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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