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Title: A guide to the shell and starfish galleries
Author: E. A. Smith
F. J. Bell
R. Kirkpatrick
Release date: May 12, 2024 [eBook #73609]
Language: English
Original publication: United Kingdom: British Museum of Natural History, 1901
Credits: Richard Tonsing, deaurider, and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive/American Libraries.)
*** START OF THE PROJECT GUTENBERG EBOOK A GUIDE TO THE SHELL AND STARFISH GALLERIES ***
[Illustration:
BRITISH MUSEUM (NATURAL HISTORY)
Ground Floor.
]
A GUIDE
TO THE
SHELL AND STARFISH
GALLERIES
(MOLLUSCA, POLYZOA, BRACHIOPODA, TUNICATA, ECHINODERMA, AND WORMS).
DEPARTMENT OF ZOOLOGY.
BRITISH MUSEUM (NATURAL HISTORY),
CROMWELL ROAD, LONDON, S.W.
_WITH NUMEROUS ILLUSTRATIONS._
PRINTED BY ORDER OF THE TRUSTEES.
1901
_All rights reserved._
LONDON:
PRINTED BY WILLIAM CLOWES AND SONS, LIMITED,
STAMFORD STREET AND CHARING CROSS.
PREFACE.
One of the large north galleries approached from the Bird Gallery is
devoted to the exhibition of the extensive Class of Mollusca. Specimens
of the types of all the principal divisions of this Class are exhibited,
either entire and preserved in spirit, or as models. However, as not the
animals, but their shells have always been a favourite object of study,
and a popular source of pleasure to collectors, the exhibition of the
species of shells has been made as complete as the space of this Gallery
admitted. It has thus proved adequate for the requirements of the
majority of visitors and students who consult this Collection. A
separate series of British Shells is exhibited in some small table-cases
along the west wall.
This Gallery also contains the exhibited series of Polyzoa, Brachiopoda,
and Tunicata.
The Starfish Gallery, so called from one of the best-known types of the
Echinoderma, contains an exhibition of the animals of this Class, as
well as of the somewhat heterogeneous assemblage of creatures which are
comprised under the popular name of Worms (_Vermes_). These animals
possess greater attraction to students of Natural History than to the
general public, and many, from their small size or the soft nature of
their body, are not suitable for exhibition. Therefore no attempt has
been made to show more than a carefully selected number of the types of
the larger groups. But the exhibition of very complete series,
supplemented by models or figures, to illustrate the remarkable life
history of some of these animals, also of specimens of the Worms which
possess a special interest from their relation to man, render this
Gallery particularly instructive to the student.
This guide has been prepared by Mr. E. A. Smith, Mr. F. J. Bell, and Mr.
R. Kirkpatrick, who have special charge of the collections described.
In conclusion, thanks are due to Messrs. A. & C. Black, Messrs. F. Warne
& Co., Messrs. Macmillan & Co., Crosby Lockwood & Son, and the Linnean
Society for kindly allowing the use of _clichés_ from illustrations in
various works published by them.
E. RAY LANKESTER.
TABLE OF CONTENTS.
THE SHELL GALLERY.
GENERAL NOTES ON MOLLUSCA 1
SYSTEMATIC ARRANGEMENT 7
AMPHINEURA (Chitons, etc.) 7
GASTROPODA (Marine Univalves, Land-Snails, etc.) 9
LAMELLIBRANCHIA (Bivalves, Oysters, Cockles, etc.) 29
CEPHALOPODA (Octopus, Cuttlefish, etc.) 45
INDEX TO PRINCIPAL GENERA OF MOLLUSCA 51
GENERAL NOTES ON POLYZOA 54
│CLASSIFICATION 57
AN ACCOUNT OF THE BRACHIOPODA 74
│CLASSIFICATION 77
AN ACCOUNT OF THE TUNICATA 83
│ARRANGEMENT 90
THE STARFISH GALLERY.
GENERAL ACCOUNT OF THE ECHINODERMA 106
CRINOIDEA (Lily Stars, etc.) 112
ASTEROIDEA (Starfishes) 113
OPHIUROIDEA (Brittle-Stars) 114
ECHINOIDEA (Sea-Urchins) 114
HOLOTHURIOIDEA (Sea-Cucumbers) 115
──────┬────────────────────────────────────────────────────────────────
VERMES│PLATYHELMINTHES (Tapeworms, etc.) 116
„ │NEMATODES (Round-worms, etc.) 118
„ │ANNULATA (Marine Worms, Earthworms, Leeches) 122
THE SHELL GALLERY.
GENERAL NOTES ON MOLLUSCA.
The Mollusca constitute one of the principal divisions of the Animal
Kingdom, and include such animals as the Octopus, Cuttlefish, Snail,
Slug, Whelk, Cockle, and Oyster.
[Sidenote: Definition.]
They may be characterized as soft, cold-blooded animals, without
distinctly marked external division into segments (as in Worms); their
cerebral ganglia (the centre of the nervous system) lie above the
commencement of the gullet, and are connected with the inferior ganglia
by nerve-chords. Their heart consists of two or more chambers, and is
situated on the dorsal side of the animal; it drives the blood into
spaces between the various organs of the body. Only the Cephalopods
possess internal cartilages, but all are without a bony internal
skeleton; in the majority this is compensated by an external hardened
shell which is formed (secreted) by the outer covering of the animal
termed the _mantle_. [Sidenote: The shell.] The shell may consist of two
parts (valves), as in the Oyster, or may be single, as in the Whelk and
Limpet, or composed of a series of plates, as in the “Coat-of-mail”
shells or _Chitons_: when well developed it is hardened by a rich
deposit of carbonate of lime; but it may be gelatinous, as in
_Cymbulia_, or altogether absent, as in _Octopus_; it may cover and
protect the body, as in the Oyster, lie within the folds of the mantle,
as in the Sea-hares (_Aplysiidæ_), or it may be quite internal, as in
the horny “pen” of the Squid. It may be elongated, as in the Elephant
Tooth-shell (_Dentalium_), cup-shaped, as in the Limpet, or spirally
coiled, as in the Snail.
[Sidenote: Description of the animal.]
The mantle may form a free fold on either side of the body, as in the
Bivalves, or it may become largely attached to the body-wall, as in the
Snail or the Slug, and so give rise to an air-chamber, which, when its
walls are richly supplied with blood, serves as a lung. The ventral
surface of Molluscs is produced into the so-called “foot,” which may be
very variously modified. The foot may be more or less hatchet-shaped, or
curved and capable of serving as a leaping-organ, or sole-shaped and
adapted for creeping; its margins may be produced into elongated
processes, as the so-called arms of the Octopus, eight in number and
provided with suckers, or of the Nautilus, where the arms are much more
numerous, but shorter and without suckers. In the Cephalopods, also,
another part of the foot may fold over from either side and form a
median funnel, through which the water of respiration is driven
outwards, causing the animal to move in the opposite direction—this part
of the foot having, therefore, still the function of an organ of
locomotion. By means of their muscular foot the _Solenidæ_, or
Razor-shells, burrow in the sand, the Pond-Snails (_Limnæidæ_) crawl on
aquatic plants and swim reversed on the surface of the water, the Limpet
clings to the rock, and the Cockles and _Trigonias_ take surprising
leaps.
[Sidenote: The operculum.]
Upon the upper surface of the foot, in many Gastropods, a flat hard
structure termed the _operculum_ is situated, which, when the animal is
retracted, partly or entirely closes the aperture of the shell. In some
cases, as in the _Turbos_, it is very strong and of a stony nature, but
in most instances it is horny. It is differently constructed in distinct
families: it may be annular and multispiral, annular and paucispiral,
subannular and ovate, or subannular and unguiculate. In the _Nerites_ it
is shelly, somewhat semicircular, closes the aperture of the shell, and
is furnished with a stout projection on the straight edge, fitting like
a hinge under the inner lip of the shell. A series of _opercula_ is
exhibited in side table-case C.
[Sidenote: The breathing-organs.]
Thread-like processes on either side of the body, the so-called
gill-filaments, often unite with those in front of and behind them, and
so give rise to plates; these, when well developed, are best seen in the
division to which the Oyster and the Mussel belong, and which,
therefore, has been called the division of the plate-gilled Molluscs, or
_Lamellibranchia_. Where the body is coiled or twisted on itself, as so
often happens, the gills of one side may be altogether lost. Sometimes,
as in _Phyllirhoë_, when the body is small and its wall thin, the gills
(_ctenidia_) disappear altogether, and there is no special
breathing-organ; in others the loss of the gill is compensated by the
formation by the mantle of a lung; this is most often seen in the forms
that live on land.
But these so-called gills may have other functions: in the
Lamellibranchs, where there is no head and no special means by which the
creature can obtain food, the delicate waving filaments or _cilia_ with
which they are covered cause currents in the surrounding water, by means
of which minute organisms are brought to the mouth.
[Sidenote: The radula.]
All Molluscs, except the Lamellibranchs, have a very remarkable
structure developed in the floor of their mouth-cavities; on a basis of
cartilage, which may be moved backwards and forwards by muscles, there
is developed a horny plate, which may be of considerable length, and
which has its upper surface covered with a number of more or less fine,
flattened, or spiny outgrowths, which are known as teeth. This is the
_odontophore_, _tongue_, _radula_, or lingual ribbon (see fig. 3).[1]
[Sidenote: The eyes.]
Eyes may be absent, as in nearly all the headless Lamellibranchs; but in
other Molluscs they are generally present, and may be more or less well
developed. An instructive series of stages is exhibited by the
Cephalopoda. In _Nautilus_ the eye remains an open pit; in
_Ommatostrephes_ two chambers appear, the anterior of which is bounded
posteriorly by the lens, and is open to the exterior, so that sea-water
enters it; in _Sepia_, finally, the anterior chamber becomes closed in
front. We may observe that the eyes of all Cephalopods are at first
pit-like, or pass through a stage which is permanent in _Nautilus_, one
of the geologically oldest types.
Cephalic eyes have recently been noticed in _Mytilus_ and _Avicula_.
Eyes of a more complicated structure, which are modified tentacles, are
sometimes found on the edges of the mantle in Lamellibranchs (e.g.
_Pecten_); these eyes resemble those of Vertebrates, and differ from
those of most invertebrate animals in having the fibres of the optic
nerve entering the distal and not the proximal ends of the retinal
cells. Eyes of a similar construction are to be found on the back of the
shellless _Oncidium_, and may be about one hundred in number.
Eyes of a remarkable character on the shells of some of the Chitons
appear to be modified from tactile organs, and are innervated like the
ordinary molluscan eye; they sometimes occur in enormous numbers, more
than ten thousand being present on one animal (see wax model, Case 2).
[Sidenote: Organ of hearing.]
In Cephalopods the ear, like the eye, is known to make its first
appearance in the form of an open pit, the mouth of which gradually
closes up, leaving only a narrow slit in communication with the
exterior. It is probable that in many forms the so-called ear is an
organ by means of which the mollusc becomes acquainted with changes in
the surface over which it is passing; it is often found deeply imbedded
in the substance of the foot, where it forms a closed vesicle.
[Sidenote: Sense of smell.]
There is no doubt that the carnivorous Gastropoda are gifted with a
sense of smell, and throughout the series we observe patches of modified
cells of the body-wall (the _osphradium_) which serve either as
olfactory organs or as an apparatus for testing the nature of the water
of respiration.
[Sidenote: The sexes and reproduction.]
The sexes are distinct in the most highly organized Mollusca, but are
united in the same individual in some of the lower forms, such as
Land-Snails, the _Opisthobranchia_ (including the Bubble-Shells,
Sea-Slugs, &c.), and in some Bivalves. The reproduction of Mollusca is
in all cases effected by means of eggs. In some instances the young are
actually hatched within the oviduct of the parent, as in the Freshwater
Snails (_Vivipara_); and apparently in most Bivalves the eggs are also
retained within the valves until hatched.
The ova of many molluscs are deposited in masses enclosed in capsules.
Some of them are very wonderful and complicated structures. Those of the
Cuttles and their allies are clustered like grapes, each capsule
containing but a single embryo; but in the Calamaries or Squids they
form a radiating mass of elongated sacks, each containing from thirty to
two hundred eggs, and it has been estimated that one of the
spawn-clusters of the Common Squid (_Loligo vulgaris_) contains as many
as 40,000 ova. Everybody knows the spawn-cases of the Common Whelk,
found so abundantly on the sea-beach, consisting of a large number of
yellowish capsules, heaped one upon another and forming an irregularly
rounded mass. As many as five or six hundred capsules may be piled
together in a single heap, each capsule containing several hundred eggs,
of which perhaps only thirty or forty are hatched.
In other genera, as _Tethys_, _Doris_, _Eolis_, &c., the eggs are
contained in a spirally rolled ribbon or strap-like structure; and some
of the _Naticæ_ build a somewhat similar capsule, composed of the eggs
cemented together by sand and a gelatinous material, the whole forming
two-thirds of a circle narrowed at the upper part.
Terrestrial Molluscs deposit, in comparison with their marine relations,
but very few eggs. They are sometimes covered by a thin soft skin, but
in certain groups, such as the large South-American _Strophochili_ and
the African _Achatinæ_, which include the largest of known
land-molluscs, they are protected by a hardened calcareous shell, in
some instances fully an inch in diameter. The freshwater forms (_Limnæa_
and _Physa_) deposit from thirty to a hundred eggs enveloped in a
gelatinous mass.
The number of eggs produced by some Bivalves is enormous. The Common
Oyster is said to produce a million or more, and the American variety
ten, or even sixty, times as many. Some of the River-Mussels are also
very prolific, as many as two millions being sometimes the product of a
single individual. A small series of the eggs of Land-Snails and of the
egg-capsules of some marine Gastropods is exhibited in side table-case C
at the side of the Gallery.
The ova of Mollusca may be gradually developed into the form of the
parent, or there may be a free-swimming larva, which has a circlet of
cilia near the anterior pole of its body (so-called “Veliger” larvæ), or
there may be special larvæ, as in the case of the Freshwater Mussel, the
“Glochidium,” as it is called, which has a toothed bivalve shell by
which it can fix itself to fishes.
[Sidenote: Duration of life.]
The limits of age of molluscs has been definitely ascertained in a few
instances only. Most Land-Snails probably live about two years, although
in confinement some have been kept alive for a much longer time. Some of
the marine forms live for a considerable period, the Common Oyster not
attaining full growth until about five years old, after which it may
continue to live for many years. The Giant Clam, a specimen of which is
placed in the upright cases near the entrance to the Gallery, must, one
would think, have a very long existence, judging from the size and
thickness of the shell. [Sidenote: Hibernation and torpidity.] All
terrestrial molluscs hibernate in cold climates, hiding themselves away
in the ground between roots and similar sheltered places. In tropical
countries some assume a state of torpidity (æstivate) during the hottest
and driest season of the year, closing up the aperture of their shells
with a temporary lid or door (epiphragm), in order to resist the dryness
of the atmosphere. Some of these “summer-sleepers” are endowed with a
remarkable tenacity of life. An Australian Pond-Mussel has been known to
live a year after being removed from the water; several Land-Snails have
revived after a captivity of from two to five years, without any food
whatever. One of the most remarkable instances of this kind occurred in
the British Museum. A specimen of _Helix desertorum_, a common
Desert-Snail from Egypt, was fixed to a tablet in March 1846, and in the
same month of the year 1850 it was discovered to be alive. It must have
come out of its shell in the interval, and finding it was unable to
crawl away, had again retired within it, closing the aperture with a new
epiphragm, but leaving traces of slime upon the tablet, which led to its
immersion in water and subsequent revival, having passed a period of
four years in a dry museum without the smallest particle of food. The
actual specimen is here figured, Fig. 1.
[Illustration:
Fig. 1.[2]
_Helix desertorum._
(See black table-case 1.)
]
[Sidenote: Economic uses.]
The _economic_ uses of molluscs to man are manifold, and will be
mentioned in the course of the description of the several families; but
here may be the place to direct the attention of visitors to side
table-cases B and D at the side of the room, containing some specimens
of articles manufactured from shells, such as cameos, flowers,
bracelets, brooches, &c.
[Sidenote: Geological history.]
Mollusca made their appearance on the globe at a very early epoch in the
history of the development of animal life, a large number of fossil
forms, such as _Nautilus_, _Lituites_, _Orthoceras_, &c., being found in
the oldest Palæozoic formations. Probably all these belonged to the
_Tetrabranchia_, of which one descendant only, the Pearly Nautilus, has
survived to our period. Some Gastropods and Bivalves coexisted with
those ancient Tetrabranchs; but these types abounded more in the later
geological epochs, many Tertiary forms being undistinguishable from
species which now exist.
[Sidenote: General distribution.]
The greater number of Mollusca are inhabitants of the sea, some passing
their whole life at the surface hundreds or thousands of miles away from
land; others at the bottom of the ocean at all depths, some having been
dredged at five miles from the surface. Many are found in much shallower
water, and a large number between tide-marks. Rivers and lakes furnish
an immense variety of forms, and vast numbers live on land in all
situations—on mountains, in valleys, forests, and deserts.
Molluscs are either animal or vegetable-feeders, the former preying
principally upon other members of their own class.
[Sidenote: Systemic arrangement.]
The following Table shows the systematic arrangement of the Mollusca
adopted in the Shell Gallery:—
Class I.—Amphineura.
Order 1.— Polyplacophora. Chitons.
„ 2.— Aplacophora. Neomenia, Chætoderma, etc.
Class II.—Gastropoda.
(Section Streptoneura.)
Order 1.— Scutibranchia. _Nerites_, Top-shells, Ear-shells, Limpets.
„ 2.— Pectinibranchia. Rock-snails, Whelks, Olive-shells,
Harp-shells, Cones, Strombs or Wing-shells, Periwinkles,
Carrier-shells, etc.
(Section Euthyneura.)
Order 1.— Opisthobranchia. Bubble-shells, Sea-hares, Umbrella-shells.
„ 2.— Pulmonata. Land and freshwater Snails, False Limpets.
Class III.—Scaphopoda. Tooth-shells (_Dentalium_).
Class IV.—Lamellibranchia. Bivalved Molluscs.
Order 1.— Protobranchia. Nucula, etc.
„ 2.— Filibranchia. Anomia, Common Mussel, Ark-shells, etc.
„ 3.— Pseudolamellibranchia. Pearl-Oyster, Hammer Oyster,
Wing-shells, etc.
„ 4.— Eulamellibranchia. Freshwater Mussels, Cockles,
Razor-shells, Ship-worms, etc.
„ 5.— Septibranchia. Poromya, etc.
Class V.—Cephalopoda.
Order 1.— Tetrabranchia. Pearly Nautilus.
„ 2.— Dibranchia. Octopus, or Poulp, Argonaut, Squids, and Cuttle
fishes.
Class I.—AMPHINEURA.
[Sidenote: Cases 1–3.]
The Molluscs of this class are characterized by bilateral symmetry. The
head and arms are situated at the opposite extremities of the elongated
body, the gills, genital ducts and circulatory organs being paired and
similar on both sides. The first order belonging to this division, the
Polyplacophora, includes the “Coat-of-mail shells,” or “Sea-woodlice”
(_Chitonidæ_). They have their back armed with eight shelly plates which
overlap one another like tiles, and, like woodlice, have the power of
rolling themselves into a ball. These plates are imbedded at the sides
into the fleshy mantle, beneath which, on each side of the foot, are
arranged the gills. A Chiton differs in many respects from other
Mollusca. It has a shell like an Isopod Crustacean, a heart down the
back like a sea-worm, symmetrical organs of reproduction on each side
like the bivalves, a head and crawling foot like a true Limpet, and a
posterior anal orifice. These several anatomical peculiarities at one
time induced certain eminent authorities to hesitate in considering them
molluscs; but now that the development from the egg has been
investigated, their association with the Mollusca may be considered
definitely settled.
[Illustration:
Fig. 2.
Coat-of-mail Shells, or Chitons.
1. _Chiton squamosus_ (upper surface).
2. _Chiton elegans_ (lower surface): _a_, mouth; _b_, foot; _c_,
mantle; _d_, gills.
]
Chitons are found in all parts of the world, the finest inhabiting
tropical countries. They live chiefly on rocks and under stones at
low-water or at moderate depths; but a few forms have been discovered by
the ‘Challenger’ Expedition at depths exceeding 2000 fathoms. The
numerous sections of the group are principally distinguished by
differences in the edges of the plates or valves which are inserted in
the mantle, and in the different kinds of ornamentation upon the upper
surface of the mantle-border. This, in some species, is quite smooth, in
others covered with a dense mass of minute grains or scales, and in
others armed with short prickly spines. In the giant _Cryptochiton_ of
Kamtschatka the plates are entirely covered over by the thick leathery
granular mantle, and in another set, _Cryptoplax_, which consists of
long slug-like animals, the plates are very small, and placed at
intervals along the back.
About five hundred living species are known, and about one-fourth that
number has been found fossil from the Silurian age downwards.
[Sidenote: Case 3.]
The second order of Amphineura, namely, the Aplacophora, comprises a few
somewhat worm-like Molluscs which are devoid of a shell, but have
instead the dorsal surface more or less studded with numerous minute
calcareous spines or spicules. _Neomenia_, _Proneomenia_, _Chætoderma_
are genera belonging to this order.
Class II.—GASTROPODA.
[Sidenote: Cases 4–135.]
In contradistinction to the preceding class these Molluscs are
asymmetrical, especially in respect of the gills and the spiral coiling
of the viscera and most of the shells. They may be divided into two
sections, _Streptoneura_ and _Euthyneura_, distinguished by differences
in the arrangement of the visceral nerve-loop.
Section STREPTONEURA.
[Sidenote: Cases 4–94.]
The Molluscs of this section are bisexual and furnished with a shell,
and generally with an operculum. The gills are in front of the heart and
the visceral nerve-loop is twisted into a figure of 8. The section
contains two groups or orders, Scutibranchia and Pectinibranchia.
Order 1.—SCUTIBRANCHIA.
The Scutibranchia have a free bipectinate gill, or the gill may be
absent (_Lepeta_, _Helicina_), and generally exhibit traces of bilateral
symmetry.
[Sidenote: Case 4.]
The _Acmæidæ_ are called False Limpets, because, although the shells are
identical with the true Limpets, the animals differ in having only a
small gill on the left side of the neck, whilst the _Patellæ_ have the
gills greatly developed all round the sides of the foot. Both the true
and the false Limpets are littoral and found on rocks between
tide-marks. They have the power of excavating the surface to which they
attach themselves, and adhere so firmly that it is easier to break the
shell than detach the animal. The largest known Limpet (_Patella
(Ancistromesus) mexicana_, case 7) inhabits the west coast of Central
America, its shell having sometimes a length of 12 inches. The Limpets
are vegetable-feeders and fond of seaweeds of various kinds, which they
rasp with their remarkable spiny tongues. That of the common English
Limpet (_P. vulgata_, Fig. 3) is longer than the shell itself, and armed
with as many as 1920 glassy hooks in 160 rows of twelve teeth each. The
Limpet is commonly used for bait in the sea-fishing off the Scottish
coast, and vast quantities are consumed as food in some parts of
Ireland. Some Limpets, such as _P. compressa_, _P. mytilina_, etc., are
found on the stems of floating seaweeds, and have the shells usually
thinner and smoother than the Rock-Limpets, which have to resist the
fury of the breaking waves.
[Illustration:
Fig. 3.
1. Radula of the Common British Rock-Limpet (_Patella vulgata_),
natural size.
2. Two transverse series of teeth: _a_, median teeth; _b_, laterals;
_c_, uncini or marginals.
]
[Illustration:
Fig. 4.
The Common Rock-Limpet (_Patella vulgata_). British.
1. Animal: _a_, foot; _b_, fringed mantle; _c_, tentacles; _d_, mouth;
_e_, eyes; _f_, gills.
2. Side view of shell, showing the impression or scar of the
attachment-muscle, _g_.
3. Upper surface of the shell.
]
[Sidenote: Case 8.]
The “Keyhole Limpets” and “Slit Limpets” (_Fissurellidæ_) resemble in
external shape ordinary Limpets, but are perforated at or near the apex,
or more or less slit at the front margin. The hole or slit gives passage
to a tubular fold of the mantle, through which the water apparently
flows to the gills. The largest species are from California and South
America, and others are found, but not abundantly, on most shores. The
animal of the large _Lucupina crenulata_ from California is eight or ten
inches in length, and almost conceals the shell, and the shell of the
South-African _Pupillæa aperta_ is also all but hidden beneath the
mantle of the animal.
[Illustration:
Fig. 5.[3]
_Pleurotomaria adansoniana._ Case 9.
⅓ natural size.
]
[Sidenote: Case 9.]
The _Pleurotomariæ_ are extremely rare in recent times, only five
species being known, whereas over a thousand fossil forms have been
described. The specimens of _P. adansoniana_ and _P. beyrichi_ exhibited
in case 9, are among the finest acquisitions to the shell collection of
recent years.
[Sidenote: Cases 9–11.]
The “Ear-shells” or “Ormers” (_Haliotidæ_) are found adhering to rocks
in most parts of the world, with the exception of South America. They
are lined with _pearl_, and many exhibit splendid colours and sculpture
externally. Like the Limpets they hold on to the rocks with such
tenacity that it is absolutely impossible to remove some of the larger
species by force without injuring the shell. Boiling water or mustard
and water poured over them will, however, soon compel them to relinquish
their hold. The shell of _Haliotis_ is pierced by a series of holes
parallel with the left margin. Through such of them as are open the
animal protrudes a slender filament or feeler, and the water also finds
its way through them to the gills beneath.
The Single British species (_H. tuberculata_) is not actually found on
the English coast, but common on rocks and stones at low-water in the
Channel Islands. It is frequently eaten by the poor of those islands and
the north of France; other species in New Zealand, China, Japan, West
Africa, and elsewhere, constitute a common article of diet among the
natives. _Haliotis_-shells are largely used in the manufacture of pearl
ornaments, and in all kinds of inlaid work.
[Illustration:
Fig. 6.
Top-shell (_Turbo petholatus_). (From the Indo-Pacific Ocean.)
_a._ Inner surface of operculum. _b._ Exterior of ditto.
]
[Sidenote: Cases 12–18.]
The _Trochidæ_, and _Turbinidæ_ are two extensive families, the animals
of which are very much alike, and mainly distinguished by the operculum,
which in the former is horny, and shelly in the latter. The shells of
these families are beautifully pearly within, and the external shelly
coat is generally brightly coloured and highly ornamented. Several very
pretty species are found on our own shores. The opercula of _Turbo
petholatus_ (Fig. 6), from the Indian and Pacific Oceans, are frequently
mounted in gold and silver as scarf-pins, ear-rings, &c.
[Sidenote: Cases 18–20.]
The _Nerites_ are mostly found in tropical countries, and, like the
Winkles, are very strongly made, to resist the force of the breaking
waves. The _Neritinas_ are partly found in the sea, and partly in fresh
water, and are less solid shells. The third section of _Neritidæ_, the
_Septariæ_, are shaped very much like Limpets, except that the apex is
at one end instead of central. They are, however, very different
animals, and furnished with a shelly operculum imbedded in the foot.
Order 2.—PECTINIBRANCHIA.
[Sidenote: Cases 22–94.]
In most cases the molluscs of this order have an attached monopectinate
gill and a single osphradium. A few are fresh or brackish water forms,
but the majority are marine.
[Sidenote: Case 22.]
The “River-Snails” (_Viviparidæ_) might be termed freshwater
Periwinkles, as the animals of both are very similar. The true
_Viviparæ_ are viviparous. They are rather sluggish, and found at the
bottom of ponds and rivers feeding on decaying animal and vegetable
matter.
[Illustration:
Fig. 7.
The Common British River-Snail (_Vivipara vivipara_).
_a_, head; _b_, tentacles; _c_, eyes; _d_, foot; _e_, operculum.
]
[Sidenote: Cases 23–25.]
The _Cyclophoridæ_ are land-shells, which, however, cannot properly be
considered true lung-breathers like ordinary snails. They have not the
closed lung-chamber of the Pulmonates, their eyes are placed at the base
of the tentacles instead of at their tips, they have a long proboscis
armed with a different rasping tongue (_radula_), a spiral operculum,
and the sexes are distinct, whereas the true Snails are hermaphrodite.
The operculated air-breathers have been divided into many sections,
chiefly on account of differences in the apertures of the shells and in
the opercula. They most abound in hot countries, but a few species are
met with in temperate regions.
[Sidenote: Cases 25–26.]
The “Apple-Snails” (_Ampullariidæ_) live in the rivers and marshes of
tropical regions, and, although represented by a large number of
species, exhibit comparatively slight variations in form and colour. The
animal has both a pectinated gill and a lung cavity, being thus enabled
to breathe either water or air.
[Illustration:
Fig. 8.
_Ampullaria canaliculata._
]
[Sidenote: Cases 27–28.]
The “Periwinkles” (_Littorinidæ_) are found almost on every known shore;
they feed upon all kinds of marine vegetation. Some species are met with
at low-water mark, others on rocks almost beyond the reach of the sea,
and some have been discovered inland nearly half a mile away from the
shore. It is calculated that 1900 tons of the “Common Periwinkle”
(_Littorina littorea_), of the value of £15,000, are annually consumed
in London alone.
[Sidenote: Cases 31–32.]
The family of _Calyptræidæ_ includes the “Slipper-Limpets” (_Crepidula_)
and the “Cup-and-saucer Limpets” (_Crucibulum_). Although furnished with
a foot, they rarely crawl about, but remain attached to rocks, stones,
or other shells, sometimes forming a shelly plate under the foot by
which they become fixed to the spot where they have taken up their
abode.
[Sidenote: Cases 32–35.]
The “Cowry-shells” (_Cypræidæ_) are remarkable for their varied markings
and splendid polish, which is produced and preserved by two flaps of the
mantle, one on each side, which fold over the back, a line down the
centre of which usually marks where the flaps meet. The animals are even
more brilliantly coloured than the shells. They have no operculum, but a
large foot, which they can withdraw entirely within their shell,
although the aperture is usually very narrow. Cowries, as is well known,
are sold as ornaments; and a small yellow species, “the money-cowry”
(_C. moneta_), which is very common in the Indian and Pacific Oceans,
passes current as coin among the negro tribes of certain parts of
Africa. The specimen of _Cypræa leucodon_ figured on p. 16 is extremely
valuable and supposed to be the only one hitherto discovered. The
“orange cowry” (_Cypræa aurora_) is worn by chiefs in the Friendly
Islands, and is considered the highest order of dignity. Only one small
species, _Trivia europæa_, is found on the British coast, and about 100
fossil forms have been discovered in the Chalk.
[Illustration:
Fig. 9.
The Tiger Cowry (_Cypræa tigris_). (From the Indo-Pacific Ocean.)
_a_, the shell; _b_, the mantle; _c_, foot; _d_, siphon; _e_,
proboscis; _f_, tentacles; _g_, eyes.
]
[Sidenote: Case 35.]
Of the _Ovulidæ_, the most curious is the “Weaver’s-shuttle” (_Radius
volva_), in which the shell is peculiarly beaked at both ends. It is
found living on barked corals (_Gorgoniidæ_), and some of the smaller
species exhibit differences of coloration, resembling the tints of the
Gorgonias upon which they are found.
[Sidenote: Cases 36–37.]
The Naticas are mostly blind, and have a very large foot, suitable for
burrowing in the sand when in quest of bivalves. They are very
voracious. This is one of the groups of shells that have continued to
exist from Palæozoic times.
[Sidenote: Case 38.]
The “Violet Snails” (_Ianthinidæ_) are found floating about in every
ocean, excepting in cold regions, with the spire of the shell downwards,
and the bottom, being more exposed to the action of light, is more
deeply tinted than the upper part. They feed upon Jelly-fish, and
construct a gelatinous raft, filled with air-bubbles, beneath which the
females attach their eggs.
[Illustration:
Fig. 10.
_Cypræa leucodon_. Case 32.
]
[Sidenote: Cases 38–41.]
The _Melaniidæ_ are freshwater Snails which abound in most tropical and
subtropical countries; about 1000 species are known. They are mostly of
dark colours, and are fond of muddy places.
[Sidenote: Cases 42–43.]
The _Cerithiidæ_ are chiefly marine forms, some, however, entering
brackish water. About five hundred fossil species have been described,
some of them gigantic in comparison with any now living, of which more
than two hundred are known.
[Sidenote: Case 44.]
The _Scala scalaris_ was formerly considered a great rarity, as much as
£40 having been given for a single specimen, which might now be
purchased for as many pence.
[Illustration:
Fig. 11.
_Scala scalaris._
Case 44.
]
[Sidenote: Cases 46–47.]
The “Worm-shells” (_Vermetidæ_) are a very peculiar family. Their shells
can scarcely be distinguished from the shelly tubes which are formed by
certain species of marine worms, _Serpula_, &c. They are free and spiral
in early life, but afterwards become distorted and generally attached to
rocks, stones, &c. A foot for walking purposes therefore would be of no
use; consequently it is more or less obsolete, serving only as a support
to the operculum.
[Sidenote: Case 48.]
The “Screw-shells” (_Turritellidæ_) have elongate tapering shells; about
100 recent and 200 fossil species are known. One species only
(_Turritella communis_) is now found living on the British coasts.
[Sidenote: Case 49.]
The _Xenophoridæ_ have the singular habit of cementing to the exterior
of their shell, stones, pieces of coral, and fragments of other shells;
hence they have been called “Carrier-shells,” and, according to the kind
of material chosen, have been named “Conchologists” and “Mineralogists.”
Beyond acting as a disguise, and consequently as a protection, there
does not appear to be any special utility in thus adding to the weight
of their own shells. The animals do not glide like most other molluscs,
but scramble along like the Strombs, the form of their foot being small,
divided into a front, expanded, and a hind, tapering portion admirably
adapted to the nature of the ground on which they live, which usually
consists of broken and dead shells.
[Sidenote: Cases 49–52.]
The “Wing-shells” (_Strombidæ_) are the largest of the Gastropods with a
proboscis or non-retractile snout. They do not crawl like most other
Gastropods, but progress by a sort of hopping movement. They act as
scavengers, feeding on decomposing animal matter.
The _Strombus gigas_, or “Fountain-shell,” occurs in great numbers in
the West Indies, and is a very heavy solid shell. It is a favourite
ornament for rockwork and fountains in gardens, and, like the
Helmet-shells, is used for cameo-carving. It is also employed in the
manufacture of porcelain, as many as 300,000 having been imported into
Liverpool in one year for that purpose.
[Sidenote: Cases 51–52.]
The Scorpion-shells, or “Spider-claws,” as they are sometimes called
(_Pterocera_), possess singular claw-like projections, which are
developed on the outer lip of the shells.
[Sidenote: Cases 53–54.]
The “Trumpet-shells” (_Lotoriidæ_) have varices or strengthening ribs at
intervals, like the Murices; the largest species, _Lotorium variegatum_,
is used by South-Sea Islanders as a horn or trumpet. A hole is made in
the upper part of the spire to blow through, and the sound produced can
be modulated or varied by inserting the hand in the aperture or mouth of
the shell.
[Sidenote: Cases 55–56.]
The “Helmet-shells” (_Cassididæ_) are used for cameo-carving; they
consist of differently coloured layers, so that the ground-colour of the
carving is of a different tint from the subject engraved. The most
artistic shell-cameos are produced in Italy, whence the art has been
introduced into France and England. The _Cassis madagascariensis_ (Fig.
12 on p. 19) is in special request by shell-carvers on account of the
strong contrast of the white upper layer with the dark ground beneath.
Extinct forms of _Cassis_ are found fossil in Tertiary formations, but
none of them equal in size the largest living species.
[Sidenote: Case 57.]
The “Tun-shells” (_Doliidæ_) are remarkable for the globoseness of the
shells, which are covered with very regular revolving ribs.
[Sidenote: Cases 58–60.]
The _Fasciolariidæ_ contains two of the largest living Gastropods:
_Megalatractus aruanus_, from North and West Australia, and _Fasciolaria
gigantea_, which is found off the coast of South Carolina, and attains
at times a length of two feet.
[Sidenote: Cases 61–64.]
The Mitras (_Mitridæ_) are great favourites with shell-collectors, on
account of their beautiful colours and varied sculpture. There are about
600 living species already known, and between one and two hundred have
been found in a fossil state. Shells of this group, like the
_Fasciolariæ_, are distinguished by a few plaits or folds on the inner
side of the aperture (the columella). Mitras are almost exclusively
found in tropical or subtropical regions, the majority being met with
either at low-water mark or in comparatively shallow water.
[Sidenote: Cases 64–66.]
The family of _Buccinidæ_ also contains a very large and various
assemblage of forms. Among them may be mentioned the Whelks
(_Buccinum_). (See Fig. 13 on p. 19.)
[Illustration:
Fig. 12.
_Cassis madagascariensis_, with cameo engraved upon it.
Side table-case B.
]
[Illustration:
Fig. 13.
The Common Whelk (_Buccinum undatum_).
_a_, siphon; _b_, foot; _c_, tentacles; _d_, eyes; _e_, operculum.
]
[Sidenote: Cases 70–76.]
The family of _Muricidæ_, or “Rock-shells,” is another extensive group,
containing many very handsome and peculiar forms. The animals of this
family have a long proboscis, at the end of which is the spiny tongue
(radula), and which is retractile within the body. The true Murices
produce at intervals ribs or varices, which in some species are
ornamented with long spines or foliations, and which indicate periods of
growth, but of what duration we do not know. They are all carnivorous,
feeding chiefly on other Mollusca, boring through the shells of bivalves
with their spiny tongue, and slowly devouring the unfortunate inhabitant
piecemeal. From certain species of _Murex_ (_M. brandaris_, &c.) found
in the Mediterranean, the ancients manufactured the celebrated Tyrian
purple dye.
[Sidenote: Cases 74–75.]
The “Purples” (_Purpura_) are found between tide-marks all over the
world. _Magilus_, belonging to the family _Coralliophilidæ_ (Case 77),
is found among coral-reefs in tropical seas, and has the remarkable
habit of lengthening the aperture of its shell into an elongate tube, in
order to keep pace with the growth of the coral, and to prevent its
being overgrown and killed.
[Sidenote: Cases 78–81.]
The “Volutes” (_Volutidæ_) are a group of shells also much sought after
by shell-collectors. Some of these attain to a very large size, the
animals inhabiting them being enormous. The Boat-shells (_Cymba_) and
Melons (_Cymbium_) are ovo-viviparous, the young being carried about by
the parent until they are an inch in length. Volutes are found chiefly
in the warmer parts of the Atlantic and Indo-Pacific Oceans, and occur
in the greatest variety on the coasts of Australia.
[Sidenote: Cases 81–83.]
The Olives (_Olividæ_) are common in most tropical seas, and are
remarkable for their beautiful polish and various patterns of colouring.
In structure and form they are very similar to each other. They burrow
in sand in quest of bivalves for food, and some species are said to have
the power of swimming by expanding the lobes of the foot.
[Sidenote: Case 84.]
The Harps (_Harpidæ_) form a small well-marked group, of which probably
nearly all the existing species have been discovered. The animals
inhabiting these beautiful shells are also brightly coloured. They have
the remarkable power of casting off a portion of the foot when
disturbed. The species are known from the Indo-Pacific Ocean, the west
coast of Central America, and West Africa.
[Sidenote: Cases 85–87.]
The next family, the “Slit-lips” (_Pleurotomatidæ_), consists of very
numerous species, over a thousand living forms having been discovered,
and almost as many fossil species from Cretaceous and Tertiary strata
have been described. The typical forms are characterized by a slit in
the outer side (lip) of the aperture. Species of _Pleurotoma_ are found
in every sea, although most abundant in the tropics, and, although so
numerous in species, the number of specimens is small in comparison with
some other genera.
[Sidenote: Cases 87–89.]
The “Auger-shells” (_Terebridæ_), like the Cones, present a great
similarity in form, but, unlike them, have a great diversity of
“_sculpture_” or external ornamentation. They are all elongate shells,
with a deep notch at the base of the aperture. Owing to the length and
comparative solidity of the shells, the animals of many of the species
do not carry their shelly structures on their backs, like most other
species, but drag them along the sandy sea-bottom.
[Illustration:
Fig. 14.
The “Glory-of-the-Sea” Cone (_Conus gloria-maris_). Case 94.
(From the Philippine Islands.)
]
[Sidenote: Cases 89–94.]
The _Conidæ_, or Cones, form one of the most beautiful portions of the
collection of Shells. This family, of which between 400 and 500 distinct
kinds are known, is a great favourite with collectors on account of the
brilliant colours and various patterns of the shells. Some, owing to
their beauty and rarity, have been sold at very high prices, as much as
£50 having been paid for a single shell. The Cones are found in all
tropical seas, but are rare in cold or temperate latitudes. None are met
with on our own shores, one species alone being known from the
Mediterranean. They occur fossil in the Chalk and Tertiary strata. These
animals are all carnivorous, and live usually in shallow water among
rocks and coral-reefs. Some of them are said to bite when handled, and
to be dangerously poisonous, the bite in some instances having been all
but fatal.
[Sidenote: Case 94.]
The _Atlantidæ_, _Pterotracheidæ_, and _Carinariidæ_, at various times
recognized as forming a distinct sub-class or order of Gastropoda, under
the name of Heteropoda or Nucleobranchiata, are now regarded as families
of aberrant Gastropods organised for swimming in the open sea. The
_Atlantas_ are found in great numbers in warm latitudes, and are
provided with a glassy, thin, flat, spiral shell, not unlike a keeled
Ammonite. The glassy shell of the _Carinaria_ is one of the most
beautiful structures of any mollusc, and at one time was such a rarity
that £100 are said to have been given for a single specimen, which at
the present time is perhaps worth only from five to ten pounds. Species
of _Carinaria_ are found in the Mediterranean and warmer parts of the
Atlantic and Indian Oceans. The animal is large, semitransparent, and
elongate, with a compressed fin-like foot which projects from the body,
and is used in swimming. The gills are placed towards the hinder part of
the back and covered by the shell. They feed on jelly-fish of various
kinds, and probably on other soft animals.
[Illustration:
Fig. 15.
Glassy Nautilus (_Carinaria lamarcki_).
_a_, proboscis; _b_, tentacles; _c_, shell; _d_, gills; _e_, foot;
_f_, sucker.
]
Section EUTHYNEURA.
[Sidenote: Cases 94–135.]
The Gastropods belonging to this sub-class have the visceral nerve-loop
straight and not twisted as in the STREPTONEURA. All the EUTHYNEURA are
hermaphrodite, and their radula is generally composed of numerous
similar denticles on each side of a median tooth. Scarcely any of these
forms are provided with an operculum in the adult state. The EUTHYNEURA
may be divided into two orders, _Opisthobranchia_ and _Pulmonata_.
Order 1.—OPISTHOBRANCHIA.
[Sidenote: Cases 94–97.]
All the Molluscs of this order are marine, some (_Tectibranchia_)
breathing by means of the ordinary Gastropod ctenidium, which is
generally behind the heart, whereas others (_Nudibranchia_) have
developed a different type of respiratory organs.
The _Opisthobranchia_ include the “Pteropods” formerly considered as a
distinct class, the “Bubble-shells” (_Bullidæ_), the “Sea-Hares”
(_Aplysiidæ_), the “Umbrella-shells” (_Umbraculidæ_), the _Nudibranchs_
and some others.
[Illustration:
Fig. 16.
Shell-bearing Pteropod (_Cavolina tridentata_). Case 96.
_a._ Shell and animal. _b._ Side view of shell. _c._ Dorsal view of
shell.
]
The Pteropods[4] are sometimes called Sea-butterflies, and are organized
for swimming freely in the ocean. They have a pair of fins developed
from the sides of the mouth or neck, which perform a flapping movement
during progression. Some Pteropods (Thecosomata) are provided with small
glassy shells; others (Gymnosomota) are naked. They exist in countless
millions in some parts of the ocean, discolouring the water for miles.
They constitute the principal food of the Baleen Whales.
[Illustration:
Fig. 17.
Shell-less Pteropod (_Clione limacina_).
_a._ Dorsal view. _b._ Ventral aspect.
]
About a hundred species are known.
[Sidenote: Case 96.]
The Sea-Hares, so called on account of a slight resemblance to a
crouching hare and not for their nimbleness of foot, are found in most
parts of the world, in pools at low water. At the hinder part of the
back two flaps of the mantle partly conceal a thin horny shell which
serves as a protection to the gills and vital organs beneath. When
molested, these animals discharge a large quantity of a purple fluid,
discolouring the surrounding water for a distance of more than a yard.
[Illustration:
Fig. 18.
Sea-Hare (_Tethys (Aplysia) punctata_). British.
_a_, labial tentacles; _b_, upper tentacles or rhinophores; _c_,
siphonal fold of the mantle near the shell; _d_, eye.
]
[Sidenote: Case 97.]
The shell of _Umbraculum_ is shaped very like that useful article, an
umbrella, of the Chinese pattern. The animal is very large, having its
breathing-organs on the right side below the shell.
[Sidenote: Case 97.]
The _Nudibranchs_ or Naked-gilled Molluscs comprise some of the most
beautiful and strange forms. They are unprovided with shells except in
the earliest stages of their existence, when they dwell in a minute
nautiloid shell, furnished with an operculum, both of which are
subsequently cast off. Unfortunately the colours of these beautiful
creatures cannot be preserved after death, and therefore a small series
of glass models is exhibited, which will give some idea of their great
variety in form and colouring. They are found in most parts of the
world, chiefly in shallow water, but a few species live upon floating
seaweed in the open sea. Over a hundred species exist on the British
coast, the majority of which are, however, very small. They are chiefly
carnivorous, feeding on other molluscs, sea-anemones, &c.
[Illustration:
Fig. 19.
The Umbrella-shell (_Umbraculum mediterraneum_).
_a_, shell; _b_, gills; _c_, tentacles; _d_, mouth; _e_, foot.
]
[Illustration:
Fig. 20.
Naked-gilled Mollusc, or Nudibranch (_Doto coronata_).
_a_, head; _b_, foot; _c_, gills; _d_, tentacle-sheath; _e_, tentacle.
]
Order 2.—PULMONATA.
[Sidenote: Cases 97–135.]
The Pulmonata are furnished with a lung cavity in place of the ordinary
gill of other Gastropods, and may be termed true air-breathers. Most of
them are provided with shells, and, with the exception of the
_Amphibolidæ_ never possess an operculum. They are divisible into two
groups or sub-orders, _Basommatophora_ and _Stylommatophora_,
characterized by a difference in the position of the eyes. The
Basommatophora, including the _Auriculidæ_, _Amphibolidæ_,
_Siphonariidæ_, and _Limnæidæ_, have a single pair of non-retractile
tentacles, at the base of which are situated the eyes. The
Stylommatophora (Land-snails, Slugs, &c.), are provided with two pairs
of retractile tentacles, with the eyes at the summit of the upper pair.
Over ten thousand species of Pulmonata are known.
(_Basommatophora._)
[Sidenote: Cases 97–98.]
The first group of the aquatic air-breathers, the _Auriculidæ_, chiefly
inhabit salt or brackish water. The largest forms are tropical and found
at the mouths of rivers, among the roots and stems of mangrove-trees, or
in damp woods near the sea.
[Sidenote: Cases 98–99.]
The “Limpet-Snails” (_Siphonariidæ_) seem at first sight to be out of
place among the Snails and Slugs, and more nearly allied to the
Rock-Limpets; but the character of the tongue (radula) and the closed
respiratory cavity indicate a close relationship with the present group.
[Illustration:
Fig. 21.
Three Rows of Teeth of the Radula of _Siphonaria_.
_c_, central; _l_, lateral teeth.
]
The shells of _Siphonaria_ may be known from Limpets by a slight bulging
on one side, caused by a radiating groove which interrupts the muscle of
attachment. They are marine, and are found on rocks between tide-marks,
chiefly in tropical countries.
[Sidenote: Cases 99–101.]
The _Limnæidæ_ are only found in fresh water. Most of them occasionally
rise to the surface to breathe, where they glide along foot uppermost,
at times suspending themselves by a glutinous thread, after the fashion
of a spider. All countries appear to have their peculiar species.
The freshwater Limpets (_Ancylus_) live attached to stones and leaves of
plants, and have not the habit of floating, but, like the rest of the
_Limnæidæ_, feed on freshwater algæ, confervæ, and decayed vegetable
matter.
[Illustration:
Fig. 22.
British Pond-Snail (_Limnæa stagnalis_).
1. Upper view: _a_, foot; _b_, tentacles; _c_, eye; _d_, muzzle.
2. Lower view: letters _a_, _b_, _c_ as above; _e_, mouth; _f_,
respiratory orifice.
]
(_Stylommatophora._)
[Sidenote: Cases 102–135.]
True Snails (_Helicidæ_, etc.) have a distinct head furnished with eyes,
tentacles, cutting upper jaws, and rasping teeth, and all are protected
by a spiral shell. They are almost exclusively vegetable-feeders,
subsisting chiefly on leaves. The sexes are not distinct. Many of the
species are beautiful objects on account of the brilliancy of their
colouration, and some are remarkable for the variation they exhibit in
this respect. Species of _Helicidæ_ are found in nearly every part of
the world and in all situations, from sea-level to an altitude of 12,000
feet. They are fond of moisture, and in hot and dry weather retire
within their shells, remaining torpid until the return of dew and rain.
_Helix pomatia_ (Case 119), which is found on the chalk in the south of
England and on the Continent, is commonly eaten in Austria, France, and
Belgium.
The eggs of Land-Snails vary in texture, size, and in numbers: they are
usually white, but in some instances yellow and pale green. Those of
some of the large South-American forms are as hard as that of a hen, and
more than an inch in length (Case 120).
[Illustration:
Fig. 23.
British Land-Snail (_Helix pomatia_).
_a_, eye-bearing tentacles (“horns”); _b_, lower or smaller tentacles.
]
Slugs (Cases 106,107) are very like Snails without external shells; most
of them, however, possess a small internal shelly plate, or a few
calcareous granules hidden beneath the skin of the back. Some have a
large slime-pore at the end of the foot, and others are slightly
phosphorescent. Like the Snails, they are fond of damp localities, and
at times become great pests to farmers in devouring the young shoots of
the growing corn. _Testacella_, which is found in this country, differs
from the Slugs in having an external shell at the tail-end of the foot.
It is not slimy, and lives under ground, feeding upon earthworms.
Class III.—SCAPHOPODA.[5]
[Illustration:
Fig. 24.
British Tooth-shell.
(_Dentalium tarentinum_).
_a._ The shell. _b._ The animal, removed from its shell; _f._ the
foot.
]
[Sidenote: Case 136.]
The “Tooth-shells” (_Dentaliidæ_) form a distinct group, the shells of
which are very unlike those of any other mollusc, but closely resembling
the shelly tubes constructed by certain kinds of marine worms. The
_Dentalia_ have neither eyes nor tentacles, or a distinct head like
Gastropods; their organs of circulation and respiration are of a
rudimentary kind, and they have no heart. The sexes are separate. Their
foot is adapted for burrowing in sand, in which they live and obtain
their food, which consists of _Foraminifera_ and minute Bivalves. One
species, _Dentalium pretiosum_, found on the shores of North-West
America, was until recently used as money by the Indians.
Class IV.—LAMELLIBRANCHIA.[6]
[Sidenote: Cases 137–204.]
The Molluscs belonging to this Class have neither head, nor cephalic
eyes, nor jaws or tongue like those of the other Classes, and are
enclosed in a shell which consists of two plates or valves held together
on one side of the margin by a horny, elastic substance, called the
“_ligament_.” Bivalves do not creep about in search of food, but find
their means of existence in the shape of minute particles, both animal
and vegetable, which happen to be contained in the water which they
breathe. Some, however, are capable of locomotion by means of a
well-developed foot, and a few swim through the water by alternately
opening and shutting their valves. The body is enclosed within two lobes
of the mantle which line the interior of the valves, and which at their
base are firmly attached to the shell, producing on the shell a scar or
impression called the “pallial line.” The gills are lamellar or
leaf-like, and placed on each side of the body. Each gill is called a
ctenidium, and consists of an axis which is partly attached to the body
of the Mollusc. This axis generally gives off two plates consisting of
hollow filaments which are parallel with one another, directed downwards
towards the ventral side, and in most cases long and refolded upon
themselves, so that each plate becomes in reality a double lamella. In a
few instances, however, the filaments are simple and not reflected. They
are connected with one another by microscopic cilia, sometimes by
vascular junctions, and the dependent and reflected portions (lamellæ)
of each filament may be connected by “interlamellar vascular junctions.”
The mouth is merely an oval aperture at the anterior end of the body,
and generally furnished on each side with soft thin flaps, or labial
palps, which have the function of conveying the food to the mouth. The
mantle secretes the substance out of which the shell is formed. The two
valves are always in contact at the _hinge_, which is generally formed
by small interlocking projections or hinge-teeth, and they are closed by
large adductor muscles, which are attached to impressions in the
interior of the shell. When these muscles cease to act, as after death,
the valves of the shell open in consequence of the elasticity of the
ligament on the dorsal margin. The majority of species have two
principal adductors, one at each end, like the Venus-shells, Cockles,
Razor-shells, &c.; but in Oysters, Scallops, and a few others, there is
but a single central muscle. All Bivalves are aquatic, and the majority
marine. They are found burrowing in sand or attached to rocks. Some
perforate stones and corals, others wood, and a few construct a sort of
nest of fragments of shells, stones, &c.
Many schemes of classification have from time to time been propounded,
based upon the presence or absence of respiratory siphons, the number
and position of the adductor shell-muscles, the character of the
shell-hinge, &c. The most recent arrangement is founded principally upon
the structure of the gills. The value of such a classification has yet
to be fully tested. Mr. Paul Pilseneer has suggested five orders of
Lamellibranchs: Protobranchia, Filibranchia, Pseudolamellibranchia,
Eulamellibranchia, Septibranchia.
[Illustration:
Fig. 25.
(From the ‘Cambridge Natural History.’ Messrs. Macmillan & Co.)
A. Protobranchia. B. Filibranchia. C. Eulamellibranchia. D.
Septibranchia.
_m._ Mantle, _v._ Body. _f′._ Foot. _e._ Outer gill-lamella; _i._
Inner gill-lamella; _e′._ Reflected portion of outer lamella; _i′._
Reflected portion of inner lamella; _s._ Septum-like gill.
]
[Illustration:
Fig. 26.
Gill of _Mytilus edulis_.[7]
A. Part of four filaments showing ciliated interfilamentar junctions
(cj).
B. Diagram of a single filament showing the two lamellæ connected at
intervals by interlamellar junctions (ilj) and the position of the
interfilamentar ciliated junctions (ep).
]
PROTOBRANCHIA. (Fig. 25, A.)
[Sidenote: Case 137.]
In this order the filaments of the gills are not reflected, but arranged
in two divergent rows, the foot being expanded and flattened beneath
with crenulated margins and with the byssal gland very slightly
developed. The _Nuculidæ_ and _Solenomyidæ_ are the only families
belonging to this order. The shells of the former are remarkable for the
numerous fine interlocking hinge-teeth, and those of the latter on
account of the strong fringed periostracum.
FILIBRANCHIA. (Fig. 25, B.)
[Sidenote: Cases 137–145.]
In this group the gills are smooth, with the filaments directed
downwards, reflected, and connected one with another by interfilamentar
ciliated junctions, but the lamellæ are not connected. The foot is
usually provided with a well-developed byssal gland. _Anomia_, _Arca_,
_Trigonia_, and _Mytilus_ belong to this order.
[Sidenote: Cases 137–138.]
The family of _Anomiidæ_ contains a number of more or less pearly shells
remarkable for a deep notch or hole in the lower or flat valve through
which a shelly plug passes, by means of which the animal attaches itself
to other shells, stones, &c. _Anomia ænigmatica_ is found adhering to
leaves in mangrove-swamps.
[Sidenote: Case 138.]
The _Placunidæ_, sometimes called Window-shells and Saddle-Oysters, are
very flat pearly shells with a remarkable hinge, which consists of two
long divergent teeth, like a =⋀=, to which the ligament is attached. The
species are few in number, and inhabit sandy shores of India, China, and
North Australia.
[Sidenote: Cases 139–141.]
The _Arcidæ_ are a family of strong ponderous shells varying much in
form and sculpture. The animals have a longish pointed foot, deeply
grooved along the bottom, no labial palpi, and free margins to the
mantle, which are not prolonged into breathing-siphons. Many of the Arks
often anchor themselves by means of a strong byssus. The shells of this
family are usually radiately ridged; and the hinge is composed of a
number of teeth arranged along the hinge-line, which is generally
straight. _Arca tortuosa_, from China, has the valves curiously twisted.
The section _Barbatia_ is remarkable for the coarse fibrous character of
the epidermis; _Scapharca_ for its unequal valves; and _Cucullæa_, from
the Indian Ocean, for the elevated ridge bounding the posterior muscular
impression. _Glycymeris_ (better known as _Pectunculus_) has the
hinge-teeth arranged in an arched series, and the shells are more
regular in growth than in many other forms of _Arcidæ_.
[Sidenote: Case 141.]
The _Trigoniidæ_ are one of those families which have all but
disappeared during our period. Only three or four living species are
known, whilst more than a hundred fossil forms have been described from
the Jurassic and Cretaceous formations. Australia, where some of the
oldest types of animal life persist, furnishes also the existing species
of _Trigonia_ (Fig. 27). The animals have a long, sharply-bent, pointed
foot like the Cockles, with which they make surprising leaps. The shells
are beautifully pearly within, and ribbed and noduled exteriorly.
[Illustration:
Fig. 27.[8]
_Trigonia margaritacea._
Case 141.
]
[Sidenote: Cases 142–145.]
The _Mytilidæ_, or Mussels, are too well known to need description. The
small foot, which is brown in the common species, is not much used in
creeping about, but has the power of spinning a byssus or bundle of
tough threads, by means of which the animals attach themselves to rocks
and one another, forming colonies of vast numbers. Mussels have always
been much eaten in this and other maritime countries, and large
quantities are brought to the London market from the Dutch coast. At
times they are unwholesome; but all the exact causes of this are not
known. Mussels seem to be found on every shore, and some of the species
are very widely distributed—the common edible Mussel, _M. edulis_, being
found on every European coast, on the shores of North and South America,
in the Arctic and Antarctic Oceans, and probably on the coasts of
Australia.
One group of Mussels (_Lithodomus_, Case 144) burrow in rocks and other
shells, forming holes just large enough to contain their shells. _L.
dactylus_ is sold as an article of diet on the shores of the
Mediterranean.
PSEUDOLAMELLIBRANCHIA.
[Sidenote: Cases 145–160.]
The gills in this order are plicate, and the two lamellæ of each plate
are furnished with conjunctive or vascular interlamellar junctions, and
the filaments are connected by interfilamentar ciliated discs in some
cases, in others by vascular concrescence. The mantle-margins are
separated all round, and the foot is either small or wanting. Only a
single adductor muscle is generally present. The Pearl-oysters, true
Oysters, and Scallops are the forms which constitute this order.
The large family of _Aviculidæ_ includes the “Wing-shells” (_Avicula_),
the “Pearl” and “Hammer Oysters” (_Meleagrina_ and _Malleus_), and
_Pinna_ (Fig. 28). Some species of _Pinna_ attain to a length of two
feet. They are found imbedded in the sand with the narrow pointed end
downwards. They form a large silky byssus, which can be woven or knitted
into gloves, socks, etc. (see side table—case B). The “Hammer-Oyster”
(_Malleus_) is so called from its rude resemblance to a hammer. The
“Pearl-Oysters” (_Meleagrina margaritifera_, Fig. 29) possess rather
heavy strong shells, lined with very thick layers of “mother-o’-pearl.”
Hundreds of tons of these shells are annually collected at the great
pearl-fisheries of North and West Australia, and imported into Europe.
The pearl-oyster of Ceylon (_M. fucata_, Case 146) is a smaller species,
and collected more for the pearls than the shells. The round pearls,
which are valued so highly, are either excrescences of the pearly layer
or are found loose in the fleshy parts of the animal. Some small foreign
body which has accidentally penetrated under the mantle and irritates
the animal is covered with successive concentric layers of nacre, thus
attaining sometimes, but rarely, the size of a small filbert. The nacre
is generally of the well-known pearly-white colour, very rarely dark,
and occasionally almost black. The action of the animal in secreting
successive layers of nacre over any foreign body which intrudes between
the mantle-folds, and thus converting it into a pearl, is strikingly
illustrated by two specimens in which, in the one case, an entire fish,
and in the other a small crab, have been so enclosed (see side
table-case E).
[Illustration:
Fig. 28.
British “Fan-Mussel” (_Pinna pectinata_): _a_, the byssus. Case 150.
]
[Illustration:
Fig. 29.
Pearl-Oyster (_Meleagrina margaritifera_). Case 146.
]
The most ancient and, even at the present day, one of the most important
of the pearl-fisheries is that carried on on the western shores of
Ceylon. “The Banks,” or spots on which the oysters grow, are at an
average depth of 30 to 60 feet, and extend several miles along the
coast. The oysters, which should be six or seven years old when
collected, are gathered in baskets by native divers and hauled up by
ropes into hundreds of small boats. The shells are then brought to land
and placed upon the ground to die and putrefy, and then minutely
examined for the pearls, which are either found loose in the shells or
imbedded in the fleshy parts of the oysters. As many as two million
oysters have been brought ashore on one day; but the number obtained
varies very much according to the state of the banks. A small proportion
of the oysters contain pearls; in some only very small ones (seed or
dust-pearls as they are called) are found, and very few contain pearls
larger than a pea, which are so highly valued. In his account of the
pearl-fishery of Ceylon the Rev. James Cordiner says that he saw the
operation of sorting the pearls performed; the produce of 17,000 oysters
weighed only ¾ lb. and was contained in a vessel smaller than a common
soup-plate. Out of that quantity there were not found two fine perfect
pearls; all of the largest were slightly deformed, rugged and uneven,
but of the smaller sizes many were round and perfect. The chief
qualities which regulate the value of pearls are size, roundness, and
brilliancy of lustre. Of the smallest kind several may be bought for a
shilling, whilst many thousand pounds have been given for a single fine
pearl of surpassing beauty.
Other important pearl-fisheries besides that of Ceylon are carried on in
the Persian Gulf, on the west coast of Central America, and especially
North-west Australia, where diving-dresses are now employed in
collecting the shells.
The Chinese obtain pearls artificially from a species of freshwater
Mussel (_Dipsas plicata_). In order to do this they keep them in tanks
and insert between the shell and the animal either small shot or small
round pieces of mother-of-pearl, which soon receive regular coatings of
nacre and assume the look of ordinary pearls. They also insert small
metal images of Buddha, which also soon become covered with pearl and
firmly cemented to the shell, the production being to the uninitiated a
supernatural testimony to the truth of Buddhism. (A shell treated in
this way is exhibited in one of the small cases (E) at the side of the
room.)
[Sidenote: Cases 152–154.]
The _Ostreidæ_, or Oysters, undoubtedly take the first rank among
molluscs as regards usefulness to mankind as an article of food. They
have no foot; the mantle is entirely open, with double edges, each being
bordered by a short fringe, and the labial palps are large and somewhat
triangular. There are on each side a pair of simple gills, which appear
closely striated; the single adductor muscle is large and nearly central
(see Fig. 30). The Oyster is, except in the very young state, entirely
incapable of locomotion, and always attached by the deeper valve to
other shells, rocks, or other substances. The common British species is
not full-grown until it is about five or seven years old. A series of
different ages, from the “spat” to the adult form, is exhibited in Case
152. During the months of May, June, and July the eggs are discharged
into the gills, where they remain until hatched; and it is during this
period that oysters are “out of season.” In the American Oyster (_O.
virginica_), on the contrary, the eggs are said to be hatched outside
the parent shell. Oysters of different kinds are found on nearly every
shore. The gigantic _O. gigas_ is said to grow to the length of three
feet in the Bay of Taichou, Japan, where it is commonly eaten. About two
hundred fossil species have already been described.
[Sidenote: Cases 154–156.]
The _Spondylidæ_, or Thorny Oysters, closely resemble the Scallops, but
the shells are more spiny, heavier, united by interlocking teeth, and
one of the valves is attached to rocks, corals, etc. Many of the species
are very brightly coloured; and from the fact that small quantities of
water are sometimes enclosed in cavities in the inner layer of the
shell, they have been called “Water-Clams” or “Water Spondyli.”
[Illustration:
Fig. 30.
Common Edible Oyster (_Ostrea edulis_).
_a_, labial palpi; _b_, gills; _c_, mantle; _d_, junction of the two
folds of the mantle; _e_, large adductor muscle; _f_, the shell.
]
[Sidenote: Case 156.]
The Limas (_Limidæ_) are very like the Pectens, but the inner edge of
their mantle is fringed with very long thread-like filaments. The shells
are always white, generally more or less oblique, and radiately ridged.
They appear to be found in most seas, and either swim about freely like
the young Scallops by flapping their valves or attach themselves by a
byssus, sometimes forming a sort of nest, consisting of pieces of coral
and shell or small stones, in which they are completely concealed.
[Sidenote: Cases 156–160.]
The Scallops or Fan-shells (_Pectinidæ_) are well known for their
beautiful colours, sculpture, and excellent flavour. The animal has a
distinct foot, which is not, however, used as a locomotive organ, but
employed in spinning a byssus of attachment when required. The young
Pectens dart through the water by opening and suddenly closing their
valves. The species are very numerous, world-wide in their distribution,
and may be found at depths from a few to _three thousand_ fathoms.
EULAMELLIBRANCHIA. (Fig. 25, C.)
[Sidenote: Cases 160–204.]
In this order the gills have interfilamentar and interlamellar
_vascular_ junctions, and there are generally two adductor muscles. The
order is very extensive, comprising nearly sixty families, of which only
the more important can be referred to.
The _Carditidæ_ and _Astartidæ_ have strong solid shells, frequently
ornamented with radiating or concentric ribbing, and usually are coated
with a dark epidermis. They have the general appearance of certain
_Veneridæ_; but the animal has no prolonged siphons, but merely a
fringed opening in the mantle. One very remarkable species, _Thecalia
concamerata_ (Case 161), has an internal cup-like process within the
valves, which serves as a nursing-pouch for the young.
[Sidenote: Cases 162–163.]
The _Lucinidæ_ are almost invariably white shells, and may generally be
recognized by the very long muscular scar in front on the inner surface
of the valves. They occur in all parts of the world; and the fossil
forms, which are still more numerous than those now living, have existed
at every epoch from the Silurian.
[Sidenote: Cases 166–177.]
Of the freshwater Mussels or _Unionidæ_ more than 1200 species have been
already discovered; they are found in most parts of the world, the
greatest number having been described from North America. In _Unio_ the
edges of the mantle are not united along the bottom and not prolonged
into siphonal tubes; at the posterior end there are two openings, of
which the upper or excretal orifice is simple, and the lower or
branchial fringed at the edge. Two “cephalic eyes” have recently been
noticed by Mr. P. Pelseneer. The foot is very large and adapted for
crawling and burrowing. The sexes are distinct; and the shells of the
females are somewhat more tumid than those of the males. _Margaritana
margaritifera_, (Case 168) which is found in this country and in Europe,
sometimes produces handsome pearls, but not equal to those obtained from
the pearl-oyster of tropical seas. The hinge in this family is extremely
variable, being in some instances delicate and toothless (_Anodonta_,
Fig. 31, B), whilst in others it is enormously thickened and furnished
with strong interlocking teeth (see Fig. 31, A).
[Illustration:
Fig. 31.
]
[Sidenote: Case 178.]
The family _Ætheriidæ_, or freshwater Oysters, consists of but three
genera: _Ætheria_ contains African, and _Mülleria_ and _Barttettia_
Indian and South-American forms. When young the shells of _Ætheria_
(which are common in the Nile) are free and not unlike an _Anodonta_,
but when adult they become attached and irregular and look like an
olive-green Oyster; they are, however, provided with two muscular
impressions instead of one, as in ordinary marine Oysters. Still more
remarkable is _Mülleria lobata_ of Colombia, which, when young, freely
moves about and has two adductor muscles, but in time becomes attached
and stationary, and then possesses but a single adductor.
[Sidenote: Cases 179–181.]
The _Tellinas_ have usually thin shells, and their two siphons are
longer and more completely separated from each other than in the
_Veneridæ_. The pallial line is widely and deeply sinuated, and the
ligament generally external. In the genus _Semele_ of the family
_Scrobiculariidæ_, it is placed within the hinge-margin. They live in
great numbers beneath the sand in shallow water, and are occasionally
used as food.
[Sidenote: Cases 183–185.]
The _Mactridæ_ have an internal ligament to the hinge, the siphons are
joined together and fringed at the ends, and the pallial line is more or
less sinuated. _Spisula solidissima_ (Case 184), the largest species
found on the coast of the United States, is a common article of diet.
[Illustration:
Fig. 32.
Common British Cockle
(_Cardium edule_).
_a_, foot; _b_, exhalant siphon; _c_, branchial or inhalant siphon;
_d_, edge of mantle; _e_, ligament; _f_, umbones or beaks of the
shell.
]
The next family, _Veneridæ_, have long respiratory siphons and a
sinuated pallial line. Many of this tribe are very beautiful in form and
colouring, and most of them have very hard strong shells. The valves are
united above by an external ligament, and the hinge-plate is toothed.
Nearly all of them live buried an inch or two beneath the sand or mud,
but a few are found burrowing in rocks. Probably the majority of the
species of this family might be used as food. _Venus verrucosa_, of our
own southern shores, is frequently eaten both in this country and
abroad; and _Venus mercenaria_ (Case 189) is commonly sold in the
markets of Philadelphia and New York. _Cytherea lusoria_ (Case 185) also
forms a favourite article of diet among the poorer classes in Japan, and
several kinds are eaten by the natives of New Zealand and other
countries.
[Sidenote: Cases 192–194.]
Some of the “Cockles” (_Cardiidæ_) from warm latitudes are highly
coloured and adorned with most beautiful sculpture. Probably the
majority are eatable, as the common cockle (_Cardium edule_) of the
British coast. The foot of these molluscs is very large, bent, and used
for leaping. The siphons are short and fringed at the margins.
[Sidenote: Cases 195–196.]
The _Tridacnidæ_, or true Clams, differ from other Bivalves with united
mantle-margins in having but a central adductor muscle. In the typical
species the animal is attached to the rocks by a “byssus,” a strong
fibrous structure which passes through an aperture at the upper part of
the shell. A species found in the Red Sea, _T. elongata_, is eaten by
the natives, and the shell employed for the manufacture of lime.
_Tridacna gigas_, the largest known bivalved mollusc, sometimes weighs
over 500 lb., that exhibited in the upright cases at the entrance of the
Gallery being 310 lb. in weight. A large pair bordered with gilt copper
are used as _bénitiers_ or holy-water vessels in the church of St.
Sulpice in Paris. _Tridacnæ_ are found associated in large numbers in
lagoons, among coral-reefs in the Eastern and Pacific Seas. The animals
are described as presenting a beautiful iridescent glare of blue,
violet, and yellow variegated with fantastic markings.
[Illustration:
Fig. 33.
Left valve of the Giant Clam (_Tridacna gigas_).
Length, 36 inches. Weight, 154 lb.; weight of the two valves, 310 lb.
]
[Sidenote: Case 196.]
The genus _Chama_ consists of tropical species, which are found fixed to
corals, rocks, etc. Nevertheless, they have a small bent foot, but what
purpose it serves is difficult to conceive.
In this place attention should be called to the _Hippuritidæ_ and
_Radiolitidæ_, very remarkable extinct families of bivalved molluscs
which occur abundantly in the cretaceous strata of southern and eastern
Europe, Egypt, etc. They are remarkable for the solidity of the shells,
the relatively small space occupied by the animal, and the complicated
character of the hinge and processes bearing the adductor muscles. They
are usually classed near to the _Chamidæ_, but their true position as
regards living Mollusca is very problematical. A fine series of these
shells is exhibited in Gallery VIII., wall-case 5, in the Geological
Department.
[Sidenote: Cases 198–199.]
The _Myidæ_, popularly known as “_Gapers_,” on account of their valves
being open at one or both ends, have the mantle united all round, except
where the small foot is protruded. The siphons are very long, united
almost to the ends, and covered with a coarse wrinkled outer skin. They
bury themselves in mud and sand at low-water mark or in shallow water.
The species are few in number, and chiefly from the shores of northern
countries. _Mya arenaria_ of our own coasts is largely eaten in some
parts of Europe and North America.
[Illustration:
Fig. 34.
British Gaper (_Mya truncata_).
_a_, foot; _b_, siphon-sheath; _c_, exhalant siphon; _d_, inhalant
siphon; _e_, umbones or beaks; _f_, anterior, _g_, posterior end of
shell.
]
[Sidenote: Case 198.]
The _Corbulæ_ (Case 198) have one valve larger than the other and are
like little _Myæ_, but the valves _are almost closed_ and their siphons
are very short.
[Illustration:
Fig. 35.
British Razor-shell (_Solen siliqua_).
_a_, foot; _b_, mantle; _c_, inhalant siphon; _d_, exhalant siphon;
_e_, shell.
]
[Sidenote: Cases 199–201.]
Many of the _Solenidæ_, or Razor-shells, possess very elongated shells,
and are remarkable for the great development of the foot, which can be
pointed or contracted as may be required for boring into sand. By means
of this powerful foot the animals, when disturbed, bore with such
rapidity and to such a depth that their capture is a matter of great
difficulty; and even when seized they hold on so tightly that at times
they suffer their foot to be torn off rather than be captured. They not
only burrow in sand, but also have the power of darting through the
water, like the Scallops. Solens were considered a dainty dish by the
ancient Greeks, and numbers are still eaten by the poorer
coast-population of this country and abroad.
[Sidenote: Cases 201–202.]
The _Pholadidæ_, or Piddocks, are very remarkable shells, of an
unusually complicated structure, some having the power of boring into
rocks, wood, mud, sand, etc. Their shells are white, adorned with
prickly sculpture, and, although thin, are strong. The foot is believed
to be the principal excavating instrument, but the shell no doubt is
used as a file to enlarge the hole as the creature grows. These animals
are brightly phosphorescent; and certain species are eaten at many
places on the shores of the Mediterranean. They appear to be indifferent
as regards the material they bore into; for the common _Pholas dactylus_
(Fig. 36) of our own shores has been found in slate-rocks, mica-schist,
coal-shale, new red sandstone, chalk, marl, peat, and submarine wood.
The siphons are long in the Piddocks, united except near the end, and
enclosed in tough skin. The species are world-wide in their
distribution, and several are found fossil in some of the Tertiary
formations.
[Illustration:
Fig. 36.
Piddock, or Borer (_Pholas dactylus_). (From the British coast.)
1. Animal in the shell: _a_, foot; _b_, siphons; _c_, inhalant
orifice; _d_, exhalant orifice.
2. Shell: _e_, accessory valves or plates.
]
[Sidenote: Case 202.]
The _Teredinidæ_, or Ship-worms, are also borers, like the Pholads, but
do not perforate rocks. They are principally wood-borers; the large
_Kuphus arenaria_, which is an exception, living buried in the sand. The
ship-worm has a long worm-like body, from 6 to 12 inches in length,
which is more or less enclosed in a thin shelly tube or sheath. The true
bivalved shell is at the thicker end, and protects the mouth, labial
palps, the liver, and other internal organs. At the opposite, or more
slender, end of the animal, the mantle is produced into two small tubes,
one of which conveys the water to the gills, whilst through the other
the water is expelled, charged with the woody pulp excavated by the
foot. At the end there is a pair of pallets, or paddles as they are
sometimes termed, which are probably used as a means of defence, in
closing the shelly tube after the contraction of the siphons.
[Illustration:
Fig. 37.
Ship-worm
(_Teredo norvegica_).
Case 202.
_a_, animal, removed from its shelly tube: _p_, _p_, pallets; _s_,
exhalant siphon; _s′_, inhalant siphon.
_b_, _c_, different aspects of the shell.
]
These animals are most destructive to ships, piers, etc.; and wood,
which is not protected by metal, when once attacked, is soon riddled
through and through. They work either with or across the grain, and
although the holes may be all but touching, they seldom appear to run
into one another.
[Illustration:
Fig. 38.
Watering-pot Shell
(_Brechites vaginifer_).
Case 204.
_a_, bivalve shell of the very young animal.
]
The “Watering-pot shell” (_Brechites_), of the family _Clavagellidæ_, is
a very remarkable structure, and unlike the shell of an ordinary
bivalved mollusc. On looking carefully, however, near the perforated end
(the rose), two small valves will be seen imbedded in the surface. They
are found with the rose downwards buried in mud or sand at low water on
the shores of the Indian and Pacific Oceans.
SEPTIBRANCHIA. (Fig. 25, D.)
[Sidenote: Case 204.]
The members of this order differ from other Lamellibranchs in having the
gill-plates represented by a muscular _septum_. They are provided with
two respiratory siphons and two adductor muscles, and the edges of the
mantle-lobes are connected at three points. The families _Poromyidæ_,
and _Cuspidariidæ_, constitute this order. The species are all small,
without colour-markings, are world-wide in their distribution and occur
at all depths.
Class V.—CEPHALOPODA.[9]
[Illustration:
Fig. 39.
A, the upper, B, the lower beak of _Architeuthis monachus_; one-third
natural size.
]
[Sidenote: Cases 205–208.]
This Class includes the Octopus or Polypus, Cuttlefish, Squid, Spirula,
the Paper and Pearly Nautilus. The body of the animal consists of a
muscular sac, in the cavity of which the viscera are placed. In front of
the body projects the head, which, in species belonging to the
two-gilled section of the Class, is surrounded by eight or ten fleshy
arms. A wide aperture below the head admits the water to the gills or
branchiæ, which are situated in the interior of the sac, whilst a short
tube, the so-called funnel or siphuncle, projects from the opening of
the mantle—the water and various excretions being expelled through this
tube, especially also an ink-like fluid, which is discharged by all
Cephalopods (except _Nautilus_) when disturbed, in order to darken the
water and thus escape their enemies. The centre of the head, between the
base of the arms, is occupied by the mouth, which is armed with two
horny or calcareous jaws, similar to the beak of a parrot (Fig. 39). The
two large eyes are placed on the sides of the head. The arms or feet are
more or less elongate, capable of movement in any direction, and, except
in _Nautilus_, furnished on one side with numerous suckers, by means of
which the animal attaches itself most securely to anything it may seize;
they are employed in capturing food and in walking. Cephalopods walk in
any direction head downwards, but can swim backwards only, being
propelled in that direction by the water which they discharge with force
through the funnel out of their branchial cavity. They are divided,
according to the number of their gills (which is either two or four),
into _Dibranchia_ and _Tetrabranchia_. Of the latter but one
representative now exists, viz., the Pearly Nautilus, all other living
Cephalopods being provided with but two gills, placed one on each side
of the body within the mantle, as may be seen in the wax model of _Sepia
officinalis_ (Case 207). The two-gilled section comprises forms with
eight arms, as _Argonauta_ and _Octopus_, and others with ten arms,
viz., the Cuttlefishes (_Sepia_) (Fig. 43), the Squids (_Loligo_,
_Ommatostrephes_, _Sepiola_, _Chiroteuthis_, etc.), and _Spirula_. The
“shell” of the Paper-Nautilus, or _Argonauta_, is too well known to
require any description. Unlike the shells of other Mollusca, it is not
attached to the animal by a special muscle, but is held on to the body
by two of the arms, which are dilated and specially adapted for this
purpose. Only the female Argonaut is provided with a shell, the male
being shellless and a much smaller creature. The Argonaut-shell is
therefore not a true shell, but simply a receptacle for the ova, serving
at the same time for the protection of the parent.
[Illustration:
Fig 40.
The Common Octopus (_Polypus vulgaris_), resting.
]
[Illustration:
Fig. 41.
_Sepiola scandica_
(Natural size). British.
]
_Chiroteuthis Veranyi_ is remarkable on account of the great length of
the tentacular arms. These are non-retractile and are employed to seize
their prey when at a distance.
[Illustration:
Fig. 42.
_Chiroteuthis Veranyi_ (much reduced).
_a_, general view of animal; _b_, magnified view of pedunculated
sucker of the terminal club of the tentacular arms; _c_, internal
shell or gladius.
]
The species of Octopus are found on the shores of almost all temperate
and tropical seas; they do not attain to a large size, and are without
the internal shell or “bone” which is found in the mantle of many
Cephalopods. That of the Cuttlefish or _Sepia_ (Fig. 43 _a_) is found in
abundance on our coasts; it is composed of numberless layers of a
friable calcareous substance. That of the Squid tribe is of quite
another character, consisting of an elongate thin horny plate, and
strengthened by one or more thickened ribs, in some species somewhat
resembling a quill-pen. Some species of this pen-bearing class related
to the Common Squid attain an immense size. One was captured off the
Irish coast in June, 1875 (probably _Architeuthis harveyi_), with the
shorter arms 8 feet in length and 15 inches in circumference at the
base, the two tentacular arms having a total length of 30 feet. The
powerful beak measured about 4 inches across. Thus from the tip of the
tail to the end of the tentacular arms this wonderful monster must have
measured something like 40 feet in length. Other very large specimens of
_Architeuthis_ have been captured on the coasts of Newfoundland and
Labrador. Two specimens stranded on the south coast of Newfoundland, in
the winter of 1870–1871, measured respectively 40 and 47 feet. Another,
cast ashore at Bonavista Bay in December, 1873, had a very stout body 14
feet long, arms 10 feet, and tentacles 24 feet in length. These are only
a few of the many instances of the capture of gigantic Cephalopods,
which occur not only in the North-Atlantic Ocean, but also in tropical
seas. Their appearance in mid-ocean may, in some instances, have given
rise to the tales of “Sea-serpents.” Specimens much smaller than those
mentioned above have attacked men, and pearl-fishers are in constant
fear of them. One of the arms of a large Squid (_Architeuthis
harveyi?_), which is supposed to have been found off the coast of South
America, is exhibited in the black upright Case A at the side of the
room.
[Illustration:
Fig. 43.
The Common Cuttlefish (_Sepia officinalis_), and its shell or bone
(_a_).
]
The shells of _Spirula_ (Fig. 44) have been long known, and are
scattered in thousands on the shores of New Zealand and other islands in
the Pacific Ocean, and they are also found in the Indian and Atlantic
Oceans, occasionally drifting on the coast of Devon and Cornwall.
Notwithstanding the abundance of the shells, very few specimens of the
perfect animal have been captured. The loosely-coiled shell resembles a
ram’s horn, and is divided into a number of segments by fine concave
partitions, like the shell of _Nautilus_, each one pierced by a slender
tube or siphon. It is placed at the hinder end of the body, and is
covered with so thin a skin, that a small portion of it appears to be
exposed both in front and behind. Absolutely nothing is known of the
habits of this very interesting creature, although probably they are
somewhat similar to those of other Cephalopods.
[Illustration:
Fig. 44.
The Spirula (_Spirula peronii_). (From the Indian and Pacific Oceans.)
1. Animal: _a_, portions of the shell exposed in front and behind;
_b_, the funnel or siphuncle. 2. Side view of shell. 3. Shell in
section, to show partitions or septa.
]
The _Nautilus_ (Fig. 45), of which several shells (Case 208) and a
perfect animal in spirit (black upright case A) are exhibited, is an
inhabitant of the Indo-Pacific Ocean, and differs from all other living
Cephalopods in being provided with four instead of two gills, and,
instead of eight or ten arms with suckers and hooks, has a number of
small retractile feelers. The Nautilus occasionally swims, like other
members of its class, at the surface of the sea, but mostly crawls about
leisurely on its feet at the bottom in search of food, which consists
chiefly of small crabs or Mollusca, which it crushes with its strong
calcareous parrot-like mandibles.
[Illustration:
Fig. 45.
The Pearly Nautilus (_Nautilus pompilius_).
_a_, body; _b_, siphuncle; _c_, eye; _d_, hood; _e_, tentacles; _f_,
muscle of attachment to the shell; _g_, siphon.
]
The chambered shell is pearly within, and covered with an external
calcareous layer. The chambers are connected by a slender tube or
siphon, the function of which is not at present thoroughly understood.
The septa, or partitions across the shell, indicate periods of growth.
When the Nautilus outgrows the capacity of the outer chamber, in which
it resides, it constructs a new one of larger size, separating the
additional chamber from the preceding one by a transverse partition.
A series of Cephalopods preserved in spirit is exhibited in the black
upright case at the side of the room.
ALPHABETICAL INDEX
OF THE
FAMILIES AND PRINCIPAL GENERA OF MOLLUSCA EXHIBITED IN THE SHELL
GALLERY.
This Index has been compiled to assist the numerous visitors, who wish
to examine and determine specimens of shells, in finding, without
trouble or loss of time, the Cases in which the genera are placed.
Subgeneric terms are omitted, as they do not fall within the scope of
this “Guide.”
Acanthina, 75
Achatina, 130–131
Achatinella, 134, 135
Acmæa, 4
Actæon, 94
Ætheria, 178
Amphibola, 98
Amphiperas, 35
Ampullaria, 25, 26
Amussium, 159
Anatina, 203
Ancylus, 99
Anodonta, 166, 167
Anomia, 137
Anostoma, 128
Aplacophora, 3
Aplustrum, 96
Aplysia = Tethys, 96
Aporrhais, 49
Arca, 139
Arctica, 162
Argonauta, 205, 206
Arion, 107
Aspergillum (= Brechites), 204
Astarte, 161
Atlanta, 94
Auriculidæ, 97, 98
Avicula = Pteria, 145
Batissa, 165
Brechites, 204
Bryopa, 204
Buccinum, 65
Buliminus, 128
Bulimulus, 122
Bulimus = Strophocheilus, 120
Bullidæ, 95
Calyptræidæ, 31, 32
Cancellaria, 77
Capulus, 31
Cardita, 160
Cardium, 192–194
Carinaria, 94
Cassis, 55–56
Cerion, 129
Cerithiidæ, 42–44
Chætoderma, 3
Chama, 196
Chitonidæ, 1–3
Chrysodomus, 64
Circe, 187
Clausilia, 129–130
Clavagella = Bryopa, 204
Columbella, 69
Conus, 89–94
Coralliophila, 76
Corbicula, 164
Corbis, 163
Corbula, 198
Crassatella, 161
Crenatula, 147
Crenella, 145
Cucullæa, 140
Cuma, 75
Cuspidaria, 204
Cyclophoridæ, 23, 25
Cyclostomatidæ = Pomatiidæ, 28, 29
Cylindrella, 127
Cypræa, 32–35
Cyprina = Arctica, 162
Cyrena, 164
Cythera = Meretrix, 185
Delphinula, 12
Dentalium, 136
Despoena, 22
Diplodonta, 163
Dolium, 56, 57
Donax, 182
Dosinia, 188
Dreissensia, 179
Eburna, 67
Emarginula, 7
Ennea, 103
Eucalodium, 129
Eulima, 46
Fasciolaria, 59
Ficula = Pirula, 57
Fissurella, 8
Fulgur, 61
Fusus, 57, 58
Gadinia, 99
Galatea, 165
Galeomma, 163
Gastrochæna, 201
Gena, 11
Glandina = Oleacina, 102
Glauconome, 192
Glycymeris, 141
Haliotis, 9–11
Haminea, 95
Harpa, 84
Helicarion, 104
Helicidæ, 107–121
Helicina, 21
Hemifusus, 61
Heteropoda, 24
Hinnites, 159
Hydatina, 96
Ianthina, 38
Isocardia, 162
Isognomon = Melina, 147
Kellia, 163
Latiaxis, 74
Latirus, 59
Leda = Nuculana, 137
Lepeta, 4
Lepton, 163
Lima, 156
Limax, 106
Limnæidæ, 99–102
Limopsis, 141
Lithodomus, 144
Littorina, 27
Loligo, 207
Lotorium, 53, 54
Lucinidæ, 162, 163
Lutraria, 199
Lyonsia, 203
Mactridæ, 183–185
Magilus, 77
Malletia, 137
Malleus, 146
Marginella, 83, 84
Melaniidæ, 38–41
Meleagrina, 146
Melina, 147
Melongena, 61
Meretrix, 185
Mesodesma, 182
Mitridæ, 61–64
Modiola, 143, 144
Modiolarca, 145
Modiolaria, 145
Monoceros = Acanthina, 75
Montacuta, 163
Murex, 70–73
Mutela, 177
Mya, 198
Myadora, 203
Mycetopus, 176
Myochama, 203
Mytilus, 142, 143
Nassa, 67, 68
Naticidæ, 36, 37
Nautilus, 208
Navicella = Septaria, 20, 21
Neæra = Cuspidaria, 204
Neomenia, 3
Nerita, 18, 19
Neritina, 19–20
Nucleobranchiata = Heteropoda, 84
Nucula, 137
Nuculana, 137
Nudibranchiata, 97
Octopus, 205
Oleacina, 102
Olividæ, 81–83
Ostrea, 152–154
Ovulum = Amphiperas, 35
Paludina = Vivipara, 22
Paludomus, 41
Pandora, 202
Panopea, 201
Partula, 126, 127
Patella, 4–7
Pectinidæ, 156–160
Pectunculus = Glycymeris, 141
Pedum, 156
Periploma, 203
Petricola, 192
Philine, 96
Pholadidæ, 201, 202
Pholadomya, 204
Pholas, 201, 202
Phorus (= Xenophora), 49
Physa, 101
Pinna, 148–152
Pirula, 57
Placuna, 138
Planaxis, 44
Planorbis, 100
Pleurotomaria, 9
Pleurotomidæ, 85–87
Plicatula, 154
Pomatiidæ, 28, 29
Proserpina = Despoena, 22
Psammobia, 197
Pteria, 145
Pterocera, 51, 52
Pteropoda, 96
Puncturella, 7
Pupa, 128
Purpura, 74, 75
Pyramidellidæ, 45
Ranella, 54
Ricinula = Sistrum, 76
Ringicula, 95
Rissoiidæ, 30
Rocellaria, 201
Rostellaria, 52
Rotella, 15, 16
Saxicava, 201
Scala, 44, 45
Scalaria = Scala, 44, 45
Scaphander, 95
Scaphopoda, 136
Scintilla, 163
Scutum, 7
Semele, 181
Sepia, 206, 207
Septaria, 20, 21
Septifer, 143
Siliquaria, 47
Siphonaria, 98, 99
Sistrum, 76
Solarium, 45
Solenidæ, 199–201
Solenomya, 137
Sphærium, 165
Spirula, 207
Spondylus, 154–156
Stenogyra, 133
Stilifer, 46
Stomatella, 11
Streptaxis, 102
Strombus, 49–51
Strophia = Cerion, 129
Strophocheilus, 120
Struthiolaria, 49
Succinea, 135
Sunetta, 187
Sycotypus, 61
Tapes, 190,191
Tellinidæ, 179–181
Terebellum, 52
Terebridæ, 87–89
Teredo, 202
Testacella, 102
Tethys, 96
Thracia, 203
Trichotropis, 37
Tridacna, 195
Trigonia, 141
Triton = Lotorium, 53, 54
Trochidæ, 12–15
Trophon, 70
Truncatella, 30
Tugonia, 198
Turbinellidæ, 60,61
Turbinidæ, 16–18
Turritella, 48
Typhis, 70
Umbraculum, 97
Umbrella = Umbraculum, 97
Ungulina, 163
Unionidæ, 166–178
Valvata, 30
Vanicoro, 49
Velutina, 37
Veneridæ, 185–191
Venerupis, 192
Venus, 188
Vermetidæ, 46, 47
Vitrina, 106
Vivipara, 22
Volutidæ, 78–81
Vulsella, 147
Xenophora, 49
Yoldia, 137
POLYZOA.
(*) An asterisk against names of species denotes that specimens of these
species are in the upright part of Case A and preserved in spirit.
[Sidenote: Upright Table-Cases A and B, at south end of Shell Gallery.]
From a casual glance at the contents of these cases, it might be
supposed that many of the specimens exhibited therein were seaweeds; but
a closer inspection, especially with a lens, will reveal structure of a
kind not to be found in any plant.
Let us select for examination _Flustra foliacea_, the Broad-leaved
Hornwrack or Sea-Mat (Fig. 1), (Case A 1), commonly to be found among
heaps of seaweed cast up on sandy shores round our coasts.
[Illustration:
Fig. 1.
_Flustra foliacea._ A, natural size; B′, portion magnified in B; B,
magnified 30 diameters.
_a_, avicularium; _o_, ovicell.
[‘The Cambridge Natural History.’]
]
The brown horny fronds, which vary in width, branch upwards from a
narrow flat stem attached at its base to stones and shells. Both
surfaces of the fronds show a fine network pattern formed by the edges
of little oblong boxes or cells termed zoœcia,[10] arranged in
longitudinal parallel rows and forming a double layer back to back. The
cells are broad and rounded above, narrow and truncate below, and each
is roofed in by a transparent membrane with a semicircular lid or
operculum situated near the upper end; four short stout spines spring
from the margin in this neighbourhood. When the surface of a living
frond is examined in sea-water, here and there a bundle of tentacles may
be observed pushing up a lid, slowly emerging and expanding into a
bell-shaped coronet; on the least alarm the tentacles are rapidly
withdrawn into the cell and the lid shut. The flexible protrusible
region of the cell is termed the tentacle sheath. The relation of the
cell to the tentacle sheath (Figs. 2, 3) may be roughly compared to a
glove finger, stiff below, but flexible at the end, and surmounted by a
crown of bristles; on pulling down the glove-finger tip, the tentacles
will also be drawn in, and will lie in a sheath formed by the
invaginated portion of glove finger. The lid which closes over the
tentacle sheath is only found in the Sub-order Chilostomata to which
_Flustra_ belongs. The area of the tentacle sheath whence the tentacles
arise is termed the lophophore.[11]
[Illustration:
Figs. 2, 3, diagrams representing polypide in cell. Fig. 2,
tentacle-sheath protruded. Fig. 3, ditto, retracted; _a_, tentacles;
_b_, tentacle-sheath; _c_, mouth; _d_, gullet; _e_, stomach; _f_,
vent; _g_, retractor muscle; _h_, funiculus; _l_, ovary; _k_,
testis; _l_, lid or operculum; nerve ganglion is between mouth and
vent. Fig. 4, polypide extracted from cell; _d_, pharynx; _e_,
stomach; _f_, anus (after Van Beneden). Fig. 5, section (partly
diagrammatic) of frond of _Flustra_, showing cells back to back.
]
The mouth is situated in the centre of the lophophore, surrounded by the
circle of tentacles; and the latter, by the action of their cilia, set
up currents which convey food to the mouth.
The mouth leads into a pharynx and gullet, the latter opening into a
stomach, whence the intestine ascends to terminate in the vent opening
below and outside the circle of tentacles; the intestines, in fact, form
a U-shaped tube (Figs. 4, 5) suspended in the body-cavity in the
interior of the cell. A cord, the funiculus, passes from the stomach to
the base of the body-cavity. A small nerve ganglion is situated within
the upper part of the loop of intestine.
The tentacles, intestines, and other organs constitute the “polypide,”
the cell being simply the protective house formed by the latter.
The body-cavity, which contains fluid, is in direct communication with
the interior of the tentacles, which are hollow, and which act as
respiratory organs by bringing the fluids of the body-cavity in
proximity to the water. In _Flustra_ the body-cavities of the cells are
shut off from each other, but pores and sieves in the partition walls
allow of the junction of the inner linings of these cavities. The male
and female reproductive elements are formed in the body-cavity. The egg
develops in a helmet-shaped brood-pouch, the ovicell, situated at the
summit of the cell and almost immersed in the cell above. The ciliated
embryo swims about for a few hours and settles down to form the first
polypide and cell; from the latter there arise buds which remain
attached, and produce other buds, till a colony like that of _Flustra_
results.
Among the ordinary cells are certain smaller cells (Fig. 1, _a_)
slightly raised above the general level, different in shape from the
ordinary kind and with thicker lids. These peculiar cells are termed
avicularia, and chiefly contain muscles for opening and shutting the
lid. They arise by modification of the ordinary cells, whereby all the
organs of the polypide have become atrophied except the muscles. The
Polyzoa[12] were so named by Vaughan Thompson, who, in 1820, discovered
that certain plant-like animals, which had previously been classed with
the zoophytes, possessed a much higher organisation, in that the
intestine was separate from the body-cavity and not continuous with it
as in Sea-Firs, Sea-Anemones, and Corals. In 1834, Ehrenberg named the
group Bryozoa[13] or Moss Animals.
With the exception of one genus (_Loxosoma_), all Polyzoa form colonies,
which arise by the continual budding of the cells, the buds remaining
attached to the parent cells. The colonies vary endlessly in form and
habit, occurring as crusts on rocks, etc., masses, broad fronds,
branching tree-like growths, bushy tufts, etc.
The texture and consistency may be gelatinous, cartilaginous, horny and
flexible, or stony.
The great majority of species are marine, but a considerable number
inhabit fresh water. The Polyzoa are classified as follows:—
{Sub-order 1. Chilostomata.[15]
{Order I. Gymnolæmata.[16] { Orifice of cell with a
{ Lophophore and tentacular { horny lid.
{ crown circular. {
{ Without a lobe over {Sub-order 2. Ctenostomata.[17]
{ the mouth. { Orifice of cell closed
{ { by a membranous
Group I. Ectoprocta.[14] { { comb-like frill.
Vent opens outside {Order II. Phylactolæmata.[18] { Always fleshy or
the circle of tentacles.{ Lophophore and tentacular { horny.
{ crown horse-shoe {
{ shaped. With {Sub-order 3. Cyclostomata.[19]
{ lobe over the mouth. { Without lid or frill;
{ Fresh water forms. { orifice of cell usually
{ circular; cells
{ always calcareous.
Group II. Entoprocta.[20]
Vent opens inside circle of tentacles.
_Sub-order 1._—CHILOSTOMATA.
[Sidenote: Cases A and B 1.]
The Chilostomata, which contain many more species than all the other
groups put together, are divided into three sections:—_A._ Cellularina,
in which the cells are more or less boat-shaped or cornucopia-shaped,
and joined together to form flexible branching colonies; _B._ Flustrina,
in which the cells are typically shaped like oblong boxes with
membranous front walls; and _C._ Escharina, in which the whole front
wall is calcified.
[Sidenote: Case A 1.]
[Illustration:
Fig. 6.
A, _Bugula turbinata_, natural size, B, portion × 50.
_a_, avicularia; _m_, mouth; _o_, ovicell.
[‘The Cambridge Natural History.’]
]
_Section A._ CELLULARINA.—_Bugula turbinata_, or the Bird’s-head
Coralline (Fig. 6) grows attached to rocks near low water mark in the
form of spiral tufts about two inches in height, composed of narrow flat
branches in which the cells are arranged from two to six abreast and all
facing upwards. Each cell is boat-shaped and with nearly the whole front
surface membranous; the globular bodies at the head of certain cells are
the ovicells. Attached to the outer edge of each cell is a remarkable
object resembling a bird’s head, and hence termed avicularium, seated on
a short stalk. The head and beak contain powerful muscles for opening
and shutting a horny lid or mandible hinged on below. In life, the
avicularium sways to and fro on its stalk, with the lower “jaw”
continually snapping up and down in the most ludicrous fashion. The beak
is capable of seizing and holding quite large objects.
The function of these curious appendages is partly to warn off
trespassers and partly to capture and retain small animals till
decomposition has set in; in the latter case, the currents set up by the
tentacles draw in the particles to the mouths of the polypides. The
avicularia have arisen by modification of the ordinary cells, in which
the muscles have developed at the expense of the degenerated polypides,
the cells have become much smaller, of different shape, and separated
out from the rest; the mandible represents the lid or operculum of the
ordinary cell. The avicularia vary greatly in size and shape in the
different genera; in _Flustra_, for instance, these organs closely
resemble the ordinary cells.
[Illustration:
Fig. 7.
_Bugula bicornis._ Cells magnified. (After Busk.)
]
[Sidenote: Case A. Upright part.]
In _Bugula bicornis_(*) (Fig. 7), from 1950 fathoms in the Southern
Indian Ocean, each cell is provided with two avicularia with remarkably
long stalks. The graceful vase-shaped _Kinetoskias cyathus_(*) (Fig. 8),
one of the treasures of the “Challenger” Expedition, was dredged from
1525 fathoms off Cape St. Vincent. The stem, which tapers gradually
upwards, rises from a tuft of root fibres. The cup is formed of slender
branches supported at the base by a delicate membrane. The branches are
composed of biserial rows of cells (Fig. 9) opening towards the interior
of the cup. The avicularia are pear-shaped and pedunclate. Probably, in
life, the cup is capable of being opened out to a considerable extent.
Specimens of this species were also obtained from 2160 fathoms in the
South Atlantic.
[Illustration:
Fig. 9.
_Kinetoskias cyathus._ A branch magnified.
_a_, an avicularium. (After Busk.)
]
[Sidenote: Case A 1.]
_Scrupocellaria reptans_, or the Creeping Coralline (Fig. 10 A, B) forms
branching colonies, creeping over rocks and seaweeds, and attached by
horny fibres often provided with curved hooks. The branches are composed
of cells arranged in a double row. Each cell has the membranous area of
its front surface protected by a branched flattened spine or operculum,
and is produced and narrowed below; at the upper outer margin is a
minute triangular avicularium. At the base of the back surface is a
small sack-shaped cell with a cleft at the upper end, in which a horny
bristle is articulated. The little cell is termed a vibracular cell, and
the bristle a vibraculum.[21] This organ has arisen by a further
modification of an avicularium, whereby the horny lid of the latter has
become a long bristle. The bristles by their motion keep off intruders,
and possibly act as scavengers by sweeping the surface of the cells.
[Illustration:
Fig. 8.
_Kinetoskias cyathus._ (From Voy. Challenger, Atlantic: Wyv. Thomson.)
]
[Illustration:
Fig. 10.
_Scrupocellaria reptans._ A. Creeping over seaweed, natural size; B.
Front surface, magnified.
_a_, branched spine covering front of membranous area; _b_,
avicularium; _c_, vibraculum.
C. Back surface; _a_, vibracular cell; _b_, vibraculum.
]
In _Caberia ellisii_ the vibracular cells are very large. The vibracula,
which are long and serrated, have been observed to move in unison like a
double row of oars.
[Sidenote: Case A 1, 2.]
_Section B._ FLUSTRINA.—In this group the colonies form leafy lamellæ,
crusts, etc., in which the individual cells are typically in the form of
oblong boxes with their front walls wholly or partly membranous.
_Flustra foliacea_ has already been described. [Sidenote: Case A 1.] In
_Flustra carbasea_ the fronds are formed of only one layer of cells, and
not of two layers back to back as in _F. foliacea_. The fine specimen of
_Flustra nobilis_ from S. Africa is so called from the large size of its
long hexagonal cells which form a honeycomb pattern clearly visible to
the naked eye.
[Sidenote: Case A 1.]
In _Flustra cribriformis_(*) (Fig. 11), from Torres Straits, the
fenestrated frond forms a beautiful spiral. _Flustra florea_, from S.
Australia, grows in the form of branching tufts of narrow spiral fronds.
_Electra pilosa_ [dry and spirit specimens exhibited] (Fig. 12) forms a
delicate silvery lace-work, encrusting shells and seaweeds (especially
red algæ) on almost every shore. The long horny spine at the base of the
membranous area of each cell gives the crust a pilose appearance. In
_Electra verticillata_ from West Africa, the cells form an elegant
branched colony, the branches being composed of regular verticils of
cells.
[Illustration:
Fig. 11.
_Flustra cribriformis._
]
[Illustration:
Fig. 12.
_Electra pilosa._ A, incrusting a seaweed, natural size; B, cells
magnified; _a_, lid or operculum.
]
[Sidenote: Case A 1.]
_Membranipora membranacea_ occurs in the form of horny incrustations on
bladder-wrack, which, owing to their flexibility, are able to adapt
themselves to the swaying of the fronds of the Fucus.
The _Selenariidae_ (Case B 2) form free colonies, usually orbicular in
shape, convex above and concave below. In _Lunulites capulus_
alternating rows of cells and vibracula radiate from the centre of the
colony.
_Section C._ ESCHARINA.—In this group, the front walls of the cells are
wholly calcareous. Many species form patches or crusts on shells etc.,
and hence the name of the section; other species, again, form stony
tree-like growths, or thick plates. Frequently one and the same species
occurs in the form of crusts or of erect lamellæ, the identity being
recognised by the characters of the individual cells.
Often a large number of species may be found on one shell. Two good
examples of this are exhibited in Case A 2.
[Sidenote: Case A 2.]
_Lepralia pallasiana_ (Fig. 13) forms sub-circular vitreous patches on
stones and shells; the cells are rather large, broadly oval, and with
the front wall punctured with pores; the aperture is squarish and with a
slight indentation on each side.
[Illustration:
Fig. 13.
_Lepralia pallasiana_, incrusting a shell. A, natural size; B, cells
magnified.
]
_Lepralia foliacea_ forms a massive coral-like growth composed of thin
contorted plates which fuse to form labyrinthine cavities, the plates
being constructed of a double layer of cells back to back. A large
specimen from the English Channel is exhibited in Case B, upright part.
In _Lepralia_ the orifice and lid of the cell have a straight lower
margin, but one large group, _Myriozoidæ_, is characterised by having a
notch in the lower margin of the orifice, (Fig. 14, _Schizoporella
unicornis_).
[Sidenote: Case B 1.]
In many of the Escharina, the front wall of the cell is produced into a
stout process or mucro at the lower margin of the orifice (genus
_Mucronella_), or, again, a collar or tube grows up round the primary
orifice, thus giving rise to a secondary orifice (_Smittia_, _Porella_,
etc., Case B 1).
[Illustration:
Fig. 14.
_Schizoporella unicornis_, magnified.
]
[Illustration:
Fig. 15.
_Retepora beaniana._
]
[Sidenote: Case B 1.]
In the _Celleporidæ_, (Case B 1) the cells are typically pitcher-shaped
and arranged vertically, and tend to be heaped up from the overcrowding.
_Cellepora pumicosa_ forms thick pumice-like masses composed of
succeeding layers of cells. The _Reteporidæ_ (Case B 1) form delicate
stony networks. The reticulate fronds may be expanded out, or may form
tubular or contorted growths (Fig. 15, _Retepora beaniana_). The
beautiful _Retepora phœnicea_ from Torres Straits is of a rich purple
colour.
The _Adeonidæ_ form thick fenestrated plates which unite to form
cavernous masses usually attached to rocks by a thick jointed stem.
Several very fine examples from Port Phillip, Victoria, are exhibited in
the upright part of Case B.
[Sidenote: Case A 2.]
The _Catenicellidæ_ are represented by a fine series of specimens from
Australia. The colonies form dense clusters of finely beaded branches.
The cells are arranged in single series, each cell being united to those
above and below by a horny joint. The cells are usually urn-shaped with
a triangular avicularium at each upper angle, and with the front surface
variously sculptured with pores or bands (Fig. 16, _Catenicella
ventricosa_).
[Illustration:
Fig. 16.
_Catenicella ventricosa._ A, natural size; B, magnified. (After Busk.)
]
Sub-order 2.—CTENOSTOMATA.
[Sidenote: Case B 2, and A upright part.]
The Ctenostomata are fleshy, horny, or membranous; never calcareous.
When the tentacles of a polypide are retracted into the cell, they are
protected above by a membranous comb-like frill.
The cells either bud off from each other or arise as buds on a stolon or
stem.
[Sidenote: Case A, upright part.]
_Alcyonidium gelatinosum_(*) (Fig. 17), so called from its resemblance
to the zoophyte Alcyonium, forms fleshy translucent growths occurring in
the form of nodulated branched masses, or of long finger-like growths.
The species is common round our coasts where it grows attached to stones
and shells near low-water mark.
[Illustration:
Fig. 17.
_Alcyonidium gelatinosum._ A, a small piece, natural size; B, the same
magnified.
]
[Sidenote: Case A, upright part.]
_Amathia_ forms bushy growths composed of slender horny branches. The
cells, which are cylindrical or squarish, rise from the branches in
biserial rows like Pan’s pipes. In _Amathia lendigera_(*) (Fig. 18) the
groups of cells are well separated from each other, but in _A.
spiralis_(*) and _A. convoluta_(*) the cells form a nearly or entirely
continuous series winding in a spiral round the slender stems. In
_Bowerbankia imbricata_(*) the cells are clustered on the stems.
_Vesicularia spinosa_(*), or the Silk Coralline, forms delicate brown
tufts resembling a filamentous alga; the cells arise separately in a
single series from the hollow tubular stems and are contracted at their
point of attachment.
[Illustration:
Fig. 18.
_Amathia lendigera._ A, natural size; B, magnified.
]
Nearly all the Ctenostomata are marine, but a few species live in fresh
water.
Sub-order 3.—CYCLOSTOMATA.
[Sidenote: Case B 2.]
In the CYCLOSTOMATA, which are all calcareous, the usually tubular
zoœcia have plain circular orifices without a lid or frill closing over
the retracted tentacle-sheath. There are two sections in this group,
viz., Articulata, in which the cells form branching colonies, the
branches being connected by horny joints; and Inarticulata, in which the
colonies may be encrusting, or erect and branching, but are without
joints.
The first section includes the _Crisiidæ_.
[Sidenote: Case B 2.]
_Crisia denticulata_ (Fig. 19) forms delicate white tufts, in which the
flat slender branches are composed of a double row of tubular cells. The
horny joints between the branches are black. The Inarticulata occur as
crusts or branching growths. In _Tubulipora flabellaris_ (Fig. 20) the
colonies form little fan-shaped crusts on seaweeds. _Lichenopora
hispida_ forms little white disks, in which rows of tubular cells
radiate from the centre. In _Idmonea_, the colony is branched, the
tubular cells being arranged in parallel rows on each side of the middle
line of the branch.
[Illustration:
Fig. 19.
_Crisia denticulata._ A, natural size; B, branches magnified.
]
The Cyclostomata are all marine.
[Illustration:
Fig. 20.
_Tubulipora flabellaris._
_a_, half of an incrusting colony, × 8; _b_, a few cells, × 44; _c_, a
colony, natural size.
]
Order II.—PHYLACTOLÆMATA.
[Sidenote: Table Case A, upright part.]
All the forms in this group inhabit fresh water, where, in the form of
creeping or erect branching growths or masses, they grow attached to
freshwater plants, tree-trunks, old wood, etc.; two species are capable
of slow movement from place to place. The lophophore and tentacular
crown of the polypide are horseshoe-shaped. The Order owes its name to
the presence of a lobe guarding the mouth.
In addition to the sexual, there is an asexual reproduction by means of
peculiar internal buds termed statoblasts (Fig. 21). When the colony
dies in the autumn, the liberated buds, securely protected in a horny
capsule, retain their vitality till the spring; in due season the valves
of the statoblast burst open, and the contents develope into a new
colony. The statoblasts, which resemble small seeds, are usually
provided with a ring of air cells, which act as a float, and in some
species spines are present.
[Illustration:
Fig. 21.
Statoblasts of Freshwater Polyzoa. A, _Fredericella sultena_ × 38; B,
_Plumatella repens_ × 38; C, _Lophopus crystallinus_ × 28; D,
_Cristatella mucedo_ × 28.
[‘The Cambridge Natural History.’]
]
[Illustration:
Fig. 22.
A, _Plumatella repens_, partly free, partly incrusting stem of
water-weed. B, Cells magnified. (After Allman.)
]
[Sidenote: Case A, upright part.]
_Plumatella repens_(*) (Fig. 22) forms brown branching colonies, wholly
or partly adherent to the surface of leaves of water plants, old wood,
etc. The individual cells are club-shaped, and about a quarter of an
inch long, each cell being attached to the upper back part of the cell
below; the statoblasts (Fig 21, B) are simple oval bodies with a zone of
air cells.
_Plumatella_ (_Alcyonella_) _fungosa_ forms thick masses, composed of
closely packed vertical tubes. A small specimen(*) surrounding a stick
from Hampstead Ponds is exhibited.
_Lophopus crystallinus_(*) occurs in the form of translucent gelatinous
blobs, often attached to the slender stems of duck-weed. The statoblasts
(Fig. 21, C) are elliptical and pointed at each end. The polypides are
comparatively large, and can be easily observed through the transparent
surface. When its delicate plumes are fully expanded, _Lophopus_ forms a
beautiful object.
_Cristatella mucedo_(*) (Fig. 23) occurs in the form of greenish
translucent oval or worm-like colonies with the polypides on the convex
upper surface. The animal slowly creeps about on its flattened under
surface.
[Illustration:
Fig. 23.
_Cristatella mucedo_, creeping over a stem of water-weed; × 6. (After
Allman.)
_a_, polypides with horseshoe-shaped crown of tentacles; _b_,
statoblasts seen through the tissues; _c_, muscular sole by means of
which the animal creeps; _d_, stem of water-weed.
]
Freshwater Polyzoa usually prefer dark places, but _Cristatella_ creeps
along on the stones and pebbles in clear water, and in the sunlight. The
polypides form three or more concentric rows on the upper surface. The
statoblasts (Fig. 21, D) are circular, provided with a zone of air
cells, and with hooked spines, the total diameter being about ¹⁄₃₀ of an
inch.
Sub-class II.—ENTOPROCTA.
[Sidenote: Table Case A, upright part.]
In this small group, both orifices of the alimentary canal open within
the circle of tentacles, and there is no tentacular sheath.
[Illustration:
Fig. 24.
_Pedicellina cernua._ × 27.
[‘The Cambridge Natural History.’]
]
The polypides are borne on contractile stalks. In the _Pedicellinidæ_
the stalks arise from a creeping stolon. In _Pedicellina cernua_(*)
(Fig. 24) a stolon, creeping over seaweeds, etc., gives rise to stalked
cups, the movements of which are vigorous: “the polypides, when excited,
dash themselves vehemently from side to side. The heads are easily
knocked off, but the decapitated stalks develop fresh ones. In
_Ascopodaria_ the stalks are swollen at the base; _A. fruticosa_(*),
from Port Phillip, Victoria, forms beautiful tree-like colonies. The
_Loxosomidæ_ do not form colonies, owing to the buds becoming detached
from the parent. The species of _Loxosoma_ are always found associated
with some other animal, such as a worm or Tunicate. The tentacles of the
polypide are arranged obliquely to the long axis of the body, hence the
name of the family (_loxos_, oblique). _Loxosoma phascolosomatum_(*)
occurs, in the form of delicate tufts, on the caudal end of the
Sipunculid worm _Phascolosoma_. The individuals resemble pins with
little white heads, and are capable of vigorous movements to and fro;
occasionally a stalk coils itself up into a spiral.
BRACHIOPODA.
[Illustration:
Fig. 1.
British Brachiopods (_Terebratula_ and _Crania_).
]
[Sidenote: Small Table Case A against the west wall to left of main
entrance.]
The Brachiopoda, though presenting a certain outward resemblance to
bivalved Mollusca, are quite distinct from this group. They are all
marine, and all possess a bivalve shell. They grow attached to rocks
(Fig. 1), usually by a horny peduncle or stalk passing between the two
valves, or through a foramen in one of the latter; or, peduncle and
foramen may be absent, one of the valves adhering by its surface to the
rocks; some species of _Lingula_ live in tubes in the sand or mud. They
occur at all depths, from shallow water up to 2900 fathoms, but the
largest number of species live at a depth of about 350 fathoms. Though
found in all seas, the localities whence they have been obtained are
comparatively few in number; but specimens are usually congregated in
considerable numbers, in places where they do occur. The surviving
species of Brachiopods constitute only a small remnant of a group that
flourished abundantly in former epochs. There are about 150 recent, and
over 6000 fossil species.
THE SHELL.—The valves of a Brachiopod shell differ from each other in
size and shape, but each valve is in itself symmetrical, _i.e._, similar
on each side of a middle line.
The valve through which the peduncle passes is termed the _peduncle_ or
_ventral valve_ (Fig. 2, A), the other being the _brachial_ or _dorsal
valve_. The peduncle valve, which is usually the larger and uppermost,
contains the bulk of the viscera; in the higher genera, calcareous bars
or loops (Fig. 2, B) attached to the inner surface of the brachial valve
form a support for the “arms” of the animal. The inner surface of the
valves presents certain markings and depressions where the muscles have
been attached (Fig. 6).
[Illustration:
Fig. 2.
_Magellania flavescens._ Australia. Interior of valves.
A. Peduncle valve: _f_, foramen for peduncle, below which are the two
small deltidial plates; _t_, hinge-teeth; _a_, _b_, _c_, muscle
scars. B. Brachial valve, showing the reflected loop for support of
the “arms.”
]
The shell is constructed of very minute prisms of calcareous substance
imbedded in an organic matrix. In _Lingula_ the shell is formed of
alternating layers of horny and calcareous substance.
The shell-valves are either hingeless, or joined by a hinge in which
teeth in the peduncle valve fit into sockets in the brachial valve. The
Brachiopoda are primarily divided into two sections, _Inarticulata_ and
_Articulata_, based on the absence or presence of a hinge.
The division into Orders is based on the relation of the peduncle to the
valves in its passage between them or through one of them. In the most
primitive Brachiopoda (_Lingulidæ_), the peduncle simply passes out
between the valves and not through a foramen or pore in one of them;
hence the group is named _Atremata_ (_a_, not, _trema_, pore). In the
next group, including the families _Discinidæ_ and _Craniidæ_, the
peduncle passes through a fissure in the edge of the peduncle valve, the
fissure in recent forms becoming closed round to form a slit-like
foramen; this group is named NEOTREMATA (_neos_, new, _trema_, pore). In
the third group, PROTREMATA (_pro_, in front of, _trema_, pore), which
includes the _Thecidiidæ_, the peduncle lies at the apex of a triangular
fissure in the peduncle valve, and secretes a calcareous plate to fill
in the gap. In the fourth group, TELOTREMATA (_telos_, final or
complete, _trema_, pore), including the _Terebratulidæ_, etc., the
triangular fissure in the peduncle valve is filled in by two calcareous
plates termed deltidia, secreted by the edges of the mantle.
The valves are hingeless in the first two Orders (Inarticulata), and
hinged in the last two (Articulata).
THE BODY.—The body usually occupies only a comparatively small space in
the posterior or peduncle end of the shell. From each side of the body
there is given off a thin expansion, the mantle which lines the inner
surface of the shell. The space between the valves is termed the
mantle-cavity. The mouth is situated in the centre of the front wall of
the body or floor of the mantle-cavity. The front wall gives rise to a
horseshoe-shaped platform surrounding the mouth and bearing on its upper
edge ciliated tentacles, or cirri, which set up currents carrying food
towards the mouth. In many genera the platform is produced into two
coiled “arms” (Figs. 3 and 5), which fill up the mantle-cavity.
The name Brachiopoda (_brachion_, arm, _pous_, foot) was given to the
group because these “arms” were supposed to be homologous with the
Molluscan “foot.”
The mouth leads into a gullet, which opens into a stomach and intestine.
In the more primitive forms the intestine terminates in a vent, but in
the higher forms the distal end of the intestine has become atrophied,
and consequently the gut ends blindly.
The body-cavity contains fluid, and is in communication with a system of
sinuses in the lobes of the mantle (Fig. 5). Bands of muscles pass
across from valve to valve. The peduncle consists of a horny outer
sheath surrounding longitudinal and transverse bands of muscles.
[Illustration:
Fig. 3.
_Magellania flavescens._ (After Davidson.)
A. Interior of dorsal valve to show the “arms”; some of cirri removed
on right side; _v_, mouth. B. Longitudinal section, with a portion
of the animal.
]
The sexes are usually separate. The reproductive cells are formed in the
body-cavity. The embryo swims freely for a short time before settling
down and becoming fixed. The specimens exhibited in the case are
arranged according to the following classification:—
Section I. INARTICULATA. │Order 1. Atremata. │Fam. _Lingulidæ_.
─────────────────────────┼──────────────────────┼──────────────────────
„ │Order 2. Neotremata. │Fam. _Discinidæ_.
„ │ „ │Fam. _Craniidæ_.
Section II. ARTICULATA. │Order 3. Protremata. │Fam. _Thecidiidæ_.
─────────────────────────┼──────────────────────┼──────────────────────
„ │Order 4. Telotremata. │Fam. _Rhynchonellidæ_.
„ │ „ │Fam. _Terebratulidæ_.
„ │ „ │Fam. _Terebratellidæ_.
Section I.—INARTICULATA.
Order 1.—ATREMATA. Family _Lingulidæ_.—The Lingulas possess emerald
green or golden brown duck-bill-shaped shells. Having no hinge, the dead
valves of dried shells easily fall apart. The peduncle, which is
sometimes over six inches in length, passes between the pointed
posterior borders of the valves. Dr. François gives a very interesting
account of the habits of _Lingula anatina_ which he found living in the
sand at Noumea, New Hebrides. The sole evidence of the animal’s
existence is the presence, on the surface of the sand or mud, of a
small, three-lobed slit (Fig. 4, upper figure). The tube (Fig. 4) in
which the _Lingula_ lives is about four inches deep, flat in the upper
half, rounded below. The walls of the upper flat portion simply consist
of the sand with a surface coating of mucous secretion; but in the lower
end the sand grains are agglutinated so as to form a distinct tube.
[Illustration:
Fig. 4.
_Lingula anatina_ in tubes in the sand; upper figure shows trilobed
opening on surface of sand. Dotted line in lower figure indicates
position in retraction. (After François.)
]
The edges of the mantle-folds are provided with setæ (bristles), which
form three funnels protruding through the three lobes of the slit-like
mouth of the sand-tube; currents enter by the lateral funnels and leave
by the central.
On the least alarm the animal is rapidly withdrawn as far as the centre
of the tube (see the dotted line of the shell in the figure), the
surface slit and upper part of the tube being obliterated. Each of the
arms forms a spiral with several coils (Fig. 5). The _Lingulidæ_ are of
exceptional interest, in that they furnish a very remarkable example of
“persistence of type.”
Shells of _Lingula_ occur in the earliest Palæozoic strata, and so
closely resemble those of the present day, that often no difference can
be observed either in the shape of the valves or in the muscular
impressions on their inner surface (Fig. 6).
_Lingula_ occurs in the Indo-Pacific, Australia, China, Japan, and the
Pacific Islands. _Glottidia_, a smaller form, with two small curved
plates on the brachial and a ridge on the peduncle valve, is found on
the American coast of the Pacific, and in the Atlantic.
[Illustration:
Fig. 5.
_Lingula anatina_, removed from shell, mantle reflected, coiled arms
separated slightly; _a_, mouth. (Marginal setæ omitted.) Ventral
aspect, three-quarter face.
]
[Illustration:
Fig. 16.
_Lingula anatina._ Interior of valves showing muscle scars.
V. Peduncle valve, D. Brachial valve.
]
Order 2.—NEOTREMATA. The _Discinidæ_ includes two genera, _Discina_ and
_Discinisca_ (Fig. 7), with orbicular conical shells, of horny
calcareous composition; both valves are conical in the former genus, but
in the latter the peduncle valve is flattened. Sometimes the embryos
settle down on the parent shells, and we see a mass of shells in various
stages of growth, as in the specimen of _Discinisca lamellosa_ from
Peru.
[Illustration:
Fig. 7.
_Discinisca lamellosa._ Peru. (After G. Sowerby.)
A group of old and young specimens; largest showing foramen in
peduncle valve, the rest showing brachial valves.
]
[Illustration:
Fig. 8.
Three specimens of _Crania anomala_ on a stone. Loch Fyne.
]
The _Craniidæ_ form small limpet-like shells (Fig. 8) closely adherent
to the rocks by the whole surface of the peduncle valve; although this
valve is so named, no peduncle or foramen is found in this family. The
Neotremata, like the _Lingulidæ_, are remarkable examples of persistence
of type, since forms very similar to the present day _Discinas_ and
_Cranias_ occur in the Palæozoic, Ordovician and Silurian strata.
A piece of rock, with several specimens of _Crania anomala_ attached, is
exhibited.
Section II.—ARTICULATA.
Order 3. PROTREMATA.—This group, formerly very abundant, is now almost
extinct, the Family _Thecidiidæ_ representing the Order at the present
day. _Thecidium mediterraneum_ (Fig. 9) forms little oval boxes about a
third of an inch in length, shaped somewhat like a pear cut in half
(peduncle valve), and with a semicircular lid (brachial valve) working
on a hinge on the upper flat surface. The foramen and peduncle are
absent; but between the pointed end of the peduncle valve and the hinge
is an area filled in by a calcareous plate characteristic of the
Protremata.
[Illustration:
Fig. 9.
_Thecidium mediterraneum._ A, natural size. B, section through shell.
Magnified.
]
The brachial valve opens like the lid of a snuff-box, and shuts down on
the least alarm with the rapidity of lightning. The peduncle valve is
fixed on the rocks by its convex surface. The species is common in the
Mediterranean in from 30 to 300 fathoms, and is also found in the West
Indies.
Order 4. TELOTREMATA.—This group, which at the present day contains the
largest number of species, includes the Lamp shells, so called from
their resemblance to an ancient lamp. The valves are joined by a
well-marked hinge, the peduncle passes through the peduncle valve
through a foramen completed by two plates secreted by the mantle edges,
and the brachial valve has attached to it a calcareous scaffolding of
processes or loops for the support of the “arms.”
The shells in this group are frequently ridged. Their colour is usually
white, but sometimes red or yellow; deep-sea forms are generally
vitreous.
_Rhynchonella psittacea_ has a black shell with a pointed incurved beak;
each of the arms forms a many coiled spiral and can be protruded beyond
the shell; the brachial skeleton is comparatively small and simple,
consisting of two separate processes.
In _Terebratulina_ the brachial skeleton forms a simple loop; in
_Magellania_ the loop is reflected on itself (Fig. 2).
The beautiful and unique specimen of _Dyscolia wyvillii_, from 390
fathoms W. Indies, is remarkable for its size, being over two inches in
length. The small vitreous specimens of _Terebratula wyvillii_ were
obtained off Chili from a depth of 2160 fathoms; specimens of the same
species were obtained also from a depth of 2900 fathoms in the North
Pacific.
TUNICATA.
[Sidenote: Wall Case to left of main entrance to Shell Gallery.]
The Tunicata are marine animals, the majority of which live, in their
adult stage, a stationary life, fixed to the rocks or sea-bottom, but a
comparatively small number are free-swimming.
[Illustration:
Fig. 1.
_Ascidia mentula_ from the right side. _at_, atrial aperture; _br_,
branchial aperture; _t_, test.
[After Herdman: _Tunicata_, Encyc. Britannica.]
]
They occur in the form of cartilaginous or leathery sacs, fleshy
incrustations, solid fleshy masses, free-swimming, barrel-shaped
animals, solitary or united into chains or hollow cylinders; or, lastly,
of minute free-swimming tadpole-shaped organisms. To explain briefly the
structure of a Tunicate, _Ascidia mentula_ (Fig. 1), is selected. The
animal, which lives on a muddy bottom, in from five to twenty fathoms,
resembles a conical sac fixed by the broader end, of grayish green
colour and about 4 inches in height. At the narrower end are two
orifices, one terminal—the branchial orifice or mouth, and the other a
little lower—the atrial orifice: the former has eight lobes and the
latter six.
When the Ascidian is undisturbed, the orifices are wide open, and
currents enter by the branchial and leave by the atrial orifice. On the
least alarm, the orifices close, jets of water being at the same time
squirted out; hence the popular name “Sea-squirts” given to these
animals.
[Illustration:
Fig. 2.
Diagrammatic section of _Ascidia_ representing the three sacs, and the
branchial sac as the pharynx or throat.
_a_, branchial; and _b_, atrial orifice; _c_, tunic or test; _d_,
mantle; _e_, branchial sac; _f_, gullet; _g_, stomach; _h_, anal
orifice; _i_, dorsal lamina; dotted line indicates the endostyle.
]
The Ascidian is orientated as follows: hold the animal with the
branchial orifice pointing forwards and the atrial upwards; the
branchial orifice will be anterior and the opposite end posterior; the
atrial orifice will lie on the upper or dorsal aspect, the opposite
aspect being lower or ventral, and the sides right and left. The
aspects, in fact, correspond with those of a vertebrate animal. A
vertical section roughly shows the animal to be formed of three
concentric sacs (Figs. 2, 3). The outermost, which is tough and
membranous, is called the Test or Tunic, the whole group owing its name
to the presence of this protective covering.
[Illustration:
Fig. 3.
Diagrammatic dissection of _A. mentula_.
_at_, atrial orifice; _br_, branchial orifice; _a_, anal orifice;
_brs_, branchial sac; _dl_, dorsal lamina; _end_, endostyle; _m_,
mantle; _ng_, nerve ganglion; _oea_, orifice of gullet; _pbr_,
peribranchial cavity; _st_, stomach; _t_, test; _tn_, tentacles.
(After Herdman: _Tunicata_, Encyc. Britannica.)
]
The middle sac, termed the Mantle, which almost corresponds in shape to
the outer, is composed of connective tissue, muscle-fibres,
blood-vessels, etc.; in spirit specimens, the mantle is shrunk away from
the test except at the orifices and at a point behind, where vessels
enter the test.
The innermost or Branchial Sac is attached behind the branchial orifice
and along the ventral edge, but otherwise hangs free in the interior,
the space around and outside of the sac being termed the atrial or
peribranchial cavity.
The delicate walls of the branchial sac, which resemble fine muslin, are
perforated by innumerable vertical slits, termed stigmata, arranged in
transverse rows (Fig. 4).
[Illustration:
Fig. 4.
_Ascidia mentula._ Part of wall of branchial sac showing stigmata.
Magnified.
]
The margins of the stigmata are lined with cilia which set up currents;
and the water which enters by the branchial orifice, passes through the
stigmata into the atrial cavity, and thence out through the atrial
orifice. The walls of the branchial sac are chiefly composed of a
sieve-like meshwork of fine blood-vessels arranged in transverse and
longitudinal rows. The currents of water passing through the stigmata
aërate the blood in the vessels. Besides the stigmata, the branchial sac
has two relatively large orifices, viz., the branchial orifice or mouth,
and, at the opposite end, the opening into the gullet. The branchial sac
is, in fact, a capacious throat or pharynx (Diagram Fig. 2 and Fig. 14).
Inside the branchial orifice is a circle of fine tentacles, which guard
the entrance to the branchial sac. The food of the animal consists of
minute animal and vegetable organisms.
It may be wondered how this food is secured, seeing that the currents of
water are continually passing through the sieve-like walls of the
branchial sac to the exterior again. Within the branchial orifice and
above the branchial sac are two circular ciliated ridges with a groove
between, which is full of viscid secretion; the cilia on the ridges
direct particles into the groove where they are retained by the mucus.
Passing backwards along the ventral edge of the branchial sac is a
thick-lipped furrow, which appears like a rod in the thin-walled sac,
and hence is called the endostyle. This organ secretes the mucus which
is carried up by ciliary action to the circular groove in front of the
branchial sac, and thence to the gullet along a fold or crest, termed
the dorsal lamina, situated along the dorsal edge of the branchial sac.
The gullet opens into a large stomach situated posteriorly on the left
side of the branchial sac. The stomach opens into the intestine, which,
after forming a loop, terminates in the anal orifice or vent opening
into the atrial cavity.
The tubular heart lies below the stomach, a remarkable feature in the
circulation consisting in the periodic reversal of the blood current. An
elongated nerve ganglion is situated between the branchial and atrial
orifices.
[Illustration:
Fig. 5.
Ascidian Tadpole with part only of the tail _C_. Magnified section.
_N_, nervous system with enlarged brain in front and narrow spinal
cord behind _n_; _N′_, cavity of brain; _O_, the single cerebral eye
lying in the brain; _a_, auditory organ; _K_, pharynx; _d_,
intestines; _o_, rudiment of mouth; _ch_, notochord or primitive
backbone.
(From Gegenbaur’s ‘Elements of Comparative Anatomy.’)
]
_Ascidia mentula_ is hermaphrodite. The egg develops into a minute
tadpole-like larva which swims about by means of its tail. Water
entering by the mouth passes out through the gill-slits. A nerve-tube
extending along the back and tail is swollen in front into a
brain-vesicle; and underneath the long nerve-tube behind the brain is a
stiff skeletal rod or axis—the notochord—which constitutes the rudiment
of a backbone. Inside the brain are two unpaired sense organs, an eye
and an organ of hearing (Fig. 5). After swimming freely for a few hours,
the larva settles down head foremost and fixes itself by papillæ on the
anterior end (Figs. 6, 7). Presently the tail becomes absorbed, and the
posterior end of the nerve-tube, and the brain with its eye and hearing
organ, undergo atrophy, the nerve ganglion of the adult alone
representing the cerebrospinal axis of the larva. The branchial sac and
intestines develop greatly, and growth proceeds in such a manner that
the mouth is pushed round to a position opposite to the fixed area, and
gradually the animal becomes the adult ascidian.
[Illustration:
Fig. 6.
Degeneration of Ascidian Tadpole to form the adult. The black pieces
represent the rock or stone to which the Tadpole has fixed its head.
]
[Illustration:
Fig. 7.
Very young Ascidian with only two gill-slits.
(Figs. 6, 7, from Lankester’s ‘Degeneration.’)
]
This wonderful metamorphosis presents a striking example of DEGENERATION
resulting from the adoption of a fixed mode of life. The active
free-swimming larva with its brain, eye, hearing organ, and muscular
tail becomes transformed into a comparatively inert sac.
[Illustration:
Fig. 8.
Tadpole of Frog and Ascidian. Surface view. (Lankester’s
‘Degeneration.’)
]
[Illustration:
Fig. 9.
Tadpole of Frog and Ascidian. Diagram representing the chief internal
organs. (Lankester’s ‘Degeneration. A chapter in Darwinism.’)
]
The tadpole of an Ascidian resembles that of a frog (Figs. 8, 9), not
merely superficially, but also in its general structure and mode of
development. The Tunicata are now generally regarded as a degenerate
offshoot from the ancestral stock of the Vertebrata, in that the larva
possesses a skeletal rod (rudimentary backbone) separating the dorsally
situated nerve-tube (cerebrospinal axis) from the ventrally situated
intestinal tube, the existence of the cerebral eye in the Ascidian
tadpole further tending to confirm the truth of this theory. Apart from
a knowledge of the course of their development, Tunicata would have been
classed among the Invertebrata, but the structure of the larva clearly
reveals the affinities of the group to the backboned animals.
_Ascidia mentula_ belongs to the group of SIMPLE ASCIDIANS which are all
fixed, and are either solitary or joined into colonies in which each
individual or ascidiozooid has a distinct test of its own. In the
COMPOUND ASCIDIANS, which form colonies by budding, the ascidiozooids
are buried in a common investing mass and have no separate tests. In a
third group, the SALPA-LIKE ASCIDIANS, the ascidiozooids are united to
form free-swimming colonies shaped like hollow cylinders open at one
end. The above three groups belong to one great Order—the ASCIDIACEA. A
second Order, THALIACEA, includes the free-swimming _Salpa_ and
_Doliolum_, which exhibit alternation of generations in their life
history. A third Order LARVACEA, includes very minute free-swimming
forms which possess a tail in the adult stage. There are sixteen
families of Tunicata.
The following is a tabular view of Prof. Herdman’s classifications:—
Order I. Ascidiacea.│Sub-order 1. Ascidiæ Simplices, 4 Families.
„ │Sub-order 2. „ Compositæ, 7 Families.
„ │Sub-order 3. „ Salpiformes, 1 Family.
Order II. Thaliacea │3 Families.
Order III. Larvacea │1 Family.
Order I.—ASCIDIACEA.
The Ascidiacea include the great majority of species. With the exception
of the one genus _Pyrosoma_, they lead a fixed or stationary life.
_Sub-order 1._—ASCIDIÆ SIMPLICES.
The Simple Ascidians are mostly solitary; in a few forms, however,
colonies arise by budding from stolons, but each individual has a
distinct test. The four families into which the sub-order is divided are
chiefly characterised by the nature of the test, the number of lobes
round the branchial and atrial orifice, and the character of the
branchial sac.
In the family _Molgulidæ_ the tough membranous test is often coated with
sand; the branchial aperture is six-lobed, the atrial four-lobed, the
branchial sac has long folds or pleats, and the stigmata are curved or
arranged in spirals.
_Molgula gigantea_, which is one of the largest of the Ascidians, and
which attains a length of over thirteen inches, forms a tough conical
sac; the branchial and atrial orifices at the upper end have six and
four lobes respectively. The test is leathery, smooth above, but coated
with sand below. The exhibited specimen, which comes from the Straits of
Magellan, has several specimens of the stalked _Boltenia legumen_
attached to the lower part of the test.
The curious _Molgula oculata_ (Fig. 10) has a soft oval or rounded body
coated with sand. The branchial and atrial orifices have respectively
six and four lobes. Specimens grow attached to the rocks and also live
free in the sand. The surface of the test is provided with hairs, which
adhere to the rocks and collect particles of sand. The adhesion not
being very firm, specimens are easily detached by currents and collected
into heaps by the eddies; when living in the sand only the two dark
orifices are visible. The sand coating has been supposed to confer
protection by mimicry of the environment; but Professor Lacaze Duthiers
found, much to his chagrin, that the sandy tests of his specimens were
of no avail in securing them from being devoured by crabs who seemed to
scent their prey from afar.
[Illustration:
Fig. 10.
_Mogula oculata._
_a_, branchial; _b_, atrial orifice.
]
In the family _Cynthiidæ_ the test is usually leathery, the branchial
and atrial apertures four-lobed, and the branchial sac folded into
longitudinal pleats.
The genera _Boltenia_ and _Culeolus_ include species in which the body
is attached to a peduncle.
The large exhibited specimen of _Boltenia pachydermatina_ is 28 inches
in length, the head being 4 and the stalk 24 inches long. The two
four-lobed apertures are along one edge, the branchial being the lower;
the body is marked with long deep furrows, and the stalk with transverse
wrinkles. _Culeolus perlucidus_, from 1600 fathoms in the Southern
Ocean, is in the form of a small pear-shaped head on a slender stalk,
the total length being 4½ inches. The branchial orifice forms a
transverse slit with raised lips near the stalk, the slit-like atrial
orifice being near the rounded end of the body. _Culeolus moseleyi_,
another slender-stalked form, was obtained from 2425 fathoms in the
Central Pacific.
[Illustration:
Fig. 11.
A. _Styelopsis grossularia_ on shell. B. Tadpoles of same, × 9. _a_,
branchial; _b_, atrial orifice. (B, after Sir J. Dalyell.)
]
The little Cynthiid _Styelopsis grossularia_ (Fig. 11), popularly known
as the “Currant Squirter,” occurs in the form of bright red
hemispherical blobs on stones and shells; when undisturbed, the
branchial and atrial orifices expand and project upwards. The eggs are
brilliant red in colour. Sir John Dalyell was the first to discover the
tadpole form, which is about ⅒ inch long (Fig. 11, B), and to observe
the tadpoles become fixed and develop into fixed Ascidians. He calls the
active little swimming larvæ “Spinulæ,” from their resemblance to small
pins.
The family _Ascidiidæ_ includes forms with a gelatinous or cartilaginous
test; the branchial and atrial orifices usually have 8 and 6 lobes
respectively; the branchial sac is without folds.
_Ascidia mentula_, described above, belongs to this family.
_Chelyosoma_ is characterised by the test forming tortoise-like horny
plates on the upper surface. The exhibited specimen of _C. macleayanum_
(Fig. 12) comes from Greenland; the upper hemispherical part of the test
is divided into 8 plates; the branchial and atrial orifices are situated
in the joints between the plates.
[Illustration:
Fig. 12.
_Chelyosoma macleayanum_, slightly enlarged. _a_, branchial; _b_,
atrial orifice.
]
The fine specimen of _Phallusia mammillata_ from Naples consists of
several individuals partly fused together; the branchial and atrial
orifices are wide open, and the mantle can be seen through the thick
knobby translucent test.
In _Rhodosoma_ the test is modified so as to form stiff plates recalling
the valves of a bivalve shell. One plate is attached to the rocks, the
other closing against the first like a lid; the anterior end of the
animal with its branchial and atrial orifices is visible only when the
lid is open. The Mediterranean species _R. callense_ (Fig. 13) grows
attached to the rocks. The little exhibited specimen is on a fragment of
shell in front of a black patch. The figure shows specimens with the lid
open and closed.
[Illustration:
Fig. 13.
_Rhodosoma callense_, × 10. A, “valve” open; B, shut. _a_, branchial;
_b_, atrial orifice. (After Lacaze Duthiers.)
]
Family _Clavelinidæ_. The body is attached to a creeping stolon or mass
of stolons, from which new individuals arise by budding. The other three
families of Simple Ascidians included solitary forms, but the
Clavelinidæ are social, and form colonies wherein each individual has
its own test.
_Clavelina lepadiformis_ (Fig. 14) forms graceful crystal vases about an
inch in height. The figure shows one individual, but usually the
processes at the base extend out as stolons whence other individuals
arise.
_Diazona violacea_, from Cornwall, forms beautiful purple disk-shaped
colonies in which the ascidiozooids arise from a basal mass of stolons.
Sometimes the ascidiozooids die down, leaving only a smooth violet pad,
which in due time produces a new crop of ascidiozooids.
[Illustration:
Fig. 14.
_Clavelina lepadiformis_; diagrammatic, showing the anatomy. The oval
bodies are the eggs; at lower end lies the tubular heart; the
root-like processes at the base grow into stolons, whence other
ascidiozooids arise.
]
[Illustration:
Fig. 15.
_Perophora listeri_; A, slightly, B, further magnified. Ascidiozooids
in right, left, and lateral aspects.
_a_, branchial; _b_, atrial orifice.
]
The remarkable _Rhopalæa neapolitana_, from Naples, may be roughly
compared to an hour-glass with a very long constriction. The test is
smooth in the upper part, but knobby and encrusted with foreign bodies
below. The upper or thoracic end contains the branchial sac, and the
lower or abdominal portion the stomach, heart, and reproductive organs,
the gullet and intestine traversing the whole length of the narrow
central region. Although from its general structure _Rhopalæa_ is a
Clavelinid, it is not certainly known to produce buds.
_Perophora listeri_ (Fig. 15) occurs in the form of little jelly-like
transparent blobs rising by short stalks from a silvery thread-like
stolon. Owing to their small size and transparency, it is possible to
examine specimens alive under the microscope, the currents passing
through the stigmata in the walls of the branchial sac, and the beating
of the heart being distinctly visible. The rapid motion of the cilia
surrounding stigmata gives the appearance of dark wheels all rotating in
the same direction. The heart beats so as to drive the blood current so
many times in one direction, and then after a short pause, in the
reverse direction.
The exhibited specimen growing on an oyster shell, is from Plymouth.
_Sub-order 2._—ASCIDIÆ COMPOSITÆ.
The Compound Ascidians are fixed forms, which give rise to colonies by
budding, the individuals being immersed in a common mass and not
possessing separate tests.
Although reduced to an extremely small size each individual or
ascidiozooid of a colony possesses the same organs as a large Simple
Ascidian, excepting that the former does not possess a separate test.
Frequently the individuals of a colony are grouped into systems, in
which the atrial orifices open into a common cloaca. The little
ascidiozooids vary greatly in shape in the different families. In the
_Polyclinidæ_, for instance, they are long, the organs being so to
speak, drawn out, and being arranged in three regions, the thoracic,
abdominal and post-abdominal, the first region containing the branchial
sac, the second the stomach, and the third the heart and reproductive
organs. In the _Distomidæ_, the body exhibits two regions, thoracic and
abdominal, the heart and reproductive organs lying alongside of the
stomach. The _Botryllidæ_ comprise only one region, the stomach and the
other organs being situated by the side of the branchial sac.
The Compound Ascidians include seven families which are characterised
chiefly by the method of bud formation, and by the arrangement of the
organs into one, two, or three regions.
It is only possible, from limits of space, to refer to a few interesting
forms.
The species of _Botryllus_ are those most commonly met with. They form
richly coloured gelatinous incrustations on rocks and seaweeds. _B.
violaceus_ (Figs. 16, 17, and 18 D) is blue with white lines; _B.
smaragdus_, green; _B. marionis_, brown with white and carmine; _B.
castaneus_, purple, and so on. The individuals are arranged in circular
systems with the branchial orifices round the circumference and the
atrial orifices opening into a common central cavity (Fig. 17), the
whole colony being composed of groups of systems.
[Illustration:
Fig. 16.
_Botryllus violaceus_ on seaweed. (After H. Milne-Edwards.)
]
The exhibited specimen of _B. violaceus_ was grown in the tanks of the
Biological Station at Plymouth. The red specimen of _B. aurolineatus_,
from Naples, shows well the branchial and cloacal orifices. In
_Botrylloides_, the individuals form elliptical or elongated systems.
_Colella thomsoni_ was obtained near the Philippines at a depth of 10
fathoms. The specimen, which is about 7 inches in length, resembles an
elongated head of clover on a thickened stalk. The individuals which
compose the head are arranged in spiral lines, the atrial orifice of
each ascidiozooid opening separately and not into a common cloaca.
[Illustration:
Fig. 17.
A. _Botryllus violaceus_, magnified, showing two systems of 6 and 7
ascidiozooids. B. One ascidiozooid extracted.
_a_, branchial; _b_, atrial orifices; _c_, branchial sac; _d_,
stomach.
(After H. Milne-Edwards.)
]
_Colella quoyi_ (Fig. 18 A), from 25 fathoms off Kerguelen Island, forms
a rounded head on a short peduncle, the total height being one inch. The
ascidiozooids are arranged in vertical lines in the “head,” each line
consisting of a double zigzag series.
[Illustration:
Fig. 18.
Colonies of _Ascidiæ compositæ_, natural size. A. _Colella quoyi._ B.
_Leptoclinum neglectum._ C. _Pharyngodictyon mirabile._ D.
_Botryllus._
(After Herdman, _Challenger_ Report and Encyclopædia Britannica.)
]
_Julinia ignota_, from the Antarctic regions, forms long narrow
colonies, which attain a length of nearly three feet. One end is
attached, the rest of the colony apparently lying along the sea-bottom.
_Amaroucium roseum_ from Naples forms translucent gelatinous masses; a
slice is exhibited, showing the long slender ascidiozooids immersed in
the mass.
[Illustration:
Fig. 19.
_Pyrosoma elegans_, natural size. A. Side view of entire colony. B.
End view of open extremity.
(Herdman: _Tunicata_, Encyclopædia Britannica.)
]
_Pharyngodictyon mirabile_ (Fig. 18 C), from 1600 fathoms in the
Southern Indian Ocean, resembles a small mushroom, and is about one inch
in height. This species is one of the few deep-sea Compound Ascidians.
_Leptoclinum albidum_ is a common and widely distributed species; it
occurs in the form of thin white crusts. The glistening white appearance
is due to the common test being densely crowded with minute stellate
spicules of carbonate of lime.
The specimen of _Leptoclinum neglectum_ (Fig. 18 B) encrusts a fragment
of sponge.
_Goodsiria pedunculata_ from the Straits of Magellan, forms a rounded
cartilaginous mass attached by a short peduncle; sometimes several
masses are attached to each other. Each of the small dark oval areas on
the surface corresponds to the branchial and atrial orifices of one
ascidiozooid.
_Sub-order 3._—ASCIDÆ SALPIFORMES.
The Salpiform Ascidians comprise only one genus, _Pyrosoma_, which
occurs in the form of free-swimming colonies shaped like hollow
cylinders closed and rounded at one end and open and truncate at the
other (Fig. 19). The wall of the cylinder is formed of a single layer of
ascidiozooids (Fig. 20), so arranged that all the atrial orifices open
into the interior of the cylinder, and all the branchial orifices on the
exterior, the two kinds of orifices being at opposite ends of the body,
and not close together, as in most simple and compound Ascidians.
Specimens vary in size from a few inches to upwards of four feet in
length, and, as the name of the genus implies,[22] they are brilliantly
phosphorescent. Sometimes they occur in innumerable multitudes, giving
rise to a zone of greenish light extending for miles. Professor Moseley
records that during the voyage of the _Challenger_ in the North Atlantic
a huge specimen of _Pyrosoma spinosum_, four feet in length, was
captured. On tracing his name on its body, the word came out in letters
of fire.
[Illustration:
Fig. 20.
Section through wall of _Pyrosoma_, magnified, showing a single layer
of ascidiozooids.
_br_, branchial; _at_, atrial orifice; _tp_, process of the test; _br
s_, branchial sac.
(Herdman: _Tunicata_, Encyclopædia Britannica.)
]
In _Pyrosoma elegans_ (exhibited), from Naples, the ascidiozooids are
arranged in verticils, and the mouth of the cylinder is surrounded by a
movable diaphragm; the outer end of each ascidiozooid is provided with a
membranous spine. Six species of _Pyrosoma_ are known. _Pyrosoma
atlanticum_ is found in the tropical Atlantic and Antarctic; _P.
giganteum_ in the Atlantic, Pacific, and Antarctic; and _P. spinosum_ in
the South Atlantic.
Order II.—THALIACEA.
The Thaliacea are free-swimming Tunicates, which exhibit alternation of
generations in their life history. There are three families, _Salpidæ_,
_Octacnemidæ_, and _Doliolidæ_.
_Salpidæ._—The Salpas are transparent barrel-shaped organisms which
occur in abundance at the ocean surface. They are so transparent that
they are rarely seen, except in calm weather from the side of small
boats; yet they frequently swarm in countless multitudes. From five to
ten bands of muscles partially or entirely surround the body, like
hoops. The branchial and atrial openings are at or near the opposite
ends of the body. The branchial sac has almost disappeared, the dorsal
lamina and ventral gutter (or endostyle) alone remaining, the interval
between the two on each side representing an enormous stigma; the dorsal
lamina, or “gill” is the transversely striated band passing obliquely
across the body and forming the only barrier between the branchial and
atrial cavities. Water enters at the mouth, and, by the contraction of
the muscle-hoops, is driven out through the atrial aperture at the
opposite end, which is then closed by a sphincter muscle. The elastic
walls of the body expand, and water again enters through the mouth, the
valve-like lips of which prevent its being driven out that way. The
_Salpa_ swims along in jerks, and along with each gulp of water takes in
Radiolaria, Foraminifera, etc., which are retained by the mucus of the
endostyle and carried to the gullet. The _Salpa_, in fact, lives, as
Professor Brooks observes, in a “living broth,” so abundant is the food
supply.
The intestines usually form an oval mass termed the “nucleus,” which is
a conspicuous object at the posterior end.
[Illustration:
Fig. 21.
Posterior part of solitary form of _Salpa democratica-mucronata_,
showing a chain of embryos nearly ready to be set free.
_gem_, young chain of _Salpæ_; _st_, stolon; _t_, test; _visc_,
visceral mass.
]
The _solitary Salpa_ above described is asexual. In the ventral region
of its body it forms a stolon which becomes segmented into a series of
buds (Fig. 21). As the stolon grows the end series of buds breaks off in
the form of a chain and swims away, other chains being detached in
succession. A chain is formed of individuals arranged in two rows, the
individuals in each row being alternate (not opposite).
Each individual of a chain differs from the solitary individual in
shape, arrangement of muscle bands, etc., but especially in having
reproductive organs. The chain Salpid is hermaphrodite; the embryo
develops into a solitary asexual _Salpa_ which produces the chains by
budding. The wonderful life history of _Salpa_ was discovered by the
poet Chamisso during a voyage round the world in 1819. He observes: “A
_Salpa_ mother is not like its daughter or its own mother, but resembles
its sister, its granddaughter, and its grandmother.” Here we have an
example of “alternation of generations,” a sexual generation (chain
form) giving rise to an asexual generation (solitary form), which latter
produces the sexual generation.[23]
Most of the species of _Salpa_ have double names owing to the chain and
solitary forms having been regarded as distinct species before they were
known to be phases in the life history of one and the same species.
_Salpa runcinata-fusiformis_, solitary form (Fig. 22 B), is
barrel-shaped, truncated at each end, with terminal orifices, and with
nine muscle-bands on the dorsal surface, some of which converge towards
each other. An individual of a chain (Fig. 22 A) is fusiform, with six
muscle-bands, and with the orifices not terminal, but at each end of the
dorsal surface.
The solitary form of _S. africana-maxima_ is barrel-shaped, with
truncated ends and terminal orifices, and with nine broad parallel
muscle-bands. The chain form is conical at one end, with six bands, and
with orifices on the dorsal surface. The exhibited specimen of the chain
form, which is in an early stage of growth, contains 202 individuals.
The solitary and chain individuals of _Salpa costata-tilesii_ attain a
length of six to eight inches. The solitary form has eighteen
muscle-bands and two large spines at the posterior end. The individual
of the chain has five muscle-bands. A chain of three individuals is
exhibited.
_Salpa pinnata_ produces a circular chain; the exhibited specimen of the
solitary form shows a small chain about to be detached; a circular chain
of six individuals is also exhibited. Species of _Salpa_ abound in all
seas, but specimens from Naples have alone been exhibited on account of
their good preservation.
Family _Octacnemidæ_ includes _O. bythius_, a deep-sea Salpid, in which
the body forms a flattened disk produced into eight radiating lobes.
[Illustration:
Fig. 22.
_Salpa runcinata-fusiformis_. A. Chain form. B. Solitary form. 1–9,
muscle bands; _em_, embryo; _m_, mantle; _visc_, visceral mass or
nucleus.
(Herdman: _Tunicata_, Encyclopædia Britannica.)
]
[Illustration:
Fig. 23.
_Doliolum denticulatum_, sexual generation, from the left side.
_m^1_-_m^8_ muscle bands; _at_, atrial; _br_, branchial apertures;
_br s_, branchial sac; _sg_, stigmata; _st_, stomach; _ng_, nerve
ganglion; _so_, sense organs.
(After Herdman, Encyclopædia Britannica.)
]
Family _Doliolidæ_. The body is cask-shaped and surrounded by circular
hoops. The branchial and atrial orifices are at the opposite ends. The
branchial sac is pierced by two oblique bands of stigmata (Fig. 23
_sg_). The life history is very complicated. The egg develops into a
tailed larva, which develops into a “nurse”; the latter is asexual, and
produces three kinds of buds on a stolon, viz. (1) nutritive buds which
provide the “nurse” with food, (2) foster forms which are set free as
cask-shaped bodies with eight broad muscle-bands, and (3) sexual forms
which are attached for a time to the foster forms, but which later
become free and give rise to the egg.
Order III.—LARVACEA.
[Illustration:
Fig. 24.
_Oikopleura cophocerca_ in its “house” (after Fol); seen from right
side, × 6. Arrows indicate course of the water; _x_, lateral
reticulated parts of the “house.”
]
The Larvacea are very minute Tunicata which live at the surface and swim
by means of a tail-like appendage, resembling in this and certain other
respects the tadpole larva of other Tunicata. They are able to form a
temporary test or “house” many times larger than the body (Fig. 24). The
organism itself, which is almost lost in its large test, is the little
hammer-shaped body in the centre of the figure; the streaked areas bound
a space in which the tail lashes vigorously. The animal can leave its
test and secrete another in a few hours.
The tail is attached to the under or ventral surface of the tiny little
barrel-shaped body, and usually points forwards; a skeletal rod, the
urochord, runs along its length. The branchial sac has two ciliated
openings or gill-clefts leading directly to the exterior, and not
opening like the stigmata of the other orders into an atrial cavity.
The order contains one family, the _Appendiculariidæ_, and four genera,
and is represented in all seas.
_Oikopleura cophocerca_, one of the largest forms, is about half an inch
in length. The exhibited specimens came from St. Andrews, Fife.
Professor McIntosh reports that occasionally specimens of this species
occur in immense quantities, the tow-nets being filled with them.
THE STARFISH GALLERY.
In the STARFISH GALLERY is exhibited a series of the animals belonging
to the class _Echinoderma_; of these the Starfishes are the best known,
while others are the Sea-Lilies, Sea-Urchins, and Sea-Cucumbers or
Sea-Slugs.
A small collection of various kinds of Worms is also exhibited in this
Gallery (Wall-cases I.–III.).
ECHINODERMA.
Six table-cases contain the dried Echinoderms arranged in systematic
order. The seventh is devoted to preparations, models, and figures
illustrative of the structure and life history of various members of the
group.
An inspection of that Case and the accompanying woodcuts will make clear
the distinctive characters of the Echinoderma. Unlike that of a Crayfish
or a Mussel, the body does not appear to be divided into two equal or
symmetrical halves, though it really is; this is due to the possession
of a number of rays, of which there are ordinarily five. The skin is
strengthened by the deposition in it of carbonate of lime, which may be
in the form of continuous plates or bars, or of separate scattered
spicules. A series of tube-feet or suckers (podia) are generally
developed along each ray, and these are supplied by a system of
water-vessels peculiar to Starfish and their allies. These rays are
often called “_ambulacra_.”
[Illustration:
Fig. 1.
A. Anchor and plate of _Synapta_. B, C. Tables of _Holothuria
impatiens_; and D. _Holothuria atra_: from various aspects. E.
Spicule from sucker of _Stichopus variegatus_, magnified about 200
times.
]
[Illustration:
Fig. 2.
Diagram of Water-vessels.
_c.c._ Circular canal, with _p.v._, its Polian vesicles; from it a
radial canal (_v.c._) is given off along the lower surface of each
arm; this supplies, by side branches, the suckers, _s_; connected
with each sucker is a contractile swelling or ampulla (_a_). The
circular canal is in connection with the exterior by _s.c._, the
stone-canal, and opens to it by the madreporite (_m_).
]
[Illustration:
Fig. 3.
Figure of a Starfish (_Asterias rubens_).
In the ray marked I. the skin has been removed from the upper surface,
and the ambulacral ossicles (_ao_) and the podia (_s_) are seen _in
situ_; the blind outgrowths (_c_) from the central stomach (_sp_)
have been dissected out. In II. the gonads (_g_) are exposed; and in
the centre above the stomach the rectal glands (_rg_) are to be
seen. The anus (_a_) is seen to be subcentral in position.
]
In the body of the Starfish (Fig. 3) the arms are seen to be continuous
with the disk and to contain portions or prolongations of the chief
organs. The middle of the arm is occupied by two rows of hard pieces
(ambulacral ossicles), the fellows of which make an open angle with each
other, and so form an open ambulacral groove; along this we find the
suckers, the water-canal that supplies them, the blood-vessel of the
arm, and a nerve-cord. At the centre of the disk is the mouth. The
ossicles at the sides of the arms bear spines, which vary in different
species; the surface of the back is supported by a network of hard
pieces, and through the intervening spaces there project membranous
pouches, which are respiratory in function. The modified plate on the
upper surface opens into a tube by means of which the water-vessels
communicate with the exterior; this plate is known as the madreporite
(Fig. 2, _m_).
The organs for masticating the food are most highly developed in the
regular Echinoids, where the complex apparatus known as the “Lantern of
Aristotle” is found (Case 38) to consist of five sets of pieces; the
tooth is strong and bevelled at its free end; it is supported by
triangular jaws on either side, a pair uniting and having the form of an
inverted pyramid; these alveoli are connected with their neighbours by
oblong pieces (_falces_); above these there are elongated bars, which
are hinged on to the inner end of the falces and have their outer ends
free. The whole lantern is connected to the test by muscles which pass
from its sides to the auricles or upstanding pillars which lie round the
mouth; and, owing to this muscular apparatus, the teeth are capable of
complicated and various movements.
In the Ophiuroids the edges of the mouth-slits are provided with short
spinous processes, varying a good deal in arrangement, but never having,
apparently, any other function than that of a filtering-apparatus; in
the Starfishes the plates round the mouth have a supporting function
only; in Crinoids and Holothurians the mouth is unarmed; the latter are
often remarkable for a deposit of calcareous plates in the walls of the
gullet, and in the former the grooves on the arms are the lines along
which food comes to the mouth.
Echinoids live on seaweeds and the animals that are found on them; such
as have no teeth, like _Spatangus_ (Case 32), use their spout-like mouth
to take up the sand and débris on which they move, and from which they
extract some nutriment. Ophiuroids live on the smaller foraminifera;
Asteroids on dead fishes (as line-fishermen well know), oysters, and
other molluscs, and even on specimens of their own particular species;
Holothurians on shell or coral débris and the minute organisms it
contains; and Crinoids on small tests of foraminifera and on the adults
of small and larvæ of larger crustacea.
In a number of Echinoids and Asteroids some of the spines are specially
modified to act as seizing-organs—the free end being divided into two,
three, or rarely four pieces, which are moved on one another by special
muscles. These minute organs were regarded by earlier observers as
parasites, and were named _pedicellariæ_; they may be movable, when they
have a stalk, or the stalk may be absent and the valves sessile.
Considerable difficulty attaches to the determination of the use that
these organs may be to their possessors; but there is reason to suppose
that they may act as cleansing-organs by removing minute particles of
dirt, and as temporary organs of fixation, while M. Prouho has observed
their use as organs of defence.
Echinoderms move but little; the unstalked Crinoids, if they cannot find
stones or worm-tubes around which to attach themselves, swim by beating
the water with their delicate arms, five being raised and five depressed
alternately. The Echinoid or Asteroid is able to move by the aid of its
podia or so-called ambulacral feet, which become erected by being filled
with water, and are then contracted; by means of this contraction
movement is effected; a similar kind of locomotion obtains with the
pedate Holothurians; in the Ophiuroids the flexible arms either serve as
the organs of movement, or act as an apparatus whereby the creature
becomes coiled round the branches of corals (see Case 20).
Echinoderms are often of exceedingly bright colours, as is shown by the
pictures on the wall, and are very conspicuous objects; this may,
apparently, be associated with disagreeable tastes or odours; sometimes
they cover themselves over with seaweed, and so hide their brilliancy;
the spines of some forms are exceedingly painful to the touch, and the
stout plates of some of the _Goniasters_ must form admirable organs of
protection. The power of restoring lost or injured parts is one of the
most remarkable points in the Echinoderm organization (see Case 6).
Echinoderms are of great geological age, and were very abundant in
earlier periods of the world’s history. Two groups (the Blastoids and
Cystids) have completely disappeared, and the Stalked Crinoids
(Lily-Encrinites) are far less common than they used to be. Echinoderms
are now found in all seas, and extend to great depths of ocean; many of
the species have exceedingly wide areas of distribution, and most are
characterized by their gregarious habits, a large number of specimens of
a single species being generally obtained by the dredge. They are most
abundant in the tropical seas.
Most Echinoderms lay their eggs in the water, where the larvæ are
developed and swim about freely; but in a few (_Hemiaster_, _Ophiacantha
vivipara_, and others) the young do not pass through any metamorphosis,
for the eggs are placed in special pouches of the body of the parent, in
which they are hatched. The free-swimming larvæ of the other Echinoderms
pass through a series of remarkable changes (Figs. 4 and 5); these are
illustrated by the twelve models of various forms of larvæ exhibited in
Case 36; in Case 35 is a set of models showing in detail the changes
undergone by a single species (_Asterina gibbosa_). A portion only of
the body of the larva is converted into the substance of the perfect
animal; the rest is either absorbed by the growing animal, or shrivels
up and disappears.
[Illustration: Fig. 4. _Pluteus._ Fig. 5. _Bipinnaria._ Developing
larvæ.]
Below the twelve models in Case 36 may be seen a representation of three
stages in the history of the Feather-star (_Antedon bifida_). The larvæ
of this Echinoderm are not free, but are attached by a stalk (Fig. 6);
in the common Feather-star and other _Comatulidæ_ the stalk is found
during larval stages only; in others, such as _Pentacrinus_, it persists
throughout life.
The presence or absence of this stalk has been taken as the first
character of importance in the classification of Echinoderma which may
be divided into two groups:—
A. PELMATOZOA,[24] or Echinoderms provided with a stalk throughout life
or in the larval stages only. To this group belong the _Crinoidea_, and
the extinct _Blastoidea_, and _Cystidea_.
[Illustration:
Fig. 6.
Pentacrinoid stage of _Antedon rosacea_.
_a_, arms; _b_, basals; _r_, radials; _s_, stalk.
]
B. ECHINOZOA, or Echinoderms without stalks at any time of their
existence. To this group belong the _Asteroidea_, _Ophiuroidea_,
_Echinoidea_, and _Holothurioidea_.
CRINOIDEA.—This Order may be described as stalked, globular, or
cup-shaped Echinoderms, in which the oral surface of the calyx or disk
looks upwards, and in which five jointed and generally branched rays
arise from the central disk. Their joints have jointed pinnules at their
sides, and the sucking-feet have the form of tentacles.
The stalked representatives of this Order are placed on tables and
brackets near the south door, and are worthy of being particularly
noticed for their fine preservation, size, and beauty. The largest
specimen of _Pentacrinus decorus_ was taken on a telegraph-wire, to the
covering of which the stalk of the Crinoid is still attached.
_Metacrinus_ is a more lately discovered genus, which appears to be
confined to the eastern seas.
A few dried unstalked Crinoids are shown in Table-case 1; these show the
leading modifications of structure in the two great genera _Antedon_ and
_Actinometra_.
[Illustration:
Fig. 7.
Comet form of _Linckia_.
]
ASTEROIDEA.—This Order comprises Echinoderms with a depressed body of
pentagonal or star-like shape, to the ventral surface of which the
ambulacral feet are confined. The rays are more or less elongate movable
arms, with skeletal structures, which consist of transversely arranged,
paired, calcareous plates, articulated with each other like vertebræ,
the series extending from the mouth to the end of the arms. The groove
in which the ambulacral feet are arranged is uncovered.
Typical specimens of this Order are exhibited in Cases 2 & 3, in which
the great variety of form in the genus _Asterias_ and beautiful examples
of _Acanthaster_ are shown. Cases 6 & 7 contain specimens illustrating
the curious habit of self-mutilation possessed by so many Echinoderms;
among Starfishes, and notably in the genus _Linckia_, the single arms
separated from the disk are able to develop a fresh disk and arms, and
so to multiply the species. Cases 9–11 contain fine series of
_Oreaster_.
OPHIUROIDEA, or “Brittle-stars.”—These Echinoderms appear to resemble
the ordinary Starfish[25]; but they differ in having the organs of
digestion, respiration, and reproduction confined to the disk, the arms
having merely the function of locomotor organs. The arms therefore are
more slender and cylindrical in form, and are sharply distinct from the
disk; the separate joints consist of two central ossicles, which leave
only a narrow canal between them, and these are covered above, below,
and at the sides by specially developed investing plates; the lateral
plates bear spines, which are always comparatively short and delicate,
as compared with the spines found at the sides of the arm in starfishes.
The principal types of this Order are exhibited in Cases 17–22; the most
exquisite of them are the forms whose arms are divided and subdivided
till they end at last in the finest threads, as in _Astrophyton_, the
so-called Basket-fish or Gorgon’s heads.
ECHINOIDEA, or “Sea-Urchins,” are Echinoderms in which the rays are not
free, as in the Starfishes or Brittle-stars, but unite to form a
compact, spherical, heart- or disk-shaped test; this test is covered
with spines, which may attain to a great length, as is shown in the fine
example of _Diadema saxatile_ from the Andaman Islands; some of the
tests are flexible and very fragile. Owing to the quantity of specimens
that are sometimes dredged at one spot, the naturalist has been able to
gain a better idea of the range of variation in the species of
Echinoderms than in some other divisions of the Animal Kingdom; an
instructive series, showing the variations of _Echinometra lucunter_, is
shown in Case 28.
The genus _Hemiaster_ offers an example of an Echinoderm in which the
eggs are laid in special pouches; the hinder ambulacra are deepened to
form pits, which are guarded by specially elongated spines (see Case
34); in these pits the young pass through all the stages of their
development.
The minute structure of the spines of Sea-Urchins is illustrated by a
series of figures on the wall.
The HOLOTHURIOIDEA, or Sea-Cucumbers, form the last order of
Echinoderms. Their body, as indicated by their English name, is
elongate, subcylindrical, with a more or less flexible integument,
according to the extent of the reduction of the calcareous skeleton; the
mouth is at one end of the body and surrounded by tentacles, the vent at
the opposite end.
As these animals cannot be shown in a dried state, some of them,
preserved in spirit, are placed in Wall-Case IV. According as they have
or have not the sucking-feet of the Echinoderma, they are ordinarily
divided into the _Pedata_ and the _Apoda_; the latter are represented by
_Synapta_, which may attain to a great length, and by _Chiridota_; the
Pedata are illustrated by the genera _Cucumaria_, _Psolus_, and
_Holothuria_. Deep-sea investigations have revealed the existence of
another group of specially modified Holothurians—the _Elasipoda_; these
are remarkable for their well-marked bilateral symmetry and the
distinctness between the dorsal and ventral portions of the body; the
prominent processes on the dorsal surface are not contractile.
An exhibition of some interest is to be found in a Table-Case against
the wall, in which there are various specimens of the edible
Holothurians—_trepang_ or _bêche-de-mer_; these were all bought in the
market at Canton, and may be taken to be typical of the kinds offered
for sale in various eastern countries.
WORMS.
By the name “Worms,” people commonly indicate a number of different
forms whose relations with one another are by no means so close as those
of a Holothurian and a Crinoid, or a Mussel and an Octopus. There are
not, indeed, any common characters by the possession of which the
worm-like animals can at once be distinguished from other animals. We
take the divisions, examples of which are here represented, either by
drawings, models, or specimens preserved in spirit separately.
The groups referred to may be enumerated as follows:—
_Platyhelmia._│Turbellaria.
„ │Trematoda.
„ │Cestoda.
_Nemertinea._ │
_Nematoidea._ │
_Chætopoda._ │
PLATYHELMIA, or Flat-Worms.—These form the lowest and simplest division
of the group.
The parasitic Platyhelmia—the Tapeworms (_Cestoda_) and the Flukes
(_Trematoda_)—occupy Case I.; the life history of the common Tapeworm
(_Tænia solium_) is shown by the aid of models and figures. A model of
the anterior end of the common Tapeworm shows the four suckers and the
crown of hooks; the unjointed neck is followed by the joints
(_proglottids_), which increase in size the farther they are from the
neck. Several entire specimens of _Tænia_ follow, showing the size of
the whole worm and the form of its joints. The structure of the body is
shown in the models of two joints. The growth and development of the
_Tapeworm_ is dependent on a migration or a change of the hosts which it
inhabits in the various stages of its life; and although the different
kinds of Tapeworm differ from each other somewhat in certain details of
their migration and development, their life history exhibits, on the
whole, the same events which we find in _Tænia solium_, a common
Tapeworm of man in Northern Europe. This worm is matured in the
intestines of man; its final joints consist merely of fertilized ova
which have already passed through the earlier stages of development;
when the joints are detached and discharged, their contents escape in
the form of embryos contained in a thick chitinous shell. If these are
now swallowed by a pig, the shell is digested by the gastric juices of
the new host, and a rounded embryo, which is provided with three pairs
of hooks, is set free; by means of these hooks the guest makes its way
through the wall of the stomach or intestine, and finally settles down
in the muscles of its host. The embryo now loses its hooks, and
gradually acquires a bladder-like form, the central cavity of which is
filled with fluid. This bladder-worm (_Cysticercus_) has its outer wall
pushed inwards at the anterior end, and on this hooks and suckers become
developed. We have now a narrow head and neck with an attached bladder,
the head being at this time hollow. If during the long time that these
bladder-worms remain alive, the pig is killed for food, its flesh is
found to be “measly”; if it is afterwards insufficiently cooked and
eaten, the worms are conveyed into the human stomach. Here the
bladder-like termination becomes absorbed, and, the neck beginning to
grow, we have the commencement of the form from which we started, and
the completion of that “vicious circle” which is so curious a
characteristic of many forms of parasitic life.
[Illustration:
Fig. 8.
_Tænia solium_: showing the head (_h_) with its suckers (_s′_) and
crown of hooks (_s_), the unjointed neck (_n_), and a few of the
succeeding joints (_j_).
]
In other Tapeworms the cyst may be more complicated than that in the
pig, as, for example, the form found in the sheep’s brain or the liver
of the horse.
Of the other Cestode parasites mention should specially be made of those
of Fishes; the vulgar notion that the parasites of these animals are
dangerous to man has been shown to be entirely erroneous.
[Illustration:
Fig. 9.
_Limnæa truncatula._
]
The _Flukes_ infest animals of all kinds; that which is most dangerous
to sheep, and the cause of much pecuniary loss (_Distoma hepaticum_), is
selected here as a type; its structure is shown by a large model, and
its life history by a series of diagrams (Figs. 10–13). Here, again, we
have a creature which infests two hosts. If the larvæ which escape from
the sheep fall on wet ground in or near a pool, they make their way to a
small pond-snail (_Limnæa truncatula_, Fig. 9), into the lung-chamber of
which they bore their way. On leaving them the larva may be, and is, too
frequently, eaten by a sheep, and makes its way into the liver of that
animal, where it causes the disease known as the “liver rot.”
The damage done by the liver-fluke may be imagined from the fact that in
the winter of 1879–80 no less than three millions of sheep died of rot
in the United Kingdom; this heavy loss is no doubt largely due to the
immense number of eggs to which a single fluke may give rise. It has
been estimated that every fluke may produce, during its life, several
thousands of eggs; and in one case Prof. A. P. Thomas found as many as
7,400,000 eggs in the gall-bladder of a sheep which was suffering from
rot, and which, at that time, had in its liver about 200 flukes.
The _non-parasitic_ Flat-worms are shown, magnified, in the upper parts
of Cases I. & II. The _Turbellaria_ proper, without any or with a simple
or a branched intestine, but without a vent, are represented by
_Convoluta_ and _Thysanozoon_: the general structure is shown by a
diagram in Case II., which is here reproduced (Fig. 14). _Planaria_,
_Thysanozoon_, and _Bipalium_ serve to illustrate the forms of members
of this group.
The Nemertine Worms (_Nemertinea_), with a straight intestine, with a
vent, and with a proboscis, may attain to a very considerable length;
_Carinella_ and _Lineus_ are represented by large figures, and various
species are shown in spirit. These forms, which used to be very
unsatisfactory to exhibit, on account of the great difficulty of
preserving them complete and uninjured, are now, with improved methods,
very satisfactorily shown, as the specimens purchased from the Marine
Biological Laboratory at Plymouth prove.
[Illustration:
Stages in the life history of the Fluke.
Fig. 10. Egg of Fluke, showing the operculum and the contained
yolk-spheres. Magnified 340 diams.
Fig. 11. An embryo forcing its way by its boring-papilla (_p_) into
the wall of the lung of a Snail (_e.p._) Magnified about 340 diams.
Fig. 12. A young _Rédia_ (natural size, ½ millimetre or ¹⁄₅₀ inch):
_pl._, pharynx; _g_, contained germs; _p_, characteristic posterior
processes of the _Rédia_.
Fig. 13. Free-swimming _Cercaria_, before the commencement of the
formation of the cyst. Magnified 100 diams.
]
[Illustration:
Fig. 14. Diagram of the structure of a Turbellarian: _ng_, nerve-
(cerebral) ganglia; _nb_, nerve-branches; _yg_, yolk-glands; _t_,
testis; _o_, ova; _ov_, ovary; _c_, cirrus; _m_, mouth; _ph_,
pharynx.
Fig. 15. Diagram of a Nemertine: _b_, brain; _m_, mouth; _n_, renal
organs; _id_, diverticula of intestine; _g_, gonads; _sn_, side
nerve-trunk; _pr_, proboscis in its dorsal sheath.
Fig. 16. Diagram of the structure of a Nematoid; _m_, mouth; _ph_,
pharynx; _a_, anus; _o_, orifice of genital tube.
]
NEMATODES (Thread-Worms or Round-Worms).—These are for the most part
parasitic, and infest plants as well as animals; the common Round-Worms
living parasitically in man (_Ascaris_, _Stronaylus_, _Trichocephalus_)
belong to this Order. Sometimes they are parasitic in their early stages
and later live a free life—such are _Gordius_ and _Mermis_. A specimen
of a Mantid is exhibited from which half the body of the infesting
_Gordius_ has already protruded (Fig. 17). One of the most remarkable
_Gordii_ is the great elongated _G. fulgur_, or “Lightning Snake,” from
Celebes. Another very large Nematode is the so-called Guinea-worm, or
_Dracunculus medinensis_, which is found beneath the skin of the leg; it
is very possible that this worm was the cause of the illness which
afflicted the Israelites in their journey through the desert from Egypt
to the Promised Land.
[Illustration:
Fig. 17.
_Gordius_ escaping from a Mantid.
]
[Illustration:
Fig. 18.
Figure of _Trichina spiralis_, showing the worms encysted in muscle.
]
Of all Nematodes the most dangerous to man is the small worm which is
known as _Trichina spiralis_ (Fig. 18); a series of models are shown
which give a good idea of the structure of the female and the smaller
male. The young make their way through the walls of the stomach of their
host, and encyst themselves among its muscles: a piece of a
sternothyroid muscle is shown, taken from a man in whose body it was
calculated there were forty millions of encysted _Trichinæ_.
Other Nematodes infesting man, such as _Filaria sanguinis hominis_, are
too small for exhibition.
Plants are not free from the attacks of Nematodes, and examples are
shown, accompanied by an illustrating figure, of the Ear-cockle gall of
wheat; this gall is due to the injuries inflicted by a minute
Thread-worm—_Tylenchus tritici_. Wheat is, of course, by no means the
only cultivated plant that is attacked by these minute worms; the
history of most has, however, still to be made out.
Holding a somewhat uncertain position in relation to the Round-worms are
the parasitic _Acanthocephali_ (Thorn-headed Worms) and the
free-swimming _Chætognatha_, or Bristle-jawed Worms; examples of both of
these groups are shown, together with diagrams illustrative of their
general structure.
ANNULATA or Chætopoda.—So-called because consisting of a series of
rings, and being provided with chætæ or bristles; they are to be
associated with the Arthropoda, under the one head “Appendiculata,” a
better name than “Articulata,” since Cuvier did not include worms in his
group. The creatures that are most familiarly called worms are to be
found in Case III.; here are a few examples of the numerous kinds of
worms that are found living freely in the sea, of earth and freshwater
Worms, and of Leeches. All these worms are distinctly characterized by
the fact that they consist of a number of definite rings (somites),
whence they have been called _Annulata_. The marine Worm and the
Earthworm differ from the Leech in that these rings are provided with
chætæ or bristles, of which there are a number in each bundle in the
marine, and a few only in the terrestrial or freshwater form: hence the
marine Worms are called _Polychæta_ and the latter _Oligochæta_.
The former are divisible into two great groups. There are those that are
free-swimming and are able to forage for themselves, such as the lovely
Sea-mouse (_Aphrodite aculeata_), the large _Eunice gigantea_, the
common _Nereis pelagica_, or the exquisitely coloured _Chloeia flava_.
Others live a more retired life, dwelling in tubes, which they fashion
for themselves; they lead either a solitary or a social life. Here we
have examples of _Sabella_, _Sabellaria_, _Serpula_; a number of forms
of worm-tubes, showing their great variety and beauty (see especially
the delicate _Filograna_), are to be seen in the small Table-cases
placed against the north wall of the Gallery. Attention should be
especially directed to Mr. A. T. Watson’s beautiful preparations of
_Terebella littoralis_. We give a figure (Fig. 19) after a drawing by
that gentleman of the home of _Panthalis oerstedi_, the tube-forming
habits of which have been carefully observed by him.
[Illustration:
Fig. 19.
Home of _Panthalis oerstedi_.
]
[Illustration:
Fig. 20.
Section across the body of an earth-worm to show the disposition of
the more important organs; the body wall (_w_) consists of dermis,
circular, and longitudinal muscles; the body cavity is divided by
membranes (_c_) into a series of chambers, in each of which opens
the mouth of a coiled nephridium (_n_). The axis of the cavity is
occupied by the intestine (_i_); above and below it is a longer
blood-vessel (_v_), and below it is also the central nerve-cord
(_nc_).
]
The _Oligochæta_ are represented by the common Earthworm, the influence
of which in the formation of mould and in the general ploughing of the
soil was carefully investigated by Mr. Darwin; and by the little
_Tubifex rivulorum_ (Bloodworm), which owes both its red colour and its
ability to dwell in mud, which is so poor in oxygen as to be unfit for
respiration, to the same chemical compound as that which gives the red
colour to our blood and carries the oxygen of respiration all over the
body.
[Illustration:
_Acanthobdella_: _e_, eyes; _ch_, chætæ; _s_, sucker.
]
The _Hirudinea_, or Leeches, are often said to be distinguished from the
_Chætopoda_ by the absence of bristles, but, as a fact, _Acanthobdella_
(Figs. 21 and 22) has very well marked bristles. They always have a
sucker at the hinder end of the body by which they are attached to their
prey; they are found in fresh water (_Piscicola_), on sea-fishes (as
_Pontobdella_), or in moist places, as the Leech (_Hirudo_). The
last-named has three jaws, armed with as many as ninety denticles.
_Trochetia subviridis_ (Land-Leech) is a species which is found rarely
and sporadically in England.
The Myzostomaria form a division of Polychæta all the members of which
live parasitically on Crinoids, and otherwise present great differences
in their habits. Some move about freely on the Crinoids they infest,
others are more sluggish and rarely move, others produce galls or cysts
on their host, and yet others are internal parasites, and live in the
alimentary canal. It is of interest to note that there are corresponding
degrees of difference between the young and old specimens of the
different groups of species.
The general organisation of Myzostomaria is shown in the accompanying
figure (Fig. 23) in which the dorsal wall of the body is supposed to be
transparent so as to allow of the chief internal organs being seen.
[Illustration:
Fig. 23.
Diagram of Myzostomum to show the general form of the body and the
marginal extensile cirri (_c_); within these and on the ventral
surface are four pairs of suckers, and more internally five pairs of
appendages each bearing two hooks; the proboscis (_p_), the
digestive tract and its ramifications, and the reproductive organs
are outlined as if seen through a transparent wall; _a_, anus.
]
The last group of Worms here represented is that of the _Gephyrea_; with
the advance of our knowledge it is probable that they will be found to
be more intimately allied to the _Annulata_ than is now generally
supposed; it will be seen indeed that _Echiurus_ has bristles at its
hinder end; _Sipunculus_ is the best known representative of the unarmed
_Gephyrea_; _Bonellia_ is interesting both from the fact that it owes
its green colour to a matter closely resembling the chlorophyll of green
plants, and from the possession by the female of a proboscis, which is
protruded from the hole in the rock occupied by the worm: the male is
very much smaller than the female, and is not nearly so well developed.
Owing to the mode of lighting the Gallery, the visitor may have to shift
his position several times before gaining a good view of the whole
length of the proboscis.
INDEX.
Acanthaster, 114
Acanthobdella, 124
Acanthocephali, 122
Acmæidæ, 9
Actinometra, 113
Adeonidæ, 66
Ætheriidæ, 39
Alcyonidium, 67
Amaroucium, 99
Amathia, 67, 68
Amphineura, 7–9
Ampullariidæ, 14
Ancylus, 27
Annulata, 122
Anodonta, 39
Antedon, 111–113
Aphrodite, 122
Aplacophora, 9
Aplysiidæ, 23
Apoda, 115
Appendiculariidæ, 105
“Apple-Snails”, 14
Architeuthis, 48
Argonauta, 46
Ascaris, 120
Ascidia, 83, 93
Ascidiacea, 90
Ascidiæ Compositæ, 96
Ascidiæ Salpiformes, 99
Ascidiæ Simplices, 90
Ascopodaria, 73
Astartidæ, 38
Asterias, 108, 114
Asterina, 111
Asteroidea, 112, 113
Astrophyton, 114
Atlantidæ, 22
Auger-shells, 21
Auriculidæ, 26
Aviculidæ, 33
Bartlettia, 39
Basket-fish, 114
Basommatophora, 26
Bêche-de-mer, 115
Bipalium, 118
Bird’s head Coralline, 58
Bladder-worm, 117
Blastoidea, 112
Bloodworm, 124
Boat-shells, 20
Boltenia, 91, 92
Bonellia, 126
Borer, 43
Botryllidæ, 96, 97
Botrylloides, 97
Botryllus, 97, 98
Bowerbankia, 67
Brachiopoda, 74–82
Brechites, 44
Bristle-jawed worms, 122
Brittle Stars, 114
Bubble-shells, 23
Buccinidæ, 18, 19
Bugula, 58, 59
Bullidæ, 23
Caberia, 62
Calamaries, 4
Calyptræidæ, 14
Cardiidæ, 40
Carditidæ, 38
Carinariidæ, 22
Carinella, 118
Carrier-shells, 17
Cassididæ, 18, 19
Catenicellidæ, 66
Cavolina, 23
Cellularina, 58
Cephalopoda, 45–50
Cerithiidæ, 17
Cestoda, 116
Cestode parasites, 116
Chætoderma, 9
Chætognatha, 122
Chætopoda, 116, 122
Chama, 41
Chelyosoma, 93
Chilostomata, 57
Chirodota, 115
Chiroteuthis, 47
Chitonidæ, 7, 8
Chloeia, 122
Clams, 41
Clavagellidæ, 44
Clavelina, 94, 95
Clavelinidæ, 94
Clione, 24
Coat-of-mail shells, 7
Cockle, 40
Colella, 97, 98
Comatulidæ, 111
Conchologists, 17
Conidæ, 21
Convoluta, 118
Coralliophilidæ, 20
Corbula, 42
Cowries, 14
Crania, 74, 80
Creeping Coralline, 60
Crepidula, 14
Crinoidea, 112
Crinoids, 109
Crisia, 69
Crisiidæ 69
Cristatella, 71, 72
Cryptochiton, 8
Cryptoplax, 8
Ctenostomata, 57, 66
Cucumaria, 115
Culeolus, 92
Cup-and-saucer Limpets, 14
Currant Squirter, 92
Cuspidariidæ, 45
Cuttlefish, 48
Cyclophoridæ, 13
Cyclostomata, 57, 68
Cynthiidæ, 91
Cypræidæ, 14
Cysticercus, 117
Cystidea, 112
Dentaliidæ, 28
Desert-snail, 6
Diadema, 114
Diazona, 94
Dipsas, 36
Discinidæ, 80
Distoma, 118
Distomidæ, 96
Doliidæ, 18
Doliolidæ, 100, 103
Doliolum, 90, 103
Dracunculus, 121
Dyscolia, 82
Ear-cockle gall, 122
Ear-shells, 12
Earthworms, 124
Echinoderma, 106–115
Echinoidea, 112, 114
Echinometra, 114
Echinozoa, 112
Echiurus, 126
Ectoprocta, 57
Elasipoda, 115
Electra, 62, 63
Elephant-tooth shell, 1
Entoprocta, 57, 73
Escharina, 64
Eulamellibranchia, 38
Eunice, 122
Euthyneura, 23
False Limpets, 9
Fan-Mussel, 34
Fan-Shells, 38
Fasciolariidæ, 18
Feather-star, 111
Filaria, 122
Filibranchia, 32
Filograna, 123
Fissurellidæ, 11
Flat-worms, 116
Flukes, 118, 119
Flustra, 54, 62
Flustrina, 62
Fountain-shell, 17
Fredericella, 71
Freshwater Limpets, 27
Freshwater Mussels, 38
Freshwater Oyster, 39
Freshwater Polyzoa, 70
Freshwater Snails, 7, 26
Freshwater Worms, 122
Gapers, 42
Gastropoda, 9–28
Gephyrea, 126
Giant Clam, 41
Glassy Nautilus, 22
“Glory-of-the-Sea” Cone, 21
Glottidia, 78
Goodsiria, 99
Gordius, 120, 121
Gorgon’s head, 114
Guinea-worm, 121
Gymnolæmata, 57
Gymnosomata, 24
Haliotidæ, 12
Hammer Oyster, 33
Harpidæ, 20
Harp-shells, 20
Helicidæ, 27, 28
Helmet-shells, 18
Hemiaster, 110, 114
Heteropoda, 22
Hippuritidæ, 41
Hirudinea, 124
Hirudo, 124
Holothuria, 107, 115
Holothurioidea, 112, 115
Ianthinidæ, 16
Idmonea, 69
Julinia, 98
Keyhole Limpets, 11
Kinetoskias, 59–61
Kuphus, 43
Lamellibranchia, 29–45
Land-Snails, 26
Larvacea, 104
Leeches, 124
Lepralia, 64
Leptoclinum, 98, 99
Lichenopora, 69
“Lightning-Snake”, 121
Lily-Encrinites, 110
Limidæ, 37
Limnæa truncatula, 118
Limnæidæ, 26
Limpet, 9
Limpet Snails, 26
Linckia, 113, 114
Lineus, 118
Lingula, 74, 75, 77–79
Lithodomus, 33
Littorinidæ, 14
Liver-fluke, 118
Loligo, 46
Lophopus, 71, 72
Lotoriidæ, 18
Loxosoma, 73
Lucinidæ, 38
Lunulites, 64
Mactridæ, 40
Magellania, 75, 77, 82
Magilus, 20
Malleus, 33
Margaritana, 39
Marine worms, 122
Melaniidæ, 17
Meleagrina, 33–35
Melons, 20
Membranipora, 63
Mermis, 120
Metacrinus, 113
Mineralogists, 17
Mitridæ, 18
Molgula, 91
Molgulidæ, 90
Mollusca, 1–53
Money-Cowry, 15
Moss Animals, 56
Mucronella, 65
Mülleria, 39, 40
Muricidæ, 20
Mussels, 33
Mussels (freshwater), 38
Myidæ, 42
Mytilidæ, 33
Myzostomaria, 125
Naked-gilled Molluscs, 24
Natica, 15
Nautilus, 45, 49
Nematoidea, 116, 120
Nemertinea, 116, 118, 120
Neomenia, 9
Nereis, 122
Neritidæ, 13
Non-parasitic Worms, 118
Nucleobranchiata, 22
Nuculidæ, 31
Nudibranchia, 23, 24
Octacnemidæ, 100
Octopus, 46, 47
Oikopleura, 104
Oligochæta, 122, 124
Olividæ, 20
Ophiacantha, 111
Ophiuroidea, 112, 114
Opisthobranchia, 23
Orange Cowry, 15
Oreaster, 114
Ormers, 12
Ostreidæ, 36
Ovulidæ, 15
Oyster, 36
Panthalis, 123
Paper-Nautilus, 46
Patella, 9
Pearl-Oyster, 33
Pearly Nautilus, 50
Pectinibranchia, 13
Pectinidæ, 38
Pedata, 115
Pedicellinidæ, 73
Pelmatozoa, 111
Pentacrinus, 111, 113
Periwinkle, 14
Perophora, 95, 96
Phallusia, 93
Pharyngodictyon, 98, 99
Pholas, 43
Phylactolæmata, 70
Piddocks, 43
Pinna, 33, 34
Piscicola, 124
Planaria, 118
Platyhelmia, 116
Pleurotomaria, 11
Pleurotomatidæ, 20
Plumatella, 71, 72
Polychæta, 122
Polyclinidæ, 96
Polyplacophora, 7
Polyzoa, 54–73
Pond-Mussel, 5
Pond-Snails, 27
Pontobdella, 124
Poromyidæ, 45
Poulp, 7
Proneomenia, 7
Protobranchia, 31
Pseudolamellibranchia, 33
Psolus, 115
Pterocera, 18
Pteropoda, 23
Pterotracheidæ, 22
Pulmonata, 25–28
Purpura, 20
Pyrosoma, 90, 99
Radiolitidæ, 41
Razor-shells, 42
Retepora, 65, 66
Rhodosoma, 93, 94
Rhopalæa, 96
Rhynchonella, 82
River-Snails, 13
Rock-shells, 20
Round Worms, 118
Sabella, 122
Sabellaria, 122
Saddle-Oysters, 32
Salpa, 90, 100–103
Salpidæ, 100
Scala, 17
Scallops, 38
Scaphopoda, 28
Schizoporella, 65
Scorpion-shells, 18
Screw-shells, 17
Scrobiculariidæ, 40
Scrupocellaria, 60, 62
Scutibranchia, 9
Sea-Butterflies, 23
Sea-Cucumbers, 106, 115
Sea-Hare, 23, 24
Sea-Lilies, 106
Sea-Mat, 54
Sea-Mouse, 122
Sea-Slugs, 106
Sea-Urchins, 106, 114
Sea-Woodlice, 7
Selenariidae, 64
Semele, 40
Sepia, 46–48
Sepiola, 46
Septibranchia, 45
Serpula, 122
Ship-worm, 43
Silk Coralline, 68
Siphonariidæ, 26
Sipunculus, 126
Slipper-Limpets, 14
Slit-Limpets, 11
Slugs, 28
Snails, 27
Solenidæ, 42
Solenomyidæ, 31
Spatangus, 109
Spirula, 49
Spondylidæ, 37
Squid, 46
Stalked Crinoids, 110, 112
Starfishes, 108
Stichopus, 107
Streptoneura, 9
Strombidæ, 17
Strongylus, 120
Styelopsis, 92
Stylommatophora, 27
Synapta, 107, 115
Tænia, 116, 117
Tapeworm, 116
Tectibranchia, 23
Tellina, 40
Terebella, 123
Terebratula, 74, 82
Terebratulina, 82
Terebridæ, 21
Teredo, 43, 44
Testacella, 28
Tethys, 24
Thaliacea, 100
Thecalia, 38
Thecidium, 81
Thecosomata, 23
Thorn-headed Worms, 122
Thorny Oysters, 37
Thread-worms, 118
Thysanozoon, 118
Tooth-shells, 28
Top-shells, 12
Trematoda, 116
Trepang, 115
Trichina, 121
Trichocephalus, 120
Tridacnidæ, 41
Trochetia, 125
Trochidæ, 12
Trumpet-shells, 18
Tubifex, 124
Tubulipora, 69, 70
Tunicata, 83–105
Tun-shells, 18
Turbellaria, 116, 118, 120
Turbinidæ, 12
Turritellidæ, 17
Tylenchus, 122
Umbraculidæ, 23, 24
Umbraculum, 24, 25
Umbrella-shells, 23
Unionidæ, 38, 39
Veneridæ, 40
Venus-shells, 30
Vermetidæ, 17
Vesicularia, 67
Violet Snails, 16
Viviparidæ, 20
“Volutes”, 20
Volutidæ, 20
Water-Clams, 37
Watering pot-shells, 44
Water Spondyli, 37
Weaver’s-shuttle, 15
Whelk, 18
Window-shells, 32
Wing-shells, 33
Winkles, 14
Worms, 116–126
Worm-shells, 17
Worm-tubes, 123
Xenophoridæ, 17
LONDON: PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, STAMFORD STREET
AND CHARING CROSS.
-----
Footnote 1:
A framed series of photographs, illustrating different kinds of
radulæ, is placed on the east wall of the gallery.
Footnote 2:
From Woodward’s ‘Manual of the Mollusca,’ published by Lockwood & Son.
Footnote 3:
From ‘The Cambridge Natural History,’ Messrs. Macmillan & Co.
Footnote 4:
From the Greek: _pteron_, wing, and _pous_, foot.
Footnote 5:
From the Greek: _scaphe_, a small boat, and _pous_, a foot—the foot of
some Scaphopods being somewhat pointed like the prow of a vessel.
Footnote 6:
The term Lamellibranchia is used instead of Pelecypoda in deference to
the wish of Professor Lankester.—E. A. S.
Footnote 7:
From the ‘Encyclopædia Britannica.’ Messrs. A. & C. Black.
Footnote 8:
From ‘The Cambridge Natural History.’ Messrs. Macmillan & Co.
Footnote 9:
From the Greek: _kephale_, head, and _pous_, foot.
Footnote 10:
_Zoon_, animal; _oikos_, house.
Footnote 11:
_Lophos_, plume; _pherein_, to bear.
Footnote 12:
_Polus_, many; _zoon_, animal.
Footnote 13:
_Bryon_, moss.
Footnote 14:
_Ektos_, outside; _proktos_, vent.
Footnote 15:
_Cheilos_, lip; _stoma_, mouth.
Footnote 16:
_Gumnos_, naked; _laimos_, throat.
Footnote 17:
_Ktenos_, of a comb.
Footnote 18:
_Phulassein_, to guard; _laimos_, throat.
Footnote 19:
_Kuklos_, circle.
Footnote 20:
_Entos_, inside; _proktos_, vent.
Footnote 21:
_Vibraculum_, a bristle.
Footnote 22:
_Pyrosoma_—_pur_, fire; _soma_, body.
Footnote 23:
It should be mentioned that one high authority, Prof. W. K. Brooks,
does not regard the life history of _Salpa_ as an example of
alternation of generations, but considers the solitary _Salpa_ to be,
not asexual, but a female which produces a chain of males; but it is
impossible to enter into a difficult question of controversy here.
Footnote 24:
From the Greek _pelma_ = a stalk.
Footnote 25:
The Asteroidea or Ophiuroidea may be counted under the name
Stelliformia.
------------------------------------------------------------------------
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