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.