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Title: The story of life in the seas
Author: Sydney J. Hickson
Release date: March 31, 2026 [eBook #78328]
Language: English
Original publication: London: G. Newnes, 1898
Other information and formats: www.gutenberg.org/ebooks/78328
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*** START OF THE PROJECT GUTENBERG EBOOK THE STORY OF LIFE IN THE SEAS ***
Transcriber’s Note
1. Certain typographic errors & hyphenation inconsistencies were
silently corrected.
2. The text version is marked up as follows:
(a) italics are indicated thus _italic_;
(b) small-caps are indicated thus +Small-Caps+;
(c) Images are indicated as [Illustration: (with narration.)]
3. Illustration captions have been standardised.
4. Table of Contents extended by adding references to the LIST OF
ILLUSTRATIONS, INDEX, and ADVERTISEMENTS.
[Illustration: _Frontispiece._
+A Jelly-fish with the fry of the Horse-mackerel.+
(The fish have been relatively very much enlarged.)]
THE
STORY OF LIFE IN THE SEAS
BY
SYDNEY J. HICKSON, D.Sc., F.R.S.
_Professor of Zoology in the Owens College, Manchester_
_WITH FORTY-TWO ILLUSTRATIONS_
LONDON
GEORGE NEWNES, LIMITED
SOUTHAMPTON STREET, STRAND
1898
OUTLINE CLASSIFICATION
OF
ANIMALS MENTIONED IN THIS BOOK.
PROTOZOA {Foraminifers.
{Radiolarians.
PORIFERA Sponges.
{Sea-anemones.
CŒLENTERATA {Corals.
{Jelly-fish.
{Many of the Zoophytes.
{_Asteroidea_ Star-fishes.
ECHINODERMA {_Echinoidea_ Sea-urchins.
{_Crinoidea_ Sea-lilies.
{_Holothuroidea_ Trepangs.
PLATYELMIA Flukes, Tape-worms, Planarians.
CHÆTOPODA Segmented Worms and Gephyreans.
{ {Copepods.
{ {Barnacles.
ARTHROPODA {_Crustacea_ {Shrimps.
{ {Crabs.
{_Insecta_ Halobates, &c.
{_Lamelli_ Bivalves.
{ _branchiata_
MOLLUSCA {_Gastropoda_ Whelks, &c., and Pteropods.
{_Cephalopoda_ Octopuses, Cuttlefishes, &c.
TUNICATA Sea-squirts, Salps, Pyrosoma.
{_Pisces_ Fish.
{_Amphibia_ Frogs.
{_Reptilia_ Turtles, Crocodiles, Snakes.
VERTEBRATA {_Aves_ Birds.
{ {Whales, Porpoises.
{_Mammalia_ {Seals,
{ {and the terrestrial Mammals.
PREFACE.
The story of the life of animals and plants in the sea is one with so
many aspects, that it is difficult to choose the points that may be
included, and those that may be omitted from a book intended for the
general reader. To some the story of the food Fishes and the Whales is
of the greatest interest; to others the beautiful shapes and colours
of shells have a predominating fascination; and to those who have
devoted themselves to geological study, the history of the animals that
contribute to the formation of the reefs and the ocean-bed present
features of special attraction. To many, then, the perusal of my book
must lead to disappointment as no one of these aspects has been treated
adequately; but if some new interest is awakened, some new train of
thought quickened into life, one of the objects I had in view will have
been gained.
The book is only intended to be a sketch of some of the most important
lines of scientific researches which are now being pursued by
zoologists in many parts of the world. Discoveries, which are of the
deepest interest to all intelligent minds, are in many cases described
in books and periodicals that do not come within the reach of the
general public. I have tried, therefore, to collect some of them into
a small compass and describe them in language which I trust will be
intelligible to those who have not been trained in the alphabet of
zoological technicalities. The use of some long words was unavoidable,
but I have endeavoured to explain them adequately either in the text or
in the index.
Some of the illustrations have been copied from the works of other
Naturalists, and the sources from which they came are acknowledged in
the list of illustrations; but the majority of them have been drawn,
specially for this work, from specimens from the Manchester Museum or
my own collections.
SYDNEY J. HICKSON.
_November 1897._
CONTENTS.
CHAP. PAGE
LIST OF ILLUSTRATIONS 8
I. OCEANOGRAPHY 9
II. SHALLOW-WATER FAUNA 23
III. SHALLOW-WATER FAUNA OF THE TROPICS 55
IV. SURFACE-SWIMMING FAUNA (INVERTEBRATES) 82
V. SURFACE-SWIMMING FAUNA (VERTEBRATES) 118
VI. DEEP-SEA FAUNA 135
VII. COMMENSALISM AND PARASITISM 146
VIII. THE ORIGIN OF THE MARINE FAUNA 172
INDEX 180
ADVERTISEMENTS 184
LIST OF ILLUSTRATIONS.
ILLUSTRATION PAGE
_Frontispiece._--A Jelly-fish with the fry of the Horse Mackerel
(from specimens captured by Mr F. W. Gamble off Valentia). 2
Fig. 1--Globigerina Shell 21
Fig. 2--Radiolarian Shells 22
Fig. 3--Common Pipe-fish (from Royal Natural History) 25
Fig. 4--Phyllopteryx (from Royal Natural History) 26
Fig. 5--Diagrams of Eyes 27
Fig. 6--A branch of the Zoophyte Obelia 30
Fig. 7--Medusa produced by Obelia 31
Fig. 8--A Bivalve Mollusc 35
Fig. 9--The common Sole 37
Fig. 10--The Angler (from Royal Natural History) 38
Fig. 11--Vertical section of a Balanus (after Claus) 40
Fig. 12--Nauplius larva of a Balanus (after Groom) 41
Fig. 13--Sea-urchin 43
Fig. 14--Smooth-shelled Gastropod 47
Fig. 15--Spiny Gastropod 47
Fig. 16--A Cuttle-fish 49
Fig. 17--The Wrasse 51
Fig. 18--The John Dory 54
Fig. 19--Polyp of a Madrepore Coral (after Fowler) 58
Fig. 20--Chætodon 62
Fig. 21--Globe-fish 64
Fig. 22--Stereosoma 67
Fig. 23--Coral-reefs 69
Fig. 24--Periophthalmus 74
Fig. 25--Free-swimming Copepod (after Claus) 88
Fig. 26--Swim-bladder of Velella 96
Fig. 27--Solitary form of Salp 99
Fig. 28--Pteropod 101
Fig. 29--Shells of Foraminifers 104
Fig. 30--Globigerina 105
Fig. 31--Young larva of a Star-fish 111
Fig. 32--Pluteus larva 113
Fig. 33--Long-spined Barnacle-nauplius (after Chun) 116
Fig. 34--Sun-fish 122
Fig. 35--The common Porpoise 129
Fig. 36--A deep-sea Fish (after Filhol) 139
Fig. 37--Hermit-crab and Sponge 149
Fig. 38--Section through a Sponge showing Hermit-crab 150
Fig. 39--A Trepang 153
Fig. 40--A Crab-gall 158
Fig. 41--A parasitic Copepod 169
THE STORY OF LIFE IN THE SEAS.
CHAPTER I.
OCEANOGRAPHY.
One of the most important facts that has been established by modern
investigations of the Sea is that there is no region in its vast extent
that is entirely devoid of animal life. The surface waters in the
Equatorial calms and the ice-cold waters between the ice-bergs of the
Arctic regions are densely populated by animals, large and small; the
heavy and heated waters of the Mediterranean and Red Seas, and the cold
and comparatively fresh waters of the Norwegian fjords, the shallow
waters of the coasts and the greatest depths of the ocean-beds all
present us with their characteristic forms of living creatures. There
is no Azoic region known to us. Wherever we use the trawl or dredge
we may expect to find some representatives of the various classes of
marine animals. But the seas exhibit so many varying conditions that,
as we might have expected, the animals that characterise one region are
absent from another; and while, in some places animal life is abundant,
in others it is very scarce; just as on land we find the grass-lands
and forests teeming with life, and the great deserts and mountain tops
inhabited only by a few solitary Lizards, Birds or Insects.
In order that we may fully understand, then, the nature of the problems
concerning the distribution of animals through the seas, it is of
importance to consider first the conditions under which they must
live in the different parts of the ocean. A knowledge of geography is
clearly necessary for those who study the distribution of terrestrial
and aërial animals, and equally necessary is it for those who wish
to learn something about the distribution of the aquatic animals,
to consider first of all the rudimentary principles of hydrography.
The principal Sea areas of our globe may be roughly divided into two
groups: namely, the great oceans--the Atlantic, Pacific and Indian
Oceans--and the Inland seas, which are partly enclosed by land, such
as the German Ocean, the Mediterranean Sea, and the Red Sea. Taking
the areas of the great oceans and the seas together, we find that no
less than 141 millions of square miles, or nearly three-quarters of the
surface of the globe, are covered with water. Moreover, these great
areas are continuous, so that it would be possible for an animal, other
conditions being favourable, to pass from any one sea, such as the
Black Sea, to any other, such as the Hudson’s Bay, without leaving the
water,--an important fact in the consideration of the distribution of
marine forms of life.
The depth of the sea varies very considerably in the different parts of
the world. In the inland seas the water is comparatively shallow, but
in the great oceans it is very deep. In the middle of the North Sea,
for example, we should not expect to find a depth much exceeding 250
fathoms, but in the Atlantic or Pacific Oceans we have to pay out more
than 2000 fathoms of the sounding-line before the bottom is reached. In
some parts of the ocean-basins a few very deep holes or furrows may be
found in which the depth exceeds 4000 fathoms, or 24,000 feet. One of
these deep holes occurs in the Atlantic Ocean, a little to the North
of the Virgin Islands in the West Indies, and there is another in the
Pacific Ocean close to the coast of Japan; but the greatest depth that
has yet been found is one recently discovered by H.M.S. “Penguin” off
the coast of New Zealand of over 5000 fathoms. Apart, however, from
the fact that these very great depths are only of local occurrence,
the areas of deep water--that is, of more than 2000 fathoms--are so
much greater than the areas of shallow water, that when we make a
calculation of the average depth of the sea we find it is no less than
2100 fathoms, or 12, 600 feet.
The temperature of the sea is another feature which undoubtedly
influences very greatly the character of its Fauna. The main source of
the heat of the sea is the sun--for the heat derived from submarine
volcanoes must be comparatively so small that we may omit it from
consideration. Consequently we find that in the Equatorial regions the
surface waters of the ocean are warmer than they are in the Temperate
regions. These, again, are warmer than in the Arctic circles. But
water is well-known to be a bad conductor of heat, and therefore
the direct influence of the sun affects only the most superficial
layers. In the Equatorial region of the Pacific Ocean, for example,
the surface temperature is sometimes as high as 80° Fahr., at 100
fathoms from the surface it is only 60°, at 400 fathoms only 45°, and
at 1000 fathoms only a few degrees above freezing point. On the land
the temperature falls as we pass from the coast to the high plateaux
and mountains, and we find snow-capped mountains in Central Africa just
as in Switzerland or Norway. In the sea the temperature falls as the
thermometer is sent deeper from the surface. Just as on the land the
snow line of the mountains is reached at high altitudes in the Tropics,
at lower altitudes in the Temperate regions, and in the Arctic circle
at the level of the sea, so in the sea the cold water that is found 500
fathoms below the surface in the Tropics, reaches a higher level in the
Temperate regions, and is at the surface in the Arctic circle.
There is however one important point of difference between the
distribution of these low temperatures on the land and in the oceans,
in that they are broken in the former, and continuous in the latter.
If we were to imagine an aquatic animal that could only live in
temperatures below 35° Fahr., it would be able to travel below the
surface from one pole to the other, or from one ocean to another; but
it would be impossible for a terrestrial animal, exhibiting the same
peculiarity, to leave the Arctic circle or the Alpine region without
traversing lands where the temperature is higher than that which is
necessary for its existence. It might be supposed from what has just
been said that the temperature of the water at the bottom is constant
for the same number of fathoms of depth. This is not, however, the
case. The temperature of the sea-bottom of the great ocean-beds is
approximately the same, varying from 28° F. in the Atlantic to 35°
F. in the Pacific; but in places where main basins occur, surrounded
on all sides by shallower ridges, the temperature of the bottom of
the basin is the same as that of the lowest ridge. For instance, the
temperature of the bottom of the Sulu Sea, lying between Borneo and
the Philippines, is 40° F. at a depth of over 2000 fathoms. Again, the
temperature of the Red Sea is as high as 70° F., although depths of
1200 fathoms occur in its central portions; and this is the same as the
temperature at the Straits of Babel Mandeb, which are 200 fathoms deep,
and form the only outlet to the open ocean. These facts probably cause
considerable modification in the character of the animals inhabiting
such enclosed basins, but further investigations are needed before we
can arrive at any very definite conclusions in the matter.
Another important element that must be taken into consideration in
studying the environment of marine animals, is the quantity and
character of the salts held in solution by the sea-water. In the first
place we must remember that the sea-water normally contains a far
greater percentage of salts in solution than the water of rivers and
lakes, and this causes it to be very much heavier. If a tumbler be
half filled with sea-water, upon which some fresh water is slowly and
carefully poured, there will be for some time very little mixture of
the two fluids, the heavier sea-water remaining at the bottom, and the
lighter fresh water at the surface. Now the density of the sea-water,
or in other words the amount of salts in solution, is not the same
over the whole world, and the differences that may be observed in
this respect are due, in most cases, to the simple physical principle
just enunciated. If we could imagine a river pouring its waters into
a perfectly calm, tideless sea, we should be able to trace the fresh
river water far away from the coast, for it would simply float on the
heavier sea-water without mixing with it to any appreciable extent. In
most cases, however, the tidal-waves, rushing up and down the river
estuaries, stir up the fresh and salt water together, and cause a very
considerable mixture, so that the water becomes either distinctly salt
or brackish. Where very large quantities of fresh water are poured
into the ocean, as, for example, at the mouth of the Amazon or the
Mississippi, the surface water remains so fresh that the salt taste
can hardly be appreciated at a distance of some miles from the coast.
This fact sufficiently indicates the influence of great rivers upon the
density or saltness of the sea-water in their neighbourhood, and the
reader will be prepared for the statement that many inland seas, such
as the Black Sea, are appreciably less salt than the great oceans.
Again, the ocean water itself is not of the same density in all
latitudes. In regions where there is a copious rainfall and the sea is
not frequently disturbed by severe storms, the rain takes some time to
mix with the heavier salt water on which it falls, and consequently
there may always be discovered in these localities a thin stratum of
comparatively fresh water on the surface of the ocean. In some inland
seas where there is considerable evaporation and a slight rainfall, as
for example, the Red Sea and the Mediterranean, the sea-water reaches
an even higher degree of concentration than it does in the open ocean.
The following table will serve to illustrate these facts:--
Density of rain-water, 1·00.
Density of the Black Sea surface, below 1·025.
Density of the Atlantic Ocean surface (west of the Canaries), 1·0275.
Density of the Mediterranean Sea, over 1·028.
Density of the Red Sea, 1·030.
Density of the bottom water of the Atlantic, 1·029 (west of the
Canaries).
That the rate of movement of the water influences very largely the
character of the animals that live in it, is a fact that it is not
necessary to discuss fully in this place; but as it is undoubtedly one
of the factors which must be taken into consideration in discussing the
character and possible origin of the Fauna of any particular region,
a brief survey must be given of some of the principal causes of the
movements of the water and the characters of the tides and currents
which are manifest in the sea. Twice every twenty-four hours the water
of the sea rises and falls. This movement is due to the attracting
influences of the sun and moon, and is, as is well-known, greater when
the moon is full and when it is new than at the intermediate times. If
the distribution of land and water on the surface of our globe were
different, and a free waterway occurred round the world, right in the
Equatorial band we should probably find a double tidal-wave rushing
round the earth every twenty-four hours. As it is, however, the great
tidal-wave is checked by the continents, and as it approaches the
coasts is retarded and diminished in force. In Archipelagoes and along
broken coast lines the tidal-waves produce true surface currents, which
frequently run with great rapidity and exert considerable corroding
action upon the rocks. In many estuaries and bays the tide rushes in
with such force that the water is heaped up to a great height against
the land. At the entrance to the Bay of Fundy, for example, the rise
at spring-tides is no less than 70 feet, and at the Cardiff docks the
difference of level between high and low spring-tides is 42 feet. The
tumultuous ebb and flow of such masses of water along the coast is
fatal to some forms of animal life and favourable to others, and so to
some extent it modifies the character of the Fauna.
In addition to the surface currents of the coast, produced by the
tidal-waves, there are also the true ocean-currents, which must be
briefly considered. They are caused by the winds which blow constantly
in a definite direction across the oceans. The prevailing winds not
only raise the sea into waves, but drive the superficial layers of the
water over the subjacent layers in one direction. In studying a map
of the great ocean-currents, we notice a well-marked one lying to the
North of the Equator in both the Atlantic and Pacific. This flows from
East to West and follows very closely the lines of the prevailing winds
in that region. Similarly in the temperate regions of the Southern
Hemisphere there is an ocean-current, flowing however in this case in
the opposite direction--from West to East--and so corresponding with
the trade-winds of that part of the world. The well-known Gulf-stream
of the North Atlantic, although modified in some respects by other more
complicated causes, also follows for part of its course the general
direction of the prevailing winds.
The currents just described are surface currents only, and do not
affect to any great extent the mass of the subjacent waters in the
ocean-basins. It is difficult to estimate the depth to which their
influence reaches, but it is not probable that it extends more than 200
fathoms below the surface. In addition to these, there is also a series
of slow currents in the deep waters flowing in definite directions.
In the Tropical regions the waters are constantly being heated by the
sun, and passed away by the trade-winds to the North and South and
ultimately towards the poles. In their long and complicated journey
they are gradually cooled down until, in the regions of the ice-bergs,
they reach a temperature just above the freezing point of sea-water.
Here the water, being colder and therefore heavier than that of the
other regions of the world, sinks to the bottom, and gradually returns
in a deep-seated mass towards the Equator, where, welling up from the
bottom, it replaces the heated layers of the surface. It is almost
impossible to determine with accuracy the rapidity and exact direction
of these deep-sea-currents. It is extremely probable that they are
immensely modified by the irregularities of the bottom and the outline
of the coast banks, but their exact topography must remain for the
present one of the secrets of the abyss that are not revealed to us.
All that can be said is, that the warm surface water which passes from
the Tropics towards the North and South is replaced by deep-seated
Polar currents, which account for the extremely cold water that is
found at great depths in the ocean-basins, and also for some of the
peculiarities of the marine Fauna, which will be referred to later on.
The character of the sea-bottom in various parts of the world must
be referred to before passing on, for there can be little doubt of
the important effect it has upon the Fauna. In the neighbourhood of
continents the bottom of the sea varies very considerably. The great
rivers bring with them in suspension the products of the wear and tear
of mountains and valleys; the coast line, washed by the continuous
ebb and flow of the tides, contributes some of its substance to the
formation of the sea-bottom; and the countless millions of animals
and plants of the shallow waters leave their skeletons and shells as
they die to form an integral part of the floor of the ocean. Thus
the sea-bottom in the neighbourhood of the land is formed partly
by terrigenous deposits, varying of course in character with the
geological nature of the land itself, and partly by the animal and
vegetable deposits of the coast. In some cases the deposits brought by
the rivers can be traced in the sea-bottom for a very great distance
from the coast. The characteristic mud of the Congo river can be traced
600 miles from its mouth, and it is said that the Arabian Sea and the
Bay of Bengal are carpeted for 1000 miles by the mud brought down by
the Ganges and the Indus. Leaving out of consideration, however, for
the moment the exceptional cases of such large rivers as these, we may
say that the influence of the land deposits upon the character of the
sea-bottom extends to a distance seawards of about 250 miles. If we
had a complete and careful survey of all the coast lines, it would be
possible for us to draw a line round the great continents marking the
limit of the deposits of river and coast mud. This line has been called
the mud-line by Mr John Murray, and, as he has clearly pointed out, it
is characterised by an abundant and extremely interesting Fauna.
The sea-bottom, then, within the limits of the mud-line, is very
largely composed of deposits from the land brought down by the rivers.
In some volcanic regions of the world this is, to a great extent,
augmented by lava and water-logged pumice, and in other districts by
the mud and stones dropped by the melting ice-bergs. The influence
of animals and plants upon the formation of the sea-bottom is often
very great indeed in shallow water, though it varies considerably in
different parts of the world. In the neighbourhood of the British
coast, for example, the sea-bottom is, in many places, carpeted with
the calcareous Sea-weed, _Lithothamnion_,--in other places the
dredge will come up crammed full of bivalve shells. But such instances
as these in which the floor of the sea is covered with animal or
vegetable shells, are comparatively rare and of small extent in the
neighbourhood of land in the Temperate regions, and in nearly all
localities the true terrigenous deposits can be readily obtained by
the use of a small meshed dredge. In the warmer regions of the world,
however, the sea-bottom in the shallow water is over great areas
completely covered by animal and vegetable products. In the West
Indies, and in some parts of the Eastern coasts of Tropical America, in
the Eastern Archipelago and the coast of East Africa and its islands,
Coral-reefs are found. These are entirely built up of the skeletons and
shells of animals, and a few Coralline Algæ. In the vicinity of these
reefs the floor of the sea is for miles carpeted with the broken-down
skeletons of these animals, sometimes in the form of a fine coralline
sand, sometimes of large lumps studded with knolls of living Corals,
Molluscs, Sea-urchins, and other creatures. We find, therefore, in the
warmer regions of the world immense areas of shallow water in which the
terrigenous deposits take but a very small part in the formation of the
sea-bottom, animal and vegetable life being so vigorous and active as
to be able to form enough shells and skeletons to cover every available
part of its surface.
[Illustration: +Fig. 1.+
Globigerina Shell from a deep-sea ooze.]
Far away from continental lands, and at great depths, the character
of the sea-bottom completely changes. At a distance of 100 miles from
the coasts of America or Europe, for example, the land deposits have
already found their resting-place, and the animal life in the depths of
the Atlantic is poor in skeleton-forming genera. However, the surface
waters of the ocean teem with creatures of all sorts which, as they
die, drop down their skeletons and shells in a gentle shower to form
a fine deposit on the bottom. When we get beyond the mud-line, then,
and use the dredging or sounding apparatus in depths of 1500 to 2500
fathoms, we find that the bottom is largely composed of the shells of
such surface animals as the Pteropods and Globigerinas, and according
to the relative abundance of these forms it is called Pteropod Ooze or
Globigerina Ooze. In still greater depths than these the character of
the bottom again changes, and we find a deposit which is commonly known
as the Red clay. The explanation of this change of character depends
upon the fact that sea-water exercises a slightly solvent action upon
carbonate of lime, and the shells of the Globigerinas and other forms
are, in seas of a depth of over 2500 fathoms, dissolved before they can
reach the bottom. The only shells that can survive this long journey
are the siliceous shells of the Diatoms and Radiolarians, and in those
parts of the ocean where these organisms live in abundance their empty
shells form an important percentage of the composition of the Red
clay. Over a very considerable area of the Pacific Ocean, however,
the Red clay contains only a minute proportion of these shells, and
its composition has given rise to a good deal of discussion among the
authorities. It may be considered to be a conglomerate of the horny
fragments of dead surface-living animals, of volcanic and meteoric
dust, and of small pieces of water-logged pumice-stone.
[Illustration: +Fig. 2.+
Radiolarian Shells from a deep-sea deposit.]
In some of the very deep holes of the Pacific Ocean the mud is
almost entirely composed of Radiolarian shells, and is then called
“Radiolarian ooze”: and in the Southern Sea a mud called Diatom ooze
has been found, which consists principally of the siliceous shells of
these minute algæ. Notwithstanding these varieties of the mud, and
others that space does not allow me to refer to at length, we may
suppose that if the floor of any one of the great oceans were exposed
it would have the appearance to a traveller of a vast desert-like
expanse, without a stone, a rock, or a cliff to vary the monotony of
the scene. At one time it was supposed to be an absolute plain, without
any important change of level from the mud-line of one continental
coast to the other; but the result of modern submarine explorations has
been to prove that in all the great ocean-basins, hills and ridges, as
well as troughs and deep holes occur, which break the monotony of the
generally smooth and level character of the bottom.
CHAPTER II.
SHALLOW WATER FAUNA.
Having considered thus briefly the general conditions under which the
animals of the sea must live, we may now consider more in detail the
special conditions of shallow water life.
In water of only a few fathoms in depth, the direct light of the sun
is capable of reaching and influencing all living things that occur,
either at the bottom, at the surface, or in the intermediate waters.
Great as the influence of direct sunlight must be upon animals, it
is even greater upon plants. Nearly all the Sea-weeds are, like the
plants of the dry land, dependent upon carbonic acid gas dissolved in
the water for one of the most important constituents of their food,
but it can only be absorbed by the plant in the presence of sunlight.
It is possible, therefore, for Sea-weeds to flourish in the shallow
waters of the sea, while they are necessarily absent from the deeper
and darker regions to which the rays of the sun cannot penetrate.
Everyone knows that in the shallow waters of our own coast there is
in many places a dense tangle of Wracks and long, flat ribbon-like
Sea-weeds growing on the bottom, and that on and amongst these weeds a
rich harvest of animals awaits the eager shore collector. It is true
that there are vast fields of sand on which the Sea-weeds are few and
far between, but we may say that wherever they can obtain a secure
foothold in the shallow waters of the British coast there they will
grow and multiply in great profusion. We must not, however, jump too
hastily to the conclusion that the same is true for all parts of the
world. The British coasts are particularly rich in Sea-weeds, in fact
a distinguished botanist once said that they probably present us with
the greatest number of genera and species of any coast line of the same
extent in the world. In the Temperate regions of both the Southern and
Northern Hemispheres there is generally a rich Sea-weed flora, but in
the warmer regions it is less luxuriant, and on the Coral-reefs of the
Tropical seas it is remarkably poor.
Turning our attention for the present to the Temperate regions, let us
consider the influence that the Sea-weeds have upon the animals of the
shallow waters. In the first place we find that they afford shelter and
support for a large number of animals which could hardly live without
them. In the roots of the weeds may be found little Crabs and Molluscs,
which occur nowhere else; and clinging to the long waving branches are
many forms of Sea-anemones, Zoophytes, flattened limpet-like Molluscs,
Ascidians, and other forms of animal life. The great forests of weeds
are also the haunts of many queer Fish, Prawns, Crabs, and Sea-slugs,
which hunt their prey or hide from their enemies amidst the shelter of
the stems and branches.
[Illustration: +Fig. 3.+
The common Pipe-fish.]
Now many of these animals, which together make up the Fauna of the
Sea-weed region, have assumed, in the course of the ages of evolution,
not only the colours of the Algæ on which they live, but in some cases
even forms which render them at first sight more like plants than
animals. The slender Pipe-fish, for example, which is not uncommonly
found amongst the bright green Sea-weeds of our coast, is wonderfully
similar, both in form and colour, to the weeds on which it lives. The
Sea-horses, which have such a curious form out of the water, in their
natural surrounding resemble the weeds so closely that they may be
easily overlooked. A still more remarkable example is to be found in
the curious fish Phyllopteryx, in which the body is provided with long
branched processes, making it we may suppose much more difficult to
distinguish in its natural haunts than even the Pipe-fish or Sea-horse.
[Illustration: +Fig. 4.+
Phyllopteryx.]
Many other examples to illustrate this feature of the Sea-weed Fauna
could be quoted, but sufficient has been said at present if we have
indicated to the reader the manner in which, by the indirect influence
of light, the form and colour of animals may be modified for their
life among the marine plants. The presence of light, however, modifies
directly the character of the animals themselves in many respects.
[Illustration: +Fig. 5.+
Diagrams of Eyes of _A_ Whelk, _B_ Lobster, _C_ Scallop, _D_ Fish.]
The statement that many of the animals of the shallow water are
provided with eyes because there is light is, when carefully examined,
found to be strictly true, however anomalous it may seem to be. There
must have been light in the shallow waters of the sea when the first
primordial forms of life made their appearance, and it was this light
which, step by step, led to the evolution of the most complicated and
perfect forms of eye from the simplest pigment spot of the Protozoan
to the eye of the Lobster and the Fish. We may say that all animals
that freely swim in the shallow waters or that crawl and creep on the
rocks and sands at the bottom are provided with eyes. The Fish are
provided with a pair of eyes which present us with the same general
features that we find in the other Vertebrates. Lobsters, Crabs and
Prawns have each a pair of stalked eyes, which are probably as perfect
in their functions as the Vertebrate eye, although built up on an
entirely different model. Whelks, Winkles, and other Gastropods,
although so slow in their movements, have each, upon or close to their
tentacles, a pair of minute eyes, which are much simpler and probably
far less perfect in form than the eyes of their more highly organised
neighbours. Even the little Jelly-fish, Star-fish, Sea-urchins, and
other creeping forms of life, are provided with specialised pigment
spots, which we have good reason to believe enable them to perceive the
rays of light.
But we must notice that it is only the animals that are capable of
locomotion from place to place that need these organs of vision,
and that the stationary forms are blind. In the large class of the
Lamellibranchs, for example, to which the Oyster and the Mussel belong,
we find that with a few exceptions there are no eyes. These animals,
after the first few stages of their life are past, settle down into
the sand, or fasten on to a rock and remain there until their life
is done. Their food, consisting of the minutest specks of animal and
vegetable life, is brought to them by the sea-currents; they do not
need nor desire to seek the society of their relations, and when their
enemies approach they resign themselves almost without an effort to the
inevitable. To such animals eyes would be useless, and so nature has
withheld them.
There are, however, a few Lamellibranchs that possess good eyes, eyes
that are almost as complicated in their structure as the Vertebrate
eye; and not a single pair only, but sixty, eighty, a hundred, or even
more, may be found on a single individual. Such a Lamellibranch is the
common Scallop which may be seen in many of our fishmongers’ shops.
Unlike most of the group, this animal is able to make long flights
through the water by the flapping together of its shells, and there
can be no doubt that on the approach of danger it uses this method of
locomotion for escape. One of its most deadly enemies is the Star-fish,
which forces the shells apart and sucks out the flesh by means of its
protrusible stomach. When a Star-fish is placed in an aquarium in which
there are some Scallops, lying, as they do, on their sides, with the
valves slightly open, showing the double row of gleaming metallic eyes
on the margin of the mantle, the Star-fish immediately moves, with what
in such an animal may be considered extraordinary rapidity, straight
in the direction of the Scallop. Before, however, it reaches its prey,
the coveted victim gives four or five vigorous flaps of its shells and
swims away to another part of the tank. This suggests that the eyes
of the Scallop are used in the light as a means of giving warning of
the approach of an enemy, and they are found in the Scallops only,
among common British bivalves, because they alone possess this power
of swimming away. Of course if there were no light in the water the
eyes would be useless, and it is an interesting fact that the Scallops
which live in the darkness of the great depths of the ocean are quite
blind.
[Illustration: +Fig. 6.+
A branch of the Zoophyte Obelia.]
Very interesting examples of the connection between the presence of
eyes and the power of locomotion are found among the sedentary or fixed
forms of life. The great class of Sea-squirts or Tunicates includes a
number of genera, most of which are, in the adult condition, immoveably
fixed to the bottom, and in that stage have no eyes; but the eggs
which they produce give rise to little creatures, like tad-poles in
form, which swim about freely in the water. Each of these tad-poles
has a large eye in its brain, which remains so long as the animal
leads a free life. As soon, however, as it settles down upon the rock
which is to become its permanent resting-place through life, the eye
and the organ of locomotion, the tail, both degenerate and ultimately
disappear. Again, we often find upon rocks, Sea-weed, old shells, and
the like, some curious delicate branching organisms called Zoophytes.
Notwithstanding their general resemblance in form to Sea-weeds, these
Zoophytes are known to be animals. Each tuft or branch is formed by
a number of delicate little Polyps, each with a crown of tentacles
round a terminal mouth. The Polyp cannot move away from the branch it
grows upon, nor the branch from the stem, nor the stem from the rock
on which it rests, and none of these minute creatures are provided
with eyes. There are, however, a few Zoophytes that give rise to buds,
which grow into the form of minute Jelly-fish (or Medusæ, as they are
called); and these, becoming detached from the parents, swim away and
lead an independent existence. These Medusæ are in many cases provided
with simple little eyes. During their short life they are drifted away
by the sea-currents to some distance from their parents, and produce a
number of eggs which are capable of developing into a new fixed colony
of Polyps.
[Illustration: +Fig. 7.+
Medusa produced by Obelia.]
We may consider it one of the maxims of Science that in a population
of animals which possess eyes, colour as well as form is of extreme
importance. As well as the alteration in form that takes place in the
animals living among Sea-weeds, we find a modification in colour in
the creatures dwelling among rocks or on the sand, so that they may
resemble the ground on which they live. No more striking example of
this could be found than in the common Shrimps of our coasts. Anyone
who has watched them in the sea-pools must have been struck with their
close resemblance to the sand. In fact it is only by close observation
that they can be distinguished from it. In an aquarium, too, it
may be observed how very much the upper sides of Soles, Flounders,
Turbots and other flat fishes are like the sand in colour, while the
under sides are almost invariably pure white. The black colour of the
Lobsters, speckled and striped with blue, has a close resemblance to
the holes and crannies of the rocks among which they live. The bright
transparent green Prawns are almost invisible as they move about among
the Sea-weeds, and Sea-slugs assume all manner of beautiful colours
according to the ground on which they feed.
On the Coral-reefs of the warmer regions of the world the pools that
are left when the tide goes down are characterised by their brilliancy
of colour. The bright purple, green, and yellow tips of the Coral
branches, the red and bright green Sponges, and the white pieces of
dead and broken Corals make up a scene of beauty which can only be
compared with a bed of variegated flowers. Here the Fish, Prawns, and
other moving animals have assumed the most gorgeous colours in patterns
of spots and stripes which verily astonish the naturalist when he
sees them for the first time. The great Sea-perches, with their sides
covered with bright red or brown blotches, the curious Trigger-fish,
with bright red, blue, or yellow bands crossing their bodies, the
banded Lobsters, and the spotted Cuttlefishes, strange and conspicuous
as they may seem when they are taken out of the water, are in life but
in harmony with their surroundings, and, in reality, less conspicuous
than they would be if less brilliantly arrayed.
I remember on one occasion as I was watching some expanded Polyps in
a little shore pool in the Tropics, I noticed something suddenly move
close to the coral block on which I was standing. As there was no
escape from the little pool, nor any holes in which the creature could
hide itself from my observation, I searched with diligence to find it
again, but for a long time without avail. Suddenly it moved again, and
then I saw, resting on the brilliantly coloured corals, a remarkable
little Shrimp called by zoologists _Stenopus_. Its body, which was
almost transparent, was marked by a number of bands of a bright red
colour, it had enormously long antennæ similarly banded, and its legs
and body were covered with short red-tipped spines. When I succeeded in
safely landing it in my collecting bottle I felt perfectly astonished
that I had been so baffled by such a lovely little jewel, so bright, so
strange, and so generally conspicuous did it then seem to be.
The colours of the animals I have referred to hitherto may be accounted
for by their need to escape the attention of their enemies, or to
avoid detection in the pursuit of their prey; but the animal colour
problem is not yet exhausted. The beautiful colours of the Anemones,
Corals, Sponges and many other sedentary forms of animal life, and
the marvellous patterns on the shells of Molluscs cannot be due to
these causes. Many ingenious theories have been brought forward to
explain away the difficulty, but none of them seem to be perfectly
satisfactory, and consequently it is unnecessary to enunciate them in
detail in this small book.
The character of the bottom of shallow water, especially in the
neighbourhood of the coasts, presents us with so many variations
that it would be a long task to consider in detail all the different
adaptations that the animals exhibit. The Fauna of the sand, shingle,
and mud at the mouths of rivers, of the rocks of iron-bound coasts,
and of the coral-reefs, each present us with many curious kinds of
modification of form and structure. A brief reference to one or two
characteristic regions must be made before passing on.
The sandy bottoms which are so prevalent, not only upon our coasts,
but in nearly all parts of the world, invariably support a Fauna with
many curiously altered forms. In walking across the sands at low water
we may have often noticed many worm-like and twisted columns of sand;
these are the casts of the common Lug-worm, which is a favourite bait
for many kinds of Fish. The Lug-worm lives in a U-shaped tube, which it
forms from a slimy secretion of the body; it feeds by swallowing the
sand in which it burrows, extracting from it as it passes through the
intestine whatever animal or vegetable food it may contain. There can
be little doubt that the sand is a very poor form of diet, and that
an immense quantity must pass through the body of the animal compared
with its weight in order to afford sufficient nourishment. It has been
reckoned that as many as 82,433 casts may be found on an acre of sand
where conditions are favourable for these Worms, and that they would
weigh together nearly 2000 tons. This would mean that the whole mass of
the sand would pass through the bodies of Lug-worms on an average of
once in every twenty-two months.
But the Lug-worm is only one of the many forms of life that burrow
in the sand. A very large number of bivalve Molluscs live with the
greater part of their bodies perpetually buried. Their organisation
is such that their food and the water that is used for respiration
can be brought to them by a tubular prolongation of the body called
the siphon. Some of these animals live much deeper down than others,
and while some have but feeble powers of moving either up or down in
their burrows, others can penetrate to great depths with extraordinary
rapidity.
[Illustration: +Fig. 8.+
A Bivalve Mollusc, showing below the foot with which it burrows into
the sand, and above the siphons.]
The shell of a Lamellibranch, called the Razor-shell (_Solen_), is
not an uncommon object of the sea-shore where stretches of sand occur.
When the animal is alive it has the power of burrowing down so quickly
that it is practically impossible for one man to capture a specimen
by digging, when it is thoroughly alarmed. Occasionally, however, the
sea itself, when lashed into fury by a storm, is a better digger than
the Solen is a burrower; and after a heavy storm in the Isle of Man I
have seen the shore littered with Solens scooped out of the sand by the
force of the waves and cast up with lacerated shells to fall an easy
prey to the Seagulls.
The sandy bottoms in shallow waters are also the haunts of many kinds
of Fish that are specially modified in form and colour to resemble
their surroundings. The large family of the Skates have their bodies
compressed from above downwards so that they can lie perfectly flat
upon the sand. Their upper surface is deeply pigmented, giving them
a general resemblance to the ground on which they lie; but to assist
them in escaping observation they have a habit of shaking their fins in
such a manner as to scatter a considerable quantity of sand over their
bodies. Thus it can well be understood that in the dim light of the
sea-bottom the little Fish and Shrimps, which form a large portion of
their food, may approach quite close to them without being in the least
aware of the danger into which they are running. The upper side of the
Skate is also armed with a number of sharp and hard spines, and in some
forms--called the Sting-rays--one of these, situated at the base of
the tail, is much larger than the others, and provided with muscles so
that it can be suddenly erected. In connection with this spine there is
a poison-gland and duct. The wound that is inflicted upon the arms or
feet of the fishermen by this formidable weapon of offence is said to
be of a very serious nature.
Some of the Skates, too, show another very interesting modification
of structure, which, however, is not directly associated with their
mode of life, but may be briefly referred to here whilst dealing with
the family. This is the electric organ. In the younger stages of the
common English Skate a small region of the muscular system on each side
of the base of the tail becomes changed into an electric organ, but
the discharges which it is able to give are so feeble that they can
only be appreciated by a galvanometer. In the Torpedo, however, the
electric organs are very large indeed, and situated one on each side
of the head. They can give a shock which is powerful enough to kill
small animals and to stun larger ones. With such a formidable weapon
of offence and defence, it is clear that the need for active movements
is considerably diminished, and the Torpedoes are described as being
exceedingly slow in progression and incapable of the violent movements
of other Skates.
[Illustration: +Fig. 9.+
The Common Sole, showing both eyes on one side of the head.]
The other Flat Fishes found on sandy bottoms belong to a different
group altogether, and are characterised by their bony skeletons and
other features. The Soles and their allies do not, like the Skates, lie
flat upon their bellies, but are _laterally_ compressed and lie upon
one side. The side which is habitually uppermost is of the colour of
the sand, and the other almost invariably pure white. In the course
of their development the Soles undergo very extraordinary changes in
form to reach the perfectly flattened condition of the adult stage.
These changes affect many of the organs of the body, but perhaps the
most interesting of all is the history of the eyes. In the young Soles,
which swim almost vertically, like the majority of fishes, there is
one eye o each side of the head, but as they grow older and gradually
take to the habit of swimming on one side, the eye of that side sinks
down into the head, and rotating as it goes passes through to the other
side. This process naturally leads to a considerable distortion of the
skull, so that the bones of the adult Sole show as complete a want of
symmetry as can be found in any Vertebrate.
[Illustration: +Fig. 10.+
The Angler.]
Some of the Bony Fish, however, that live on the sand are flattened
dorso-ventrally like the Skates. The Angler or Fishing-frog, for
example, is a Flat Fish, which is perfectly symmetrical. Like the other
Flat Fish its upper surface is coloured in such a manner as to resemble
the ground on which it lives. Its great mouth, armed with formidable
rows of sharp-pointed teeth, is directed upwards, and it receives its
name from a curious tentacle terminating in a brightly-coloured knob
which dangles over its mouth. The brightly-coloured knob looks no doubt
a tempting morsel to the little Fish upon which the Angler preys, but
the greed or curiosity, whichever it may be, that induces them to
inspect the bait leads them to the fate which follows one snap of the
great tooth-armed jaws.
The Fauna of the shallow waters where rocks abound also possesses many
peculiarities. In the first place we must remember that the rocks,
being firm and hard, present a basis upon which many of the sedentary
forms of life that would be swept away or smothered if they attempted
to live on the ever-shifting sands, can fix themselves. Consequently
the rocky bottom is characterised by a rich Fauna of those groups of
animals, which, in the adult condition, are immoveably fixed. If a
large stone or a water-logged piece of timber that has been at a depth
of a few fathoms for some months or years be captured in the dredge and
brought on board for examination, it is sure to present the observer
with a multitude of firmly-fixed creatures. Among them there is almost
certain to be found a number of small conical shells, made up of a
series of triangular plates, fixed to the rock by their bases. These
are commonly spoken of as Barnacles (_Balanus_), and they pass
through an interesting history. For many years they were considered,
from the character of their shells, to be allied to the Molluscs, but
an examination of the soft parts of the animal shows that, unlike any
Molluscs, they are provided with six pairs of jointed legs; and a still
further study of their anatomy proves beyond a doubt that they can no
longer be classified, with any pretence of scientific accuracy, with
that group.
[Illustration: +Fig. 11.+
Vertical section of a Balanus, showing animal
_in situ_ in its shell.]
The secret of their true relationship was not discovered until the
story of the development was worked out, when it was found that the
eggs they discharged each gave rise to a little larva called Nauplius,
which is provided with three pairs of legs like the larvæ of some of
the Prawns and their allies. The result of these observations then was
to prove that the Barnacles are really Crustaceans, notwithstanding the
fact that, unlike most of that group, they are, in the adult stage,
permanently fixed to the rocks.
On the same piece of stone there will most probably be found several
twisted or coiled tubes of lime, formed by a little Sea-worm called
Serpula. When living in the water the head of this worm projects from
its shell and expands a circlet of delicate tentacles, by means of
which the food is brought to the mouth. One of these tentacles is
specially modified and enlarged at the extremity to form a conical
knob, which, when the animal is retracted into the shell, closes the
aperture like a stopper.
[Illustration: +Fig. 12.+
Nauplius larva of a Balanus, enormously magnified.]
Then there may be found some spherical or lobate masses of a fleshy
consistency, white, pale pink or yellow in colour, and studded
with numerous star-shaped apertures. When these are allowed to
remain in a basin of fresh sea-water for some time, each one of
the star-like apertures opens, and a beautiful transparent little
Polyp with eight-feathered tentacles gradually unfolds itself, only
to be slowly withdrawn into the mass when the vessel is shaken or
otherwise disturbed. These Polyps form colonies, known as _Alcyonium
digitatum_.
Now it is to be noticed that none of these three examples of the
sedentary Fauna can move in the least degree from the rock or shell
to which they are fixed. When once the young larva has taken up its
position there it must remain until old age or some disaster brings
its life to an end. When they are first hatched from the egg that
is thrust into the water by the parent, they pass through a larval
stage that, like the Nauplius of the Barnacle, is active and free.
Then they are carried away from the parent stock partly by their own
active movements, but more particularly by the tides and currents of
the sea-water. At last a change in their structure occurs. They sink
to the bottom, become attached to a rock or stone, complete their
metamorphosis, and remain anchored to the spot for the rest of their
lives.
The number of different forms of animal life which constitutes this
sedentary Fauna of rocky coasts is very great indeed. In addition to
the Barnacles, Worms and Alcyoniums, there are numerous species of
Sea-anemones, Sponges, Corals, Zoophytes, Sea-squirts, and other forms,
and in _all_ these cases the eggs give rise to free-swimming
larvæ, by which the distribution of the species is effected.
Another group of animals, which forms an important feature of some
rocky coasts, are the Boring-molluscs. The Rock-borers belong to
many different species. Some of them, such as the Pholases, make long
cylindrical holes in chalk, and even in harder rocks. The Teredo is the
borer, commonly known as the Ship-worm, on account of its powers of
penetrating into timber. The long calcareous tube which it forms as it
works its way into the wood, gives it a superficial resemblance to a
large sedentary worm, but it is in reality a bivalve Mollusc, specially
modified in structure for its peculiar habits.
[Illustration: +Fig. 13.+
Sea-urchin with large thick spines.]
The next group of animals we have to consider in the Fauna of the rocks
includes all those that slowly creep or crawl, without possessing
any powers of rapid locomotion. Amongst these may be mentioned
the Sea-urchins and Star-fishes. The former possess spherical or
heart-shaped bodies covered with a formidable array of spines, among
which there protrude several rows of soft transparent tubes terminating
in little cup-like discs. Some of these tubes--or tube-feet as they are
called--are fixed to a rock, and the heavy body is slowly dragged after
them; another set is then attached, whilst the former is released to
obtain another hold a little further on. The progress is slow, but the
Urchin is able to climb the smooth face of an absolutely perpendicular
rock with perfect ease or to get over any other obstacles that may be
in its path. The Star-fishes are similarly provided with tube-feet,
but in their case these are confined to the lower surface of the body,
the upper side being entirely devoid of them. Star-fishes have a very
wide distribution in the sea, and occur on sandy shores as well as
among rocks and shingles. If a specimen be watched gliding slowly and
smoothly over the sea-bottom and then the mouth be examined with a
pocket lens, a doubt might arise in the mind of the young naturalist
as to the justice of the charge that is made against these animals of
their being the principal enemies of the hard-shelled Oysters. But the
charge is well founded, for if a Star-fish be placed in an aquarium
with an Oyster or a Cockle, or, in fact, almost any bivalve Mollusc,
it may be seen to clasp its prey in its arms and slowly but firmly and
surely force open the shells, and then protrude on to the soft parts
a long tubular stomach which gradually digests and absorbs them. The
Star-fishes, then, are undoubtedly to be reckoned among the most
voracious and destructive inhabitants of the shallow waters, and it is
probable that the covering of spines which we find so commonly among
shallow water animals is an adaptation to prevent or render difficult
the operations of these creatures.
The Gastropod Molluscs form another large and important group of
creeping animals of rocky coasts. On nearly all our own coasts numerous
Periwinkles may be seen clinging to the rocks at low tides, and if a
search be made in the deeper pools and on the rocks nearest to the
low-water marks many other species will be found of animals with
spirally-coiled shells which are included in this group of Gastropods.
The Periwinkles on the rocks might at first sight be thought to
belong to the sedentary group of animals, but when the water covers
them again, or when they are put into an aquarium, they may be seen
to protrude a head and an elongated slimy foot, which, gliding over
the surface of the rock, drags the great shell and its contents with
it. On the approach of danger the foot and head are withdrawn into
the shell, and the animal rests secure from many enemies that might
otherwise have found it a dainty morsel. Some of the Gastropods are
purely vegetable-feeders, but most of those living in shallow sea-water
feed upon Molluscs and other animals. It might well be a matter for
wonderment when the soft head and little mouth of a Gastropod, such as
a Whelk, are examined, that it is carnivorous and attacks and devours
animals as large as itself. But the anatomist shows us that hidden in
the recesses of that mouth there is a ribbon beset with numerous sharp
little teeth, which by a complicated mechanism can be worked backwards
and forwards in such a manner that it can bore a hole through very
thick and dense shells; and, the soft parts being reached, a tube is
protruded which dissolves and sucks them up into the animal’s stomach.
Many people must have noticed that numbers of the bivalve shells
that are cast up on the sand at low tide are perforated close to the
hinge by a neat little round hole. This is the hole made by some
predaceous Gastropod which, having killed its prey and devoured all
that is digestible of it, leaves its empty shells at the mercy of the
waves. Amongst the rocks numerous species of Gastropods are found,
some undoubtedly carnivorous, others herbivorous. The many beautiful
forms and colours that their shells assume may be seen in any good
museum or conchological cabinet. Some of them are very minute, others
are provided with a shell more than a foot in length; some are marked
with numerous coloured spots, others with bands or lines; some have
perfectly smooth shells, others are ribbed or spiny. It is extremely
difficult to account for all these modifications, partly because it
is impossible to study the animals alive in their natural habitats a
few fathoms below the surface of the sea, and partly because life in
the shallow waters must be so complicated that we are at a loss to
understand the value to a species of slight modifications in structure
such as these. The difficulty that has been found in explaining these
various forms and colours has led some naturalists to the belief that
they are of no importance to the species in the struggle for existence,
that they are, as it were, the accidental result of some process of
excretion, and not the outcome of a long series of slight changes,
bringing about at length an adaptation of form most suitable to the
habits of the animal. Such views are, however, to be accepted with
great caution, and most zoologists will be contented to wait until our
knowledge is much greater than it is at present, before wholly agreeing
with them.
[Illustration: +Fig. 14.+
Smooth-shelled Gastropod.]
[Illustration: +Fig. 15.+
Spiny Gastropod.]
Another great class of animals which has many representative forms
among the rocks is the group of Crustaceans. The Lobster, the Prawn
and the Crab are all familiar examples of this class. They may be
found by searching rock pools at low-water, or can be captured in
basket-work traps in places beyond low-water mark. When undisturbed
they crawl slowly over the rocks and weeds by their long jointed
legs, searching for their prey, but when alarmed the Lobster and
the Prawn can, by violent flapping movements of their tails, dart
rapidly backwards through the water, while the Crab beats a hasty
retreat sideways into some shelter among the rocks. Like many of the
Molluscs, the Crustaceans have a hard covering or shell to protect
them from many of the dangers to which soft-bodied animals would
be exposed, but a momentary glance at them would be sufficient to
satisfy the most inexperienced eye that there are many and important
differences in the character of the shells of these two great groups
of animals. One important distinction between them, however, might
well escape observation, and that is, that whilst in the Mollusc the
shell increases gradually in size during the life of the animal, in
the Crustaceans it cannot do so. In the Lobsters and Crabs the shell
is periodically cast off entirely, and for a day or two at each period
the skin of the animal is quite unprotected. A new shell is gradually
formed, and this is hardened and thickened until it assumes a form
similar to that of the one that has been lost, but larger. During the
moult the Crustacean usually hides itself in a hole in the rocks and
waits patiently until the new shell has grown.
[Illustration: +Fig. 16.+
A Cuttle-fish.]
The animals included under the popular names of Cuttlefishes, Squids
and Octopuses are also capable of crawling about among the rocks by
their long feeler-like arms; but they are in the habit, as well, of
making prolonged journeys through the water, by pumping the sea-water
through a tubular siphon situated on the under side of their bodies.
These animals possess in such a remarkable degree the power of changing
colour that they might be called the Chamæleons of the sea. As they
pass slowly through the water from one part of the coast to another
the colour of the skin changes so as to resemble the colour of the
rocks or weeds which are below them. These changes are brought about by
numerous little bladders in the skin which are filled with different
coloured fluids, and are worked by a complicated system of muscles
under the control of special nerves from the brain. When the colour
blue is predominant, it is found that all the bladders containing blue
fluids are dilated, the others being constricted; when the colour is
red the red bladders only are dilated, and so on; and as the nervous
response to the colour of the rocks perceived by the eye is practically
instantaneous, the change in the general colour of the body brought
about by the dilatations of these vesicles is extremely rapid. Many
other animals have the power of changing colour, but in no group is the
alteration more rapid and remarkable than in this order of Cuttlefishes
and Squids. Another very interesting feature presented by these animals
is their ability to discharge suddenly a cloud of inky substance into
the water. Their principal enemies are the Whales, Porpoises and some
of the larger Sharks and other Fish. When these animals approach, or
any other danger is feared, the Cuttlefishes discharge into the water
from a special bag, called the ink-sac, a quantity of black or brown
pigment which, diffusing rapidly, forms a cloud round their bodies, in
the obscurity of which they frequently escape pursuit. The well-known
Sepia of painters is obtained from these ink-sacs.
The last group of animals occurring among the rocks are those capable
of vigorous swimming movements. Many Crustaceans, such as the Lobsters
and Prawns, are capable, as has just been pointed out, of swimming
rapidly through the water by means of their powerful tails. But this
swimming power is only accessory to that of crawling or creeping, and
is used merely when the animals are disturbed. Cuttlefishes and their
allies seem to spend a considerable portion of their time in floating
or swimming in the water, but still they do crawl about among the
rocks, and very probably attack and feed upon their prey entirely upon
the sea-bottom.
The members of the Rock-fauna which belong to the class of Fishes
very rarely rest upon the sea-bottom at all. They are not, as a rule,
provided with limbs which are capable of crawling or creeping; and
their mouths are adapted for catching food that is swimming, or of
browsing upon or nibbling at fixed forms of life while their bodies
are still floating in the water. Nearly all the animals living among
the rocks that we have hitherto spoken of, have some organs or some
specialised portion of the body-wall for resting upon or for attaching
themselves to the bottom.
[Illustration: +Fig. 17.+
The Wrasse.]
The Anemones are attached by their bases; the Sea-urchins and
Star-fishes crawl by means of their tube-feet; the Gastropod creeps
over the rocks by its broad flat foot, and the Octopus stretches out
its muscular arms and drags its body along by the numerous suckers they
bear. In the Flat Fishes of the sandy and gravelly shores we usually
find a white under surface on which they rest when waiting for their
prey. Among the Fish which frequent the rocks, however, such as the
Cods, the Whitings, and the Wrasse, there are no such surfaces. The
body of the Fish is usually more rounded in form, and no well-marked
limit can be assigned to the coloured upper surface and the pale
silvery under side. These Fish are in nearly all cases rapid and
powerful swimmers, rushing through the water after their prey, or away
from their enemies, by vigorous lateral movements of their tails.
A curious exception to this general rule among the Fish occurs in the
family of the Lump-suckers. These Fish are found on the English, but
more commonly on the Scottish coast, and are distinguished by the
presence of a sucker, formed by the throat fins, on the under side of
the head. By means of this the Lump-sucker is able to attach itself
so firmly to rocks and stones that it can only with considerable
difficulty be removed from the object to which it is attached.
Of the Fish that are commonly found among the rocks, a very
considerable number migrate from time to time to other parts of the
sea, and may be caught in the trawl on sandy or shingly bottom, or even
in the drift nets at the surface of the sea. A large number of Fish
belonging to the family of the Codfishes frequent the rocks during a
part of their lives. The Pollack is distinguished from most of the
others by the absence of a barbel on the lower jaw, and is one of the
persistent rock frequenters. In the adult condition it feeds almost
entirely upon other Fish, although in the younger stages of its life
Crustaceans, Worms, and other Invertebrates seem to form the bulk of
its food.
The true Cod and the Haddock seem to have a much wider range,
occurring on shingly bottoms, where they are frequently caught in the
fishermen’s trawls, as well as in the neighbourhood of rocks. The Hake
feeds principally upon Pilchard, Herrings and Sprats near the surface
of the sea.
It is an interesting fact that the Fish belonging to this one family
have very different methods of feeding. The Cod and the Pollack both
hunt their prey principally by day-light. The Pollack is guided by
its sight alone, the Cod-fish is assisted by its barbel, which acts
as a delicate feeler or organ of touch. The Hake, on the other hand,
retires into deep water during the day-time, and only comes to the
surface at night to feed. Similarly the Rockling hides away in holes or
under stones during the day-time, and only comes out to hunt for the
Crustaceans and little Fish upon which it feeds at night.
The development of these Fish presents some features of interest, as
showing us the changes which occur in habit during their life-history.
The eggs of the Cod are buoyant, rising to the surface of the sea as
soon as they are spawned. In twelve or fourteen days, according to
the temperature of the water, the larvæ are hatched and swim about in
large numbers just below the surface, feeding upon minute Crustaceans
and other animals. A little later the young Cod frequently shelter
themselves under large Jelly-fishes, feeding upon the numerous
parasites which infest those creatures. When they are about a year old
they are found feeding among the sea-weeds on rocky coasts, and they
migrate into deeper water when they reach their full size.
[Illustration: +Fig. 18.+
The John Dory.]
The John Dory is a remarkable Fish, by no means confined to the rocks,
as its food often consists largely of Pilchards and Herrings. It
differs from most of the Fish of similar habits in being remarkably
flattened from side to side. This feature seems to be of service to
it in the peculiar manner it has of securing its prey. Mr Cunningham,
to whom we are indebted for this interesting observation, says:--“It
does not overtake (its prey) by superior speed like the mackerel, nor
lie in wait for it like the angler, but stalks it and approaches it by
stealth. It is able to do this in consequence of the extreme thinness
of its body, and the peculiar movement of its hinder dorsal and ventral
fins. The dory places itself end on towards the fish it desires to
devour, and in this position it is evident that it excites no alarm on
the part of its prey. The appearance of the dory seen in this way is
a mere line in the water, to which no particular significance can be
attached. I have not particularly noticed the effect of the ribbons of
membrane, which project from the dorsal fin. But I have observed that
the movements of the dory are very gradual except in turning; it alters
the position of its body by a turn of the tail or side fins, and then
slowly swims forward by vibrating the second dorsal and ventral, a
movement which causes very slight disturbance of the water. The whole
appearance of the dory in these actions is suggestive of suppressed
excitement, his eyes being fixed on his prey.”
* * * * *
We have now considered some of the chief features of animal life in
the shallow seas, the illustrations being taken principally from the
regions of our home shores. The shallow waters of the tropics present
us with so many phenomena of striking interest and importance, that
the subject would be most incompletely treated if they were left out
of consideration altogether, and, therefore, our next chapter will be
devoted to them.
CHAPTER III.
SHALLOW WATER FAUNA OF THE TROPICS.
The shallow waters of the Tropical seas present us with so many
different conditions of tides, of coast lines, of temperature, of
liability to storms, and of other natural phenomena, that we find an
infinite variety in the general character of their Fauna. Just as on
land, we find in one part of the Tropics a dense forest, and in another
a dry desert, so in the Tropical seas we find on one coast a crowded
population of animals and plants, and on another a sandy bottom, which
is, comparatively speaking, lifeless. In order to bring before the
reader some of the principal characters of animal life in the shallow
waters of the Tropics it will be well to confine his attention to
one part of the world which is fairly well-known--namely, the Malay
Archipelago--and refer only in passing to other localities. The most
characteristic feature of Tropical coasts is the Coral-reef, and
nowhere in the world may it be seen in more exquisite variety than in
the archipelagoes of the East. Although, however, these vast structures
are so abundant on some coasts, others seem to be entirely without
them. They are not found at all on the Western coasts of America or of
Africa, and even in some regions of the larger islands in the Pacific
and the Indian Oceans, many miles of coast line may be devoid of
them. These curious and interesting variations in the distribution of
the reefs can be explained, but the explanation will be more easily
understood when their general features have been described.
It is a well-known fact that the great masses of limestone which
compose the reefs are formed by the activity of countless thousands
of minute animals, but the popular idea of the general form of these
animals has been very much misled by the unfortunate term “Coral
insect,” which has crept into many books of travel, and the leading
articles in the newspapers. The word “insect” is used by zoologists
as a general term for certain air-breathing animals that are widely
distributed over the surface of the earth. Many of them are extremely
tiny, and hence the natural mistake has arisen in the untrained mind
that all minute animals are insects. It might clear the ground a little
if the reader would note at once that “insects” are very rarely indeed
marine in habit. If there is a need for a popular word for the animals
that form coral, it should be Coral “polyps” or Coral “anemones.” The
word “coral” has, from the zoologist’s point of view, a very indefinite
meaning, for it is applied to the hard skeleton of carbonate of lime
formed by certain Sea-weeds, Sponges, and Worms, as well as to that of
Coral-anemones and other Polyps. In many places on the British coasts
the sea-bottom is very largely composed of a branching Coral formed
by a true Sea-weed called _Lithothamnion_, and in other places
very large lumps of rock are made by a Worm named _Filograna_. In
the Tropical regions, too, the well-known Nullipores, which in many
places play an important part in the formation of Coral-reefs, are of
vegetable, and not animal origin.
However, the greatest part of the Coral-reef is made by animals closely
related to the Sea-anemones, living together in colonies; and of all
the different kinds of Coral-polyps, by far the most prolific as a
reef-builder is one which will be referred to in these pages as the
Madrepore.
[Illustration: +Fig. 19.+
Polyp of a Madrepore Coral, showing the canals by
which it is connected with its fellows.]
Let us consider now the manner in which the Polyps form the Coral. In
a very old work on the natural history of Corals a statement is made
to the effect, that the Polyps construct the Coral in much the same
way as Bees build their hive, or a Bird its nest. This very erroneous
view coincides closely with ideas which might easily be gained by a
casual observation of corals in a museum. The lime is not, however,
collected as such from the sea by the Coral-polyps and plastered
round their bodies to form a house or shelter, but it is formed as a
secretion by the activity of certain organs of the animal’s body, and
is consequently a true shell or skeleton. In a Coral, which is formed
by a colony of numerous Polyps, the shell secreted by each individual
fuses on to those formed by its neighbours, and thus a communal
shell is formed which may assume a most complicated branching, bushy
form, according to the species of the Coral and the conditions that
are favourable or unfavourable to the nourishment and growth of the
different parts of the colony. In such a Coral as the Madrepore every
individual Polyp is connected with its neighbours by a system of
branching canals; and as spaces are left for these when the shell is
formed, the dried Coral is perforated by numerous tubular pores, and
has a soft, spongy texture which can be easily crushed into a powder if
trodden upon.
In other Corals the canals of communication between the Polyps are
entirely at the surface, and the shell that is formed is much harder
and imperforate. In others again colonies are not formed, but each
individual grows to a considerable size and remains independent of its
fellows all its life.
These, then, are three of the more important varieties of Corals found
on the reef, the Perforate, the Imperforate, and the Solitary Corals;
but it must be remembered that all the Corals of the reef cannot be
included in these three groups. The varieties are much more numerous
and in many cases much more complicated and difficult to understand.
Although the Coral-reefs of the Tropical world have a general
resemblance to one another, the differences in detail are so great that
it is impossible to describe any one as typical. In sailing over the
edge of a reef near the coast on a calm day, when the water is so clear
that the bottom can be seen at a depth of 8 or 9 feet, the reef may,
in some regions, be observed to change in character every few minutes.
In one spot there may be clumps of living Corals surrounded by beds of
fine white sand; in another there will be great stretches of branching
Madrepores; in another Madrepores, Mushroom-corals, the Imperforate
Brain-corals, Sponges, and many other forms of life will be clustered
together; while further on the predominant features will be the soft
and slimy Sarcophytums, looking like large green toad-stools, some
lumps of Organ-pipe Corals and a few colonies of the Blue-coral.
On other coasts I have wandered for miles along a reef mainly composed
of endless tangles of Madrepores, with very little variation indeed in
the general form of the Corals, in the character of the Sponges and
Sea-weeds that grow with them, in their colour or in any other detail.
Anyone reading the many accounts of Coral-reefs that have been written
by travellers, must be struck with their inconsistency as regards
many particulars; and in no one point are they more inconsistent than
in the description of the colours--some writing in glowing terms of
the beauties of the sea-gardens, and others complaining that their
charms have been grossly exaggerated. As a matter of fact some reefs
have a prevailing dull green or brown tone, while others exhibit all
the colours of the rainbow in their more brilliant shades and tints.
Another cause of the discrepancy is that some reefs can only rarely be
approached in a small boat owing to the breakers that dash over them;
whilst in the Tropical calms a tiny canoe can with perfect safety be
manœuvred over the reefs during nine months of the year. From my own
experience I can assert that it would be difficult indeed to exaggerate
the glorious beauty of some of the reefs in the Malay Archipelago,
more particularly of those where many different kinds of Corals may be
seen in close proximity to one another. On such a reef, for example,
there may be seen Madrepores with bright violet growing-points to their
branches, orange or red Fan-corals, bright brick-red Sponges, yellow
Sarcophytums, emerald green Organ-pipe Corals and dozens of other
forms of animal life in every imaginable colour. When seen from a boat
through two or three feet of water, these portions of the reef look
more like a beautifully planted flower-bed than a mass of animals; but
the simile is not a complete one, for the branches of the Madrepores,
the great knobs and lumps of the Brain and Organ-pipe corals, the
fronds of the Gorgonias and other forms make a wild mass of organisms
resembling a tangled thicket or a miniature forest. At low-water of
spring-tides the living reef is partly left exposed, and then the scene
changes, for the Polyps retract their tentacles after the manner of the
Sea-anemones and retreat as far as possible into the shelter afforded
by their shells.
[Illustration: +Fig. 20.+
Chætodon.]
The interest of the living reefs is, however, by no means confined to
such organisms as the Corals and Sponges, which are immoveably fixed to
the bottom; for numerous brightly-coloured Star-fishes, Sea-urchins,
Brittle-stars, Sea-slugs, and their allies crawl about among the
branches and the débris of the dead Corals; while Crabs, Lobsters and
Shrimps of many kinds may be seen swimming or crawling in search of
their prey, and the marvellously striped and spotted Coral fishes dart
hither and thither in the thicket, or remain hovering in the water
among the Corals. The whole scene is most fascinating. As the boat
slowly drifts along, new and strange creatures are constantly coming
into sight and disappearing again. Here the writhing arms of a bright
blue Brittle-star may be seen embracing the stem of a Coral-branch;
there a curiously flattened Chætodon, with its body marked by great
diagonal yellow bands, is nibbling at the young, tender branchlets;
in another place four or five Sea-urchins with very long and slender
spines are lying apparently motionless on the bottom; while a little
further on a long black Slug-like creature, the famous “Trepang” of
commerce, is slowly wending its way across the reef. Now and again a
large shoal of little Fish or a small party of Cuttlefishes may be
seen, and these may rapidly be dispersed in all directions by the
sudden dash of a Sea-perch or a small Shark. The interest is so varied,
so many-sided, in these scenes of animal life that the attention of the
naturalist is with difficulty kept to any particular point. The feature
which is perhaps the most striking is, however, the wonderful variety
in the colours of the animals and of the character of their markings.
If we consider the Fishes alone, we find some of them have broad
yellow bands running diagonally across their bodies, others have thin
longitudinal stripes of blue and yellow, some have a uniform bright
red colour, and others again have their red skins speckled with blue
spots. It would take more than a whole chapter of this book to describe
even the principal varieties of pattern found on one such Coral-reef,
but the main fact that has to be related is that where the reefs are
built by brightly-coloured Polyps, there we find these curiously marked
Fishes. There can be little doubt that the marking and colouring does
give a certain amount of protection to them. Numerous individual cases
have been mentioned of Fishes which resemble some particular Sea-weed
or Coral; but this general statement is the important one, that on
a parti-coloured background the striped and speckled Fish are less
conspicuous than those that are modestly attired.
The Fishes of the reefs, however, have other means of protection than
that afforded by their colours. The Trigger-fish and Coffer-fish, for
instance, have a body encased in closely fitting hard, thick scales,
so that they might almost be called “armour-plated fish”; and the
Globe-fish bristle all over with long and extremely sharp spines. In
the Trigger-fish there is a curious modification of the three front
spines on the back, to which their name is due. It is not known
precisely how they act, but they probably form an effective weapon of
defence. In the same family of Fish we frequently find on each side of
the tail two or three rows of sharp spines, which may also be regarded
as defensive. In the family of the Surgeons there is only one of these
spines on each side of the tail, and it is much larger than any of
those in the Trigger-fish, and capable of being folded down into a
case in the skin like a clasp-knife. It is said that these spines are
connected with a poison-gland, and can give very severe and painful
wounds, like the poison spines of our European Sting-rays and Weavers.
[Illustration: +Fig. 21.+
Globe-fish.]
A few days’ hunting on a Coral-reef will reveal the fact that it
supports a very considerable population of Crustaceans. They are not
at all obvious at first to one who has had no previous experience of
reef-work, partly because they resemble the general appearance of
their surroundings very closely, and partly because of their habit
of remaining perfectly motionless when first alarmed. In form, many
of them are, like the Lobsters, Crabs and Prawns of our own coasts,
but their colours and markings are, like those of the reef-fishes,
characterised by their brilliancy and their arrangement in bands and
stripes. The smaller ones can be caught after a little practice with
a simple hand-net, but the larger ones are more easily captured by a
rattan noose in the pools, after the blocks of Coral are loosened by a
pick-axe and slowly turned over.
The Coral-reef is a favourite hunting-ground for the conchologist,
some of the largest and most beautiful shells in the world being found
amongst the Corals. In Celebes, the giant bivalves, the huge Tridacnas,
which are sometimes two feet across, and whose shells have been known
to weigh as much as 500 lbs., may be seen wedged in among the Corals.
The mantle of the living animal presents to the observer a wonderful
display of colour as it lies in the shallow water with its shells open.
The animal is eaten by the Malays, who roast it on a tripod spit over
a fire, and cut it into steaks. A fair-sized Tridacna will afford a
good meal for four or five men. The great Cowries, Helmet-shells, and
many other species may be found in hunting over the reefs, but their
beauties are frequently hidden, when alive, by the coal-black mantle
which folds back over the shell as they crawl along. It must not be
supposed, however, that all the shells of the reefs reach to such
enormous size as those we have hitherto mentioned, for a rich harvest
of species awaits the eager conchologist who hunts for the smallest
shells he can find in the pools. Within the last few years a large
number of new species of small Molluscs have been described from the
coral seas, many of which do not attain to a total length of more than
⅛ of an inch when perfectly adult, so that the range in size of this
class of animals is very great indeed.
So much has been said about the Madrepores, the Imperforate Corals and
the solitary Corals of the reef, that the impression might be left
that all the Polyps of the Tropics differ from those of the Temperate
regions in the fact that they form shells or skeletal structures.
This is by no means the case, for there are many species of true
Sea-anemones and other Polyps to be found on Coral-reefs which make
no shell at all, and others in which the body-wall is strengthened by
numerous, but very minute spicules or grains of lime which, on the
decomposition of the animal’s body, fall down into a shapeless powder
or sand.
True Sea-anemones are not very abundant on the reefs of North Celebes,
but many species have been found on the Barrier-reef of the Australian
coast, and among them specimens reaching the gigantic size of two
feet in diameter--the largest size attained by single individuals of
the class of animals to which the Anemones belong. Another family of
Polyps called the Clavulariidæ belonging to the Alcyonarians has some
species which make no skeleton of calcium carbonate. An illustration
of one of these is given below (Fig. 22). Another species, called
_Clavularia viridis_, which forms a few spicules in its body-wall,
has a very wide distribution in the East Indies. In some places patches
of it may be seen several square yards in extent, and the crowds of
little Polyp heads with their eight-feathered tentacles waving to and
fro with the pulsations of the tide, is a sight that excites immense
interest and admiration in the mind of the observer.
[Illustration: +Fig. 22.+
Stereosoma, one of the Clavulariidæ.]
One word more about the Corals. Where they are so abundant in number
and species, where rocks hundreds of miles in extent are mainly
composed of their shells and skeletons, it might be thought that a
rich profit could be gained by collecting ship-loads of the Coral that
is used for making beads and brooches by our jewellers. Any expedition,
however, fitted out for this purpose would end in disastrous
failure, for the Precious-coral is not known to occur anywhere in
the neighbourhood of Coral-reefs, but the fishery is confined to
certain parts of the Mediterranean Sea. Species closely allied to the
Precious-coral, but of an inferior colour, have been found in the
Japanese waters. None of the Coral structures found on the reefs have
the same delicate salmon-pink colour and probably none of them are hard
enough to take a good polish.
The Coral-reefs which occur in different parts of the Tropical world
were considered by Darwin under three heads,--Barrier-reefs, Atolls,
and Fringing-reefs. The distinction between these three kinds of
reef is not one that can be insisted upon scientifically, but the
arrangement is convenient for purposes of description.
The Barrier-reefs (Fig. 23, _B_) are situated at a distance of one
to eight miles from the coast, and are separated from it by a lagoon
of moderately deep water. The Barrier-reef of New Caledonia is said
to be 400 miles in length and it follows the general contour of the
coast line.
The Atolls (Fig. 23, _C_) are ring-shaped islands composed of
coral limestone with a lagoon of salt water within them, situated in
the sea without any definite relation to other existing land.
The Fringing or shore-reefs (Fig. 23, _A_) are situated at a
distance of 100 yards or less from the beach and separated from it by
a shallow lagoon which is frequently left as a dry sand-bank at low
tides.
[Illustration: +Fig. 23.+
_A_, Fringing-reef; _B_, Barrier-reef; _C_, Atoll.
_a_, sea; _b_, reef; _c_, rocks of the coast.]
Many forms of reefs are found in different parts of the world, but
they may all be looked upon as special modifications of one of these
three groups. The facts which Darwin collected about Coral-reefs in
his memorable voyage round the world in the _Beagle_, suggested
to this great observer that all the different forms of reef must be
related to one another, and he formulated an ingenious theory to show
how, by the gradual sinking of the crust of the earth, Fringing-reefs
may have become, in the course of a long period of time, either Atolls
or Barrier-reefs. Some doubts have recently been expressed as to the
truth of Darwin’s famous “subsidence theory.” But, whether it is true
or not, to Darwin is due the credit of bringing home forcibly to our
minds the fact that Coral-reefs are slowly undergoing changes of growth
and of destruction, which must lead to most important and far-reaching
alteration in the character of the Tropical seas. It is not intended
in this work to enter into a discussion of the various alternative
theories of Coral-reef formation, but a few words may be added on the
method of formation of Coral limestone.
In studying any one particular form of reef-building Coral we can find
a long series of specimens from one inch in length up to a certain
maximum, which varies with the species and the reef, but may reach
over five feet in diameter as can be seen in the specimens now in the
British Museum at South Kensington. Beyond this maximum size--let
us say four feet in diameter--the Coral rarely grows, because at a
certain age, probably when the vitality of the Coral is on the wane,
the stalk of attachment becomes so bored with parasitic Sponges, Worms,
Fungi and other organisms, that it is thoroughly rotten. If a large
block be picked up from the reef, and with the help of two or three
strong natives carried ashore and broken up with a hammer, a most
interesting migration of Crabs, Worms and other creatures occurs, and
the collecting bottles may be filled with a rich variety of animals
parasitic on the Coral. Now the time comes when the stalk becomes so
brittle that a heavy wave breaks it in two, and the Coral topples
over and dies. If it falls into the sand, either on the inside or the
outside of the reef and becomes buried, it may be preserved fairly
complete, but if it lodges between other Corals the waves and the
parasites between them dissolve it and break it up into thousands of
pieces. This constant disintegration leads to the formation of great
quantities of coralline sand which fills up the interstices between
the living Corals, or gets washed over into the lagoon, or falls as a
talus over the seaward slope of the reefs. In the latter case lumps of
Coral torn off from the reef become embedded in it and form with it a
bank which gives support to more living Polyps on the seaward slope.
Consequently, in the course of many years, a reef which was at one
time only fifty yards from the beach may have extended to a distance
of a hundred yards or more, growing, as it were, on the skeletons or
shells of the Corals that have died. There can be little doubt that
Coral-reefs do grow seawards in this manner in some places, but they
may also be either beaten back or kept for a long time perfectly
stationary if the tides are too strong or too slack.
What the precise conditions are which favour the growth seawards of
Coral-reefs has not yet been systematically investigated; but we may
suppose that if the tides are too strong the sand has no opportunity
of settling between the Coral blocks and forming a solid limestone
rock, and if they are too slow the Coral-polyps do not get sufficient
nourishment to allow them to build fast enough to counteract the
solvent action of the water.
An interesting point connected with the Coral-reefs is the manner in
which they are formed at first. A volcanic upheaval gives rise to a new
island which, in the course of time, is surrounded by a Fringing-reef.
How does this reef begin?
The answer to this question has been recently given by the observation
of the formation of new Corals on the shores of the island of Krakatoa
which is situated in the Sunda straits, and was the seat in 1884 of
one of the most violent volcanic eruptions of historical times. After
the eruption the sea-bottom round the island was found to consist of a
fine volcanic mud, in which it may be believed no Coral embryos could
find a secure foothold. Now it is known that living Corals give rise to
a number of very minute larvæ which for a period of time swim freely
in the water, eventually settling down on some solid stone or shell
to give rise by growth and budding to the Coral blocks. These larvæ
frequently settle down on a piece of floating pumice-stone and after
a time grow to such a size that they sink it. If, in sinking, they
fall upon the bottom in shallow water they form together a substratum
on which other larvæ can settle and flourish. This is apparently the
manner in which Coral clumps are beginning on the slopes of Krakatoa
and these will undoubtedly give rise in time to a more or less complete
Fringing-reef.
Any further discussion on this point would lead us into subjects
beyond the scope of this book, but enough has been said to indicate
to the reader the manner in which the countless Coral-polyps may, in
the course of time, change the position of the reefs on Coral shores,
thereby altering the set of the tides there, changing the position of
the sand-banks, affecting the rate of erosion of the cliffs, and in
other ways causing important modifications of the coast line.
I have mentioned that the ground on the growing edge of the Coral-reef
is carpeted with Corals, Sponges and many other forms of animal life;
in the water swim countless Fish, and the branches of the Corals yield
to the naturalist innumerable forms of creeping and crawling creatures.
The shallow waters of the Tropics, as a whole, however, do not possess
a particularly rich Fauna,--in fact, the distinguished Dr Kükenthal,
who has had great experience of marine work, says that, in his opinion,
the Tropical seas are not richer in littoral animals than the Arctic
seas. Between the reef and the sea-beach there is a lagoon, of varying
breadth, with a sandy bottom, which is almost as barren of animal life
as a desert. A few Worms and Crabs, here and there a Star-fish and some
shells of Foraminifers, are all the spoils which fall to the bag of
the naturalist after many hours’ wading on this unprofitable ground.
The reason for this is, perhaps, not far to seek. When the tide goes
down many stretches of sand are left dry, and others retain only a
few inches of water. The exposure to the heat of a Tropical sun soon
kills and dries up any living animal that is unable to burrow deeply in
the sand, and the water in the shallow pools rises in temperature to
a degree that the human hand or foot can only just bear, so that the
little Fish that escape into them run the risk of being slowly cooked
alive.
On the inside of the lagoon there is, in many places, a broad belt
of Mangrove trees, forming the “Mangrove-swamp,” which contains
some interesting and important animals. These trees have a peculiar
spreading and branching system of roots which are left exposed when
the tide goes down, and form with one another a kind of network or web
a foot or more above the ground, upon which it is possible, with care,
to walk from place to place, at low tide. Between the roots there is a
slimy black sand or ooze, sometimes hard enough to walk upon, but more
commonly soft and treacherous. At high tide the water rises to a height
of two or three feet, completely covering the roots and giving the
swamp the appearance of a forest growing in the sea. Of the animals,
aërial and terrestrial, that haunt the swamps it is not necessary to
say more than a few words, although they too form a study of great
interest to the enthusiastic naturalist. But the marine zoologist who
visits the swamps cannot fail to take note of the millions of Ants,
Flies and Mosquitoes which torment him at every step, and make a
prolonged stay an impossibility.
[Illustration: +Fig. 24.+
Periophthalmus.]
One of the first creatures to be seen on entering a Mangrove-swamp
at low tide is a curious little Fish called _Periophthalmus_.
In some places hundreds may be seen at one time resting on the roots
of the Mangroves, or skipping over the pools of water from one root
to another. There are many varieties of _Periophthalmus_ in
different parts of the world, and their habits are not exactly the
same, so, to give an accurate description, our attention will be
confined to the form occurring in N. Celebes. This little Fish is about
three inches in length and remarkable for its very peculiar eyes, which
are of a bright yellow colour, situated quite close together on the top
of the head, and projecting so much from their sockets that the outline
of more than two-thirds of the eye-ball can be seen. These eyes are
extraordinarily moveable, and frequently revolve quite independently
of one another, like the eyes of a Chamæleon, giving the animal a most
grotesque and even ludicrous expression. These Fish seem to swim in
the water very seldom; when undisturbed they may be seen clinging to
rocks or trees by their fore-fins with their tails only in the water,
but from time to time they spring into the air to catch a Fly on the
wing, or a small Crab which has come unwarily within their range. Their
fore-fins are peculiarly adapted to their habits, in that they have a
very muscular base and a distinct elbow joint.
These creatures are not easy to capture, as the ground on which they
live is not adapted for rapid pursuit, and it is impossible to get
close enough to them to catch them in a hand-net with a long handle.
When kept in an aquarium it is seen that although they are rapid
swimmers when they do go below the surface, they seem to prefer to
live with their head and shoulders out of the water; and when chased
in their natural haunts they very rarely, if ever, seek to escape by
plunging into the water, but they execute a series of rapid jumps with
extraordinary rapidity from root to root or rock to rock, and so avoid
their pursuers. Their existence is really an amphibious one, and their
food consists partly of Insects on the wing. Their gills are very much
reduced in size, and it seems probable, from observations that have
been recently made, that their respiration is partly carried on by the
thin skin between the rays of the tail-fin.
Another animal extremely abundant in the Mangrove-swamps of Celebes,
and, like _Periophthalmus_, having a very wide distribution in
similar places in other tropical countries, is the “Caller-crab”
_Gelasimus_. These Crabs are about an inch in breadth across the
back, and are remarkable for possessing one very much enlarged and
brightly-coloured claw, the others being normal in size and dull in
colour like the rest of the body. On first entering the swamp at low
tide there may be seen on the mud between the roots of the trees a
number of bright yellow, blue or green objects, which, as the traveller
approaches, disappear one by one into holes in the ground. When the
eyes are accustomed to the gloom of the swamp these bright objects are
seen to be the great claws of the “Caller-crabs,” the rest of the body
being inconspicuous owing to its close resemblance in colour to the
slimy ground.
These are the principal and most abundant marine inhabitants of the
swamps, and as has been pointed out, all of them are more or less
amphibious in habit. More locally distributed, Oysters and other
bivalves may be found attached to the roots of the trees. Several
species of marsh Gastropods occur, some of them in great abundance in a
few localities. Occasionally a Sea-anemone, with remarkable powers of
burrowing rapidly in the sand when disturbed, may be found, and to the
microscopist a harvest of Foraminifers and other minute forms of life
awaits investigation and description in the Mangrove-swamp.
Whenever the tide is high a considerable number of Fish-fry,
Jelly-fish, and other forms of floating and swimming life
characteristic of these waters, drift into the swamps; and some being
caught by the tangle of roots are left behind, either in the pools, or
high and moderately dry upon the sand when the tide ebbs. Upon these
victims of the retreating tide swarms of Ants and Flies descend from
the trees, Crabs from the shore and from their holes in the sand are on
the watch for them, Kingfishers and Sandpipers are ready to pounce upon
those which are most to their taste, so that before the friendly waters
of the ocean return to the swamp, scarcely one of them is left.
These constitute what may be called in the swamp the _extraneous_
Fauna, which if it is not truly indigenous is nevertheless necessary
for the continued existence and well-being of the true inhabitants.
The character of the sea-bottom on the outer side of the living
Coral-reefs varies so much in different parts of the world that an
adequate treatment of the Fauna in that region would have to be one of
greater length than is possible in this book.
The living edge of the Coral-reef is in some cases situated on the
top of a submarine precipice of very considerable height, and in
many places the sounding-line goes down to a depth of five or six
hundred feet a few yards beyond the limits of the reef. The practical
difficulties in the way of determining the character of the Fauna of
any sea-bottom that shelves in this manner are very great, but where it
is partly composed of massive lumps of solid Coral they are at present
insurmountable. Every time the dredge or trawl reaches the bottom it
becomes entangled in the Coral branches, and is liable to be seriously
torn, or even lost. Swabs and iron hooks and fish traps may yield some
scraps of information, but speaking generally, the Fauna of these steep
slopes is scarcely known at all.
The most important question, from the geological point of view, that
has to be determined is the depth of water in which reef-building
Corals can live and thrive. This is still a matter of uncertainty owing
to the practical difficulties met with in the attempts to investigate
it. Darwin estimated that the limit of vigorous coral growth was
between 20 and 30 fathoms, but in recent years, owing to the discovery
of luxurious Coral patches in 44 fathoms on the Tizard and Macclesfield
banks, there is a pretty general opinion that his estimate is too low.
Whatever the exact limit may be, it is quite clear that in many parts
of the world the sea-bottom quite close to the outer edge of the
reef cannot support a vigorous Coral fauna. Here and there patches
of peculiar deep-sea species of Corals occur, but they do not form
in such depths anything of the nature of a reef. They are usually
isolated specimens, similar to those that are found in deep water on
the Norwegian coast and other parts of the world outside the limits of
the Tropics; but these specimens really belong to the deep-sea Fauna,
of which we shall learn more in another chapter.
In many places, however, the water at the base of the outer edge of
the reef is not very deep, and may slope away gradually towards the
bed of the ocean. The Fauna of such slopes in the Tropics is not
characteristically rich, as my own experiences of dredging in such
waters have proved. Time after time the dredge that was used in 15 to
20 fathoms off the coast of Talisse, came up with nothing but sand
or gravel. Occasionally a Brittle-star or a branch of dead Coral,
with a few Zoophytes growing on it, came up; and in some places a
few beautiful Lily-stars or Crinoids relieved the monotony of the
investigation. But, on the whole, the animals found in this region were
not numerous, nor of a character to excite any particular interest.
Before bringing this chapter to a close, a brief reference must be made
to one of the most remarkable phenomena in the animal kingdom,--this
is the history of the Palolo worm. On certain parts of the coast of
the Samoan islands the Palolo worm appears in great abundance in the
early morning hours of one or two days at the beginning of the third
quarter of the moon in the months of October and November. As the
worm is regarded as a very great delicacy by the natives, the day of
its appearance is looked upon as one of the most important red-letter
days of the year. Weeks before the worms are expected the advent of
the Palolo is discussed, stories are told of the fisheries of bye-gone
years, anecdotes of the last year are remembered and rehearsed, and the
whole population is prepared for the great event as for a feast.
When the grand day arrives the boats are decorated, the girls put on
all their finery, and everyone who can find a seat in a boat goes off
to the fishery amid a merry chorus of song and laughter. It must,
indeed, be a strange sight to see the flotilla of canoes with their
eagerly expectant and gaily-bedecked crews, waiting in the dim light
of the half moon for the day to break and the exciting fishery to
begin. As soon as it is light enough to see into the water, a few
writhing Worms may be distinguished at the surface, which increase in
number with such extraordinary rapidity, that in a little while it is
impossible to see anything below three or four inches owing to the
multitude of Palolos. As quickly as possible the fishery proceeds,
every man, woman, and child gathering the harvest of Worms during the
precious moments of the sunrise. When at last the sun rises well above
the horizon the Worm disappears again, and the boats hasten to the
shores with their booty.
This remarkable swarming process of the Palolo, occurring as it does,
only once or twice a year, in constant relation with a particular
phase of the moon, and lasting on each occasion only a few minutes in
time, is not the only noteworthy feature of the animal.
The Palolo worm, as it is caught, varies in length from an inch to a
foot or more, and is about a quarter of an inch in breadth, but it
readily breaks up into pieces when handled. It is composed of numerous
rings or segments, each provided with a pair of processes bearing
bristles, but there is no head. Astonishing as it may seem to those
unacquainted with the natural history of Worms, it is nevertheless a
fact that when the Palolo swarms it leaves its head behind among the
Corals, where, in all probability, it regenerates a new body. This
accounts for the fact that while the body of the Palolo is frequently
brought home to our Museums in England, its head is a rarity. The
colour of the Worm varies very considerably. The pieces bearing eggs
are usually of some shade of green, hence the specific name _Palolo
viridis_ that is given to it by scientists, but the males are
usually white. In connection with the appearance of the Worm there is a
curious statement that once in every four years it is exactly one lunar
month late, so that the time of year of its occurrence is constant. The
natives are also forewarned of the advent of the Palolo worm by the
movements of the land Crabs, which, it is reported, come down from the
fields and forest a few days before the Palolo feast and plunge into
the sea.
The precise habitat of the Worm when it is not swarming is still a
matter of some doubt. A few rare specimens have been found in the
Coral blocks in shallow water, but it is generally supposed that the
majority of them live in deep water on the outer side of the reefs. It
is not confined by any means to Samoa. It occurs also in Fiji, Tonga
and other Pacific islands. A Worm similar to the Palolo in habits was
described years ago by Rumphius in the Malay Archipelago, and Saville
Kent mentions a little Nereid worm with similar spawning habits on the
great Barrier-reef of Queensland.
CHAPTER IV.
SURFACE-SWIMMING FAUNA (INVERTEBRATES).
Everyone of an observant turn of mind must have noticed that in the
wake of a boat that is passing through the water on a calm summer’s
night, sparks of bright phosphorescent light may be seen to appear, to
remain for a few seconds, and then become extinguished again. Sometimes
the breaking of the ripples on the surface of the water seems to be
sufficient to cause these sparks to appear, but occasionally streaks
and flashes of pale blue light arise and disappear without apparently
any such mechanical disturbance.
The phosphorescence of the sea, as this phenomenon is called, is common
enough on our coasts, but it never reaches the degree of brilliancy and
beauty which is so remarkable in the open Atlantic Ocean, the South
Seas, and some other parts of the world. In the Atlantic Ocean the
phosphorescence is sometimes so bright that it is possible to read a
book on deck by its light alone; and on a dark night in the Banda seas
the water is often like a huge expanse of pale blue smoke studded with
diamonds and other lustrous gems.
These lights are mainly produced by animals which float and drift about
on the surface of the water. It is not, as is very commonly supposed,
only one or two different kinds of animals that are phosphorescent,
but a vast number belonging to many widely different families and of a
great variety of form and structure. When the day breaks many of these
animals sink down a few fathoms into the darker and cooler strata of
water, but a considerable number remain so close to the surface that
they can be easily caught in a muslin net dragged after a boat.
Some of these animals, such as the Jelly-fish, can, during the day, be
observed clearly enough from the boat, others can only be seen when the
contents of the net are emptied into a glass bottle, and others again
are so minute that it requires a strong magnifying glass to detect them
at all. Such animals that float or drift in the water without powers
of swimming vigorously in one direction or the other, are collectively
called the Plankton. In every sea, from the Arctic regions to the
Equator, a Plankton will be found. Sometimes it is mainly composed
of one species, in other cases it consists of many different species
living together. Under certain conditions the water is simply crowded
with these organisms, and in different circumstances the Plankton is
represented by only a few individuals.
The variations of the Plankton in different parts of the world have,
of recent years, been subjected to many searching investigations, but
although many important facts have been recorded, the explanation of
the principal phenomena remains a mystery.
One of the most interesting facts, perhaps, is the extraordinary
local variations to be observed. To give a single example as an
illustration of this point the case of the common white Jelly-fish may
be mentioned. On occasions the surface of the water in our bays and
estuaries contains so many of these animals, that the sea appears to be
little more than a mass of jelly. In other seasons not more than a few
isolated individuals will be seen all through the summer months.
With all the resources of modern scientific investigation no adequate
explanation has been given to account for this fact. It may be that the
variation is due to the prevailing winds or tides, to the temperature
of the water, to the roughness or smoothness of the sea, to disturbance
of the ground where the eggs have settled, or to some other hitherto
unforeseen conditions. Not only seasonal, however, but even diurnal
variations occur, of a most remarkable and inexplicable character.
On one occasion for example I was collecting a number of Jelly-fish in
Southampton Water, and for nearly two hours specimens were obtained as
fast as they could be hauled into the boat. Suddenly a change came,
and in a few moments the water that had been alive with these animals
seemed to contain not one. Another time, after dredging nearly all
the afternoon at Lulworth for _Hormiphora_, with the very poor
success of a half dozen specimens, the net came up simply choked full
of these little round jelly-like Ctenophores, and for the remaining
hours of day-light there appeared to be an abundance of them all along
the coast. One morning in the Tropics, at about an hour after sunrise,
I was looking over the side of a steam-boat, and saw that the surface
waters were full of beautiful and rare species of floating animals. In
less than half an hour afterwards, when a boat was put off, scarcely
one of them could be found. Anybody who is accustomed to working with a
tow-net can give similar experiences.
In each of these cases a simple explanation might be suggested. In the
first case it might have been the change in the tide which effected the
disappearance of the Jelly-fish; in the second it might have been the
approach of nightfall that caused the Hormiphoras to rise; and in the
last case it might have been the approach of the heat of day; but when
carefully considered such explanations are not sufficient, in that they
do not account for the suddenness of the change.
The fact is that the conditions of life in the surface waters are
so complicated that it is extremely difficult for us to accurately
estimate the balance of the forces which act upon these organisms. The
direct heat of the sun, the light of both the sun and the moon, the
tranquillity or roughness of the sea, the conditions of the tides and
winds which cause changes in the surface temperature of the water,
independently of the direct heat of the sun, all influence the delicate
tissues of which these animals’ bodies are composed, and cause them to
change their position.
The animals which compose the surface Plankton may be considered under
two heads--those that are adult, and those that are the larvæ of
sessile and crawling forms of life which in the adult stage live at the
bottom.
Those belonging to the former group frequently occur far out in the
open ocean as well as in the neighbourhood of the land, and have as a
rule a wide geographical distribution. Those belonging to the latter
group are more usually found within a few miles of the coast line,
although winds and tides may occasionally drift them far out into the
sea, where their larval existence is prolonged for an abnormally long
time. Leaving out of consideration for the moment the many interesting
exceptions, we may say that the Plankton of the open oceans differs
from that of the neighbourhood of the coasts, by the larger proportion
of adult forms that it bears.
A great variety of animals pass the whole of their lives in the surface
waters of the sea, but the commonest and most widely distributed of all
probably are the Copepods belonging to the class _Crustacea_.
The Copepods are minute creatures, rarely exceeding a quarter of an
inch in length, which row themselves through the water by a pair of
long antennæ, projecting from the head end of the body. They occur in
fresh water as well as in the sea, and so abundant are they that if a
glass tumbler be filled with the water from a pond, a lake, or the sea,
and examined with a magnifying glass, a number of specimens are almost
sure to be seen. They occur in abundance at the surface of the sea in
nearly all climes, and very often are the sole representatives of the
Plankton that are found in the hauls of the tow-net.
Attention has already been called above to the fact that in the Tropics
the surface-floating animals gradually sink down into the depths as
the heat of the day approaches, but even on fine calm days a few
Copepods will be found at the surface. Although they sometimes occur in
Temperate seas in such vast numbers that the water is quite discoloured
with them, more variety of form, or, in other words, more distinct
genera and species are found in the warm and Tropical parts of the
world.
The study of this group reveals to the microscopist some of the most
marvellously beautiful displays of colour and form that can be found
in the animal kingdom. Sometimes the body and legs are beset with an
immense number of extremely fine and delicate spines, which are in some
cases provided with rows of still finer spinelets, giving them the
appearance of a most minute feather. Sometimes the body contains large
granules of a bright red colour, and at others smaller granules of a
bright blue are seen scattered among the organs. The female Copepods
usually carry, securely fastened to their tails, two little pear-shaped
sacks of eggs, which are sometimes bright green, blue or red.
[Illustration: +Fig. 25.+
A free-swimming Copepod.]
Endless are the varieties of form and colour presented by these little
creatures, and endless are the beauties which the study of their
structure reveals; but as we have mentioned them first as inhabitants
of the surface waters of the seas, we must pause to consider here
how these organisms, which excite so much wonder and admiration,
are adapted or fitted for their peculiar mode of life. But it must
be remarked that these statements apply only to the free-swimming
Copepods, for many animals classed in this group by zoologists are
parasites, and as such are so profoundly changed that they might at
first sight be relegated to another class of organisms altogether.
Now we must remember that animals that live in the surface waters must
be prepared to keep afloat for the whole period of their lives--from
the time they are hatched until they fall a prey to some voracious
enemy. Under ordinary circumstances, they never find an opportunity of
resting, either on the sea-bottom or on any floating substance.
If a Copepod is watched in a tumbler of water it will be seen to give a
number of strokes with its long antennæ and then to rest suspended for
a few seconds; a few more strokes follow and then another pause, and so
on. During the period of rest the body sinks slowly, sometimes almost
imperceptibly, but never so much that it cannot recover its position in
the water after the first few strokes.
It must be clear to the reader that the less it sinks during the pause
the less will be the muscular activity required to regain its position,
and that, consequently, every mechanical contrivance that its body
possesses to diminish its tendency to sink will be a saving of muscular
and nervous energy.
A very simple experiment will demonstrate that a body which presents a
considerable surface to the water, sinks more slowly than one of the
same weight that is round and compact. If we take two equal pieces of
silver paper and roll one of them into a tight little ball, leaving
the other as a flat sheet, and then let them sink together in a tall
jar of water, the former will reach the bottom long before the latter.
Similarly the body of an animal which possesses a dense armature of
spines, as it presents more surface to the water, sinks much more
slowly than the body of an animal of the same weight that is smooth and
compact.
The spininess or hairiness of the Copepod body, then, may be regarded
as one of its adaptations to the environment in which it lives. But of
course this character is not by any means confined to the Copepods.
Very many of the surface-swimming Crustaceans, and more particularly
their larvæ, have remarkably spiny bodies, and among many of the
Foraminifers, Radiolarians, Worms, Molluscs and even Fish we find
some similar extension of the surface of the body which lowers the
sinking rate. Another means by which the bodies of many of the animals
composing the Plankton are buoyed up, is the secretion into a special
chamber or reservoir of some gas or oil of a lesser weight than the
sea-water. This is what may be called the balloon principle. In such
animals we may regard the heavy muscles, skeleton, skin and viscera
as the car and the freight of the balloon, while the gas reservoir
corresponds to the whole silk case containing the coal-gas.
Such an animal might also be compared to a man in the sea clinging to
an india-rubber life-belt. The body of the man by itself is heavier
than the water, and in the absence of the muscular exercise of swimming
sinks rapidly to the bottom; but the body of the man and the life-belt
taken together are lighter than water and float continuously without
any action of the muscles. If the life-belt were considerably smaller
than usual the man and belt would sink, but much less rapidly than the
man alone; and the muscular energy required to keep himself afloat
would be far less with the belt than without it, consequently he
would be able to keep afloat much longer with the same expenditure of
muscular energy. The bodies of many of these surface-swimming animals
may then be best compared with a man assisted by a _small_
life-belt. When dead or still they slowly sink, but a slight amount
of muscular energy expended in swimming is sufficient to keep them
afloat. In what has been said above about the body of the Copepod,
reference has been made to certain bright red granules. These are in
all probability little globules of some oily or fatty substance lighter
in weight than the sea-water, which serve to buoy up the body of the
little creature. It is difficult to say why they should have such
bright colours. We have no record of observations that show that the
colours can be of any use to them as a protection from their enemies,
nor is there any physical explanation of the colours of these granules
any more than of the blood, the bile and other products of animal and
vegetable vital processes. The eggs contained in the egg-sacks of the
Copepods also bear a certain amount of oily substance very frequently
different in colour from that of the other parts of the body, and this
probably acts in the same manner upon the body of the parent or on
that of the little larvæ when they are first hatched.
Thus we find in the body of the Copepods at least two important
modifications of structure, which render them fit or suitable for their
life-long swim in the surface waters of the sea.
Let us now consider another important group that has the same habit but
differs from the Copepods in size and form, namely, the Jelly-fish.
The Jelly-fish, or Medusæ, as they are usually called by zoologists,
are disc or bell-shaped animals of a very soft gelatinous texture.
From the centre of the disc or bell there hangs down a tube of varying
length bearing the mouth, and the margin is often provided with a row
of thin delicate tentacles like a fringe. (See Fig. 7). When watched
on a calm summer’s evening they may be seen to slowly sink a few
inches or more from the surface, and then with a series of convulsive
contractions of the bell to rise to the surface again. Sometimes these
contractions may be observed to continue perfectly rhythmically for a
long time.
In one of the commonest of the English Medusæ four rings of a bright
pink or orange colour may be observed in the disc. These are eggs
and male spawn, and when shed they give rise to multitudes of tiny
little larvæ which sink to the bottom and become fixed to some rock
or sea-weed. After the larva has securely fixed itself it becomes
changed into a little Polyp which gives rise, in the course of time,
to a number of small discs, arranged one above another like a pile of
saucers. These discs break away from the base and from the parent stock
to grow into the form and size of the adult Jelly-fish.
We have here an example in the life-history of the common Jelly-fish,
of what is known as “alternation of generations.” The eggs give rise to
sessile Polyps, and these produce a number of buds which, when fully
grown, give rise in their turn to the eggs; or, in other words, the
egg-producing generation of large surface-swimming Jelly-fish regularly
alternates with the small sedentary bud-producing generation. Now as
the bud-producing or Polyp generation of the common Jelly-fish referred
to is fixed to the bottom, the proximity to a coast, or at any rate
to a shallow water area, is a necessity for the continuation of the
species. Many of the Jelly-fish are undoubtedly drifted out into the
open ocean by the tides, but the larvæ they produce, after swimming
about in search of something solid to which they can attach themselves,
must at last perish. It is only those larvæ which are hatched near
enough to the shore to be able to reach the bottom during the tenure of
their lives, that can continue the generation of these Jelly-fishes.
But even in the open ocean far away from shallow water or a coast
line, Jelly-fish, belonging of course to different species from those
of the coasts, are found. What is their natural history? How is their
life different from that of the Jelly-fish of the shore? Some of
them produce larvæ very similar to those described above but they
seek, instead of the rocks or sea-weed, other Jelly-fish and attach
themselves to them as parasites.
In other species, however, the “alternation of generations” is entirely
lost, and the egg gives rise directly to a free-swimming little
Jelly-fish which in time grows to be like its parent in size and shape.
In this case the fixed or sessile form in the life-history is, as it
were, omitted in order that the animal may lead a life independent of
the coast and sea-bottom.
The Jelly-fish, then, present us with an interesting example of a
manner in which the life-history of an animal may be modified for or
adapted to this surface-swimming habit.
There is also another point of interest about these creatures in this
connection. In writing about the Copepods I pointed out the mechanical
contrivances they exhibit for keeping themselves afloat, namely, the
spines, hairs and oil globules. Jelly-fish have neither spines nor oil
globules of the same nature, but still their bodies are very light in
the water and in the absence of muscular movements sink but slowly to
the bottom. This lightness is due to the fact that all the tissues and
organs of which it is composed are very largely distended with water.
When the body of a Jelly-fish is analysed it is found that over 95 per
cent. of it consists of water. This power of absorbing large quantities
of fluid into the tissues, while it increases the size of the body,
proportionately diminishes its weight in water.
It has also another effect. It makes the tissues of the body much more
transparent and gives them that soft jelly-like consistency which is
so characteristic of the surface-swimming forms.
The popular term ‘Jelly-fish’ is one that is frequently applied to
many forms of surface-swimming animals that are really very different
in structure and general composition from the true Medusæ. The Salps,
for example, to which reference will be made presently, although soft
and transparent in texture like the Medusæ, belong to a very widely
separated group of animals, and to the anatomist it would be as absurd
to classify them together, as to put the Butterflies and the Fish in
the same group.
These remarks are necessary because in the treatment adopted in this
little book the animals that live together are considered in the same
chapter, and it is important that the reader should bear in mind that
they are not as a consequence anatomically related to one another.
It is indeed remarkable that animals which are so different from one
another, in their anatomy, development and life-history, as, for
example, the Salps and the Medusæ, and which have had such a widely
different ancestry, should, as a matter of fact, resemble one another
so closely in form and texture as to be given collectively the same
name by the unscientific observer.
Among the heterogeneous crowd of animals that are popularly called
Jelly-fish there is one particular group which presents us with some
very interesting members. These are the Siphonophores. In many parts of
the temperate and warmer seas of the world the surface may be covered
with thousands of little creatures which, when brought upon the deck,
seem to be little else than coloured bladders of air. The scientific
name of these animals is _Physalia_. When placed in a glass of
water, however, it will be seen that, from the under side of the
bladder which floats freely on the water, numerous delicate tentacles
and Polyps hang down. These creatures are kept at the surface by an
air-bladder float and no muscular energy is required to sustain them in
that position.
[Illustration: +Fig. 26.+
The swim-bladder of Velella.]
Another Siphonophore called Velella has a bladder of a more complicated
character in the shape of a disc with a semi-circular or triangular
sail on its upper side. There can be no doubt of the advantage of
this float to the species. It not only enables them to keep afloat
without the expenditure of muscular energy, but as the wind catches
the sail they are drifted along over great areas of the ocean and thus
distributed far and wide from the spot on which they were hatched.
Still the float has undoubtedly its disadvantages, for it exposes them
to the danger of being blown ashore by a steady wind and so perishing
in thousands. Agassiz says that on the coast of Florida the beach is
sometimes marked with lines of Velellas that have been stranded in
this manner, and I have seen in Celebes four or five rows of bright
blue Physalias stretching for miles along the shore.
In the Mediterranean and Eastern Atlantic Ocean a very large Physalia
occurs which has received the popular name of the “Portuguese
man-of-war,” and is famous for its stinging powers. The stinging
is produced by a number of very minute sacs, which shoot out, when
they are touched, a long pointed thread that penetrates the skin
and conveys an irritant poison. These are called the thread-cells,
and the “Portuguese man-of-war” is not by any means peculiar in
possessing them. All the Medusæ and Siphonophores, all the true Corals
and Sea-anemones have them--in fact, all those creatures which are
classified together by the zoologist as _Cœlenterata_ may be said
to be stinging animals. The thread-cells, however, vary very much in
size in this group, and in the great majority of cases the thread is
too feeble to perforate the skin of the human hand, and consequently
their owners have not acquired a bad reputation.
People do not warn their children not to touch the Sea-anemones on the
rocks or the Jelly-fish stranded on the beach, and yet they are both
dependent for their food upon their stinging powers; and indeed many of
the British Medusæ which may be handled with impunity, are capable of
stinging quite severely the more delicate skin of the back and arms of
unwary bathers.
Besides the two forms of Siphonophores which have been described,
there are many others to be found at or near the surface of the seas
of all climes. Some of them possess great floats like Physalia and
Velella, but the majority of them have either no floats at all or such
as are too small to do more than assist in keeping the animal near the
surface. All of these Siphonophores are provided with little bells,
which, contracting rhythmically like a Jelly-fish, drag the animal
along, sometimes to the surface, sometimes a few fathoms below it.
Some of these forms are extremely graceful, being like long strings of
jelly, with numerous clusters of Polyps and long feathery tentacles,
towed through the water by one or two exquisitely delicate little bells
situated at the leading end of the string.
A few words must now be said about the Salps, because in some seas
the water is on occasions so full of them that they seem to be packed
together ready for preserving. The simplest form of Salp is like a
small sac or barrel of transparent gelatinous substance open at both
ends. Running round the barrel are five or seven bands of a less
transparent nature, appearing to the unaided vision like milky white
streaks. These streaks are bands of muscles by which the movement
of the body through the water is assisted. Sometimes they are seen
swimming about independently of one another, sometimes Salps very
similar to them in general appearance are seen to be attached to one
another in long chains. At first it was supposed by naturalists that
the former or Solitary Salps were of a different species to the latter,
or Chain-salps as they are called; but it has been discovered that
these two forms are but stages in the life-history of one species.
When the anatomy of a Chain-salp is minutely examined it is found to
contain a single egg, which gives rise to a young Salp similar in
nearly all details to the solitary one. This escapes from its parent’s
body when it is old enough to take care of itself, and leads an
independent existence. After it has grown to its full size it gives
rise to a stalk which divides up into a number of young Salps, attached
to one another in a very characteristic manner.
[Illustration: +Fig. 27.+
Solitary form of Salp, bearing a young stalk of Chain-salps.]
Here, then, we have another instance of alternation of generations
similar in this respect to the example previously quoted among the
Jelly-fish, in that the one generation produces an egg, and the other
numerous buds; but differing from it in the fact that in the case of
the Salps both generations are adapted for freely swimming at the
surface of the sea.
Space does not allow us to say more in detail about the other animals
of the Plankton that belong to the same group as the Salps; of the
wonderfully interesting life-history of _Doliolum_; of the
extraordinary bright light emitted by _Pyrosoma_, or of the
remarkable little _Fritillaria_, shaped like a tad-pole, living
in its house of jelly. The story of each of these might take a whole
chapter to itself and still be only partly told.
Anyone who is acquainted with the general appearance of the Whelks
and Periwinkles, and other Gastropods of our shores might be well
astonished when he saw, for the first time, many of the Gastropods of
the high seas. The shell is either absent altogether or consists of a
thin little papery cap far too small to afford protection to the body.
The head and foot, and, indeed, the greater part of the body, are
transparent, soft and gelatinous like a Jelly-fish, in fact the whole
appearance is so different that it is not until the internal anatomy is
carefully studied that their true position in the animal kingdom can be
assigned to them.
Here, then, we find another instance of a profound modification of
structure associated with the surface-swimming habit; the modification
being due very largely to the absorption of considerable quantities of
water into the tissues of the body, which has the effect of rendering
them transparent, and, at the same time, of reducing their weight in
the water.
The transparency of the body of so many of the animals of the Plankton
has suggested the theory that by rendering them less conspicuous to
their enemies it is of the nature of a protection to them. We ought to
hesitate before accepting this theory until we know more accurately
what are the enemies that they endeavour to protect themselves against.
It is very probable that none of the Fish will feed upon any of the
transparent Jelly-fish, neither is there any evidence that the Salps
and the pelagic Gastropods form a favourite food for them. There is no
good reason for supposing that the Sea-birds would, if they could see
them better, prey upon them, so long as there are Fish in the sea to
provide a more substantial and satisfactory meal. The Whales, as they
dash through the water with their huge mouths wide open, undoubtedly
swallow them in thousands, but it can not be reasonably supposed that
the Whale can be guided by sight in the selection of its food. We ought
not, perhaps, to go so far as to say that it is no protection to them,
for Prof. Moseley states that the Turtle sometimes feeds upon the
Velellas, but at the same time we may consider that the transparency
is an effect produced by the large amount of water in their tissues,
which is there for the purpose of reducing their specific gravity and
assisting in that manner in their floatation.
[Illustration: +Fig. 28.+
Pteropod, showing the so-called wings.]
The only Gastropod found in the open seas which retains in its
characteristic form the large coiled shell, is the beautiful blue
_Janthina_, famous for its habit of constructing a little raft
which floats on the surface of the sea. To the underside of this it
attaches its eggs and spends its life in pushing or dragging the raft
about.
No account of the Molluscs of the Plankton would be complete without
some reference to the Pteropods. These creatures are provided with a
pair of muscular lobes of the body, which have been compared to wings.
By means of these they are able to swim through the water. Some of them
are provided with delicate little glassy shells, but in others the body
is quite naked. We may regard the Pteropods as the most highly modified
forms of Gastropods adapted for a pelagic life.
In both the Arctic and Antarctic seas this group occurs in immense
numbers, and it is supposed to form not an inconsiderable proportion of
the food of the gigantic Right-whales. They also occur in the Temperate
and Tropical zones, and indeed there are actually more genera and
species there than in the colder regions to the North and South.
The Insect world is represented at the surface of the ocean by a
curious little Bug called _Halobates_. It is not uncommonly found
in tropical or subtropical seas feeding upon dead Salps or Jelly-fish,
and when disturbed scuds over the surface after the manner of many of
the Insects living on our inland ponds and lakes. It has been described
as an “ivory-legged fellow, covered with a bluish-white down.” As it is
essentially an air-breather like all adult insects, its usual habitat
is ‘_on_’ the sea and not _in_ it, so that strictly speaking
it is not a member of the Plankton. There is no doubt that under
certain circumstances it can and does dive into the water, and on these
occasions it carries with it for respiratory purposes a layer of air
attached to the ‘bluish-white down’ covering the body.
There are no traces of wings on its thorax, and it is therefore
incapable of flight. Very little is known at present of its
development, and practically nothing of its internal anatomy, so that
its proper position in the order of the Bugs or _Hemiptera_ is a
matter of conjecture, but it is an interesting little creature, in the
fact that it is the only member of its class that has a purely pelagic
life-history.
Among the microscopic forms of life found in the Plankton of the sea,
the Radiolarians and Foraminifers are perhaps the most important. The
Radiolarians are very minute specks of protoplasm, usually protected
or supported by an elaborate skeleton of a substance closely allied
to flint. The form of this skeleton varies so much in the numerous
species that have been described, that it is quite impossible in a few
words to give an adequate idea of the principal types. (See Fig. 2). We
may say, however, that in a considerable number of them the skeleton
has the form of a hollow sphere, perforated by numerous round holes
and supporting outside a number of long thin needles. The anatomy of
the Radiolarians is extremely simple. Their bodies are built entirely
of protoplasm which performs all the vital functions. There is no
definite head, mouth, brain, nor muscular organ. This being the case,
the question arises, How do these animals provided with a skeleton
of such a heavy substance as flint manage to support themselves in
the water without muscular appendages? The answer to this question
is two-fold--In the first place, the elaborate form of the skeleton
presents an enormous surface to the water in proportion to its
weight, and consequently sinks slowly; and secondly, the protoplasm
is provided with numerous vacuoles containing a watery fluid, and in
many cases at least one larger vacuole containing oil. If the liquids
in these vacuoles are lighter than sea-water, and there is good reason
to suppose that some at least of them are, then they are of the same
nature as the oil chambers of the Copepods, and are hydrostatic in
function.
[Illustration: +Fig. 29.+
Shells of Foraminifers living at the bottom of the sea.]
Among the Foraminifers very few genera strictly belong to the surface
Fauna. Most of them have heavy, compact shells of carbonate of lime,
and they live among the sand or the rocks at the bottom of the sea. The
best known of the surface-dwelling forms is _Globigerina_, and
this, in accordance with its habits, possesses a shell which, like that
of the Radiolarians, is very light, perforated by numerous large holes
and provided with long delicate spines. The shell of _Globigerina_
might well be mistaken for that of a Radiolarian were it not for the
fact that it is composed of carbonate of lime instead of flint.
The Radiolarians in some waters, and the _Globigerinidæ_ in
others, are present in enormous numbers, and as they die their shells
fall in a gentle rain from the surface towards the sea-bottom, where
they frequently, form a very large part of the abysmal mud.
In speaking of the organisms of the surface of the sea no mention has
yet been made of the plant world. Of the large conspicuous Sea-weeds
that are often found far out in the open ocean the best known is the
Sargasso or Gulf-weed of the Atlantic. It forms in some cases great
floating patches, of very considerable area, and is, when alive,
of a bright yellow colour. The Sargasso patches are, however, of
great interest to the zoologist, because they support a considerable
population of animals specially adapted by their form and colour to
live among the Sea-weeds. They present us, in fact, with a peculiar
Fauna, containing representatives of all the most important groups of
marine animals.
Besides the large conspicuous weeds like the Sargasso, the surface
of the sea supports a large Flora of minute plants of very lowly
organisation, and it is not at all uncommon for them to be present in
such numbers as to cause a distinct discolouration of the water.
[Illustration: +Fig. 30.+
Globigerina living at the surface of the sea.]
The banks that they form on the coast of Brazil and elsewhere
were called “Sea-sawdust” by Sir Joseph Banks. Moseley says that
“when tracts of the sea are passed through, which are full of this
_Trichodesmium_, the water lighted up by sunlight, when looked
down into, appears as if full of small particles of mica or some such
substance, so strongly is the light reflected from the minute bundles
of the Algæ”; and again, he says, “so abundant is _Trichodesmium_
in some seas that one of the explanations of the name of the Red Sea is
that the term was derived from the discolouration of the water by vast
quantities of _Trichodesmium erythræum_.”
In addition to this “Sea-sawdust,” Diatoms, the still more minute
organisms, the Bacteria, and the debateable particles called
Coccospheres and Rhabdospheres, add to the number of the floating Flora
of the seas.
The importance of these organisms to the zoologist is that they must
ultimately form the food supply of the animals of the Plankton. Some
of the larger animals may feed upon the smaller ones, and the smaller
ones may, in their turn, feed upon still smaller ones, but we must
come eventually, in descending the scale, to the animals that are
vegetable-feeders and prey upon the minute plants that have just been
mentioned.
Now that we have considered very briefly some of the principal forms of
life that compose the floating and drifting population of the surface,
we may return to the subject with which the chapter opened, namely, the
phosphorescence of the sea.
It need hardly be mentioned that it is a subject which is beset
by innumerable difficulties. Even when the sea is extremely
phosphorescent, and the observer is provided with an excellent
microscope and all the necessary scientific appliances, he finds
it difficult to answer the question--“What is the cause of the
phosphorescence tonight?” The sample of water he takes may reveal to
him a multitude of different organisms, many of which are so small that
they can only be seen with a strong artificial light, and then it is
impossible to say which are and which are not phosphorescent.
Some of the Copepods are known to possess an organ emitting a blight
blue star-like light which shines for a time and is then suddenly
extinguished. In the Malay Archipelago several of these bright lights
may be seen near the surface of the water on calm mornings just before
sunrise, and it is extremely interesting to watch them gradually
sinking down into deeper water as the day dawns, and then suddenly
going out one after the other.
Some of the large Jelly-fishes, such as _Pelagia noctiluca_,
glow with a soft blue light. The curious pelagic Tunicate colony
_Pyrosoma_ receives its name from the fact that it emits a bright
light. A giant _Pyrosoma_ was caught by the _Challenger_
in the deep-sea trawl, and, to quote the words of Professor Moseley
once more, “It was like a great sac, with walls of jelly about an inch
in thickness. It was four feet in length and ten inches in diameter.
When a Pyrosoma is stimulated by having its surface touched, the
phosphorescent light breaks out at first at the spot stimulated,
and then spreads over the surface of the colony as the stimulus is
transmitted to the surrounding animals. I wrote my name with my finger
on the surface of the giant Pyrosoma as it lay in a tub at night, and
the name came out in a few seconds in letters of fire.”
All of these animals are sufficiently large to be easily seen by the
naked eye, and the phenomena of their phosphorescence can be carefully
observed. But many of the more minute forms of life also exhibit this
peculiarity, and contribute in no small degree to the bright light of
the sea.
For instance, when the sea on our coasts shows a dull blue light,
flashing into greater intensity where the ripples break, it will be
found to contain immense numbers of very minute creatures called
_Noctiluca_. Each of these has a gelatinous consistency, and is
the shape of a microscopic cherry, bearing a short whip-like process,
called the flagellum, which propels the organism slowly through the
water. There seems to be no doubt that, on these occasions, the light
is caused by these _Noctilucas_, but there are many other minute
forms which abound on the surface and give off a pale phosphorescent
light at night.
We do not know for certain what may be the use of the phosphorescent
light to the organisms that possess the power of emitting it. If we
assume that the transparency of the bodies of the pelagic animals has
a protective value in the day-light, it is difficult to understand why
many of them should become so attractive, as the phosphorescent light
makes them, at night. It is probable that the star-like lights of many
of the Copepods may serve to attract to one another the two sexes, as
it does with the Glow-worms and Fire-flies, but such an explanation as
this cannot well be accepted in the case of _Pyrosoma_, which is
hermaphrodite, or the _Noctilucas_, which live together in immense
numbers. There can be little doubt, however, that there is some good
reason for it, as it occurs in so many different animals belonging to
widely separated families.
In the neighbourhood of coasts or in shallow water, the surface of the
sea usually supports a very large number of animals in a larval or
immature state. These creatures live only a portion of their lives in
a free-swimming condition, and then a change occurs during which they
sink to the bottom and gradually assume the adult characters.
Nearly everybody is acquainted with the general appearance of the Crab
and Star-fish, but few would guess that the young stages of these
animals are to be found among the minute transparent floating Fauna of
the surface waters of the sea.
The habits of the young and of the old, of these animals are widely
different; the former must constantly support themselves in the water,
they must feed upon and have means for catching and devouring minute
floating organisms and must in other ways be adapted for life with the
Plankton; the latter being unable to swim are capital crawlers and
walkers over the rocks and sand of the bottom, have heavy bodies which
sink rapidly in the water and, in other ways, are adapted for life with
the shallow water Benthos.
The conditions of life at the surface and at the bottom being, as
I have previously pointed out, so different and the adaptations of
structure to suit each set of conditions so great, we have, as a
result, a long series of animals in which the young larval stages of
life are absolutely unlike the adult and mature stages.
No better examples to illustrate these changes could be given than
those chosen from the group of the Echinoderms. Take, for instance,
the common Star-fish with its thick heavy skin studded with plates
of carbonate of lime, and its dense opaque body drawn out into five
finger-like processes. These features of the animal indicate at once
that its life is spent crawling on the sand or rocks at the bottom
of the sea. If a Star-fish that has been caught in a lobster pot or
brought to the surface attached to the bait on a fishing line, is
cast into the sea it sinks to the bottom at once without any apparent
effort to swim, to keep afloat, or to arrest its rapid descent. It is
therefore clearly unfitted for a surface-swimming existence, but its
eggs give rise to larvæ which are admirably adapted to it, and can
indeed only exist at or near the surface of the sea. These larvæ are,
as a rule, when first hatched, covered with a number of very minute
vibratile cilia, by means of which they swim with considerable rapidity
through the water. After a time a number of bands appear, which are
covered by specially long cilia and then the smaller cilia on the
intervals between the bands disappear.
[Illustration: +Fig. 31.+
Young larva of a star-fish before the Brachiolaria stage is reached.]
The precise arrangement of the bands differs in the different species,
but from being at first perfectly circular in contour they become more
and more curved and twisted, sometimes fusing with one another and in
parts degenerating, until, at last, when the larval stage reaches its
full development, the bands have assumed an elaborate and somewhat
fantastic pattern.
The body of the larva is, like that of so many surface-swimming
creatures, extremely transparent. The uniform oval shape which it
has when first hatched becomes changed as it develops by the formation
of a certain number of short blunt processes or arms, and it was the
presence of these which caused the older naturalists to call this larva
the Brachiolaria.
If one of these minute Brachiolaria larvæ be caught and examined with a
microscope it is not difficult to see that it has a little round mouth
leading into a short digestive canal which opens to the exterior by a
vent. It is therefore clearly capable of feeding itself and leading
a perfectly independent existence. In the older larvæ there will be
noticed an appearance which has, under a low magnifying power, the form
of an incomplete and rather opaque ring round the stomach. This opaque
ring becomes larger and larger, it exhibits five projections radiating
from its centre, and at last gives rise to all the organs of the fully
formed Star-fish. As the ring develops the larva sinks from the surface
and loses the power of independent feeding, and then, when all is
ready, the skin is cast off and a small but perfectly formed Star-fish
emerges.
The Trepangs, the Brittle-stars, the Sea-urchins and other Echinoderms
have, as a general rule, life-histories similar to that of the
Star-fish, but there is one point of difference in detail which is of
sufficient interest to be mentioned before passing on. The larva of
the Brittle-stars and of some of the Sea-urchins has a number of arms
which are much longer, in proportion to the whole size of the larva,
than they are in the Brachiolaria, and on account of the manner in
which these arms are inclined towards the apex, the larva has a rough
resemblance to the form of a painter’s easel. This type of larva is
called the Pluteus. The main point of interest about the Pluteus,
however, is that the arms are supported by delicate bars of carbonate
of lime which are connected together at the apex and form a very
definite larval skeleton.
[Illustration: +Fig. 32.+
Pluteus larva.]
This larval skeleton is cast off with the skin when the metamorphosis
takes place, and it is consequently of great interest to scientists in
the fact that it is one of those structures which are formed to meet
the exigencies of larval life only, and is perfectly useless for the
adult. In considering the manifold questions which arise in the study
of the relation of animals to their surroundings we are often inclined
to fix our attention too exclusively upon the adaptations that are
manifested in the adult form. In the case of some classes in which the
immature stages of life are passed through very rapidly and under the
protection of the parents, this is not to be deprecated; but in most
cases it is important to remember that in the struggle for existence
there is such danger of extermination that each stage of life may have
acquired special characters for adaptation to its particular mode of
existence. The peculiar markings and colours of the Caterpillars is
a familiar example of the special characters of larval forms among
terrestrial and air-breathing animals, but in none of these do we find
so great a specialisation in larval characters as in some of the marine
forms of life.
It is said above that the Echinoderms as a general rule have
free-swimming larvæ, but there are exceptional cases which have an
interest for us quite as great as that of the ordinary life-history.
Many Echinoderms are found living in very great depths of the ocean
and it is difficult for us to believe that any of these can have
pelagic larvæ similar to those of their shallow water relatives. The
difference in pressure between that of the bottom of the deep-sea and
of the surface is, by itself, sufficient to convince us that a delicate
organism like a Brachiolaria or Pluteus could not make the upward
journey unharmed; but when we add to that the great distance of two or
even three miles in a direct line, the difference in temperature and in
light, we must realise that the ordinary transformations of the shallow
water Echinoderms is an impossibility for the deep-sea varieties.
As a matter of fact we know very little about the life-history of
deep-sea Echinoderms, and this is not a matter for wonder when the
reader reflects upon the great difficulties that have to be overcome
in obtaining a few specimens of the adult forms; but at least one of
the Star-fish of the Abyss has been found to bear little pouches or
pits in which the young are fostered until they are ready to lead an
independent life in the form of the parents.
It has also been shown that in some of the Arctic Star-fishes the
larval life is in a similar manner abbreviated and protected, and
it seems probable that this may be accounted for by the fact that
the surface waters, where the larval forms would live if they were
liberated, are for very long periods covered with ice.
The great group of the Crustaceans also presents us with many
interesting larval forms specially adapted to surface life. In a
previous chapter I have pointed out that the Barnacles of our coast
give birth to curious little free-swimming, six-legged larvæ called
Nauplii, which after having undergone two or three further changes,
settle down on a rock and assume the adult features (see Fig. 12).
It is not known how long these changes take in the ordinary course
of nature, but it is quite probable that the larval life is a
comparatively short one.
Some Barnacles, however, live far out at sea on drifting wood or
parasitic on the skin of Whales, and it is reasonable to suppose that
when their larvæ are hatched a very considerable time may elapse before
they find a suitable resting-place to complete their metamorphosis.
The life-histories of these species are not at present accurately
known, but a few remarkable Nauplii have been found which, there is
reason to believe, are really the Nauplii of some kind of Barnacle and
are specially adapted to a long life at the surface by the enormous
length of their spines.
In the specimen discovered by Chun in the Canary Islands, of which
a figure is given here, the spines were seven or eight times the
length of the body, the eye was remarkably small, and the muscles were
feebly developed. It may be that this is the larva of some species of
Barnacle, which, from the character of the host or home where it lives
when adult, must be prepared to wait a long time in its larval habitat
before the chance comes for it to find a suitable resting-place.
[Illustration: +Fig. 33.+
Long-spined Barnacle-nauplius.]
Many of the Crabs and Prawns have remarkable larvæ, characterised
either by two or three extremely long spines or in some cases by a
festoon of shorter and many branched spinous processes spreading out
from their carapace, tail and limbs. These spines may be regarded
partly as a device for assisting in the floatation of the body,
and partly, perhaps, as a protection against some of the creatures
that feed upon them; but in both respects they are special larval
adaptations to the pelagic life. It is extremely interesting to find
that in this class of animals the same characters are not constant
in the larvæ. A Prawn called _Palinurus_ has a larva the body
of which becomes extremely expanded and flattened, so as to resemble
a very thin sheet of glass, the eyes and the limbs at the same
time undergoing remarkable modifications. Another larva becomes
extraordinarily distended by the absorption of water into its tissues
so as to resemble in texture a small Jelly-fish.
A great deal more might be said about the story of Crustacean larvæ,
as it is one which is full of interest and wonder, but throughout the
whole of it we see, wherever there is a larval history at all, that
some one or more of those characteristic features have been evolved,
which were previously noted in adult animals as an adaptation to their
free-swimming pelagic life.
In many other groups of marine animals we find the same alternation of
a transparent larval life at the surface and an opaque adult life at
the bottom.
The Oysters, Clams and Mussels, the Winkles and other Gastropods, the
Worms, the Sponges and many other forms of life that creep among the
Sea-weeds and are fixed upon the rocks or burrow in the sand, produce
exquisite and delicate transparent little larvæ which for a certain
length of time at least float and drift about in the light of the
sunshine in the surface water. They have, of course, many varieties of
form and many peculiar organs for locomotion and floatation, so that it
is possible for a competent zoologist to tell without much difficulty
the group of animals, if not the actual genus and species, to which any
particular larva belongs.
It might be thought that, as so many of the animals living near the
coast line in shallow water have pelagic larvæ, the Plankton of the
neighbourhood of the coasts would differ from that of the open oceans
in the fact that a considerable proportion of it consists of these
larval forms. But many of the larvæ seem to be able to live a long time
without further change than an increase in size, and being drifted out
to sea by the winds and tides are often found in the open ocean at very
great distances from any coast line.
It would be interesting to know more of these larvæ which go thus
astray. How long can they go on waiting for the opportunity to cast off
their childish clothes and assume the garments of the adult? Do they in
time undergo changes which bring about a kind of childish old age, or
do they suddenly perish with all the characters of youth upon them?
These and many other questions connected with this most fascinating
chapter in the story of the sea have still to be answered by the
investigations of scientific men in the future.
CHAPTER V.
SURFACE-SWIMMING FAUNA (VERTEBRATES).
In the preceding chapter we have considered only those animals of
the surface of the sea, which, owing either to their small size or
the transparency of their bodies, are not as a rule conspicuous to a
passenger on board a mail-steamer. Such a passenger might cross the
ocean many times without realising in the least the wealth of animal
life that there is in every wave that breaks upon the ship, and yet
be impressed with what he has seen of the Whales and Porpoises, the
Sharks, Bonitos and Flying-fish.
It is to these groups of animals that a few lines must be devoted
before closing our story of the surface-swimming Fauna. If the young
and immature stages be for the moment left out of consideration, it may
be said that nearly all the Fish and all the Whales and Porpoises are
large, opaque in appearance, and perfectly conspicuous. Moreover, they
are all strong and rapid swimmers, capable of roaming over wide areas
of the sea in search of prey, and independent of, except in so far
as their prey are influenced by, the currents and winds. It is clear
that they cannot be said to ‘float’ and ‘drift’ about in the ocean,
and consequently they do not strictly belong to the Plankton. The term
used in speaking of them collectively is the Nekton, which means the
swimming population.
The greatest number of the Fish of the sea are shore Fish; that is to
say, they habitually feed at, or close to, the bottom of the shallow
waters near the coasts or sunken banks.
But there is a very considerable number that are strictly pelagic,
living and feeding far away from the shores, bringing forth their young
alive, or shedding floating eggs, and in every way independent of the
shore and of the bottom.
It is difficult to give any general features by which they are
characterised, as so much variety may be observed among them; but as
a general rule they are elongated in form, round or oval in section,
in colour green or gray above, with silvery white bellies. Some of
these, such as the Flying-fish and the Flying-gurnard, are capable of
making very considerable flights in the air, their pectoral fins being
enormously elongated, and when fully expanded somewhat similar to the
wing of an Insect.
The Flying-fish occur in shoals in nearly all tropical and subtropical
seas. When disturbed by a ship on a calm day it is said that they
spring out of the sea, expand their fins, describe a regular parabolic
curve in the air, and then fall with a splash into the water. There is
a considerable controversy raging on the question of the use of their
fins in this flight through the air, some observers believing that
the fins are used only as a kind of parachute, and others that they
are used like wings for raising the body above the water. It is very
difficult to decide which view is correct.
In the Indian Ocean I watched the Flying-fish for several days during
rather rough weather, and my impression was most distinctly that in
the middle of the flight the fins are vigorously flapped four or five
times, the flapping being followed by a decided rise in the air. On the
other hand, it may be that this flapping appearance is caused by the
wind catching the wings in a certain position, and not by the muscles
of the fish. Whether the flying is actually assisted by the flapping
of the wings or not, it is certain that the Fish do rise in gusty
weather to a very considerable height, frequently falling on to the
decks of steamers twenty feet above the water line.
At night, these Fish fly at the ships and not away from them, as
they do in the day-time, and the natives in some parts of the Malay
Archipelago catch them in large numbers by holding up a torch by the
side of a large sheet, when the Fish flying at the bright light and
striking against it, fall into the bottom of the canoe. Flying-fish are
excellent to eat, their flesh being similar in taste to that of the
Herring, but an epicure would probably say that it is not quite so good.
The Bonito is a fish occurring over a wide area of the tropical and
temperate seas, which sometimes makes tremendous jumps out of the
water. I have seen it frequently on the coast of Celebes jump to a
height which I roughly estimated as at least fifteen feet. It is said
to feed upon the Flying-fish, and it is probable that it has acquired
the power of springing out of the water in the pursuit of its prey.
The pelagic Fish, which has acquired the widest reputation, and that
an evil one, is the “blue-Shark.” This occurs in the tropical and
occasionally in temperate seas. Its usual size is from twelve to
fifteen feet, but, according to Dr Günther, individuals of twenty-five
and more feet are occasionally captured. It is extremely voracious,
attacking anything of a fleshy nature that it observes in the water.
One of the most remarkable animals of the open ocean is the Sun-fish.
It has a very wide distribution in the tropical and temperate
regions. In the adult condition it is almost circular in outline and
considerably flattened from side to side like a John Dory. It sometimes
reaches a size of eight feet in diameter and a very great weight. From
the little that is known of its development it apparently undergoes
some extraordinary changes in shape before it reaches the adult form.
[Illustration: +Fig. 34.+
The Sun-fish.]
In addition to these and several other Fish of a large size which may
be found at the surface of the open ocean, there are several species
known to science which never grow to a length of more than a few
inches. Many of these are characterised by remarkably long fin rays,
by their large eyes, or by other features which may be regarded as
special modifications for their peculiar habits.
A very interesting genus is _Scopelus_, which is found very widely
distributed in tropical and other seas. Some of the species live in
very deep water, and are purely abysmal in habit, but most of them rise
to the surface at night, when they may be caught in immense numbers.
In form they are not unlike a small Sprat, but they exhibit on each
side of the body a series of minute eye-like organs, which emit a
phosphorescent light.
In addition to these Fish which are found far out in the open ocean,
there are several genera, which form an important feature of the
surface waters in the neighbourhood of the coasts. Among them we
find such valuable food-fish as the Herrings, Sprats, Mackerels, and
Pilchards.
The complete history of the Herring has yet to be written, for,
notwithstanding the laborious investigations of several naturalists,
working independently, or as officers of the Marine Biological
Association and similar Institutions, there are some facts and stages
which have, up to the present time, escaped observation.
The Herring species is divided into a number of races, which, differing
from one another only slightly in anatomical characters, have different
seasons for depositing their eggs. This fact has only recently been
thoroughly established; and while it assists us greatly in the task
of completing the history of the Fish, it definitely destroys the
validity of many theories which were prevalent among fishermen and
others before the days of the more exact scientific treatment of
fishery questions.
It is well-known that from the end of the month of June to December
immense shoals of Herrings are found in the North Sea. It is quite
impossible to estimate the numbers of Fish in these shoals, but they
are so great that if they could be counted it would probably be found
that the Fish that are annually caught by all the fishing boats, form
but an insignificant fraction of the whole. All of these Fish are in
such a condition that it is evident their spawning time is close at
hand. When they are ready they approach the coasts, the exact time
varying according to the race of Herrings, and the spawn is deposited
on or close to the ground, the eggs becoming attached to stones and
other objects on the bottom. After the Herrings have spawned, they seem
to disperse, or, at any rate, to disappear from the surface waters of
the North Sea. What actually becomes of these shoals of spent Herrings
is not known, but it is a fact that in the spring there are so few
Herrings to be found in the narrower part of the North Sea that it does
not pay the fishing boats to go after them. It is possible, however,
that, after the spawning process, the Herrings migrate to the deeper
water of the Norwegian coasts, in order to feed on the Crustaceans and
other forms of life that are to be found there in abundance.
The Mackerel do not apparently make such extensive migrations as the
Herrings. They spawn in the open sea, five or ten miles from the
coast, during the spring time. The egg of the Mackerel, unlike that of
the Herring, does not sink to the bottom when it is spawned, but, being
provided with a large oil globule, it is light enough to remain on the
surface until the young larva is hatched. After the spawning has taken
place the Mackerel approach the coast, and will even enter bays and
narrow inlets on the shores in pursuit of the young Sprats and other
small Fish upon which they prey.
A great deal could be written on the history of the Pilchards, the
Anchovies, the Sprats, and other Fish which frequent the surface waters
of the sea in the neighbourhood of the European coasts. No two species
seem to have precisely the same habits, and what is known about them
presents us with many curious and remarkably interesting facts. For
further details, however, I must refer the reader to the larger and
more comprehensive books dealing specially with the subject, for space
must still be found for a few words on another group of animals which
play a conspicuous part in the story of life in the seas.
The animals composing the class of Mammals are distinguished from other
Vertebrates by the fact that the females are capable of providing milk
for their young ones after birth. Most of the Mammals are strictly
terrestrial, but three orders, namely, the _Cetacea_, the _Sirenia_,
and the _Carnivora_ contribute to the surface-swimming population of
the sea.
The _Cetacea_ are all aquatic. The order includes the many genera
of Whales, Porpoises and Dolphins.
The Right-whales are distinguished by the enormous size of the mouth
and the absence of the little triangular fin in the middle of the back
which is found in the other Whales.
These animals have no teeth in the adult condition, but are provided
with a series of plates situated at the sides of the mouth which are
used as strainers to catch the small Pteropods and other animals
living in the water which pass through the great gape. The plates are
composed of a substance called “Baleen,”--the well-known whale-bone
of commerce,--they are triangular in shape, and frayed out into a
brush-like edge on the side that faces the cavity of the mouth. The
Greenland Right-whale attains to a size of fifty feet in length when
fully grown, and it is usually found in shoals among the ice floes of
the far north.
In former times many Right-whales belonging to species allied to
the Arctic form occurred in the temperate regions of the Atlantic
and Pacific Oceans, but in consequence of the valuable fishery they
afforded they are now becoming very scarce.
The largest of all the Whales--in fact, the largest existing animal--is
the Blue-whale, which attains to the enormous length of 86 feet. It
spends the winter in the open seas, and approaches the coast of Norway
in the spring.
The Whales, like all the animals of the Class to which they belong,
are air-breathers. They are able, however, to hold their breath for a
considerable time under water. When they come to the surface to renew
the air-supply in their lungs, they first make a violent expiratory
effort from the nostril, and drive a column of spray many feet into
the air above them. This phenomenon is called by the whale fishers
“spouting,” and it was erroneously supposed by them to be a column of
water forced from the mouth into the nostril, and then expelled at the
surface.
The Dolphins and the Porpoises are distinguished from the true Whales
by the fact that they are provided with teeth, on one or both of their
jaws, and there is no “whale-bone.”
The Sperm-whale has probably been called a “Whale” from its enormous
size, but it is anatomically very widely separated from the true whales
and more closely allied to the Dolphins. It has no “whale-bone,” and
the lower jaw is provided with a row of sharp-pointed teeth set in a
groove in the bone. Its great value is due to the fact that there is
a large cavity situated above the skull, containing an oily substance
from which “Spermaceti” is made.
The principal food of these large “Toothed-whales” seems to be
Cuttlefishes, and the examination of the contents of their stomachs,
which has been carried out on board the Prince of Monaco’s private
steam-ship when engaged on a scientific cruise, has yielded some new
forms of these giant Molluscs. The Sperm-whales, however, do not
disdain a Fish diet as well.
The Porpoises are found on our own coasts. They may often be seen
following the “schools” of Whales as they approach the coast in the
summer months, and they occasionally chase their prey some distance up
the estuaries of the English rivers. The habit that Porpoises have of
accompanying ships for long distances affords us many opportunities
of watching their graceful movements, and of estimating the very
great speed with which they can swim through the water for hours at a
stretch. It is difficult to understand the meaning of this habit of
following ships. It has been suggested that the Porpoises mistake the
ship for a huge Sperm-whale, and hope to benefit by stray morsels of
large fish that fall from its jaws. But this theory does not account
for the fact that the Porpoises so often go in front of the ship. The
following extract from Darwin’s “Voyage of the Beagle” gives, in a few
words, a vivid picture of Porpoise life:--
“In our passage to the Plata, we saw nothing in particular, excepting
on one day a great shoal of Porpoises, many hundreds in number. The
whole sea was in places furrowed by them; and a most extraordinary
spectacle was presented, as hundreds, proceeding together by jumps,
in which their whole bodies were exposed, thus cut the water. When
the ship was running nine knots an hour, these animals could cross
and recross the bows with the greatest ease, and then dash away right
ahead.” This description reminds me very forcibly of a sight I once saw
in the Talaut Islands, south of the Philippines. In passing between two
of the islands the strait seemed to be alive with Porpoises tearing
through the water at a terrific pace. They accompanied the steamer for
about six hours and then suddenly disappeared. My impression was that
they were attracted to the ship not from a desire for more food, for
there was an abundance of Herrings in the straits at the time, but from
sheer curiosity. I think the feeling of curiosity, that is to say, the
desire to go and look at something strange or unusual, is much more
prevalent among animals than we generally suspect.
[Illustration: +Fig. 35.+
The common Porpoise.]
It is an interesting fact about the Porpoise that, although it is so
fish-like in shape, it should present some features which remind us of
the Pig. The English word is probably derived from the two French words
_porc_ and _poisson_, and therefore means “Pig-fish.” The
Germans call it Meeresschwein, meaning “Sea-pig,” and the Malay word
for it is “Babi-laut,” which also means “Sea-pig.” But if we make some
allowance for those who call these Cetaceans “Pigs,” we must make none
for those who call them “Fish.”
Like all the other members of their order the Porpoises have a
fish-like tail, but the flaps are placed horizontally and not
vertically as they are in Fish. The skin is quite naked, having no
scales of any kind, and there are no gills or gill-openings. Like
all other Mammals the Cetaceans bring forth their young alive, suckle
them, and breathe air by means of lungs. But there can be no doubt that
they are extremely modified for their aquatic life. The characteristic
covering of Mammals--the hairs--is, in the adult condition of the
Whales, entirely wanting, and is represented in other members of
the Class by only a few bristles on the snout. The heat of the body
is maintained by a thick coat of fat, called the blubber, lying
immediately beneath the skin, and this yields, on boiling, a valuable
oil, which helps to support the whale-fishermen. One of the most
striking modifications, however, is the loss of the hind limbs. It is
only in some species that even rudiments of these have been found. All
of these facts indicate that the Cetaceans must have taken to a mode
of life in the water a very long time ago, and the study of the rocks
proves the existence of Whales as far back as Eocene times, but it is
of interest to note that, in some respects, the oldest fossil Cetaceans
are less specialised than those that are now living.
The class of Mammals called the _Carnivora_ includes the Cats,
Dogs, Ferrets and many other animals which are purely terrestrial,
but one of its divisions is entirely composed of those well-known
aquatic animals the Seals and the Walrus. If we take the common Seal
as an example of this group, and compare it with the Porpoise, as a
representative of the Cetaceans, we find that in habits as well as in
anatomy the former is less completely changed than the latter. The Seal
frequently comes to land to bask in the sun, or to produce and care
for its young, and it is capable of making some progress over the rocks
by the help of its flipper-like fore-limbs; the Porpoise, on the other
hand, never leaves the water of its own free will. Unlike the Porpoise
the body of the Seal is covered with a thick coat of hairs, and the
hind limbs are retained. Although there is a general resemblance in
the form of the body between these two animals--this form being, in
all probability, mechanically the best for rapid progress through
the water--a glance at their skeletons shows great and important
differences, which the merest tyro in anatomy could point out. In
expression, too, there is a marked difference, for while the Porpoise
has a certain cast of countenance which, when seen at a distance,
deserves the epithet “pig-faced,” the face of the Seal, with its large
round eyes, its small nose and high intelligent brow, is almost human
in expression.
The Seal has a habit of raising its head above the water and staring
at an approaching boat, and when doing this it may readily be mistaken
at first sight for a man overboard, but no one could ever mistake a
Porpoise for a human being.
The common Seal has a very wide range occurring near the coast of
both the Atlantic and Pacific Oceans. It is found on some of the more
sequestered parts of the British shores, but not in large numbers,
for the common Seal, unlike many of its allies, does not appear to
congregate in large shoals at any time of the year. They are described
as being timid, inoffensive creatures, easily tamed, passionately fond
of their children and taking an intelligent interest in music.
There are several animals closely related to the Seal, occurring
in different parts of the world; and a few words may be said about
the remarkable animal called the “Sea-elephant,” which is found on
Kerguelen island in the Antarctic Ocean. The popular name was given
to the animal in consequence of the fleshy protuberant nose which has
been compared with the trunk of an Elephant and is possessed only by
the male. The late Professor Moseley, who came across a small herd of
them when the _Challenger_ was at Kerguelen, says: “The trunk is
produced by an inflation of a loose tubular sac of skin placed above
the nostrils, just as is the ‘Cap’ in the northern Bladder-nose Seal.
The trunk is evidently, as appears from both the drawings, sacculated,
and hence irregular in form when inflated.”
The Sea-lions and Sea-bears or the Eared Seals, as they are sometimes
called, form a very distinct family. The one that is best known to
the general public is the Californian Sea-lion, as it often lives in
captivity in the European menageries for many years and attracts the
attention of the visitors by the tricks which it is taught to perform.
In the spring months of the year these creatures may be seen in great
numbers on the rocky islands off the coast of California, where they
come to breed.
The most important of them all, from a commercial point of view, is
the Fur-seal from the Northern Pacific. In the month of May these
animals approach the Prybilov islands in the East, or the Commander
islands in the West of the Behring Sea. The first to arrive on the land
are the old males. These choose for themselves certain areas or ‘homes’
on the shore and fight desperately for their possession with all who
dare to come within their reach. When matters are at length somewhat
settled the time arrives for the females to approach the shore.
The fighting then begins again with renewed vigour, and desperate
encounters take place for the possession of a goodly stock of wives for
the season.
There seems to be little in the way of courtship in the domestic
economy of the Fur-seals, the wives being simply “captured” by the
scruff of their necks when they come within reach of a would be
husband, and retained in his harem just so long as he can prevent
any one of his neighbours from stealing her. The Fur-seal, like all
his relations, is polygamous, but the number of wives that each male
appropriates to himself seems to vary very considerably. Mr Elliott
mentions a case in which there were as many as forty-five females in
one home, but, as a general rule, the number is much less. As there is
only one male to every twelve or thirteen females, there are numerous
males over, which cannot found a home for themselves. These bachelors,
together with a number of the young females, resort to a separate
piece of ground, where they spend their time in playing games. The
play-grounds are however the scene of the tragedies of Seal life, for
they are resorted to by the hunters, who slaughter immense numbers of
the larger males for the sake of their valuable skins. As the skins of
the old male Seals are not of very much value and as it is important,
for the perpetuation of the race, to preserve the females from injury,
the breeding grounds are usually not molested. It is therefore the
bachelor seal of from two to five or six years of age that has to
supply the market. Those naturalists who have visited the Seal
rookeries on these islands say that the numbers of these animals that
can be seen at one time is almost incredible. We can form some estimate
of them when we learn that over a hundred thousand skins are exported
from the Prybilov islands alone every year.
The Seals leave the rookeries in the month of August, and after
swimming about for some time in the neighbourhood of the islands,
eventually depart into the open ocean in search of the food their
famished bodies need so much after the fasting and fighting of the
breeding months.
The largest of all these aquatic Carnivores is the Walrus, which lives
within a short distance of the shores of the lands in the Arctic
regions. It is easily distinguished from the Seals by its great size,
the males reaching a length of 10 or 12 feet, and by the enormous
canine teeth in the upper jaws, which project downwards from the cover
of the lips in the form of two large pointed tusks. These tusks are
used for hoisting the bodies of the animals on to the ice, for digging
in the sand in search of the Mussels upon which they feed, and for
general fighting purposes.
It is said that in former times the Walruses lived in immense herds
in regions much further south than they do now; but the ravages of
the hunter, who chased them for their ivory tusks and their oil, have
driven them into regions where they are rarely visited by anyone but
the Arctic explorer; and to those of my readers who wish to learn more
of their habits, I can but say that in the pages of Dr Nansen’s book,
“Farthest North,” he will find the story of the Walrus written in a
manner which no man living could have done more vividly and brilliantly
than the great Norwegian zoologist and explorer.
CHAPTER VI.
DEEP-SEA FAUNA.
Some of the most important conditions under which life at the bottom
of the deep-sea occurs have been mentioned in the first chapter. We
have pointed out that the pressure is enormous, that the temperature
is only a few degrees above the freezing point, and that, except in
those places where phosphorescent animals emit a faint light, it is
absolutely dark, no rays of direct sunlight being able to penetrate
such a mass of water as lies between the bottom of the ocean and its
surface.
With such conditions to contend with it is not surprising that the
naturalists at the beginning of the century believed that no animals
could possibly live on the floor of the great oceans. Their beliefs,
however, merely afford us an example of the danger of prophesying
without knowing, for the great expeditions which have investigated
the ocean-bed during the last thirty years have proved the existence
of a rich and peculiar Fauna in all the great depths that have been
dredged. The general results of these investigations have been recently
summed up by Dr John Murray in the last volume of the _Challenger_
Reports. He points out the extraordinary variety of life in the deep
sea as shown by the contents of the dredge. “At Station 146 in the
Southern Ocean, at a depth of 1375 fathoms, the 200 specimens captured
belonged to 59 genera and 78 species.” He can find no record of species
equal to this in depths of under 50 fathoms, and concludes that the
evidence at present before us is sufficient to warrant the belief that
the great depths of the ocean are as a general rule extremely rich in
species.
From what has been already said, it may be gathered that nearly all
the most important groups of marine animals have representatives
in the deep-sea. There are Fish, Tunicates, Crustaceans, Molluscs,
Echinoderms, Worms, Cœlenterates and Protozoa. Nearly all of these are
so modified, either in form or colour, or the structure of their organs
of sense, or in other particulars, that they could be recognised at
once in a collection as deep-sea animals; but there is a small minority
which seem to have undergone but little change in adapting themselves
to their strange environment.
We may commence our study of this remarkable Fauna by a few remarks on
their colour. The first and most striking feature is that the animals
are almost invariably uniform in colour. If they are dark-brown they
are dark-brown all over, if they are red they rarely exhibit bands of
white or spots of blue. Moreover, they are not always in harmony with
the colour of their surroundings.
In the shallow waters the animals that live among the green Sea-weeds
are green, those that live on the sand are coloured like the sand, and
many of those that live among the rocks are darkly pigmented with black
and blue. In the abyss of the ocean, where there is any light at all,
the colour is, in all probability, fairly uniform over wide tracts, and
yet we may find in one haul of the dredge, black Fish, red Crustaceans,
and purple Trepangs.
There seems to be no particularly predominant colour among the deep-sea
animals. Most of the Fish are black or dark-brown, but many are light
violet, some are pale rose and others bright red. Among the Crustaceans
bright red seems to be the prevailing shade, just as the darker tints
of black and brown are among the Fish.
Among the Echinoderms we find white, purple, yellow, red, and pink
forms, and among the Jelly-fish and Corals, red, violet, and green. In
fact it would be necessary to describe every class of animals in turn,
and then almost every genus in each class, to give an adequate idea of
the variety of colour met with in the Fauna of the deep-sea.
It is inconceivable that each of these animals can live amid
surroundings of a colour similar to its own, and therefore we may
without much hesitation believe, that the colour of deep-sea animals is
not, as a general rule, of use as a protection.
Next to the peculiarities of colour, the most striking features of
the more highly organised inhabitants of the bottom of the sea are
the modifications of the organs of special sense. The Fish, the
Crustaceans and the Molluscs almost invariably exhibit some remarkable
modifications of the eyes. In their natural haunts there must be
either absolute darkness, or the faint and usually intermittent light
emitted by phosphorescent animals. How intense this light may be it
is impossible to judge. The light that is emitted by animals on the
deck of a ship can afford no criterion of the light they emit under
a pressure of two tons to the square inch. However, the fact that
the deep-sea animals have either very large eyes or no eyes at all,
suggests forcibly that this light is not sufficient to cause a general
illumination.
Some of the Fish are quite blind, and although most of these have a
very small and rudimentary eye, in at least one Fish, _Ipnops_,
which is peculiar to deep water, no trace of an eye is to be found.
In some genera with a very wide distribution, a very interesting series
of stages may be found, indicating the changes that may have taken
place in the history of the blind Fish of the abyss. In the genus of
deep-sea Cods (_Macrurus_) for example, those species which live
in water of less than a thousand fathoms depth have very large eyes,
and those that are found in greater depths have much smaller ones.
The same in general is true of the Crustaceans. The deep-sea Cray-fish
have lost not only their eyes, but also the stalks which supported
them. In _Bathynomus_ (a Crustacean belonging to the group
_Isopoda_), however, there is a pair of enormous eyes. But as a
rule the eyes of Crustaceans degenerate and disappear in shallower
water than the eyes of Fishes. At depths of 500 fathoms or greater,
the eyes of the Crustaceans usually show signs of reduction in size or
other retrogressive changes, and in the greatest depths they are nearly
always wanting altogether.
[Illustration: +Fig. 36.+
A deep-sea Fish showing very elongated fins.]
Accompanying the loss of eyesight in deep-sea animals we often find
a very remarkable development of organs, which may be regarded as
especially tactile in function.
Many of the deep-sea Fish, for example, with rudimentary eyes possess
long barbels, and in some cases the paired fins are enormously
elongated to form delicate pointed organs like the tentacles of a
Polyp. Among the blind Crustaceans, too, we often find enormously long
antennæ, and even the claws and legs are so long and delicate that they
bring to mind the appendages of a Daddy-long-legs or a Harvest spider.
Just as a blind man acquires a remarkably acute sense of touch, so, it
seems, in the course of generations, these blind animals of the abyss
have acquired extremely delicate tactile organs.
The deep-sea Fauna is also remarkable for the great number of animals
which are phosphorescent. As in the surface-swimming creatures the
phosphorescence is not confined to a few classes, but probably occurs
to a greater or less extent in all the more important groups. The word
“probably” must be used in the previous sentence, because it is not
yet scientifically proved that many forms which are supposed to be
phosphorescent are actually so; but the evidence is conclusive that
phosphorescence is a common and widespread character of most of the
deep-sea Fauna.
The Fish exhibit, perhaps more than any other group, peculiar organs
which are supposed, and in many cases proved to be, used for the
purpose of generating light. In the _Stomiatidæ_, a family of Fish
related to the Salmons, there are often numerous little organs, like
minute bull’s-eye lanterns, arranged in rows on the sides of the body
from the head to the tail, and in addition to these in some species one
or more pairs of larger organs are seen on the upper lip just in front
of or below the large eyes. It is not certain what the colour of the
light is that is emitted by these organs, but it is very probable that
if the Fish could be seen in their natural haunts they would have an
extremely beautiful effect.
In describing the general characters of the shallow water Fauna in
Chapter II. reference has been made to the remarkable lure at the end
of the tentacle of the Angler-fish. In the Angler of the great depths
this is also found, but in the obscurity of their surroundings a lure,
such as that of the shore species, would be useless, and it is actually
replaced by an organ which is supposed to be phosphorescent. The mouth
is enormous and armed with ferocious-looking teeth, the body is rounded
and adapted for burrowing in the ooze, and we can well frame in our
minds a picture of the little Fishes and other creatures attracted
by the “will-o’-the-wisp” light, meeting with a sudden death in the
cavernous jaws of this voracious deep-sea Fish.
It is possible that in addition to the light given off by definite
organs, the slime secreted by the skin either over the whole surface
or certain circumscribed regions, may be phosphorescent, but how
far this may serve as a means of illumination must remain a matter
of conjecture. The Crustaceans are in some cases known to emit a
phosphorescent fluid. The naturalists of H.M.S. _Investigator_
found a brilliantly phosphorescent liquid in the glands at the base of
the antennæ and elsewhere in certain deep-sea Shrimps, and one of the
Cray-fish from great depths is said to have two definite spots on the
body that emit a phosphorescent light.
Several of the Star-fish and Brittle-stars from the abyss are known to
be brilliantly phosphorescent, and there are some vivid accounts of the
light given off by Worms and various kinds of deep-sea Polyps found in
the dredge.
It is possible that some of the more minute forms of life that occur on
the mud at the bottom may also be phosphorescent. The _Phæodaria_,
a family of Radiolarians peculiar to deep water, provided with thick
heavy shells, have a curious organ in their bodies which may be capable
of emitting light. If this is the case, it is not unreasonable to
suppose that the vast tracts on the bed of the ocean may be faintly
luminous like the surface of the sea on a calm night.
Among the other characters must be mentioned a very prevalent
deficiency in the salts contained in the skeletons of these animals.
The skin of the Fish is usually soft and velvety to the touch, the
scales being either very thin and few in number or altogether missing;
the bones are described as being so soft that it is easy to pass a
needle through them. The shells of the Crustaceans, although frequently
drawn out into numerous long and pointed spines, are usually deficient
in carbonate of lime. The shells of the Molluscs are, when compared
with those that live in shallower water, thin and brittle. The Corals
do not seem in this respect to show much variation from their shallow
water relations. Some of the solitary forms seem to have rather thinner
shells, but the colonial genera have, as a rule, as good a support of
carbonate of lime in the abyss as elsewhere.
Before proceeding to the next character, it is necessary to digress a
little to consider the food of the animals in the abyss. In the absence
of any direct sunlight there can be no vegetable growth, all of the
animals must therefore be carnivorous. The food must be either the
living bodies of the truly abysmal animals, or the dead bodies of those
that fall from the surface waters.
It is probable that the bodies of Fish and the larger Invertebrates
only rarely reach the bottom, as they have to run the gauntlet of many
different forms of life living within 100 fathoms of the surface. When,
therefore, such a prize does fall to the luck of a deep-sea Fish,
it is important that it should have accommodation for it before the
neighbours come to share the meal. This may be the cause of the fact
that deep-sea Fish have, as a general rule, jaws and stomach that are
extravagantly large, even for a carnivorous creature. The width of the
gape and the extensibility of the stomach reach their highest grade
in some of the deep-sea Eels, which have been found containing Fish
actually larger than themselves. In these cases the stomach and the
body wall hang down from the under side of the Eel’s body in the form
of an enormous membranous sac containing the prey.
We have now considered very briefly some of the principal modifications
of structure exhibited by the animals of the deep-sea, but before
leaving the subject altogether it is necessary to refer to a few of
the more characteristic and remarkable forms.
Although it may be considered to be one of the greatest scientific
triumphs of the century to have discovered the existence of animal life
in a region, which nearly all the distinguished men of science of the
last generation believed to be as lifeless as the moon, the revelations
of the dredge brought with them a certain amount of disappointment.
The study of the crust of the earth has revealed to us the fact that in
times long since gone by, there existed not only the hairy Mammoths,
the Iguanodons, and many other terrestrial monsters; but that the sea
was peopled with certain Reptiles, Fish, Molluscs, Echinoderms and
Crustaceans, which are now believed to be extinct.
When it was first discovered that some forms of animal life had
attached themselves to a telegraph cable lying in 1200 fathoms, and
that it was therefore a fact that life existed in very deep water,
a successful application was made by scientific men to the British
Government to assist them in a thorough survey of this hitherto unknown
field of investigation.
The result of the voyages of H.M.S. _Lightning_ and H.M.S.
_Porcupine_ was to prove the existence in water of 1000 fathoms in
depth of a rich Fauna of rare and very remarkable animals. Among them
were several new genera of Sea-lilies and a very curious Heart-urchin.
The Sea-lilies that were then known to live in shallow water were very
few in number, and nearly all of them were free and unattached. Now,
in past times in the history of the earth different genera and species
of stalked Crinoids or Sea-lilies were very plentiful, and from their
abundance in certain geological deposits, it is believed that they
lived in enormous numbers. The discovery of the new genera of stalked
Crinoids in the abyss suggested that possibly there might be found
several other families of extinct animals still surviving in the deep
sea. This view was supported by the Heart-urchin, whose shell showed
some striking peculiarities that were only known in fossil genera.
But the hopes that were felt, even if they were not always expressed,
were doomed to disappointment. No living Ichthyosauruses or
Plesiosauruses, none of the remarkable Ganoid fish of Devonian times,
no Trilobites, no Cystoids nor Blastoids,--in fact none of the most
interesting of the fossil types rewarded the investigators of the
_Challenger_ and subsequent expeditions.
It is perfectly clear to us now that, taken as a whole, the deep-sea
Fauna is not more ancient in character than any other Fauna. It is
true that a few genera, such as those just referred to, have survived,
probably from very ancient times, without much modification; but the
vast majority of forms are simply shallow water animals, which have
been profoundly modified in structure, and adapted to the peculiar
conditions of existence in the great depths of the ocean.
CHAPTER VII.
COMMENSALISM AND PARASITISM.
The term Symbiosis has been applied by naturalists to the phenomenon of
the living together for mutual help or protection of different species
of animals or plants. It is a well-known fact, to all those who have
taken an interest in any large group of animals, that some species are
nearly always associated with other species, belonging perhaps to a
different class altogether, and very frequently mimicking them in form
or colour. At first it might be thought that most of these cases could
be dismissed as cases of parasitism; but when the careful observer
notices that neither of the species is injured by the association, the
conditions of the partnership are evidently very different to those of
a blood-sucking parasite and its ungracious host.
Besides the words Symbiosis and Parasitism, the terms Commensalism
and Mutualism have been applied to various cases of association of
different species of animals; but with the increase of our knowledge
of the habits of animals, it is becoming more and more difficult to
classify all known cases under these four heads, and the words are
consequently often used with widely different meanings.
It will be perhaps the best plan to adopt in this book, to avoid
any attempt to give definitions of these terms until a few cases
illustrative of each have been described.
One of the commonest objects of the sea-shore is the Hermit-crab.
From the open mouth of what is apparently an empty shell a bundle of
claws and legs may be seen to protrude; turn the shell over and it
will scamper away into the deeper parts of a rock pool. This is an
association of a living Crab with the shell of an animal that is dead;
but if the Hermit-crab be extracted it will be seen that it has a soft
and twisted tail, quite unlike that of the shore Crabs, and that it
could not possibly live for any length of time without the shelter and
protection afforded by the shell that it has appropriated to itself.
The Hermit-crab in the course of its life increases in size, and when
it gets too big for the shell it is living in, it goes in search of
another a little bit larger and changes, until at last it attains to
the size and dignity that requires a full-grown Whelk shell.
In the waters of our coast just beyond the low tide mark we often find
that the shell containing a Hermit-crab bears a Sea-anemone which
belongs to a species rarely found anywhere excepting in association
with a Hermit-crab. Moreover, the Anemone is always seated in a
definite position on the shell, so that its mouth is turned towards
the jaws of the Hermit-crab when it is extended, enabling it to catch
any morsels of food that escape the mouth of its comrade. When the
Hermit-crab has grown too large for its shell, and moves into a new
one, the Anemone moves too, and takes up the same position on the
new shell that it occupied on the old one, and the companionship is
continued in this manner throughout life.
The advantage of this arrangement to the Anemone is obvious, for it can
not only obtain its food after the manner of the other Anemones, but
it also gains a share of the food of the Hermit-crab. The advantage to
the Crab is not so apparent, but it is probable that the Anemone, being
very distasteful to many Fish and other animals, acts as a protector to
it. The facts that Hermit-crabs are extremely shy, darting back into
their shells when there is the slightest sign of danger, and that they
are extremely good bait for many kinds of Fish, suggest very forcibly
that they have many enemies among the inhabitants of the deep. Any
such covering as that afforded by the Anemone, which hides to a great
extent the character of the shell, would be of protective value, but
when to that is added the fact that the Anemone, which affords this
covering, is avoided as uneatable and distasteful by carnivorous Fish,
there can be no doubt whatever of the assistance that it renders to the
Crab in return for its board. If any of my readers are sceptical about
the distastefulness of Sea-anemones I would ask them to think of any
instance in which Sea-anemones are used for bait, and then to try the
experiment of offering pieces of them to the Fish in an aquarium.
An observation by Prof. Möbius in the Indian Ocean affords another
example of the use of Sea-anemones in this respect. He discovered a
little Crab called _Melia tesselata_ which carried about in each
of its claws a Sea-anemone. When the Crab was alarmed it held them up
in much the same way that a man holds a torch, as if it would call
attention to the fact that it had these terrible weapons at hand. When
the Anemones were removed it carefully searched for them, and held them
up again when found, and even when the Anemone was cut into pieces the
Crab diligently collected them, arranged them as far as possible in
their proper places, and held them up together.
[Illustration: +Fig. 37.+
Hermit-crab protruding from its hole in the sponge.]
But Sea-anemones are not the only animals that seem to be generally
distasteful to Fish. Many of the Sponges are free from attack, and
could serve as a protection to the Hermit-crabs. On our own coast
a small brown Sponge is not infrequently brought up in the dredge
surrounding and protecting a Hermit-crab; and hidden somewhere in the
substance of the sponge, there may always be found a small shell which
lies at the end of the hole in which the Crab lives.
This association is, from the Crab’s point of view, a more advantageous
one than that with the Sea-anemone, for it does away with the
necessity of any changes of shell, the Crab and the Sponge growing up
together. The history of the companionship is probably as follows:--A
small Hermit-crab takes for its shelter a small Gastropod shell, and
upon this shell a Sponge larva settles, grows and spreads, until it
surrounds the whole of it except the hole from which the Crab emerges.
As the Sponge grows still further in thickness the margin over-lapping
the aperture of the shell expands, leaving a conical cavity leading
from the exterior to the shell, surrounded, of course, by Sponge
structure, in which the Crab lives. Thus as the Hermit-crab increases
in size it is ever provided with a wider hole to accommodate its body
by the growth of the Sponge, and the little shell wholly deserted
remains as a token of the past history of the pair. But in the later
stages of growth a third creature is taken into the partnership, in
the person of a small segmented Worm which lives in the hole with
the Hermit-Crab. The need for this third person seems to be one of a
sanitary character. The cleanliness of the Hermit-crab, which has
no sponge to protect it, is provided for by the simple expedient of
frequent changes into a new home. In this case it is arranged for by
taking into the home, on what we may call board wages, an efficient
scavenger.
[Illustration: +Fig. 38.+
Section through a sponge (_D_) showing _A_, the little shell; _B_,
the worm; _C_, the Hermit-crabs in their natural positions.]
In this remarkable association, then, no less than four species
belonging to four different groups of animals are concerned. First of
all there is the _Gastropod_ Mollusc, which forms a shell for the
_Crustacean_ Hermit-crab to commence life in, then there is the
_Sponge_ which protects, and afterwards forms a shelter and home
for the Hermit-crab, and lastly, there is the _Annelid_ worm,
which helps in its way to keep the house clean in return for the scraps
of food that fall from the head partner’s table.
A very similar association has recently been described by Bouvier from
the Gulf of Aden and Red Sea waters. A number of simple solitary Corals
were thrown into an aquarium by a French naturalist, some falling on
their sides and some on their crowns, but he noticed that, after the
lapse of some time, they were all in the erect position again with
their crowns of tentacles expanded in the water. On carefully watching
them he observed that at the base of each Coral there was a little hole
from which emerged a small unsegmented Worm, belonging to a family that
usually exhibits sand-burrowing propensities. These Worms were found to
be the agents which restored the Corals to their erect positions. The
advantage of this arrangement to the Worm was two-fold: it brought it
into direct contact with the sand in which it searches for its food,
and, at the same time, it brought the Coral into such a position as
to hide and protect it from its enemies above in a most effectual
manner. To the Coral it was obviously an advantage, in that it placed
it in a position to expand its tentacles in search of the food it seeks
in the water and prevented a death from suffocation. A more minute
investigation of the Coral, however, revealed the facts that hidden
in its substance there was a small Gastropod shell on which we may
suppose both the Coral larva and the Worm settled when the partnership
began, and that in association with the Worm there was a small bivalve
Mollusc which probably acts as a scavenger in the manner of the Worm in
the last mentioned case. Here again, then, there are three different
species living together to their mutual advantage and commencing their
association on the shell of a fourth species belonging to a different
class of animals. What words can we apply to these associations? The
Hermit-crab and the Anemone feed at “the same table” and therefore
they afford a case of “Commensalism”; the Coral and the Worm are of
advantage to one another, the former in shielding and protecting the
latter and the latter in keeping the former in an upright position,
but as they do not feed “at the same table” it is not a case of
“Commensalism” but rather one of “Mutualism.”
There are many cases, however, of the association of animals in which,
although the advantage to one of the partners is clear, it is extremely
difficult to say what benefit is derived by the other.
Living in a tube on our coasts is a very common Worm called Sabella,
and at the mouth of the tube a little Polyp may frequently be found
which has received the fanciful name of the “Household god of the
Sabellids” (_Lar Sabellarum_). The Polyp undoubtedly benefits
by the currents of water which the Worm sets up when feeding, but it
is difficult to see what advantage, if any, the worm gains from the
presence of the Polyp.
Again, some of the Trepangs are frequently inhabited by a little Fish
called _Fierasfer_, which comes out from time to time to feed and
“take the air,” but rapidly retreats into the body of the Trepang on
the slightest alarm.
[Illustration: +Fig. 39.+
A Trepang, or sea-cucumber.]
The large stinging Sea-anemones of the Coral often afford protection
of a similar kind to a little Fish. Saville Kent gives a beautiful
picture of a little bright red Fish swimming about on the disc of a
large purple Sea-anemone, and he says that it darts into the mouth when
alarmed. On our own coasts we may often observe a number of little
Fish generally belonging to the Cod-family swimming round the disc and
tentacles of the large Jelly-fishes, and these, when frightened, swim
vigorously toward the under surface of the umbrella and seek security
there. Sometimes as many as a hundred or more of them may be seen
hovering round one large Jelly-fish, and we can hardly estimate how
valuable to our sea fisheries is the protection afforded by these great
Medusæ to the young Fish-fry. (_See Frontispiece._)
It was not my purpose in writing this book to point out the practical
value of scientific investigation, but this history of the Jelly-fish
and Codling cannot be passed without comment. The Jelly-fish might
readily be regarded by the ignorant not only as useless to man, but,
in so far as they sometimes choke his fishing nets and sting his hands
and arms, a positive nuisance to him. Scientific investigation when
pursued by properly qualified persons for its own sake, and not for
any definite commercial results that may possibly come out of it,
frequently reveals facts of the utmost importance, such as the one that
has just been mentioned.
There are some other cases of association which would on first
consideration be called undoubtedly cases of parasitism, but as this
term has been used somewhat vaguely in popular English, it would be
well, before proceeding further, to place before the reader a definite
statement of the sense in which the word is used in this book.
In many of the cases that we have mentioned hitherto of animals
living together, no apparent injury is inflicted upon either of the
associates, but a very definite and decided advantage accrues to each
of them, by the association.
In other cases, however, whilst no apparent injury is inflicted on
either, the advantage of the partnership falls entirely to one of them.
In a third set of cases one of the associates feeds upon the blood or
tissues of the other without rendering it any service in return, and
consequently inflicts either temporary or permanent injury. These are
cases of parasitism. In such an association the animal that inflicts
the injury is called the “parasite,” and the one that receives it, the
“host.”
One difficulty the naturalist has to contend with in trying to use
these terms correctly is that of finding out whether in any particular
case an injury is inflicted or not; another is that of determining
whether those animals should be called parasites which injure, alter,
or destroy the tissues of their hosts without feeding upon them.
A few cases will throw more light upon the subject than any further
discussion of the difficulties surrounding the application of these
terms.
One of the commonest Corals to be found upon the coral-reefs of both
the Old and New world is one called _Millepora_. In the Millepores
of the Pacific region we very frequently find a number of Barnacles
(called _Pyrgoma milleporæ_) so deeply buried in the substance of
the Coral that their presence is indicated only by a small oval hole on
the surface. There can be little doubt that in the course of the growth
of these Barnacles they distort, if they do not actually destroy, some
of the connecting canals of the Coral in their immediate neighbourhood,
but their food is derived entirely from the water that surrounds
the Coral and not any portion of it from the cells or tissues of the
Coral-polyps themselves.
There is a great deal of difference in the Millepores from one and the
same coral-reef, in the extent to which the Barnacles have attacked
them. In some specimens large areas of the Coral are beset with the
little holes, in others only one or two may be found on the whole
colony, whilst others again are quite free from them. Now when we
compare carefully the anatomy of those Millepores with the Barnacles
and those without them, no single sign or symptom can be found that the
vigour or strength of the former is in any way impaired. If then there
is no evidence that the Barnacles are parasitic, in the sense that they
are injurious to the Millepores, we must next inquire whether they
could possibly be of any service to them.
The polyps of the Millepores feed after the manner of the polyps of
other Corals, upon minute organisms floating in the sea; these they
paralyse and capture by means of tentacles bearing stinging cells. The
food is in the ordinary course brought within reach of the tentacles
by the tides that sweep over the reefs. The Barnacles also feed upon
minute organisms of the same kind, but they are provided with six pairs
of long feathery legs which by a curious vibratory movement create
currents in the water. When there are many Barnacles in close proximity
to one another it is quite probable that the water is considerably
disturbed by these currents, and the constant and rapid flow of fresh
water bearing food-organisms benefits, not only the Barnacles, but also
the Millepore polyps in their neighbourhood.
Thus the Barnacles _may_ be a benefit to the Millepores in which
they live. It cannot be asserted, however, that this probability is
a proved fact. A great deal more knowledge about the rate of growth
of the Corals which are and are not affected, must be acquired before
such an assertion could be made: but the _probability_ that the
Barnacles may be of service is sufficient to cause us to hesitate
before branding them with the epithet of “parasites.”
This particular case, which has been given above in some detail, may
be regarded, in a sense, as a test case, because other animals besides
the Barnacles, which gain their food by producing currents, are found
in Corals. Such are the tubicolous Worms, bivalve Molluscs, and certain
Sponges. So plentiful are these on the older branches of some Corals,
that quite a rich Fauna belonging to several groups of animals may
be found by carefully studying them. These might all be dismissed as
parasites by the non-inquisitive mind, but many of them, at any rate,
may be regarded by the more cautious naturalist as not injurious, and
others perhaps as positively beneficial to the Coral on which they
live. There is a very curious case of symbiosis mentioned by Semper,
which may be related here as similar in some respects to those above
quoted.
[Illustration: +Fig. 40.+
A Crab-gall on a branch of a Seriatopora.]
On the shores of the Philippine Islands and in other parts of the
Pacific Ocean there is a very common coral named _Seriatopora_.
It is composed of numerous delicate branches, terminating in fine
pointed extremities, forming hemispherical shrub-like masses, six or
eight inches in diameter. Semper noticed that on some of the branches
of these Corals there were little heart-shaped swellings, which had the
appearance of malformations or structures corresponding to the galls on
the leaves and branches of trees. Each of these swellings contained a
cavity, communicating with the exterior by two minute holes, in which
there was imprisoned a small Crab.
By the examination of a large number of specimens, Semper came to
the conclusion that the history of these structures was somewhat as
follows. The young Crab, when it settled down on the branch, produced
an irritation which in some way caused a gall-like growth of the
tissues of the Coral. This growth continued until it formed at first a
case or sheath for the protection of the Crab, and eventually, as the
Crab increased in size, a cage from which it could not do more than
protrude its tentacles and claws when feeding.
It seems very improbable that these cage-like swellings upon the branch
can be of any great disadvantage to the Coral. It is true that they
destroy the beautiful symmetry of the branch, and give it a distorted
and diseased appearance; but this is only an æsthetic disadvantage,
which does not probably count for much in the struggle for existence
on the Coral-reef. To the Crab the arrangement is undoubtedly an
advantage, as it gives it a secure position, free from the attack of
its ordinary foes, where food is probably abundant and easily obtained.
The skin of Whales is often beset with Barnacles; in fact some species
of them are found nowhere else but on these Mammals. They are usually
deeply embedded in the skin, only a small round hole through which the
legs can be protruded, communicating with the exterior. These Barnacles
do not feed upon the tissues and juices of the Whale, but, in the
usual manner of the non-parasitic Barnacles, upon organisms that swim
freely in the water. The advantage to the Barnacles is obvious, as the
movements of the Whale through the water must bring them in reach of
constant fresh supplies of food, but the benefit to the Whale is not
so clear. It cannot be supposed for a moment that the Barnacles assist
the Whales in their search for food, nor can they be regarded, when
present in great numbers, as a protection to the skin by the strength
afforded by their thick calcareous shells; at the same time there is no
reason to suppose that their presence is an inconvenience or in any way
harmful to the Whales.
These cases of animals bearing on their bodies other creatures which
are not in the strictest sense of the word parasites, are but instances
of a phenomenon that is very widely spread among marine organisms.
There are many cases, however, in which plants and inorganic foreign
bodies play a very important part in the economy of animals.
In our chapter on the free-swimming organisms of the ocean, mention has
been made of the delicate and beautiful creatures called Radiolarians.
Many years ago it was discovered that each of these animals bears
in its protoplasm a number of little cells, which from their colour
received the name of “the yellow cells.” It was clear from observation
and experiment that they were neither organs nor products of the
Radiolarian, but independent organisms belonging to the Vegetable
Kingdom.
More recently cells similar to these have been found in many of the
Corals, in Worms, and other animals, and there can be no doubt now that
when present they perform very important physiological functions which
materially assist their host in its growth and development.
So numerous are these “yellow cells” in some Polyps and so important
must be their influence on their vital processes, that it may be
confidently asserted that the Polyps could not continue to exist
for long without them. In the genus _Millepora_, for example,
no single specimen and no single fragment of a specimen that I have
examined was devoid of them; and although the numbers vary considerably
the most superficial canals of this Coral may in all cases be
described as crowded with “yellow cells.” But as the “yellow cells” are
certainly of great physiological importance to the Millepore, it is
equally certain that the secretions and the protection afforded by the
Millepore are of extreme importance to the “yellow cells.” In fact it
is not going too far to say that the Millepore and its “yellow cells”
are dependent upon one another for their existence, and the naturalist
might say with a great deal of truth that this particular Coral is not,
strictly speaking, animal in nature, but rather an animal and vegetable
combination.
Many years ago there was a bitter controversy among learned men on
the question of the animal or vegetable nature of Corals. The great
naturalist Linnæus, who was appealed to for his support by both parties
to the controversy, took up a middle position, asserting that they were
partly of the nature of animals and partly of the nature of plants,
and hence the term “zoophytes,” _i.e._ animal-plants, came to be
applied to them. There can be no doubt that in the end the position
in the controversy, assumed by Linnæus, became untenable, and the
supporters of the animal view of zoophytes won all along the line. It
is curious, therefore, that we are now in a position, not to support
the view of Linnæus, but to assert that some Corals are essentially a
combination of animals and plants.
Plants are of use to marine animals, however, in another manner.
Mention has already been made of the way in which many animals
resembling in colour, and even in form, certain kinds of sea-weeds,
escape the attention of their enemies and hide for safety among the
plants they simulate. Sometimes, however, the weeds will grow upon
the shells of the animals, and thus hide them even more effectually.
One of the most remarkable instances occurs in a Spider-crab that is
common upon our own coasts. The _Inachus_, as it is called, is
usually covered with a little forest of algæ, which do not grow there
naturally, but are actually placed on the carapace by the Crab itself.
If the plants be scraped off artificially the Crab will go in search
of fresh ones, carefully chew the bases until they are soft, and then
deliberately decorate the carapace with them as before.
There are some Molluscs that artificially decorate themselves with
little shells and other objects in such a manner as to completely
hide their general form. One of the most remarkable instances of this
occurs in the Gastropod _Xenophora_, which covers its own shell
with numbers of others belonging to a smaller species, so that in
the natural state it has the appearance of a conglomerate of shells.
The manner in which the smaller shells are fixed has not yet been
described, but from the orderly arrangement which they exhibit in some
cases there can be little doubt that they are deliberately placed in
position by the Gastropod itself and not attached by accidental contact.
In both these cases it is clear that the reason for the phenomena
described is that of affording a covering or mantle, which hides or
obscures the real form and character of the living animals.
Many of the Worms use little bits of shell and grains of sand to
build up a tube for the protection of their bodies. One of these--the
_Terebella_--is very common on our shores, the sandy tubes ending
in a tuft of fine filaments, and decorated all over with tiny little
stones or shells, projecting an inch or two from the surface of the
sand. In some localities these tubes may be found in thousands when the
tide is low.
Another form--_Pectinaria_--constructs much firmer tubes, which
retain their cylindrical shape after the death of the animal. In the
process of construction this Worm must carefully select the grains
of sand, for when the tube is examined with a magnifying glass the
particles will be seen to be of almost exactly the same size, and
arranged in their places with a mathematical precision.
But Worms are not by any means the only animals that use the sand in
this manner for the protection of their bodies. There are some kinds
of Polyps, belonging to the family _Zonathidæ_, a peculiar group
of Sea-anemones, in which the body-wall is considerably strengthened
by foreign bodies of various kinds. The _Zoanthus_ does not, like
the Terebella-worm, form a tube or case in which the body can freely
move up or down, but sticks the grains of sand into its skin, so that
they become in the older forms deeply buried in the tissues and give a
considerable support to the body-wall.
The _Cerianthus_--another Sea-anemone--forms a tube which is
partly composed of a matted network of stinging threads, and partly of
the mud in which the animal lives.
The use of foreign inorganic substances for the protection or
concealment of animals is not, strictly speaking, however, a part
of the subject-matter of this chapter, which was intended for the
consideration of the associations of two different kinds of living
organisms.
The subject of Parasitism must now be considered, a subject which
presents so many features of interest that it is possible here only
to touch on a few points of general importance. It is a well-known
truism to say that Parasitism, whether in human society or in animal
life, leads to degeneration; but there are degrees of parasitism among
animals, and consequently degrees of degeneration exhibited by animal
parasites. We may roughly divide them into two classes, the outside or
skin parasites and the internal parasites, the latter being invariably
far more modified in structure and in development than the former.
Among the terrestrial animals we find a great number of external
parasites, such as the Fleas and the Bugs, which are only slightly
modified, as in the loss of their wings, owing to their habits, and can
live an active, if not a very prosperous, life for some length of time
apart from the society of their hosts. There are others, such as the
Mosquitoes, Ticks, and Leeches, which are only occasional parasites;
that is to say, they will suck the blood of another animal when the
opportunity is presented, but failing that, are able to continue their
life and their race independently. It is not surprising, however, that
the terrestrial Vertebrates should be thus subjected to the attacks of
these parasites, as their feathery or hairy skin affords a shelter and
a foothold, from which the efforts of their hosts to dislodge them are
exercised in vain.
The skin of Fish, although covered with over-lapping scales, is smooth
and slippery, and with the rapid movement through the water many of the
forms of parasites of the types we meet on land would, if they existed
in the sea at all, find a difficulty in securing an attachment. It is,
however, provided with another means of defence against skin parasites,
in the possession of numerous mucous glands which keep the body bathed
with a slimy fluid.
Everyone must have noticed the slime that exudes from freshly killed
Fish, and if the finger be pressed along the skin it is possible to
see the openings of the glands as the slime is squeezed out. In the
majority of Fish the openings of the glands are most easily seen on the
jaws and the flap of the gill cover.
We must remember that the sea is in most places teeming with the larvæ
of Worms, Barnacles and Zoophytes, and the spores of Algæ and Fungi
of various kinds. Logs of wood, the iron supports of piers, and the
bottoms of ships become covered with various fixed forms of animal and
vegetable life when submerged in the sea-water for even a few weeks.
How is it, then, that the bodies of the Fish are usually so clean and
wholesome? The answer to this question is probably to be found in the
slime which, passing continuously over the skin, removes the larvæ and
the spores before they can secure a firm attachment.
The Crabs, Lobsters and other Crustaceans free themselves from their
skin parasites at every moult, but in some of the large, old Lobsters
and Crabs that are caught a considerable number of Worms, Barnacles and
weeds are frequently found firmly-fixed to the carapace and claws. The
Limpets and Winkles of our rock pools are often covered with a little
forest of Algæ.
The shells of other Molluscs are, however, kept remarkably clean, and
the method by which they destroy the spores, etc. that settle upon them
is not yet fully understood.
One of the most serious of the external parasites is the Hag-fish. This
remarkable animal is eel-like in shape, although very different indeed,
anatomically, from all the true Fishes, and buries its head in the skin
of the Cod and other Fishes as it feeds upon their flesh. In some cases
the whole body of the Hag gets inside the host, and it thus becomes an
internal parasite. It causes an immense destruction of valuable food
fish in some districts.
Closely related to the Hag is the marine Lamprey, which fastens itself
to Salmon by its suctorial mouth, causing considerable wounds. This
parasite sometimes reaches to a length of two feet, and is often
carried many miles up the river by the host to which it is attached.
Most of the Leeches occur either in fresh water or in damp forests and
marshy places. There is one, however, named _Pontobdella_, which
is found only in sea-water. It is difficult to give an exact statement
as to its size, because, like all its relations, it is capable of
very extensive movements of expansion and contraction, but the
_Pontobdella_ is large for a Leech, and when moderately contracted
it may be two or three inches in length. The body of this Leech is
covered with small tubercles, and it has a large round sucker at each
end. Its favourite hosts are the Sharks and Rays, but as it usually
drops back into the water when these Fish are hoisted on to the deck it
is not very commonly seen in the fishermen’s boats.
The most common external parasites of Fish are the Fish-lice. Most of
these are little Crustaceans, belonging to a group which includes the
Wood-louse. They have curiously flattened bodies, provided with short,
bent legs, terminating in sharp hooks, by which they adhere to the body
of the Fish and crawl about over the skin. Some of these parasites seem
to prefer the tongue as a resting-place, the genus _Glossobius_,
for example, being found in this position on the Flying-fish of both
the Pacific and Atlantic Oceans. In _Glossobius_ we find a very
remarkable difference in size and form between the males and females,
a condition of affairs which is of very common occurrence among the
parasitic Crustaceans. The male in this particular case is so small
that it is entirely concealed beneath the tail of the female. In
another genus a still more interesting condition has been observed, the
small young forms which are males growing up, and changing in later
life into females.
There is a remarkable parasite called _Sacculina_ which may
sometimes be found on the under side of the tail in Crabs. In shape it
is like a small pea or bean, and is attached to its host by a number
of root-like processes, which penetrate through the skin and burrow
deeply into the subjacent tissues. It would be quite impossible to tell
to what group of animals this parasite belongs by the study of the
adult form alone. It is, in fact, little more than a skin full of eggs.
When the development of the eggs is watched, however, it is observed
that the young _Sacculina_ as it is hatched is very much like
the Nauplius larva of a Barnacle. The later stages of the development
prove that whatever may happen to the adult the _Sacculina_ must
be related to the group of the Cirripedia. Later on it is found that
the females settle down on a Crab, lose all their limbs and other
Cirripedian characters, and finally degenerate into a mere palpitating
sac of eggs.
The males never pass beyond the second stage of development known as
the Cypris stage. Several of them may usually be found attached to the
female, and although they always remain extremely minute they do not
lose entirely their Crustacean features.
For those who are in search of parasites, however, there is no more
fruitful ground than the gills. That these organs should be a good
place for attack is not surprising, when we consider that to maintain
the respiration of the animal a constant flow of sea-water over them
must be kept up, and this must bring with it many larval forms which
may take the opportunity to attach themselves as they pass through the
meshes of the gill filaments. Moreover, it is in the gills particularly
that the blood current comes into closest contact with the water, and
it requires but a little puncture on the part of the young parasite to
reach a constant supply of this nourishing fluid.
It is in the gill-chambers that we find most frequently representatives
of that interesting group of animals, the parasitic Copepods.
[Illustration: +Fig. 41.+
A parasitic Copepod.]
It would be difficult to recognise them as Copepods if we were to judge
by their adult characters alone. Unlike the brisk, brightly-coloured
creatures with long rowing antennæ that we have described above as
living a free life in the surface waters of the ocean, these parasites
have a white sac-like body, with short blunt processes representing the
legs, no eye, and generally two long thread-like bags of eggs attached
to the sides of the rudimentary tail. As we found in the case of the
_Sacculina_, the true zoological position of these parasites can
only be determined by reference to their developmental history.
In the gill-chamber of the Prawns we find a very much modified
parasite, which is closely allied to those skin parasites of Fishes
mentioned above. Many of my readers may have noticed that in some
Prawns there is a wart-like swelling on one side of the neck. If
the skin be removed it will be observed that this is in reality a
cup-shaped protrusion on the wall of the gill-chamber covering a
little, flat, soft animal. In past times it was thought that this
was a young flat fish, and a wonderful story of its development was
fabricated on the strength of this error. It is now known to be one of
these extremely degenerate Isopod parasites called _Bopyrus_.
It is a curious fact that there is very rarely indeed more than one
of these parasites on a single Prawn. If there is one in the right
gill-chamber there are none in the left, and _vice versâ_. It
is difficult to find a satisfactory explanation for this, for it is
not at all probable that, during the lives of the many hundreds of
Prawns that have been examined, only one larva has passed through the
gill-chamber of each individual. The explanation must be looked for
in some hitherto unknown influence which the parasite has upon the
constitution of the host, rendering it unsuitable for the attachment
of another _Bopyrus_ of the same habits. The case is by no means
unique. There are several instances of Fish and other animals that bear
one, and never more than one, parasite of a particular species.
A few words must now be added about the internal parasites of marine
animals. The subject is really an immensely wide one; for the
intestines, body cavities, and even blood-vessels of Fish are liable
to the attacks of many different forms of Flukes, Tape-worms, and
other kinds of parasites which are not even known by name, perhaps
fortunately, to the general public.
The life-history of some Flukes that occur in terrestrial animals
has been satisfactorily worked out, and we know that, in most cases,
they must infest two different hosts before they can reach maturity.
The first of these hosts is usually an Invertebrate, and the second a
Vertebrate animal. Moreover, it is known that the larvæ are extremely
particular in their choice of the first host, attacking one species,
and one species only, of Snail or Slug, or whatever Invertebrate
its first host may be. If the first host dies a natural death or
is swallowed by any other animal than the parasite’s proper second
host, it--that is to say, the parasite--dies. It seems probable that
the Flukes that infest the intestines of marine animals pass through
some similar life-history, but owing to the great difficulties that
confront the observer their development has not yet been thoroughly
investigated. Similarly the life-histories of the Tape-worms, with
which a very large number of marine animals are infested, are not
yet known to us. It is comforting to know, after looking through the
volumes of papers on these internal parasites of marine animals, that
none of them have been shown to be, even occasionally, parasitic upon
man, and we can continue our Fish diet without any misgivings on that
score. An exception must, however, be made to this statement for the
semi-marine Salmon and Sturgeon, which are suspected of being the
first hosts of a human Tape-worm.
It is perhaps unsatisfactory to dismiss the internal parasites of
marine animals with so few words, but I feel compelled to do so, not
only because I have nearly outrun the limits of space, but because
we possess so little positive information on the subject, which is of
greatest interest to us here, of their developmental history. Lists
of species infesting different Fish and Whales could be published, a
statement of the points of anatomical importance which distinguish
the families could be written, but they would present few features of
interest to the general reader.
It may be well to point out before the chapter is closed, however, that
there is probably no branch of our subject that is so little known and
presents such a wide and important field for future investigation as
the life-histories of these marine parasites.
CHAPTER VIII.
THE ORIGIN OF THE MARINE FAUNA.
When we survey the distribution of living organisms over the surface
of the globe, we cannot fail to be impressed with the enormous range
in the characters of the physical conditions which are capable of
supporting animal and vegetable life. Thus we find Birds flying in the
sunlight of the cold and very light atmosphere of the mountain tops,
and Fishes swimming in the chill darkness of the depths of the ocean,
supporting a pressure of two tons to every square inch of their bodies.
We find Algæ, which give the snow sometimes the name of “Red snow,”
flourishing at temperatures below the freezing point of water, and we
meet with Insect larvæ swimming freely in the water of the hot springs.
Some sea-water animals can only be induced to live in the aquarium when
the water is kept as pure as it is in the open sea, and languish and
die as soon as any impurity occurs; on the other hand, several of the
Crustaceans seem to flourish best in stinking and putrescent pools.
The desert, the forest, the swamp, the lake, the river, as well as the
surface and the bottom of the sea have each their characteristic set of
animals and plants modified in structure and form to support life in
their natural habitats.
There can be no doubt that at the time when animals and plants first
made their appearance upon the earth, their distribution was far more
limited than it is now, and that all the adaptations to life in special
and extraordinary conditions have been acquired in the course of
evolution by organisms which originally existed in one particular zone
of the earth.
The reasons which have led scientific men to this opinion are manifold,
but not the least important of them are those based upon the presence
of organs or rudiments of organs of animals of the present time, which
could only have been called into existence at a period when their
ancestors had an altogether different habit of life.
For example, in the Birds and Reptiles, as well as in the Mammals,
the presence of openings in the throat during the early stages of
development, similar in their position, in their blood-vessels and in
other respects to the openings of the gills in Fishes, indicates that
their ancestors in remote periods lived in water and not on dry land.
Again the presence of rudimentary eyes in the Mole and other
subterranean animals indicates that at one time its ancestors must have
lived in the light of the day. The characters of the embryos of some
of the land and fresh water Snails proves that they are derived from
ancestors that lived in the sea.
When we collect together all the evidence of this kind and place it
side by side with the facts revealed to us by Geology, the irresistible
conclusion is arrived at that all animals are originally derived from
ancestors that lived in the sea. And when we consult the botanists and
find that they are agreed that all plants must have had a marine origin
also, the case for the sea being the original home of living organisms
may be said to be completed.
It is difficult to picture to ourselves the condition of the earth in
those very distant times, when the dry land bore no forests nor grass,
the air supported no Birds nor Butterflies, and in the rivers and lakes
swam no Fish nor Frogs. It must have been “dry” land indeed, when
there were no trees to attract the rain clouds and no herbs or mosses
to retain the moisture on the ground. The rivers must have risen and
fallen with great rapidity as they carried away the rain that fell in
cloud-bursts on the mountain tops.
But speculation on the character of the land in those times is not
within the scope of this work, and we must turn again to the sea to
inquire where the primordial animals and plants lived in the days of a
lifeless land.
We have seen that in the sea there are three possible habitats for
animals and two for plants. The surface waters of the great oceans
bear a characteristic population of animals and plants, the bottom of
the sea supports a considerable number of animals but no plants, and
lastly the shallow waters exhibit an immense variety of Sea-weeds,
Fish, Worms, and other creatures. Which of these three was the original
cradle of the great classes of animals and plants?
The early discovery of certain animals in very deep water which are
closely allied to, if not identical with, some fossils of early
geological strata, suggested the idea that a very primordial set of
creatures might be found at the bottom of the sea when it was more
thoroughly investigated; but as I have pointed out in a previous
chapter the hopes of those who anticipated the discovery of a rich
Fauna of “living fossils” were doomed to disappointment.
It is not probable, however, that the abyss of the oceans could have
been the cradle of life, even if it had shown a more ancient Fauna than
it actually does.
We cannot tell in what form life first appeared upon the earth.
Whether the unstable living substance called Protoplasm was in the
earliest conditions of the globe formed spontaneously by the chance
combination of its elements, or whether some germ or other made a
hazardous journey through space from another planet enwrapped in the
casing of a meteorite, are questions upon which no light has yet been
thrown by scientific observation or speculation; but this can be said,
that at a very early period in the history of life upon the earth the
simple green plants must have played an important part. It is on the
substances that are formed by the activity of this green coloured
substance that all plants and animals are directly or indirectly
dependent for their food in the present-day economy of Nature, and we
are forced to believe that, whatever may have been the form of the
earliest living things, Chlorophyll--the green coloured substance of
plants--must have had an extremely ancient origin.
Now, in the darkness of the ocean depths Chlorophyll does not and
cannot exist; for it is one of its characteristic features that it
is active only in the rays of direct sunlight; and, therefore, it is
extremely improbable that the cradle of the marine Fauna could have
been there. We are then left with two alternatives. It must have been
either at the bottom of the shallow waters or on the surface of the
seas.
Both of these sites have had their advocates, but the balance of
opinion has now turned decidedly in favour of the first of them--the
shallow waters. It is not easy to explain the reasons for this view
without assuming a fairly complete knowledge on the part of the reader
of the various forms of life that are found in the sea, but still a
few words of explanation may be written to indicate that the view is a
reasonable one.
In the first place we find, when we take a general survey of the
animals that live in the surface water, that they are all specially
modified in some way or another in structure or development in
adaptation to the peculiar conditions of their life. The long spines
of the Foraminifers and the Crustacean larvæ, the air-bladders of the
Portuguese men-of-war, the oil drops of the Copepods, the raft of the
Mollusc _Janthina_ are, as we have seen, among the characters
which distinguish this peculiar Fauna. Now, when we compare these
surface-dwelling forms with their nearest relations in the shallow
waters, the conclusion we come to is that these features have been
acquired by the ancestors of the former, which may have been similar in
some respects to those now living in shallow water.
Some of the Gastropods of the shore-waters have a simple cup-shaped
shell like that of the common Limpet, but the great majority of them
have a shell that is twisted up into a spiral form. This twisting of
the shell is, of course, due to the twisting of the mantle or fold of
skin which secretes the calcium carbonate of which the shell is mainly
composed; and, when we study the internal anatomy of the animal we find
that the shape of the mantle is associated with a loss of the organs
of one side of the body. To put a long story into a few words, we may
say that the Gastropods with twisted shells are lop-sided. Now when we
examine the shells of the Gastropods that live in the surface waters
of the ocean we notice that their shells are (with a few exceptions
such as _Janthina_) perfectly symmetrical, and we might jump to
the conclusion that this was due to a corresponding symmetry of the
internal organs.
Such a conclusion would, however, be an erroneous one, for the
results of the careful anatomical study of these Molluscs proves most
definitely, that although a false symmetry of the organs is often
shown, there is a general suppression of the organs of one side of the
body. A study of the development of these animals also shows, that
in the early stages of their life, the shell is not symmetrical like
that of the adult, but twisted into a spiral like that of a Whelk or
a Periwinkle. These facts indicate that the surface-swimming Molluscs
have passed through a stage in their evolution when their bodies
were twisted up into a spiral shell, and that the false symmetry,
which they exhibit in the adult condition, is an adaptation to
their peculiar habits of life. The study of the group of Gastropods
alone then does not give us any evidence in favour of the view that
the surface-swimming Fauna is primitive; in fact, it proves almost
conclusively that its share in the Fauna has been contributed from the
shallow water districts.
The group of the Tunicates affords similar evidence. There is no
good reason for believing that the Salps and _Pyrosoma_ which
drift about in the surface waters are more primitive than the fixed
Sea-squirts of the rocks and Sea-weeds, in fact, the view is gaining
ground, as our knowledge increases, that all the free Tunicates must
have passed through a sessile ancestry. The evidence afforded by the
Cœlenterates is not so potent. Several naturalists believe that some
free-swimming form of Jelly-fish was the ancestor, and that the fixed
Zoophyte was a stage introduced into the life-history at a later period
in the evolution of the group. Others believe that the Zoophyte-stage
came first and that the Jelly-fish was introduced, for the purpose
of distributing, over a wide area, the eggs of the species. My own
researches lead me to incline towards the latter view, but I feel that
it is still far from being proved.
A great deal more could be written upon this fascinating speculation
about the origin of Life in the Sea. But it is still a speculation,
and all that can be done at present is to weigh the evidence carefully
and see in which way the scale seems to point. If I have succeeded in
making clear to the general reader the nature of the evidence we can
use in judging this question, and have indicated to him the direction
in which it seems to _me_ to point, my task has been accomplished.
INDEX.
A.
Air-bladders in Plankton, 90.
Alcyonarians of Coral-reef, 67.
Alcyonium, 42.
Alternation of Generations, 31, 93, 99.
Angler-fish, 39.
Azoic, without animal life, 9.
B.
Barbel, a tentacle on the lower jaw of Fish such as the Cod, 52, 139.
Barnacles, 40;
on Whale, 159.
Benthos, the animals living on the bottom of the sea, 110.
Blue-shark, 121.
Blue-whale, 126.
Bonito, 121.
Boring Mollusc, 43.
Bopyrus, 170.
Brachiolaria, 112.
C.
Caller-crab, 76.
Cetacea, 125.
Cilia, minute vibratile hair-like processes on the body of some small
animals, 111.
Cirripedia, a group of Crustacea to which the Barnacles belong, 168.
Cod-fish Family, 52.
Coffer-fish, 63.
Colour in shallow water animals, 31;
in deep-sea animals, 137.
Copepods, 86;
parasitic, 169.
Corals, 57.
Coral and Worm, 151.
Coral-reefs, 56;
shells of, 65;
Anemones of, 66;
alcyonarians of, 67;
different forms of, 68;
Fauna, of outer edge of, 78.
Crab-galls, 158.
Crab and Sea-weed, 162.
Crustaceans of the rocks, 48.
Ctenophores, a group of Cœlenterates, 85.
Currents, 16.
Cuttle-fish, 49.
D.
Density of Sea-water, 13.
Depths of the Sea, 10.
Diatoms, minute unicellular, plants, 23.
E.
Electric Organ, 37.
Eyes, 27;
of Scallops, 29;
of larval Tunicate, 30;
of Medusæ, 30;
of deep-sea animals, 138.
F.
Fauna, the animals living in a particular region considered as
a whole.
Filograna, 57.
Fish of the rocks, 51;
of the Coral-reefs, 63;
of the surface waters, 119;
of the deep-sea, 139.
Fish-lice, 167.
Flying-Fish, 120.
Fur-seal, 133.
G.
Gas reservoirs in Plankton, 90.
Gastropods of shallow water, 45;
of Plankton, 100.
Gelasimus, 76.
Globe-fish, 63.
Globigerina, 21, 104.
H.
Hag-fish, 166.
Halobates, 102.
Hermit-crab, 147.
Herrings, 123.
Hormiphora, 85.
I.
Inachus, 162.
Ipnops, 138.
Isopoda, a group of Crustaceans to which the Wood-louse belongs, 139.
J.
Janthina, 101.
Jelly-fish, 84, 92.
John Dory, 54.
L.
Lamprey, 166.
Larva, a young immature free individual differing in form from
the Parent.
Larvæ in surface waters, 109.
Leeches, 166.
Lithothamnion, 20, 57.
Lug-worm, 34.
Lump-sucker, 52.
M.
Mackerel, 124.
Madrepore, 57.
Mangrove-swamp, 73.
Medusæ, 31, 92.
Melia tesselata, 148.
Millepore and Barnacle, 155.
Mud-line, 19.
N.
Nauplius larva, 40, 115.
Nekton, 119.
Noctiluca, 109.
O.
Ooze, 21.
P.
Palolo Worm, 79.
Parasitism, 164.
Periophthalmus, 74.
Periwinkles, 45.
Pholas, 43.
Phosphorescence, 82, 107;
of deep-sea fauna, 140.
Phyllopteryx, 26.
Physalia, 97.
Pipe-fish, 25.
Plankton floating or drifting animals of a region considered as
a whole, 83.
Pluteus, 113.
Porpoise, 127.
Prawn, parasite of, 170.
Pteropod, 21, 102.
Pyrosoma, 108.
R.
Radiolarians, 22, 160.
Razor-shell, 35.
Red clay, 21.
Right-whale, 126.
Rocks, Fauna of, 39.
S.
Sabella, 153.
Sacculina, 168.
Salps, 98.
Sandy shores, Fauna of, 34.
Sargasso, 106.
Scallop, eyes of, 29.
Scopelus, 123.
Sea-anemones, 66, 163.
Sea-bottom, 18.
Sea-elephant, 132.
Sea-horse, 26.
Sea-lilies, 144.
Sea-lion, 132.
Sea-sawdust, 106.
Sea-urchin, 43.
Sea-weeds, 24, 106.
Seal, 130.
Serpula, 41.
Shells of Coral-reef, 65.
Ship-worm, 43.
Shrimps, 31.
Siphonophores, 95.
Skates, 36.
Slime glands of Fish, 165.
Sole, 37.
Solen, 35.
Sperm-whale, 127.
Spines in Plankton, 90.
Sponge, Hermit-crab and Worm, 150.
Star-fish, 44;
method of feeding of, 29.
Stenopus, 33.
Sting-ray, 36.
Stinging of Cœlenterates, 97.
Sun-fish, 122.
Swamp-fauna, 74.
Symbiosis, 146.
T.
Temperature of sea-water, 11.
Teredo, 43.
Thread-cells, 97.
Tides, 15.
Torpedo, 37.
Trepang, 153.
Trichodesmium, 106.
Tridacna, 65.
Trigger-fish, 63.
Tunicates, eye of, 30.
V.
Velella, 96.
W.
Walrus, 134.
Worm-tubes, 163.
X.
Xenophora, 162.
Y.
Yellow cells, 160.
Z.
Zoophytes, 30, 161.
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IV.
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