Second suborbital with an internal lamina supporting the
globe of the eye; entopterygoid present; palate usually toothless; teeth
often either cutting in front, or molar-like on the sides. Vertebrae with
transverse processes from the second or third; all the ribs attached to the
transverse processes. Two nostrils on each side. Gill-membranes free from
isthmus; 5 to 7 branchiostegal rays; gills 4, a slit behind the fourth;
pseudobranchiae present. Lower pharyngeal bones separate. Soft portion of
dorsal fin not much more developed than the anal. Ventral fins with 1 spine
and 5 soft rays.
The Sea-Breams embrace some 200 species, distributed over the coasts of
nearly the whole world. Some are herbivorous, but the majority are
carnivorous.
Principal genera: _Scolopsis, Dentex, Synagris, Caesio, Maena, Oblata,
Melambaphes, Girella, Doydixodon, Cantharus, Box_, {665}_Crenidens,
Pachymetopon, Dipterodon, Sargus, Charax, Lethrinus, Sphaerodon, Sparus,
Pagrus, Pagellus_.
Abundantly represented in Eocene and Miocene beds by remains of _Sargus,
Sparus, Pagrus, Pagellus_, and by the extinct genera _Ctenodentex,
Sparnodus_, and _Trigonodon_. Some species grow to a length of three feet,
such as the "Sheep's-Head" of North America, one of the best salt-water
fishes of the United States, and the "Schnapper" (_Sparus unicolor_), of
Australia, also much esteemed. Some of the Atlantic and Mediterranean
species of _Box, Sargus, Charax, Sparus_, and _Pagellus_ are known to be
normally, or at least very frequently, hermaphrodite.
[Illustration: FIG. 406.—Gilt-head Sea-Bream (_Pagrus auratus_). A, its
dentition. (After Cuvier and Valenciennes.)]
FAM. 24. MULLIDAE.—The "Red Mullets" are very nearly related to the
Sparidae, with which they agree in the structure of the vertebral column
and the presence of a subocular shelf. They differ in the very weak
dentition, the presence of a pair of hyoid barbels, the reduced number (4)
of branchiostegal rays, and the double perforation of the scapula. Two
short dorsal fins, remote from each other, the anterior with weak spines.
Small marine and brackish-water fishes, feeding on animalcules and
decomposing matter; inhabitants of nearly all the tropical seas and
extending to Northern Europe. About 50 species are known, referred to 5
genera: _Upeneoides, Upeneichthys, Mullus, Mulloides_, and _Upeneus_.
{666}The British species are _Mullus barbatus_ and _M. surmuletus_,
remarkable for their beautiful pink or red colour, and much valued on the
market, although no longer held in the high estimation for which they were
noted by the Romans.
[Illustration: FIG. 407.—Scapular arch of _Mullus surmuletus_. _cl_,
Clavicle; _cor_, coracoid; _pt_, pterygials; _ptcl_, post-clavicle; _pte_,
post-temporal; _sc_, scapula; _scl_, supra-scapula.]
FAM. 25. SCORPIDIDAE.—Second suborbital with an internal lamina supporting
the globe of the eye; entopterygoid present; palate toothed. Ribs sessile,
behind the parapophyses when these are present. Two nostrils on each side.
Gill-membranes free from isthmus; 7 branchiostegal rays; gills 4, a slit
behind the fourth; pseudobranchiae present. Lower pharyngeal bones
separate. Ventral fins, if present, with 1 spine and 5 soft rays.
This family embraces 12 species from the coasts of Africa, Southern Asia,
Australia, and New Zealand, referable to 5 genera: _Scorpis_,
_Atypichthys_, _Atyposoma_, _Henoplosus_, _Psettus_. The fish here figured
(_Psettus sebae_, Fig. 408) is remarkable for the excessive depth of the
body, which is greater than in any other species.
_Aipichthys_, one of the few Acanthopterygian types known to have existed
in the Cretaceous period, appears to belong to the family Scorpididae as
here defined, and not to the Carangidae.
FAM. 26. CAPROIDAE.—Characters of Scorpididae, but supratemporal completely
ankylosed to the skull.
The Boar-Fish (_Capros aper_) of the Atlantic and Mediterranean is
occasionally found on our southern coasts, and is highly remarkable for the
hair-like bristles with which its scales are {667}covered, an extreme
exaggeration of the "Ctenoid" type. The mouth is very protractile, and the
vertebrae are only 22 or 23 in number. _Antigonia_, with a single species
found at remote points in the Atlantic, Pacific, and Indian Oceans, is
probably allied to _Capros_, with which it is believed to be connected
through the fossil genus _Proantigonia_, from the Upper Miocene of Croatia.
[Illustration: FIG. 408.—_Psettus sebae_, from the West Coast of Africa, ×
½.]
FAM. 27. CHAETODONTIDAE.—Closely allied to and evidently derived from the
more generalised types of the Scorpididae, differing in the attachment of
the gill-membranes to the isthmus. Post-temporal more or less firmly united
with the skull, sometimes indistinctly bifurcate. Mouth small; palate
toothless; soft portions of vertical fins usually covered with scales; ribs
usually strong and blade-like; body short and deep.
A large group of about 200 marine carnivorous fishes from the tropics,
mostly of small size, remarkable for their singular forms and markings and
brilliant coloration. They are particularly abundant about volcanic rocks
and coral reefs.
{668}An Atlantic species of _Ephippus_ (_E. faber_) is extremely
remarkable, when adult, for an enormously enlarged globular bony mass on
the back of the head, formed by hypertrophy of the frontal and
supraoccipital bones.
Principal genera: _Ephippus, Parapsettus, Scatophagus, Chaetodon, Chelmo,
Heniochus, Holacanthus, Pomacanthus, Platax_.
_Chaetodon, Holacanthus, Pomacanthus, Scatophagus, Ephippus_, and _Platax_
were represented in the Eocene of Europe.
FAM. 28. DREPANIDAE.—The genus _Drepane_, with a single species from the
Indian Ocean, is very closely related to the Chaetodontidae, but it lacks
the subocular shelf, and it is distinguished externally by the very
elongate, falciform pectoral fin.
FAM. 29. ACANTHURIDAE.—A more or less developed subocular shelf;
entopterygoid present. Mouth very small, not or but slightly protractile,
the maxillary more or less firmly attached or ankylosed to the
praemaxillary; teeth conical, bristle-like, or incisor-like. Palate
toothless. Vertebrae 22 or 23, the praecaudals with strong transverse
processes commencing from the first; ribs and epipleurals inserted on the
transverse processes. Post-temporal not distinctly forked, ankylosed to the
skull. Two nostrils on each side. Gill-membranes broadly attached to the
isthmus; 4 or 5 branchiostegal rays; gills 4, a slit behind the fourth;
pseudobranchiae present. Lower pharyngeal bones separate. Body covered with
minute, often rough scales. Dorsal and anal fins elongate, with more or
less strong spines. Ventrals with 1 spine and 2 to 5 soft rays.
A family of about 80 species, mostly herbivorous, from the tropical seas,
referred to 6 genera: _Zanclus, Ctenochaetus, Acanthurus, Colocopus,
Prionurus, Naseus_. They form a connecting link between the Chaetodontidae
and the Plectognathi.
Remains from the Eocene of Europe have been referred to _Zanclus,
Acanthurus_, and _Naseus_, and to the extinct genera _Aulorhamphus_ and
_Apostasis_.
FAM. 30. TEUTHIDIDAE.—No subocular shelf; entopterygoid present. Mouth very
small, beak-like, not protractile, with incisor-like teeth; maxillary
ankylosed to the praemaxillary. Palate toothless. Two nostrils on each
side. Gill-membranes broadly attached to the isthmus; 5 branchiostegal
rays; gills 4, a slit behind the fourth; pseudobranchiae present. Lower
pharyngeal bones separate. Supratemporal forked. Vertebrae {669}23, with
sessile ribs and no parapophyses, the epipleurals inserted on the ribs.
Body covered with very small scales. Vertical fins elongate, with strong
spines, 6 or 7 in the anal. Ventrals with 2 spines and 3 soft rays between
them.
A single recent genus, _Teuthis_, with about 30 species, herbivorous fishes
from the Indian and Western Pacific Oceans. According to Bottard[715] the
sting from the spines of these fishes is much dreaded. _Archaeoteuthis_,
from the Oligocene of Switzerland.
FAM. 31. OSPHROMENIDAE.—Second suborbital with a more or less developed
internal lamina; entopterygoid present; palate toothed. Most of the
praecaudal vertebrae with transverse processes, to which the ribs are
attached. Two nostrils on each side. Gill-membranes attached to isthmus; 4
to 6 branchiostegal rays; gills 4, a slit behind the fourth;
pseudobranchiae absent. Lower pharyngeal bones separate. Vertical fins very
variable in extent, the spines sometimes very numerous, sometimes absent.
Ventral fins with not more than 5 soft rays, sometimes reduced to a
filamentous ray. A superbranchial respiratory organ, situated in a cavity
above the gills.
Freshwater fishes having much in common with the Anabantidae, and likewise
confined to South-Eastern Asia and Africa. Only 22 species are known,
referable to 7 genera: _Helostoma_, _Polyacanthus_, _Osphromenus_,
_Trichogaster_, _Luciocephalus_, _Betta_, and _Micracanthus_. The latter,
the only African representative of the family (one species from the Ogowe),
hardly differs from the Malay genus _Betta_. Most of the Osphromenidae are
notable as aquarium fishes. The largest species, the Gourami (_Osphromenus
olfax_), growing to a length of 2 feet, from the Malay Archipelago, is one
of the best flavoured fishes of the Far East and has been acclimatised in
India, the Guianas, and Mauritius. A domesticated variety of the Chinese
_Polyacanthus opercularis_, known as _Macropodus viridi-auratus_,
remarkable for the beauty of its form and colour, readily breeds in our
aquariums. Like the Gourami, the male constructs a nest of air-bubbles,
strengthened by a buccal secretion, and watches over the eggs and young.
The little _Betta pugnax_, from South-Eastern Asia, derives its name from
its excitable nature, which causes specimens to be kept by the Siamese in
glass vessels where they engage in fights, special breeds being cultivated
for the purpose. According to Cantor, {670}the Siamese in 1840 were as
infatuated with the combats of these fishes as the Malays are with their
cock-fights, and the licence to exhibit them was farmed, bringing in a
considerable annual revenue to the king.
FAM. 32. EMBIOTOCIDAE.—Second suborbital with an internal lamina supporting
the globe of the eye; entopterygoid present; palate toothless. Ribs
sessile, above and behind the parapophyses, where these are present. Two
nostrils on each side. Gill-membranes free from isthmus; 5 or 6
branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae
present. Lower pharyngeals united, with conical or pavement-like teeth.
Anal fin, with three spines. Ventral fins with 1 spine and 5 soft rays.
[Illustration: FIG. 409.—_Ditrema temminckii_, from Japan. × ⅓. (After
Jordan.)]
Small or moderate-sized fishes inhabiting California and Japan, mostly
marine, one species, however, inhabiting fresh waters, whilst another
descends to a great depth. They feed mostly on crustaceans, but one genus
(_Abcona_) is herbivorous. The name "Surf-Fishes," by which they are
generally known, refers to the fact that most species are found in the surf
along sandy beaches. All are viviparous in the strictest sense of the term,
the young remaining for a long time closely packed in a sac-like
enlargement of the oviduct analogous to a uterus; they are of relatively
large size at birth, and quite similar in form to the parent, whilst at an
earlier period they differ in having the vertical fins much more elevated.
Twenty-four species are known.[716] Principal genera: _Hysterocarpus_,
_Abcona_, _Cymatogaster_, _Embiotoca_, _Ditrema_.
FAM. 33. CICHLIDAE.—No subocular shelf; entopterygoid {671}present; palate
toothless; lower pharyngeal bones more or less completely united, with
median suture. Vertebrae with parapophyses from the third; ribs most
frequently sessile or subsessile. A single nostril on each side.
Gill-membranes free from isthmus; 5 or 6 branchiostegal rays; gills 4, a
slit behind the fourth; pseudobranchiae absent. Dorsal fin, with numerous
spines; anal with 3 spines or more. Ventral fins with 1 spine and 5 soft
rays.
[Illustration: FIG. 410.—_Tilapia dardennii_, from Lake Tanganyika. ⅓ nat.
size.]
Fresh or brackish-water fishes, variable in form and dentition, some
carnivorous, others chiefly herbivorous. In some American forms (_Cichla_)
the males and females differ during the spawning season, the male
developing a hump on the top of the head, which disappears afterwards. The
eggs and young are cared for by the parents; either the male or the female,
according to the species, sheltering them in the mouth or pharynx.[717]
These fishes, often designated as "Chromides," a name which properly
pertains to members of the following family, inhabit Africa, Madagascar,
Syria, India and Ceylon, and Central and South America, from Texas to
Uruguay. About 45 genera are distinguished, based mostly on the number of
anal spines and the dentition, which for variety of types is comparable to
that of the Characinidae. Of these 45 genera, 30 are African. 150 species
are known from Africa (with Syria and Madagascar), 140 from America, and 3
from India and Ceylon.[718] Principal genera—African: _Lamprologus_,
_Hemichromis_, _Paratilapia_, _Xenotilapia_, _Tropheus_, _Tilapia_,
{672}_Asprotilapia_, _Eretmodus_, _Plecodus_, _Pseudetroplus_. American:
_Acara_, _Heros_, _Hygrogonus_, _Cichla_, _Crenicichla_, _Chaetobranchus_,
_Geophagus_, _Symphysodon_, _Pterophyllum_. Indian: _Etroplus_.
No part of the world surpasses Lake Tanganyika in variety of generic and
specific types of Cichlidae, the fish-fauna of this great lake being in
great majority made up of members of this family.
[Illustration: FIG. 411.—Distribution of the Cichlidae.]
_Priscacara_, from the Eocene of North America, is the only extinct genus
which can be referred to this family.
FAM. 34. POMACENTRIDAE.—A subocular shelf; entopterygoid present; palate
toothless; teeth conical or incisor-like; lower pharyngeals completely
united into one bone. Vertebrae with transverse processes from the fourth
or fifth; ribs inserted on the transverse processes, when these are
present. A single nostril on each side. Gill-membranes free from the
isthmus; 5 to 7 branchiostegal rays; gills 3½; pseudobranchiae present.
Dorsal fin elongate, with numerous strong spines; anal with 2 spines only.
Ventral fins with 1 spine and 5 soft rays.
Small fishes of the tropical and warm seas, resembling the Chaetodontidae
in form and mode of life, likewise usually of brilliant coloration; in
structural characters intermediate between the Cichlidae and the Labridae.
They feed chiefly on small marine animals, but the species with
incisor-like teeth are entirely or mainly herbivorous. Over 150 species are
known.
Principal genera: _Heliastes_, _Azurina_, _Amphiprion_, _Premnas_,
_Dascyllus_, _Pomacentrus_, _Glyphidodon_, _Microspathodon_.
{673}The family is supposed to be represented in the Upper Eocene and Lower
Miocene of Italy by the extinct genus _Odonteus_.
FAM. 35. LABRIDAE.—No subocular shelf; entopterygoid present; palate
toothless; anterior teeth of the jaws usually strong and canine-like,
lateral teeth often soldered at the base; lower pharyngeals completely
united into one bone, with conical or tubercular teeth. Vertebrae with
transverse processes from the third; all the ribs attached to the
transverse processes. Lips thick. Two nostrils on each side. Gill-membranes
free or joined to the narrow isthmus; 5 or 6 branchiostegal rays; gills
three and a half; pseudobranchiae present. Dorsal fin elongate, with
numerous, usually slender, spines. Ventral fins with 1 spine and 5 soft
rays.
The "Wrasses" form a large family of mostly brilliantly coloured marine
fishes, inhabiting all the tropical and temperate coasts. Their regime is
partially herbivorous, partially carnivorous. About 400 species are known.
Principal genera: _Labrus_, _Tautoga_, _Malacopterus_, _Ctenolabrus_,
_Chaerops_, _Platychaerops_, _Heterochaerops_, _Labrichthys_, _Cossyphus_,
_Cirrhilabrus_, _Chilinus_, _Epibulus_, _Anampses_, _Platyglossus_,
_Novacula_, _Julis_, _Gomphosus_, _Chilio_, _Coris_.
[Illustration: FIG. 412.—Upper and lower pharyngeal bones of _Labrus
maculatus_.]
Some of the members of this family have been observed to build nests for
the protection of their eggs and young. These nests, in the European
_Labrus_, are made of seaweeds, zoophytes, corals, broken shells, etc., and
are the work of both the male and the female.[719] It is also in this
family that sleep was first observed in fishes, and this has been fully
verified by Möbius[720] {674}on _Labrus rupestris_ in an aquarium, the fish
seeking a sleeping place at night and laying itself down to rest on one
side.
As first pointed out by Günther, the Labridae found in temperate regions
have a higher number (30 to 41) of vertebrae than those inhabiting the
tropics (23 to 29), a rule which applies more or less to other families of
Acanthopterygians. Remains of _Labrus_ and _Julis_ occur in the Eocene and
Miocene of Europe. An allied fossil genus, _Labrodon_, is represented by
numerous species in Tertiary beds of Europe and North America. _Phyllodus_,
_Egertonia_, _Platylaemus_, and _Pseudosphaerodon_, from the Eocene and
Miocene, are referred, with doubt, to this family.
FAM. 36. SCARIDAE.—Closely allied to the preceding, with which they have
usually been united, but differing in the more or less coalescent teeth,
forming often a parrot-like beak, the lower pharyngeals united into a
concave or spoon-shaped bone with flat, tessellated teeth; and in the
development of transverse processes from the first vertebra. Vertebrae 24
or 25.
[Illustration: FIG. 413.—Upper and lower pharyngeal bones of _Scarus
strongylocephalus_. (After Jordan and Evermann.)]
Curious, mostly brilliantly-coloured fishes of the tropical seas and the
Mediterranean, especially abundant about coral-reefs. "Parrot-Wrasses" feed
mostly on vegetable matter, corals, and on hard-shelled Mollusca, for
crushing which their dentition is well adapted. The largest reach a length
of 4 feet. Some are much valued as food, whilst others are reputed
poisonous. About 110 species are known, referable to 8 genera:
_Cryptotomus_ (_Calliodon_), _Calotomus_, _Sparisoma_, _Scarus_,
_Pseudoscarus_, _Odax_, _Coridodax_, _Siphonognathus_. The latter is very
aberrant in shape, the head and body resembling those of a Pipe-Fish.
_Scarus_ is reported from the Eocene and Miocene of Europe.
{675}DIVISION II.—SCOMBRIFORMES.
No bony stay for the praeopercle. Spinous dorsal, if distinct, formed of
short or feeble, slender spines. Epipleurals usually attached to the centra
when ribs are sessile, or to the parapophyses of the vertebrae, rarely to
the ribs. Pectoral arch similar to that of the Perciformes, but pterygials
sometimes more abbreviated. Ventral fins thoracic. Caudal fin, if well
developed, with very numerous rays deeply forked at the base.
[Illustration: FIG. 414.—Caudal fin of _Sarda orientalis_. _h.s_, Hypural
spine.]
Although bound by natural ties, the series of families that cluster round
the Mackerel offer so many modifications of structure that it is almost
impossible to draw up a diagnosis differentiating every one of its members
from the Perciformes, with which they are closely connected, and from which
they hardly deserve to be separated. Even after removing many genera which
have been united with them by my predecessors, and which will now be found
scattered among various groups of the system, no good definition of the
Scombriformes can be given. The Mackerel and Horse-Mackerel are taken as
the pattern-forms around which more or less aberrant types are located,
types yet not so aberrant as to be traced back to these familiar forms
through a number of intermediate grades. As {676}regards external features,
it may be stated that the dorsal and anal spines, if present, are either
weak and slender, or, if strong, short and detached; the caudal peduncle is
constricted, and the caudal fin, if well developed, is usually deeply
forked, and with the forked bases of the very numerous rays much longer
than in most of the Perciformes, embracing at least a considerable portion
of the expanded hypural bones, a character by which the Chaetodontidae,
Acanthuridae, and several extinct types which have been placed with the
Carangidae are at once excluded. All are carnivorous and marine, and many
are pelagic and of very wide distribution. No praetertiary members of this
division, as here defined, have yet been found.
SYNOPSIS OF THE FAMILIES
I. Praemaxillaries more or less protractile, not beak-like; scales small
or absent, sometimes with enlarged lateral scutes; spinous dorsal
fin short or replaced by a series of isolated spines; anal usually
with one or two spines detached from the rest of the fin.
Praecaudal vertebrae with transverse processes behind which the ribs
are attached 1. _Carangidae_.
Praecaudal vertebrae without well-developed parapophyses, ribs and
epipleurals inserted close together on the centra
2. _Rhachicentridae_.
II. Praemaxillaries not protractile; scales usually small or absent;
body more or less elongate; dorsal fin elongate, single or divided,
without free spines; no free anal spines.
A. Pseudobranchiae present.
Vertebrae without transverse processes; soft dorsal fin longer than
the spinous; pectoral fins high up the sides 3. _Scombridae_.
Vertebrae without transverse processes; soft dorsal fin shorter than
the spinous, if the latter be distinct; pectoral fins low down
the sides 4. _Trichiuridae_.
Vertebrae without transverse processes; snout produced into a spear
5. _Histiophoridae_.
Vertebrae with transverse processes bearing the ribs; snout produced
into a sword; no ventrals 6. _Xiphiidae_.
Vertebrae without transverse processes; gill-membranes attached to
isthmus; dorsal and anal fins formed of unarticulated, widely set
rays; dentition very feeble 7. _Luvaridae_.
B. Pseudobranchiae absent; no well-developed transverse processes to
the praecaudal vertebrae; the ribs and the epipleurals inserted
close together on the centra; snout short and very deep
8. _Coryphaenidae_.
III. Praemaxillaries not protractile, or if slightly protractile,
scales large; dorsal and anal fins elongate, without distinct spinous
division; most of the praecaudal vertebrae with strong haemapophyses,
to which the ribs are attached 9. _Bramidae_.
{677}FAM. 1. CARANGIDAE.—Praemaxillaries more or less protractile.
Vertebrae 24 to 26; ribs behind the parapophyses; epipleurals on the
parapophyses, rarely on the ribs.[721] Body covered with small scales, or
naked, often with enlarged scutes on each side of the body or of the tail;
dorsal spines few, or slender or rudimentary; a more or less developed
spine adnate to the soft portion of the anal, often preceded by a pair of
spines separated from the rest of the fin. Pseudobranchiae usually present.
Inhabitants of the seas of the temperate and tropical regions, many of the
species having a very wide range. About 150 species are known.
Principal recent genera: _Caranx_, _Chloroscombrus_, _Selene_, _Mene_,
_Apolectus_, _Nematistius_, _Seriola_, _Seriolichthys_, _Naucrates_,
_Trachynotus_, _Zalocys_, _Lichia_, _Paropsis_, _Chorinemus_. Species of
_Caranx_, _Mene_, and _Seriola_ have been described from the Eocene and
Miocene of Europe, in which occur also the fossil genera named
_Vomeropsis_, _Archaeus_, _Carangopsis_, _Carangodes_, _Ductor_, and
_Semiophorus_.
The family is represented on our coasts by the common Horse-Mackerel,
_Caranx trachurus_. The young of this species keep together in small bands
in the neighbourhood of medusae, under which they seek shelter when
disturbed. The Pilot-Fish, _Naucrates ductor_, is a truly pelagic fish of
wide distribution, which occasionally appears on our coasts, accompanying
large sharks and ships. Much has been written on the marvellous habits of
this little fish, which is said to lead the shark like a pilot, directing
it to its food, in exchange for which services the pilot enjoys protection
from the fear which the proximity of its formidable companion inspires to
its enemies among other carnivorous fishes, and an abundance of food from
the shark's excrements.[722]
FAM. 2. RHACHICENTRIDAE.—Praemaxillaries slightly protractile. Vertebrae 25
(11 + 14), without well-developed parapophyses; ribs and epipleurals
inserted close together on the centra. Body covered with very small scales;
a series of short isolated dorsal spines; soft dorsal and anal long;
pectorals {678}inserted low down. A single genus, _Rhachicentrum_
(_Elacate_), with a single species from the coasts of the tropical and
warmer parts of the Atlantic and of the Indian Ocean.
FAM. 3. SCOMBRIDAE.—Praemaxillaries large, not protractile, beak-like.
Vertebrae 30 to 50, without transverse processes, but some of the hinder
praecaudals with haemal arches; ribs inserted on the centra or on the
haemal arches when these are present; epipleurals all on the centra. Scales
cycloid and usually very small (except in _Gastrochisma_), sometimes
absent. A spinous dorsal fin formed of slender spines, folding into a
sheath; soft dorsal longer and broken up into finlets, similar to the anal;
pectoral inserted high up the sides. Hypural bones completely embraced by
the forked bases of the caudal rays. Pseudobranchiae present.
[Illustration: FIG. 415.—Tunny (_Thunnus thynnus_). (After Cuvier and
Valenciennes.) × ⅛.]
About 50 species, referred to the following genera:—_Scomber_, _Auxis_,
_Thunnus_, _Sarda_, _Cybium_, _Acanthocybium_, _Gastrochisma_
(_Lepidothynnus_). Numerous fossil representatives in Tertiary beds,
belonging to _Scomber_, _Auxis_, _Thunnus_, _Cybium_, and to the extinct
genera _Eothynnus_, _Isurichthys_, _Palimphyes_, _Scombrinus_,
_Sphyraenodus_, _Scombramphodon_.
These fishes, elegant in form and often in colour, are among the swiftest
of the inhabitants of the sea. Some are migratory, like the Mackerel
(_Scomber scombrus_) of the North Atlantic, whilst others are remarkable
for their wide distribution. The Tunny (_Thunnus thynnus_), for instance,
the largest member of the family, reaching a length of 10 feet, inhabits
the Atlantic, Pacific, and Indian Oceans, extending as far north as the
British {679}seas, Newfoundland, California, and Japan. It supplies
important fisheries in France and Italy. The Tunnies are the only fish
known to be warm-blooded.
FAM. 4. TRICHIURIDAE.—Praemaxillaries not protractile. Vertebrae 32 to 160,
without transverse processes; ribs sessile, on the centra or on the haemal
arches when these are present; epipleurals, if well developed, on the
centra. Scales very small or absent. Spinous portion of dorsal fin much
longer than the soft, the spines more or less feeble. Pectoral fins
inserted low down the sides. Pseudobranchiae present.
The members of this family show a great variation in the shape of the body,
which, although always strongly compressed, is not unlike that of a
Mackerel in the more normal types, such as _Thyrsites_ and _Ruvettus_,
whilst, through a chain of genera, it generally assumes an extremely
elongate form; concurrently with this elongation of the body, the dorsal
fin loses its differentiation into two portions, the ventrals become
reduced and disappear, as in the Scabbard- or Frost-Fish (_Lepidopus
caudatus_), while the caudal fin decreases in size, loses its fork-shape,
and is finally lost in _Trichiurus_, in which the body is ribbon-shaped and
tapers to a point.
About 25 species are known, pelagic and widely distributed, many descending
to great depths.
Principal living genera: _Ruvettus_, _Thyrsites_, _Epinnula_, _Nesiarchus_,
_Nealotus_, _Promethichthys_, _Dicrotus_, _Gempylus_, _Aphanopus_,
_Lepidopus_, _Euoxymetopon_, _Benthodesmus_, _Eupleurogrammus_,
_Trichiurus_.
Remains of several species referred to _Thyrsites_, _Lepidopus_, and to the
extinct genera _Thyrsitocephalus_, _Hemithyrsites_, and _Trichiurichthys_,
have been found in the Oligocene and Miocene of Europe.
FAM. 5. HISTIOPHORIDAE.—Praemaxillaries not protractile; snout produced
into a spear-shaped rostrum; a praedentary bone; teeth minute. Body
elongate, covered with small or rudimentary scales. Vertebrae 24 or 25,
without transverse processes; ribs sessile; no epipleurals. One or two
dorsal fins, without a distinctly spinous portion. Pectoral fin low down
the side. Pseudobranchiae present.
The Sail-Fishes are large oceanic fishes, endowed with great strength and
swiftness, occurring in the tropical and sub-tropical {680}seas. Four or
five species are distinguishable, and are referable to two genera:
_Histiophorus_, with a single dorsal fin and 2 or 3 ventral rays, and
_Tetrapturus_, with the dorsal divided into two parts and a single ventral
ray.[723]
Fossil Histiophoridae are known from the Eocene and later beds in Europe
and America. Dr. A. S. Woodward observes that the known fossils are too
imperfect to be referred with certainty to their respective genera. Most of
them probably belong to _Histiophorus_, but at least one genus from the
Eocene (_Xiphiorhynchus_) appears to be well distinguished.
[Illustration: FIG. 416.—_Tetrapturus belone_, from the Mediterranean.
(After Cuvier and Valenciennes.) × ⅒.]
The imperfectly known extinct family PALAEORHYNCHIDAE, from the seas of the
Eocene, Oligocene, and Miocene periods, with the genera _Palaeorhynchus_
and _Hemirhynchus_, is probably closely related to the Histiophoridae. The
vertebrae number 50 to 60, and the ribs completely encircle the body. In
_Palaeorhynchus_ both jaws are equally produced into an ensiform weapon.
_Blochius_, from the Eocene, with diamond-shaped, slightly overlapping bony
scutes on the body, is perhaps also to be placed near this family.
{681}FAM. 6. XIPHIIDAE.—Differs from the preceding in the absence of
praedentary bone, and in the vertebrae (26 in number), which in the
praecaudal region are provided with short but well-developed transverse
processes, to which the short ribs are attached. Ventral fins absent, the
pectorals being inserted in the place usually occupied by them. Adult
without teeth or scales.
A single species, the Sword-Fish (_Xiphias gladius_), of nearly world-wide
distribution, occurring occasionally on the coasts of Great Britain and
Ireland.
FAM. 7. LUVARIDAE.—Mouth small, praemaxillaries not protractile, with very
feeble dentition. Gill-membranes attached to the isthmus. Vertebrae 23,
without transverse processes; ribs blade-like, inserted on the centra; no
epipleurals. Body rough, with minute scales. Dorsal and anal fins elongate,
formed of unarticulated, widely set rays. Pectoral fins inserted rather low
down; ventrals much reduced, the two halves of the pelvis fused into a
single bone. Supraclavicle fused with the forked post-temporal. Hypural
bones completely embraced by the forked bases of the caudal rays.
Pseudobranchiae present.
_Luvarus imperialis_, a rare pelagic fish from the Atlantic, Mediterranean,
and Pacific, growing to a length of 6 feet, and occasionally captured on
our coasts, is the only representative of this family. Very little is known
of the habits of this strange fish. The excessive length of the intestines
and the feeble dentition point to its feeding partly on vegetable matter,
partly on minute animals; the circumstances under which certain specimens
were captured tend to indicate that they follow up streams of pelagic life
such as engage the attention of the Basking Shark, of similar distribution.
FAM. 8. CORYPHAENIDAE.—Praemaxillaries small, not protractile. Vertebrae 30
to 33, without transverse processes; ribs and epipleurals attached close
together on the centra. Body elongate; scales small, cycloid or elongate
lanceolate. Dorsal and anal fins much elongate, without distinct spines.
Pectoral fins inserted rather low down the side. Pseudobranchiae absent.
The "Dolphins" (_Coryphaena_), of which only two species can be
distinguished, are large pelagic fishes, of carnivorous habits, pursuing
the Flying-Fish. They grow to a length of 6 feet, and their flesh is much
valued. Their deep head, with short snout, {682}and their long posteriorly
attenuate body ending in a large forked caudal fin, give them a peculiar
appearance.
FAM. 9. BRAMIDAE.—Praemaxillaries small, not or but feebly protractile;
maxillaries large, scaly. Vertebrae 42 to 47, the praecaudal without
transverse processes, but mostly with hæmal arches to which the ribs are
attached, the epipleurals being inserted on the centra. Body deep; scales
moderate or large, strongly imbricate, with processes which, in certain
parts at least, serve to connect the rows of scales. Dorsal and anal
elongate, some or all of the rays simple, but not forming true spines.
Pectoral inserted rather low down the side, freely movable upwards and
downwards. Pseudobranchiae present.
Pelagic fishes, often descending to great depths. About 12 species are
known,[724] referable to 6 genera: _Brama_, _Taractes_, _Pterycombus_,
_Pteraclis_, _Bentenia_, and _Steinegeria_. _Taractes_, often confounded
with _Brama_, differs from it not only in the larger, keeled scales, but
also in the protractile mouth and in the much greater development of most
of the ribs, which form curved lamellae of great width.[725] _Pteraclis_ is
very remarkable for the enormous, sail-like dorsal and anal fins.
DIVISION III.—ZEORHOMBI.
Aberrant, strongly compressed Perciformes, with very short praecaudal
region, modified much as in the Flat-Fishes, culminating in asymmetrical
forms, and characterised by the combination of an increased number (7 to 9)
of ventral rays, with absence of hypural spine (by which the Berycidae are
excluded), _or_ by asymmetry of the skull in the forms in which the spine
of the ventral fin has been lost.
Among the symmetrical forms, the existing Zeidae agree with the Berycidae
in having more than five soft rays to the ventral fins, and are probably
derived, together with the Eocene Amphistiidae, from some common ancestral
group still to be discovered in Cretaceous beds. These Zeidae have much in
common with the Pleuronectidae,[726] and might be regarded as {683}forming
part of the family out of which the latter have sprung, were it not that
they have lost the last half-gill. _Amphistium_ is probably more nearly
related to the Pleuronectidae, which may have been directly derived from
the family of which it is as yet the only known representative.[727]
This division embraces three families only:—
A distinct spinous dorsal fin; anal spines detached from the soft
portion; a ventral spine; gills three and a half, four slits between
them 1. _Zeidae_.
Dorsal and anal spines few, continuous with the soft rays; a ventral
spine 2. _Amphistiidae_.†
No spines; cranium twisted in front, with the two orbits on one side;
gills 4, a slit behind the fourth 3. _Pleuronectidae_.
† Extinct.
FAM. 1. ZEIDAE.—No subocular shelf; praemaxillaries strongly protractile.
Gill-membranes free from isthmus; 7 or 8 branchiostegal rays; gills 3½;
pseudobranchiae well developed. Lower pharyngeal bones separated. Vertebrae
30 to 46, the anterior with sessile ribs, the posterior praecaudals with
long neural spines bent forwards and with transverse processes directed
downwards, forming haemal arches and bearing the ribs at their extremity;
epipleurals much reduced or absent; hypural large, without the basal spine
or knob present in most Perciformes and all Scombriformes and Percesoces,
bearing fewer than 20 rays. Dorsal and anal fins elongate, the former with
a distinct spinous portion, the latter with 1 to 4 spines detached from the
soft portion. Pectoral fin supported by 4 pterygials, of which 3 are in
contact with the perforated scapular bone; post-temporal forked and solidly
attached to the skull. Ventral fin with 1 spine and 6 to 8 soft rays.
Scales small or minute, sometimes hard and rough and firmly joined in
vertical series; bony plates may be present along the base of the vertical
fins. Air-bladder present.
Twelve species are known from the Atlantic and Pacific Oceans, referable to
5 genera: _Grammicolepis_, _Cyttus_, _Cyttopsis_, _Zenion_, and _Zeus_.
_Oreosoma_ was founded on a young form of a fish allied to _Cyttus_.
Remains of _Zeus_ occur in the Oligocene, and _Cyttoides_, from the same
period, has been compared with _Cyttus_.
The well-known John Dory (_Zeus faber_) is much valued for the table.
{684}FAM. 2. AMPHISTIIDAE.—The only known representative of this family,
the Upper Eocene _Amphistium paradoxum_, originally described as a
_Pleuronectes_, has much in common with the Zeidae, from which it differs
in the smaller number of vertebrae (10 + 14), and in the dorsal and anal
spines being more reduced, adnate and continuous with the series of soft
rays; the scales are more normal and imbricate; ventral fins with 1 spine
and 8 soft rays. This fish appears to realise in every respect the
prototype of the Pleuronectidae before they had assumed the asymmetry which
characterises them as a group.
[Illustration: FIG. 417.—Restoration of _Amphistium paradoxum_. × ½.]
FAM. 3. PLEURONECTIDAE.—Head asymmetrical, the skull twisted in front, with
the two orbits on one side in the adult; the side of the body bearing the
eyes and turned upwards in life being coloured, the other side colourless
and blind. Mouth more or less protractile. Gills 4, a slit behind the
fourth; pseudobranchiae present. Lower pharyngeal bones usually separated,
rarely imperfectly united. Vertebrae 24 in the most generalised form
(_Psettodes_), varying from 28 to 65 in others, the praecaudals mostly with
more or less developed transverse processes, which may be directed
downwards and become converted into haemal arches; ribs and epipleurals
present. Caudal fin, if well developed, supported by a large hypural
usually without basal spine or knob. Dorsal and anal fins much elongate,
without spines, the former often extending on the head. Paired fins often
reduced, {685}sometimes absent; if fully developed and normally formed, the
bones of the pectoral and pelvic girdles as in the Zeidae. Ventral fins
usually with 5 to 7 soft rays.
Scales usually imbricate, cycloid or ctenoid; rarely absent; bony tubercles
sometimes present. Air-bladder absent.
Most species, and even genera, are either sinistral or dextral, but this is
inconstant in some, including the most generalised genus, _Psettodes_. The
very young are transparent and symmetrical, with an eye on each side, and
swim in a vertical position like other Fishes. These larval forms have been
described as distinct genera, under the names of _Peloria_, _Bibronia_,
_Charybdia_, etc. As they grow, the eye of one side moves by degrees to the
other side, where it becomes the upper eye. If at that age the dorsal fin
does not extend to the frontal region, the migrating eye simply moves over
the line of the profile, temporarily assuming the position which it
preserves in _Psettodes_, _Atheresthes_, and _Platysomatichthys_; in other
genera, the dorsal fin has already extended to the snout before the
migration takes place, and the eye, passing between the frontal bone and
the tissues supporting the fin, appears to pass from side to side through
the head, as was believed by some of the earlier observers.[728]
Flat-fishes are a large group of some 500 species, mostly marine, a few
species related to the Soles being confined to the fresh waters of South
America and the Malay Archipelago. They range from the Arctic Circle to the
southern coasts of the Southern Hemisphere; many occur at great depths
(_Citharichthys dinoceros_ down to 955 fathoms). Well-preserved remains
referred to _Psetta_ occur in the Upper Eocene, and a species of _Solea_ is
known from the Lower Miocene.
{686}[Illustration: FIG. 418.—Outlines of various Pleuronectids, showing
differences of form. A, _Psettodes erumei_; B, _Pleuronectes platessa_; C,
_Psetta maxima_; D, _Solea vulgaris_; E, _Cynoglossus lingua_.]
{687}A satisfactory classification of the Pleuronectidae is still a
desideratum, and cannot be attempted until the osteology of the very
numerous forms has been thoroughly studied. Even the division into two
principal groups, regarded by some recent authors as families,
Pleuronectidae and Soleidae, is based on characters which the examination
of a large number of generic types shows not to be constant. Thus the
former have been defined as having the praeopercular margin distinct
externally, the snout not projecting beyond the mouth, the nostrils of the
two sides on the coloured side or those of the blind side high up near the
dorsal line of the head; the latter as having the praeoperculum hidden
under the skin, the snout projecting more or less beyond the mouth, and the
nasal organ of the blind side similarly situated to that of the eyed side.
However, the genera _Aphoristia_ and _Peltorhamphus_, and others among the
Soles, show exceptions to this definition.[729]
Principal genera: _Psettodes_, _Atheresthes_, _Platysomatichthys_,
_Hippoglossus_, _Hippoglossoides_, _Hippoglossina_, _Poecilopsetta_,
_Chascanopsetta_, _Paralichthys_, _Pleuronectes_, _Glyptocephalus_,
_Citharus_, _Rhomboidichthys_, _Psetta_ (_Rhombus_), _Arnoglossus_,
_Zeugopterus_, _Lepidorhombus_, _Ammotretis_, _Rhombosolea_, _Solea_,
_Achirus_, _Achiropsis_, _Soleotalpa_, _Synaptura_, _Ammopleurops_,
_Aphoristia_, _Cynoglossus_, _Symphurus_ (_Plagusia_).
The following are the principal British representatives which are valued as
food:—The Halibut (_Hippoglossus vulgaris_), by far the largest of all
Flat-Fishes, growing to a length of 10 feet or more; the Long Rough Dab
(_Hippoglossoides limandoides_); the Plaice (_Pleuronectes platessa_); the
Flounder (_P. flesus_), which ascends streams; the Dab (_P. limanda_); the
Smear Dab, often called Lemon Sole (_Glyptocephalus microcephalus_); the
Witch (_G. cynoglossus_); the Megrim or Whiff (_Lepidorhombus megastoma_);
the Turbot (_Psetta maxima_); the Brill (_P. laevis_); and the Sole (_Solea
vulgaris_).
DIVISION IV.—KURTIFORMES.
No bony stay for the praeopercle. Dorsal spines feeble, few. Scapula
absent, the coracoid supporting four small pterygials. Ventral fins
thoracic.
FAM. 1. KURTIDAE.—The genus _Kurtus_, with a single species, from the
Indian and Pacific Oceans, forms an isolated, very {688}aberrant group. The
strongly compressed body is covered with minute, rudimentary scales; the
dorsal is short, with few, graduated spines, and the anal much elongate,
with 2 small spines; the ventrals are formed of 1 spine and 5 soft rays.
The vertebral column consists of 24 vertebrae; the ribs of the third and
fourth are free and slender, whilst the following are immovably fixed
between rings formed by the ossification of the outer membrane of the
elongate air-bladder in a manner unique among fishes. The skull is peculiar
for its very strong, denticulate, occipital crest, which ends posteriorly
in a curved spine bent forwards; the suborbitals are slender and do not
emit a subocular lamina. _Kurtus indicus_ does not exceed a length of 5
inches.
[Illustration: FIG. 419.—Skeleton of _Kurtus indicus_.]
DIVISION V.—GOBIIFORMES.
No bony stay for the praeoperculum. Basis cranii simple. Spinous dorsal, if
present, formed of few, flexible rays. None of the epipleural bones
attached to the centra of the vertebrae in the praecaudal region. Scapula
and coracoid more or less reduced or even vestigial; pterygials large, 4 or
5 in number, forming together a thin plate which is in contact with or
narrowly separated from the clavicle; one or two of the pterygials in
contact with the coracoid. Ventral fins thoracic.
The Gobiidae, which alone constitute this division, are not very remote
from the Perciformes, and may have evolved out of a type not very different
from the Percidae.
{689}FAM. 1. GOBIIDAE.—Suborbital arch ligamentous or absent.
Gill-membranes more or less broadly attached to isthmus; 4 to 6
branchiostegal rays; gills 4, a slit behind the fourth; pseudobranchiae
often present. All or most of the praecaudal vertebrae with transverse
processes bearing the ribs, to which epipleurals are attached.
Post-temporal forked, as in normal Perciformes. Ventral fins with 1 feeble
spine and 4 or 5 branched rays, often united to form a sucking disk, a
transverse fold of skin at their base completing the cup.
Head usually more or less depressed, body varying from short and stout to
elongate and eel-shaped, but never with a very high number of vertebrae,
these varying from 24 to 37 (10-14 + 13-24); scales cycloid or ctenoid, or
absent; no lateral line; mouth moderate or large, dentition various; soft
dorsal and anal fins nearly equally developed, varying from very short to
very elongate; usually a large anal papilla.
[Illustration: FIG. 420.—_Gobius ruthensparri_. Nat. size. (From Holt and
Byrne, _Report Fisheries Ireland for 1901_.)]
A large family of some 600 species, the great majority marine, mostly
carnivorous and of small size. The largest form (_Eleotris marmorata_, from
the rivers of Siam, Borneo, and Sumatra) grows to nearly 3 feet, whilst the
smallest (_Mistichthys luzonensis_, from the Philippines) measures only 12
to 14 millimetres, and is believed to be the smallest known Vertebrate.
Gobiids occur in all the seas outside the Arctic and Antarctic circles, and
they have representatives in the fresh waters of all parts of the world.
The genera are numerous but difficult of definition. The following are the
principal: _Eleotris_, _Oxymetopon_, _Vireosa_, _Rhyacichthys_, _Gobius_,
_Crystallogobius_, _Aphia_, _Gobiosoma_, _Gobiodon_, _Benthophilus_,
_Typhlogobius_, _Luciogobius_, _Sicydium_, _Lentipes_, _Periophthalmus_,
_Boleophthalmus_, _Amblyopus_, _Trypauchen_, {690}_Trypauchenichthys_.
_Oxuderces_, which has been made the type of a distinct family, appears to
differ from _Trypauchen_ only in the absence of ventral fins. Fossils
referred to _Gobius_ have been described from the Upper Eocene and Miocene
of Europe, but there is no satisfactory evidence that they really belong to
this family.
_Gobius_, of which several species are of common occurrence on our shores,
have attracted special interest from their habits during the much prolonged
breeding season. The male, usually more brilliantly coloured than the
female, mounts guard over the eggs, which are either simply fixed by the
female to the under surface of stones or weeds, or in a sort of nest built
and kept in constant repair by him. This nest is usually made of a shell of
_Cardium_, _Patella_, _Haliotis_, etc., or of the carapace of a crab, with
the convexity turned upwards and covered with sand; the sand underneath is
hollowed out, and a round opening at the side, coated by a mucus secreted
by the skin of the male fish, gives access to the interior; the eggs, which
are elongate and pyriform, are stuck to the inner surface of the shell
forming the roof.[730] A curious British form is _Aphia pellucida_, two
inches long which, from its transparent and almost colourless body, has
long been erroneously supposed to be the fry of some larger fish. Among
exotic forms, mention should be made of the Blind Goby (_Typhlogobius
californiensis_), two inches long, uniform light pink, scaleless, with the
eyes very small, reduced to mere vestiges, covered by skin, and functional
only in the young, living like a slug under rocks between tide marks on the
coast of California;[731] and to the Walking-Fish or Jumping-Fish
(_Periophthalmus_), of which various species are found in great abundance
on the mud-flats at the mouths of rivers in tropical Africa, Asia, and
North-West Australia, skipping about by means of the muscular, scaly base
of their pectoral fins, with the head raised and bearing a pair of strongly
projecting versatile eyes close together.[732]
{691}DIVISION VI.—DISCOCEPHALI.
Highly aberrant Acanthopterygians with the anterior dorsal fin modified
into a suctorial, transversely laminated oval disk[733] on the head, the
skull being very much flattened and with simple basis cranii. The pectoral
rays are inserted on the small, perforate, scapula and on four
hour-glass-shaped pterygials, three of which are in contact with the
coracoid. Ventrals thoracic.
FAM. 1. ECHENEIDIDAE.—Maxillary slender, adnate to the upper surface of the
praemaxillary; suborbital arch slender. Pectoral fin inserted high up;
supraclavicle much reduced; ventral fin with one spine and five soft rays.
Body elongate and covered with small scales; soft dorsal and anal fins
elongate and opposed to each other. All the praecaudal vertebrae with very
strong parapophyses, the anterior with diapophyses as well; ribs and
epipleurals nearly equally developed, both inserted at the extremity of the
parapophyses.
[Illustration: FIG. 421.—_Remora brachyptera_. (After Goode.) × ½.]
In spite of a superficial external resemblance to the genus _Elacate_, the
Sucking-Fish bear certainly no affinity to that genus nor to other
Scombriformes, as first observed by Gill. They are probably derived from
Perciformes, but from which family it is impossible to suggest. Three
genera may be distinguished: _Opisthomyzon_, from the Upper Eocene of
Switzerland, with a very small suctorial disk and 23 or 24 vertebrae;
_Echeneis_, with large disk and 30 vertebrae; and _Remora_, distinguished
from the second by a shorter body with only 27 vertebrae. These remarkable
fishes, of which about 10 species are distinguished, are distributed all
over the tropical and warm seas, and exceptionally carried as far north as
the south coast of England. They feed on other fishes, and attach
themselves by means of their cephalic {692}sucker to boats or to sharks,
turtles, cetaceans, and other large swift-swimming animals. On the East
Coast of Africa they are employed by the natives for catching turtles, to
the carapace of which they stick with extraordinary tenacity, being held by
a line attached to a metal ring round the caudal peduncle.[734] The largest
Sucking-fish grows to a length of three feet.
DIVISION VII.—SCLEROPAREI.
Second suborbital bone more or less produced towards or ankylosed with the
praeoperculum ("suborbital stay").[735] Ventral fins thoracic.
[Illustration: FIG. 422.—Skull of _Ophiodon elongatus_. _sor_, Suborbital
stay.]
The "Cheek-armoured Acanthopterygians," "Joues cuirassées" of Cuvier, after
the exclusion of the Sticklebacks, form a perfectly natural association,
evidently derived from the Serranidae, with which the more generalised
forms have much in common. From the Perch-like genus, _Sebastes_, a
continuous series can be traced towards the Triglidae, especially through
such forms as _Apistus_, _Minous_, and _Choridactylus_, in which one or
more of the lower pectoral rays are detached from the rest of the fin.
Through the Comephoridae the Scorpaenidae are connected with the Cottidae,
whilst the latter merge insensibly into the still more aberrant
Cyclopteridae. These conclusions, which are apparent enough from a mere
comparison of the external characters, become fortified by a study of the
skeletons. The passage between the various groups here accepted as families
is so complete that no {693}serious objection could be raised to their
union in one great family with a number of minor divisions.
[Illustration: FIG. 423.—Left pectoral arch of A, _Sebastes percoides_; B,
_Scorpaenichthys marmoratus_; C, _Dactylopterus volitans_. _cl_, Clavicle;
_cor_, coracoid; _pcl_, post-clavicle; _p.r_, pectoral rays; _ptr_,
pterygials; _ptte_, post-temporal; _sc_, scapula; _scl_, supraclavicle.]
The character from which the Scleroparei derive their name is subject to
many modifications. The second suborbital (the third if the praeorbital be
regarded as the first) may be merely enlarged and prolonged over the cheek
towards the praeoperculum (_Sebastes_, _Anoplopoma_), or firmly ankylosed
to the latter (_Scorpaena_, _Platycephalus_), or form part of the external
armature of the head (_Trigla_, _Dactylopterus_). The structure of the base
of the pectoral fin appears to afford important characters for the
definition of the families, as first pointed out by Gill; these
{694}characters have, however, not yet been tested on a sufficient number
of the very numerous forms grouped under Cottidae, some of which I have
already transferred to the Comephoridae.
SYNOPSIS OF THE FAMILIES.
I. Head not completely cuirassed.
A. Ventral fins not widely separated; none of the pectoral pterygials
in contact with the clavicle.
Two nostrils on each side; basis cranii double; gill-membranes free
from isthmus 1. _Scorpaenidae_.
A single nostril on each side; basis cranii double; gill-membranes
free from isthmus 2. _Hexagrammidae_.
Two nostrils on each side; basis cranii simple; gill-membranes free
or narrowly attached to isthmus 3. _Comephoridae_.
Two nostrils on each side; basis cranii simple; gill-opening
narrow, above base of pectoral 4. _Rhamphocottidae_.
B. Ventral fins, if present, not widely separated; one or several of
the pterygials in contact with the clavicle.
Ventral fins distinct; gill-clefts wide 5. _Cottidae_.
Ventral fins united into a sucking disk; gill-opening narrow, above
base of pectoral 6. _Cyclopteridae_.
C. Ventral fins widely separated; none of the pterygials in contact
with the clavicle.
Ventral fins behind base of pectorals; praecaudal vertebrae without
transverse processes 7. _Platycephalidae_.
Ventral fins a little in front of base of pectorals; praecaudal
vertebrae with transverse processes 8. _Hoplichthyidae_.
II. Head completely cuirassed.
Ventral fins narrowly separated; no pectoral appendages; pterygials
short and broad 9. _Agonidae_.
Ventral fins widely separated; 2 or 3 lowermost rays of pectoral
fin detached as feelers; pterygials short and broad
10. _Triglidae_.
Ventral fins narrowly separated; pectoral fin divided into two
portions; pterygials elongate 11. _Dactylopteridae_.
FAM. 1. SCORPAENIDAE.—Head not or but incompletely cuirassed, usually with
spines; basis cranii double; parietal bones often meeting on the median
line, over the supraoccipital; two nostrils on each side. Gill-membranes
free from isthmus; gills 3½ or 4; pseudobranchiae present. Vertebrae 24 to
37, the anterior praecaudals with sessile ribs bearing epipleurals, the
posterior with transverse processes, often directed downwards, or forming
haemal arches, bearing the rib and the epipleural. Post-temporal more or
less distinctly forked, more or less firmly ankylosed to the skull; scapula
and coracoid well developed, in contact with each other or separated by
cartilage; pectoral rays inserted {695}on the scapula and on 3 or 4 large,
hour-glass or anvil-shaped pterygials, two of which are in contact with the
coracoid. Ventral fins close together, with 1 spine and 3 to 5 soft rays.
Spinous dorsal strong, usually longer than the soft, sometimes extending on
the head; anal usually with 3 spines. Body covered with scales or naked.
A large family of carnivorous marine fishes, some descending to great
depths, of nearly world-wide distribution, represented by three extinct
genera (_Ampheristus_, _Histiocephalus_, _Scorpaenoides_) in the Eocene and
by several species of _Scorpaena_ in later formations. About 250 recent
species are known. Principal genera: _Sebastes_, _Setarches_, _Scorpaena_,
_Pterois_, _Apistus_, _Minous_, _Pelor_, _Choridactylus_, _Centropogon_,
_Gymnapistus_, _Amblyapistus_, _Pentaroge_, _Tetraroge_, _Gnathacanthus_
(_Holoxenus_), _Agriopus_, _Synancia_, _Polycaulus_.
[Illustration: FIG. 424.—_Scorpaena grandicornis_. (After Valenciennes.) ×
½.]
Great variety of form obtains in this family, from the Perch-like
_Sebastes_ to the extraordinary-shaped _Tetraroge_, _Pelor_, and
_Synancia_. Many of its members are excellent examples of mimetic
adaptation to the surrounding, resembling the rocks among which they live
and being covered with dermal appendages simulating weeds. An interesting
example of commensalism has been discovered by A. Alcock[736] in _Minous
inermis_, off the coasts of India, which, wherever found, is always more or
less incrusted with the {696}Gymnoblastic Hydroid _Stylactis minoi_. Many
of the _Sebastes_ and their allies are of large size and used as food; some
are viviparous, the young being produced in great numbers and very small in
size. _Scorpaena_, _Pterois_, _Pelor_, and _Synancia_ are dangerous for the
stings from their dorsal spines, which are provided with poison
glands.[737]
FAM. 2. HEXAGRAMMIDAE.—Head not cuirassed, without strong spines; basis
cranii double; a single nostril on each side. Gill-membranes free from
isthmus; gills 4; pseudobranchiae present. Vertebrae 42 to 57, most of the
praecaudals with transverse processes bearing the ribs and the epipleurals.
Post-temporal forked; scapula and coracoid well developed, in contact with
each other; pectoral rays inserted on the scapula and 4 anvil-shaped
pterygials, 2 of which are in contact with the coracoid. Ventral fins close
together, more or less behind the pectorals, with 1 spine and 5 soft rays.
Spinous dorsal of usually rather feeble rays, nearly as long as or longer
than the soft; anal elongate, with or without spines. Body covered with
small scales.
Carnivorous fishes, mostly of large size, from the rocky coasts of the
North Pacific. Some are highly valued as food. Twelve species, referable to
6 genera: _Hexagrammus_, _Pleurogrammus_, _Agrammus_, _Ophiodon_,
_Zaniolepis_, _Oxylebius_. _Hexagrammus_ and _Pleurogrammus_ are remarkable
in having 4 or 5 lateral lines on each side.
FAM. 3. COMEPHORIDAE.—Head not cuirassed, without spines; basis cranii
simple; two nostrils on each side. Gill-membranes free or narrowly attached
to isthmus; gills 4; pseudobranchiae present or absent. Vertebrae 42 to 64,
some or most of the praecaudals with transverse processes bearing the ribs
and the epipleurals. Post-temporal forked; scapula and coracoid well
developed, in contact with each other or separated by cartilage; pectoral
rays inserted on the scapula and on 4 anvil-shaped or plate-like
pterygials, 2 of which are in contact with the coracoid. Ventral fins, if
present, close together, with 1 spine and 3 to 5 soft rays. Spinous dorsal
of rather feeble rays, as long as or shorter than the soft; anal spines
feeble or absent. Body covered with small scales or naked.
{697}Four genera, each with a single species: _Anoplopoma_
(_Scombrocottus_), from the North Pacific from Unalaska to California;
_Triglopsis_, from deep water in Lakes Michigan and Ontario;
_Cottocomephorus_, from Lake Baikal, and _Comephorus_ from the greatest
depths of that lake. As in many bathybial forms, _Comephorus_ is colourless
and provided with very large eyes; ventral fins are absent and the skeleton
is very thin and papery. As a result of this condition, the second
suborbital is not produced over the cheek, a unique exception to the main
characteristic of this division; but no doubt can be entertained as to the
propriety of referring it to the neighbourhood of _Anoplopoma_, since the
recently discovered _Cottocomephorus_ may be regarded as a connecting link
between the two genera. _Comephorus_ is viviparous, and dies after
parturition.[738] Jordan regards _Triglopsis_ as a relic of a former Arctic
marine fauna.
FAM. 4. RHAMPHOCOTTIDAE.—Head incompletely cuirassed, with spines; basis
cranii simple; two nostrils on each side. Gill-opening narrow, above the
base of the pectoral; gills 3½. Vertebrae 24. Post-temporal short and flat,
ankylosed to the skull; scapula and coracoid well developed, separated by
cartilage; pectoral rays inserted on the scapula and on 4 plate-like
pterygials, 2 of which are in contact with the coracoid. Ventral fins close
together, behind the pectorals, with a rudimentary spine and 3 soft rays.
Spinous dorsal shorter than the soft; no anal spines. Body densely covered
with small prickly scales.
_Rhamphocottus richardsonii_, a small fish 3 inches in length, from the
north-west coast of North America, is the only representative of this
family.
FAM. 5. COTTIDAE.—Head not or but incompletely cuirassed, usually with
spines; basis cranii simple; parietal bones often meeting on the median
line; two nostrils on each side. Gill-membranes free or attached to
isthmus; gills 3½ or 4; pseudobranchiae usually present. Vertebrae 24 to
50, the anterior praecaudals with sessile ribs, the posterior with
transverse processes, often directed downwards, or forming haemal arches,
bearing ribs and epipleurals. Post-temporal more or less distinctly forked;
scapula and coracoid separated from each other {698}by the intervention of
the plate-like pterygials, of which one, two, or three are in contact with
the clavicle; the coracoid more or less reduced. Ventral fins close
together, with 1 spine and 2 to 5 soft rays (absent in _Ereunias_). Spinous
dorsal usually shorter than the soft, sometimes quite indistinct; anal
without spines. Body naked, partially scaly, or with prickles or bony
plates.
Mostly small carnivorous fishes, the largest (_Scorpaenichthys_) growing to
about 3 feet. Some species inhabit fresh waters, but the majority are
marine, a few descending to great depths. Nearly all are from the northern
regions, but a genus allied to _Cottus_ (_Sclerocottus_) is from South
Georgia, in the Antarctic region. Fossil Cottidae are known from the Upper
Eocene and Miocene (_Eocottus_, _Lepidocottus_), and are distinguished from
the modern forms in the smaller number of vertebrae (24 or 26 instead of 30
to 50). At least 220 species are known. Principal genera: _Jordania_,
_Scorpaenichthys_, _Icelus_, _Triglops_, _Cottus_, _Cottunculus_,
_Blepsias_, _Pseudoblennius_, _Hemitripterus_, _Synchirus_, _Ascelichthys_,
_Psychrolutes_, _Ereunias_. The little freshwater "Miller's Thumb" (_Cottus
gobio_) and the larger marine "Bull-heads" (_C. bubalis_ and _C. scorpius_)
are the most familiar British representatives of this family. The eggs are
deposited on stones, weeds, or other submerged objects, or in a sort of
nest, and are guarded by the male, which in most species is distinguished
by a large genital papilla; this, in some forms, acts as an intromittent
organ.
FAM. 6. CYCLOPTERIDAE.—Very closely related to the preceding, with which
they are connected through _Psychrolutes_, and it is even doubtful whether
they deserve to be separated from them. The only important distinctive
characters reside in the structure of the ventrals, which, if present
(absent in _Paraliparis_, a close ally of _Liparis_), are united to form a
sucking disk, and the small size of the gill-cleft. The body is short,
tumid, tadpole-like, naked or tubercular; the spinous dorsal, if present,
is short. Vertebrae 28 to 60, the skeleton feebly ossified.[739]
Sluggish fishes, feeding on small animals and plants, from the North
Atlantic and Pacific Oceans, and the Arctic and Antarctic seas, many
descending to great depths (1800 fathoms). About fifty species are
distinguished. Principal genera: _Cyclopterus_, _Cyclopterichthys_,
_Liparops_, _Liparis_, _Careproctus_, _Paraliparis_.
{699}The common Lump-Sucker of our coasts (_Cyclopterus lumpus_) is the
largest member of the group, growing to a length of 2 feet or more. The
male makes pits in the sand between stones, in which the female deposits
the eggs; he watches over the eggs and also over the young, which cling to
his body with their suckers. The "Sea-Snails" (_Liparis_), are represented
by two species on the British coasts.
[Illustration: FIG. 425.—_Cyclopterus lumpus_. × ⅓.]
FAM. 7. PLATYCEPHALIDAE.—Head not cuirassed, much depressed, with spines;
basis cranii simple; two nostrils on each side. Gill-membranes free; gills
4; pseudobranchiae present. Vertebrae 27; ribs all sessile, bearing the
epipleurals. Post-temporal forked; scapula and coracoid well developed, in
contact with each other; pectoral rays inserted on the scapula and on 4
short and broad pterygials, 2 of which are in contact with the coracoid.
Ventral fins widely separated, behind the pectorals, with 1 spine and 5
soft rays. Spinous dorsal shorter than the soft; anal without spines. Body
covered with small scales.
The single genus _Platycephalus_, with some 40 species, inhabits the coasts
of the Indian Ocean and the Western Pacific.
FAM. 8. HOPLICHTHYIDAE.—Head incompletely cuirassed, much depressed, with
spines; basis cranii simple; two nostrils on each side. Gill-membranes
attached to isthmus; gills 4; pseudobranchiae present. Vertebrae about 30,
the praecaudals with transverse processes. Post-temporal fused with the
skull; scapula and coracoid in contact with each other; pectoral rays
inserted on the scapula and on 3 plate-like pterygials. Ventral fins widely
separated, a little before the pectorals, with 1 spine and 5 soft
{700}rays. Spinous dorsal shorter than the soft; anal without spines. Back
and sides with bony, prickly plates.
_Hoplichthys_, with a single species from the coasts of Japan and China.
_Bembras_, with two species from the coasts of Japan, appears to be related
to it, but the skeleton is still unknown; it differs in having the body
covered with small scales and the gill-membranes free.
FAM. 9. AGONIDAE.—Head completely cuirassed, usually with spines; basis
cranii simple; two nostrils on each side. Gill-membranes free or attached
to isthmus; gills 3½; pseudobranchiae present. Vertebrae 35 to 50; ribs
sessile. Post-temporal fused with the skull; scapula and coracoid in
contact with each other, or separated by a cartilaginous space; pectoral
rays inserted on the scapula and 3 or 4 plate-like pterygials. Ventral fins
close together, with 1 spine and 2 soft rays. Spinous dorsal shorter than
the soft, or absent; anal without spines. Body covered with bony plates.
Small fishes, mostly from the coasts of the Northern Atlantic and Pacific,
extending into the Arctic Ocean; one species from the coast of Chili.
_Bathyagonus_ occurs in the North Atlantic between 350 and 477 fathoms.
About 40 species are known. Principal genera: _Agonus_, _Agonopsis_,
_Bathyagonus_, _Aspidophoroides_. The "Pogge," or Armed Bullhead (_Agonus
cataphractus_), is the only British species of this family.
FAM. 10. TRIGLIDAE.—Head completely cuirassed, with spines; basis cranii
double; parietal bones meeting on the median line; two nostrils on each
side. Gill-membranes free; gills 4; pseudobranchiae present. Vertebrae 25
to 40, the anterior praecaudals with sessile ribs, the posterior with
transverse processes. Post-temporal fused with the skull; scapula and
coracoid separated by a cartilaginous space; pectoral rays inserted on the
scapula and on 4 large plate-like pterygials, of which two are in contact
with the coracoid; 2 or 3 of the lower pectoral rays detached, forming
feelers. Ventral fins widely separated, with 1 spine and 5 soft rays.
Spinous dorsal shorter than the soft; anal without spines. Body covered
with scales or bony plates.
Marine fishes from all warm and temperate regions, some occurring in deep
water. They are remarkable for the {701}finger-like appendages of the
pectoral fins, which are employed to feel the ground in search of
crustaceans and other small animals on which they feed; also for the
grunting sounds which they utter by the contraction of the air-bladder.
About 50 species are known, referable to 4 genera: _Prionotus_, _Trigla_,
_Lepidotrigla_, _Peristedion_. Fossil remains referred to _Trigla_ have
been found in Miocene and later formations. British species are the Grey
Gurnard (_Trigla gurnardus_), the Red Gurnard (_T. cuculus_), the Tub or
Sapphirine Gurnard (_T. hirundo_), the Piper (_T. lyra_), the Long-finned
Gurnard (_T. obscura_), and the Streaked Gurnard (_T. lineata_).
[Illustration: FIG. 426.—_Dactylopterus volitans_. (After Gill.) ⅓ nat.
size.]
FAM. 11. DACTYLOPTERIDAE.—Head completely cuirassed; basis cranii simple;
parietal bones meeting on the median line; two nostrils on each side.
Gill-cleft broadly separated by scaly isthmus; gills 4; pseudobranchiae
present. Vertebrae 20-22 (8-9 + 12-13), the first very elongate and formed
by the fusion of three or four; ribs sessile, no transverse processes.
Post-temporal fused with the skull; no supraclavicle; scapula and coracoid
well developed, in contact with each other; pectoral rays divided into two
parts, inserted on the scapula and on 4 elongate pterygials, of which 3 are
in contact with the coracoid. Ventral fins close together, with 1 spine and
4 soft rays. Spinous dorsal shorter than the soft; anal without spines.
Body covered with hard, rough scales.
The "Flying Gurnards," of which four species are known, belonging to a
single genus (_Dactylopterus_), are inhabitants of the tropical and warm
parts of the Atlantic and the Indian Ocean and Archipelago. They are
remarkable, when adult, for the {702}wing-like portion of the pectoral
fins, by which they are able to move in the air like _Exocoetus_, but for
shorter distances, and, unlike them, the wings are moved rapidly, the mode
of flight resembling that of many forms of grasshoppers;[740] the young,
however, have comparatively short pectorals, and were formerly regarded as
belonging to a distinct genus (_Cephalacanthus_).
DIVISION VIII.—JUGULARES.
No bony stay for the praeoperculum. Ventral fins jugular or mental.
Gill-openings in front of the pectoral fin, the base of which is vertical
or subvertical.
In a recently published note[741] I have alluded to the group of
Physoclistous fishes for which I proposed to revive the old name Jugulares,
pointing out that some of the forms previously grouped together as
Trachinidae agree with the Gadidae, not only in the jugular position of the
ventral fins, but also in the condition of the scapula and coracoid. Mr.
Regan[742] has since been able to show that the Gadidae and Macruridae
possess certain characters in common by which they may be separated not
only from the other Jugulares, but even from the Acanthopterygians, and, as
mentioned above (p. 646), the Müllerian Sub-order Anacanthini may be
maintained, after excluding the Pleuronectidae. That the Blenniidae are
akin to _Lycodes_ and its allies has long been admitted, and authors who
have placed them in different divisions of their systems have had to
confess the difficulty of referring certain genera to the one family rather
than to the other. The fact that _Lycodes_ and many forms previously
associated with the Ophidiidae agree with the Macruridae and Gadidae in the
diphycercal vertebral column and in the absence of spines to the fins is
merely, it seems to me, the result of degradation; they probably form the
terminal group of a series in which the vertebral column was originally
homocercal and fin-spines were present, as is the case in most of the
Blenniidae and Trachinidae and their near allies. All these families may be
assumed to have evolved in several series, often on parallel lines, from
some group closely related to the {703}Berycidae; and the resemblance which
their terminal forms bear to the Anacanthini is, as pointed out by Regan,
probably to be ascribed to convergence, not to any close genetic affinity,
as hitherto believed by many authors.
[Illustration: FIG. 427.—Pectoral arch and pelvis (left side) of A,
_Trachinus draco_; B, _Percophis brasilianus_; _cl_, clavicle; _cor_,
coracoid; _pelv_, pelvis; _pt_, pterygials; _ptcl_, post-clavicle; _pte_,
post-temporal; _sc_, scapula; _scl_, supraclavicle.]
The character of the position of the scapular foramen, either in the
scapular bone or between it and the coracoid, which obtains in many genera
of this division as well as in most of the Anacanthini, has proved to be
unreliable even for the purpose of family definition; it is, however, of
assistance in determining the relation of certain obscure, degraded forms
placed by some authors with the Anacanthines, by others with the Blenniids.
SYNOPSIS OF THE FAMILIES.
I. Pectoral rays attached to the scapula and to a series of pterygials
of which only one or two are in contact with the scapula (see
Fig. 427); ventral fins jugular, with 1 spine and 4 or 5 soft rays;
anterior dorsal rays usually spinous or not articulated, often forming
a detached fin.
A. Epipleurals present.
1. Second suborbital produced inwards to support the eye-ball.
Ventrals close together; scales very small, cycloid, forming
oblique bands 1. _Trachinidae_.
Ventrals widely separated 2. _Percophiidae_.
2. No subocular shelf.
Ventrals widely separated; two nostrils on each side
3. _Leptoscopidae_.
Ventrals widely separated; a single nostril on each side
4. _Nototheniidae_.
Ventrals close together; scales very small, forming oblique bands;
head partly covered with bony plates 5. _Uranoscopidae_.
B. No epipleurals. {704}
Post-temporal forked, articulated to the skull; soft dorsal and anal
much elongate 6. _Trichodontidae_.
Post-temporal closely adnate to the skull; soft dorsal and anal
short (with only 7 to 10 rays) 7. _Callionymidae_.
Post-temporal simple, articulated to the skull; soft dorsal and anal
short; a ventral sucker 8. _Gobiesocidae_.
II. Pectoral rays all attached to the pterygials, of which two or three
are in contact with the scapula; ventral fins, if present, jugular or
mental, composed of 1 to 4 rays.
A. Ventrals jugular or absent.
Post-temporal distinctly forked; praecaudal vertebrae with
transverse processes; some or all of the dorsal rays spinous
or not articulated; caudal fin usually distinct 9. _Blenniidae_.
Post-temporal small and ankylosed to the skull; praecaudal
vertebrae without well-developed transverse processes; a very
short spinous dorsal; caudal fin distinct 10. _Batrachidae_.
Post-temporal distinctly forked; praecaudal vertebrae with haemal
arches; dorsal rays all spinous; caudal fin distinct
11. _Pholididae_.
Post-temporal distinctly forked; praecaudal vertebrae with
transverse processes; dorsal rays all articulated, or a few of
the posterior spinous; no distinct caudal fin 12. _Zoarcidae_.
Post-temporal forked, ankylosed to the skull; praecaudal vertebrae
with transverse processes; no spines; no distinct caudal fin
13. _Congrogadidae_
B. Ventrals mental (just behind the chin); no spines 14. _Ophidiidae_.
III. Pectoral rays attached to an undivided cartilaginous plate
representing the pterygials; ventral fins jugular, reduced to a
filament formed of two adnate rays; fins without spines
15. _Podatelidae_.
FAM. 1. TRACHINIDAE.—Second suborbital with an internal lamina, supporting
the globe of the eye; mouth large, protractile. Ribs and epipleurals nearly
equally developed, sessile; posterior praecaudal vertebrae with short
parapophyses. Gill-membranes free from isthmus; 6 branchiostegal rays;
gills 4, a slit behind the fourth; pseudobranchiae well developed. Scapula
and coracoid well developed, a foramen between them; pectoral rays attached
to the scapula and to three short and broad pterygials, two of which are in
contact with the coracoid. Ventral fins jugular, close together, with 1
spine and 5 soft rays. Body elongate, covered with small cycloid scales
forming oblique bands. A short spinous dorsal and a long soft dorsal and
anal. Vertebrae 35-43 (10-11 + 25-32). No air-bladder.
This family includes but one genus (_Trachinus_), the Weevers, with 4
species, occurring on the coasts of Europe, the {705}Mediterranean, and
West Africa north of the Equator. A fossil species has been described from
the Upper Miocene of Croatia. The two British species, _T. draco_ and _T.
vipera_, are well known for the painful wounds which they are able to
inflict through their sharp, grooved dorsal and opercular spines, which
convey a very active poisonous fluid secreted by small glands at their
base. As these fish like to bury themselves partially in the sands in
shallow water, people bathing occasionally tread on them with, as a rule,
at least violent pain as a result.[743] The flesh is not bad eating, and
great numbers of the larger species (_T. draco_), are brought to the Paris
market.
FAM. 2. PERCOPHIIDAE.—_Percophis_, with a single species from the coast of
Brazil, differs from the Trachinidae in the scapular fenestra being
situated entirely in the scapula, in the ventral fins being rather widely
separated at the base, and in the quincuncial disposition of the scales.
Vertebrae, 57 (22 + 35). _Bleekeria_ and _Embolichthys_, from the Indian
and Japanese seas, with the ventral fins rudimentary or absent, which have
been placed in the Ammodytidae, appear to be related to _Percophis_.
FAM. 3. LEPTOSCOPIDAE.—Differ from the preceding in the absence of a
subocular shelf. Scapular fenestra either in the scapula or between the
scapula and the coracoid. Mostly Marine Fishes, various in form, from the
tropics to the Antarctic circle, some occurring at great depths. About 25
species, referable to 7 genera: _Leptoscopus_, _Parapercis_, _Neopercis_,
_Pteropsaron_, _Bembrops_, _Pleuragramma_, _Chimarrhichthys_. The latter,
from New Zealand, is the only freshwater form of the family, and is
remarkably adapted for living in alpine torrents. _Pleuragramma
antarcticum_, brought home by the Southern Cross Expedition, comes from 78°
35´ S. lat., the farthest point at which fishes have yet been obtained in
the Antarctic region. _Macrius amissus_, from the Pacific Ocean at a depth
of 1000 fathoms, which, judging from a very imperfect description, probably
belongs to this family, measures 5 feet, and is the largest known deep-sea
Teleostean.
FAM. 4. NOTOTHENIIDAE.—Also closely allied to the Trachinidae. No subocular
shelf; a single nostril on each side; ventrals {706}widely separated;
pectoral arch usually as in the Trachinidae, but scapular fenestra
sometimes in the scapula (_Trematomus_). Body varying much in shape
according to the genera, the form sometimes suggestive of the Cottidae;
scales usually ctenoid, sometimes absent; anterior (spinous) dorsal
sometimes absent; lateral line often double, or even triple. Mostly from
the Southern seas and the Antarctic circle. About 40 species, referable to
19 genera, of which the following are the principal:—_Notothenia_,
_Trematomus_, _Chaenichthys_, _Champsocephalus_, _Cryodraco_,
_Acanthaphritis_, _Eleginops_, _Bovichthys_, _Gymnodraco_, _Gerlachia_,
_Bathydraco_, _Racovitzaia_, _Harpagifer_, _Draconetta_.
FAM. 5. URANOSCOPIDAE.—Agree with the Trachinidae in general structure, and
in the closely approximated ventrals. Scales very small, in oblique bands,
or absent. Pterygials much reduced, fused with the scapula and the
coracoid; scapular fenestra in the scapula. Parapophyses strongly developed
on the praecaudal vertebrae, with the ribs attached to their upper surface.
The head is very large, broad, partly covered with bony plates; cleft of
the mouth vertical; eyes on the upper surface of the head. Vertebrae 25 to
30 (12-14 + 13-16). Four genera: _Uranoscopus_, _Anema_, _Cathetostoma_,
_Ariscopus_, with 15 species, from the tropical seas, northwards to the
Mediterranean and Japan, southwards to South Australia and New Zealand.
FAM. 6. TRICHONOTIDAE.—Small elongate fishes very nearly related to the
Callionymidae, with which they agree in the arrangement of the bones at the
base of the pectoral fins and the absence of epipleurals; but post-temporal
more distinctly forked and detached from the skull, suborbital arch
ossified (without subocular shelf), gill-openings wide, a single long
dorsal fin, a long anal fin, and body covered with scales. Vertebrae 48-53.
Five marine species, referable to 3 genera: _Trichonotus_ and
_Taeniolabrus_ from the Indian Ocean, and _Hemerocoetes_ from New Zealand.
FAM. 7. CALLIONYMIDAE.—Suborbital arch ligamentous; entopterygoid absent;
basis cranii simple; mouth rather small, protractile. Vertebrae few (7 +
14), the last two much enlarged; most of the vertebrae with bifid neural
processes, simulating a "spina bifida"; first vertebra ribless,[744] second
to fourth {707}with sessile ribs and no transverse processes, fifth to
seventh with ribs inserted on short transverse processes; no epipleurals.
Post-temporal forked, but completely adnate to the skull; scapula separated
from the coracoid by a fenestra; pectoral rays attached to the scapula and
to three broad pterygials, all three in contact with the coracoid. Ventral
fins jugular, widely separated from each other, with 5 soft rays in
addition to a short spine. Gill-openings very narrow, generally reduced to
a foramen on the upper side of the operculum; 6 branchiostegal rays; gills
4, a slit behind the fourth; pseudobranchiae well developed. Body naked.
Two dorsal fins, the first composed of a few flexible spines; second dorsal
and anal rather short (7-10 rays).
Small marine fishes, referable to 2 genera; _Callionymus_, with about 45
species, nearly cosmopolitan, and _Vulsus_, with a single species from
Amboyna and Celebes. In the common British species, the Dragonet
(_Callionymus lyra_), the male acquires very marked secondary characters,
the snout becoming more elongate, the second dorsal fin much produced, and
the body ornamented with yellow and blue bands. The courtship and pairing
have been described by E. W. L. Holt,[745] who observes that this curious
fish offers the only instance of a definite sexual intercourse among
Teleosteans propagating by pelagic ova. In the Indian _C. carebares_ it is
the female that is the more brightly coloured.
FAM. 8. GOBIESOCIDAE.—Suborbital arch absent; entopterygoid absent; basis
cranii simple; mouth moderate, protractile. Vertebrae numerous, 27-31
(14-16 + 11-21), the first, if present, rudimentary,[746] the third and
following praecaudals with long parapophyses[747] bearing the ribs at their
extremity; no epipleurals. Post-temporal simple, articulated to the skull;
scapula with a foramen, coracoid much reduced; pectoral rays inserted on
the scapula and on four large pterygials, two of which are in contact with
the scapula; an adhesive ventral disk, simple or double, supported in front
by the clavicles, in the {708}middle and at the sides by the enlarged
pelvic bones and fins, and behind by the enlarged lamellar post-clavicles,
which are formed of two pieces. Ventral fins jugular, widely separated from
each other, formed of 1 short spine and 4 or 5 soft rays. Gill-openings
narrow; 5 or 6 branchiostegal rays; gills 3 or 3½; pseudobranchiae well
developed. Body naked. Dorsal and anal fins short, composed entirely of
soft branched rays.
First placed with the Acanthopterygians by J. Müller, notwithstanding the
absence of spinous rays in the vertical fins, and removed from the vicinity
of the Cyclopteridae by Günther, raised to the rank of a Sub-order
(Xenopteri) near the Anacanthini by Gill, the exact systematic position of
this curious type of Fishes has long been a matter of uncertainty. The
position of the ventral fins suggests, at first glance, affinity with the
Callionymidae, and a comparison of the skeletons of these two types has
convinced me that they are really related to each other, although both
highly modified in different directions.
[Illustration: FIG. 428.—_Sicyases sanguineus_, natural size. _a_, Anus;
_op_, opercle; _pf_, pectoral fin.]
The Cling-Fishes are curious small, carnivorous, Marine Fishes, usually
found between tide-marks among loose stones and shells, to which they
adhere firmly by means of the adhesive ventral disk. They can live a long
time out of water. About 50 species are known, from various parts of the
world, extending as far north as Scotland and Vancouver Island, and
southwards to {709}New Zealand. Three or four species, belonging to the
genus _Lepadogaster_, are known to occur on the British coasts. The
principal genera are _Gobiesox_, _Chorisochismus_, _Sicyases_, _Cotylis_,
_Lepadogaster_, _Trachelochismus_, _Diplocrepis_, _Crepidogaster_, and
_Leptopterygius_.[748]
FAM. 9. BLENNIIDAE.—Suborbitals often forming a more or less distinct
subocular shelf; mouth moderate or large, more or less protractile, often
bordered to a considerable extent by the maxillaries. Most of the
praecaudal vertebrae with strong transverse processes supporting the ribs,
which may bear epipleurals. Gill-membranes usually attached to isthmus; 6
or 7 branchiostegal rays; gills 4, a slit behind the fourth;
pseudobranchiae usually present. Post-temporal forked; scapula and coracoid
more or less developed, sometimes much reduced, the former pierced by a
foramen; pectoral rays attached to 4 or 5 hour-glass-shaped pterygials, one
or two of which are in contact with the coracoid. Ventral fins jugular,
with not more than 4 rays, or absent. Body more or less elongate, sometimes
Eel-shaped, naked or with small scales. Dorsal and anal fins elongate, the
former constituted entirely of spines, or anteriorly of spines or
non-articulated rays, and posteriorly of soft rays. Caudal fin usually
distinct, with expanded hypural.
A large family, mostly of small Marine Fishes, the arrangement of which
still offers great difficulties. Whether the aberrant genera _Cerdale_ and
_Ptilichthys_ deserve to be regarded as the types of distinct families
cannot be decided until the skeleton has been examined. The species number
about 350, from nearly all the seas, a few inhabiting fresh waters, and are
referred to numerous genera, of which the following are the
principal:—_Gadopsis_, _Enneanectes_, _Heterostichus_, _Acanthoclinus_,
_Clinus_, _Emmnion_, _Blennius_, _Chasmodes_, _Petroscirtes_, _Xiphasia_,
_Anarrhichas_, _Pataecus_, _Salarias_, _Ophioblennius_, _Anoplarchus_,
_Xiphistes_, _Opisthocentrus_, _Chaenopsis_, _Pholedichthys_, _Lumpenus_.
Remains of _Clinus_ and _Blennius_ have been described from the Miocene,
and the extinct genus _Pterygocephalus_, from the Upper Eocene, is regarded
as allied to _Clinus_.
The Blenniidae are mostly carnivorous, but a few are herbivorous; some are
viviparous (_Clinus_), others oviparous. Species {710}of _Blennius_ occur
in abundance on our coasts, and are among the most familiar tenants of
small rock-pools. Their habits have been admirably described by
Guitel.[749] The male makes a sort of nest, and defends the brood. Numerous
species of the genus _Salarias_ occur in the tropics; these little fish, as
their name implies, are remarkable for the long leaps they are able to
make. The largest of the Blenniids are the "Wolf-Fishes," often named
"Cat-Fishes" (_Anarrhichas_), of which one species (_A. lupus_) is common
on the British coasts, growing to a length of 5 or 6 feet. "It is
impossible," says Brown Goode, "to imagine a more voracious-looking animal
than the Sea Cat-Fish, with the massive head and long sinuous, muscular
body, its strongly rayed fins, its vice-like jaws, armed with great
pavements of teeth, those in front long, strong, pointed, like those of a
tiger. It has been known to attack furiously persons wading at low tide
among the rock-pools." Its flesh is excellent eating, but generally
despised in this country owing to the unprepossessing appearance of the
animal.
FAM. 10. BATRACHIDAE.—Suborbital arch absent; basis cranii simple; mouth
very large, slightly protractile, bordered to a great extent by the
maxillaries. Vertebrae numerous, 29-46 (11-12 + 17-34), without ribs, with
sessile epipleurals, simulating ribs;[750] parapophyses rudimentary or
absent. Post-temporal small and ankylosed to the skull; scapula and
coracoid much reduced, 4 or 5 elongate pterygials, dilated distally, the
two lower in contact with the coracoid. Ventral fins jugular, with 1 spine
and 2 or 3 branched rays. Gill-openings narrow, the gill-membranes broadly
grown to the isthmus; gills 3; pseudobranchiae absent. Head broad and
depressed; body naked or with small scales. Spinous dorsal very short, soft
dorsal and anal long.
This family is on the whole intermediate between the Blenniidae and the
Pediculati. Sluggish, voracious, carnivorous Fishes from the shores of
tropical and warm seas, some of them ascending rivers. The species number
about 20, referable to {711}5 genera: _Batrachus_, _Opsanus_,
_Thalassophryne_, _Thalassothia_, and _Porichthys_. The eggs of _Batrachus
tau_ are very large, ¼ inch in diameter, and are deposited in a little
retreat provided by the parent; the male assumes the care of the brood; the
young fasten themselves to rocks by means of an adhesive ventral disk,
which soon disappears.[751]
In _Thalassophryne_, from the coasts of Central America, the opercular
spine and the two dorsal spines are perforated, and convey poison from
subcutaneous sacs situated at their base.[752] In the American genus
_Porichthys_ the head and body bear series of greatly developed mucous
pores, some of which simulate the photophores of _Scopelus_, but are not
luminous.[753]
[Illustration: FIG. 429.—_Porichthys porosissimus_. (After Goode and Bean.)
½ nat. size.]
FAM. 11. PHOLIDIDAE.—Suborbitals not forming a subocular shelf; mouth
scarcely protractile, with thick lips. Praecaudal vertebrae similar to the
caudals, without transverse processes, with hæmal arches; ribs sessile.
Gill-membranes free from the isthmus; 4 or 5 branchiostegal rays; gills 4,
a slit behind the fourth; pseudobranchiae present. Scapular arch as in
Blenniidae. Ventral fins jugular and rudimentary, or absent. Body elongate,
compressed, with very small scales. Dorsal and anal fins elongate, the
former constituted entirely of non-articulated rays or spines. Caudal fin
distinct, with expanded hypuraL.
Small shore fishes of the Northern Seas, differing from the Blenniidae in
the structure of the praecaudal vertebrae, in spite of the external
resemblance which the two known genera, _Pholis_ (_Centronotus_) and
_Apodichthys_, bears to _Anoplarchus_ and _Xiphistes_. Species about 10.
{712}A well-known British fish of this family is the little Gunnel or
Butter-Fish (_Pholis gunnellus_), remarkable for the manner in which the
female protects her offspring, coiling herself round the eggs, which she
rolls up into a ball about the size of a Brazil nut, in holes of the boring
Mollusc (_Pholas_). The male sometimes assists the female.
FAM. 12. ZOARCIDAE.—Suborbitals not forming a subocular shelf; mouth feebly
protractile. Praecaudal vertebrae with strong transverse processes bearing
ribs and epipleurals. Gill-membranes usually more or less broadly united to
isthmus; 5 to 8 branchiostegal rays; gills 4, a slit behind the fourth;
pseudobranchiae present or absent. Scapular arch as in Blenniidae. Ventral
fins jugular or absent; if present, with 1 to 4 rays. Body more or less
elongate, naked or with very small scales. Dorsal and anal fins elongate,
all the rays articulated, or a few of the posterior dorsals spinous.
Usually no distinct caudal fin.
[Illustration: FIG. 430.—_Typhlonus nasus_ × ½. (After Güther.)]
These fishes have usually been placed, in part at least, near the Gadids,
but they have more in common with the Blenniids, as pointed out by Jordan
and Evermann, and may be regarded as degraded forms descended from the
latter.[754] The family is widely distributed in all seas, many of the
forms being specially adapted to live at great depths. The species known
number about 130. Principal genera: _Scytalina_, _Zoarces_, _Lycodes_,
_Gymnelis_, _Lycocara_, _Melanostigma_, _Derepodichthys_, _Bathyonus_,
_Porogadus_, _Bythitis_, _Neobythitis_, _Cataetyx_, _Selachophidium_,
_Acanthonus_, _Typhlonus_, _Aphyonus_, _Tauredophidium_, _Rhodichthys_,
_Brosmophycis_, _Brotula_, _Lucifuga_, _Lamprogrammus_, _Diplacanthopoma_,
_Hephthocara_.
{713}Some are oviparous, others (_Zoarces_, _Diplacanthopoma_,
_Hephthocara_, _Lucifuga_) viviparous. The eyes are absent, or at least not
visible externally in some of the bathybial forms (_Typhlonus_, _Aphyonus_,
_Tauredophidium_), as well as in the only known freshwater forms, the Cuban
Cave-Fishes _Stygicola_ and _Lucifuga_, which are evidently allied to the
marine _Brotula_, whilst the blind Cave-Fishes of North America (cf. p.
618) are derived from freshwater types. It is believed that blind fishes
are found also in caves of the island of Jamaica, but no specimens have
been seen by naturalists. The largest Cuban Cave-Fish is 5 inches
long.[755]
FAM. 13. CONGROGADIDAE.—Eel-shaped Fishes without ventrals, allied to the
Blenniidae, but with all the rays soft and articulated, the post-temporal
small and ankylosed to the skull, and the sub-orbitals produced into
laminae supporting the eyeball. Lips much developed; gill-membranes free
from isthmus; scales very small.
A single genus, _Congrogadus_, with three species from the Australian and
East Indian coasts. The recently described Japanese genus _Hierichthys_ has
been referred to this family.
FAM. 14. OPHIDIIDAE.—Degraded Blenniids, closely related to the Zoarcidae,
with pseudobranchiae, with tapering tail without distinct caudal fin, and
with the ventral fins each reduced to a pair of filaments or a bifid ray
inserted just behind the chin at the extremity of the clavicle, which is
produced forwards as a slender rod.
Small marine, carnivorous fishes, from the Atlantic and Southern Pacific
coasts as well as from great depths in the Atlantic, Pacific, and Indian
Oceans. About 25 species are known. Genera: _Ophidium_, _Lepophidium_,
_Genypterus_.
FAM. 15. PODATELIDAE.—Mouth inferior, protractile, toothless or with minute
teeth. Praecaudal vertebrae with transverse processes, to which the ribs
are attached. Gill-membranes narrowly attached to isthmus; 8 or 9
branchiostegal rays; gills 4; no pseudobranchiae. Supratemporal loosely
attached by ligament to the skull; scapula cartilaginous, perforate,
bearing the base of the pectoral fin, which is an undivided cartilaginous
plate; coracoid small, ossified. Ventral fins jugular, each reduced to a
single stout filament made up of two intimately coherent rays. Body short,
tail elongate and tapering, {714}compressed; no scales. A short dorsal fin,
without spines, situated above the pectorals; a long anal fin, continuous
with the caudal.
[Illustration: FIG. 431.—Pectoral arch of _Podateles indicus_. _cl_,
Clavicle; _cor_, coracoid; _pelv_, pelvis; _ptr_, pterygial; _ptte_,
post-temporal; _sc_, scapula; _scl_, supraclavicle.]
The genus _Podateles_ (_Ateleopus_) comprises only two species from the
deep sea, one from Japan and one from India.
DIVISION IX.—TAENIOSOMI.
Exceedingly compressed, more or less elongate, often ribbon-like fishes of
doubtful affinities, probably related to the earlier Acanthopterygians, the
ventral fins, when well developed, comprising as many as 7 to 9 rays.
Dorsal fin extending from the head to the end of the tail, its rays simple
(separable into lateral halves), the anterior often prolonged; anal fin
very short or absent. Pectoral fin with horizontal or nearly horizontal
base, the rays supported by the scapula and by three short pterygials, all
three, or two at least, of which are related to the coracoid. Ribs small
and slender, or absent. Post-temporal simple and solidly attached to the
skull. Scales minute or absent.
Deep-sea or pelagic fishes from the Atlantic and Mediterranean and from the
Pacific; the life-histories are still very imperfectly known, and great
changes of form take place with growth. Only two families.[756]
{715}FAM. 1. TRACHYPTERIDAE.—Mouth very protractile; ventral fins more or
less developed, with 6 to 9 rays, or reduced to a single long ray; no anal
fin; vent about the middle of the body; caudal rays, if present, divided
into two fascicles, the upper sometimes much prolonged and directed
upwards.
[Illustration: FIG. 432.—_Trachypterus iris_. × 1. (After Cuvier and
Valenciennes.)]
Two genera. The most generalised is _Trachypterus_, of which probably only
10 forms are entitled to specific distinction. The best known species is
_T. arcticus_, the Deal-Fish or Northern Ribbon-Fish, which reaches a
length of 8 feet or more, and of which a few specimens have been stranded
on the coasts of Scotland. Nilsson, who has observed these fishes alive on
the Scandinavian coast, says they approach the shore at flood-tide on sandy
shelving bottoms, and are often left by the retreating waves; that they
move with one side turned obliquely upward, and that they lie on the side
like Flat-Fishes on the bottom in 2 or 3 fathoms of water. _Regalecus_
differs in the presence of a single ray to the ventral and the absence of
the caudal fin. Some 5 or 6 species may be distinguished. _R. glesne_, the
Oar-fish, or "King of the Herrings," is the best known and the largest
species, reaching a length of over 20 feet. About 25 {716}specimens are
known to have occurred on the British coasts. Some of the accounts of
"Sea-Serpents" are probably based on this fish, which has been observed to
swim with undulating motion and with a small portion of the head as well as
the crest-like anterior part of the dorsal fin above the water.
The fish named _Stylophorus chordatus_, which has been referred to this
family, is known from a single specimen too imperfectly preserved to afford
a clear idea of its affinities.
FAM. 2. LOPHOTIDAE.—Mouth moderately protractile; ventrals very small, if
distinct, with 4 or 5 rays; abdominal cavity extending nearly the whole
length of the much elongated body, the vent very far back and followed by a
short anal fin; caudal fin small, not divided.
A single genus, _Lophotes_, with 3 or 4 species, from the Mediterranean,
the tropical Atlantic, the Cape of Good Hope, Japan, and New Zealand,
reaching a length of 6 feet or more. The dorsal fin commences with an
extremely long and strong spine on the head, which is much elevated and
truncate in front.
SUB-ORDER 11. OPISTHOMI
Air-bladder without open duct. Opercle well developed, hidden under the
skin; supraoccipital in contact with the frontals, separating the
parietals. Pectoral arch suspended from the vertebral column, far behind
the skull; no mesocoracoid. Vertical fins with spines. Ventral fins absent.
This division stands in the same relation to the Acanthopterygii as do the
Apodes to the Malacopterygii. The single family is possibly derived from
the Blenniidae.
FAM. 1. MASTACEMBELIDAE.—Body more or less Eel-shaped; a series of short
spines detached from the very elongate dorsal fin, which is more or less
confluent with the likewise very elongate anal fin. A single nostril on
each side. Mouth not protractile, bordered by the praemaxillaries, to the
upper border of which the maxillaries are attached. Gill-cleft inferior;
gills 4; branchiostegal rays 6; no pseudobranchiae. Vertebrae numerous
(72-95), the praecaudals with transverse processes bearing the ribs. Scales
very small.
Carnivorous fishes, from fresh and brackish waters of Southern Asia and
Tropical Africa. 33 species are known, referable to {717}two genera:
_Mastacembelus_ and _Rhynchobdella_. The largest species reach a length of
three feet. Little is known of their habits. Of the Indian _Rhynchobdella
aculeata_, Day says it conceals itself in the mud and becomes drowned in
water if unable to reach the surface, as it apparently requires to respire
air directly.
[Illustration: FIG. 433.—_Mastacembelus maculatus_. × ½.]
SUB-ORDER 12. PEDICULATI.
Air-bladder without open duct. Opercle large, hidden under the skin;
supraoccipital in contact with the frontals, separating the parietals.
Pectoral arch suspended from the skull; no mesocoracoid. No ribs, no
epipleurals. Ventral fins jugular. Gill-opening reduced to a foramen
situated in or near the axil, more or less posterior to the base of the
pectoral. Body naked or covered with spines or bony tubercles.
A small, natural group, connected with the Acanthopterygii Jugulares
through the Batrachidae, in which the elongate pterygials of the pectoral
fin foreshadow the kind of arm ("pseudobrachium") which is more or less
characteristic of these highly aberrant Fishes. As in the Batrachidae, the
post-temporal is flat and ankylosed to the cranium, and the suprascapula is
much elongated. The pterygials, two or three in number, are separated from
the small scapula and coracoid by a broad ligament, the arm-like pectorals
being more or less distinctly geniculated and inserted far back behind the
cranium. The head is large, the basis cranii simple. The gills are reduced
to 2, 2½, or 3. The spinous dorsal, if present, consists of a few rays,
which may be modified into tentacles inserted on the head. Vertebrae 17 to
31.
{718}Five families:—
I. Gill-opening in or behind lower axil of pectoral; mouth large,
terminal or directed upwards.
Pectoral fin scarcely geniculated; ventrals present 1. _Lophiidae_.
Pectoral fin scarcely geniculated; ventrals absent 2. _Ceratiidae_.
Pectoral fin strongly geniculated; ventrals present 3. _Antennariidae_.
II. Gill-opening behind lower axil of pectoral; mouth large, inferior;
ventrals absent 4. _Gigantactinidae_.
III. Gill-opening above axil of pectoral; mouth rather small,
subterminal or inferior; pectoral fin strongly geniculated; ventrals
present; spinous dorsal absent or reduced to a small tentacle lodged
in a cavity under the snout 5. _Malthidae_.
FAM. 1. LOPHIIDAE.—Mouth extremely large, terminal, with very strong
cardiform teeth. Gill-opening in lower axil of pectoral; pseudobranchiae
present. Pectoral fin scarcely geniculated, with two pterygials. Ventral
fin with 1 spine and 5 branched rays. Spinous dorsal present. Skin naked.
Twelve species, referable to three genera (_Lophius_, _Chirolophius_, and
_Lophiomus_) living on the bottom of the Atlantic, Indian, and Pacific
Oceans, at moderate or great depths. _Lophius_ was represented in the Upper
Eocene of Monte Bolca.
[Illustration: FIG. 434.—_Chirolophius naresii_. (After Günther.) × ⅓.]
The Fishing-Frog or Angler (_Lophius piscatorius_) has a wide distribution,
occurring on the coasts of Europe and North America. The first dorsal ray,
inserted on the snout, is very long, movable in every direction, and
terminates in a dermal flap, which is supposed to be used by the "Angler"
as a bait, attracting other fishes, which are soon ingulfed in the enormous
gape. It grows to a length of over 5 feet. The ventral rays are very
elongate in the young.
{719}FAM. 2. CERATIIDAE.—Mouth extremely large, terminal, with strong
cardiform teeth. Gill-opening in lower axil of pectoral; pseudobranchiae
absent. Pectoral fin scarcely geniculated, with three pterygials. Ventral
fins absent. Spinous dorsal fin usually present, sometimes reduced to a
single tentacle on the snout. Skin naked.
The members of this family, about 25 in number, are all inhabitants of
great depths (300-2600 fathoms). The colour of the body is usually a deep
black, and the first dorsal spine, on the head, may terminate in a luminous
bulb with or without filaments. "The Bathybial Sea-devils," writes Günther,
"are degraded forms of _Lophius_; they descend to the greatest depths of
the ocean. Their bones are of an extremely light and thin texture, and
frequently other parts of their organisation, their integuments, muscles,
and intestines are equally loose in texture when the specimens are brought
to the surface. In their habits they probably do not differ in any degree
from their surface representative, _Lophius_."
[Illustration: FIG. 435.—_Himantolophus reinhardti_, outline and skeleton.
(After Lütken.) × 1.]
Principal genera: _Ceratias_, _Aceratias_, _Oneirodes_, _Himantolophus_,
_Aegaeonichthys_, _Melanocetus_, _Liocetus_, _Linophryne_, _Caulophryne_,
_Dolopichthys_.
{720}FAM. 3. ANTENNARIIDAE.—Mouth large, vertical or very oblique, turned
upwards, with cardiform teeth. Gill-opening in or behind lower axil of
pectoral; pseudobranchiae absent. Pectoral fin forming an elbow-like angle,
with three pterygials. Ventral with 4 or 5 rays. Spinous dorsal present.
Skin naked or spinulose.
About 40 species, referable to 5 genera: _Pterophryne_, _Antennarius_,
_Brachionichthys_, _Saccarius_, and _Chaunax_.
The species of _Antennarius_ live mostly in coral groves, where they lie in
wait for prey, well concealed by their protective coloration and the
harmonising aspect of their integument and appendages. To this genus also
belongs the "Marbled Angler" (_A. marmoratus_), carried about in mid ocean
among the _Sargassum_ weed, to rest on which, from its peculiar arm-like
pectoral fins, it is specially fitted; there it makes its wonderful nest of
silk-like fibres, probably secreted by the parent as in the Sticklebacks,
with large bundles of eggs hanging like grape clusters.[757] The deep-sea
_Chaunax_ inflates its abdomen like _Tetrodon_.
FAM. 4. GIGANTACTINIDAE.—Mouth inferior, snout produced into a long
tentacle directed forwards, and bearing a luminous organ. Body covered with
small spines. Otherwise as in the Ceratiidae. _Gigantactis vanhoeffeni_, of
Brauer, from the Indian Ocean, at depths of about 1000 fathoms.
FAM. 5. MALTHIDAE.—Mouth rather small, subterminal or inferior, with
villiform or cardiform teeth. Gill-opening above pectoral; pseudobranchiae
absent. Pectoral fin forming an elbow-like angle, with three pterygials.
Ventral with 5 rays. Spinous dorsal absent, or reduced to a more or less
developed tentacle lodged in a cavity under the snout. Head and body with
bony tubercles or spines.
About 30 species are known, mostly from the deep sea within the tropics
(down to 1270 fathoms). Principal genera: _Coelophrys_, _Malthe_,
_Malthopsis_, _Halieutaea_, _Halicmethes_, _Dibranchus_.
The "Bat-Fish" (_Malthe vespertilio_), common in shallow water about the
West Indies, is said to assume an almost toad-like attitude on the ground,
the head being directed slightly upwards, while the pectorals take on the
function of hind legs and the ventrals of fore legs.
{721}SUB-ORDER 13. PLECTOGNATHI.
Air-bladder without open duct. Opercular bones more or less reduced;
supraoccipital in contact with the frontals, separating the parietals;
maxillary and praemaxillary bones often firmly united. Pectoral arch
suspended from the skull. No ribs. Ventral fins thoracic and much reduced
if present; the pelvic bones, if present, more or less completely
co-ossified. Gill-opening much reduced. Body covered with more or less
osseous scales, bony scutes, or spines, or naked.
A highly aberrant group, closely connected with the Acanthopterygii through
the Acanthuridae, as pointed out long ago by Dareste.[758] The skeleton is
often feebly ossified and the vertebrae much reduced in number, but the
jaws, although short, are very strong, usually with large sectorial teeth
which may be confluent into a beak; the post-temporal is short and simple,
suturally united to the squamosal. These fishes have usually been arranged
in three divisions: _Sclerodermi_, _Ostracodermi_, and _Gymnodontes_, but
Regan,[759] whose classification is here followed, has shown that the
latter include a type (_Triodon_) which, in spite of its beak-like teeth,
is more nearly related to the Sclerodermi, whilst the Ostracodermi have
much more in common with the latter than with the Gymnodontes. It therefore
appears best to admit only two divisions, the first with 4, the second with
3 families:—
I. SCLERODERMI.—Supraclavicle vertical; pectoral arch of the
Perciform type; all the vertebrae with a single neural spine.
A. Body covered with hard or spinous scales; epipleurals present;
pelvis present.
Teeth separate; spinous dorsal present; ventrals paired; pelvis
immovable 1. _Triacanthidae_.
A beak; spinous dorsal and ventrals absent; pelvis movable
2. _Triodontidae_.
Teeth separate; spinous dorsal present; ventrals absent or
represented by a single short spine; pelvis movable
3. _Balistidae_.
B. Body encased in a carapace; no epipleurals; spinous dorsal,
pelvis, and ventrals absent 4. _Ostraciontidae_.
II. GYMNODONTES.—Supraclavicle oblique or nearly horizontal; lower{722}
three pterygials enlarged and immovably united to the coraco-scapular
cartilage; anterior vertebrae with bifid divergent neural spines;
pelvis absent.
Beak with a median suture; interoperculum not connected with
suboperculum; caudal fin present; body inflatable.
1. _Tetrodontidae_.
Beak without median suture; interoperculum attached posteriorly to
suboperculum; caudal fin present; body inflatable.
2. _Diodontidae_.
Beak without median suture; interoperculum attached posteriorly to
suboperculum; caudal fin absent, the body non-inflatable,
truncate posteriorly, with the dorsal and anal fins confluent.
3. _Molidae_.
[Illustration: FIG. 436.—Left side of pectoral arch of A, _Triacanthus
brevirostris_, and B, _Tetrodon mbu_. _cl_, Clavicle; _cor_, coracoid;
_pcl_, post-clavicle; _p.r_, pectoral rays; _ptr_, pterygials; _sc_,
scapula; _scl_, supraclavicle.]
DIVISION I.—SCLERODERMI.
Supraclavicle vertical; pectoral pterygials not enlarged, movably attached
by ligament to the scapula and coracoid, three to the former and one to the
latter. All the vertebrae with the neural arches forming a single spine.
Basis cranii more or less distinctly double; dentary and articular
completely co-ossified.
FAM. 1. TRIACANTHIDAE.—Praemaxillaries protractile, free from the
maxillaries; teeth in the jaws separate, conical or incisor-like; palatine
arch firmly united to the skull. Gills 4. Praecaudal vertebrae with
parapophyses; epipleurals present. Spinous dorsal fin with 2 to 6 spines.
Ventral fins each represented by a strong spine, with an inner basal knob
which locks it when everted, rarely with the addition of 1 or 2 rudimentary
soft rays; pelvis present, firmly united to the pectoral arch. Scales
small, sometimes spinous or bony. Vertebrae 20.
Marine fishes from the Indian and Western Pacific Oceans. Ten species,
referable to three genera: _Triacanthus_, _Triacanthodes_,
{723}_Halimochirurgus_. The latter, remarkable for its long, tube-like
snout, is the only deep-sea form of this Sub-order; it was recently
discovered in the Gulf of Manaar, at a depth of 143 fathoms. Fossil genera
are _Acanthopleurus_, Oligocene, and _Spinacanthus_, Eocene.
FAM. 2. TRIODONTIDAE.—Praemaxillaries not protractile, firmly united to the
maxillaries; teeth coalescent into a beak, the upper jaw divided by a
median suture, the lower simple. Praecaudal vertebrae with or without
parapophyses; epipleurals present. No spinous dorsal fin. No ventral fins.
Abdomen with a dilatable sac, kept expanded by the very long movable
pelvis. Body covered with small, spiny, subimbricate, bony laminae.
Vertebrae 20.
A single species, the curious _Triodon bursarius_ of the Indian Ocean and
Archipelago.
[Illustration: FIG. 437.—_Monacanthus_, sp., with enlarged views of dorsal
scales (_b_) and ventral spine (_c_).]
FAM. 3. BALISTIDAE.—Praemaxillaries not protractile, firmly united to the
praemaxillaries; teeth incisor-like; palatine movably articulated with
ectopterygoid, or entirely free from it. Gills 4. Praecaudal vertebrae with
well-developed parapophyses, to which epipleurals are attached. Spinous
dorsal fin with 1 to 3 spines. Ventral fins, if present, represented by a
single short {724}rough spine at the end of the long, movable pelvis. Body
covered with juxtaposed movable scutes or with minute rough scales.
About 100 species are known from the tropical and warm seas, one species
(_Balistes capriscus_) occasionally wandering as far north as the south
coast of England. Genera: _Balistes_, _Monacanthus_, _Paraluteres_,
_Pseudaluteres_, _Pseudomonacanthus_, _Aluteres_, _Psilocephalus_. The
Oligocene genus _Acanthoderma_ is closely allied to _Balistes_.
The "File-Fishes" or "Trigger-Fishes" (_Balistes_), the largest species of
which grow to nearly 3 feet, have a powerful dentition, which enables them
to break off pieces of corals, on which they feed, and to bore holes in the
hard shells of Mollusca in order to extract the soft parts; they are
themselves well protected by a mail of hard, rhomboidal scales. The
herbivorous _Monacanthus_ is less favoured in this respect, the rough
scales being so small as to give the skin a velvety appearance.
_Psilocephalus_ differs from _Monacanthus_ in its very elongate head and
body, the very feeble dorsal spine, the presence of a mental barbel, and
its more numerous vertebrae (29 or 30 instead of 18 to 21). The flesh of
many of these fishes is poisonous.[760] The drumming sounds produced by
_Balistes_ have been described by Möbius.[761]
FAM. 4. OSTRACIONTIDAE.—Praemaxillaries not protractile, firmly united to
the praemaxillaries; teeth incisor-like; palatine immovable. Gills 4.
Praecaudal vertebrae with very feeble parapophyses and no epipleurals. No
spinous dorsal fin. Clavicles, coracoids, and post-clavicles much expanded.
No ventral fins. Body encased in a carapace formed of large, juxtaposed,
mostly hexagonal bony plates. Vertebrae 14 to 16.
The species of "Trunk-Fishes" number about 20, and are referable to 3
genera: _Aracana_, _Ostracion_, _Lactophrys_; all belong to the tropical
seas, living near the bottom in shallow water. The genus _Ostracion_ is
represented by one species in the Upper Eocene.
The rigid box in which these fishes are encased entails more use of the
dorsal and anal fins for progression than is customary {725}among fishes.
According to Brown Goode, "the propelling force is exerted by the dorsal
and anal fins, which have a half rotary, sculling motion, resembling that
of a screw propeller; the caudal fin acts as a rudder, save when it is
needed for unusually rapid swimming, when it is used as in other fishes;
the chief function of the broad pectorals seems to be that of forming a
current of water through the gills, thus aiding respiration, which would
otherwise be difficult on account of the narrowness and inflexibility of
the branchial apertures. When taken from the water, one of these fishes
will live for two or three hours, all the time solemnly fanning its gills,
and when restored to its native element seems none the worse for its
experience, except that, on account of the air absorbed, it cannot at once
sink to the bottom." "No group of tropical fishes," says the same author,
"is so thoroughly worked out in the writings of the fathers of natural
history as this one. Over 200 years ago every species of trunk-fish now
taken from the Atlantic was known to and described by the naturalists, and
it is a well-deserved tribute to their discrimination as zoologists to say
that none of the many efforts which have since been made to subdivide their
species have been at all successful."
[Illustration: FIG. 438.—_Ostracion quadricornis_. × ½.]
DIVISION II.—GYMNODONTES.
Supraclavicle oblique, sometimes nearly horizontal; lower three pectoral
pterygials enlarged and immovably united to the coracoscapular cartilage;
upper pterygial small, suturally united to the scapula. Anterior vertebrae
with bifid divergent neural spines. Basis cranii simple; suture between
dentary and articular evident. Pelvis absent.
The spinous dorsal and the ventral fins are constantly absent, the
praemaxillaries are united to the maxillaries, and the teeth {726}are
coalescent, forming a beak; parapophyses are not developed, and epipleurals
are absent.
FAM. 1. TETRODONTIDAE.—Beak with a median suture. Interoperculum a long
rod, attached to inner face of praeoperculum, sometimes connected with
operculum, never with suboperculum. Gills 3. First 4 or 5 praecaudal
vertebrae with bifid neural spine and closed neural arch. Skin naked or
with movable spines, rarely with bony plates; belly inflatable. Vertebrae
17 to 29.
The "Puffers" or "Globe-Fishes" comprise about 60 species, referable to 5
genera: _Tetrodon_, _Ephippion_, _Tropidichthys_, _Xenopterus_,
_Chonerhinus_. They inhabit all the tropical and warm seas, a few species
being confined to fresh water. Remains of _Tetrodon_ have been found in
Upper Eocene and later formations. They are remarkable for the manner in
which they inflate themselves with air.[762] The flesh of most species is
poisonous.
FAM. 2. DIODONTIDAE.—Beak without median suture. Interoperculum rod-like,
attached posteriorly to the rod-like anterior limb of the suboperculum.
Gills 3. All the praecaudal vertebrae with bifid neural spines. Skin with
movable spines; belly inflatable. Vertebrae 21 or 22.
[Illustration: FIG. 439.—_Diodon geometricus_. (After Goode.)]
Only two genera appear capable of clear definition: _Diodon_ and
_Lyosphaera_; species about 15. Numerous species have been described from
the Upper Eocene and later formations.
"Porcupine Fishes" are confined to tropical seas, and have attracted
attention from the earliest times, being frequently preserved as
"curiosities." Their flesh is regarded as poisonous.
FAM. 3. MOLIDAE.—Beak without median suture. Interoperculum rod-like,
attached posteriorly to the rod-like anterior limb of the suboperculum.
Gills 4. Anterior praecaudal vertebrae with divergent bifid neural spines
and neural canal not roofed in. {727}Body non-inflatable, truncate
posteriorly, without caudal peduncle; caudal fin absent, the dorsal and
anal fins confluent. Skin rough or tessellated. Vertebrae 17 in
_Orthagoriscus_.
The very young are armed with spines.
[Illustration: FIG. 440.—_Orthagoriscus mola_. (After Goode.) × 1/20.]
The "Sun-Fish" are extraordinary creatures found in the open sea or
descending to great depths, and of wide distribution. The number of species
is still very uncertain, but two generic forms, _Orthagoriscus_ or _Mola_
and _Ranzania_, are easily distinguished. Examples of both occur now and
then on our coasts. _Orthagoriscus mola_ grows to upwards of 8 feet and to
a weight of 1800 pounds. It has been observed to swim slowly about, near
the surface, the high dorsal above the water. Its food is said to consist
chiefly of jelly-fish and larval fishes; its mode of reproduction and
places of breeding are still unknown.
{729}INDEX
Every reference is to the page: words in italics are names of genera or
species; figures in italics indicate that the reference relates to
systematic position; figures in thick type refer to an illustration; f. =
and in following page or pages; n. = note.
Abbott, 193 n.
_Abcona_, _670_
Abdominal pores, 401 f.
_Abramis_, _582_
_Abyssascidia_, _73_
_Acanthaphritis_, _706_
_Acanthias_, _455_, 298;
vertebral column, 198;
uterine nutrition, 434;
_A. vulgaris_, 264, 455;
pectoral fin, 243
_Acanthicus_, _595_
_Acanthistius_, _659_
_Acanthocepola_, _662_
_Acanthoclinus_, _709_
_Acanthocybium_, _678_
Acanthodei, _440_ f., 148
_Acanthoderma_, _724_
_Acanthodes_, _442_;
_A. wardi_, 441
Acanthodidae, _441_
_Acanthodopsis_, _442_
_Acanthonus_, _712_
_Acanthophthalmus_, _582_
_Acanthopleurus_, _723_
_Acanthopoma_, _589_
Acanthopterygii, _650_ f., 159, 306, 543;
diagram showing relationships of groups, 651
Acanthuridae, _668_, 357, 651, 652, 654
_Acanthurus_, _668_;
_A. chirurgus_, 357
_Acara_, _672_
Accessory respiratory organs, 292
_Acentronura_, _634_
_Acentrophorus_, _498_
_Aceratias_, _719_
_Acerina_, _659_;
_A. cernua_, 659
_Acestra_, _595_;
_A. gladius_, 595
_Acestrorhamphus_, _575_
_Acestrorhynchus_, _575_
_Achilognathus_, _582_
_Achiropsis_, _687_
_Achirus_, _687_
_Acipenser_, _492_, 149, 262, 264, 273, 274, 276, 282, 348;
ribs, 201;
lymph follicles, 261;
spiral valve, 268;
gills, 283, 284;
air-bladder, 298, 299;
vascular system, 319, 322, 328, 334;
spleen, 343;
gonoducts of female, 400, 405;
distribution, habits, and food, 493;
breeding, 494;
economic value, 494;
_A. huso_, 494;
_A. rhynchaeus_, pectoral fin, 243;
_A. ruthenus_, 493;
scales, 187, 188;
brain, 376;
micropyles, 411;
larva, 494;
_A. sturio_, 493, 494;
vertebral column, 200;
supra-renals, 346;
micropyles, 411
Acipenseridae, _486_, 489, 495
Acipenseroidei = Chondrostei, _q.v._
Acrania, _113_
Acrartete, 636 n.
_Acrochordonichthys_, _588_
_Acrodus_, _445_
_Acrogaster_, _656_
_Acrognathus_, _611_
_Acropoma_, _659_
Acropomatidae, _659_, 654
_Acrotus_, _644_
Actinistia, _477_ n.
Actinopterygii, _476_
Actinotrocha, 28 f., 29
_Acysis_, _588_
Adipose fins, 163
_Aegaeonichthys_, _719_
_Aelurichthys_, _588_
_Aethalion_, _546_
_Aetheolepis_, _498_;
scales, 187
Aetheospondyli, _497_
_Aëtobatis_, _465_, 466;
spines, 177
Agassiz, 558 n., 685 n., 720 n.
_Ageniosus_, _589_;
_A. valenciennesi_, 589
Agnathostomata, _145_;
characters of, 147
_Agoniates_, _575_
Agonidae, _700_, 694
_Agonopsis_, _700
Agonus_, _700_; {730}
_A. cataphractus_, 700
_Agrammus_, _696_
_Agriopus_, _695_
_Aida_, _639_
_Ailia_, _588_;
air-bladder, 302
_Ailiichthys_, _588_
_Aipichthys_, _666_
Air-bladder, as an accessory respiratory organ, 291 f.;
_Amia_, 291;
_Lepidosteus_, 291, 299;
_Sudis gigas_, 291;
_Erythrinus taeniatus_ and _E. braziliensis_, 291;
_Neoceratodus_, 291, 300;
_Protopterus_, 291, 301, 302;
_Lepidosiren_, 291;
structure, 297;
in different Fishes, 298;
"red bodies," 307;
"red glands," 307, 308, 309;
gases of the air-bladder, 309;
functions, 309;
in locomotion, 310;
as a vocal organ, 358;
connexion with the auditory organ, 388, 389, 390
_Albula_, _548_, 541, 549;
vestigial conus arteriosus, 329, 548;
_A. conorhynchus_, 548;
larva, 548
Albulidae, _547_, 544
_Alburnus_, _582_;
_A. lucidus_, 167, 583
Alcock, 695
Alder and Hancock, 38
Alepidosauridae, _614_, 606
_Alepidosaurus_, _614_;
_A. ferox_, 614
Alepocephalidae, _569_, 544
_Alepocephalus_, _570_;
_A. rostratus_, 570 n.
_Aleposomus_, _570_
_Alestes_, _575_
Alimentary canal, of Hemichordata, 11, 25;
of Tunicata, 54, 67;
of Amphioxus, 120;
of Fishes, 252 f.;
regions, 252;
mesenteries, 258;
histology, 259 f.;
glands, 270
_Allabenchelys_, _588_
Allis Shad, 564
Allman, 22 n., 705 n.
_Alopecias vulpes_, _451_, 452;
spiral valve, 265
_Aluteres_, _724_
_Amaroucium_, _88_;
_A. proliferum_, stomach, 88
Ambassinae, _660_
_Ambassis_, _660_
_Ambloplites_, _657_
_Amblyapistus_, _695_
Amblyopsidae, _618_, 361, 395, 606
_Amblyopsis_, _618_;
_A. spelaea_, 166, 361, 618, 619
_Amblyopus_, _689_
_Amblypharyngodon_, _582_
_Amblypterus_, _487_
_Amia_, _499_, 149, 160, 262, 273, 274, 276, 283, 291, 393, 609 n.;
fossil, 501;
_A. calva_, 499, 500;
scales, 189;
vertebral column, 201, 202, 203;
skull, 228;
median fins, 235;
pectoral fin, 243, 244;
spiral valve, 268;
pseudobranch, 284;
air-bladder, 297, 299, 310;
heart, 328;
arteries to air-bladder, 337;
sensory canal, 385;
nephrostomes, 401, 402;
segmentation of ova, 409;
distribution and habits, 499;
spawning, 500;
nest, 500, 501;
larvae, 501
Amiidae, _499_, 299, 497
_Amiopsis_, _501_
_Amiurus_, _588_, 261;
liver and pancreas, 273;
_A. catus_, rectal valve, 254;
renal portal system, 319;
_A. nebulosus_, 587 n., 592;
_A. nigrilabris_, blind, 394
Ammocoetes, _428_, 46, 262, 272, 280, 280 n., 327, 343, 428;
metamorphosis, 429;
protective value of the skin, 429
_Ammocrypta_, _659_
_Ammodytes_, _639_, 275;
_A. lanceolatus_, 639;
_A. tobianus_, 639
Ammodytidae, _639_, 637
_Ammopleurops_, _687_
_Ammotretis_, _687_
Amniota, _145_
_Ampheristus_, _695_
_Amphioxus_, _112_ f., 4, 37, 46, 110;
general characters, 113;
external characters, 114;
anatomy, 116, 117;
musculature, 117;
skeleton, 119;
notochord, 119;
alimentary canal, 120;
branchial bars, 122;
endostyle, 123;
coelom, 123;
vascular system, 124, 125;
renal organs, 125, 127;
nervous system, 127;
sense-organs, 128;
gonads, 129;
embryology and life-history, 130, 130, 131, 132, 133, 134, 135, 136;
compared with Hemichordata, 11 f., 16, 29 f.
—see also _Branchiostoma_
Amphipnoidae, _598_
_Amphipnous_, _598_;
respiratory air-sacs of, 598;
_A. cuchia_, 294, 598
_Amphiprion_, _672_
_Amphisile_, _633_;
_A. strigata_, 633
Amphisilidae, _633_, 628
Amphistiidae, _684_, 683
_Amphistium_, _684_;
_A. paradoxum_, 684
Amphistylic, 222, 223
Anabantidae, _645_, 292, 637;
distribution of, 645
_Anabas_, _645_, 355;
_A. scandens_, 645;
labyrinthiform organ, 293
_Anableps_, _616_, 419;
intromittent organ, 414;
development of embryos in ovisacs, 418;
_A. tetrophthalmus_, 617
Anacanthini, _646_ f., 543, 702, 703
_Anacyrtus_, _575_
Anadromous, 413
Anallantoidea, _145_
Anamniota, _145_
_Anampses_, _673_
_Anapterus_, _611_
_Anarrhichas_, _709_;
teeth, 251;
_A. lupus_, 710;
eggs, 408
Anaspida, _531_ f., 149
_Anchinia_, _96_, 100;
_A. rubra_, 100;
polymorphism, 100
Anchovy, 564 {731}
_Ancylodon_, _663_
_Ancylostylus_, _549_
_Andersonia_, _588_, 589
Andrews, 113
_Anema_, _706_
Angel-Shark, 456
Angler, 718
_Anguilla_, _601_;
_A. vulgaris_ (Common Eel), 601;
larva, 602;
red glands, 307;
renal portal circulation, 319, 320
Anguillidae, _600_, 163, 405, 649 n.
_Anodonta_, _Rhodeus_ in, 584
_Anogmius_, _549_
Anomalopidae, _660_;
photophores, 178
_Anomalops_, _660_
_Anomalopterus_, _570_
_Anoplarchus_, _709_
_Anoplogaster_, _656_, 655
_Anoplopoma_, _697_, 693
_Anoplopterus_, _588_
Anostominae, _576_
_Anostomus_ (Characinidae), _576_;
(Mugilidae), _640_
Ansorge, 560
Antennariidae, _720_, 718
_Antennarius_, _720_;
nest, 414;
_A. hispidus_, scales, 191;
_A. marmoratus_, 720;
scales, 191
_Anthias_, _659_, 660
Antiarchi, _532_ f., 149
_Antigonia_, _667_
_Antimora_, _648_
_Anurella_, _78_;
development of, 78;
_A. roscovita_, larva, 78
_Apateodus_, _611_
_Apeltes_, _630_
_Aphanopus_, _679_
_Aphareus_, _660_
_Aphia_, _689_;
_A. pellucida_, 690
_Aphiocharax_, _575_
_Aphoristia_, _687_
_Aphredoderus_, _656_, 655;
_A. sayanus_, 656
_Aphyonus_, _712_, 713
_Apistus_, _695_, 692
_Aplidium_, _88_;
_A. zostericola_, stomach, 88
Apodes, _599_ f., 306
_Apodichthys_, _711_
_Apogon_, _660_
_Apolectus_, _677_
_Apostasis_, _668_
_Appendicularia_, _68_, 37;
_A. sicula_, 66
Appendicularians, 64
Appendiculariida, _65_, 66
Appendiculariidae, _68_, 38;
nervous system, 53
Appendix digitiformis, 276
_Aprion_, _660_
_Aprionodon_, _448_
_Apua_, _582_
_Aracana_, _724_
_Arapaima_, _557_;
_A. gigas_, 556, 558
Arch-centra, 196
Archaeomaenidae, _545_, 544
_Archaeomenes_, _545_
_Archaeoteuthis_, _669_
_Archaeus_, _677_
Archenteron, 20, 56, 130, 132
_Archeobatis_, _446_
Archinephric duct, 397, 398
_Arctoscopus_, _663_
Argenteum, 168
_Argentina_, _565_, 569
_Arges_, _595_
Arginae, _595_
_Argyriosus vomer_, _363_
_Argyropelecus_, _571_
_Ariscopus_, _706_
Aristotle, 36
_Arius_, _588_, 587, 589;
_A. australis_, 593;
deposition of eggs, 415;
_A. commersonii_, 593
Armed Bullhead, 700
_Arnoglossus_, _687_
_Arripis_, _663_
Arthrodira, _535_ f., 149
Arterial system, 329 f.
Artificial pearls, 167
_Ascelichthys_, _698_
_Ascidia_, _72_, 73;
structure, 39 f., 43, 45;
test, 40, 41;
body-wall, 42;
mantle, 42;
branchial cavity, 43, 48;
atrial or peribranchial cavity, 43;
other cavities of body, 44;
tentacles, 44;
endostyle, 46;
branchial sac, 47;
heart and circulation, 49, 51;
blood, 49;
neural gland, 52;
dorsal tubercle, 52;
nervous system, 53;
sense-organs, 53;
alimentary canal, 54;
renal organ, 54;
reproductive organs, 55;
embryology and life-history, 55, 57, 60;
metamorphosis, 61;
_A. challengeri_, dorsal tubercle, 79;
_A. mentula_, structure, 39 f., 40;
test, 42;
endostyle, 46;
pharynx, 47;
nervous system, 52;
larva, 78;
_A. meridionalis_, dorsal tubercle, 79;
_A. pyriformis_, dorsal tubercle, 79;
_A. translucida_, dorsal tubercle, 79;
_A. virginea_, 39 f.
Ascidiacea, _70_ f., 64
Ascidiae Compositae, _80_ f., 64;
structure, 81
Ascidiae Luciae, _90_ f., 86
Ascidiae Simplices, _71_ f.
Ascidians, 35, 64, 70
Ascidiella aspersa, 77
Ascidiidae, _72_, 64, 110
Ascidiinae, _72_
Ascidiozooids, 35, 84
Ascopera gigantea, branchial sac, 77;
dorsal tubercle, 79
_Asineops_, _656_
Aspidophora, 5 n.
_Aspidophoroides_, _700_ {732}
Aspidorhynchidae, _502_
_Aspidorhynchus_, _502_;
_A. acutirostris_, 502
_Aspius_, _582_
Aspredinidae, _596_, 575
_Aspredo_, _596_, 416
_Aspro_, _659_
_Asprotilapia_, _672_
_Asteracanthus_, _445_
Asterolepidae, _534_
_Asterolepis_, _534_
Asterospondylic, 198
_Astroblepus_, _595_
_Astronesthes_, _571_, 570, 572;
_A. niger_, photophores, 178
_Asymmetron_, _137_, 129;
distribution, 138;
_A. bassanum_, 137;
_A. caudatum_, 137, 138;
_A. cingalense_, 137;
_A. cultellum_, 137;
_A. hectori_, 137;
_A. lucayanum_, 137, 138;
_A. maldivense_, 137
Ateleaspidae, _528_
_Ateleaspis_, _528_, 530;
_A. tessellata_, 528
_Ateleopus_, _714_
_Atheresthes_, _687_, 685
_Atherina_, _639_, 640
_Atherinella_, _639_
_Atherinichthys_, 641
Atherinidae, _639_, 637
_Atherinops_, _639_
_Atherinopsis_, _639_
_Atopochilus_, _588_
Atrial cavity, 43, 44, 59, 63, 67
Atriopore, 113, 115, 117
Atrium, of Amphioxus, 113, 118, 121, 135
_Atypichthys_, _666_
_Atyposoma_, _666_
_Auchenaspis_, _529_
_Auchenipterus_, _589_;
_A. nodosus_, elastic-spring-apparatus, 358
_Auchenoglanis_, _588_
Auditory organs, 387 f., 388;
connexion with the air-bladder, 389, 390
Audouin, 37
_Aulacocephalus_, _659_
_Auliscops_, 632
_Auliscus_, _631_
_Aulolepis_, _560_
_Aulopus_, _611_
_Aulopyge_, _582_
_Aulorhamphus_, _668_
Aulorhynchidae, _631_, 628
_Aulorhynchus_, _631_, 632
_Aulostoma_, _632_;
_A. coloratum_, 632
Aulostomatidae, _632_, 628
_Aulostomatomorpha_, _570_
Autostylic, 223
_Auxis_, _678_
_Azurina_, _672_
_Badis_, _658_
_Bagarius_, _588_
_Bagrichthys_, _588_
Bagrinae, _588_
_Bagroides_, _588_
_Bagropsis_, _588_
_Bagrus_, _588_
Balanoglossus, 3, 5 f., 24, 25, 28, 30 f., 123;
embryo, 8;
history of name, 17
_Balanoglossus_, _17_, 5, 6, 13, 16;
_B. aurantiacus_, 7, 11, 15;
_B. biminiensis_, 18, 21 n.;
_B. clavigerus_, 6, 17;
_B. gigas_, 5
_Balistes_, _724_, 264, 354;
coloration, 174;
scales, 190;
sound production, 357;
_B. aculeatus_, 357;
_B. capriscus_, 724;
_B. vetula_, 361
Balistidae, _723_, 652, 721, 163
Ballowitz, 591 n.
Bancroft, 85
Band-Fish, 662
Barbels, 154
_Barbus_, _582_, 584;
_B. mosal_, 584;
_B. tropidolepis_, 583;
lower pharyngeals, 583;
_B. viviparus_, 584
_Barilius_, _582_
Barracudas, 642
Barramunda, 558
Barrois, 39
Basking Shark, 453
Bat-Fish, 720
Bateson, 5, 6, 11, 14, 20, 30 n.
_Bathyagonus_, _700_
_Bathyclupea_, _657_, 656
_Bathydraco_, _706_
_Bathygadus_, _647_
_Bathylaco_, _571_
_Bathylagus_, _566_, 565, 569
_Bathylychnus_, _571_
_Bathymaster_, _661_
_Bathymyzon_, _426_
_Bathyoncus_, _74_
_Bathyonus_, _712_
_Bathypterois_, _611_, 613;
_B. dubius_, 162, 612;
_B. longipes_, 613
_Bathysaurus_, _611_
_Bathythrissa_, _548_
_Bathytroctes_, _570_
Batoidei, _457_ f., 148
Batrachidae, _710_, 651, 704, 361;
photophores, 179
_Batrachus_, _711_;
_B. tau_, 711, 361
Baudelot, 584 n.
_Bdellostoma_, _423_;
external characters, 151;
skull, 220, 221;
pancreas, 273;
gill-sacs, 282, 423;
vascular system, 315 n.;
distribution, 423;
habits, 423;
eggs, 424;
_B. stouti_, 423;
embryo, 425
Bdellostomatidae, _423_
_Belodontichthys_, _588_
_Belone_, _638_, 411;
_B. annulata_, 638
_Belonesox_, _616_
_Belonoglanis_, _588_, 589
Belonorhynchidae, _485_, 488
_Belonostomus_, _502_ {733}
_Bembras_, _700_
_Bembrops_, _705_
Beneden, Van, 37, 39
Benham, 113
_Bentenia_, _682_
_Benthodesmus_, _679_
_Benthophilus_, _689_
_Benthosaurus_, _611_
Berycidae, _655_, 651, 652, 653, 703, 303, 361, 389
_Beryx_, _656_;
_B. affinis_ and _B. mülleri_, coloration, 165;
_B. splendens_, 655
_Betta_, _669_;
_B. pugnax_, 669
Bib, 649
_Bibronia_, _685_
_Birkenia_, _531_, 149;
_B. elegans_, 532
Birkeniidae, _531_
Bitterling, 584, 416
Black Bass, 657
Black-Fish, 610, 643
Blastocoele, 44, 49, 52, 130
Blastogenetic acceleration, 84
Blastopore, 20, 130
Blastosphere, 20, 130
Blastozooid, 84, 93
Blastula, 56
Bleak, 583, 167
_Bleekeria_, _705_
Blenniidae, _709_, 651, 702, 703, 704, 163, 271, 302, 418
_Blennius_, _709_, 710
_Blepsias_, _698_
Blind Fishes, 394, 713;
of Mammoth Cave, 619
_Blochius_, _680_
Blood, of Hemichordata, 15;
of Tunicata, 49;
of Amphioxus, 124;
of Fishes, 341
Blood-glands, 342
Blue Shark, 448
Boar-Fish, 666
Boas, 548 n.
_Bola_, _582_
_Boleophthalmus_, _689_
_Boleosoma_, _659_
_Boltenia_, _75_;
_B. pachydermatina_, _B. tuberculata_, dorsal tubercle, 79
Bolteninae, _75_
Bombay-duck, 613
Bonnet Shark, 450
Bonnier and Pérez, 95
Borgert, 99
_Bothriolepis_, _534_;
_B. canadensis_, 534
_Botia_, _582_
Botryllidae, _88_, 81, 82, 84, 110;
vessels, 42;
neural gland, 52 n.;
reproductive organs, 56;
ascidiozooid, 82
_Botrylloides_, _88_
_Botryllus_, _88_, 36, 90;
stigmata, 59;
diagram of budding, 90;
_B. schlosseri_, 80;
_B. violaceus_, 89
Bottard, 669, 696 n.
Boulenger, 459 n., 477 n., 481 n., 551 n., 584 n., 586 n., 593 n., 683 n.
_Boulengerella_, _575_
Bourne, 19
Boveri, 113, 126
_Bovichthys_, _706_
Bow-Fin, 499 = _Amia_, q.v.
_Box_, _664_, 665, 264, 276;
_B. vulgaris_, rectal caecum, 254
_Brachionichthys_, _720_
Brachiopoda, 35
_Brachychalcinus_, _575_
_Brachymystax_, _565_
_Brama_, _682_;
_B. longipinnis_, 682 n.;
_B. raii_, 682 n.
Bramidae, _682_, 676
Branchial bars, of Amphioxus, 120, 122
Branchial cavity, of Tunicata, 43, 101
Branchial clefts, 155, 277 f.
Branchial sac, 43, 44, 45, 53;
of Appendicularians, 67;
of Thaliacea, 96, 104, 105
Branchial septa, of Hemichordata, 12;
of Amphioxus, 120
_Branchiostoma_, _137_, 112, 138;
distribution, 138;
_B. belcheri_, 137, 138;
_B. californiense_, 137;
_B. capense_, 137;
_B. caribbaeum_, 137;
_B. elongatum_, 137, 138;
_B. indicum_, 137;
_B. lanceolatum_, 112, 114 n., 115, 137, 138;
transverse section, 118, 121;
nephridium, 126;
nervous system, 128, 129;
gonads, 129;
_B. nakagawae_, 137, 138;
_B. pelagicum_, 129 n., 137, 138
—see also Amphioxus
Branchiostomatidae, _137_, 112
Brauer, 571
Breathing sounds, 357
Breathing-valves, 288
_Bregmaceros_, _648_
_Breitensteinia_, _588_
_Brevoortia_, _563_
Bridge, 555, 558 n.
Bridge and Haddon, 574 n., 587 n., 590 n.
Brill, 687
Brooks, 39, 101 n., 105
_Brosmius_, _648_
_Brosmophycis_, _712_
_Brotula_, _712_, 713
_Brychaetus_, _557_
_Brycon_, _575_;
_B. fulcatus_, mouth, 577
_Bryconaethiops_, _575_
_Bryconodon_, _575_
_Bryconops_, _575_
_Bucklandium_, 589
Budding, in Pterobranchia, 24, 27;
in Tunicata, 71, 80, 81 f., 90, 97, 103
Budgett, 482, 483 n., 514, 552, 558
Bull-heads, 698
Bullhead Sharks, 444
_Bunocephalichthys_, _596_
_Bunocephalus_, _596_
Burbot, 649 {734}
Bursa Entiana, 254
Bury, 31 n.
_Butirinus_, _548_
Butter-Fish, 712, 415
_Bythitis_, _712_
_Cachius_, _582_
Cadophore, 97
Caecum cloacae, 276
_Caenotropis_, _576_
_Caesio_, _664_, 264
_Caesioperca_, _659_
_Calamichthys_, _484_, 158, 202;
distribution, 484;
_C. calabaricus_, 484
_Calamostoma_, _634_
_Callanthias_, _659_
_Callichrous_, _588_
Callichthyinae, _588_
_Callichthys_, _588_, 587, 153, 302;
intestinal respiration, 292;
_C. littoralis_, 592;
_C. paleatus_, 414, 592
_Calliodon_, _674_
Callionymidae, _706_, 704
_Callionymus_, _707_;
_C. carebares_, 707;
_C. lyra_, 707, 420 n.
_Callomystax gagata_, stridulating mechanism, 356
_Callophysus_, _588_
_Callopristodus_, _446_
_Callopterus_, 204
_Callorhynchus_, _471_;
vertebral column, 199;
frontal clasper, 223;
branchial blood-vessels, 334;
distribution, 471;
egg-case, 471;
_C. antarcticus_, 470
_Calotomus_, _674_
Campanula Halleri, 393, 394
_Campostoma_, _582_
Candiru, 593
_Canobius_, _487_
_Cantharus_, _664_
Capelin, 568
_Capoëta_, _582_
_Caprodon_, _659_
Caproidae, _666_, 653
_Capros_, _666_;
_C. aper_, 666, 357
Carangidae, _677_, 652, 676, 158, 303
_Carangodes_, _677_
_Carangopsis_, _677_
_Caranx_, _677_, 161, 284;
with Medusae, 643;
_C. hippos_, 363;
_C. rhonchus_, 363;
_C. trachurus_, 677, 307
_Carapus_, _579_
Carbonnier, 592
_Carcharias_ (_Carcharinus_), _448_;
succession of teeth, 250;
_C. glaucus_, 448;
_C. nicaraguensis_, 448
Carchariidae, _448_, 449
_Carcharinus_—see _Carcharias_
_Carcharodon rondeletii_, _451_, 452
_Careproctus_, _698_
Cariba, 578
Carp, 583, 584;
Leather-, 584
_Carpiodes_, _581_
Caryo-enteric, 102, 108
Castle, 56 n.
Cat-Fishes, 710, 587
_Cataetyx_, _712_
_Cathetostoma_, _706_
_Catla_, _582_;
_C. buchanani_, 584
_Catlocarpio_, _582_
_Catopra_, _658_
_Catoprion_, _576_
Catopteridae, _488_, 486
Catosteomi, _626_ f., 306
Catostominae, _581_
_Catostomus_, _581_
_Caturus_, _499_;
_C. furcatus_, 499;
vertebrae, 203
Caudal fin, 156, 159, 237
Caullery, 85
_Caulolatilus_, _661_
_Caulolepis_, _656_
_Caulophryne_, _719_
Cave-Fishes, 394 f., 618 f., 713
Centrarchidae, _657_, 653
_Centrarchus_, _657_
_Centrina_, _455_;
_C. salviani_, 455
Centriscidae, _633_, 628, 357
_Centriscus_, _633_, 154;
_C. scolopax_, _633_;
scales, 189, 190;
stridulation, 357
_Centrogenys_, _659_
_Centrolepis_, _487_
_Centrolophus_, _643_;
_C. britannicus_, 643;
_C. niger_, 643
_Centromochlus_, _588_, 361
_Centronotus_, _711_
_Centrophorus_, _455_
_Centropogon_, _695_
Centropominae, _660_
_Centropomus_, _660_
_Centropristes_, _659_
_Centroscyllium_, _455_
_Cephalacanthus_, _702_
Cephalaspidae, _528_
_Cephalaspis_, _529_, 149;
_C. lyelli_, 529;
_C. magnifica_, 529;
_C. murchisoni_, 528
Cephalochordata, _112_ f., 4, 38
_Cephalodiscus_, _21_ f., 5, 28, 29, 31, 32;
distribution, 23;
budding, 24;
structure, 24 f.;
males, 26;
_C. dodecalophus_, 22, 23, 24, 25
_Cepola_, _662_;
_C. rubescens_, 662
Cepolidae, _661_, 653
_Ceratias_, _719_;
phosphorescent organ, 174, 178;
_C. bispinosus_, 174
_Ceratichthys_, _582_
Ceratiidae, _719_, 718
Ceratodontidae, _507_
_Ceratodus_, _508_, 519
_Ceratoptera_, _465_;
_C. vampyrus_, 466
Ceratotrichia, 234
_Cerdale_, _709_
_Cestracion_ = _Heterodontus_, q. v.
_Cetengraulis_, _563_ {735}
Cetomimidae, _614_, 606
_Cetomimus_, _614_
_Cetopsis_, _588_, 587
Cetorhinidae, _453_
_Cetorhinus_, see _Selache_
_Chaca_, _588_;
_C. lophioides_, 589
_Chaenichthys_, _706_
_Chaenobryttus_, _657_
_Chaenopsis_, _709_
_Chaerops_, _673_
_Chaetobranchus_, _672_
_Chaetodon_, _668_
Chaetodontidae, _667_, 654, 361;
coloration, 166
_Chaetostomus_, _595_;
_C. cirrhosus_, sexual differences, 594;
_C. gigas_, 595
_Chalceus_, _575_;
_C. angulatus_, mouth, 577
_Chalcinus_, _575_
Chamisso, 36
_Champsocephalus_, _706_
_Champsodon_, _641_;
_C. vorax_, 642
Chaninae, _563_
_Channa_, _645_
_Channalabes_, _588_, 589
_Channomuraena_, _605_;
_C. vittata_, 605
_Chanoides_, _563_
_Chanos_, _563_, 563;
_C. salmoneus_, gill-helix, 294
_Characidium_, _576_
Characinidae, _575_, 574, 294;
distribution, 576;
teeth, 250;
sound production, 361;
Weberian ossicles, 389, 573
_Characodon_, _616_
_Charax_, 665
_Charitosomus_, _572_
Charr, 567
_Charybdia_, _685_
_Chascanopsetta_, _687_
_Chasmodes_, _709_
_Chatoessus_, _563_, 563
Chauliodontinae, _571_
_Chauliodus_, _571_, 570, 572
_Chaunax_, _720_
_Cheiracanthus_, _442_
_Cheirodus_, _488_;
_C. granulosus_, 488
_Cheirolepis_, _485_, 487
_Chela_, _582_
_Chelaethiops_, _582_
_Chelidoperca_, _659_
_Chelmo_, _668_
_Chelyosoma_, _73_;
_C. macleayanum_, 72
Chiaje, Delle, 17
_Chiasmodon_, _641_, 642
Chiasmodontidae, _641_, 637
_Chilinus_, _673_
_Chilio_, _673_
_Chilobranchus_, _598_
_Chilodactylus_, _664_
Chilodipterinae, _660_
_Chilodipterus_, _660_
_Chiloglanis_, _588_
_Chilorhinus_, _601_
_Chiloscyllium_, _446_, 447;
pectoral girdle, 239;
pectoral fin, 239;
pelvic girdle, 240;
pelvic fin, 240;
brain, 374
_Chimaera_, _469_, 382;
vertebral column, 199;
skull, 223;
frontal clasper, 223, 469;
gills, 282, 283;
lateral sensory organs, 386;
auditory organ, 387, 388;
distribution, 469;
egg-case, 470;
segmentation of egg, 474;
fossil, 474;
_C. affinis_, 469;
_C. colliei_, 469, 473;
_C. monstrosa_, 469;
vertebral column, 199;
skull, 223;
pectoral fin, 243;
spiral valve, 267
Chimaeridae, _468_, 474
_Chimaeropsis_, _468_
_Chimarrhichthys_, _705_
Chirocentridae, _561_, 544
_Chirocentrites_, _561_
_Chirocentrodon_, _563_
_Chirocentrus_, _562_;
vestigial spiral valve, 269;
_C. dorab_, skull and pectoral arch, 562
_Chirodon_, _575_
_Chirolophius_, _718_;
_C. naresii_, 718
_Chironemus_, _664_
_Chirostoma_, _639_
Chirothricidae, _615_, 606
_Chirothrix_, _615_;
_C. libanicus_, 615
Chlamydoselachidae, _443_
_Chlamydoselachus_, _443_, 279;
rectal gland, 276;
branchial clefts, 277;
efferent branchial vessels, 332 n.;
_C. anguineus_, 443, 444;
_C. lawleyi_, 443
_Chlamydothorax_, _17_ n.
_Chlorophthalmus_, _611_;
_C. gracilis_, 613
_Chloroscombrus_, _677_
_Chologaster_, _618_;
_C. agassizii_, 618;
_C. cornutus_, 618;
_C. papilliferus_, 618
_Chondrostachys_, _85_
Chondrostei, _485_ f., 149
Chondrosteidae, _489_, 486
_Chondrosteus_, _489_;
_C. acipenseroides_, 489, 490
_Chondrostoma_, _582_
_Chonerhinus_, _726_
Chorda-centra, 196
Chordata, characters of, _3_ f., 35, 38
_Choridactylus_, _695_, 692
_Chorimycterus_, _576_
_Chorinemus_, _677_
_Chorisochismus_, _709_
_Chorizocormus_, _89_;
_C. reticulatus_, 90
Choroid gland, 393, 394
Chromatophores, 166
Chromides, _671_
_Chrysichthys_, _588_
_Chrysophrys auratus_, 420
_Cichla_, _672_, 671
Cichlidae, _670_, 654;
distribution, 672;
eggs carried in the mouth, 416
_Cichlops_, _661_
Ciliation, of the alimentary canal, 262 {736}
_Cimolichthys_, _609_
_Ciona_, _72_;
development, 56 n.;
_C. intestinalis_, 73, 77
_Cirrhilabrus_, _673_
_Cirrhites_, _660_
_Cirrhitichthys_, _660_
Cirrhitinae, _660_;
coloration, 166
Cirri, of Amphioxus, 128
_Citharichthys dinoceros_, _685_
_Citharidium_, _576_
Citharininae, _576_
_Citharinus_, _576_, 578;
_C. geoffroyi_, 579
_Citharus_, _687_
Cladistia, _481_ f., 477
_Cladocyclus_, _561_
_Cladodus_, _438_;
_C. neilsoni_, _438_
_Cladoselache_, _438_, 148, 153, 160, 162, 197;
pectoral and pelvic girdles, 239;
pectoral and pelvic fins, 242, 245;
characters, 436 f.;
_C. fyleri_, 437
Cladoselachidae, _438_
_Clariallabes_, _588_
_Clarias_, _588_, 153, 302;
accessory respiratory organs, 293, 294;
_C. anguillaris_, 590;
_C. lazera_, 590
Clariinae, _588_
Clarke, 608
_Clarotes_, _588_
Claspers, 162, 246, 414, 432, 469, 470, 471
_Clavelina_, _71_, 83;
nervous system, 53;
stigmata, 59;
hibernation, 88;
_C. lepadiformis_, 71, 72
Clavelinidae, _71_, 81, 83, 109, 110
_Climatius_, _441_
Climbing Perch, 645, 292
Cling-Fishes, 708
_Clinus_, _709_
Cloaca, of Tunicata, 44, 45, 81, 82, 92;
of Fishes, 156, 256
Cloacal aperture, of Tunicata, 80
Cloacal cavity, of Hemimyaria, 101
Club-shaped gland, 134
_Clupea_, _563_, 564;
_C. alosa_, 564;
_C. finta_, 564;
_C. harengus_, 564, 307; = Herring (_q.v._);
_C. pilchardus_, 564;
_C. sprattus_, 564
_Clupeichthys_, _563_
Clupeidae, _562_, 544;
gill-helix, 294;
connexion of air-bladder with auditory organ, 303, 389;
eggs, 412
Clupeinae, _563_
_Cnidoglanis_, _588_
Coal-Fish, 649
Cobitidinae, _582_, 585
_Cobitis_, _582_, 271;
_C. taenia_, 358
_Cobitopsis_, _639_;
_C. acuta_, 639
_Coccodus_, _498_
_Coccolepis_, _487_
_Coccosteus_, _535_, 149, 536;
_C. decipiens_, 535, 536
_Cochliodon_, _595_
Cochliodontidae, _445_;
teeth, 251
_Cochliodus_, _445_
_Cochlognathus_, _582_
Cod, 648, 648; = _Gadus morrhua_, q.v.
Coelacanthidae, _480_, 481
_Coelacanthus_, _481_, 160
Coelocormidae, _86_
_Coelocormus huxleyi_, _86_;
reproductive organs, 56
Coelolepidae, _524_, 530
Coelom (body-cavity), in Hemichordata, 8, 21, 24 f.;
in Tunicata, 44;
in Amphioxus, 123, 132 f.;
in Fishes, 397
_Coelonotus_, _634_, 635
_Coelophrys_, _720_
Coffer-Fishes, 152, 361
_Coilia_, _563_, 563
Cole, 25 n.
Cole and Johnstone, 687 n.
_Colella_, _85_, 83;
_C. pedunculata_, 85;
_C. quoyi_, 80
Collar-pore, 9, 25, 27
Collett, 714 n.
_Coloconger_, _601_
_Colocopus_, _668_
Colour, of Fishes, 164 f.;
brilliancy and variety, 164;
cause of, 166;
changes, 169;
protective value, 171;
aggressive and alluring, 173;
warning, 174
_Columbia_, _621_;
_C. transmontana_, 621
Comephoridae, _696_, 692, 694, 418
_Comephorus_, _697_, 692 n.
Compound Ascidians, 35, 36 f., 70, 71, 80 f., 110;
eggs, 56
_Conchopoma_, _507_
_Conger_, _601_;
_C. vulgaris_, skull and pectoral arch, 600
Congrogadidae, _713_, 704
_Congrogadus_, _713_
_Congromuraena_, _601_
_Conocara_, _570_
_Conorhynchus_, _588_
Conte and Vaney, 27 n.
Cope, 477 n., 542, 543, 573, 619, 626
_Copidoglanis_, _588_
_Copodus_, _446_
Coprolites, 268
_Coregonus_, _565_, 568, 311;
_C. clupeoides_, 568;
_C. lavaretus_, 568;
_C. oxyrhynchus_, 568;
_C. pollan_, 568;
_C. vandesius_, 568
_Corella_, _73_;
stigmata, 48;
_C. japonica_, branchial sac, 73;
_C. parallelogramma_, 73
Corellinae, _73_
_Coreoperca_, _659_
_Coridodax_, _674_
_Coris_, _673_
_Corvina_, _663_;
_C. lobata_, air-bladder, 304
_Corydoras_, _588_;
_C. paleatus_, 592
_Corynascidia_, _73_;
_C. suhmi_, 72;
branchial sac, 73
_Corynopoma_, _575_ {737}
_Coryphaena_, _681_
Coryphaenidae, _681_, 676
_Coryphaenoides_, _647_
_Cossyphus_, _673_
Costa, 112
Coste, 630 n.
Cottidae, _697_, 692, 694
_Cottocomephorus_, _697_, 692 n.
_Cottunculus_, _698_
_Cottus_, _698_;
_C. bubalis_, 698;
_C. gobio_, 698;
_C. scorpius_, 698, 357
_Cotylis_, _709_
Cranial nerves, 378, 380
Craniata, _4_, 38;
characters of, 141;
classification, 145
_Cranoglanis_, _588_
_Creagrutus_, _575_
_Crenicichla_, _672_
_Crenidens_, _665_
_Crenuchus_, _575_
_Crepidogaster_, _709_
Crista acustica, 60
_Cromeria_, _573_
Cromeriidae, _573_, 545
_Cromileptes_, _659_
Crossognathidae, _565_
_Crossognathus_, _565_
_Crossopholis_, _492_
Crossopterygii, _476_ f., 149;
distribution, 483
_Crossorhinus_, _447_
_Cryodraco_, _706_
_Cryphiolepis_, _487_
_Cryptopterus_, _588_
_Cryptotomus_, _674_
_Crystallaria_, _659_
_Crystallogobius_, _689_
_Ctenochaetus_, _668_
_Ctenodentex_, _665_
Ctenodontidae, _505_
_Ctenodus_, _506_
_Ctenolabrus_, _673_
_Ctenolates_, _659_
_Ctenothrissa_, _560_;
_C. vexillifer_, 559
Ctenothrissidae, _559_, 544
_Cubiceps_, _643_
Cuchia, 598, 294
Cuénot, 584 n.
_Culeolus_, _75_;
spicules, 87;
_C. murrayi_, 42;
_C. moseleyi_, dorsal tubercle, 79;
_C. wyville-thomsoni_, 75
_Culter_, _582_
Cunningham, 565 n., 602 n., 687 n.
_Curimatus_, _576_
Cutaneous sense-organs, of Fishes, 383 f., 385, 386
Cuvier, 36
_Cyathaspis_, _527_
Cyathozooid, 91, 93, 94
_Cybium_, _678_
_Cyclobatis_, _462_
Cyclomyaria, _95_, 101
_Cyclopterichthys_, _698_
Cyclopteridae, _698_, 692, 694, 302
_Cyclopterus_, _698_, 321, 408, 415;
sucker, 162;
_C. lumpus_, 699;
rectal valve, 254
_Cyclosalpa_, _108_;
_C. pinnata_, 102, 108
Cyclospondylic, 198
Cyclostomata, _421_ f., 145 f.;
characters, 146;
external features, 150;
skeleton, 197 f.;
teeth, alimentary canal and digestive glands, 247 f.;
respiratory organs, 279 f.;
vascular system, 315 f.;
blood-glands, 343;
nervous system, 367 f.;
organs of special sense, 383 f.;
kidneys and reproductive organs, 399 f.
_Cyclothone_, _571_
_Cymatogaster_, _670_
_Cynoglossus_, _687_;
_C. lingua_, 686;
_C. semilaevis_, 393
_Cynolebias_, _616_
_Cynthia_, _75_;
branchial sac, 74;
_C. cerebriformis_, dorsal tubercle, 79;
_C. formosa_, 76;
dorsal tubercle, 79;
_C. papietensis_, dorsal tubercle, 79
Cynthiidae, _74_, 64, 110;
branchial sac, 48;
neural gland, 52 n.;
reproductive organs, 55;
eggs, 56;
dorsal tubercle, 79
Cynthiinae, _75_;
tentacles, 75
Cyphosidae, _657_, 653
_Cyphosus_, _657_
Cyprinidae, _581_, 575, 271;
ventral suckers, 162;
absence of jaw-teeth, 251;
air-bladder, 302;
breathing sounds, 358;
connexion of auditory organ with air-bladder, 389
Cyprininae, _582_, 583
_Cyprinion_, _582_
_Cyprinodon_, _616_
Cyprinodontidae, _616_, 606, 163, 275, 414, 418;
distribution, 617
_Cyprinus_, _582_, 155;
pharyngeal teeth, 252;
_C. auratus_, eyes, 155;
coloration, 171;
_C. carassius_, 585;
_C. carpio_, 583, 307, 358
Cystoarian, 403
_Cystodytes_, _85_;
spicules, 87
_Cyttoides_, _683_
_Cyttopsis_, _683_
_Cyttus_, _683_
Dab, 687;
Long Rough, 687;
Smear, 687
_Dactylopogon_, _611_
Dactylopteridae, _701_, 694, 361
_Dactylopterus_, _701_, 693;
_D. volitans_, 161, 355, 357, 361, 399, 701;
pectoral arch, 693
_Dallia_, _610_;
_D. pectoralis_, 610, 611
Dalliidae, _610_, 606
_Danio_, _582_
_Dapedius_, _498_, _541_ n.;
_D. politus_, 498
_Dapedoglossus_, _557_;
_D. testis_, 556
Dareste, 721 {738}
Darters, 659
_Dascyllus_, _672_
_Dasyatis sabina_, _464_
_Dasyscopelus_, _611_
_Datnioides_, _658_
Dawydoff, 16 n.
Day, 569 n., 593 n.
Deal-Fish, 715
Delhez, 551
_Deltodus_, _445_
_Deltoptychius_, _445_
Demersal, 408
_Dendrodoa_, _76_
_Dentex_, _664_
Dentition, 247 f.;
dentinal tissues, 249;
fixation of teeth, 249;
succession and replacement, 250;
shape, 251;
sexual differences, 251;
pharyngeal teeth, 251;
Cyclostomata, 247;
_Petromyzon_, 248;
Fishes, 248 f.;
_Scyllium_, 249;
_Carcharias_, 250
Dercetidae, _623_, 622
_Dercetis_, _623_
_Derepodichthys_, _712_
_Derichthys_, _601_
Dermal denticles, 183 f.;
structure and development, 183, 184, 185
_Diagramma_, _664_
_Dibranchus_, _720_
_Dicerobatis_, _465_
_Dicrotus_, _679_
Didemnidae, _86_, 82, 110
_Didemnum_, _87_
_Didymaspis_, _530_
_Dinichthys_, _537_, 149
_Dinolestes_, _659_
_Dinoperca_, _659_
_Diodon_, _726_, 163, 354, 361;
scales, 191;
teeth, 251;
_D. geometricus_, 726;
_D. hystrix_, 364
Diodontidae, _726_, 722
Diphycercal, 159
Diplacanthidae, _441_
_Diplacanthopoma_, _712_, 713
_Diplacanthus_, _441_
_Diplesium_, _659_
_Diplocrepis_, _709_
_Diplomystes_, _588_, 587
_Diplomystus_, _563_
_Diplophos_, _571_
_Diplophysa_, _582_
_Diplopterus_, _477_
_Diplosoma_, _87_;
section, 87
Diplosomatidae, _87_, 110
_Diplosomoides lacazii_, larva, 78
Diplospondylic, 198
_Diplurus_, _481_, 161
Dipneusti—see Dipnoi
Dipnoi (Dipneusti), _505_ f., 149;
distribution, 512
_Dipterodon_, _665_
_Dipterus_, _506_, 519;
_D. valenciennesi_, 506
_Diptychus_, _582_
Disartete, 636 n.
Discocephali, _691_ f., 651, 652
_Discognathus_, _582_
_Discopyge_, _464_
_Distaplia_, _85_;
_D. magnilarva_, larva, 78
Distichodontinae, _576_
_Distichodus_, _576_;
_D. niloticus_, mouth, 577
_Distoma_, _85_
Distomatidae, _85_, 81, 82, 83, 86, 110;
reproductive organs, 56;
ascidiozooid, 82;
transverse section, 86
_Ditrema_, _670_;
_D. temminckii_, 670
Dog-Fishes, _446_
_Dolchinia_, _96_, 100;
_D. mirabilis_, 100
_Dolichoglossus_, _17_, 5, 13;
_D. kowalevskii_, 6, 7, 9, 20;
_D. otagoensis_, 11
_Dolichorhynchus_, _137_;
_D. indicus_, 137
Doliolidae, _96_, 39, 110
_Doliolum_, _96_, 37;
structure, 96;
life-history, 97, 98;
occurrence, 99;
budding, 97;
polymorphism, 98;
_D. nationalis_, 99;
_D. tritonis_, 96, 99
Dollo, 518
_Dolopichthys_, _719_
Dolphins, 681
Doradinae, _588_
Dorado, 578
_Doras_, _588_, 590, 592, 363;
intestinal respiration, 292;
stridulation, 357;
elastic-spring-apparatus, 359;
_D. maculatus_, air-bladder, 303;
nature of sounds, 362
Dorsal lamina, 45, 47, 52, 101, 105
Dorsal pore, 18, 19, 31
Dorsal tubercle, 52, 79
_Doryichthys_, _634_, 635
_Doumea_, _588_
_Doydixodon_, _664_
_Draconetta_, _706_
Dragonet, 707
Drasche, 38
Drepanaspidae, _525_, 530
_Drepanaspis gemündenensis_, _526_, 530, 525, 526
_Drepane_, _668_
Drepanidae, _668_, 654
Drum, 663, 304, 362
_Ductor_, _677_
Dufosse, 355
_Dussumieria_, _563_, 563
Dybowski, 697 n.
_Dysichthys_, _596_
_Dysomma_, _603_
_Dysommatopsis_, _603_
Eagle Rays, _465_
Echeneididae, _691_
_Echeneis_, _691_;
cephalic sucker, 161
_Echidna_, _605_ {739}
_Echidnocephalus_, _624_
_Echinorhinus_, _455_;
_E. spinosus_, 456
_Echiostoma_, _571_
_Ecteinascidia_, _71_
_Edaphodon_, _474_
Edinger, 271
Eels, 599 f., 601 f., 163, 405
Eel-Fares, 602
_Egertonia_, _674_
Eggs, of Hemichordata, 20, 26;
of Tunicata, 55, 73, 84, 97, 106;
of Amphioxus, 130;
of Fishes, 408 f., 423, 424, 428, 432, 433, 435, 456, 470, 471, 494,
504, 510;
alecithal and telolecithal, 410;
holoblastic and meroblastic segmentation, 409, 410;
micropyles, 411, 412;
deposition, 411;
attachment, 411, 424, 433, 434;
demersal and pelagic, 411, 412;
spawning, 412;
relative fecundity, 412;
fertilisation, 413;
sexual relations, 413;
sexual congress, 414, 427, 428, 432;
time of hatching, 417, 433, 494, 500, 504;
influence of temperature, 417;
of _Aspredo_, 596;
of _Batrachus tau_, 711
Ehrenbaum, 685 n.
Eigenmann, 602 n., 619 n., 670 n.
_Eigenmannia_, _579_
_Elacate_, _678_
Elaeoblast, 107, 107
Elasmobranchii, _431_ f., 148
_Elasmodus_, _474_
_Elassoma_, _657_
Electric, Cat-Fish, 591;
Eel, 580, 365 f.;
Rays, 462;
Mormyridae, 550;
organs, 365 f.
_Eleginops_, _706_
_Eleotris_, _689_;
_E. marmorata_, 689
_Ellipesurus_, _465_
_Elonichthys_, _487_
Elopidae, _546_, 544
_Elopopsis_, _547_
_Elops_, _547_;
_E. lacerta_, 547;
_E. saurus_, 547
Elvers, 602
_Embiotoca_, _670_
Embiotocidae, _670_, 654, 418, 419
_Embolichthys_, _705_
Embryology (development), of Hemichordata, 18;
of Tunicata, 55;
of Amphioxus, 130;
of Fishes, 417
Embryonic nutrition;
food-yolk, 409, 417;
ovarian secretion, 419;
oviducal or uterine secretion, 434, 435;
placentas, 98, 101, 107, 434
Emery, 622, 623, 685 n., 714 n.
_Emmnion_, _709_
_Empetrichthys_, _616_
_Encheliophis_, _625_;
_E. vermicularis_, 625
_Enchelycore_, _605_
Enchodontidae, _608_, 606
_Enchodus_, _609_
End-buds, 383
Endostyle, 46, 53, 58, 67;
of Amphioxus, 122
Engraulinae, _563_
_Engraulis_, _563_;
_E. encrasicholus_, 564
_Enneanectes_, _709_
Entero-epicardiac budding, 82
Enteropneusta, _5_ f., 30 n.;
distribution, 5, 6;
coloration, 7;
habitat, 6, 7;
divisions of body, 7;
burrowing, 7;
body-cavities, 8;
body-wall, 9;
nervous system, 9;
alimentary canal, 11;
vascular system, 15;
excretion, 15;
reproductive organs, 16;
regeneration, 16;
development, 18 f.
_Entersphenus_, _426_
_Eocottus_, _698_
_Eothynnus_, _678_
_Epapterus_, _589_
_Ephippion_, _726_
_Ephippus_, _668_;
_E. faber_, 668
_Epibulus_, _673_
Epicardiac budding, 81
Epicardium, 44, 60, 71, 83
_Epigonus_, _659_
_Epinephelus_, _659_, 660
_Epinnula_, _679_
Epipharyngeal groove, 123
_Eques_, _663_
_Equula_, _663_
_Eremophilus_, _589_
_Eretmodus_, _672_
_Ereunias_, _698_
Erythrininae, _575_
_Erythrinus_, _575_, 578;
air-bladder, 305, 306;
_E. taeniatus_, _E. braziliensis_, air-bladder as a respiratory organ,
291
Esocidae, _609_, 606, 275;
distribution, 610
_Esox_, _609_;
teeth, 250, 262;
_E. lucius_, 609, 307;
skeleton, 609;
_E. nobilior_, 609;
_E. lepidotus_, 610
Essence orientale, 585
_Esunculus_, _548_
_Etelis_, _660_
_Etheostoma_, _659_
_Etroplus_, _672_
_Etrumeus_, _563_
_Euanemus_, 359
_Eucalia_, _630_
_Euchilichthys_, _588_, 587
_Eucirrhichthys_, _582_
_Eugnathichthys_, _576_
Eugnathidae, _498_, 497
_Eugnathus_, _499_
_Eugyra_, _77_, 78;
_E. glutinans_, 79;
_E. kerguelenensis_, branchial sac, 77
_Eukeraspis pustulifera_, 529
_Eumeda_, _588_
_Euoxymetopon_, _679_
_Euphanerops_, _532_
_Eupleurogrammus_, _679
Eurycormus_, _499_; {740}
_E. speciosus_, vertebrae, 203
_Eurynotus_, _487_, 488;
_E. crenatus_, 487
_Eurypharynx_, _604_
_Eurypholis_, _609_
_Eusthenopteron_, _478_, 202, 246;
_E. foordi_, 479
_Euthacanthus_, _441_
_Euthynotus_, 204
_Eutropiichthys_, _587_
_Eutropius_, _588_
_Exocoetoides_, _615_
_Exocoetus_, _638_, 161, 173, 353, 411
_Exoglossum_, _582_
_Exostoma_, _588_, 586, 587
External characters, Cyclostomata, 150 f.;
Fishes, 152 f.
Eycleshymer, 592 n.
Eye, 155, 393, 394, 395;
degeneration in deep-sea and cave Fishes, 394;
telescopic, 395
Eyelids, 395
Facciola, 685 n.
Fatio, 569 n.
Felchen, 568
_Fierasfer_, _625_, 622, 399;
_F. acus_, 156, 626
Fierasferidae, _625_, 623
File-Fishes, 724
Filippi, 690 n.
Fins, of Fishes, 156;
median, 156;
paired, 157;
pectoral, 157;
lobate, 157;
pelvic, 158;
caudal, 159;
modified to form suckers, 161;
degeneration and atrophy, 162;
endoskeletal elements, 234, 235, 236, 237, 239, 240, 241, 242, 243,
244, 245, 246;
exoskeletal elements, 158, 234, 236
Fin-rays, of Amphioxus, 120;
of Fishes, 158, 234, 236
Fishes, 141 f.;
systematic position and classification, 141;
external features, 152;
coloration, 164;
poison glands, 176;
phosphorescent organs, 178;
skin and scales, 182;
skeleton, 193;
dentition, alimentary canal, and digestive glands, 247;
respiratory organs, 277;
air-bladder, 297;
vascular system, lymphatics, and blood-glands, 313;
muscular system, locomotion, sound-producing organs, and electric
organs, 349;
nervous system and organs of special sense, 367;
kidneys and reproductive organs, breeding, 397;
systematic, 431 f.
Fishing-Frog, 718
_Fistularia_, _632_, 154;
_F. tabaccaria_, 632
Fistulariidae, _632_, 628
Flat-Fishes, 685
Flounder, 687;
coloration, 167, 170
Flute-mouths, 154
Flying-Fish, 638, 161, 173, 353, 411;
fresh-water, 559
Flying Gurnards, 701, 355, 361
Fol, 38
_Forbesella_, _75_;
_F. tessellata_, 76;
dorsal tubercle, 79
Forskål, 36
Fowler, 23
Frilled Sharks, 443
_Fritillaria_, _69_, 70;
_F. furcata_, 70;
_F. megachile_, 66
Fritsch, 551, 591 n.
Frost-Fish, 679
_Fundulus_, _616_
Gadidae, _647_, 702, 158, 303, 307, 361, 389, 412;
photophores, 179
_Gadomus_, _647_
_Gadopsis_, _709_
Gadow, 193 n.
_Gadus_, _648_, 168;
_G. aeglefinus_, 649, 308, 361;
_G. luscus_, 649;
_G. merlangus_, 649;
coloration, 168;
pyloric caeca, 275;
_G. morrhua_, 648, 648;
vertebrae, 205;
skull, 230;
pseudobranch, 284;
air-bladder, 303;
red gland, 307, 308;
vascular system, 320, 323, 336, 337;
spleen, 343;
sounds, 361;
_G. pollachius_, 649;
_G. virens_, 649;
sensory canals, 386;
tail, 646
_Gagata_, _588_
_Galaxias_, _607_, 608;
_G. attenuatus_, 607, 608;
_G. brevipinnis_, 607
Galaxiidae, _607_, 606, 163, 405;
distribution, 607
_Galeichthys_, _588_, 587, 593
_Galeocerdo_, _448_;
_G. arcticus_, 448;
_G. tigrinus_, 448
_Galeoides_, _641_
_Galeus_, _449_, 298;
_G. canis_, 449
_Gambusia_, _616_;
development of embryos in ovisacs, 418
_Ganodus_, _474_
Ganoidei, _149_
Gar-Fish, 411 (_Belone_)
Gar-Pike, 638 (_Belone_);
503 (_Lepidosteus_)—see also _Lepidosteus_
Garman, 616 n., 619, 698 n.
Gastric glands, 270
_Gastrochisma_, _678_
_Gastromyzon_, _582_;
_G. borneensis_, 586;
sucker, 162
_Gastropelecus_, _575_
Gastrosteidae, _629_, 627;
distribution, 631
_Gastrosteus_, _630_, 163, 169, 271, 288;
_G. aculeatus_, 357, 630;
pectoral arch of, 630;
_G. pungitius_, 630;
_G. spinosus_, 354;
_G. texanus_, 631 n.
_Gastrostomus_, _604_
_Gastrotoceus_, _634_, 635
Gastrula, 20, 56, 130, 131
_Gavialiceps_, _603_ {741}
_Gazza_, _663_
Gegenbaur, 5 n., 37, 570 n.
_Gempylus_, _679_
_Genidens_, _588_
Genital pores, 403
Genital wings, of Enteropneusta, 16, 10
_Genyomyrus_, _551_, 549
_Genypterus_, _713_
Geoffroy, 677 n.
_Geophagus_, _672_
_Geotria_, _426_
_Gephyroberyx_, _656_
Gephyrocercal, 161
_Gephyroglanis_, _588_
Gerbe, 673 n.
_Gerlachia_, _706_
_Gerres_, _663_
Gerridae, _663_, 654
Giard, 38, 39
Gigantactinidae, _720_, 718
_Gigantactis vanhoeffeni_, _720_
Gilbert, 548
_Gilbertia_, _659_
Gilchrist, 571
Gill, of _Salpa_, 105
Gill, T., 542, 570 n., 599 n., 629
Gill-helix, 294
Gill-slits, 4;
of Enteropneusta, 11, 12, 20, 30;
of Pterobranchia, 25, 27;
of Tunicata, 47, 67, 105;
of Amphioxus, 120;
of Fishes, 155, 277 f.
Gillaroo Trout, 260
Gills—see Respiratory organs
_Ginglymostoma_, _447_
_Girardinus_, _616_;
sexual congress, 414
_Girella_, _664_
_Glandiceps_, _17_, 5, 6, 13, 14, 15;
_G. abyssicola_, 6;
_G. hacksi_, 6;
_G. talaboti_, 5
Glandicipitidae, _17_
Glandula pterygopodia, 182
_Glanidium_, _588_
_Glaniopsis_, _582_
_Glaucosoma_, _660_
Globe-Fishes, 726, 152, 163
Glomerulus, of Enteropneusta, 15;
of Fishes, 398, 399
Glomus, 398
_Glossobalanus_, _17_, 5, 13;
_G. minutus_, 10, 11;
_G. ruficollis_, 17 n.;
_G. sarniensis_, 5, 10
_Glossodus_, _446_
_Glyphidodon_, _672_
_Glyptocephalus_, _687_;
_G. cynoglossus_, 687;
_G. microcephalus_, 687
_Glyptolepis_, _480_
_Glyptopomus_, _477_
_Glyptosternum_, _588_
_Gnathacanthus_, _695_
_Gnathonemus_, _551_, 549;
_G. curvirostris_, 550;
_G. numenius_, 550
Gnathostomata, _145_;
characters of, 147
Gobiesocidae, _707_, 704
_Gobiesox_, _709_
Gobiidae, _689_
Gobiiformes, _688_ f., 651, 652
_Gobio_, _582_
_Gobiodon_, _689_
_Gobiosoma_, _689_
_Gobius_, _689_, 690;
_G. minutus_, care of eggs, 415;
_G. ruthensparri_, 689
Gold-Fish, 585, 155, 171
_Gomphosus_, _673_
Gonads (reproductive organs), of Hemichordata, 16, 25;
of Tunicata, 55, 67, 76, 93, 97, 105;
of Amphioxus, 129;
of Fishes, 402 f., 400, 403
Gonoducts, of Fishes, 404 f.;
Elasmobranchs, 400, 404;
Teleostei, 400, 404, 406;
Holocephali, 405;
Dipnoi, 405, 406, 407;
Crossopterygii, 405;
Chondrostei, 405;
Holostei, 405
Gonorhynchidae, _572_, 545
_Gonorhynchus greyi_, _572_, 572
_Gonostoma_, _571_
Gonostomatinae, _571_
Gonozooid, 99, 100
Goode, Brown, 680 n., 710, 725
Goode and Bean, 614 n.
Goodrich, 29, 113, 126
_Goodsiria_, _89_;
_G. placenta_, 89
Gourami, 669
_Grammicolepis_, _683_
_Grammistes_, _660_
Grammistinae, _660_
Grassi, 21 n.
Grassi and Calandruccio, 602
Grayling, 568
Greene, 711 n.
Greenland Shark, 455
Gressin, 705 n.
Grey Mullets, 640
Grobben, 39
Groove of Hatschek, 128
Guanin, 167
Guaninkalk, 167 n.
Guitel, 690 n., 709 n., 710
Gunnel, 712
Günther, 543, 557, 569 n., 621 n. 632, 674, 711 n., 719
Gurnards, 701—see also _Trigla_
Gwyniad, 568
_Gymnallabes_, _588_
_Gymnapistus_, _695_
Gymnarchinae, _551_
_Gymnarchus_, _551_;
larval gills, 290, 418, 419;
abdominal pores, 401;
size of eggs, 408;
_G. niloticus_, 552;
embryo, 419
_Gymnelis_, _712_
Gymnoarian, 403
_Gymnocypris_, _582_
Gymnodontes, _725_ f., 721, 272, 361
_Gymnodraco_, _706_ {742}
Gymnotidae, _579_, 574, 163, 256, 302, 389
_Gymnotus_, _579_, 581, 649 n.;
electric organs, 365, 366;
_G. electricus_, 580
_Gyrinochilus_, _582_
_Gyrodus_, _498_
_Gyrolepis_, _487_
_Gyroptychius_, _479_
_Gyrosteus_, _489_
Haddock, 649, 308, 361
Haddon, 289
_Haemulon_, _664_
Hag-Fishes, 421
Hake, 649, 308
Haldeman, 19 n.
_Halec_, _609_
Half-Beak, 154, 414
Halibut, 687
_Halicmethes_, _720_
_Halieutaea_, _720_
_Halilophus mirabilis_, _21_
_Halimochirurgus_, _723_
_Haloporphyrus_, _648_
_Halosaurichthys_, _624_
Halosauridae, _623_;
photophores, 178
_Halosauropsis_, _624_;
_H. macrochir_, 621 n., 624
_Halosaurus_, _624_
Hammer-head Sharks, 449
Hancock, 53, 591 n., 592
Haplistia, _477_ n.
_Haplochilus_, _616_
_Haplochiton_, _608_
Haplochitonidae, _608_, 606
Haplodactylidae, _664_, 650, 654
_Haplodactylus_, _664_
Haplomi, _605_ f., 651, 306
_Haplonotus_, _663_
Harmout, 590
_Harpagifer_, _706_
_Harpodon_, _611_;
_H. nehereus_, 613;
_H. squamosus_, 613
_Harrimania_, _17_, 5;
_H. kupfferi_, 20 n.
Harrimaniidae, _17_
_Harriotta_, _471_, 154, 223;
distribution, 473;
_H. raleighana_, 472;
young, 473
Hartmeyer, 38
Hasselt, van, 36
Hatching, of eggs, 417
Hatschek, 113;
grove of, 128
Haus, of Appendicularians, 66
Head, shape and relative size, 153
Heart, of Tunicata, 49, 67;
of Fishes, 327, 328
Heart-vesicle, of Hemichordata, 15
Heincke, 565 n.
_Helgia_, _582_
_Heliastes_, _672_
_Helicophagus_, _588_
Heller, 38
_Helodus_, _445_
_Helogenes_, _589_
_Helostoma_, _669_
_Hemerocoetes_, _706_
Hemibranch, 278
Hemibranchii, _627_, 629
Hemichordata, _4_ f., 3, 38;
affinities, 30 f.
_Hemichromis_, _671_
_Hemiexocoetus_, _638_
Hemimyaria, _101_ f., 95, 96
Hemiodontinae, _576_
_Hemiodus_, _576_
_Hemipimelodus_, _588_
_Hemirhamphus_, _638_;
beak, 154;
intromittent organ, 414
_Hemirhynchus_, _680_
_Hemisilurus_, _588_
_Hemithyrsites_, _679_
_Hemitripterus_, 698
Hemprich and Ehrenberg, 558 n.
_Heniochus_, _668_
_Henoplosus_, _666_
Hensel, 593 n.
_Hephthocara_, _712_, 713
_Heptanchus_, _443_;
branchial clefts, 277;
_H. cinereus_, 443;
skull, 222
_Heptapterus_, _588_
Herdman, 38, 108 n., 109 n.
Hermaphrodite Fishes, 420
Hermaphroditism, 420
_Hermosilla_, _657_
_Heros_, _672_
Herring, 564, 389;
coloration, 173;
gonads, 403;
eggs, 412;
influence of temperature on time of hatching, 417
_Heterobranchus_, _588_, 590;
accessory gill, 293
_Heterochaerops_, _673_
_Heteroconger_, _601_
Heterodont, 251
Heterodontidae, _444_;
teeth, 251;
persistent nephrostomes, 401, 445
_Heterodontus_, _444_;
skull, 223;
teeth, 251;
spiral valve, 267;
_H. philippi_, 445
Heteromi, _621_ f., 306
_Heteropleuron_, _137_, 138;
_H. bassanum_, 137;
_H. cingalense_, 137;
_H. cultellum_, 137;
_H. hectori_, 137;
_H. maldivense_, 137
_Heterostichus_, _709_
Heterostraci, _524_ f., 149
_Heterothrissa_, _563_
_Heterotis_, _557_, 555;
larval gills, 290, 418;
size of eggs, 408;
_H. ehrenbergii_, gill-helix, 294;
_H. niloticus_, 556, 558
Hexagrammidae, _696_, 694
_Hexagrammus_, _696_
_Hexanchus_, _443_;
branchial clefts, 277;
_H. griseus_, 443
Hickson, 289, 690 n.
_Hierichthys_, _713_ {743}
_Himantolophus_, _719_;
_H. reinhardti_, 719
Hincks, 23 n.
_Hippocampus_, _634_, 635, 154, 156, 283;
prehensile tail, 163;
_H. brevirostris_, 361, 362;
_H. guttulatus_, 635
_Hippoglossina_, _687_
_Hippoglossoides_, _687_;
_H. limandoides_, 687
_Hippoglossus_, _687_;
_H. vulgaris_, 687
_Histiocephalus_, _695_
Histiophoridae, _679_, 676
_Histiophorus_, _680_
_Histiopterus_, _660_
_Histiothrissa_, _563_
Hjort, 39, 83
_Holacanthus_, _668_, 361
_Holargyreus_, _648_
_Holaspis_, _527_
Holmwood, 692 n.
Holoblastic, 97, 409, 410
Holobranch, 278
_Holocentrum_, _656_, 655;
_H. sogho_, 361
Holocephali, _466_ f., 148
Holoptychidae, _479_, 202
_Holoptychius_, _480_;
_H. leptopterus_, 236;
_H. flemingi_, 479
Holosomata, _88_ f., 81, 82, 110
Holostei, _495_ f., 149, 497
Holothurians, _Fierasfer_ in, 626;
_Syngnathus_ and _Doryichthys_ in, 635
_Holoxenus_, _695_;
_H. cutaneus_, coloration, 165
Holt, 690 n., 707
_Homaloptera_, _582_
Homalopterinae, _582_, 585
Homocercal, 160, 237
_Homosoma_, _643_
_Homosteus_, _536_
Hoplichthyidae, _699_, 694
_Hoplichthys_, _700_
Hoplognathidae, _662_, 653
_Hoplognathus_, _662_
_Hoplopagrus_, _660_
_Hoplopteryx_, _656_;
_H. lewesiensis_, 656
Horse-Mackerel, 677, 158
Houting, 568
Humboldt, 581, 595
Hundsfisch, 610
Huot, 627
Huxley, 30 n., 37, 112
_Hybodus_, _445_;
vertebral column, _197_
_Hydrocyon_, _575_;
_H. goliath_, 578
Hydrocyoninae, _575_
_Hydrolagus colliei_, _469_
_Hygrogonus_, _672_
_Hymenocephalus_, _647_
_Hyodon_, _553_;
_H. alosoides_, skull and pectoral arch, 553
Hyodontidae, _552_, 544, 389, 405
Hyostylic, 222
_Hyperlophus_, _563_
_Hyperopisus_, _549_;
_H. bebe_, 552
Hyperpharyngeal groove, 123
_Hypnos_, _464_;
electric organs, 365;
_H. subnigrum_, 258
Hypobranchial groove, 46
Hypobythiinae, _72_
_Hypobythius_, _72_;
_H. calycodes_, 72, 72;
_H. moseleyi_, _72_
_Hypomesus_, _566_
Hypopharyngeal groove, 122, 123
_Hypophthalmichthys_, _582_;
_H. molitrix_, 584
Hypophthalminae, _589_
_Hypophthalmus_, _589_
Hypophysial canal, 58
Hypophysis, 129, 370
_Hypoprion_, _448_
_Hypoptopoma_, _595_
_Hypoptychus_, _639_
_Hypostoma_—see _Plecostomus_
Hypostomides, _628_
_Hypsocormus_, _501_;
_H. insignis_, 502
Hyrtl, 558 n., 599 n.
_Hysterocarpus_, _670_
_Iasis_, _108_;
_I. cordiformis-zonaria_, 108
_Icelus_, _698_
Ichthyoborinae, _576_
_Ichthyoborus_, _576_
_Ichthyocampus_, _634_
_Ichthyococcus_, _571_
_Ichthyodectes_, _561_
Ichthyodorulites, 159, 435, 446, 467
_Ichthyomyzon_, _426_
Ichthyopsida, _145_
Ichthyotomi, _438_ f., 148
_Icichthys_, _644_
Icosteidae, _644_, 637
_Icosteus_, _644_;
_I. enigmaticus_, 644
_Iguanodectes_, _575_
Ihering, 593 n.
Ikeda, 29
_Ilyophis_, _601_
Infundibular organ, 127
Inter-renal bodies, 346, 346
Intromittent organs, 414
_Ipnops_, _611_, 613;
_I. murrayi_, 612;
photophore, 179
Iridocytes, 166
_Ischnacanthus_, _441_
_Ischyodus_, _474_
_Isichthys_, _551_
_Iso_, _639_
Isospondyli, _543_, 620
_Istieus_, _549_
_Isurichthys_, _678_
Jacoby, 601, 602 n.
_Janessa_, _446_
_Jenynsia_, _616_
Jobert, 593 n.
John Dory, 683, 308, 361, 363
Jones, 720 n. {744}
Jordan, 542
Jordan and Evermann, 569 n., 620
Jordan and Goss, 687 n.
_Jordania_, _698_
_Joturus_, _640_
Joues cuirassées, 692
Jugulares, 702 f., 651, 652, 306
Julin, 38, 39, 52
_Julis_, _673_, 674
Jullien, 23
Jumping-Fish, 690
Kalymmocytes, 40, 56, 93, 107
Kelb-el-Bahr, 578
Kidneys, 397 f.;
development, 397;
in different Fishes, 399
Kilch, 311
King-Fish, 628
King of the Herrings, 715
Kner and Steindachner, 626
_Kneria_, _616_;
_K. angolensis_, 616;
_K. spekii_, 616 n.
Kneriidae, _615_, 606
Korotneff, 39
_Kowalevskia_, _68_
Kowalevskiidae, _68_
Kowalevsky, 7 n., 37, 39, 94 n., 113
Krohn, 37
_Krohnius_, _647_
Krukenberg, 271 n.
Kuhl, 36
_Kuhlia_, _657_
Kupffer, 38
Kurtidae, _687_
Kurtiformes, _687_ f., 651, 652
_Kurtus_, _687_;
_K. indicus_, skeleton of, 688
Kyle, 687 n.
_Labeo_, _582_;
_L. falcifer_, 583
_Labidesthes_, _639_
_Labrichthys_, _673_
Labridae _673_, 654, 271, 275;
coloration, 164, 166
_Labrodon_, _674_
_Labrus_, _673_, 674;
pharyngeal teeth, 252, 288;
_L. labrax_, 275;
_L. maculatus_, pharyngeal bones, 673;
_L. rupestris_, 674
Labyrinthiform organs, 292, 293
Lacaze-Duthiers, 38
Lactariidae, _663_, 654
_Lactarius delicatulus_, _663_
_Lactophrys_, _724_
_Laemargus_, _455_;
_L. borealis_, 455, 456;
intestinal caeca, 274;
eggs, 435
_Laïs_, _588_
Lamarck, 36
_Lamna_, _451_;
_L. cornubica_, 451
Lamnidae, _450_
_Lampetra_, _426_;
_L. wilderi_, 427
Lampreys, 425 f.
Lamprididae, _628_, 627
_Lampris_, _628_, 629;
_L. luna_, 628;
coloration, 164
_Lamprogrammus_, _712_
_Lamprologus_, _671_
_Lanarkia_, _524_, 525;
_L. spinosa_, 524
Lancelet, 112
Lang, 31 n.
Langerhans, 112
Lankester, 5 n., 23, 112, 113
_Larimus_, _663_
Larvacea, _64_ f., 64, 110;
structure, 65;
habits, 65;
tail, 66;
classification, 68;
occurrence, 69
Larval Fishes—Teleosts, 417 f., 418, 419;
Cyclostomata, 428;
Crossopterygii, 290, 483, 484;
Chondrostei, 494;
Holostei, 501, 504;
Dipnoi, 514, 515, 517, 518
Larval gills, 290 f.;
Elasmobranchii, 289, 290;
_Polypterus_, 290, 483, 484;
_Heterotis_, 290;
_Gymnarchus_, 290, 418, 419;
_Misgurnus_, 290;
Salmon, 290;
_Protopterus_, 291, 515, 515;
_Lepidosiren_, 291, 517, 518
Larval organs, 418;
adhesive or cement organs, 418, 494, 501, 504, 514, 515, 517, 518;
external gills, 289, 290, 419;
cutaneous gills, 290, 483, 484, 517, 518;
defensive spines, 418
_Lasanius_, _532_;
_L. problematicus_, 532
_Lateolabrax_, _659_
Lateral line sensory organs, 163, 384, 385, 386, 387
_Lates_, _660_
_Latilus_, _661_
Latrididae, _663_, 654
_Latris_, _663_;
_L. hecateia_, nocturnal colour-changes, 170
Launce, 639
Lavaret, 568
Leather Carp, 584
_Lebiasina_, _575_, 578
Lefevre, 83
_Lefua_, _582_
Lemon Sole, 687
_Lentipes_, _689_
_Lepadogaster_, _709_, 191
_Lepidocephalichthys_, _582_
_Lepidocottus_, _698_
_Lepidomeda_, _582_
_Lepidopus caudatus_, _679_
_Lepidorhombus_, _687_;
_L. megastoma_, 687
_Lepidosiren_, _511_, 149, 153, 261, 401;
pectoral fins, 157;
skull, 231, 233;
gills, 286;
larval gills, 291, 517, 518;
lung, 291, 301;
vascular system, 527;
thyroid, 344;
growling sounds, 363;
relations with other Dipnoi, 518, 519;
_L. paradoxa_, 516;
distribution, habits, and food, 516;
nocturnal colour change, 517;
hibernation, 517; {745}
nest, 517;
filaments on pelvic limb of male, 517;
larva, 517, 518
Lepidosirenidae, _511_
Lepidosteidae, _502_
Lepidosteoidei, _495_ f.
_Lepidosteus_, _503_, 149, 153, 160, 258, 262, 273, 274, 283, 284, 291;
scales, 185, 186, 188;
vertebral column, 201, 202, 204;
skull, 229;
alimentary canal, 257;
spiral valve, 268;
pyloric caeca, 274;
air-bladder, 297, 299, 310;
spiracular pseudobranch, 334, 335;
branchial circulation, 334 n., 336;
brain, 376;
gonads, 400, 402, 405, 407;
segmentation of the egg, 409;
distribution, 503;
habits, 503;
breeding, 503;
larvae, 504;
fossil, 504;
_L. osseus_, 503, 504;
_L. platyrhynchus_, pectoral fin, 243;
_L. platystomus_, 503;
_L. viridis_, 503
_Lepidothynnus_, _678_
Lepidotrichia, 234
_Lepidotrigla_, _701_
_Lepidotus_, _498_;
_L. minor_, 497
_Lepomis_, _657_
_Lepophidium_, _713_
_Leporinus_, _576_
_Leptagoniates_, _575_
_Leptecodon_, _623_
_Leptichthys_, _563_
_Leptobarbus_, _582_
Leptocephalid, of _Albula_, 548
_Leptocephalus_, 600, 21 n.;
_L. brevirostris_, 602
_Leptochilichthys_, _570_
_Leptoclinum_, _87_;
section, 86;
spicules, 87;
_L. neglectum_, 80
_Leptoderma_, _570_
_Leptodoras_, _588_
Leptolepididae, _546_, 544
_Leptolepis_, _546_;
_L. dubius_, 546
_Leptopterygius_, _709_
Leptoscopidae, _705_, 703
_Leptoscopus_, _705_
_Leptosomus_, _611_
_Leptotrachelus_, _623_
_Lethrinus_, _665_
_Leucaspius_, _582_
_Leuciscus_, _582_, 288
Leuckart, 37
_Leucosomus_, _582_
Levinsen, 23
Leydig, 584 n.
_Lichia_, _677_
Ling, 649
_Linophryne_, _719_
_Liocassis_, _588_
_Liocetus_, _719_
_Lionurus_, _647_
_Liopropoma_, _659_
_Liparis_, _698_, 699, 411
_Liparops_, _698_
Lipogenyidae, _624_, 622, 623
_Lipogenys_, _624_;
_L. gillii_, 624
_Liposarcus_, _595_
_Lirus_, _643_;
_L. medusophagus_, 643;
_L. perciformis_, 643
_Liuranus_, _601_
Liver, 271;
of Amphioxus, 121, 123
Loaches, 585, 261, 290, 292;
Pond Loach, 585.
_Lobotes_, _658_
Lobotidae, _658_, 653
Locomotion of Fishes, 349 f., 351
Lohmann, 66
Longchamps, de Selys, 29
Lophiidae, _718_
_Lophiomus_, _718_
_Lophius_, _718_, 153;
_L. piscatorius_, 718, 355;
lure, 161, 173;
teeth, 250
Lophobranchii, _628_, 543, 629
_Lopholatilus_, _661_;
_L. chamaeleonticeps_, 661
_Lophotes_, _716_
Lophotidae, _716_
Lorenzini's ampullae, 384
_Loricaria_, _595_, 292
Loricariidae, _594_, 575, 256, 292
Loricariinae, _595_
_Lota_, _648_;
_L. vulgaris_, 649
_Lotella_, _648_
Löwig and Kölliker, 37
_Lucifuga_, _712_, 713, 395;
_L. subterranea_, 361
_Luciobrama_, _582_
_Luciocephalus_, _669_
_Luciogobius_, _689_
_Lucioperca_, _659_
_Luciosoma_, _582_
Lump-Sucker, 699, 162, 321, 408, 415
_Lumpenus_, _709_
Lunel, 635, 682 n.
Lutjaninae, _660_
_Lutjanus_, _660_
Lütken, 614 n., 682 n., 714 n.
_Lutodeira_, 256
Luvaridae, _681_, 676
_Luvarus imperialis_, _681_
_Lycocara_, _712_
_Lycodes_, _712_, 702
_Lycodontis_, _605_
_Lyconus_, _647_
_Lycoptera_, _546_
Lymph-hearts, 342
Lymphatic system, 342
Lymphoid tissue, 348
Lyomeri, 622
_Lyosphaera_, _726_
MacBride, 16, 31 n., 113, 133 n.
M‘Intosh and Masterman, 565 n.
Mackerel, 678, 168, 275, 302
_Macquaria_, _659_
_Macrius_, _705_;
_M. amissus_, 705
_Macrodon_, _575_; {746}
_M. trahira_, mouth, 577
_Macrones_, _588_
_Macropharynx_, _604_
_Macrophthalmia chilensis_, _426_
_Macropodus viridi-auratus_, _669_
_Macropoma_, _481_, 202, 268
Macrosemiidae, _498_, 497
_Macrosemius_, _498_
_Macrostomias_, _571_
Macruridae, _647_, 702
_Macruronus_, _647_
_Macrurus_, _647_;
_M. carminatus_, 647
_Maena_, _664_
Mahaseer, 584
Maigre (= Meagre), 361, 362, 663
_Malacanthus_, _661_
_Malacichthys_, _659_
_Malacocephalus_, _647_
_Malacopterus_, _673_
Malacopterygii, _543_ f., 159, 306
_Malacosarcus_, _619_
_Malacosteus_, _571_;
_M. indicus_, 570;
_photophoreo_, 178
_Mallotus_, _566_;
_M. villosus_, 568, 569
Malm, 685 n.
Malopterurinae, _588_
_Malopterurus_, _588_, 593, 359;
electric organs, 365, 366;
_M. electricus_, 591, 362
_Malthe_, _720_;
scales, 190, 191;
_M. vespertilio_, 720
Malthidae, _720_, 718
_Malthopsis_, _720_
Mantle, 42
_Marcusenius_, _551_
Marsupial pouches, 416
Mastacembelidae, _716_
_Mastacembelus_, _717_;
_M. maculatus_, 717
Masterman, 23 n., 25, 27, 28, 29, 31 n.
_Maurolicus_, _571_, 570
Mazza, 714 n.
M‘Biriki, 583
Meagre (= Maigre, _q.v._)
_Meda_, _582_
_Medialuna_, _657_
Median fins, 156, 234 f.
Medusae, with _Caranx_, 643
Meek, 714 n.
_Megalichthys_, _477_, 478;
spiral valve, 268
_Megalocercus_, _68_;
_M. abyssorum_, 66, 68
_Megalops_, _547_;
_M. atlanticus_, 547;
_M. cyprinoides_, 547
_Megalurus_, _501_
Megrim, 687
_Melambaphes_, _664_
_Melamphaes_, _656_, 620, 655
_Melanocetus_, _719_
_Melanostigma_, _712_
_Melanostoma_, _659_
_Melanotaenia_, _639_
Membrana nictitans, 395
Membranellae, 19
_Menaspis_, _445_
_Mene_, _677_
_Merluccius_, _648_, 648 n.;
_M. vulgaris_, 649;
red gland, 308
Meroblastic, 93, 409, 410
Merosomata, _85_, 81, 110
Mertens, Von, 66
_Mesacanthus_, _442_
_Mesiteia_, _447_
_Mesoborus_, _576_
Mesocoracoid arch, 543, 573
_Mesodon_, _498_
Mesonephros, 397, 398, 400
_Mesoprion gembra_, radialia of the dorsal fin, 235
Mesorchium, 402
Mesovarium, 402
_Mesturus_, _498_
Metcalf, 38, 52, 109
Metschnikoff, 19, 31 n., 39
_Metynnis_, _576_
Meyen, 677 n.
_Micracanthus_, _669_
_Micralestes_, _575_
_Microbrachius_, _534_
_Microcoelia_, _611_
_Microcosmus draschii_, dorsal tubercle, 79
_Microdon_, _498_
_Micropogon undulatus_, air-bladder and its muscles, 360, 361
_Micropterus_, _657_
Micropyles, 411, 412
_Microspathodon_, _672_
_Microstoma_, _566_, 565
Miller's Thumb, 698
Milne-Edwards, 37
Minnow, 288
Minot, 30 n.
_Minous_, _695_, 692;
_M. inermis_, 695
_Misgurnus_, _582_, 290;
intestinal respiration, 261, 292;
oviducts, 405;
_M. fossilis_, 358, 585
_Mistichthys luzonensis_, _689_
Möbius, 630 n., 673, 724
_Mochocus_, _588_
_Mola_, _727_
_Molgula_, _77_;
_M. citrina_, 79;
_M. oculata_, 77, 79;
_M. pyriformis_, branchial sac, 77;
dorsal tubercle, 79
Molgulidae, _77_, 64, 110;
nervous system, 53;
reproductive organs, 55
Molidae, _726_, 722
_Mollienesia_, _616_
Molluscoidea, 35
_Molva_, _648_;
_M. vulgaris_, 649
_Monacanthus_, _724_;
coloration, 165;
warning colours, 174;
scales, 190;
sounds, 354, 357;
_M_. sp., 723;
_M. pardalis_, 361;
_M. scopas_, scale, 191
Monascidiae, 35 {747}
Monocentridae, _656_, 653
_Monocentris_, _656_
_Monocirrus_, _658_
Monogamy, 413
_Monopterus_, _597_;
_M. javanensis_, head, skull, pectoral arch, vertebrae, 598
Moon-eyes, 553
Moon-fish, 579
_Morchellium_, _88_;
_M. argus_, stomach, 88
_Mordacia_, _426_
Morgan, 11 n., 20, 21 n., 31 n., 32
_Moringua_, _601_
Mormyridae, _549_, 544, 154;
larval gills, 290;
abdominal pores, 401
Mormyrinae, _551_
_Mormyrops_, _551_, 549
Mormyrs, 551
_Mormyrus_, _551_;
_M. caballus_, 549;
electric organs, 365, 366, 401
_Morone_, _659_, 660
Moseley, 95, 613, 702 n.
_Motella_, _649_, 288
Mouth, position, size, and shape;
of Cyclostomata, 150;
of Fishes, 153 f.
_Moxostoma_, _581_
Mud-Fish, 610
_Mugil_, _640_, 264;
_M. capito_, 640
Mugilidae, _640_, 637, 256;
gizzard, 260
Müller, H., 37
Müller, J., 19, 31, 112, 129
Müller, O. F., 36
Müllerian duct, 398, 400, 404
Mullets, Grey, 640;
Red, 665
Mullidae, _665_, 654
_Mulloides_, _665_
_Mullus_, _665_;
_M. barbatus_, 666;
_M. surmuletus_, 666;
pectoral arch, 666
_Muraena_, _605_;
_M. helena_, 600, 605
_Muraenesox_, _601_
_Muraenichthys_, _601_
Muraenidae, _604_, 163, 259
Muraenolepididae, _649_
_Muraenolepis_, _649_
Murray, 613
Muscular system, 350
_Mustelus_, _448_, 298;
placenta, 434, 449;
_M. antarcticus_, radialia of the dorsal fin, 234;
vascular system, 316, 317, 330, 331, 334;
placenta, 435;
_M. laevis_, 449;
_M. vulgaris_, 449
_Myletes_, _576_
_Myleus_, _576_
Myliobatidae, _465_;
teeth, 251
_Myliobatis_, _465_, 466;
_M. aquila_, 465, 466
_Mylostoma_, _537_
Myocoele, 397, 398
Myocomma, 117
_Myomyrus_, _551_
Myotome, 117, 133
Myriacanthidae, _468_
_Myriacanthus_, _468_;
_M. granulatus_, 468
_Myripristis_, _656_
_Myroconger_, _605_
_Myrophis_, _601_
_Myrus_, _601_
_Myxine_, _422_, 147;
external characters, 151;
thread-cells, 182;
vertebral column, 197;
teeth, 248;
liver, 273;
pancreas, 273;
gill-sacs, 281;
blood corpuscles, 341;
brain, 372;
auditory organ, 387, 388;
pituitary involution, 391;
degenerate eyes, 394, 395;
pronephros, 399, 400;
genital pore, 403;
distribution, 422;
habits, 422;
hermaphroditism, 423;
_M. glutinosa_, 151, 422, 423
Myxinidae, _422_
Myxinoides, _421_ f.
_Myxus_, _640_
Nandidae, _658_, 653
_Nandus_, _658_
_Nannaethiops_, _576_
_Nannobrachium_, _611_
_Nannocampus_, _634_, 635
_Nannocharax_, _576_
_Nanoglanis_, _588_
_Nanognathus_, _576_
_Nanostomus_, _576_
_Nansenia_, _566_, 565
_Narcetes_, _570_
_Narcine_, _464_
_Naseus_, _668_
_Naucrates_, _677_;
_N. ductor_, 677
_Nealotus_, _679_
_Neatherina_, _639_
_Nebris_, _663_
_Nedystoma_, _588_
Needle-Fish, 634
_Nemachilus_, _582_
_Nematabramis_, _582_
_Nematistius_, _677_
_Nematogenys_, _588_, 587
_Nematonotus_, _611_
Nemichthyidae, _603_
_Nemichthys_, _603_
_Nemopteryx_, _648_
_Neobola_, _582_
_Neoborus_, _576_
_Neobythitis_, _712_
_Neoceratodus_, _508_, 259, 291;
skull, 231;
pectoral fin, 244;
gills, 285, 286;
lung, 291, 299, 300;
vascular system, 323, 324, 329, 338;
grunting sounds, 363;
brain, 377;
abdominal pores, 401;
Müllerian ducts in the male, 407;
distribution and habits, 508, 512;
spawning, 510;
eggs, 510;
young, 510, 511;
relations with other Dipnoi, 518, 519;
_N. forsteri_, 508, 509
_Neochanna_, _608
Neolebias_, _576_ {748}
_Neopempheris_, _657_
_Neopercis_, _705_
_Neoscopelus_, _611_
_Neosilurus_, _588_
Nephridia, of Amphioxus, 125
Nephrostomes, 398;
persistent, 401
Nephrotome, 397, 398
_Nerophis_, _634_, 635
Nerve eminences, 383
Nervous system, of Chordata, 4;
of Hemichordata, 9, 25, 30;
of Tunicata, 53, 58, 66;
of Amphioxus, 127 f., 131;
of Fishes, 367 f.
_Nesiarchus_, _679_
Nesting habits of Sticklebacks, 630
Nests of Fishes, 414 f., 427, 500, 501, 514, 515, 517;
of _Arius_, 593;
of _Doras_ and _Callichthys_, 592;
of _Gobius_, 690;
of _Gymnarchus_, 552;
of _Heterotis_, 558;
of _Spinachia_, 631
_Nettastoma_, _601_
_Nettenchelys_, _601_
_Nettophichthys_, _601_
Neural gland, 52
Neurenteric canal, 57, 131, 133
Neuromeres, 195 n.
Neuropore, 10, 58, 59
Nilhechte, 550
Nilsson, 715
_Niphon_, _659_
Nishikawa, 685 n.
Noll, 584 n.
_Nomeus_, _643_;
_N. gronovii_, 643
Norman, 23
Notacanthidae, _624_
_Notacanthus_, _625_;
_N. bonapartii_, 625
_Notagogus_, _498_
Notidanidae, _442_, 279;
persistent nephrostomes, 401
_Notidanus_, _443_, 345;
spiral valve, 267;
_N. cinereus_, branchial clefts, 277;
skull, 222;
_N. griseus_, branchial clefts, 277
Notochord, 3;
of Enteropneusta, 14;
of _Cephalodiscus_, 24, 25;
of _Rhabdopleura_, 27;
of Actinotrocha, 28;
of Tunicata, 57, 60, 61;
of Amphioxus, 119, 118, 119, 132;
of Fishes, 193 f.
_Notoglanidium_, _588_
_Notoglanis_, _588_
_Notogoneus_, _572_
Notopteridae, _554_, 544, 303, 389, 405
_Notopterus_, _555_, 153, 305, 306;
_N. afer_, skeleton, 554;
_N. chitala_, 555
_Notothenia_, _706_
Nototheniidae, _705_, 704
_Noturus_, _588_, 590
_Novacula_, _673_
Nucleus, of Thaliacea, 101, 105
_Nuria_, _582_
Nurse, 97
Oar-Fish, 715, 163
_Oblata_, _664_
Ocelli, 53
Octacnemidae, _108_, 101
_Octacnemus_, _109_, 101;
_O. bythius_, 109;
_O. patagoniensis_, 109
_Odax_, _674_
_Odaxothrissa_, _563_
_Odontaspis_, _451_
_Odonteus_, _673_
_Odontonectes_, _660_
_Odontostomus_, _611_, 613
_Oenoscopus_, _545_
Oesophageo-cutaneous duct, 281
Oikoplasts, 65
_Oikopleura_, _68_, 67, 69;
transverse section, 67;
longitudinal section, 68;
_O. cophocerca_, 66;
_O. dioica_, 70;
_O. flabellum_, transverse section, 69
Olfactory organs, 155, 390 f., 391, 392
Olfactory pit, 129
Oligopleuridae, _545_, 544
_Oligopleurus_, _545_
_Oligorus_, _659_
_Oligosarcus_, _575_
_Oligotrema_, _78_, 111 n.
Olt, 584 n.
_Olyra_, _588_
Omble Chevalier, 567
_Omiodon_, _611_
_Omosudis_, _611_
_Oncorhynchus_, _566_
_Oneirodes_, _719_;
_O. eschrichtii_, 174
_Onus_, _648_;
coloration of larvae, 175
Oozooid, 84, 91, 93
Opah, 628, 164
Operculum, 25, 155, 278, 282, 283
_Ophichthys_, _601_
Ophidiidae, _713_, 651, 702, 704, 163, 361
_Ophidium_, _713_, 361;
air-bladder, 302
_Ophioblennius_, _709_
Ophiocephalidae, _644_, 637, 163, 293, 361;
distribution, 645
_Ophiocephalus_, _645_;
labyrinthiform organ, 293;
_O. marulius_, 361;
_O. gachua_, 361
_Ophiodon_, _696_;
_O. elongatus_, skull, 692
_Opisthocentrus_, _709_
_Opisthognathus_, _661_
Opisthomi, _716_ f., 651, 306
_Opisthomyzon_, _691_
_Opisthonema_, _563_
_Opisthoproctus soleatus_, telescopic eyes, 395
_Opisthopteryx_, _611_
_Opostomias_, _571_;
_O. micripnus_, photophores, 178, 179, 181
_Opsanus_, _711_
_Opsariichthys_, _582_
Oral hood, 116, 119, 136
_Oreinus_, _582
Oreosoma_, _683_ {749}
_Orestias_, _616_
_Orodus_, _445_
_Orthagoriscus_, _727_, 153, 354;
spinal cord, 367;
_O. mola_, 602, 727, 348, 357
Ortho-enteric, 102, 108
_Osmeroides_, _547_
_Osmerus_, _566_, 565;
_O. eperlanus_, 568;
gonoducts, 405, 411
Osphromenidae, _669_, 654;
labyrinthiform organ, 293
_Osphromenus_, _669_, 293;
_O. olfax_, 669
_Osphyolax_, _634_
Ostariophysi, _573_ f., 306, 389
_Osteochilus_, _582_
_Osteogeniosus_, _588_, 593
Osteoglossidae, _555_, 544, 294, 405;
larval gills, 290;
distribution, 557
_Osteoglossum_, _557_;
_O. bicirrhosum_, 556
Osteolepida, _477_ f.
Osteolepidae, _477_
_Osteolepis_, _477_;
_O. macrolepidota_, 477
Osteostraci, _527_ f., 149
_Ostracion_, _724_, 152, 361;
colours, 165, 174;
scales, 191;
_O. ornatus_, 165;
_O. quadricornis_, 725;
_O. trigonus_, 362
Ostraciontidae, _724_, 721
Ostracodermi, _522_ f., 149, 721
_Otocinclus_, _595_
_Otolithus_, _663_;
air-bladder, 304;
_O. regalis_, 361
Oviducal gland (shell-gland), 400, 407
Ovipositor, 408
_Oxuderces_, _690_
_Oxydoras_, _588_, 359
_Oxygnathus_, _487_
_Oxylebius_, _696_
_Oxymetopon_, _689_
Pachycormidae, _501_
_Pachycormus_, _501_
_Pachylebias_, _616_
_Pachymetopon_, _665_
Pachyrhizodontidae, _569_
_Pachyrhizodus_, _569_
_Pachystomias_, _571_;
_P. microdon_, photophores, 178, 179, 180
_Pachyula_, _588_
Paddle-Fish, _491_; = _Polyodon_, q.v.
_Pagellus_, _665_;
_P. centrodontus_, 346
_Pagrus_, _665_;
_P. auratus_, 665
_Palaeolycus_, _609_
_Palaeomylus_, _468_
Palaeoniscidae, _486_, 485;
range in time, 487
_Palaeoniscus_, _486_, 487;
_P. macropomus_, 486
Palaeorhynchidae, _680_
_Palaeorhynchus_, _680_
_Palaeoscyllium_, _447_
_Palaeospinax_, _445_;
vertebral column, 197
Palaeospondylidae, _521_ f., 149
_Palaeospondylus gunni_, _521_, 522
_Palimphyes_, _678_
Pallas, 112
Pancreas, 273
_Pangasius_, _588_, 305, 359;
_P. micronema_, 359 n.
_Pantodon buchholzi_, _558_, 559
Pantodontidae, _558_, 544
Papillae, adhering, 61
_Paracentroscyllium_, _455_
_Paradiplomystes_, _588_
_Paragoniates_, _575_
_Parailia_, _588_
_Paralepis_, _611_
_Paralichthys_, _687_
_Paraliparis_, _698_
_Paraluteres_, _724_
_Paranthias_, _659_
_Parapegasus_, _636_
_Parapelecus_, _582_
_Parapercis_, _705_
_Paraphago_, _576_
_Paraphractura_, _588_, 589
_Parapriacanthus_, _657_
_Parapsettus_, _668_
_Parascopelus_, _611_
Parasitism, of _Rhodeus_, 584, 416;
of _Stegophilus_, 594
_Paratilapia_, _671_
_Paratrachichthys_, _656_, 655
_Paratrygon_, _465_
Parental care, 415, 416, 500, 501
_Parexus_, _441_
Parietal budding, 82
Parietal eye, 395 f., 396
_Pariolius_, _588_
Parker, T. J., 714 n.
Parker, W. N., 705 n.
_Parodon_, _576_
_Paropsis_, _677_
Parrot-Wrasses, 674
_Pataecus_, _709_
Pearl Oysters, _Fierasfer_ in, 625
Pectoral fins, 157, 242, 243, 244
Pectoral girdle, 239, 240
Pediculati, _717_ f., 651, 306
Pegasidae, _635_, 626, 628
_Pegasus_, _636_
_Pegea_, _108_;
_P. scutigera-confoederata_, 108
Pelagic ova, 408
_Pelargorhynchus_, _623_
_Pelecus_, _582_
Pellegrin, 671 n., 724 n.
_Pellona_, _563_, 563
_Pellonula_, _563_
_Pelonaia_, _74_;
branchial sac, 74;
_P. corrugata_, 76
_Pelor_, _695_
_Peloria_, _685_
Pelvic fins, 158, 245 f., 246
Pelvic girdle, 239 f., 240, 241 {750}
Pempheridae, _656_, 653
_Pempheris_, _657_;
_P. muelleri_, 657
_Penetopteryx_, _634_, 635
_Pentaceropsis_, _660_
_Pentaceros_, _660_
Pentacerotinae, _660_
_Pentanemus_, _641_;
_P. quinquarius_, 162
_Pentapus_, _664_
_Pentaroge_, _695_
_Peprilus_, _643_
_Perca_, _659_, 262;
alimentary canal, 275;
veins, 323;
thyroid, 343;
auditory organ, 388;
_P. fluviatilis_, 659, 321, 323
_Percalates_, _659_
_Percarina_, _659_
Percesoces, _636_ f.
Perch, 659;
deposition of eggs, 411; = _Perca_, q.v.
_Percichthys_, _659_, 660
Percidae, _658_, 654, 412;
coloration, 166
Perciformes, _652_ f., 651
_Percilia_, _659_
_Percina_, _659_
Percophiidae, _705_, 703
_Percophis_, _705_;
_P. brasilianus_, pectoral arch of, 703
Percopsidae, _620_, 606
_Percopsis_, _621_
Peribranchial budding, 82
Peribranchial cavity, 43, 44, 59, 63;
of Thaliacea, 95, 101
Pericardium, of Hemichordata, 15;
of Tunicata, 44, 49;
of Fishes, 327
Perihaemal spaces, 9
_Periophthalmus_, _689_, 690, 355;
eyes, 155;
pectoral fins, 161;
tail as a respiratory organ, 289;
_P. koelreuteri_, 690 n.
Peripharyngeal bands, 45, 46, 52, 53
Peripharyngeal spaces, 9
_Peristedion_, _701_
Péron, 36
_Perophora_, _72_, 84;
_P. listeri_, 72
Petalodontidae, _446_
_Petalodus_, _446_
_Petalopteryx_, _498_
Petersen, 690 n.
_Petersius_, _575_
_Petrocephalus_, _551_
_Petromyzon_, _426_, 147, 258, 367, 382;
external characters, 150;
skull, 217;
teeth, 247, 248;
spiral valve, 264;
liver, 272;
pancreas, 273;
gill-sacs, 279, 280;
heart, 327;
arteries, 329, 330;
blood corpuscles, 341;
thyroid, 343, 344;
supra-renal bodies, 346;
brain, 371, 372, 392;
spinal nerves, 378;
auditory organ, 387;
olfactory organ, 391, 392;
naso-pituitary involution, 391;
pituitary caecum, 392;
parietal eye, 395, 396;
kidneys, gonads, and genital pores, 399, 400, 403;
distribution, 426;
_P. fluviatilis_, 426;
_P. marinus_, 426;
_P. planeri_, ova, 428;
_P. wilderi_, spawning, 427;
larva, Ammocoetes stage, 428
Petromyzontes, _425_ f.
Petromyzontidae, _426_
_Petroscirtes_, _709_
_Phago_, _576_
_Phallusia_, _72_
_Phaneropleuron_, _506_, 519;
_P. andersoni_, 506
_Phanerosteon_, _487_
Pharyngeal teeth, 251
_Pharyngodictyon_, _87_;
_P. mirabile_, 80
Pharyngognathi, _543_
Pharyngopneusta, _30_ n.
Phisalix, 705 n.
_Phlyctaenaspis_, _536_
_Pholedichthys_, _709_
Pholididae, _711_, 704
Pholidophoridae, _545_, 541 n., 544
_Pholidophorus_, _545_
_Pholis_, _711_, 712;
_P. gunnellus_, 712, 415
Phoronidea, _27_ f., 5
_Phoronis_, _27_ f., 5;
regeneration, 16, 30;
_P. buskii_, 28
Phorozooid, 99, 100
Phosphorescent organs, 178
_Photichthys_, _571_, 570
_Photoblepharon_, _660_
_Photonectes_, _571_
Photophores, 178, 612, 613, 624, 711
_Photostomias_, _571_
Phractolaemidae, _560_, 544
_Phractolaemus ansorgii_, _560_, 560
_Phractura_, _588_
Phthinobranchii, _629_
_Phycis_, _648_
_Phyllodoce_, nephridium, 127
_Phyllodus_, _674_
_Phyllopteryx_, _634_;
_P. eques_, 635
_Phylogephyra_, _565_
_Physailia_, _588_
_Physalia_, with _Nomeus_, _643_
_Physiculus_, _648_
Physoclisti, _306_, 307, 311
_Physopyxis_, _588_
Physostomi, _543_, 306, 311
_Piabucina_, _575_;
_P. argentina_, mouth, 577
Pike, 609, 250, 307
Pike-Perch, 659
Pilchard, 564, 389
Pilot-Fish, 677
_Pimelepterus_, _657_
_Pimelodina_, _588_
_Pimelodus_, _588_, 589, 361
_Pimephales_, _582_
Pipe-Fish, 634, 154
_Piramutana_, _588_ {751}
Piranha, 578
Pirate Perch, 656
_Piratinga_, 361
_Pirinampus_, _588_
Pit-organs, 383
Pituitary body, 129
Placenta, of _Doliolum_, 98;
of Hemimyaria, 101, 107;
of Fishes, 434
Plagiostomi, _442_ f., 148
_Plagusia_, _687_
_Plagyodus_, _614_
Plaice, 687
_Platax_, _668_
Platycephalidae, _699_, 694, 650
_Platycephalus_, _699_, 693
_Platychaerops_, _673_
_Platycormus_, _643_
_Platyglossus_, _673_
_Platylaemus_, _674_
_Platypoecilus_, _616_
_Platyrhinoidis_, _460_
_Platysomatichthys_, _687_, 685
Platysomidae, _487_, 485
_Platysomus_, _488_
_Platystoma_, _594_;
air-bladder and its extrinsic muscles, 360, 361, 362;
_P. coruscans_, 594
_Platytroctes_, _570_
_Plecodus_, _672_
_Plecoglossus_, _566_, 569
_Plecostomus_, _595_, 256, 292;
(_Hypostoma_) scales, 190;
_P. commersonii_, 192
Plectognathi, _721_ f., 543, 651, 205, 231, 275, 306, 354, 418
_Plectromus_, _656_
_Plectropoma_, _659_;
_P. richardsoni_, coloration, 165
_Plesiops_, _659_
_Plethodus_, _549_
Pleuracanthidae, _440_
_Pleuracanthus_, _440_, 159, 235, 236, 438;
vertebral column, 197;
pectoral and pelvic girdles, 239;
pectoral fin, 242;
pelvic fins, 245;
_P. ducheni_, 439
_Pleuragramma_, _705_;
_P. antarcticum_, 705
_Pleurogrammus_, _696_
_Pleuronectes_, _687_;
_P. flesus_, 687;
coloration elements, 167, 170;
_P. limanda_, 687;
_P. platessa_, 686, 687
Pleuronectidae, _684_, 683, 152, 264, 275, 284, 302, 412;
protective coloration, 172
_Pleuropholis_, _545_
_Pleuroplax_, _445_
Pleuropterygii, _436_, 148
Plication, of branchial sac, 48
_Plotosus_, _588_, 587, 408
_Podateles_, _714_;
_P. indicus_, pectoral arch of, 714
Podatelidae, _713_, 704
_Poecilia_, _616_
_Poecilodus_, _445_
_Poecilopsetta_, _687_
Poey, 713 n.
Pogge, 700
_Pogonias_, _663_;
_P. chromis_, 663, 361;
air-bladder, 304, 305;
drumming sounds, 362
Poison, of _Trachinus_, 705
Poison glands, 176
Pollack, 649
Pollan, 568
_Polyacanthonotus_, _625_
_Polyacanthus_, _669_, 293;
_P. opercularis_, 669
Polyandry, 413
_Polycarpa_, _74_, 76;
reproductive organs, 55;
_P. aurata_, 76;
dorsal tubercle, 79;
_P. comata_, 76;
_P. glomerata_, larva, 78;
_P. pedata_, 76;
_P. tinctor_, 76;
dorsal tubercle, 79
Polycarps, 76
_Polycaulus_, _695_
_Polycentropsis_, _658_
_Polycentrus_, _658_
Polyclinidae, _87_, 81, 83, 110;
ascidiozooid, 82
_Polyclinum_, _88_;
_P. molle_, stomach, 88
_Polycyclus renieri_, larva, 78
Polygamy, 413
_Polyipnus_, _571_
_Polymixia_, _656_
Polynemidae, _640_, 637
_Polynemus_, _641_;
_P. quadrifilis_, 641;
shoulder-girdle and pelvis of, 640;
_P. vereker_, coloration, 165
_Polyodon_, _491_, 154, 284, 405;
vestigial dermal denticles, 188;
ribs, 201;
skull, 225;
pelvic fin, 245;
teeth, 249;
pyloric caeca, 274, 276;
gills, 282;
spiracle, 283;
gill-rakers, 288;
_P. folium_, 491;
habits and distribution, 491;
breeding, _492_
Polyodontidae, _491_, 486, 489
Polyphyodont, 250
_Polyprion_, _659_
Polypteridae, _481_;
distribution, 483
_Polypterus_, _482_, 149, 323;
scales, 185, 187;
vertebral column, 202;
ribs, 206;
skull, 226, 227, 229;
median fins, 235;
pelvic girdle, 241;
pectoral fin, 243;
tongue, 252;
spiral valve, 268;
pyloric caecum, 274;
spiracle, 283;
pseudobranch, 284;
larval gills, 290;
air-bladder, 298;
arteries to air-bladder, 337;
sounds, 362;
habits and food, 482;
breeding, 483;
_P. bichir_, 482;
_P. congicus_, 290;
_P. lapradei_, 483;
larvae, 483, 484;
_P. senegalus_, 482, 483
_Polyrhizodus_, _446_
Polystyelidae, _89_, 81, 110
Polyzoa, 35 {752}
_Pomacanthus_, _668_
Pomacentridae, _672_, 654;
coloration, 166
_Pomacentrus_, _672_
Pomatominae, _660_
_Pomatomus_, _660_
_Pomoxys_, _657_
Pope, 659
Porbeagle Sharks, 450
Porcupine Fishes, 726, 163
_Porichthys_, _711_;
photophores, 179;
_P. porosissimus_, 711
_Porogadus_, _712_
_Portheus_, _561_
Post-abdomen, 83
Pouchet, 7 n.
Pout, 649
Powell, 714 n.
Prebranchial zone, 45
_Premnas_, _672_
Prenadillas, 595
Priacanthinae, _660_
_Priacanthus_, _660_
Prince, 630 n.
_Prionodon_, _448_
_Prionolepis_, _609_
_Prionotus_, _701_, 361
_Prionurus_, _668_
_Priscacara_, _672_
Pristidae, _459_, 458
_Pristigaster_, _563_
Pristiophoridae, _457_, 458
_Pristiophorus_, _457_
_Pristipoma_, _664_, 361
Pristipomatidae, _664_, 654
_Pristis_, _460_, 272;
rostral denticles, 184;
_P. antiquorum_, 459
_Pristiurus_, _446_, 447
_Proantigonia_, _667_
Proboscis-gland, 15
Proboscis-pore, 9, 24, 27
Proboscis-skeleton, 14
_Procatopus_, _616_
_Prochanos_, _563_
_Prochilodus_, _576_
_Prolates_, _660_
_Prolebias_, _616_
Proles gregaria, 102
Proles solitaria, 102
_Promethichthys_, _679_
_Promyliobatis_, _466_
Pronephros, 397, 400
_Pronotacanthus_, _622_
Propagation, of Eel, 601
_Propoma_, _659_
_Propristis_, _460_
_Protaulopsis_, _638_, 632 n.
_Prothymallus_, _565_
_Protocampus_, _634_
Protocercal, 159
_Protopterus_, _511_, 149, 153, 259, 261, 262, 272, 273;
nostrils, 155;
pectoral fins, 157;
cutaneous glands, 182;
skull, 231, 232;
pelvic girdle, 241;
alimentary canal, 257;
spiral valve, 268;
pancreas, 274;
gills, 286;
larval gills, 291, 515;
lung, 291, 300, 301, 302;
vascular system, 326, 327, 329, 339, 340;
thyroid, 343;
thymus, 345;
sounds, 362;
brain, 376, 377;
auditory organ, 387;
abdominal pores, 401;
testis, 406;
Müllerian ducts in male, 407;
distribution, 511, 512;
habits, 512;
summer sleep, 513;
cocoon, 513;
nest, 514, 515;
larva, 515;
relations with other Dipnoi, 518, 519;
_P. aethiopicus_, 512;
_P. annectens_, 509, 512;
_P. dolloi_, 512
_Protosphyraena_, _502_
Protospondyli, _497_
Protostigmata, 59
Protosyngnathidae, _631_, 628
_Protosyngnathus sumatrensis_, _632_
_Prototroctes_, _608_
Prototunicata, 110
Psammodontidae, _446_
_Psammodus_, _446_
_Psammoperca_, _660_
Psammosteidae, _526_, 530
_Psammosteus_, _527_
_Psenes_, _643_
_Psenopsis_, _643_
_Psephodus_, _445_
_Psephurus_, _492_;
_P. gladius_, 492
_Psetta_ (= _Rhombus_), _687_, 685;
_P. laevis_, 687;
_P. maxima_, 686, 687
_Psettodes_, _687_, 684, 685;
_P. erumei_, 686
_Psettus_, _666_;
_P. sebae_, 666, 667
_Pseudaluteres_, _724_
_Pseudecheneis_, _588_, 589
_Pseudetroplus_, _672_
_Pseudeutropius_, _588_
_Pseudobagrus_, _588_
_Pseudoberyx_, _563_
_Pseudoblennius_, _698_
Pseudobrachium, 717
Pseudochromididae, _661_, 653
_Pseudochromis_, _661_
_Pseudocorynopoma_, _575_
_Pseudogobio_, _582_
_Pseudomonacanthus_, _724_
_Pseudomugil_, _639_
_Pseudoplesiops_, _659_
_Pseudopriacanthus_, _660_
_Pseudoscarus_, _674_
_Pseudoscopelus_, _641_, 642
_Pseudosphaerodon_, _674_
_Pseudosyngnathus_, _634_
_Pseudotriakis microdon_, _447_
_Pseudoxiphophorus_, _616_
_Psilocephalus_, _724_
_Psilorhynchus_, _582_
_Psychrolutes_, _698_
_Pteraclis_, _682_
Pteraspidae, _527_, 530 {753}
_Pteraspis_, _527_, 530, 149;
_P. rostrata_, 527
_Pterichthys_, _534_, 149;
_P. milleri_, 533
Pterobranchia, _21_ f., 5, 31
_Pterois_, _695_
_Pterophryne_, _720_
_Pterophyllum_, _672_
_Pteroplatea_, _464_;
_P. micrura_, intra-uterine nutrition, 435
_Pteropsarion_, _582_
_Pteropsaron_, _705_
_Pterothrissus_, _548_
_Pterycombus_, _682_
_Pterygocephalus_, _709_
_Pterygoplichthys_, _595_
_Ptilichthys_, _709_
_Ptychobarbus_, _582_
_Ptychodera_, _17_, 5, 13, 16, 25 n.;
_P. bahamensis_, 5, 10;
_P. flava_, 10, 11
Ptychoderidae, _17_
Ptyctodontidae, _468_
_Ptyctodus_, _468_
Puffers, 726
Punnett, 17 n., 21 n.
Putnam, 619 n., 625
Pycnodontidae, _498_, 497
_Pycnodus_, _498_
_Pycnosterinx_, _656_
Pygochord, 15
Pyloric caeca, 274, 275
_Pyrosoma_, _91_, 91, 36, 37, 70, 86, 108;
structure, 91 f., 92;
development and life-history, 93, 94;
occurrence, 94;
_P. aherniosum_, 94;
_P. atlanticum_, 94;
_P. elegans_, 94;
_P. excelsior_, 95;
_P. giganteum_, 94;
_P. minatum_, 94;
_P. spinosum_, 94
Pyrosomatidae, _91_, 81, 110
_Pyrrhulina_, _575_
_Pythonichthys_, _605_
Quinnat Salmon, 566, 569
Raad, 591
_Raconda_, _563_
_Racovitzaia_, _706_
Räderorgan, 129
Raffaele, 614 n.
Rag-Fishes, 644
_Raia_, _461_;
dermal spines, 184;
pectoral fin, 243;
teeth, 251;
spiral valve, 265, 266;
inter-renal bodies, 347;
electric organs, 365;
distribution, 461;
coloration, 461;
sexual dimorphism, 462;
_R. abyssicola_, 462;
_R. alba_, 462;
_R. batis_, 462;
_R. circularis_, 462;
_R. clavata_, 462;
_R. fullonica_, 462;
_R. macrorhynchus_, 462;
_R. maculata_, 267, 462;
_R. mamillidens_, 462;
_R. microcellata_, 462;
_R. murrayi_, 461;
_R. oxyrhynchus_, 462;
_R. radiata_, 462
Raiidae, _461_
Ramsay, 635
_Raniceps_, _648_
_Ranzania_, _727_
_Rasbora_, _582_
_Rathbunella_, _661_
Rays = _Raia_, q.v.
Rectal gland, 276
Red-currant Squirter, 74
Red Mullets, 665
_Regalecus_, _715_;
_R. banksii_, prehensile tail, 163;
_R. glesne_, 715
Regan, 542, 595 n., 643 n., 646, 702, 721
Reinhardt, 579, 594
_Remora_, _691_;
cephalic sucker, 161;
_R. brachyptera_, 691
Renal organ, of Ascidians, 54;
of Fishes—see Kidneys
Reproductive organs, of Hemichordata, 16, 25;
of Tunicata, 55, 67, 76, 93, 97, 105;
of Amphioxus, 129;
of Fishes, 402 f., 400, 403
Respiration, mechanism of, 288
Respiratory organs—Elasmobranchii, 276, 278;
Cyclostomata, 279, 280, 281;
Holocephali, 282, 283;
Teleostomi, 282, 283;
Dipnoi, 285, 286;
mechanism of respiration, 288;
larval gills, 289, 290;
air-bladder as a respiratory organ, 291;
accessory organs of respiration, 292, 293, 294, 295
_Retropinna_, _566_, 565
Retzius, 113
_Rhabdocynthia_, _75_;
spicules, 87
_Rhabdopleura_, _21_ f., 5, 26 f.;
_R. compacta_, 23;
_R. grimaldii_, 23;
_R. manubialis_, 23;
_R. normani_, 22, 23, 26
Rhachicentridae, _677_, 676
_Rhachicentrum_, _678_
_Rhamphichthys_, _579_
Rhamphocottidae, _697_, 694
_Rhamphocottus richardsonii_, _697_
_Rhamphognathus_, _640_
_Rhamphosternarchus_, _579_;
_R. curvirostris_, 580;
_R. tamandua_, 580
_Rhamphosus_, _633_
_Rhina_, _457_;
_R. squatina_, 456, 457
_Rhineaster_, _589_
_Rhinelepis_, _595_
_Rhinellus_, _611_
_Rhinichthys_, _582_
Rhinidae, _456_, 458;
persistent nephrostomes, 401
Rhinobatidae, _460_, 458
_Rhinobatus_, _460_;
_R. granulatus_, 460
_Rhinodon_, _454_;
_R. typicus_, gill-rakers, 287
Rhinodontidae, _454_
_Rhinodoras_, 359
_Rhinogobio_, _582_
_Rhinoptera_, _465_, 466
Rhipidistia, _477_ n. {754}
Rhizodontidae, _478_, 202
_Rhizodopsis_, _479_, 228;
_R. sauroides_, skull, 478
_Rhizodus_, _478_, 202
_Rhodeus_, _582_;
_R. amarus_, 584;
oviducal tubes, 408;
embryos in gill-cavities of _Unio_, 416
_Rhodichthys_, _712_
_Rhodosoma_, _73_;
_R. callense_, 72
_Rhombatractus_, _639_
_Rhomboidichthys_, _687_
_Rhombosolea_, _687_
_Rhombus_, _687_, 264;
_R. aculeatus_, 262;
_R. maximus_, 264, 275
—see also _Psetta_
_Rhyacichthys_, _689_
_Rhynchobatus_, _460_
_Rhynchobdella_, _717_
_Rhynchodus_, _468_
_Rhypticus_, _660_
Ribbon-Fish, 715, 152, 160
Ribs, 205
Ridewood, 542 n.
_Rita_, _588_;
_R. crucigera_, 303
Ritter, 8 n., 39, 83, 690 n.
_Rivulus_, _616_
Rocklings, 649, 288
Rohde, 113
_Rohteichthys_, _582_
Rolph, 113
_Rondeletia_, _614_
Rosette-organ, 97
Roule, 38
Rowntree, 579 n., 621 n.
Rudder-Fish, 643
_Ruvettus_, _679_
Ryder, 711 n.
_Saccarius_, _720_
Sacchi, 696 n.
_Saccobranchus_, _588_, 590;
air-sacs, 295;
_S. fossilis_, 295;
_S. singio_, 295
_Saccodon_, _576_
Saccopharyngidae, _603_
_Saccopharynx_, _604_;
_S. ampullaceus_, 604
Sachs, 581 n.
Sagemehl, 542, 573, 575, 579 n., 586 n., 649 n.
_Sagenodus_, _507_
Sail-Fishes, _679_
_Salanx_, _566_
_Salarias_, _709_, 710
Salensky, 39, 411
_Salminus_, _575_;
_S. orbignianus_, 578
_Salmo_, _565_, 566 f., 567, 290;
pyloric caeca, 275;
spiracles in the embryo, 283;
pseudobranch, 284;
_S. alpinus_, 567;
_S. fario_, 567;
caudal portion of the vertebral column, 237;
pectoral girdle, 240;
dissection to show the internal organs, 255;
brain, 375;
section of the eye, 394;
_S. fontinalis_, 567;
_S. irideus_, 567;
_S. salar_ (Salmon), 567, 323;
skull, 211 f., 212, 213, 214;
kidneys, gonoducts, and abdominal pores in the female, 405, 406;
deposition of eggs, 415;
length of hatching, 417;
_S. salvelinus_, pectoral fin, 243;
_S. stomachicus_, gizzard, 260;
_S. trutta_, 567
Salmon, 566 f.—see also _Salmo salar_.
Salmonidae, _565_, 544, 269;
distribution, 566;
liver, 272;
abdominal pores, 402;
oviducts, 405;
eggs, 411
Salmopercae, _620_
_Salpa_, _101_, 36, 37;
arrangement of zooids, 102, 103;
structure, 104, 105;
alternation of generations, 105;
development and life-history, 106;
classification, 108;
_S. democratica-mucronata_, young, 107;
_S. hexagona_, 106;
_S. pinnata_, endostyle and stolon, 103;
_S. runcinata-fusiformis_, 102, 108
—see also _Cyclosalpa_, _Iasis_, _Pegea_, and _Thalia_
Salpians, 64
Salpidae, _101_, 39, 110
_Salvelinus_, _567_
Sand-Eel, 639, 275
Sander, 659
_Sarcodaces_, _575_
_Sarda_, _678_;
_S. orientalis_, caudal fin of, 675
Sardine, 564
_Sardinioides_, _611_
_Sardinius_, _611_
_Sargus_, _665_
Sars, G. O., 21, 23
Sars, M., 21
_Saurenchelys_, _601_
_Saurida_, _611_
Saurie, 638
_Saurocephalus_, _561_
_Saurodon_, _561_
Saurodontidae, _561_, 544
_Saurogobio_, _582_
Sauropsida, _145_
_Saurus_, _611_
Saury Pike, 411
Savi's vesicles, 384
Savigny, 36
Saw-Fishes, 457, 459
Scabbard-Fish, 679
Scales, 185 f.;
_Acipenser ruthenus_, 187;
_Antennarius hispidus_, 191;
_Balistes capriscus_, 191;
_Centriscus scolopax_, 190;
_Hypostoma commersonii_, 192;
_Lepidosteus_, 188;
_L. osseus_, 186;
_Monacanthus scopas_, 191;
_Salmo fario_, 190;
as an index of age, 191
_Scapanorhynchus_, _453_
_Scaphirhynchus_, _495_;
_S. platyrhynchus_, 495
Scaridae, _674_, 651, 654
_Scarus_, _674_; {755}
teeth, 251;
_S. strongylocephalus_, pharyngeal bones, 674
_Scatophagus_, _668_
_Scaumenacia_, _506_, 519
_Schilbe_, _588_
_Schilbichthys_, _588_
Schimkewitsch, 21 n.
_Schizocardium_, 17, 5, 6, 13, 14;
_S. brasiliense_, 6, 13, 14
_Schizothorax_, _582_
Schlosser and Ellis, 36
Schmidt, 705 n.
Schmidt, C., 37
Schnapper, 665
Schultz, 29, 30
_Sciades_, _588_
_Sciaena_, _663_;
_S. aquila_, 663, 362, 361
Sciaenidae, _663_, 653, 361, 363;
branching of the air-bladder, 303
_Scissor_, _575_
_Sclerocottus_, _698_
Sclerodermi, _722_ f., 721, 357
_Sclerognathus_, _581_
_Scleropages_, _555_;
_S. leichardti_, 556, 558
Scleroparei, _692_ f., 651, 652
_Sclerorhynchus_, _460_
_Scoliodon_, _448_
_Scolopsis_, _664_
_Scomber_, _678_, 272;
_S. brachyurus_, 357;
_S. colias_, 302;
_S. pneumatophorus_, 302;
_S. scombrus_, 678, 275, 302;
coloration, 168
_Scombramphodon_, _678_
Scombresoces, _636_ n.
Scombresocidae, _637_, 636
_Scombresox_, _638_, 411
Scombridae, _678_, 676, 158, 303, 412
Scombriformes, _675_ f., 651, 652
_Scombrinus_, _678_
_Scombroclupea_, _563_
_Scombrocottus_, _697_
_Scombrocypris_, _582_
_Scombrops_, _660_
_Scopelarchus_, _611_
_Scopelengys_, _611_
Scopelidae, _611_, 606, 302, 396;
photophores, 178
_Scopelogadus_, _656_
_Scopeloides_, _611_
_Scopelosaurus_, _611_
_Scopelus_, _611_, 613, 620;
_S. benoitii_, photophores, 178, 179, 181;
_S. crocodilus_, 612
_Scorpaena_, _695_, 693;
_S. grandicornis_, 695
_Scorpaenichthys_, _698_;
_S. marmoratus_, pectoral arch of, 693
Scorpaenidae, _694_, 692, 153, 418
_Scorpaenoides_, _695_
Scorpididae, _666_, 651, 652, 653
_Scorpis_, _666_
_Scyllaemus_, _565_
Scylliidae, _446_, 401, 402;
fossil, 447
_Scylliogaleus_, _449_
_Scylliorhinus_—see _Scyllium profundorum_
_Scyllium_, _446_, 198, 271, 446;
dermal denticles, 184;
teeth in embryo, 249;
internal organs, 253;
spleen, 343;
fossil, 447;
_S. canescens_, 447;
_S. canicula_, 446;
vertebral column, 193, 194, 195;
skull, 207 f., 208;
spiral valve, 267;
liver, 273;
pancreas, 273;
lateral veins, 318 n.;
brain, 373;
abdominal pores, 401;
oviparous, 433;
attachment of eggs, 434;
supra- and inter-renal bodies, 346, 433;
_S. catulus_, 446;
_S_. (_Scylliorhinus_) _profundorum_, 447
_Scymnus_, _455_, 456;
vertebral column, 198
Scyphophori, _543_
_Scytalina_, _712_
Sea-Breams, 664
Sea-Horse, 634, 154, 163, 361, 362
Sea-Snails, 699, 411
_Sebastes_, _695_, 692, 693;
_S. norvegicus_, 419;
_S. percoides_, pectoral arch, 693
_Sectator_, _657_
Seeliger, 39
_Selache_ (_Cetorhinus_), _453_, 264;
fossil, 454;
_S. aurata_, gill-rakers, 287;
_S. maxima_, 453;
gill-rakers as a plankton-filter, 287;
distribution and food, 453, 454
Selachii, _442_, 148
_Selachophidium_, _712_
_Selene_, _677_
Selenichthyes, _627_, 629
Semionotidae, _497_
_Semionotus_, _498_
_Semiophorus_, _677_
_Semiplotus_, _582_
Semon, 510, 593
Semper, 625
Sense-organs, of Ascidians, 53;
of Amphioxus, 128;
of Fishes, 383
_Seriola_, _677_
_Seriolella_, _643_
_Seriolichthys_, _677_
Serranidae, _659_, 651, 652, 653, 361, 389
Serraninae, _659_
_Serranus_, _659_, 660;
hermaphroditism, 420;
_S. cabrilla_, 660, 420;
_S. hepatus_, 660, 420;
_S. scriba_, 660, 420
_Serrasalmo_, _576_;
_S. niger_, 578;
_S. rhombeus_, mouth, 577
Serrasalmoninae, _576_
_Serrivomer_, _603_
_Setarches_, _695_
Sexual dimorphism, 419 f., 432, 462, 469, 471, 483, 500
Shad, 564
Shape of the body, in Fishes, 152
Sheep's-Head, 665
Shipley, 20 n.
_Sicyases_, 709;
_S. sanguineus_, 708;
sucker, 162
_Sicydium_, _689_ {756}
Siel-Smelt, 569
Sik, 568
Sillaginidae, _662_, 653
_Sillago_, _662_
_Silondia_, _588_
_Siluranodon_, _588_
_Silurichthys_, _588_
Siluridae, _586_, 575, 163, 251, 275, 283;
barbels, 154;
intestinal respiration, 292;
accessory respiratory organs, 293, 294;
air-bladder, 302, 303, 305;
sound-production, 356-361, 363;
connexion of the air-bladder and auditory organ, 389, 390;
nests, 416;
protection of young, 593
Silurinae, _588_
_Silurodon_, _588_
_Silurus_, _588_, 342;
_S. glanis_, 592, 593, 358
Silvestri, 594
_Simenchelys_, _601_;
_S. parasiticus_, 603
Simple Ascidians, 35, 36 f., 39 f., 70, 71, 110
_Siniperca_, _659_
Siphons, 43, 53
_Siphonognathus_, _674_
_Siphonostoma_, _634_;
_S. typhle_, protective coloration, 172
Sirenoidei, _149_
_Sisor_, _588_, 589
Skates = _Raia_, q.v.
Skeleton, of Enteropneusta, 14;
of Amphioxus, 119;
of Fishes, 193 f.
Skin, of Cyclostomata, 182;
of Fishes, 182
Skull, composition of, 206;
development, 206;
_Scyllium canicula_, 207, 208;
_Salmo salar_, 211, 212, 213, 214;
Cyclostomata, 216 f.;
_Petromyzon marinus_, 216 f., 217;
_Bdellostoma_, 220 f., 220, 221;
Elasmobranchii, 222 f.;
_Notidanus cinereus_, 222;
_Chimaera monstrosa_, 223;
Chondrostei, 224 f.;
Sturgeon, 224;
_Polyodon_, 225;
Crossopterygii, 226 f.;
_Polypterus_, 227;
Holostei, 228 f.;
Teleostei, 229 f.;
Dipnoi, 231 f.;
_Protopterus_, 231 f., 232;
_Lepidosiren_, 231 f., 233
Sluiter, 38
Smelt, 568, 411
Smitt, 569 n., 714 n.
Social Ascidians, 71
Sole, 687
_Solea_, _687_, 685;
_S. vulgaris_, 686, 687
Solenocytes, 29, 126, 127
_Solenognathus_, _634_
_Solenorhynchus_, _634_
Solenostomidae, _633_, 626, 627, 628;
marsupial pouches, 416
_Solenostomus_, _633_
_Soleotalpa_, _687_
Sollas, I., 522
Sollas, W. J., 522
Sörensen, 355, 590 n.
_Sorubim_, _588_, 361
_Sosia_, _588_
Sound-producing organs, 355 f.
Spalt-papillen, 384
_Spaniodon_, _563_, 564 n.
Sparidae, _664_, 654, 303, 389;
teeth, 251
_Sparisoma_, _674_
_Sparnodus_, _665_
_Sparus_, _665_;
_S. unicolor_, 665
_Spathiurus_, _545_
_Spathodactylus_, _561_
Spengel, 5, 7, 8, 13 n., 16, 17, 18 n., 19
_Spengelia_, _17_, 5, 6
Spermatozoa, 413
_Sphaerodon_, _665_
_Sphenacanthus_, _445_
_Sphenocephalus_, _656_
_Sphyraena_, _642_
Sphyraenidae, _642_, 637
_Sphyraenodus_, _678_
_Sphyrna_, _449_, 153;
liver, 272;
fossil, 450;
distribution, 449;
_S. malleus_, 266;
spiral valve, 266, 267;
_S. tiburo_, 450;
_S. tudes_, 449;
_S. zygaena_, 450
Sphyrnidae, _449_
_Spinacanthus_, _723_
_Spinachia_, _630_;
_S. vulgaris_, 631
Spinacidae, _454_;
photophores, 179;
fossil, 456
Spinal cord, 367
Spinal nerves, 378
_Spinax_, _455_
_Spinivomer_, _603_
Spiny Dog-Fishes, 455
Spiracle, 279, 283
Spiracular pseudobranch, 279, 284;
in Teleostomi, 284;
blood supply, 335
Spiral valve, 264 f.
Spix, 558 n.
Spleen, 342
Spoon-Bill, 491; = _Polyodon_, q.v.
Sprat, 564
_Squaliobarbus_, _582_
_Squaloraia_, _468_
Squaloraiidae, _468_
Star-gazers, 155
Starks, 627 n.
_Steatogenys_, _579_
Steenstrup, 36, 685 n.
_Stegophilus_, _589_;
_S. insidiosus_, _594_
_Stegostoma_, _446_;
_S. tigrinum_, 446
_Steindachneria_, _647_
_Steinegeria_, _682_
_Stenodus_, _565_
Stephanoberycidae, _619_, 606
_Stephanoberyx_, _619_;
_S. gillii_, 620;
_S. monae_, 620
_Stereobalanus_, _17_, 5;
_S. canadensis_, 16, 17
_Stereolepis_, _659_, 660
Sterlet, 493
_Sternarchus_, _579_; {757}
_S. albifrons_, 580;
_S. macrostoma_, 580
Sternoptychinae, _571_
_Sternoptyx_, _571_;
_S. diaphana_, 571, 178
_Sternopygus_, _579_
_Stichonodon_, _575_
Sticklebacks, 630, 163, 169, 288, 354, 357
Stigmata, 47;
of Appendicularians, 67;
of Cyclomyaria, 95
_Stigmatophorus_, _634_, 635
Stolon, 71, 93, 95, 97, 103
Stolonial budding, 81
_Stomatorhinus microps_, _551_
_Stomias_, _571_, 572
Stomiatidae, _570_, 545;
photophores, 178
Stomiatinae, _571_
Stomodaeum, 59, 120, 136
Storms, 691 n.
_Stratodus_, _623_
_Strepsodus_, _479_
Stridulation, 356
Stromateidae, _643_, 637
_Stromateoides_, _643_
_Stromateus_, _643_
Sturgeon, 154, 334, 493; = _Acipenser_, q.v.
_Styela_, _74_, 76;
branchial sac, 74;
_S. bythia_, 76;
_S. clava_, 76;
_S. etheridgii_, dorsal tubercle, 79;
_S. squamosa_, 76;
_S. whiteleggii_, dorsal tubercle, 79
Styelinae, _74_;
tentacles, 75
_Styelopsis_, _74_;
branchial sac, 74;
_S. grossularia_, 74, 76
_Stygicola_, _713_
_Stygogenes_, _595_
_Stylophorus_, _716_
Sucker-Fish (Sucking-Fish), 691, 161
_Sudis_, _611_;
_S. gigas_, air-bladder as a respiratory organ, 291
Sun-Fish, 657 (_Lepomis_);
727 (_Orthagoriscus_)—see also _Orthagoriscus_
Supra-pericardial organs, 344
Supra-renal bodies, 346, 346
Surf-Fishes, 670
Swinnerton, 573, 627, 636 n.
Sword-Fish, 681, 154
Symbranchidae, _597_, 283, 294
Symbranchii, _597_ f.
_Symbranchus_, _597_, 598, 156
_Symphurus_, _687_
_Symphysodon_, _672_
_Synagris_, _664_
_Synagrops_, _659_
_Synancia_, _695_;
_S. verrucosa_, poison-glands, 177
Synaphobranchidae, _603_
_Synaphobranchus_, _603_
Synapticula, 13, 12, 122
_Synaptura_, _687_
Synascidiae, _80_, 35
_Synchirus_, _698_
_Synechodus_, _445_
Syngnathidae, _634_, 627, 628, 163, 205, 251, 354, 361, 416;
colours, 166;
protective coloration, 172
_Syngnathus_, _634_, 627, 635;
mouth, 154;
_S. acus_, 262, 271;
_S. intestinalis_, 635;
_S. pelagicus_, 634
_Synodontis_, _588_, 590, 359;
_S. batensoda_, 591;
_S. decorus_, 591;
_S. membranaceus_, 591
_Syntegmodus_, _549_
Tadpole, Ascidian, 59, 62
_Taeniolabrus_, _706_
Taeniosomi, _714_ f., 651, 652
Tamiobatidae, _462_
_Tamiobatis vetustus_, _462_
_Taractes_, _682_
Tarpon, 547
Tarrasiidae, _477_ n.
_Tarrasius problematicus_, _477_ n.
Tattersall, 137 n.
_Tauredophidium_, _712_, 713
_Tauroglossus_, _17_ n.
_Tautoga_, _673_
Taylor, 599 n.
Tectospondylic, 198
Teleostei, _504_, _541_ f.;
classification, 542, 543, 149
Teleostomi, _475_ f., 149
_Telepholis_, _615_
Telescope Fish, 585
_Telescops_, _659_
_Tellia_, _616_
_Telmatherina_, _639_
Telolecithal, 409, 410
Tench, 583, 171, 260, 320, 321
Tentacles, of Ascidians, 44;
of Amphioxus, 116
Test, 40, 63;
of Appendicularians, 65
Testa-cells, 56
_Tethyum_, 36
_Tetragonopterus_, _575_
Tetragonuridae, _642_, 637
_Tetragonurus cuvieri_, _642_
_Tetranematichthys_, _589_
_Tetrapturus_, _680_;
_T. belone_, 680
_Tetraroge_, _695_
_Tetrodon_, _726_, 152, 354, 361;
warning colours, 163;
scales, 191;
teeth, 251;
_T. honckenii_, 362;
_T. mbu_, pectoral arch of, 722
Tetrodontidae, _726_, 722
_Tetronarce_, _464_
_Teuthis_, _669_
Teuthididae, _668_, 654
_Thalassophryne_, _711_;
_T. reticulata_, poison glands, 177
_Thalassothia_, _711_
_Thaleichthys_, _566_
_Thalia_, _108_;
_T. democratica-mucronata_, 108
Thaliacea, _95_ f., 38, 64, 81, 110
_Thaumatostomias_, _571_ {758}
_Thaumaturus_, _565_
_Thelodus_, _524_;
_T. pagei_, 524;
_T. tulensis_, 525
_Therapon_, _660_, 361
Thilo, 682 n.
_Thoracopterus_, _545_
Thoracostei, _627_, _629_
Thread-cells, in Myxinoids, 182
_Threpterius_, _664_
Thresher Shark, 451, 452
_Thrissopater_, _562_
Thrissopatrinae, _562_
_Thrissops_, _546_
_Thryptodus_, _549_
_Thunnus_, _678_, 275;
_T. thynnus_, 678
_Thursius_, _477_
_Thylacium_, _89_
_Thymallus_, _565_;
_T. vexillifer_ = _vulgaris_, 568
Thymus, 344, 345
_Thyrina_, _639_
Thyroid gland, 46, 343, 344
_Thyrsites_, _679_
_Thyrsitocephalus_, _679_
_Thyrsoidea_, _605_;
_T. macrura_, 605;
_T. meleagris_, skull, 604
_Tilapia_, _671_;
_T. dardennii_, 671;
_T. dolloi_, 152
Tile-Fish, 661
_Tinca_, _582_;
_T. vulgaris_, 583, 171, 260, 320;
renal portal system, 320, 321
_Titanichthys_, _537_
Todaro, 39
Tongue, 252
Tongue-bars, 12, 12, 13, 120, 135
Tope, 449
Tornaria, 18, 18 f., 28, 31;
_T. agassizii_, 21;
_T. grenacheri_, 19;
_T. krohni_, 19
Torpedinidae, _462_
_Torpedo_, _463_;
electric organs, 365, 366;
_T. hebetans_, 463;
_T. marmorata_, 463;
_T. narce_, 463;
_T. ocellata_, 463
_Toxotes_, _658_;
_T. jaculator_, 658
Toxotidae, _658_, 653
_Trachelochismus_, _709_
_Trachelyopterus_, _589_, 361
_Trachichthys_, _656_, 655
Trachinidae, _704_, 703
_Trachinops_, _659_
_Trachinus_, _704_;
warning colours, 174;
poison-glands and spines, 176;
_T. draco_, 705, 176;
pectoral arch of, 703;
_T. vipera_, 705, 176
_Trachyglanis_, _588_
_Trachynotus_, _677_
_Trachypoma_, _659_
Trachypteridae, _715_, 152
_Trachypterus_, _715_;
_T. arcticus_, 715;
_T. iris_, 715;
_T. taenia_, 160
_Trachyrhynchus_, _647_
Traquair, 436, 523, 530, 537, 687 n.
Traustedt, 38, 101 n.
Tremataspidae, _530_
_Tremataspis_, _530_
_Trematomus_, _706_
Tremblador, 580
Triacanthidae, _722_, 721
_Triacanthodes_, _722_
_Triacanthus_, _722_;
scales, 190;
sound-production, 357;
_T. biaculeatus_, 361;
_T. brevirostris_, 361;
pectoral arch of, 722
_Trichiurichthys_, _679_
Trichiuridae, _679_, 676
_Trichiurus_, _679_
_Trichodon_, _663_
Trichodontidae, _663_, 654, 704
_Trichogaster_, _669_, 293
Trichomycterinae, _589_
_Trichomycterus_, _589_
Trichonotidae, _706_
_Trichonotus_, _706_
Trigger-Fishes, 724, 174, 357
_Trigla_ (Gurnards), _701_, 693;
liver, 272;
air-bladder, 305;
sound-production, 361, 363;
_T. cuculus_, 701, 363;
_T. gurnardus_, 701, 363;
rectal valve, 254;
_T. hirundo_, 701, 363;
_T. lineata_, 701;
_T. lyra_, 701, 363;
_T. obscura_, 701
Triglidae, _700_, 694;
coloration, 164;
sounds, 361, 363
_Triglops_, _698_
_Triglopsis_, _697_
_Trigonodon_, _665_
_Triodon_, _723_, 721;
_T. bursarius_, 723
Triodontidae, _723_, 721
_Triplophos_, _571_
_Tripterophycis_, _648_
_Trissolepis_, _487_
_Tristichopterus_, _478_;
_T. alatus_, 479
_Tristychius_, _445_
_Troglichthys rosae_, _619_
_Tropheus_, _671_
Trophozooid, 98, 100
_Tropidichthys_, _726_
Troschel, 682 n.
Trout, 566 f., 567;
Brook-, 567;
Brown, 567;
Gillaroo, 260;
Sea-, 567
Trumpet-Fish, 154
Trunk-Fishes, 724
_Trygon_, _464_;
spines, 177;
uterine nutrition, 434;
_T. pastinaca_, 464;
_T. sabina_, 464
Trygonidae, _464_, 465
_Trygonorhina_, _460_, 461
_Trypauchen_, _689_
_Trypauchenichthys_, _689_
Tunic, 63
Tunicata, _63_, 35, 36 f., 39, 4, 30 n.;
affinities, 62;
external form, 64
Tunicine, 37
Tunny, 678, 678
Turbot, 687
Turner, 593 n.
Twait Shad, 564 {759}
_Typhlichthys_, _618_;
_T. rosae_, 619;
_T. subterraneus_, 619
_Typhlogobius_, _689_, 394;
_T. californiensis_, 690
_Typhlonus_, _712_, 713;
_T. nasus_, 712
Typhlosole, 54
Uljanin, 39
_Ulocentra_, _659_
_Umbra_, _609_;
_U. crameri_, 610;
_U. limi_, 610
_Umbrina_, _663_;
_U. cirrhosa_, 361
_Undina_, _481_, 161;
_U. gulo_, 480
_Unio_, _Rhodeus_ in, 584
_Upeneichthys_, _665_
_Upeneoides_, _665_
_Upeneus_, _665_
Uranoscopidae, _706_, 703
_Uranoscopus_, _706_;
eyes, 155
_Urenchelys_, _601_
_Urocampus_, _634_
Urochord, 66
Urochordata, _4_, _63_, 38
_Uroconger_, _601_
_Urogymnus_, _464_
_Urolepis_, _487_
_Urolophus_, _464_
Uronemidae, _507_
_Uronemus_, _507_, 519
_Urosphen_, _632_
Ussoff, 38
Vaillant, 596 n., 621 n., 720 n.
Vallentin, R., 608
_Vandellia_, _589_;
_V. cirrhosa_, entering urethra, 593
Vascular system, 313 f.;
general, 313;
Cyclostomata, 315, 327, 329, 330;
Elasmobranchii, 316, 318, 327, 328, 330, 331, 333, 334;
Teleostomi, 319, 320, 321, 322, 328, 333, 335, 336, 337;
Dipnoi, 323, 324, 326, 329, 338, 340;
blood, 341;
blood-glands, 342
Velum, 120
Vendace, 568
Venous system, 315 f.
Vermiform process, of Enteropneusta, 14, 17
Vertebral column, 193 f.;
_Scyllium canicula_, 193, 194, 195;
Cyclostomata, 197;
Elasmobranchs, 197, 198;
_Chimaera monstrosa_, 199;
Dipnoi, 199;
Chondrostei, 200;
_Acipenser sturio_, 200;
Crossopterygii, Holostei, and Teleostei, 201 f.;
_Amia_, 201, 203;
_Lepidosteus_, 201, 204;
_Caturus furcatus_, 203;
_Eurycormus speciosus_, 203;
_Gadus morrhua_, 205
Vertebrates, characters of, 3 f.
Vessels, of Ascidians, 41—see also Vascular system
_Vidalia_, _546_
Vipan, 592
_Vireosa_, _689_
Visceral arches, 207
Viviparous Fishes, 418 f., 433, 435
Vogt, 37
_Vomeropsis_, _677_
_Vulsus_, _707_
Wagner, 595 n.
Waite, 643
Walking-Fish, 690
_Wallago_, _588_
Warington, 630 n.
Watase, 585 n.
Weber, E. H., 573
Weber, Max, 661
Weberian ossicles, 389, 573 f.
Weevers, 704, 174
Weiss, 568 (= _Coregonus_)
Weiss, F. E., 126
Wels, 593
_Wertheimeria_, _588_
Wetterfisch, 585
Whiff, 687
Whip-tailed Rays, _464_
Whitebait, 564, 608
White-Fish, 568, 569
Whiting, 649, 168, 275
Whymper, 595 n.
Willey, 5, 7, 9, 11, 13 n., 15 n., 16, 19 n., 58 n., 113, 633
_Willeyia_, _17_ n.
Williamson, 602 n.
Wilson, 113
Witch, 687
_Wodnika_, _445_
Wolf-Fishes, 710, 251, 408
Woodward, A. Smith, 458, 477 n., 480 n., 497, 541 n., 542, 543, 622, 624,
626, 680
Wrasses, 673, 164, 166
Wright, Ramsay, 587
Wyman, 593 n., 596 n.
Xanthochroism, 171
_Xenichthys_, _659_
_Xenocharax_, _576_, 578
_Xenocypris_, _582_
_Xenocys_, _659_
_Xenodermichthys_, _570_
_Xenomystus_, _555_
_Xenopholis_, _498_
Xenopteri, _708_
_Xenopterus_, _726_
_Xenotilapia_, _671_
_Xiphasia_, _709_
_Xiphias_, _681_;
beak, 154;
_X. gladius_, 681
Xiphiidae, _681_, 676
_Xiphiorhynchus_, _680_
_Xiphistes_, _709
Xiphostoma_, _576_ {760}
Xiphostominae, _576_
Yarrell, 112
_Zalocys_, _677_
_Zanclus_, _668_
_Zaniolepis_, _696_
_Zapteryx_, _460_
Zeidae, _683_, 361
_Zenion_, _683_
Zeorhombi, _682_ f., 651, 652
_Zeugopterus_, _687_
_Zeus_, _683_;
_Z. faber_, 683, 361, 363;
red gland, 308
_Zoarces_, _712_, 713;
_Z. viviparus_, 419
Zoarcidae, _712_, 704, 361, 395
Zograf, 697 n.
_Zygaena_, _449_
END OF VOL. VII
_Printed by_ R. & R. CLARK, LIMITED, _Edinburgh_.
THE CAMBRIDGE NATURAL HISTORY
Edited by S. F. HARMER, Sc.D., F.R.S., Fellow of King's College,
Cambridge, Superintendent of the University Museum of Zoology; and A. E.
SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge, University
Lecturer on the Morphology of Invertebrates.
_To be completed in Ten Volumes. 8vo. 17s. net each_.
Intended in all respects to be a Standard Natural History accurate enough
to be of use to the Student, and at the same time popular enough for the
general reader who desires trustworthy information as to the structure
and habits of all members of the Animal Kingdom, from the Protozoa to the
Mammals. The Volumes are fully illustrated by original figures drawn
where possible from nature. When complete the Series is one which should
be indispensable in all Libraries, whether public or private.
WORMS, LEECHES, ETC.
VOLUME II.
FLAT WORMS. By F. W. GAMBLE, M.Sc. Vict., Owens College.—NEMERTINES. By
Miss L. SHELDON, Newnham College, Cambridge.—THREAD-WORMS, ETC. By A. E.
SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge.—ROTIFERS.
By MARCUS HARTOG, M.A., Trinity College, Cambridge, D.Sc. Lond.,
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WORMS. By W. BLAXLAND BENHAM, D.Sc., Hon. M.A. Oxon., Professor of
Biology in the University of Otago.—EARTH-WORMS AND LEECHES. By F. E.
BEDDARD, M.A. Oxon., F.R.S., Prosector to the Zoological Society,
London.—GEPHYREA, ETC. By A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's
College, Cambridge.—POLYZOA. By S. F. HARMER, Sc.D., F.R.S., Fellow of
King's College, Cambridge.
_CAMBRIDGE REVIEW_.—"Several of the groups treated of in this volume are
unknown by sight even, to the general reader, and possess no popular name
whatsoever; and as only a few insignificant details are known of the
habits of the animals composing them, their treatment in the volume
before us has necessarily been to a large extent anatomical. This
circumstance renders the book of especial value to students, more
particularly as in some cases the articles on the groups in question are
the first comprehensive ones dealing with their respective subjects....
Most of the articles are of a very high order of merit—taken as a whole,
it may be said that they are by far the best which have as yet been
published.... We may say with confidence that the same amount of
information, within the same compass, is to be had in no other zoological
work."
_NATURAL SCIENCE_.—"This second volume of the Cambridge Natural History
is certain to prove a most welcome addition to English Zoological
literature. It deals with a series of animal groups, all deeply
interesting to the specialist in morphology; some important from their
economic relations to other living things, others in their life-histories
rivalling the marvels of fairy-tales. And the style in which they are
here treated is also interesting; history and the early observations of
the older writers lend their charm; accounts of habits and mode of
occurrence, of life, in a word, from the cradle to the grave, are given
in ample detail, accompanied by full references to modern and current
literature. The whole is admirably illustrated."
SHELLS
VOLUME III.
MOLLUSCS AND BRACHIOPODS. By the Rev. A. H. COOKE, M.A., A. E. SHIPLEY,
M.A., F.R.S., and F. R. C. REED, M.A.
_TIMES_.—"There are very many, not only among educated people who take an
interest in science, but even among specialists, who will welcome a work
of reasonable compass and handy form containing a trustworthy treatment
of the various departments of Natural History by men who are familiar
with, and competent to deal with, the latest results of scientific
research. Altogether, to judge from this first volume, the Cambridge
Natural History promises to fulfil all the expectations that its
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_FIELD_.—"We know of no book available to the general reader which
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_KNOWLEDGE_.—"If succeeding volumes are like this one, the Cambridge
Natural History will rank as one of the finest works on natural history
ever published."
_ATHENÆUM_.—"The series certainly ought not to be restricted in its
circulation to lecturers and students only; and, if the forthcoming
volumes reach the standard of the one here under notice, the success of
the enterprise should be assured."
INSECTS AND CENTIPEDES
VOLUME V.
PERIPATUS. By ADAM SEDGWICK, M.A., F.R.S.—MYRIAPODS. By F. G. SINCLAIR,
M.A.—INSECTS. Part I. By DAVID SHARP, M.A. Cantab., M.B. Edin., F.R.S.
_FIELD_.—"Although written for the student and the specialist, the book
is not the less adapted to all intelligent readers who wish to make
themselves thoroughly acquainted with the habits, structure, and the
modern classification of the animals of which it treats. To such it
cannot be recommended too strongly."
_SCIENCE GOSSIP_.—"Every library, school, and college in the country
should possess this work, which is of the highest educational value."
_Prof. RAPHAEL MELDOLA, F.R.S., F.C.S., in his Presidential Address to
the Entomological Society of London, said_:—"The authors of this volume
are certainly to be congratulated upon having furnished such a valuable
contribution to our literature. When its successor appears, and I will
venture to express the hope that this will be at no very distant period,
we shall be in possession of a treatise on the natural history of insects
which, from the point of view of the general reader, will compare most
favourably with any similar work that has been published in the English
language."
_ENTOMOLOGIST'S MONTHLY MAGAZINE_.—"We venture to think the work will be
found indispensable to all who seek to extend their general knowledge
beyond the narrowing influence of exclusive attention to certain orders
or groups, and that it will take a high position in 'The Cambridge
Natural History' series."
INSECTS—PART II.
VOLUME VI.
HYMENOPTERA _continued_ (TUBULIFERA AND ACULEATA), COLEOPTERA,
STREPSIPTERA, LEPIDOPTERA, DIPTERA, APHANIPTERA, THYSANOPTERA, HEMIPTERA,
ANOPLURA. By DAVID SHARP, M.A., F.R.S.
_SATURDAY REVIEW_.—"Dr. Sharp's treatment is altogether worthy of the
series and of his own high scientific reputation. But in a work of this
sort it is not only necessary that information should be accurate, but
also that it shall be presented to the eye, so far as illustrations and
printing are concerned, in such a way as to render its matter as easily
intelligible as possible, and readily usable for purposes of reference.
Under both these heads we have nothing but commendation for Mr. Sharp's
treatise. The illustrations are indeed beautiful, and the use of the
heavy type for the headings of the various sections and leading
paragraphs materially helps the reader in the progress of his study.
Certainly this is a book that should be in every entomologist's library."
_DAILY NEWS_.—"It would be hard to say too much in praise of this most
admirable volume. It is too often the case that scientific books are
written in a dull and uninteresting style. The reader will find nothing
of that kind to complain of here. The descriptions are clear, the
illustrations are excellent; while, as in the previous volumes of the
series, printing and paper are all that could be desired."
FISHES
VOLUME VII.
HEMICHORDATA. By S. F. HARMER, Sc.D., F.R.S. ASCIDIANS AND AMPHIOXUS. By
W. A. HERDMAN, D.Sc., F.R.S. FISHES. By T. W. BRIDGE, Sc.D., F.R.S., and
G. A. BOULENGER, F.R.S.
AMPHIBIA AND REPTILES
VOLUME VIII.
By HANS GADOW, M.A., F.R.S.
_FIELD_.—"The work is worthy of the series in which it appears, and we
cannot give it higher praise."
_SCIENCE GOSSIP_.—"More than maintains the high scientific reputation of
this series. The herpetologists, or students of the Amphibia and
Reptiles, have now a standard work of the highest class."
_LANCET_.—"An account of both Amphibia and Reptiles which should satisfy
the expert, and at the same time entertain the reader who is merely
interested in the tit-bits of natural history.... A book full of accurate
information and pleasant reading."
_MORNING POST_.—"A delightful as well as a serviceable book.... Herein
perhaps lies the great charm and merit of Dr. Gadow's book, that, while
satisfying all the inquiries of the student, it is also in great part
written for the ordinary intelligence, and the naturalist in the crowd
may, while necessarily gliding over distressing technicalities, find in
its pages many hours of profitable and entertaining study of the habits
of the classes under notice."
_NATURE_.—"In concluding the review we would express the opinion that by
this handsome volume a very important addition to science has been made;
that the beautiful illustrations, together with the clear and charming
accounts of the life-histories which it contains, will do much to
popularise the study of a rather neglected section of zoology; and that
lovers of Reptiles, of which there are more than one generally thinks,
will feel that the new knowledge imparted to them emanates from one who
is thoroughly in sympathy with their enthusiasm."
BIRDS
VOLUME IX.
By A. H. EVANS, M.A., Clare College, Cambridge. With numerous
Illustrations by G. E. LODGE.
_IBIS_.—"Mr. Evans has produced a book full of concentrated essence of
information on birds, especially as regards their outer structure and
habits, and one that we can cordially recommend as a work of reference to
all students of ornithology."
_NATURE NOTES_.—"We venture to predict that, of the ten volumes of which
this excellent series is planned to consist, none will secure a wider
popularity than Mr. Evans's treatise on birds. Strange as it may appear,
among the many books on birds that have appeared of late years, we do not
recall any that covers the same ground.... We are grateful to the author
for the mine of valuable information which he has crowded between his two
covers."
_SCIENCE GOSSIP_.—"General readers will find this work most useful in
obtaining a proper understanding of birds, and will be assisted by the
effective diagram of a hawk in the introduction, showing the recognised
names of every part of the exterior appearance. The expressions used in
naming the various portions are fully explained on the adjoining page. As
we have already said, the illustrations are admirable. The book is a
useful addition to any library, as it treats of nearly every known kind
of bird throughout the world."
_SATURDAY REVIEW_.—"The expert and the novice alike must be at once
delighted by the accuracy and the beauty of the illustrations.... It is
astonishing to note the mass of information the author has been able to
bring together.... With a little practice any observant person would soon
learn by the help of this volume to track down any bird very nearly to
its ultimate place in classification."
MAMMALS
VOLUME X.
MAMMALIA. By FRANK EVERS BEDDARD, M.A. Oxon., F.R.S., Vice-Secretary and
Prosector of the Zoological Society of London.
_NATURE_.—"Cannot fail to be of very high value to all students of the
Mammalia, especially from the standpoints of morphology and
palæontology."
_ATHENÆUM_.—"Mr. Beddard has produced a volume equal in interest and
value to the others in the Cambridge series."
_LAND AND WATER_.—"A notable book, the result of long study, patient
labour, sound reasoning, and careful selection, for which we are deeply
indebted to the author."
_DAILY NEWS_.—"A volume which, for the interest of its contents and for
its style and method of treatment, is not only worthy of its
predecessors, but may be regarded as one of the most successful of a
brilliant series."
_KNOWLEDGE_.—"In this volume Mr. Beddard has undoubtedly made an
important contribution to the history of mammals, his text-book being the
only one which can be said to be up to date and to contain notices of the
many important types—both recent and fossil—discovered during the last
few years."
_FIELD_.—"Its utility to the working zoological student can hardly be
overrated. It is exceedingly well illustrated."
_CONCLUDING VOLUMES IN PREPARATION_.
VOLUME I.
PROTOZOA, MARCUS HARTOG, M.A., Sc.D., Trinity College (Professor of
Natural History in the Queen's College, Cork); SPONGES, W. J. SOLLAS,
Sc.D., F.R.S., St. John's College (Professor of Geology in the University
of Oxford); JELLYFISH, SEA-ANEMONES, ETC., S. J. HICKSON, M.A., F.R.S.,
Downing College (Beyer Professor of Zoology in the Owens College,
Manchester); STAR-FISH, SEA-URCHINS, ETC., E. W. MACBRIDE, M.A., St.
John's College (Professor of Zoology, McGill University, Montreal).
VOLUME IV.
SPIDERS, MITES, ETC., C. WARBURTON, M.A., Christ's College (Zoologist to
the Royal Agricultural Society); SCORPIONS, TRILOBITES, ETC., M. LAURIE,
B.A., King's College, D.Sc. (Edinb.), (Professor of Zoology in St.
Mungo's College, Glasgow); PYCNOGONIDS, ETC., D'ARCY W. THOMPSON, C.B.,
M.A., Trinity College (Professor of Zoology in University College,
Dundee); LINGUATULIDA AND TARDIGRADA, A. E. SHIPLEY, M.A., F.R.S.;
CRUSTACEA, W. F. R. WELDON, M.A., F.R.S., St. John's College (Linacre
Professor of Comparative Anatomy in the University of Oxford).
[_In the Press_.
MACMILLAN AND CO., LTD., LONDON.
----
NOTES
[1] Bateson, _Quart. J. Micr. Sci_. xxv. Suppl. 1885, p. 111.
[2] Gegenbaur, _Grundzüge vergl. Anat_. 2 ed. 1870, p. 158.
[3] Lankester, _Quart. J. Micr_. Sci. xvii. 1877, p. 448 (= ASPIDOPHORA,
Allman, _J. Linn. Soc_. xiv. 1879, pp. 490 n., 586).
[4] Spengel, _Zool. Jahrb. Syst_. xv. 1902, p. 209.
[5] _Fauna u. Flora G.v. Neapel_, 18 Monogr. 1893 (reviewed by MacBride
in _Quart. J. Micr. Sci_. xxxvi. 1894, p. 385); _Zool. Jahrb. Anat_.
xviii. Pt. ii. 1903, p. 271.
[6] _Quart. J. Micr. Sci_. xxiv. 1884, p. 208; xxv. _Suppl_. 1885, p. 81;
xxvi 1886, pp. 511, 535.
[7] _Zool. Results_, Part iii. Cambridge, 1899, p. 223.
[8] = 1 inch.
[9] _Quart. J. Micr. Sci_. xxiv. 1884, p. 209.
[10] _Quart. J. Micr. Sci_. xxv. _Suppl_. 1885, p. 91.
[11] Pouchet, _C. R. Ac. Sci_. cii. 1886, p. 272.
[12] Kowalevsky, _Mém. Ac. St. Petersb_. (7) x. No. 3, 1866, p. 7.
[13] Spengel, _Monogr_. p. 474.
[14] _Zool. Res_. Pt. iii. 1899, p. 256.
[15] See also Ritter, _Biol. Bull_. iii. 1902, p. 255.
[16] _Zool. Res_. iii. 1899, pp. 273, 280.
[17] Morgan, _J. Morphol_. v. 1891, p. 422; ix. 1894, pp. 44, 48, 72.
[18] Willey, _Zool. Res_. Pt. iii. 1899, p. 245.
[19] _Zool Res_. Pt. iii. p. 228.
[20] Spengel, _Monogr_. pp. 179, 187; Willey, _Zool. Res_. iii. p. 236.
[21] Willey.
[22] _Quart. J. Micr. Sci_. xl. 1898, p. 601; xliii. 1900, p. 351.
[23] _Monogr_. p. 684, Pl. xxvi. Figs. 14-18; see also Willey, _Zool.
Res_. iii. p. 245, and Dawydoff, _Zoolog. Anz_. xxv. 1902, p. 551.
[24] _Zool. Jahrb. Syst_. xv. 1902, p. 209. The Harrimaniidae =
_Balanoglossus_ of the Monograph (1893): _Glossobalanus_ =
_Ptychodera_, s.str., 1893: _Balanoglossus_ = _Tauroglossus_, 1893:
_Ptychodera_ = _Chlamydothorax_, 1893.
[25] Punnett ("Enteropneusta," in Gardiner's _Fauna and Geogr. Maldive and
Laccadive Arch_. ii. Pt. ii. 1903) finds small liver-sacs in
_Spengelia_, and describes _Willeyia_, a new genus of
Glandicipitidae.
[26] Exc. _G. ruficollis_, Willey.
[27] Spengel, _Monogr_. p. 370 f.
[28] Cf. Spengel, _Monogr_. p. 363 f.
[29] Bourne, _J. Mar. Biol. Ass_. (N.S.), i. 1889-90, p. 63.
[30] This closely resembles _T. grenacheri_, but see Willey, _op. cit_. p.
285.
[31] Haldeman, _Johns Hopkins Univ. Circ_. vi. No. 54, 1886, p. 45.
[32] For Vertebrates see Shipley, _Quart. J. Micr. Sci_. xxvii. 1887, p.
340.
[33] The largest known eggs are those of _Harrimania kupfferi_ (1.3 mm.).
The eggs of _Dolichoglossus kowalevskii_ measure .37 mm., while the
youngest Tornaria found by Morgan, already transparent and with their
tissues distended by water, were only about two-thirds that size.
[34] _Quart. J. Micr. Sci_. xxiv. 1884, p. 208; xxv. Suppl. 1885, p. 81;
xxvi. 1886, pp. 511, 535.
[35] Agassiz, Bourne, Spengel, Morgan (in _T. agassizii_).
[36] Morgan (in _Balanoglossus biminiensis_).
[37] A similar shrinkage takes place in the metamorphosis of the larva
(_Leptocephalus_) of Eels, as has been shown by Grassi, _Quart. J.
Micr. Sci_. xxxix. 1897, p. 374.
[38] Schimkewitsch, _Zool. Anz_. xi. 1888, p. 283; Morgan, _J. Morphol_.
ix. 1894, p. 60; Punnett (_op. cit_. p. 661) believes that they are
ectodermal.
[39] Allman's name (_Quart. J. Micr. Sci_. ix. 1869, p. 57 f.) replaces
that given by Sars, because the latter gave no description by which
the organism could be recognised.
[40] "Remarkable forms of Animal Life," i. _Christiania Univ.-Program for
the first half-year_, 1869; and _Quart. J. Micr. Sci_. xiv. 1874, p.
23.
[41] _Quart. J. Micr. Sci_. xxiv. 1884, p. 622.
[42] _Proc. Roy. Soc_. lii. 1893, p. 132; _Festschr. 70 ten. Geburtstage
R. Leuckarts_, 4to, Leipzig, 1892, p. 293.
[43] Hincks, _Hist. Brit. Marine Polyzoa_, vol. i. 1880, p. 581.
[44] _Rés. Camp. Sci. Prince de Monaco_, Bryozoaires, 1903, p. 23.
[45] _Challenger Reports_, Part lxii. 1887. See also Masterman in _Quart.
J. Micr. Sci_. xl. 1898, p. 340; xlvi. 1903, p. 715; _Rep. Brit.
Ass_. (1898), 1899, p. 914; _Tr. R. Soc. Edinb_. xxxix. 1900, p. 507;
and the notes in the _Zool. Anz_. xx. 1897, pp. 342, 443, 505; xxii.
1899, pp. 359, 361; and xxvi. 1903, pp. 368, 593.
[46] Cf. Cole, _J. Linn. Soc_. xxvii. 1899-1900, p. 256.
[47] Two dorsal portions of this region, which are regarded by Masterman
as lateral notochords, appear to me to represent the dorsal part of
the pharynx of _Ptychodera_.
[48] The diameter of a single individual removed from its tube is given by
Fowler as .123 mm.
[49] See, however, Conte and Vaney, _C. R. Ac. Sci_. 135, 1902, pp. 63,
748.
[50] Pp. 450-462.
[51] _Quart. J. Micr. Sci_. xl. 1898, p. 281; xliii. 1900, p. 375; xlv.
1902, p. 485.
[52] Cf. p. 19.
[53] _J. Coll. Japan_, xiii. Pt. iv. 1901, p. 507.
[54] _Arch. Biol_. xviii. 1902, p. 495; _Wiss. Meeresuntersuch_. vi. Abt.
Helgoland, Heft 1, 1903.
[55] _Quart. J. Micr. Sci_. xlvii. Pt. i, 1903, p. 103.
[56] _Zeitschr. wiss. Zool_. lxxv. 1903, pp. 391, 473.
[57] Vol. II. p. 459.
[58] Huxley, in 1877 (_Man. Anat. Invert. Animals_, p. 674), proposed to
unite the Enteropneusta with the Tunicata as Pharyngopneusta, in
allusion to the gill-slits connected with the pharynx; but the view
was first defended in detail by Bateson.
[59] See, for example, Minot, _Amer. Nat_. xxxi. 1897, p. 927.
[60] See MacBride, _Quart. J. Micr. Sci_. xl. 1898, p. 589; xliii. 1900,
p. 351.
[61] Bury, _Quart. J. Micr. Sci_. xxix. 1889, p. 409; xxxviii. 1896, p.
125; MacBride, _ibid_. xxxviii. p. 395; Masterman, _Tr. R. Soc.
Edinb_. xl. Pt. ii. No. 19, 1902, p. 403.
[62] This view was definitely formulated by Metschnikoff in 1881 (_Zool.
Anz_. iv. 1881, pp. 139, 153).
[63] Cf. Morgan, _J. Morphol_. v. 1891, p. 445; ix. 1894, pp. 64-66.
[64] Cf. Lang, _Jena. Zeitschr_. xxv. 1891, p. 1.
[65] _J. Morphol_. ix. p. 72.
[66] _Mém. Mus. Paris_, ii. 1815.
[67] _Mém. s. l. Anim. s. Vert_. Pt. ii. Paris, 1816.
[68] _Zoologia Danica_, iv. 1806.
[69] _Hist. Nat. d. Anim. sans Vert_. Paris, 1815-1822, t. iii.
[70] _Mém. Instit. Paris_, xviii. 1842.
[71] _Zur vergl. Physiol. Wirbellos. Thiere_, Brunswick.
[72] _Comptes Rendus_, Paris, xxii; and _Ann. Sci. Nat_. ser. 3 (Zool.) v.
[73] _Phil. Trans_. 1851; _Trans. Linn. Soc_. xxiii. 1860.
[74] _Mém. Acad. St. Pétersbourg_ (7), x. 1866.
[75] _Mém. Instit. Paris_, xviii. 1842.
[76] _Arch. mikr. Anat_. vi. 1872.
[77] _Mém. Soc. Phys. Hist. Nat. Genève_, xxi. 1872.
[78] _Arch. Zool. Expér_. i. 1872.
[79] _Synascidien der Bucht von Rovigno_, Wien, 1883.
[80] _Challenger Reports_, Tunicata, Part i. vol. vi. 1882; Part ii. vol.
xiv. 1886; Part iii. vol. xxvii. 1888.
[81] _Ann. Mag. Nat. Hist_. (3) xi. 1863, p. 153; _Journ. Linn. Soc_.
1868, etc.
[82] _Denkschr. Akad. Wiss. Wien_, 1875 and 1877.
[83] _Arch. Zool. Expér_. iii. 1874, and vi. 1877; _Mém. Instit. Paris_,
xlv. 1892.
[84] _Vid. Medd. Nat. For_. Copenhagen, 1880, 1882, 1884, etc.
[85] _Nat. Tijdschr. Ned.-Indie_, 1885, etc.
[86] _Journ. Linn. Soc. Zool_. xv. xxiii. and xxiv.; _Cat. of Tunicata in
Australian Museum_, 1899; also _Challenger Reports_ (see _note_ 80).
[87] _Arch. de Biol_. ii.
[88] "The Genus _Salpa_," _Mem. J. Hopkins Univ_. 1893.
[89] _Zeits. wiss. Zool_. 1876, 1878; _Mitth. Zool. Stat. Neapel_, 1883,
etc.
[90] _Jen. Zeitschr_. 1886, 1888, etc.; also Bronn's _Thier-Reich_.
[91] _Mitth. Zool. Stat. Neapel_, 1893 and 1897; and _Zeits. wiss. Zool_.
1895 and 1896.
[92] _Journ. Anat. Phys_. Paris, xxi. 1885.
[93] _Fauna and Flora G. v. Neapel_, Monogr. x. 1884.
[94] These sphincters close the only openings through the tough test so
effectually that when collectors are preserving Ascidians in alcohol
it is advisable to make one or more slits in the test to allow the
sea-water to escape and the spirit to enter.
[95] Except in Cynthiidae and Botryllidae where it is dorsal.
[96] The early stages of _Ciona_, of which Castle has given a very
complete account (_Bull. Mus. Comp. Zool_. xxvii. No. 7, 1896),
differ in some points from those of _Ascidia_ described here.
[97] Possibly the diverticulum may be wholly derived from the neural tube
(see Willey, _Quart. J. Micr. Sci_. 1893).
[98] See Lohmann, _Schrift. Naturw. Ver. Schlesw.-Holst_. xi. 1899, 347.
[99] _Arch. de Biologie_, vi. 1887.
[100] See _Journ. of Morphology_, xii.-xiv. 1896-1898.
[101] _Bull. Mus. Comp. Zool_. xxxv. No. 4, 1899, p. 59.
[102] _Journ. Morph_. xii. 1896, p. 149.
[103] See Pizon, _Ann. des Sci. Nat_. 7^e sér. Zool. xiv. 1892.
[104] "Oozooid" and "blastozooid" have not always been used in the same
sense. It is best to regard as oozooid the first member of a new
colony derived from an embryo formed by the fertilisation of an ovum,
and to call the remaining ascidiozooids produced by gemmation the
blastozooids.
[105] According to Kowalevsky. Salensky, however, considers that the atrial
aperture closes, and that a new surface depression appears later.
[106] See Barrois, _Journ. d'Anat. et Physiol_. 1885.
[107] _Mitth. Z. Stat. Neapel_, x. 1891.
[108] The most useful works on the Salpidae are Traustedt, _Vid. Selsk.
Skr_. ii. 8, 1885, Copenhagen; and Brooks's "The genus Salpa," _Johns
Hopkins Biolog. Memoirs_, ii. 1893.
[109] According to Metcalf, _Salpa cylindrica_ is protandrous.
[110] For a more detailed account of these subdivisions of the Salpidae,
and other groups, see Herdman's "Revised Classification of Tunicata,"
_Journ. Linn. Soc_., Zool., xxiii. 1891, p. 558.
[111] See Herdman, _Challenger Report_ on Tunicata, part iii. 1888, p. 88;
and Metcalf, _Johns Hopkins Univ. Circ_. No. 106, 1893, and _Zool.
Jahrb. Abth. Anat_. xiii. 1900, p. 572.
[112] Although the correct systematic name of the commonest species is
_Branchiostoma lanceolatum_ (Pallas), it is convenient in
non-systematic usage to employ the term "Amphioxus," which is in
general use in zoological laboratories.
[113] _Quart. Journ. Micr. Sci_. xlv. March 1902, p. 493.
[114] The cerebral eye and the pigment spots of the spinal cord are
especially prominent in the oceanic species _Branchiostoma
pelagicum_, Günther.
[115] The mesoblastic somites in Figs. 84 and 85 are all derivatives of the
larger posterior pair of coelomic pouches, the smaller more anterior
ones not being shown. For further details in regard to the coelomic
pouches see MacBride, _Quart. Journ. Micr. Sci_. xliii. p. 351, 1900.
[116] I have to thank Mr. Walter Tattersall, B.Sc., working in my
laboratory, for a detailed summary and discussion of the various
published schemes from which this table has been drawn up. He has
also filled up for me the map (Fig. 90) showing the geographical
distribution of the species. (See also _Trans. Biol. Soc. Liverpool_,
vol. xvii. 1903, p. 269.)
[117] A coelom formed by the union of one or more pairs of primitively
distinct coelomic cavities.
[118] Cf. p. 129.
[119] Cf. p. 391.
[120] Gadow, _A Classification of Vertebrata_, 1898, p. 4.
[121] Sucker-like modifications of the ventral surface of the body, in
which the paired fins take no part, are present on the throat in many
Fishes which frequent hill-streams, as in some small African and
Asiatic Cyprinidae (e.g. _Discognathus_) and a few Siluridae (e.g.
_Euglyptosternum_).
[122] Saville Kent, _The Naturalist in Australia_, London, 1897, p. 150.
[123] _Ibid_. p. 167
[124] _Ibid_. p. 168.
[125] _Ibid_. p. 173.
[126] _Ibid_. p. 188.
[127] Cunningham and MacMunn, _Phil. Trans_. 184, 1893, p. 765, where
references to many other papers are given.
[128] _Ablette_ is the French name for the Bleak.
[129] Either singly or in combination with lime (Guaninkalk), guanin is
often present in the tissues of Fishes (air-bladder, gall-bladder,
subcutaneous connective tissue, muscle-fasciae, peritoneum, and the
retinal epithelium and tapetum of the eye). For references see
Cunningham and MacMunn, _op. cit_. p. 781 _et seq_.
[130] Cunningham and MacMunn, _op. cit_. pp. 768 and 771.
[131] A. Agassiz, _Bull. Mus. Comp. Zool_., Camb. U.S.A., xxiii. 1892, p.
189.
[132] Cunningham and MacMunn, _op. cit_. p. 791, _et seq_.
[133] _Ibid_. p. 800.
[134] Saville Kent, _Nat. Austr_. p. 163.
[135] Poulton, _The Colours of Animals_, Internat. Scientific Series,
London, 1890, p. 82.
[136] Percy St. John, quoted by Day, _Fishes of Great Britain and Ireland_,
London, 1880-84, ii. p. 58.
[137] Poulton, _op. cit_. p. 82.
[138] _Ibid_. p. 86.
[139] Cunningham and MacMunn, _op. cit_. p. 773.
[140] C. Stewart, quoted by Poulton, _op. cit_. p. 67.
[141] Saville Kent, _op. cit_. p. 186, describes the colours of the living
Fish as "various shades of light crimson and lilac."
[142] Günther, _Study of Fishes_, London, 1880, p. 524.
[143] Poulton, _op. cit_. p. 72.
[144] For another view of the use of the "lure," see Cunningham,
_Marketable British Marine Fishes_, London, 1896, p. 338.
[145] Günther, _Chall. Reports, Zool_. vol. xxii. 1887, p. 50.
[146] Suggested by Lütken; Günther, _l. c_.
[147] Garstang, quoted by Poulton, _op. cit_. p. 165.
[148] See p. 364.
[149] E. Ray Lankester, _Proc. Roy. Soc_. 1873, p. 70.
[150] Cunningham and MacMunn, _op. cit_. p. 781.
[151] W. Newton Parker, _P.Z.S._ 1888, p. 359.
[152] Günther, _Trans. Zool. Soc_. vi. 1869, p. 437.
[153] Ibid., _Study of Fishes_, Edinburgh, 1880, p. 191.
[154] _Ibid_. p. 192.
[155] _Ibid_. p. 190.
[156] Lendenfeld, _Chall. Reports, Zool_. xxii. 1887, p. 277. For
references to papers by Leydig, Ussow, Emery, and others, see
Lendenfeld, _op. cit_.
[157] Moseley, _Challenger Reports, Zool_. xxii. 1887, p. 267.
[158] C. W. Wilson, _Journ. Morph_. xv. 1899, p. 667.
[159] Burckhardt, _Ann. Mag. Nat. Hist_. (7), vi. 1900, p. 568.
[160] Ibid. _op. cit_. p. 558.
[161] Wiliamson, _Phil. Trans_. cxxxix. 1849, p. 435; Hertwig, _Morph.
Jahrb_. ii. 1876, p. 328; v. 1879, p. 1; vii. 1882, p. 1; Klaatsch,
_ib_. xvi. 1890, p. 97 _et seq_., p. 209 _et seq_.
[162] Klaatsch has since affirmed the epidermic origin of the scleroblasts,
_ibid_. xxi. 1894, p. 153.
[163] Klaatsch, _Morph. Jahrb_. xvi. 1890, p. 125; Nickerson, _Bull. Mus.
Comp. Zool. Harvard_, xxiv. 1893, p. 115.
[164] Ryder, _Proc. Acad. Nat. Sci. Philadelphia_, 1892, p. 219; Smith
Woodward, _Nat. Sci_. iii. 1893, p. 448.
[165] Smith Woodward, _op. cit_. p. 449.
[166] O. Hertwig, _Morph. Jahrb_. ii. 1876, p. 374; Klaatsch, xvi. 1890, p.
146.
[167] Klaatsch, _op. cit_. p. 178.
[168] O. Hertwig, _Morph. Jahrb_. vii. p. 15.
[169] O. Hertwig, _Morph. Jahrb_. vii. p. 7.
[170] _Ibid_. vii. p. 29.
[171] _Ibid_. ii. p. 334.
[172] Hoffbauer, "Die Altersbestimmung des Karpfen an seiner Schuppe."
_Jahresb. des Schlesischen Fischerei-Vereins_, 1899; J. Stewart
Thomson, _Journ. Marine Biol. Assoc_. vi. No. 3, 1902, p. 373.
[173] Günther, _Phil. Trans_. clxi. 1871, p. 516; Klaatsch, _op. cit_. p.
209.
[174] This portion of the chapter is mainly based on the important
researches of Dr. Gadow and Miss Abbott. See _Phil. Trans_. 186,
1895, p. 163 _et seq_. where copious references to the work of other
writers are given.
[175] Neuromeres are body-segments defined and limited by the exits of the
successive pairs of spinal nerves from the neural canal.
[176] Gadow, _op. cit_. p. 190.
[177] Schneider, _Beitr. z. vergl. Anat. u. Entwickl. Wirbelth_., Berlin,
1879, p. 51; also Gadow, _op. cit_. p. 196.
[178] Smith Woodward, _Brit. Mus. Cat. Fossil Fishes_, Pt. i. 1889, p.
xvii.
[179] Hasse, _Das natürliche Syst. d. Elasmobranchier_; etc., Jena, 1879,
p. 30, _et seq_.
[180] Gadow, _op. cit_. p. 194; Ridewood, _Journ. Linn. Soc. Zool_. xxvii.
1899, p. 46.
[181] Günther, _Phil. Trans_. 161, 1872, p. 526; Wiedersheim, _Morph.
Studien_, Jena, 1880, Pt. i. p. 65; Gadow, _op. cit_. p. 198.
[182] Bridge, _P.Z.S._ 1897, p. 722.
[183] Gadow, _op. cit_. p. 201, _et seq_.
[184] See Budgett, _Trans. Zool. Soc_. xvi. Pt. vii. 1902, p. 315.
[185] Zittel, _Handb. d. Palaeontologie_, iii. 1887-1890, p. 137 _et seq_.;
Gadow, _op. cit_. p. 208.
[186] F. M. Balfour and W. N. Parker, _Phil. Trans_. 173, 1882, p. 388.
[187] As additional primary cranial elements mention may be made of a pair
of independently developed "alisphenoid" cartilages, which lie in
front of the parachordals between the brain and the eyes, and above
the trabeculae, and form a considerable part of the inter-orbital
region of the cranium. See Sewertzoff, _Anat. Anz_. xiii. 1897, p.
413; ibid., _Kupffer Festschrift_, Jena, 1899, p. 281.
[188] W. K. Parker, _Trans. Zool. Soc_. x. 1878, p. 189.
[189] Huxley, _P.Z.S._ 1876, p. 40.
[190] W. K. Parker, _Phil. Trans_. 163, 1873, p. 95.
[191] M‘Murrich, _Proc. Canadian Inst_. (N.S.) ii. Toronto, 1884, p. 278;
Cole, _Trans. Linn. Soc_. vii. Pt. v. 1898, p. 131.
[192] W. K. Parker, _Phil. Trans_. 174, Pt. ii. 1883, p. 411; Huxley,
_Journ. Anat. and Phys_. x. 1876, p. 412; Howes, _Trans. Biol. Soc.
Liverpool_, vi. 1891, p. 122.
[193] Huxley, _op. cit_. p. 421.
[194] W. K. Parker, _Phil. Trans_. 174, 1883, pp. 376-405; Ayers and
Jackson, _Journ. Morph_. xvii. 1901, p. 193.
[195] Huxley, _P.Z.S._ 1876, p. 40, _et seq_.
[196] Dollo, _Bull. Soc. Belge Géol_. etc. ix. 1895, p. 110.
[197] Hubrecht, _Niederländ. Archiv f. Zool_. iii. 1877, p. 255.
[198] W. K. Parker, _Phil. Trans_. 173, 1882, p. 139; Bridge, _Phil.
Trans_. 169, 1878, p. 683.
[199] Traquair, _Journ. Anat. and Phys_. v. 1871, p. 166; Bridge, _Proc.
Birm. Phil. Soc_. vi. 1888, p. 118; Budgett, _Trans. Zool. Soc_. xvi.
Pt. vii. 1902, p. 315.
[200] Budgett, _Trans. Zool. Soc_. xv. 1900, p. 334.
[201] Sagemehl, _Morph. Jahrb_. ix. 1884, p. 177.
[202] W. K. Parker, _Phil. Trans_. 173, 1882, p. 443.
[203] Bridge, _P.Z.S._ 1895, p. 302.
[204] Sagemehl, _Morph. Jahrb_. x. 1885, p. 1; xxvii. 1891, p. 489.
Swinnerton, _Quart. J. Micr. Sci_. xlv. 1902, p. 503.
[205] Günther, _Phil. Trans_. 161, 1871, p. 521; Huxley, _P.Z.S._ 1876, p.
31; Wiedersheim, _Morph. Stud_. i. Jena, 1880, p. 46; Bridge, _Trans.
Zool. Soc_. xiv. 1898, p. 350.
[206] Ridewood, _P.Z.S._ 1894, p. 632.
[207] _Ibid_. p. 638.
[208] Traquair, _Ann. Mag. Nat. Hist_. (5), ii. 1878, p. 1.
[209] Thacker, _Trans. Connecticut Acad_. iii. 1877, p. 281; Mivart,
_Trans. Zool. Soc_. x. 1879, p. 439; Bridge, _Linn. Soc. Journ.
Zool_. xxv. 1896, p. 530.
[210] Goodrich, _Quart. Journ. Micr. Sci_. 47, 1903-1904, p. 465.
[211] Smith Woodward, _Nat. Sc_. i. 1892, p. 29.
[212] Smith Woodward, _Brit. Mus. Cat. Foss. Fishes_, ii. 1891, p. 335.
[213] W. K. Parker, _Shoulder-girdle and Sternum of Vertebrata_, Ray Soc.
1868; Gegenbaur, _Untersuch. Vergl. Anat. Wirbelth_. Pt. ii. Leipzig,
1865; Wiedersheim, _Das Gliedmassenskelet d. Wirbelth_. Jena, 1892.
[214] Traquair, _Nature_, 62, 1900, p. 502.
[215] Traquair, _Trans. Roy. Soc. Edin_. xxxix. 1899, p. 843.
[216] It is more probable that in most existing Teleostomi the pelvic
girdle has undergone complete suppression, in which case these
cartilages are vestiges and not rudiments.
[217] See, however, Goodrich, _Quart. Journ. Micr. Sci_. xlv. 1901, p. 311.
[218] Bashford Dean, _Anat. Anz_. xi. 1896, p. 673.
[219] Budgett, _Trans. Zool. Soc_. xvi. Part vii. 1902, p. 328.
[220] Thacker, _Trans. Connecticut Acad_. iv. 1877, p. 233.
[221] Traquair, _Geol. Mag_. vii. 1890, p. 15; Goodrich, _l.c._
[222] Haswell, _Proc. Linn. Soc. N.S.W._ ix. 1884, p. 71; Howes, _P.Z.S._
1887, p. 3.
[223] Warren, _Quart. Journ. Micr. Sci_. xlv. 1902, p. 631.
[224] See Ridewood, _Nat. Sci_. viii. 1896, p. 391, for references.
[225] Ridewood, _op. cit_. p. 390.
[226] For references see Howes, _Linn. Soc. Journ. Zool_. xxiii. 1890, p.
381.
[227] Howes, _op. cit_.
[228] Macallum. Reprinted from _Proc. Canadian Instit. N.S._ ii. 1884, p.
387.
[229] Howes, _op. cit_.
[230] Howes, _P.Z.S._ 1890, p. 669.
[231] Balfour and Newton Parker, _Phil. Trans_. 173, 1882, p. 425.
[232] Newton Parker, _Trans. Roy. Irish Acad_. xxx. 1892, p. 140.
[233] Günther, _Phil. Trans_. 161, 1871, pp. 542-543.
[234] Owen, _Anat. Phys. Vertebrates_, London, 1866, i. p. 424.
[235] For the histology of the alimentary canal and its glands in Fishes,
see Leydig, _Lehrb. d. Histol. d. Menschen u. d. Tiere_, 1857; Id.
_Beitr. zu mikrosk. Anat. u. Entwickl. d. Rochen u. Haie_, Leipzig,
1852; Id. _Anat.-histol. Untersuch. üb. Fische u. Reptilien_, Berlin,
1853; Molin, _Sitz. d. k. Akad. d. Wiss. zu Wien_, v. 1850, p. 416;
Macallum, _Proc. Canadian Inst_. N.S. ii. 1884, p. 387; Id. _Journ.
Anat. and Phys_. xx. 1886, p. 604; N. Parker, _Trans. Roy. Irish
Acad_. xxx. 1893, p. 109; Ayers, _Jen. Zeitsch_. xviii. 1885, p. 479;
Edinger, _Archiv f. mikr. Anat_. xiii. 1876, p. 651; Trinkler,
_Archiv f. mikr. Anat_. xxiv. 1884, p. 174. Also Oppel, _Lehrb. d.
vergl. mikrosk. Anat. d. Wirbeltiere_, i.-ii. Jena, 1896-97, where
numerous other references are given.
[236] Owen, _op. cit_. p. 418.
[237] Owen, _l.c._
[238] Hyrtl, _Lepidosiren paradoxa. Abhand. d. böhm. Gesell. d. Wiss_.
1845, p. 629.
[239] Newton Parker, _op. cit_.
[240] Paul Mayer, _Mitt. zool. Stat. zu Neapel_, viii. 1888, p. 307.
[241] Wiedersheim, _Lehrb. d. vergl. Anat. d. Wirbelthiere_, ed. ii. Jena,
1886, p. 576.
[242] Owen, _op. cit_. p. 415.
[243] T. Jeffery Parker, _Trans. Zool. Soc_. xi. 1879, p. 49.
[244] Jeffery Parker, _op. cit_. pl. xi. Fig. 5.
[245] _Ibid_. p. 58.
[246] _Ibid_. p. 58.
[247] _Ibid_. p. 59.
[248] _Ibid_. p. 58, pl. xi. Fig. 6.
[249] Günther, _op. cit_. p. 544.
[250] Newton Parker, _op. cit_. p. 141.
[251] Macallum, _Journ. Anat. and Phys_. xx. 1886, pp. 618, 619.
[252] Owen, _op. cit_. p. 424.
[253] Macallum, _l.c._
[254] Balfour and Newton Parker, _op. cit_. p. 425.
[255] Cuvier and Valenciennes, _Hist. Nat. d. Poiss_. xix. 1846, p. 151.
[256] Rathke, _Üb. d. Darmkanal u. d. Zeugungsorgane d. Fische_, Halle,
1824, pp. 62 f.
[257] Edinger, _op. cit_. p. 678.
[258] T. Jeffery Parker, _op. cit_. p. 55.
[259] _Archiv f. mikr. Anat_. xiii. 1876.
[260] Krukenberg, quoted by Miss Alcock, _Journ. Anat. and Phys_. xiii.
(N.S.), 1899, p. 613.
[261] Stannius, _Handb. d. Zool_., Berlin, 1854, ii. p. 201; Owen, _op.
cit_. p. 425.
[262] Macallum, reprinted from _Proc. Canadian Institute_, N.S. ii. 1884,
p. 407.
[263] Newton Parker, _op. cit_. p. 138.
[264] Macallum, _Journ. Anat. and Phys_. xx. 1886, p. 632.
[265] Legouis, _Ann. Sci. Nat_. (5), xvii. 1873, Art. 8; and xviii. 1873,
Art. 3. Also Macallum, _op. cit_. p. 629.
[266] Newton Parker, _op. cit_. pp. 138-139.
[267] Turner, _Journ. Anat. and Phys_. vii. 1873, p. 233.
[268] Stannius, _op. cit_. pp. 197, 198; Owen, _op. cit_. p. 428, _et seq_.
[269] For references, see Macallum, _Journ. Anat. and Phys_. xx. p. 624 _et
seq_.
[270] Wiedersheim, _op. cit_. p. 556.
[271] Howes, _op. cit_. p. 393.
[272] Günther, _Challenger Reports_, "Zool." xxii. 1887, p. 3; Garman,
_Bull. Mus. Comp. Zool. Camb. Mass_. xii. 1885, p. 20.
[273] Günther, _op. cit_. p. 545; Newton Parker, _op. cit_. p. 137.
[274] Howes, _op. cit_. p. 393 _et seq_.
[275] Graham Kerr, _P.Z.S._, 1901, ii. p. 484.
[276] In those Elasmobranchs which have more than five branchial clefts
there is a corresponding increase in the number of branchial arches
and hemibranchs.
[277] Ridewood, _Anat. Anz_. xi. 1895, p. 425.
[278] Garman, _Bull. Mus. Comp. Zool. Harvard_, xii. 1885, p. 1; Günther,
_Challenger Reports_, "Zool." xxii. 1887, p. 2.
[279] In the _Ammocoetes_ stage the gill-sacs open directly into the larval
pharynx, which is retained as the branchial canal, the oesophagus of
the adult being an independent and later formation.
[280] Dohrn, _Mitth. Zool. Stat. Neapel_, vi. 1886, p. 49.
[281] Shipley, _Quart. J. Micr. Sci_. xxvii. 1887, p. 350.
[282] _Cf_. p. 343.
[283] See p. 423.
[284] Howes (_P.Z.S._ 1893, p. 730) has described certain remarkable
variations in the respiratory organs of _Petromyzon_ and _Myxine_.
[285] In certain Teleosts more or fewer of the branchial arches may lose
their gills. This reduction attains its maximum in the singular
Indian amphibious Fish, _Amphipnous cuchia_, where only the second
arch has a biserial gill, the remaining arches being wholly devoid of
gills (_cf_. p. 598).
[286] Balfour, _Comp. Embryol_. ii. 1881, p. 62.
[287] Ramsay Wright, _Journ. Anat. and Phys_. xix. 1885, p. 476.
[288] Sagemehl, _Morph. Jahrb_. ix. 1884, p. 213.
[289] Ramsay Wright, _op. cit_. p. 482.
[290] See p. 335.
[291] F. W. Müller, _Arch. Mikrosk. Anat_. xlix. 1897, p. 463.
[292] Ramsay Wright, _op. cit_. p. 492.
[293] F. Maurer, _Morph. Jahrb_. ix. 1884, p. 229; xiv. 1888, p. 175.
[294] Günther, _Phil. Trans_. clxi. 1871, p. 511; Baldwin Spencer, _Macleay
Memorial Volume_, 1892, p. 1.
[295] Newton Parker, _Trans. Roy. Irish Acad_. xxx. 1892, p. 161; Bridge,
_Trans. Zool. Soc_. xiv. 1898, p. 361.
[296] Boas, _Morph. Jahrb_. vi. 1880, p. 345. See Fig. 201, p. 340.
[297] Bischoff, _Lepidosiren paradoxa_, Leipzig, 1840; Hyrtl, _Abhand. d.
böhm. Gesellsch_. 1845, p. 637; also Bridge, _op. cit_. pp. 344, 345.
[298] Turner, _Journ. Anat. and Phys_. xiv. 1879, p. 273. For references to
other writers see Turner, _op. cit_.
[299] For this information, which was based on an examination of a
specimen, parts of which are now in the Cambridge University Museum,
I am indebted to Dr. Harmer.
[300] Van Beneden, quoted by Turner, _op. cit_. p. 282.
[301] Andrew Smith, also quoted by Turner, _op. cit_. p. 281.
[302] Dahlgren, _Zool. Bull_. ii. 3, Boston, 1898; Allis, _Anat. Anz_.
xviii. 1900, p. 257.
[303] M‘Kendrick, _Journ. Anat. and Phys_. xiv. 1879, p. 461.
[304] _Naturalist in Celebes_, London, 1889, p. 30.
[305] _Nature_, xxxix. 1889, p. 285.
[306] Budgett, _Trans. Zool. Soc_. xvi. Pt. ii. 1901, p. 126.
[307] Götte, quoted by Balfour, _Comp. Embryol_. ii. 1881, p. 62.
[308] Steindachner, _Sitz. d. k. Akad. d. Wiss_. i. 1869, p. 103; Hyrtl,
_ibid_. p. 109; Budgett, _op. cit_. p. 118.
[309] Boulenger, _P.Z.S._ 1899, p. 554.
[310] Semon, _Zoologische Forschungsreisen in Australien_, Pt. i. p. 44,
and Atlas.
[311] Burt G. Wilder, _Proc. Amer. Ass. Adv. Sci_. 1875, p. 151; _ibid_.
1877, p. 306.
[312] _Ann. d. Sci. Nat_. sér. 6, vii. 1878, Art. 5.
[313] Baldwin Spencer, _op. cit_. p. 3.
[314] Sörensen, _Journ. Anat. and Phys_. 1894, p. 127-138.
[315] Moreau, _Ann. d. Sci. Nat_. sér. 6, _Zool_. iv. 1876, Art. 8, p. 62.
[316] Erman, _Gilbert Ann. d. Physik_. xxx. 1808, p. 113.
[317] Jobert, _op. cit_.; _ibid_. v. 1877, Art. 8.
[318] Zograff, _Quart. J. Micr. Sci_. xxviii. 1888, p. 501.
[319] Hyrtl, _Sitz. d. k. Akad. Wiss_. x. 1853, p. 148.
[320] Hyrtl, _Denksch. k. Akad. Wiss. Wien_. xxiii. 1863, p. i.; _ibid_. x.
1855, p. 48.
[321] Hyrtl, _ibid_. viii. 1854, p. 185.
[322] Hyrtl, _ibid_. xxi. 1863, p. 7; Sagemehl, _Morph. Jahrb_. xii. 1887,
p. 307.
[323] Taylor, _Edin. Journ. Sci_. v. 1831, p. 33; Hyrtl, _Denksch. k. Akad.
Wiss. Wien_. xiv. 1858, p. 39.
[324] Hyrtl, _SB. Akad. Wiss. Wien_. xi. 1853, p. 302; Day, _Linn. Soc.
Journ. Zool_. xiii. p. 198.
[325] Burne, _Journ. Linn. Soc. Zool_. xxv. 1894, p. 48.
[326] For much interesting information on aerial respiration in Fishes, see
Day, _op. cit_.; also _P.Z.S._ 1868, p. 274; and Dobson, _ibid_.
1874, p. 312.
[327] Semper, _Animal Life_, Intern. Sci. Series, London, 1881, p. 172.
[328] Miklucho-Maclay, _Jen. Zeitsch_. iii. 1867, p. 448.
[329] Wiedersheim, _Lehrb. d. vergl. Anat. d. Wirbelth_. ed. 2, Jena 1886,
p. 616.
[330] The glottis is furnished with a structure analogous to the
epiglottis-like plate of _Protopterus_ (Wiedersheim, _op. cit_. p.
616).
[331] Balfour and Newton Parker, _Phil. Trans_. 173, Part ii. 1883, p. 425.
[332] Günther, _Phil. Trans_. 161, 1871, p. 511; Baldwin Spencer,
_Zoologische Forschungsreisen in Australien_ (Semon), i. Jena 1898,
p. 53.
[333] Newton Parker, _Trans. Roy. Irish Acad_. xxx. 1892, p. 109; Baldwin
Spencer, _op. cit_. p. 54.
[334] Henle, quoted by Howes, _P.Z.S._ 1887, p. 501; also Wiedersheim, _op.
cit_. p. 622 and Fig. 483.
[335] Bischoff, _Ann. d. Set. Nat_. (2) _Zool_. xiv. 1840, p. 136.
[336] Stannius, _Handb. d. Zool_. Berlin ii. 1854, p. 220.
[337] Günther, _Study of Fishes_, Edinburgh, 1880, p. 457.
[338] Bridge and Haddon, _Phil. Trans_. B, 184, 1893, p. 209.
[339] Reinhardt, quoted by Stannius, _op. cit_. p. 225.
[340] Cuvier and Valenciennes, _Hist. Nat. d. Poissons_, xxi. 1848, p. 139;
Bridge, _Journ. Linn. Soc. Zool_. xxvii. 1900, p. 503.
[341] Day, _P.Z.S._ 1871, p. 703.
[342] Sörensen, "Lydorganer hos Fiske," Copenhagen, 1884, p. 85; Kner, _SB.
k. Akad. Wiss. Wien_, xi. 1853, p. 138.
[343] Cuvier and Valenciennes, _op. cit_. v.
[344] Günther, _Brit. Mus. Cat. Fishes_, ii. 1860, p. 313.
[345] Moreau, _Compt. Rend_. lix. 1864, p. 436.
[346] J. Müller, _Ber. d. k. Akad. d. Wiss. Berlin_, 1842, p. 177.
[347] Bridge and Haddon, _op. cit_. p. 234, Pl. II. Fig. 18.
[348] _Ibid_. p. 216.
[349] Weber, _De aure et auditu Hominis et Animalium_, Leipzig, 1820, p.
73.
[350] Moreau, _Compt. Rend_. lxxx. 1875, p. 1247.
[351] Coggi, _Mitth. Zool. Stat. Neapel_, vii. 1887, p. 381; Swale Vincent
and Stanley Barnes, _Journ. Anat. and Phys_. xxx. 1896, p. 545.
[352] For the blood-supply of the air-bladder see Chap. XII.
[353] See Chaps. XIV. XIII. and X.
[354] Moreau, _Ann. d. Sci. Nat_. (6) iv. 1876, Art.
[355] Moreau, _op. cit_.
[356] Bridge and Haddon, _op. cit_. p. 286.
[357] Semper, _Animal Life_, Internat. Sci. Series, London, 1881, p. 321.
[358] Moreau, _op. cit_. pp. 3, 4.
[359] J. Müller, _Vergl. Anat. d. Myxinoiden_, Pt. iii. (1839), Berlin,
1841, p. 186. For an account of the vascular system of _Bdellostoma_
see Jackson, _Journ. Cincinnati Soc. Nat. Hist_. xx. 1901, p. 13.
[360] T. Jeffery Parker, _Phil. Trans_. 177, Pt. ii. 1886, p. 702. For
references to Elasmobranchs in general, see Parker, _op. cit_. p.
725.
[361] In the common Dog-Fish (_Scyllium canicula_) each lateral vein joins
the posterior cardinal near the junction of the latter with the
Cuvierian duct, the subclavian vein from the pectoral fin opening
directly into the corresponding Cuvierian duct.
[362] Jourdain, _Ann. Sci. Nat_. (4), xii. 1859, p. 321; M‘Kenzie, Reprint
from the _Proc. Canadian Institute_ (N.S.) ii. 1884, p. 428. For
references to Hyrtl and other writers, see Jourdain, _op. cit_.
[363] Balfour, _Comparative Embryology_, London, ii. 1881, pp. 66, 91, and
96.
[364] A subintestinal vein is also present in adult Holocephali (e.g.
_Callorhynchus antarcticus_), T. Jeffery Parker, _op. cit_. p. 706.
The persistence of this vein in adult Fishes is associated with the
presence of a well-developed spiral valve.
[365] Budgett, _Trans. Zool. Soc_. xiv. Pt. vii. 1901, p. 332.
[366] Günther, _Phil. Trans_. 161, 1872, p. 535; Baldwin Spencer, _Macleay
Memorial Volume_, 1894, p. 17.
[367] Baldwin Spencer, _op. cit_. pp. 24, 30-31. Not represented in Fig.
191.
[368] Hochstetter, _Morphol. Jahrb_. xiii. 1888, p. 153.
[369] The vertebral vein, which is present only on the right side, may
represent the reduced anterior portion of the right posterior
cardinal, as Baldwin Spencer (_op. cit_.) has suggested.
[370] As an abnormality the adult Frog may retain the embryonic connexion
of the right anterior abdominal vein with the heart (Buller, _Journ.
Anat. and Phys_. iii. 1896, p. 211).
[371] Newton Parker, _Trans. Roy. Irish Acad_. xxx. 1892, p. 179.
[372] Hyrtl, _Abhand. d. Böhm. Gesellsch_. 1845, p. 643.
[373] There is an incomplete auricular septum in the Holocephali (e.g.
_Chimaera monstrosa_), see Ray Lankester, _Trans. Zool. Soc_. x.
1879, p. 502.
[374] Stannius, _Handb. d. Anat. d. Wirbelth_. Berlin, ii. 1854, p. 235;
Boas, _Morphol. Jahrb_. vi. 1880, p. 527.
[375] Boas, _Morphol. Jahrb_. vi. 1880, p. 321.
[376] _Ibid. op. cit_.
[377] J. Müller, _Vergl. Anat. d. Myxinoiden_, Pt. iii. (1839) Berlin 1841
p. 179.
[378] T. Jeffery Parker, _Phil. Trans_. 177, Pt. ii. 1886, p. 686; cf. H.
Ayers, _Bull. Mus. Comp. Zool. Harvard_, xvii. No. 5, 1889, p. 191.
[379] _Chlamydoselachus_ is more primitive in this respect, and has but a
single efferent vessel for the two hemibranchs of each arch, which
corresponds with the more anterior of the two in _Mustelus_ (Ayers,
_op. cit_.).
[380] Cf. footnote to p. 332.
[381] Note, however, that in the young _Lepidosteus_ there are two efferent
vessels in each arch, which, nevertheless, differ from those of
_Mustelus_ in uniting to form an epibranchial artery before joining
the dorsal aorta (F. W. Müller, _Arch. Mikr. Anat_. xlix. 1897, p.
463).
[382] T. Jeffery Parker, _op. cit_. p. 691.
[383] Cf. Figs. 195 and 196.
[384] Ramsay Wright, _Journ. Anat. and Phys_. xix. 1885, p. 482; F. W.
Müller, _op. cit_.
[385] These vessels are not to be regarded as homologous with the primitive
paired aortae of Amphioxus and the embryos of higher Vertebrates. The
true dorsal aorta sometimes persists as a median vestigial vessel
which traverses the circulus cephalicus.
[386] For the relations of the efferent branchial vessels to the cephalic
circle and the median dorsal aorta in different Teleosts, see
Ridewood, _P.Z.S._ 1899, p. 939.
[387] Only one of the two internal carotid arteries is shown in Fig. 199.
[388] J. Müller, _U. d. Bau u. d. Grenzen d. Ganoiden_, Berlin, 1846, p.
43; Ramsay Wright, _Standard Nat. Hist_. iii. pp. 48, 49.
[389] Baldwin Spencer, _Macleay Memorial Volume_, 1892, p. 1.
[390] Newton Parker, _Trans. Roy. Irish Acad_. xxx. 1892, p. 173.
[391] According to Boas; for reference, _see_ p. 329.
[392] This structure may prove to be a hemibranch of the first branchial
arch.
[393] Newton Parker, _op. cit_. p. 167.
[394] Newton Parker, _op. cit_. p. 138.
[395] De Meuron, _Recherches sur le développement du Thymus et de la glande
thyreoïde_, Inaug. Dissert. Genève, 1886; Maurer, _Morph. Jahrb_. xi.
1886, p. 129; W. Müller, _Jen. Zeitsch_. vi. 1871, p. 428; vii. 1873,
p. 327; Dohrn, _Mitth. Zool. Stat. Neapel_, vi. 1886, p. 49; vii.
1887, p. 301.
[396] _Cf_. p. 280.
[397] Newton Parker, _op. cit_. p. 135.
[398] Quoted by N. Parker, _l.c._
[399] Van Bemmelen, _Anat. Anz_. iv. 1889, p. 400.
[400] W. Müller, _op. cit_.
[401] Dohrn, _Mitth. Zool. Stat. Neapel_. v. 1884, pp. 141-151; see also
the previously cited works of De Meuron and Maurer.
[402] Dohrn, _op. cit_.
[403] See pp. 120 and 135. Willey, _Amphioxus and the Ancestry of the
Vertebrates_, New York, 1894, pp. 30, 31.
[404] Beard, _Anat. Anz_. ix. 1894, p. 485.
[405] Newton Parker, _op. cit_. p. 135.
[406] Giacomini, quoted by Swale Vincent, _Journ. Anat. and Phys_. xxxviii.
1903, p. 41.
[407] Vincent, _Trans. Zool. Soc_. xiv. Part iii. 1897, p. 41. For
bibliography see Vincent, _Internat. Monatsschr. f. Anat. u. Phys_.
xv. 1898, p. 319.
[408] Giacomini, quoted by Swale Vincent, _Journ. Anat. and Phys_. xxxviii.
1903, p. 41.
[409] Vincent, _op. cit_. pp. 32, 33.
[410] Balfour, _Quart. J. Micr. Sci_. xxii. 1882, p. 12.
[411] Swale Vincent, _op. cit_. p. 78.
[412] _Ibid_. pp. 77, 78.
[413] Balfour, _op. cit_. p. 16.
[414] See Chapter XI.
[415] Pettigrew, _Animal Locomotion_, Internat. Sci. Series, London, 1874,
p. 64; Gadow, _Science for All_ (Cassell), v. p. 302.
[416] Bridge, _Journ. Linn. Soc. (Zool.)_, xxv. 1896, p. 530.
[417] Sörensen, _Om Lydorganer hos Fiske_, Copenhagen, 1884; Dufossé, _Ann.
d. Sci. Nat_. Ser. 5, xix. Art. 5, 1874, and xx. Art. 3, 1874. For
references to earlier papers see Sörensen, _op. cit_.
[418] Haddon, _Journ. Anat. and Phys_. xv. 1881, p. 322; Bridge and Haddon,
_Phil. Trans_. 184, 1893, p. 168.
[419] Möbius, _Sitz. d. Berlin. Akad. d. Wiss_. 1889, p. 999.
[420] _Notes by a Naturalist on H.M.S. "Challenger,"_ London, 1879, p. 51.
[421] The elastic-spring-mechanism has been described by several writers,
who had assigned to it various functions, but Sörensen (_op. cit_.
pp. 85-91) was the first to discover its vocal function by
observations and experiments on _Doras maculatus_.
[422] The mechanism is apparently absent in one species of _Pangasius_ (_P.
micronema_). Bridge and Haddon, _op. cit_. p. 220.
[423] Moreau, _Compt. Rendus_, lix. 1864, p. 436; _Ann. d. Sci. Nat_. (6)
iv. 1876, p. 65.
[424] Sörensen, _Lydorganer_, p. 82, _et. seq_.
[425] Cf. Mettenheimer, _Arch. f. Anat. u. Physiol_. 1858, p. 302.
[426] Günther, _Phil. Trans_. 161, 1871, p. 542.
[427] Pappe, _Synopsis of the Edible Fishes at the Cape of Good Hope_,
Capetown, 1853, p. 8.
[428] Günther, _Study of Fishes_, Edinburgh, 1880, p. 427.
[429] Day, _Fishes of Great Britain and Ireland_, London, i. 1880-1884, p.
151.
[430] Sörensen, _op. cit_.
[431] Moreau, _op. cit_.
[432] Ewart, _Phil. Trans_. 179 (B), 1888, pp. 399, 410, and 539; 183 (B),
1893, p. 389.
[433] Ballowitz, _Arch. Mikr. Anat_. l. 1897, p. 686; Carl Sachs,
_Untersuchungen am Zitteraal_, Leipzig, 1881.
[434] Ballowitz, _Das Electrische Organ des Africanischen Zitterwelses_,
Jena, 1899.
[435] Gotch, _Phil. Trans_. 178, 1888, p. 487.
[436] Id. _op. cit_. p. 535.
[437] Cf. p. 580.
[438] Graham Kerr, _Quart. Journ. Micr. Sci_. xlvi. 1902, p. 417.
[439] For the nomenclature of the brain and its cavities see T. J. Parker,
_Nature_, xxxv. 1886, p 208; and Parker and Haswell, _Text-Book of
Zoology_, London, 1897, ii. p. 94.
[440] It is possible that the prosencephalon is merely the bulging anterior
part of the thalamencephalon; if this be so the hemispheres are
really paired outgrowths from the thalamencephalon.
[441] In Lizards _either_ of the two vesicles may become a parietal eye
(Dendy, _Quart. Journ. Micr. Sci_. xlii. 1899, p. 111).
[442] Holm, _Morph. Jahrb_. xxix. 1901, p. 365.
[443] The sacci probably secrete the fluid contents of the ventricles.
[444] Haller, _Morph. Jahrb_. xxvi. 1898, p. 345.
[445] Goronowitsch, _Morph. Jahrb_. xiii. 1888, p. 427.
[446] Burckhardt, _Das Central-Nervensystem v. Protopterus annectens_.
Berlin, 1892.
[447] Sanders, _Ann. Nat. Hist_. (6) iii. 1889, p. 157.
[448] See Gaskell's important paper, _Journ. Physiol_. vii. 1886, p. 1.
[449] Herrick, _Journ. Neur_. ix. p. 153; Cole, _Trans. Roy. Soc. Edinb_.
xxxviii. 1896, p. 631; Id. _Trans. Linn. Soc_. vii. 1898, p. 115, to
which an excellent bibliography is appended.
[450] For a discussion of the relations of "end-buds" to the sense of taste
in Fishes, see Bateson, _Journ. Marine Biol. Ass_. i. (N.S.) 1890, p.
225; and Herrick, _U.S. Fish Commiss. Bull_. 1902, p. 237. In the
latter paper a bibliography of the subject is given.
[451] These fibres are included in the visceral sensory or "communis"
system by Herrick.
[452] See previously cited papers by Herrick and Cole; also Ewart, _Trans.
Roy. Soc. Edinb_. xxxvi. 1892, p. 59; Collinge, _Quart. Journ. Micr.
Sci_. xxxvi. 1894, p. 499; and Herrick, _Journ. Comp. Neurology_, xi.
1901, p. 177.
[453] Allis, _Journ. Morph_. ii. 1889, p. 463.
[454] Johnston, _Journ. Comp. Neurology_, xii. 1902, p. 2.
[455] Fuchs (_Archiv f. d. ges. Physiol_. lix. 1895, p. 454) has suggested
that these organs may be concerned with the perception of pressure
variations. It has also been argued that they are concerned with
equilibration and the co-ordination of the movements of the fins.
(See _American Journ. Physiol_. i. p. 128.)
[456] Burckhardt, _Das Central-Nervensystem v. Protopterus_, Berlin, 1892,
p. 32.
[457] Bridge, _Journ. Linn. Soc_. xxvii. 1900, p. 503.
[458] Ridewood, _Journ. Anat. and Phys_. xxvi. 1892, p. 26.
[459] E. H. Weber, _De aure et auditu Hominis et Animalium_. Pars i. _De
aure Animalium Aquatilium_, Leipzig, 1820; Bridge and Haddon, _Phil.
Trans_. 184, 1893, p. 65.
[460] Sagemehl, _Morph. Jahrb_. x. 1885, p. 22.
[461] The Weberian ossicles are modified components of certain of the
anterior vertebrae. The scaphium represents the neural arch of the
first vertebra; the intercalarium is the arch of the second vertebra;
while the tripus is probably the rib of the third vertebra. In the
Characinidae and the Cyprinidae an additional ossicle, the
"claustrum" is present.
[462] See also Sörensen, _Journ. Anat. and Phys_. xxix. 1895, p. 399; and
Bridge, _Journ. Linn. Soc_. xxvii. 1900, p. 531.
[463] Bridge and Haddon, _op. cit_. p. 261.
[464] Id. _Proc. Roy. Soc_. lii. 1892, p. 139.
[465] Kupffer, _Stud. vergl. Entwickl. d. Kopfes d. Kraniaten_, iii. 1895,
p. 6.
[466] In _Bdellostoma_ the olfactory organ arises as a _pair_ of outgrowths
from the pituitary involution (Bashford Dean, Kupffer's
_Festschrift_, Jena, 1899, p. 269).
[467] Kyle, _Journ. Linn. Soc. (Zool.)_, xxvii. 1900, p. 541.
[468] Beer, _Wien. klin. Wochenschr_., No. xlii. 1898, p. 11.
[469] Chun, _Aus den Tiefen des Weltmeeres_, Jena, 1900, p. 534.
[470] Ritter, _Bull. Mus. Comp. Zool_. xxiv. 1893, p. 51.
[471] Eigenmann, _Arch. f. Entwickelungsmech_. viii. 1899, p. 545.
[472] Studnička, _Sitzber. k. böhm. Ges. Wiss_., 1899, No. xxxvii.
[473] Chun, _loc. cit_. p. 536.
[474] It is probable that the archinephric duct is derived from the
embryonic epiblast; hence the suggestion that in the primitive
Vertebrates the duct was a longitudinal groove in the superficial
skin into which the pronephric tubules opened externally.
[475] W. Müller, _Jen. Zeitsch_. ix. 1875, p. 107; Semon, _Carl Gegenbaur's
Festschrift_, Leipzig, 1896, iii. p. 169.
[476] Jungersen, _Zool. Anz_. xxiii. 1900, p. 328.
[477] Bridge, _Journ. Anat. and Phys_. xiv. 1879, p. 81; Bles, _ib_. xxxii.
1898, p. 484; _Proc. Roy. Soc_. lxii. 1898, p. 232.
[478] Max Weber, _Morph. Jahrb_. xii. 1886, p. 336.
[479] Ewart, _Journ. Anat. and Phys_. x. 1876, p. 488.
[480] Burne, _Linn. Soc. Journ. Zool_. xxvi. 1898, p. 487.
[481] Semper, _Centralblatt f. Med. Wiss_. 1875, No. 29; F. M. Balfour,
_Journ. Anat. and Phys_. x. 1875, p. 17; Id. _Comparative
Embryology_, London, 1881, ii. p. 568.
[482] Huxley, _P.Z.S._ 1883, p. 132.
[483] Budgett, _Trans. Zool. Soc_. xv. 1901, p. 323; xvi. 1902, p. 315.
[484] Graham Kerr, _P.Z.S._ 1901, p. 484; _Proc. Phil. Soc. Cambridge_, xi.
Pt. v. 1902, p. 329.
[485] Graham Kerr, _op. cit_.
[486] Hirota, _Journ. Coll. Sci. Imp. Univ. Japan_, vii. 1895, p. 367.
[487] For the eggs of Cyclostomes see Chapter XVI.
[488] For a description of the eggs and breeding habits, and the larval
development and migrations of British Marine Fishes, see M‘Intosh and
Prince, _Trans. Roy. Soc. Edin_. 1890; M‘Intosh, _Ann. Report Fishery
Board for Scotland_, 1892; Cunningham, _Marketable Marine Fishes of
the British Islands_, London, 1896; M‘Intosh and Masterman,
_Life-Histories of the British Marine Food-Fishes_, London, 1897;
also numerous papers by Cunningham, Holt, Garstang, and Allen, in the
_Journ. Marine Biol. Assoc. Plymouth_, vols. i.-vi.
[489] See Chapter XVII.
[490] Cunningham, _op. cit_. p. 69.
[491] Guitel, _Arch. Zool. Expér. et Gén_. (3), i. 1893, p. 611.
[492] Garman, _Mem. Mus. Comp. Zool_. xix. 1895, No. 1, p. 11.
[493] Cunningham, _op. cit_. p. 358.
[494] H. v. Jhering, _Zeitschr. wiss. Zool_. xxxviii. 1883, p. 468.
[495] See p. 592.
[496] Olt, _Zeitschr. wiss. Zool_. lv. 1893, p. 643.
[497] _Cf_. p. 584.
[498] Budgett, _Trans. Zool. Soc_. xvi. Pt. ii. 1901, p. 130.
[499] See Chap. XVII. p. 434.
[500] Eigenmann, _Bull. Fish Comm_. (U.S.), 1892, p. 381; _Arch.
Entwickelungsmech_. iv. 1896, p. 125; Cunningham, _op. cit_. p. 356,
_et seq_.
[501] For a general account of Sexual Dimorphism in Fishes, see
Cunningham's _Sexual Dimorphism in the Animal Kingdom_, London, 1900,
pp. 178-227. Some of the more striking examples of Sexual Dimorphism
are mentioned in the chapters dealing with the different families of
Fishes.
[502] Holt, "On the Breeding of the Dragonet (_Callionymus lyra_),"
_P.Z.S._ 1898, p. 281.
[503] Howes, _Linn. Soc. Journ. Zool_. xxiii. 1891, p. 539, where
references are given to the literature of the subject.
[504] The American Hags probably belong to a distinct species, _M. limosa_
Girard; Bashford Dean, _Science_ (N.S.), xvii. 1903, p. 433.
[505] Bashford Dean, Kupffer's "_Festschrift_," Jena, 1809, p. 227 _et
seq_.
[506] _Journ. Morph_. xvii. 1898, p. 213.
[507] Jordan and Evermann, _Bull. U.S. Nat. Mus_. No. 47; _The Fishes of
North and Middle America_, Pt. i. 1896, p. 6.
[508] B. Dean, _op. cit_. p. 230 _et seq_.
[509] Jordan and Evermann, _op. cit_. p. 9 _et seq_.
[510] Plate, _Sitzungsb. d. Gesellsch. Naturforsch. Freunde Berlin_, No. 8,
1897, p. 137.
[511] Bashford Dean and F. B. Sumner, _Trans. N. Y. Acad. Sci_. xvi. 1897,
p. 321.
[512] Dohrn, _Mitth. Zool. Stat. Neapel_, vi. 1886, p. 59; Shipley, _Quart.
Journ. Microsc. Sci_. xxvii. 1887, p. 325.
[513] R. Alcock, _Journ. Anat. and Phys_. xiii. (N.S.), 1899, p. 623.
[514] Day, _Fishes of Great Britain and Ireland_, Lond. ii. 1880-84, p.
360.
[515] _Den Danske Ingolf-Expedition_, ii. No. 2, Copenhagen, 1898.
[516] Cunningham, _Marketable Marine Fishes_, London, 1896, p. 64.
[517] Wood-Mason and Alcock, _Proc. Roy. Soc_. 49, 1891, p. 359.
[518] Leydig, _Mikrosk. Anat. u. Entwick. d. Rochen u. Haie_, Leipzig,
1852, p. 90 _et seq_.
[519] T. J. Parker, _Trans. New Zealand Instit_. xxii. 1889 (1890), p. 331.
[520] Smith Woodward, _Vertebrate Palaeontology_, Cambridge, 1898, p. 32.
[521] B. Dean, _Journ. Morph_. ix. 1894, p. 87. _Trans. New York Acad.
Sci_. xiii. 1894, p. 115.
[522] Traquair, _Geol. Mag_. (3), v. 1888, p. 81; _Trans. Geol. Soc.
Glasgow_, xi. 1897, p. 41.
[523] For references see Zittel's _Text-Book of Palaeontology_ (Eng. trans.
ed. by C. R. Eastman), London and New York, ii. 1902, pp. 22-23.
[524] See also restoration of _Pleuracanthus gaudryi_ from the
Coal-Measures of Commentry, Allier, France, by C. Brongniart; Zittel,
_op. cit_. p. 23.
[525] A. Fritsch, _Fauna der Gaskohle in Böhmen_, ii. Prague, 1889; Kner,
_SB. Akad. Wiss. Wien Math.-Naturw. Cl_. lvii. Pt. i. 1868, p. 290;
Traquair, _Geol. Mag_. (3), v. 1888, p. 511, and (4) i. 1894, p. 254.
[526] Günther, _Study of Fishes_, Edin. 1880; _British Mus. Cat. Fishes_,
viii. 1870; Müller and Henle, _Syst. Beschr. d. Plagiost_. Berlin,
1841. Hasse, _Natürl. Syst. d. Elasmobr_. Jena, 1879. Goode and Bean,
_Oceanic Ichthyology_, Washington, 1895. Jordan and Evermann, _Fishes
of North and Middle America_, Washington, 1896, Pt. i. Smith
Woodward, _Vertebrate Palaeontology_, Cambridge, 1898; id. _Brit.
Mus. Cat. Foss. Fishes_, i. 1889, ii. 1891; Zittel, _op. cit_.
[527] Garman, _Bull. Mus. Comp. Zool. Harvard_, xii. No. 1, 1885, p. 1;
Günther, _Chall. Rep. Zool_. xxii. 1887, p. 2.
[528] Smith Woodward, _Nat. Science_, i. 1892, p. 671.
[529] Günther, _Study of Fishes_, p. 328.
[530] Goode and Bean, _op. cit_. p. 23.
[531] Müller and Henle, _op. cit_.
[532] Boulenger, _Ann. Mag. Nat. Hist_. (7), x. 1902, p. 51.
[533] Day, _British Fishes_, London, 1880-84, ii. p. 294.
[534] Cantor, quoted by Günther, _op. cit_. p. 318.
[535] T. J. Parker, _P.Z.S._ 1887, p. 27.
[536] D. S. Jordan, _California Acad. Sci_. (3), _Zool_. i. 1898; Bashford
Dean, _Science_ (N.S.), xvii. 1903, p. 630.
[537] Smith Woodward, _Ann. Mag. Nat. Hist_. (7), iii. 1899, p. 487.
[538] Kershaw, _Victorian Natural_. xix. 1901, p. 62; Waite, _Rec. Austral.
Mus_. iv. 1901, p. 263.
[539] Alcock, _Ann. Mag. Nat. Hist_. (6), iv. 1889, p. 379.
[540] Day, _op. cit_. p. 324. See also Stead, _Journ. Mar. Biol. Ass_. iv.
1895-97, p. 264.
[541] _Vertebrate Palaeontology_, Cambridge, 1898, p. 32.
[542] I am indebted to Mr. Boulenger for these observations.
[543] Alcock, _Ann. Mag. Nat. Hist_. (6), iv. 1889, p. 380.
[544] Jordan and Evermann, _op. cit_. p. 76.
[545] Day, _op. cit_. p. 336.
[546] Zittel, _op. cit_. p. 41.
[547] Jordan and Evermann, _op. cit_. p. 85.
[548] Günther, _op. cit_. p. 348.
[549] Duméril, quoted by Jordan and Evermann, _op. cit_. p. 92.
[550] Rohon, _Verhandl. k. Min. Ges. Petersburg_, xxxiii. 1895, p. 1.
[551] Smith Woodward, _Proc. Zool. Soc_. 1886, p. 527; and 1887, p. 481.
[552] Id., _Ann. Mag. Nat. Hist_. iv. (6), 1889, p. 275.
[553] Günther, _Ann. Mag. Nat. Hist_. (6) iv. 1889, p. 415.
[554] See also an account of the egg-case of a Chimaeroid dredged from a
depth of 516 fathoms in the Bay of Bengal (Wood-Mason and Alcock,
_Ann. Mag. Nat. Hist_. (6) viii. 1891, p. 21).
[555] Goode and Bean, _op. cit_. p. 32.
[556] Mitsukuri, _Zool. Mag. Tokyo_, 1895, quoted in _Nat. Sci_. viii.
1896, p. 10.
[557] Günther, _Chall. Reports, Zool_. xxii. 1887, p. 12.
[558] Bashford Dean, _Mem. New York Acad. Sci_. ii. Pt. i. 1899, p. 28;
_Biol. Bull_. iv. 1903, p. 270.
[559] E. T. Newton, _Mem. Geol. Surv. Monogr_. iv. 1878; Riess,
_Palaeontogr_. xxxiv. 1887, p. 1; Smith Woodward, _Brit. Mus. Cat.
Foss. Fishes_, ii. 1891, p. 52; Zittel, _Text-Book of Palaeontology_,
English ed., London and New York, ii. 1902, p. 46.
[560] Hence the name "Teleostomi" or "perfect-mouthed" Fishes.
[561] Boulenger, _Poissons du Bassin du Congo_, Bruxelles, 1901, p. 2.
Smith Woodward (_Brit. Mus. Cat. Foss. Fishes_, ii. 1891, p. 317; and
_Vert. Palaeont_. Cambridge, 1898, p. 78), following Cope, recognises
four sub-orders, the Haplistia, Rhipidistia, Actinistia, and
Cladistia. The first sub-order is reserved for the Tarrasiidae, a
family which includes only the little known _Tarrasius problematicus_
from the Lower Carboniferous of Scotland.
[562] Traquair, _Trans. Roy. Soc. Edinb_. xxvii. 1875, p. 383.
[563] Whiteaves, _Trans. Roy. Soc. Canada_, vi. 1888, p. 77.
[564] Traquair, _Trans. Roy. Soc. Edinb_. xxx. 1881, p. 169.
[565] Traquair, _Proc. Roy. Soc. Edinb_. xvii. p. 388.
[566] Reiss, Die Coelacanthinen, _Palaeontogr_. xxxi. 1888, p. 1; Smith
Woodward, _Brit. Mus. Cat. Foss. Fishes_, ii. 1891, p. 394.
[567] See also _Kurtus indicus_, p. 688.
[568] Smith Woodward, _op. cit_. p. 412.
[569] Boulenger, _Poiss. Bass. Congo_, p. 10. For a list of the more
important papers, see pp. 18-19 of that work.
[570] Mr. Boulenger informs me that he regards these spines as modified
ridge scales or fulcra. The latter are median spine-like or Λ-shaped
scales in relation with the anterior margins of the median fins in
some Crossopterygii (_e.g._ Osteolepidae) and in many Chondrostei and
Holostei.
[571] Boulenger, _op. cit_. p. 20 _et seq_.; id. _Ann. Mus. Congo, Zool_.
(1), i. Fasc. 4, Bruxelles, 1899, p. 61; ii. Fasc. 2, 1902, p. 23.
[572] _Proc. Camb. Phil. Soc_. x. 1900, p. 236; _Trans. Zool. Soc_. xvi.
Pt. ii. 1901, p. 115.
[573] _Amer. Nat_. xxxiii. 1899, p. 721; _Science_ (2), ix. 1899, p. 314.
[574] Budgett, _Trans. Zool. Soc_. xv. Pt. vii. 1901, p. 330.
[575] _Trans. Zool. Soc_. xvi. Pt. ii. 1901, p. 118; also footnote on p.
317.
[576] p. 290.
[577] Traquair, _Journ. Geol. Soc. Ireland_ (2), 1871, p. 249.
[578] Boulenger, _Les Poissons du Bassin du Congo_, Bruxelles, 1901, p. 27.
[579] Traquair, _Monogr. Palaeont. Soc. 1877_; _Quart. Journ. Geol. Soc_.
xxxiii. 1877; _Trans. Roy. Soc. Edinb_. xxx. 1883, p. 22; _Ann. Mag.
Nat. Hist_. (4) xv. 1875, p. 237; Smith Woodward, _Mem. Geol. Surv.
N. S. Wales, Palaeont_. No. 4, 1890, and No. 9, 1895.
[580] Traquair, _Trans. Roy. Soc. Edinb_. xxix. 1879, p. 343.
[581] Smith Woodward, _Brit. Mus. Cat. Foss. Fishes_, iii. 1875, p. 7.
[582] Traquair, _Geol. Mag_. (3) iv. 1887, p. 248; Smith Woodward, _Brit.
Mus. Cat. Foss. Fishes_, iii. 1895, p. 23.
[583] Jordan and Evermann, "Fishes of North and Middle America," _Bull.
U.S. Nat. Mus_. No. 47, Pt. i. 1896, p. 101.
[584] Jordan and Evermann, _op. cit_. p. 102.
[585] Day, _Fishes of Great Britain and Ireland_, ii. 1880-84, p. 282.
[586] Id., _op. cit_. p. 279.
[587] _Brit. Mus. Cat. Foss. Fishes_, iii. pp. 48, 415.
[588] Bashford Dean, _Q.J.M.S._ xxxviii. p. 413.
[589] This genus also occurs in the Cretaceous of Brazil (Smith Woodward,
_A.M.N.H._ (7) ix. 1902, p. 87.)
[590] It is possible that a similar articulation is present in _Lepidotus_
(Smith Woodward, _Brit. Mus. Cat. Foss. Fishes_, iii. p. 79).
[591] Jordan and Evermann, _op. cit_. p. 108, _et seq_.
[592] Alex. Agassiz, _Proc. Amer. Acad. Arts and Sc_. xiii. 1878, p. 65;
Mark, _Bull. Mus. Comp. Zool. Harvard_, xix. 1890, p. 1.
[593] Mark, _op. cit_. p. 3.
[594] Pander, _Ueber die Ctenodipterinen des Devonischen Systems_, St.
Petersb. 1858.
[595] Traquair, _Ann. Mag. Nat. Hist_. (5), ii. 1878, p. 1; _Geol. Mag_.
(3), vi. 1889, p. 97; Smith Woodward, _Brit. Mus. Cat. Foss. Fishes_,
ii. 1891, p. 235 _et seq_.
[596] Traquair, _Journ. Roy. Geol. Soc. Ireland_ (N.S.), iii. 1873, p. 41;
_Proc. Roy. Soc. Edinb_. xvii. 1890, p. 393.
[597] Kner, _SB. k. Akad. Wiss. Math.-Naturw. Cl_. lvii. Pt. ii. 1868, p.
279.
[598] See Chaps. XI. XII. and XIV.
[599] Miall, _Palaeont. Soc_. 1878; Teller, "Ueber Ceratodus sturi," _Abh.
k. k. Geol. Reichsanst_. Wien. xv. 1891.
[600] Günther, _Phil. Trans_. 161, 1871, p. 511.
[601] Semon, _Zool. Forsch. im Australien_, i. Jena, 1893, p. 13 _et seq_.
[602] Semon, _op. cit_. p. 115.
[603] For a list of the more important papers on _Protopterus_, see
Boulenger, _Les Poissons du Bassin du Congo_, Bruxelles, 1901, pp.
40-42.
[604] Traquair, _Rep. Brit. Ass_. 1871 (2), p. 143; Boulenger, _P.Z.S._
1891, p. 147.
[605] Newton Parker, _Trans. Roy. Irish Acad_. xxx. 1892, p. 201.
[606] _Trans. Zool. Soc_. xvi. Pt. ii. 1901, p. 119.
[607] Bohls, _Gött. Nachrichten_, 1894, p. 84; Lankester, _Trans. Zool.
Soc_. xiv. Pt. i. 1896, p. 11; Goeldi, xiv. Pt. vii. 1898, p. 413;
Graham Kerr, _Phil. Trans_. (B), 192, 1900, p. 299.
[608] Hunt, _P.Z.S._ 1898, p. 41.
[609] Lankester, _Nature_, 49, 1894, p. 555; id. _Trans. Zool. Soc_. xiv.
Pt. i. 1896; Graham Kerr, _op. cit_. p. 306.
[610] For further information about the development of _Lepidosiren_, see
Graham Kerr's valuable paper, _op. cit_.
[611] Dollo, _Sur la Phylogénie des Dipneustes_, Bruxelles, 1895.
[612] For critical remarks, see Traquair, _Brit. Ass. Reports_, 1900, p.
776 _et seq_.
[613] Compare Figs. 301 and 304.
[614] It is worthy of note that _Protopterus dolloi_ approaches
_Lepidosiren_ in the more Eel-like shape of its body, and in the
large number of pairs of ribs (54) which it possesses (Boulenger,
_op. cit_. p. 37).
[615] Traquair, _Ann. Nat. Hist_. (6) vi. 1890, p. 485; _Proc. Roy. Phys.
Soc. Edinb_. xii. 1893, p. 87; _ibid_. p. 312; _P.Z.S._ 1897, p. 314;
Bashford Dean, _Trans. New York Acad. Sci_. xv. 1896, p. 101; _Mem.
New York Acad. Sci_. ii. 1900, p. 1.
[616] In a recently published and important contribution to our knowledge
of _Palaeospondylus_, by Professor and Miss Sollas (_Phil. Trans_.
196, 1903, p. 343), they describe structures on the ventral surface
of the head, which they maintain to be branchial arches, as well as
others which, in their view, may represent hyomandibular and
mandibular elements.
[617] Graham Kerr, _Proc. Camb. Phil. Soc_. x. 1900, p. 298.
[618] Lankester, _Nat. Sci_. xi. 1897, p. 45.
[619] Traquair, _Trans. Roy. Soc. Edinb_. xxxix. 1899, pp. 595 and 828.
[620] Id. _Trans. Roy. Soc. Edinb_. xxxix. 1899, p. 844; _Geol. Mag_. vii.
1900, p. 153; ix. 1902, p. 289; _Trans. Roy. Soc. Edinb_. xl. Pt. iv.
1903, p. 723.
[621] Lankester, _Monogr. Palaeont. Soc_. 1868, 1870; _Geol. Mag_. x. 1873,
p. 241; Smith Woodward, _Brit. Mus. Cat. Foss. Fishes_, ii. 1891, p.
159.
[622] Traquair, _Trans. Roy. Soc. Edinb_. xxxix. 1899, p. 834.
[623] Lankester, _Monogr. Palaeont. Soc_. 1868 and 1870; Smith Woodward,
_Brit. Mus. Cat. Foss. Fishes_, ii. 1891, p. 176.
[624] Traquair, _Proc. Roy. Phys. Soc. Edinb_. xii. 1894, p. 269.
[625] _Trans. Roy. Soc. Edinb_. xxxix. p. 843 _et seq_.; _Rep. Brit. Ass_.
1900, p. 768.
[626] See critical remarks by Smith Woodward, _Geol. Mag_. vii. 1900, p.
66.
[627] Lankester, _Nat. Sci_. xi. 1897, p. 46.
[628] Traquair, _op. cit_. p. 837.
[629] Smith Woodward, _Ann. Nat. Hist_. (7), v. 1900, p. 416.
[630] Traquair, _Monogr. Palaeont_. Soc. 1894.
[631] Traquair, _Ann. Nat. Hist_. (6), ii. 1888, p. 485.
[632] Traquair, _Proc. Roy. Phys. Soc. Edinb_. xi. 1891-92, p. 283.
[633] Traquair, _Ann. Nat. Hist_. (6), v. 1890, p. 125.
[634] Traquair, _Geol. Mag_. (3), vii. 1890, p. 55; _Proc. Roy. Phys. Soc.
Edinb_. x. p. 227.
[635] Id. _Geol. Mag_. (3), vi. 1889, p. 1.
[636] Newberry, _The Palaeozoic Fishes of North America, Mon. U.S. Geol.
Survey_, xvi. 1889; Bashford Dean, _Fishes, Living and Fossil_, New
York, 1895, p. 129 _et seq_.; _New York Acad. Sci. Mem_. ii. 1901, p.
87; Eastman, _Amer. Journ. Sci_. (4), ii. 1896, p. 46; _Amer. Geol_.
xviii. 1896, p. 222; _Bull. Mus. Comp. Zool_. xxxi. 1897, p. 19.
[637] _Rept. Brit. Assoc_. 1900, p. 779.
[638] The natural position of the Teleostei in the series of Fishes is
indicated on p. 149.
[639] This exists in _Dapedius_, as pointed out by A. S. Woodward. But this
genus should certainly be removed from the vicinity of _Lepidotus_,
and it seems to bear affinity with the Pholidophoridae.
[640] A synopsis of the classification followed in this work has been
published in the _Annals and Magazine of Natural History_ (7), xiii.
1904, p. 161. Some corrections have been introduced, chiefly due to
the investigations of Dr. W. G. Ridewood.
[641] See p. 553, Fig. 333, B.
[642] Cf. Boas, _Morph. Jahrb_. vi. 1880, p. 527, who has found the conus,
but in a still more rudimentary condition, and with a single row of
valvules, in _Heterotis_ and _Notopterus_ also.
[643] For a general account of the Fishes of this family, cf. Boulenger,
_P.Z.S._ 1898, p. 775, and _Poissons du Bassin du Congo_, p. 49
(1901), where a bibliographical index to the principal anatomical and
physiological publications will be found.
[644] _Trans. Zool. Soc_. xvi. 1901, p. 126.
[645] _Journ. Linn. Soc_. xxvii. 1900, p. 503.
[646] On the Anatomy, cf. Agassiz, in Spix, "Pisc. Brasil." p. 32; Hyrtl,
_Denkschr. Ak. Wien_, viii. 1855, p. 73; Hemprich and Ehrenberg,
"Symb. Phys." Zootom. pls. viii. and ix.; Bridge, _P.Z.S._ 1895, p.
302.
[647] I have not been able to convince myself of the existence of an
intergular plate in this genus, but I am satisfied that the
postclavicle rests on the outer side of the clavicular arch. The bone
that has been regarded as a small intergular plate in _Spaniodon_ is,
in my opinion, the glossohyal.
[648] On the life-histories of the British Clupeids, cf. Heincke,
"Naturgeschichte des Herings" (_Abh. Deutsch. Seefisch. Ver_. ii.
1898); J. T. Cunningham, "Life-History of the Pilchard" (_J. Mar.
Biol. Ass_. [2] iii. 1894, p. 148), and the manuals of the latter
author (_Marketable Fishes of Great Britain_, 1896) and of M‘Intosh
and Masterman (_British Marine Food-Fishes_, 1897).
On the accessory branchial organs of some genera, see p. 294.
[649] For important contributions to our knowledge of European and American
Salmonids since the publication of Günther's account in the British
Museum Catalogue, cf. F. Day, _British and Irish Salmonidae_ (1887),
Smitt, _Krit. Förteckn. Riksmus. Salmonider_ (1886), Fatio, _Faune
des Vertébrés de la Suisse_, v. (1890), and Jordan and Evermann,
_Fish. N. America_, i. (1896).
[650] In _Anomalopterus_, however, a sort of adipose fin exists, as a fold
or cushion on the back, but _in front_ of the rayed dorsal.
[651] A detailed description of the skull of _Alepocephalus rostratus_ has
been given by Gegenbaur, _Morphol. Jahrb_. iv. Suppl. 1878, p. 1.
[652] As pointed out by Gegenbaur. These forms are, however, placed by Gill
in a division characterised by the atrophy or absence of the
mesocoracoid.
[653] See above, p. 178.
[654] _Zool. Jahrb. Anat_. xviii. 1903, p. 58.
[655] _Morphol. Jahrb_. x. 1885, p. 22.
[656] For the nomenclature of these ossicles, cf. Bridge and Haddon, _Proc.
Roy. Soc_. xlvi. 1889, p. 310.
[657] On the anatomy of the Characinidae, cf. Sagemehl, _Morphol. Jahrb_.
x. 1885, p. 102, and xii. 1887, p. 307, and Rowntree, _Tr. Linn.
Soc_. ix. 1903, p. 247.
[658] The end of the tail, when injured, is easily reproduced. As in
Lizards, the axis of the regenerated part is an undivided calcified
tube.
[659] Cf. Reinhardt, _Arch. f. Naturg_. 1854, p. 159.
[660] For the anatomy and physiology, cf. C. Sachs's posthumous work,
_Untersuchungen am Zitteraal_, edited by E. du Bois-Reymond (Leipzig,
1881).
[661] For an illustrated account of the principal types of pharyngeal
teeth, cf. Heckel, _Russegger's Reisen_, i. p. 1001, pl. i. (1843).
On their variations in certain European species, cf. Heincke,
_Leuckart Festschrift_, p. 85 (1892).
[662] Cf. Baudelot, _Ann. Sci. Nat_. (5), vii. 1867, p. 339, and Leydig,
"Unters. Anat. u. Histol. d. Thiere" (1885).
[663] Cf. Noll, _Zool. Gart_. 1869, p. 257, and 1877, p. 351; Olt,
_Zeitschr. wiss. Zool_. lv. 1893, p. 543; Cuénot, _Bull. Soc. Zool.
France_, 1898, p. 53.
[664] Boulenger, _Ann. and Mag. Nat. Hist_. (7), viii. 1901, p. 186.
[665] Watase, _Journ. Coll. Sci. Japan_, i. 1887, p. 247.
[666] On the anatomy of the Cyprinids, cf. Sagemehl, _Morphol. Jahrb_.
xvii. 1891, p. 489.
[667] Cf. Boulenger, "Poissons du Bassin du Congo," p. 238 (1901).
[668] In _Exostoma_ these bones are two in number and so elongate as to
resemble the condition characteristic of the Pediculati.
[669] _Proc. Canad. Inst_. (2) ii. 1884, p. 376.
[670] Cf. Bridge and Haddon, _Phil. Trans. R. Soc_. clxxxiv. 1893, p. 65.
[671] The absence of these fishes from the United States west of the Rocky
Mountains is very remarkable. _Amiurus nebulosus_ was introduced
about 1877 into some parts of California, where it is said to be now
excessively abundant.
[672] Cf. Sörensen, _C. R. Ac. Sci_. lxxxviii. 1879, p. 1042, and
"Lydorgane hos Fiske" (Copenhagen, 1884); Bridge and Haddon, _P.R.S._
lv. 1894, p. 439.
[673] Cf. Hancock, _Zool. Journ_. iv. 1829, p. 242.
[674] Cf. G. Fritsch, "Die Elektrischen Fische, I. _Malopterurus_"
(Leipzig, 1887); E. Ballowitz, "Das elektrische Organ des
Afrikanischen Zitterwelses" (Jena, 1899).
[675] Cf. Eycleshymer, _Amer. Nat_. 1901, p. 911.
[676] _Zool. Journ_. iv. 1829, p. 245.
[677] _P.Z.S._ 1836, p. 330.
[678] _Bull. Soc. Zool. France_, 1880, p. 288.
[679] _Zool. Forsch. Austral_. v. ii. 1895, p. 273. See also Wyman, _Amer.
Journ. Sci_. (2) xxvii. 1859, p. 12; Hensel, _Arch. f. Nat_. 1870, p.
70; Turner, _J. Anat. and Physiol_. i. 1867, p. 78.
[680] Cf. H. v. Ihering, _Biol. Centralbl_. viii. 1888, p. 298.
[681] Cf. Boulenger, _P.Z.S._ 1891, p. 148.
[682] Cf. Day, _Fish. Ind_. 1878, p. 456.
[683] Cf. Boulenger, _P.Z.S._ 1897, pp. 901 and 920; Jobert, _Arch. de
Parasitol_. i. 1898, p. 493.
[684] _Vidensk. Meddel_. (Copenhagen), 1858, p. 79.
[685] A monograph of these Fishes, by Mr. C. T. Regan, will shortly appear
in the _Transactions of the Zoological Society_.
[686] Cf. Moritz Wagner, _Abh. Akad. Münch_. x. 1866, p. 101, and Whymper,
_Trav. Andes Ecuador_, 1892, p. 251.
[687] Cf. Wyman, _Amer. Journ. Sci_. (2) xxvii. 1859, p. 9, and Vaillant,
_C. R. Ac. Sci_. cxxvi. 1898, p. 544.
[688] Cf. Taylor, _Edinb. Journ. Sci_. v. 1831, p. 33; Hyrtl, _Denkschr.
Ak. Wien_, xiv. 1858, p. 39. On the osteology, _cf_. Gill, _Proc.
U.S. Nat. Mus_. xiii. 1890, p. 299.
[689] _Cf_. L. Jacoby, _Zeitschr. Ges. Naturw_. 1867, p. 257.
[690] The biology of the Eel embraces an enormous literature. The following
general recent accounts should be consulted:—L. Jacoby, _Die
Aalfrage_ (Berlin, 1880), translated in _Rep. U.S. Fish Comm_. 1882,
p. 463; H. C. Williamson, _Rep. Fish. Board Scotl_. xiii. 3, 1895, p.
192; G. B. Grassi, _Proc. R. Soc_. lx. 1896, p. 260, and _Mon. Zool.
Ital_. viii. 1897, p. 233; C. H. Eigenmann, _Trans. Amer. Micr. Soc_.
xxiv. 1902, p. 5. For a summary of our knowledge of the larval forms
of European species, _cf_. J. T. Cunningham, _Journ. Mar. Biol. Ass_.
(2) iii. 1895, p. 278.
[691] Forming, with the bases of the neurapophyses, the cross-shaped
arrangement which has been described in the Pike as well as in
_Amia_.
[692] _Cf_. Raffaele, _Mitth. Zool. Stat. Neap_. ix. 1889, p. 179; Lütken,
"Spolia Atlantica," ii. 1892; Goode and Bean, "Ocean. Ichthyol." p.
70 (1895).
[693] _K. spekii_ has been described as from Central Africa, but the only
known specimens were obtained by Speke in Uzaramo, a district on the
coast of German East Africa, just south of Zanzibar.
[694] The most recent account of the Cyprinodonts, with much information on
the habits, development, and anatomy, is by S. Garman, _Mem. Mus.
Comp. Zool_. xix. No. 1, 1895.
[695] On the history and habits of the Blind Fishes of the Mammoth Cave,
_cf_. Putnam, _Amer. Nat_. 1872, p. 6, and _Proc. Boston Soc_. xvii.
1875, p. 222. For a recent account of the eyes of the Amblyopsidae,
_cf_. C. H. Eigenmann's paper in _Arch. f. Entwickelungsmech_. viii.
1899, p. 545, to which is appended a complete bibliographical index
to the subject.
[696] Vaillant was inclined to take a different view, but with considerable
diffidence, owing to his inability actually to trace an open duct. I
believe Günther to be right on this point, as well as in his account
of the suspension of the pectoral arch in _Notacanthus_, which I have
been able to verify. Besides, Mr. W. S. Rowntree, who has great
experience in these matters, has kindly examined at my request a
well-preserved example of _Halosauropsis macrochir_, and informs me
that "the air-bladder passes anteriorly into a tapering band of
tissue which ends in a thread-like ligament attached to the stomach
near its posterior end and in the mid-dorsal line—not to the
oesophagus; no trace of an open communication could be found."
[697] _Fauna u. Flora d. Golf. v. Neap_. ii. 1880.
[698] _Quart. Journ. Micr. Sci_. xlv. 1902, p. 503.
[699] _Ann. Sci. Nat_. (8), xiv. 1902, p. 197.
[700] _Ann. Mag. Nat. Hist_. (7) x. 1902, p. 147.
[701] E. C. Starks, in an important paper on "The Shoulder Girdle and
Characteristic Osteology of the Hemibranchiate Fishes" (_Proc. U.S.
Nat. Mus_. xxv. 1902, p. 619), has shown that the so-called
infraclavicle of Sticklebacks and allies does not exist as a distinct
element. The definition of the Catosteomi, as I had originally drawn
it up, has accordingly had to be modified.
[702] _Proc. U.S. Nat. Mus_. xxvi. 1903, p. 915.
[703] On the nesting habits, _cf_. Coste, _Mém. Acad. Sci. Paris_, x. 1848,
p. 575, Pl.; Warington, _Ann. Mag. Nat. Hist_. (2) x. 1852, p. 276;
Prince, _Ann. Mag. Nat. Hist_. (5) xvi. 1885, p. 487, Pl. xiv. On the
spinning organ: Möbius, _Arch. Mikr. Anat_. xxv. 1886, p. 554, Pl.
xxii.
[704] Dr. Sauvage has described a _Gastrosteus texanus_, but the locality
is probably incorrect, as recent American works do not mention the
occurrence of Sticklebacks in Texas.
[705] _Protaulopsis_, from Monte Bolca, appears to me to belong to the
Scombresocidae. The anterior vertebrae are equal in size; long,
slender ribs are present, the body is scaly, and the so-called
infraclavicles are absent. The rostrum is so much crushed that no
opinion can be formed as to its structure.
[706] Swinnerton (_Quart. J. Micr. Sci_. xlv. 1902, p. 554) has pointed out
that the skull of the Scombresoces belongs to what he terms the
Acrartete type (_i.e_. in which the attachment of the palatine
cartilage or its derivates is confined to the pre-ethmoid cornua),
whilst the other Percesoces examined by him, as well as the
Cyprinodonts are Disartete (the attachment being at the parethmoid
and pre-ethmoid cornua); but the character is so indistinctly defined
in some adult Cyprinodonts that I feel some diffidence in making use
of this character for systematic purposes in the present state of our
knowledge.
[707] Kükenthal, _Abh. Senck. Ges_. xxii. 1896, p. 9; Möbius, _Zeitschr.
wiss. Zool_. xxx. Suppl. 1878, p. 343, and _Arch. Physiol_.
(Leipzig), 1889, p. 348; Jordan and Evermann, _Fish. N. Amer_. p.
730.
[708] A revision of these fishes has recently been published by C. T. Regan
in _Ann. Mag. Nat. Hist_. (7) x. 1902, p. 115.
[709] _Rec. Austral. Mus_. iv. 1901, p. 40. Cf. also S. Garman, _Bull.
Labor. Univ. Iowa_, iv. 1896, p. 81.
[710] _Ann. Mag. Nat. Hist_. (7), x. 1902, p. 295.
[711] _Ibid_. (7), xi. 1903, p. 460.
[712] In the very aberrant Hake (_Merluccius_) ribs are absent on the
vertebrae bearing the strongly expanded, plate-like parapophyses.
[713] The increased number of pectoral pterygials has been regarded by
Sagemehl (_Morphol. Jahrb_. x. 1885, p. 17) as indicating
generalisation, and has been a great stumbling-block in his
discussion of the affinities of _Gymnotus_ with the other
Ostariophysi, and especially the Characinidae. The fact that the same
feature is repeated in three such distinct families as the
Gymnotidae, Anguillidae, and Muraenolepididae, and occurs in genera
which are in all other respects more specialised than their
neighbours, goes far to prove that Sagemehl was mistaken in his
interpretation of this character.
[714] _Siboga Expedition_, Introd. 1902, p. 108.
[715] _Poissons venimeux_ (Paris, 1889), p. 169.
[716] For recent accounts of the anatomy, embryology, and ethology, cf. C.
H. Eigenmann, _Bull. U.S. Fish Comm. for_ 1892, p. 381, and _Arch.
Entwickelungsmech_. iv. 1896, p. 125.
[717] It has recently been ascertained, on a large number of specimens,
that in the African species the female alone performs the buccal
nursing duties.
[718] Cf. Monograph by J. Pellegrin (Paris, 1904).
[719] Gerbe, _Rev. et Mag. de Zool_. xvi. 1864, p. 255.
[720] _Zool. Garten_, 1867, p. 148. See also Verrill, _Amer. Journ. Sci_.
(4) iii. 1897, p. 136.
[721] _Naucrates_. In this genus most of the epipleurals of the praecaudal
region are inserted on the ribs, but the hinder ones are on the
centra.
[722] Cf. Geoffroy, _Ann. du Mus_. ix. 1807, p. 473; F. J. F. Meyen, _Reise
um die Erde_, i. p. 56 (1834).
[723] For a detailed account of these fishes and of _Xiphias_, cf. Brown
Goode, _Proc. U.S. Mus_. iv. 1881, p. 415, and _Rep. U.S. Fish Comm.
f. 1880_, 1883, p. 289.
[724] Monographs by Lunel, _Mém. Soc. Phys. Genève_, xviii. 1865, p. 165,
and by Lütken, _Spolia Atlantica_, i. 1880, p. 491.
[725] Troschel, _Sitzb. Ver. Preuss. Rheinl_. xx. 1863, p. 51 (_Brama raii_
and _B. longipinnis_).
[726] Cf. Thilo, _Zool. Anz_. 1902, p. 305.
[727] Cf. Boulenger, _Ann. Mag. Nat. Hist_. (7), 1902, p. 295, and _C. R.
Ac. Sci_. cxxxvii. 1903, p. 523.
[728] Cf. Steenstrup, _Vid. Selsk. Skr_. 1863, p. 253, _Ann. Mag. Nat.
Hist_. xv. 1865, p. 361, and _Overs. Selsk. Skr_. 1876, p. 174; Malm,
_Svensk. Vet. Ak. Handl_. vii. 1868, No. 4, p. 28; A. Agassiz, _P.
Amer. Ac_. xiv. 1878, p. 1; Emery, _Mitth. Zool. Stat. Neap_. iv.
1883, p. 413; Facciola, _Natural. Sicil_. iv. 1885, p. 261, and vi.
1887, p. 74; Ehrenbaum, _Wiss. Meeresunters_. (2), ii. 1897, p. 255;
Nishikawa, _Annot. Zool. Japan_, i. 1897, p. 73.
[729] On the morphology and classification, cf. Traquair, _Tr. Linn. Soc_.
xxv. 1865, p. 263; Jordan and Goss, _Rep. U.S. Fish Comm. f. 1886_
(1889); Kyle, _Rep. Fish. Board Scotland_, 1900, p. 335. Also the
Monographs of the Sole, by J. T. Cunningham (Plymouth, 1890, 4to),
and of the Plaice by Cole and Johnstone, _Liverpool M.B.C. Memoirs_,
viii. 1901.
[730] On the breeding habits and eggs, cf. F. de Filippi, "Mem. s. sviluppo
del Ghiozzo" (_Ann. Univ. Med. Milano_, 1841); Holt, _Ann. Mag. Nat.
Hist_. (6); vi. 1890, p. 34; Petersen, _Vid. Meddel_. 1891, p. 243;
Guitel, _Ann. Mag. Nat. Hist_. (6) viii. 1891, p. 407, and _Arch.
Zool. Expér_. (2), x. 1892, p. 499, and (3) iii. 1895, p. 263.
[731] Cf. W. E. Ritter, _Bull. Mus. Harvard_, xxiv. 1893, p. 51.
[732] For a good figure from life of _Periophthalmus koelreuteri_ and an
account of its habits, cf. S. J. Hickson, _A Naturalist in North
Celebes_ (London, 1889).
[733] For the theories on the formation of the disk, cf. R. Storms, _Ann.
Mag. Nat. Hist_. (6), ii. 1888, p. 67.
[734] Cf. Holmwood, _Proc. Zool. Soc_. 1884, p. 411.
[735] This character suffers one exception, to be found in _Comephorus_, a
degraded form otherwise closely related to _Cottocomephorus_, in
which the skeleton is typical of the present division.
[736] _Ann. Mag. Nat. Hist_. (6), x. 1892, p. 212; and _Zool. Gleanings
Investigator_, 1901, p. 41.
[737] Cf. Bottard, _Poissons Venimeux_ (Paris, 1889, 8vo), and Marie
Sacchi, _Atti Soc. Ligust_. vi. 1895, p. 89.
[738] Cf. Dybowsky, _Verh. zool.-bot. Ges. Wien_, xxiii. 1873, p. 475, and
_Zool. Centralbl_. viii. 1901, p. 475; Zograf, _Tagebl. zool. Congr.
Berlin_, No. 8 (1901), p. 9.
[739] The anatomy and external characters of these fishes have been fully
monographed by S. Garman, _Mem. Mus. Comp. Zool_. xiv. No. 2, 1892.
[740] Moseley, _Notes Natur. Challenger_, 2nd edition, p. 495.
[741] _Ann. Mag. Nat. Hist_. (7) viii. 1901, p. 261.
[742] _Op. cit_. xi. 1903, p. 460.
[743] Cf. Allman, _Ann. Mag. Nat. Hist_. vi. 1841, p. 161; Schmidt, _Nord.
Med. Ark_. vi. No. 2, 1875; Gressin, _Contribution à l'étude de
l'appareil à venin chez les Poissons du genre Vive_ (Paris, 1884,
8vo); W. N. Parker, _P.Z.S._ 1888, p. 359; Phisalix, _Bull. Mus.
Paris_, 1899, p. 256.
[744] This is really the second, the first having entirely disappeared, as
in some Gobiesocidae.
[745] _P.Z.S._ 1898, p. 281.
[746] The vertebral column in this family shows that the first segment has
been lost in _Callionymus_, as could be deduced from the fact that,
in that genus, the first rib is on the second vertebra instead of on
the third as is usual in Teleosteans. In the Gobiesocidae, as in
_Callionymus_, there are three occipital condyles on a straight
transverse line, the outer, formed by the exoccipitals, invariably
articulating with the second vertebra.
[747] Which have been described as ribs by Günther and by Guitel.
[748] On the habits and anatomy of the French species, cf. Guitel, _Arch.
Zool. Expér_. (2) vi. 1888, p. 423.
[749] _Arch. Zool. Expér_. (3) i. 1893, p. 325.
[750] What has been described as the rib of the first vertebra is an
ossified ligament, probably homologous with the first epipleural,
which extends from the clavicle to the neural arch of the first
vertebra (_ligamentum scapulo-occipitale_ of Siebenrock).
[751] On the breeding habits and development of this fish, cf. J. A. Ryder,
_Bull. U.S. Fish Comm_. vi. 1886, p. 4, and _Proc. Acad. Philad_.
1890, p. 407.
[752] Cf. Günther, _Trans. Zool. Soc_. vi. 1869, p. 437.
[753] Cf. C. W. Greene, _Journ. of Morphol_. xv. 1899, p. 667.
[754] It is in fact, in some cases, difficult to decide whether a genus
should be referred to the Gadidae or to the Zoarcidae.
[755] Cf. Poey, _Mem. Cuba_, ii. p. 96 (1860).
[756] On the general structure, anatomy, and metamorphoses, cf. L. Powell,
_Tr. N. Zeal. Inst_. ii. 1878, p. 269; Emery, _Atti Ace. Lincei_,
iii. 1879, p. 390; Lütken, _Vid. Meddel_. 1881, p. 190, and _Overs.
Vid. Selsk. Skr_. 1882, Suppl. p. 21; Collett, _Forh. Vid. Selsk.
Christ_. 1883, No. 16; T. J. Parker, _Tr. Z. S_. xii. 1886, p. 5;
Smitt, _Bih. Fören. Förh_. i. 1889, p. 17; A. Meek, _Stud. Mus. Univ.
Coll. Dundee_, i. 1890, No. vi.; F. Mazza, _Int. Monatschr. Anat_.
xviii. 1901, p. 129.
[757] Cf. A. Agassiz, _Amer. Journ. Sci_. (3), iii. 1872, p. 154; J. M.
Jones, _Nature_, xix. 1879, p. 363; Vaillant, _C. R. Soc. Biol_. (8),
iv. 1887, p. 732.
[758] _Ann. Sci. Nat. Zool_. (3), xiv. 1850, p. 105, and _C. R. Ac. Sci_.
lxxiv. 1872, p. 1527.
[759] _Proc. Zool. Soc_. 1902, ii. p. 284.
[760] Cf. Pellegrin, _Poissons Vénéneux_ (Paris, 1900, 8vo), which contains
a very full _résumé_ of what is known of the toxic properties of the
various Plectognaths.
[761] _Sitzb. Akad. Berl_. 1889, p. 999.
[762] Cf. Thilo, _Anat. Anz_. xvi. 1899, p. 73.
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