MOLLUSCA, THE SHELL-FISH
The shell-fish are called Mollusca, the soft-bodied animals. It will
easily be seen that this name was intended to point out the distinction
between them and the Arthropoda, as regards the way in which the skin is
protected. In the latter, as we have seen, the skin itself is hardened.
In the shell-fish, the skin secretes a covering which lies outside it.
Just as our skins pass out superfluous moisture to the outside, in the
form of perspiration, so the skin of the mollusc continually passes to
the outside the solid substances which the body has taken in from the
sea-water; and by the continual accumulation of these, the shell is
formed. This, at least, is the view taken by modern authorities of the
formation of the shell in most instances.
The juvenile shell-collector usually begins his knowledge of the
classification of the Mollusca, by learning that shells are classified
as Univalves and Bivalves. This distinction is useful as a beginning.
Univalves, that is to say shells which consist of one piece, are those
of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to
spell it. Bivalves, or shells which consist of two flaps, are those of
the Lamellibranchiata or animals with plate-like gills, such as the
mussel or oyster.
Let us begin with the former. Everybody knows the snail. The snail
proper bears a typical univalve shell: though in its relatives (the
slugs), the shell is more or less suppressed. The name, Gasteropoda
(stomach-footed animals), is supposed to be descriptive of the way in
which a snail crawls. Half getting out of its shell, so to speak, it
does its best to lay its body to the ground, and its so-called "foot" is
an extensive muscular expansion underlying its body, not just a muscular
organ thrust out of the shell, as in some other groups. The shell, the
mode of crawling, and the "horns," tipped with eye-specks, and directed,
intelligently and inquisitively, towards things of interest--these make
up, for most people, the idea of Snail. But the most distinctive feature
of the class is a less obvious feature, namely, the structure of the
tongue. We may see, on any damp day or dewy evening, the snail working
away with its tongue at some tender leaf. Its tongue is practically a
file with which it files away the substance of the leaf, the resulting
green mash being thus made ready in minute quantities for the snail to
swallow. Thus are made the too familiar holes which disfigure the leaves
of plants in our garden. When seen under the microscope, the file-like
structure of the tongue is visible; indeed, in large tongues, it may, to
some extent, be made out with the naked eye. Across the tongue, which is
a flat ribbon-like structure, there runs a pattern of small teeth,
bilaterally symmetrical, and this pattern is repeated over and over
again throughout the whole length of the tongue. It might be thought
that snails' tongues, being so much alike in their mode of use, would
not need to be very various in pattern: but far from this, they vary in
appearance as much as the shell. Not only is there a different pattern
for every different order of the class, but a different pattern for
every genus; nay, there are even distinctions between the tongues of
different species in the same genus. Consequently some authorities on
shell-fish prefer to classify them by their tongues, a classification
which for the most part holds good. So characteristic is the tongue
of the Gasteropod, that when new animals have turned up which were
difficult to classify by means of the structure of the body, they have
been finally recognised as Molluscs, somewhat related to the snails, by
the tongue. This file-like tongue-ribbon of the snails is often called
the Odontophore or Tooth-Carrier; sometimes the part which actually
bears the teeth receives the name of the radula.
The snail and its relative, the slugs, belong to the Pulmonate (_i.e._
air-breathing) division of the Gasteropoda. The sea-slugs, in which,
like the land slugs, the shell is absent or reduced, are relatives of
the land snails. Some of those found on our own shores are handsome
creatures, brilliantly coloured. Both groups fall under the division
Euthyneura, while the majority of the marine univalves belong to the
division Streptoneura (_i.e._ Gasteropods with twisted nerves). The
Gasteropods, in the course of the evolution of their shell, have had
the body thrown crooked by the burden of carrying it; the Streptoneura
are the forms in which this crookedness is most pronounced; in the
Euthyneura it is less so. There are degrees of crookedness even among
the Streptoneura; and the limpet is less crooked than the periwinkle
(see Table, p. 30).
The older classifications of the Gasteropoda were largely founded on the
characters of the shell; but these, though in the main they hold good,
have required some modifications in recent times. Conchology, the study
of shells, was at one time the hobby of many collectors whose knowledge
of the animals possessing the shells was not of a very extensive kind;
and consequently the very name of conchology is often enough to ruffle
the feelings of the zoologist of the present day. Yet many interesting
problems of variation may be studied from shells alone, by those whose
circumstances forbid them to study the living animal. Nor is there any
branch of zoology which is more useful to the teacher who wishes to
catch the eye and the attention of the beginner in the study of natural
history, especially if the beginner is young, as beginners ought to
be. Therefore we must by no means undervalue the past labours of
conchologists, or the valuable collections which their industry has
brought together and set in order for the benefit of the world.
For example of the most crooked, or Azygobranchiate division of the
Streptoneura, turn now to Fig. 33, in which we see a typical Gasteropod
shell, _Murex ramosus_, the Branchy Murex, aptly enough named from the
many prickly branches which beset it. These rough points are probably
assumed for protective purposes; any animal that might wish to dine upon
the _Murex ramosus_ would think twice before trying to swallow it--the
morsel of shell-fish is so small, its shelly case so large and so
prickly. If we look for its nearest English relative, that is _Murex
erinaceus_, the Hedgehog Murex, or Sting-winkle. This, though a
comparatively plain shell, has still enough rough ridges upon it to have
secured it a comparison to the prickly hedgehog. Perhaps the most
prickly member of the genus, however, is _Murex tenuispina_, sometimes
called Venus' Comb, because the crowded parallel spines which decorate
the elongated front of the shell somewhat resemble the parallel teeth of
a comb.
[Illustration: FIG. 33.--The Branchy Murex, _M. ramosus_, a typical
specimen of the shell of the Carnivorous Gasteropods. _Sp._, spire or
posterior end of the shell; _S_, siphon or anterior end of the shell.
Fig. A, shows the mouth of the shell; Fig. B, the exterior only. Less
than one-half the natural size.]
How does the _Murex_ get its living? Let us notice the shape of the
shell, drawn out to a point, at the end opposite to the spire. According
to the older classification of the Mollusca, now somewhat fallen out of
use, this point marks the shell as belonging to one of the
Siphonostomata (shell-fish with a siphon at the mouth of the shell,
_i.e._). These shell-fish are, with few exceptions, carnivorous; not
that the siphon shape of the shell has any direct connection with the
animal's way of feeding. Just as the snail files among soft vegetable
substances, so the Murex and many of its relations file away much
harder things. A Sting-winkle, or a Dog-whelk, can sit down over a
helpless bivalve shell-fish, and patiently file away, until it has
worked a neat round hole in the protecting shell of the latter. You may
find, among the dead shells on any sandy part of the English coast, any
number of bivalve half-shells with a neat little round hole in them,
indicating unmistakeably how the tenant came to its death. There is some
controversy as to the spot chosen by the assailant for its attack. Some
authorities have stated that the predatory mollusc is so wise that it
knows where to find a weak spot, and makes a hole just over some vital
organ of the bivalve, or else above its adductor muscles, so that, when
these are cut, the half-shells cannot be drawn tightly together and kept
shut. Recently this has been denied, and statistics of the attacks of
_Purpura_, the common small whelk, a relation of the _Murex_, on
_Mytilus edulis_, the Common Mussel, have shown that the perforation
occurs in every part of the shell. It is possible, however, that the
Mussel, from the peculiar shape of its shell, offers an exceptional
case; and I am inclined to think that in the case of bivalves of a more
flattened shape, the earlier statement holds true. At South Shields,
England, perforated half-shells of the Common _Venus_ (Fig. 34) are so
abundant that the children string them for necklaces; yet I have never
been able, by the most industrious search, to find more than one or two
specimens in which the hole is at all near the lip of the shell. It is
possible that these exceptional instances were the work of a young and
inexperienced univalve mollusc, or a stupid one. It is possible, also,
that the mode of attack differs somewhat according to the species of
the assailant. (It should perhaps be explained, for the benefit of those
who have no experience in the ways of children or of shell necklaces,
that the hole must be moderately near the beak of the shell, to enable
the shell to "sit" properly on a string. Every unit in the necklace may
therefore be counted as one in favour of the older theory.) Many of the
Siphonostomatous molluscs are surprisingly active and strong, so that
they are well fitted for a predatory existence. In fact, they not only
eat bivalves, but occasionally attack the vegetable-feeding univalves
when nothing better is to be got, so that occasionally the shells of
these also may be found displaying the deadly little round hole we have
described.
[Illustration: FIG. 34.--Half Shells of the Common Venus, several of
them perforated by carnivorous molluscs. From South Shields, England.]
Let us contrast with the _Murex_ one of the shells which are
"holostomatous," _i.e._ possessing an unindented shell-mouth--that is to
say, one without a "siphon." The common edible periwinkle, _Littorina
littorea_, may be taken as an example. No shell is more familiar; even
the town-dweller, who has never found it on the sea-shore, has seen it
often on stalls in the slums. The mouth of the shell is quite round and
unindented, and in this case the character holds good as the mark of a
vegetable-feeder--a non-predatory sea-snail. It is hardly necessary to
remind the reader that its name (the shore-shell) is given it because it
lives where the tide leaves the rocks exposed during part of the day.
Another common species of _Littorina_, which frequently lives a little
lower down, where the large sea-weeds grow, has been described in
Chapter II.; and another, _L. rudis_, lives a little higher up, so that
it spends most of its time in a dry state, and is fast on its way to
become a land-shell. At most of the familiar English seaside resorts one
may see dozens of it baking in a hot July sun on rocks where only the
highest tides can reach them: and yet under these conditions they
continue to live and flourish. The periwinkles are remarkable for the
great length of the tooth-ribbon, in comparison with the size of the
animal. The number of separate teeth upon it has been estimated at 3500.
A familiar feature of the common periwinkle is the lid or stopper
(Operculum), with which the animal can close the mouth of the shell.
This is developed and carried by the outside of the animal's foot. In
the periwinkle and other English molluscs it is comparatively soft and
semi-transparent, and reminds one of a thin slice of horn. In many
tropical molluscs, however, it is hard and shelly. The large tropical
shells named _Turbo_ have massive lids of considerable weight. These
shells, which are nearly allied to the pearly Top-shells (_Trochus_) of
the English shores, are sold as ornaments, the outer coat of the shell
being partly scraped off to show the inner coat of pearl: it is rarely,
however, that the purchaser obtains a lid, or even knows that the
creature had one. The reverse is the case with some of the smaller
kinds, the lids of which, being brightly coloured, are imported without
the shell, and sometimes set as articles of jewellery. Some of these are
of a bright green hue.
While the lids of the Holostomata are rounded in shape, those that
belong to the Siphonostomatous shells are necessarily more or less
modified so as to fit the mouth of the shell, and are consequently oval
or even claw-like in shape. The Sting-winkle already spoken of, the
common small whelk, _Purpura lapillus_, and the large whelk, _Buccinum
undatum_, are common shell-fish in which the elongated lid may be
studied. The lid is not, however, like the tongue-ribbon, an essential
feature of the structure of every univalve mollusc.[D] Not only are
there special instances in which it is greatly modified, but also
there are whole groups of univalve molluscs in which it is absent.
[D] There are one or two exceptional cases of gasteropod molluscs that
have no tongue-ribbon. The majority of these are parasitic forms, which
can get their food without the trouble of filing it down.
A curious suggestion has been made with regard to the lids of univalve
shell-fish; namely, that the snapping to of the lid is capable of
producing a sound, which may perhaps be audible at a distance under the
water. Various molluscs have been credited with producing sounds, either
by muscular movements or by the grating of the shell as the animal
walks. The common Tortoise-shell Snail, _Helix aspersa_, sometimes
makes a most alarming noise when crawling over a window. It has been
disputed whether the sounds thus made are produced by the grating of the
creature's tongue-ribbon on the glass, as it files off small particles
of algae and vegetable moulds, which are invisible to our eyes: or
whether they are sounds due to suction of the muscular surfaces, such as
may be produced by drawing a wet finger across glass. The noise, however
produced, is, as I can testify from experience, sufficiently loud and
weird to be very startling, if heard in the dead of night.
Turn now to the Bivalves or Lamellibranchiate molluscs, which include
the familiar oyster, cockle, and mussel. These are also known as the
Pelecypoda, and as the Aglossa, or molluscs without a tongue-ribbon. The
name Lamellibranchiate refers to the shape of the gills--"plate-like,"
or flat; the name Pelecypoda to the shape of the foot, "hatchet-foot."
The animal usually chosen as a type of these, the fresh-water mussel, is
rather a dull sort of creature, so we have chosen a prettier and more
lively specimen as a representative of the class; namely, one of the
Scallops, _Pecten opercularis_, sometimes called the Quin, the shell of
which is shown in the frontispiece of the book. This is one of the most
beautiful, perhaps the most beautiful, of the English shells. The
generic name, _Pecten_, the Comb-shell, probably refers, not to the
shape of the gills, which is somewhat peculiar, but to the marking of
the shell, which presents raised ridges, side by side. Anyone familiar
with shells will see at once that this is an unusual pattern. There are
plenty of bivalve shells with concentric ridge markings, comparatively
few with radiating ridges. We shall see presently that there is a good
reason for this. The specific name "opercularis," lid-like, refers to
the neat round shape of the shell. Each half of the shell has a pair of
"ears," so-called. The person who first gave this name to these flaps of
shell, three of which are three-cornered and the fourth nondescript,
must have been familiar in his youth with books afflicted with the
"dog's-ear" disfigurement; for certainly there is no other kind of ear
which greatly resembles these. The notch beneath the irregularly shaped
ear is called the "byssal notch": many Pectens spin a byssus or thread,
like that spun by the common Sea-Mussel, and thus anchor themselves to
fixed objects for a time; this notch is the place where threads of this
kind leave the shell.
The two valves of the shell differ in depth, one being flatter than the
other; and the "ears" of the two valves differ in shape. The inside of
the shell shows muscular impressions, but these cannot be seen in a
photograph. The picture, however, shows the strong hinge-ligament which
joins the halves of the shell, and the difference in depth and shape of
the two valves. The valve on which the animal usually lies is the
lighter in colour of the two, and has one ear much longer than the
other.
The creature swims by means of the "mantle," or muscular margin of the
body. It contracts this suddenly, after first opening the shell and
taking in as much water as possible. Thus the water is squeezed out
again, and the effect of this is to propel the animal in an opposite
direction.
Now we are in a position to understand a little more about the shape of
the shell. These curious "ears" possessed by the two valves, together
form a straight, strong edge, which cuts the water as the animal flies
along. It reminds us of a ship's prow, and not without reason, for the
use of each is the same. A boat's sharp prow, compared with the rounded
front of a "tub," makes all the difference in the possibilities of
straight steering, and favours the putting on of speed: the ears of the
shell are not less useful to our Scallop. The following account of the
swimming powers of this species of Scallop, quoted by Woodward, was
given by the Rev. D. Landsborough, who observed young specimens, about
the size of the small ones in our picture, swimming about in a pool of
sea-water, left by the ebbing tide. "Their motion was rapid and zigzag;
they seemed, by the sudden opening and shutting of their valves, to have
the power of darting like an arrow through the water. One jerk carried
them some yards, and then by another sudden jerk they were off in a
moment on a different tack." To the sharp prow, the _Pecten_ owes this
capability of arrow-like flight. Its eyes are situated on the fringe of
its mantle, and consequently near the wide end of the shell; its
peculiar mode of progression, therefore, enables it to back away
instantly from any enemy it sees.
Something must be said regarding the interior of the shell. The majority
of bivalve shells have a complicated system of so-called "teeth," or
interlocking projections, at the hinges of the shell: these exhibit
great variety in different kinds of shell, and are therefore often a
ready means of distinguishing one shell from another. The Scallop,
however, is very deficient in this respect, as are also some of its near
relations, for instance the oyster and its family group. The Fresh-water
Mussel also gains its name, _Anodon_, or _Anodonta_, the Toothless One,
from the same circumstance. The name often puzzles the beginner, who
asks, bewildered, "But do Bivalves ever have any teeth?" True teeth, of
course, they have none--it is the shell-hinge that has teeth, not the
animal inside it. Not only have the bivalve shell-fish no teeth indeed,
or tongue-ribbon, but furthermore they have no head. For this reason the
group has not only received the name already mentioned, of Aglossa, the
Tongue-less Ones, but also that of Lipocephala, _i.e._ Molluscs in which
the head is not developed. The reason of its absence is not far to
seek--a head would be no use inside such a shell. The snail-shell, so
differently built, allows freedom for the head; the bivalve mollusc,
squeezed in between its valves, has room only for a mouth.
We have referred above to the ridges on the outside of the shell. Now
that we have learnt that the _Pecten_ is a very active animal, and moves
in the manner described, we see that these ridges run parallel to the
direction in which it moves as it darts away ears foremost. Let us try
to realise what is the effect of this.
Take a mat with parallel stripes and move it along the floor or table
in the direction of the stripes; then try moving it in an opposite
direction across the stripes. It is easy to perceive that in the former
case one's eye does not detect the movement nearly so soon as in the
latter case. To explain this would necessitate a lengthy digression on
the subject of optical illusions: that the fact is so everyone may
easily ascertain by experiment. The ridges, therefore, converging in the
direction towards which the shell is going, are a protective decoration,
enabling it to slip away more easily from under the eyes of its foes.
The reader will readily recall a parallel instance in the common Cockle.
This also is a very active creature; it takes leaps by means of a strong
muscular foot; and the ridges on the shell, like those of the Scallop,
converge towards the hinges, that is to say, in the direction in which
the shell moves. Another instance of a very active shell-fish with
similar markings is afforded by certain kinds of Lima, a near relative
of the Scallops. It may be added that all Scallops are not equally
active, nor all Limas; and various modifications of their form and
colour might be pointed out which lead us to suspect that in the less
active kinds the pattern of ridges is often somewhat obscured by means
of these differences.
Now, take up a comb and draw it over your fingers, firstly _along_ the
teeth, and secondly _across_ them, and you will be able to estimate the
gain in speed and comfort to the comb-shell, Pecten, and to the common
Cockle, from having its ridges set in the direction in which it is
going. Were the ridges concentric, as is so often the case in bivalve
shell-fish of a more sluggish disposition, the friction caused by the
ridges would seriously delay the progress of the shell.
Something must be added regarding the colouring of the shell, which is
vivid, corresponding with that of the animal within. It is capable of
great variety, though perhaps not so great as in some of the smaller
_Pectens_. The predominant shades are pink, crimson and yellow, either
separately or mixed; that is to say, some shells are pure pink, some
almost pure yellow, some almost pure crimson, while others present every
imaginable shade of pinkish yellow, reddish brown and brownish crimson.
Local variation of colour is so marked that we may suspect the
variations in tint to be in some degree protective. The shell also
varies considerably in size and strength according to the neighbourhood
in which it has grown.
This scallop-shell is but one of many: a number of other species are
found on our own shores, and many others again in foreign seas.
One shell of the English coast is very annoying to the juvenile
shell-collector who gathers specimens on the shore. This is _Pecten
pusio_, a very small and delicate kind, with a raised pattern of fine
markings upon the ridges, which are very narrow. A good specimen of the
deeper valve is common enough, but the shallow valve, if of any size, is
distorted into all manner of shapes, as if it had been squeezed and
crumpled. The disappointing character of these specimens, from an
æsthetic point of view, is explained when we learn that it not only lies
on its shallow valve, but becomes fixed in this position, instead of
hopping about freely like the _P. opercularis_. It therefore has
frequently to adapt its shape to the nature of the ground where it has
happened to fix itself. Thus arises the disfigurement of the shell.
So far we have only considered two great groups of the Mollusca, two
which are represented by common shells, familiar to everybody. We must
not leave the subject of the Mollusca without referring to their most
aristocratic group, the Cephalopoda. These are represented in museums by
the shells of the Pearly Nautilus, and of its not very near relative,
the Paper Nautilus; and they are represented on English shores by the
cuttle-fishes. All these agree with the Gasteropoda in the possession of
a tongue-ribbon, and in classification are therefore treated with them
under the name Glossophora.
With the Pteropods, transparent forms found swimming over the surface
of the deep sea, the reader is not likely to have much to do. In
classification they are now placed near the Sea-Slugs.
The Placophora, or Polyplacophora, wholly different from our usual idea
of a shell-fish, may be named as creatures which the reader is quite
likely to meet with. Though not very common, they are widely distributed
over our coasts, and may be found near low-tide mark clinging to stones.
Imagine a wood-louse without any apparent head which has taken to
clinging to the rock like a limpet, so that it cannot be removed without
injury, and you have a rough idea of their general appearance. _Chiton_
is the name of these animals, which have received the group name of
Polyplacophora, carriers of many plates, because their external covering
consists of an armour of successive shelly plates. These also belong to
the Glossophora or Tongue-ribbon Carriers, of which they present a
comparatively primitive form.
Reference has already been made to the labours of the earthworm and of
the insects, and to their important effects upon the vegetable world.
Although the Mollusca include but one terrestrial group, the Snails,
they, too, have played an appreciable part in modifying plant life. If
we owe our flowers to the insects, we have probably to thank the snail
for our medicines. For the snail dislikes bitter-tasting leaves, and
lets them alone, thus exercising an artificial selection in favour of
the survival of medicinal plants. In the same way the snail has favoured
the survival of hairy and thorny plants, upon which it cannot easily
crawl.
The larval forms of the Mollusca differ considerably from the adult.
That of _Anodon_, the fresh-water mussel, at first received, in
consequence, a different name, that of Glochidium, by which it is still
known, although it has now been long identified as a larval form. It is
exceptional in the fact that it is parasitic on fish.
The usual Molluscan larva is a ciliated creature which has been compared
to a modified trochosphere. It is preceded by a gastrula stage, and it
develops later on into what is called a "Veliger," or "veil-carrying"
larva, so called because it has in front a broad two-lobed ciliated
expansion, the velum. This larva is adapted for swimming, which is
accomplished by means of the velum. In terrestrial molluscs, the
development is necessarily much more direct. It is worthy of note that
the periwinkle mentioned above, which lives high and dry (_L. rudis_)
has no larval form, while its relatives that live under water develop
in the usual way.
The eggs of Mollusca are often enclosed in tough cases, calculated to
resist waves and weather. Some of these are shown in miniature, in the
group of eggs of various kinds, Fig. 35.
[Illustration: FIG. 35.--Eggs (reduced to half the natural size).
_A_, Egg-Capsules of _Murex_. _B_, Frog's Eggs. _C_, Eggs of large
Land-snail. _D_, Eggs of Snail placed on a leaf. _E_, Cockchafer's Eggs.
_F_, Egg-case of Cockroach. _G_, Egg-cases of Locust. _H_, _I_, _J_,
Eggs of Gasteropod Molluscs. _H_, _Sycotypus_ (_Pyrula_). _J_,
_Fusus_.]
TABLE SHOWING THE CLASSIFICATION OF THE MOLLUSCA
{ AGLOSSA: the LAMELLIBRANCHIATA, also called
{ CONCHIFERA, and PELECYPODA.
=MOLLUSCA.= {
{
{ GLOSSOPHORA { GASTEROPODA.
{ CEPHALOPODA.
CHAPTER X
THE BRACHIOPODA OR LAMP-SHELLS
These were at one time included under the Mollusca, on account of
their possession of a bivalve shell. This shell, however, is placed
practically back and front of the animal, not to the right and left of
it, as is the case with the shells of the bivalve Mollusca.
The name, arm-footed, was given them in reference to a pair of special
structures called the arms, bearing a large number of tentacles; it is
now more frequently spoken of as the lophophore (see p. 122), and
regarded as comparable to the lophophore of the Polyzoa, spread out into
two portions. With the latter group the Brachiopods were formerly united
by Huxley, under the name of Molluscoidea. This name is now obsolete,
because it is understood that all these creatures are widely different
from Molluscs; but the theory of relationship of the Brachiopoda to the
Polyzoa, implied in it, still holds good.
The chief importance of this group lies in its fossil forms, which are
exceedingly numerous, particularly in the Mountain Limestone of the
Carboniferous Period; it is crowded with their shells, especially a form
named, from its elongated shape, Productus. The shells of Brachiopods
are equal-sided; that is to say, the right and left valves match; but
they are inequivalve, the ventral valve being much the biggest. It often
contains a foramen, or hole, at the beak, for the passage of the
pedicle, or stalk, by which the animal is attached to the ground (_e.g._
_Terebratula_, _Rhynchonella_). Sometimes, however, the pedicle passes
out between the valves (_Lingula_, _e.g._), in which case there is no
foramen; or it may be arranged in other ways. Sometimes the shell is
merely attached to the ground by its side, like an oyster. Some forms
are enormously widened in a lateral direction, _e.g._ _Spirifera_, and
the _Productus_ above named. _Lingula_, among others, is remarkable as
being a form that has survived from the earliest geological period to
the present day.
TABLE SHOWING THE CLASSIFICATION OF THE BRACHIOPODA
{ TESTICARDINIS.--Shell Calcareous, with hinges.
{ Skeleton present in the arms.
=BRACHIOPODA.= {
{ ECARDINES.--Shell comparatively soft, composed
{ of Chitin, only strengthened by deposits
{ of lime, without hinges. No skeleton in
{ the arms.
The larva, in its best known forms, passes through the typical larval
stages of the animal kingdom. It is first a one-layered larva, then a
two-layered form, and then becomes a ciliated animal. In this three
regions may be distinguished, representing respectively the head, body,
and pedicle.
The shells of the Brachiopoda, including the kinds above named, may be
seen by the reader in any geological museum.
CHAPTER XI
THE POLYZOA; MOSS-CORALS AND SEA-MATS
We have already described the creatures which are popularly known
as Corallines. Modern zoologists have long separated off from the
Corallines of the older writers, a group of animals known as the
Sea-Mats, which also are colonies made up of unit individuals. The
common Sea-Mat, _Flustra foliacea_, may be picked up on almost any part
of the English coast, being often torn up "by the roots" and washed in
by the tide. When fresh it has a pleasant scent, which has been compared
to that of Lemon Verbena, and a pinkish colour, due to the presence of
the little inhabitants in their cells. When dry it has no odour,
the cells are empty, and the colour a pale drab like that of a dead
Coralline. Its texture is, however, much more crisp and brittle, and
less horny, than that of a dead Coralline: it grows in flat, forked
expansions, much resembling in outline the fronds of several common
seaweeds; and each side of these is covered with a diamond pattern of
little cells. This crowded arrangement of the cells, with a tendency to
assume a geometrical pattern, is the readiest feature by which the
beginner may distinguish a Sea-Mat from a Coralline. The latter arrange
their cells in a free-growing, tree-like or fernlike form, without any
crowding of the units into a geometrical pattern. The division of the
flat leaf-like colony by two, resulting in bifurcated branches, is
another obvious feature of the Sea-Mat.
Covering--and to the botanist's eye disfiguring--the branches of many
sea-weeds, and growing upon oyster-shells, tangle-roots, and other fixed
objects, we may find many little incrustations which remind us of the
lichens of the land: the diamond pattern of little cells shows us,
however, that these things are relations of the Sea-Mats. The name of
Bryozoa, Moss-Corals, was formerly given to these growths. Many of
them bear long hair-like processes at regular intervals; these, which
are large enough to be plainly seen with the naked eye, afford a ready
means of recognising these creatures.
TABLE SHOWING THE CLASSIFICATION OF THE POLYZOA
{ ECTOPROCTA, with excretory aperture outside
{ the ring of tentacles, _e.g._, _Flustra_.
=POLYZOA.= {
{ ENDOPROCTA with excretory aperture inside
{ the ring of tentacles.
The Polyzoa include freshwater as well as marine forms. They have a
free-swimming larva, which becomes fixed after a time, and gives rise
to the adult Colonial forms. The zooids of the latter have each an
independent head with a crown of tentacles, called the Lophophore
(Crest-carrier); but the fixed ends of their bodies communicate with one
another. The hard covering of the colony, which retains its form after
the animal is dead, is a kind of hardened skin: the apparent "cells" are
the openings through which the individual zooids protrude themselves.
Sometimes certain of the zooids undergo modification for special
purposes: in this way are formed the "avicularia," snapping appendages,
probably defensive in purpose, so called because they open and shut like
a bird's beak. There are two divisions of the Polyzoa, the Ectoprocta
and the Endoprocta. Among the latter there is found a form which is not
colonial.
_Phoronis_, a curious worm-like animal, which has a larval form called
_Actinotrocha_ is sometimes placed in classification near the Polyzoa,
which it resembles in possessing a crown of tentacles (Lophophore).
CHAPTER XII
THE ECHINODERMATA
Everybody knows the Star-fish and many people know the Sea-Urchin.
An "urchin" is not a name for a naughty little boy, but the French
(_oursin_) for a hedgehog. A Sea-Urchin is therefore a "Sea-Hedgehog,"
a name very appropriate for a creature armed with prickles. The Greek
word _echinos_ also means a hedgehog, so that the long name given to the
group means simply hedgehog-skinned. The prickles attain their maximum
in the Sea-Urchin, but they are well represented in the Star-fish, while
in the Sea-cucumber the general tendency to "prickliness" is much
reduced, and represented only by "spicules" (needles) of shelly stuff
underneath the skin of the animal.
The largest and the most beautiful of the Sea-Urchins of the English
coast is known as the Purple-tipped Sea-Urchin, on account of the
beautiful colour of the spines. It lives on rocky coasts, and during
very low tides may be seen at home, although it usually takes care not
to stray above the water-line. It is a shelly ball with a flat base; its
surface is covered with long spines. Its mouth, which is in the centre
of the base, shows five wicked-looking teeth peeping out. The shell is
pierced by what look like hundreds of minute pin-holes, arranged in a
complicated pattern; these are the holes through which it pokes its
feet, which greatly resemble those of a Star-fish, being white suckers
with a disc at the end. When thrown out to their full length they are,
however, much longer than those of the Starfish, for they are naturally
obliged to be thrown out to a distance longer than the length of the
animal's own prickles. When moored by all its feet, extended from all
sides of the shelly ball, the animal presents a curious and pretty
sight. Large specimens are almost as big as a child's head, but smaller
ones are more common. There is a considerable range of variation
in colour; not only are various shades of purple found, but also
purplish-red and red. The spines are mounted on something resembling a
ball and socket joint, with a ring-shaped pad, so that they have a wide
range of movement; if any of the spines are touched they are immediately
set back over a considerable part of the neighbouring surface.
Other kinds may be found upon a more sandy shore. These are heart-shaped
and much lighter in colour. The shell is thinner and of less weight.
These adaptations for lessening the animal's weight enable it to move
over sand: the species above described has no occasion for such
precautions. When it crawls over rocks and the strong seaweeds that grow
on them, there is no fear of its sinking in. The sand-dweller, on the
contrary, must take care that it is not swallowed up.
[Illustration: FIG. 36.--The Five-holed Sand Cake, _Mellita pentapora_,
a flat sea-urchin from the east coast of tropical North America. _A_,
upper surface; =B=, lower surface; _C_, side view.]
There are Sea-Urchins that carry their precautions against sinking to an
extreme degree. These are the Shield-Urchins or Clypeastridæ, so-called
from their flat shape; they include the American forms popularly known
as "sand-cakes." The diagram (Fig. 36) shows one of the most curious of
these flattened forms adapted for moving over fine sand and ooze, and
literally "as flat as a pancake." The mouth is approximately in the
centre of the lower surface, _B_; the upper surface, _A_, shows a
rosette pattern on the top of the shell. This is formed by the rows of
holes for the very minute tube feet. In the English Sea-Urchin above
described, which is one of the group called (for that reason) Regulares,
the rows of holes are uniformly continued all along the rounded sides of
the body down to the neighbourhood of the mouth. Here they are much
restricted, forming merely a rosette at the top of the shell: hence
they are described as circumscript or "petaloid." The excretory aperture
is shown in the photograph as a smaller dot on one side of the mouth,
while in the Echinus, on the contrary, it is at the top of the shell.
The five odd-looking, elongated holes are a curious individual
peculiarity of this Sea-Urchin. It has already been explained that the
Shield-Urchins are flattened in order to distribute their weight; these
holes are a contrivance for still further reducing the weight in
comparison with the area. This is when the animal is lying quiet at the
bottom of the water, but when it moves about what effect will the
presence of the holes produce? Flattened animals are usually supposed
to derive an advantage from the fact that they sink more slowly
through depths of water; as in lying upon the ground, their weight is
distributed, and they float, as it were, in the same stratum of water
without sinking further down. This creature, on the contrary, has
apparently feared lest it should move too slowly when it moves in a
vertical direction, and it presents us with an arrangement by means of
which its sinking through water is facilitated. Water will pass readily
through the five holes as the animal goes either up or down, and the
resistance of the whole flat area to the water is thus reduced and
vertical movement rendered more easy. Thus, by one and the same
contrivance, the animal has lessened its weight when lying quiet, and
diminished the resistance it meets with when it moves. The distribution
of the holes, moreover, is such as to regulate the animal's position in
sinking, and to prevent it from falling "headlong." For although the
creature has, strictly speaking, no "head," yet the end nearest the
mouth is the thickest and heaviest part of the "cake," and would
naturally tend downwards. This tendency is counteracted by the fact that
the thicker end is unperforated, while the thinner and lighter end has a
large central hole to diminish its resistance and enable it to sink more
rapidly.
Adapted for living in sand rather than on rocks, but not so extreme
in the peculiarity of their form as the Shield-Urchins, are the
Heart-Urchins, already referred to, shaggy-looking creatures whose fine
yellowish-white spines give them almost the appearance of being clothed
with fur. The excretory aperture is at the narrow end of the "heart,"
and the mouth at one side of the lower surface towards the wide end. The
complicated apparatus of teeth found in other Sea-Urchins is absent in
these. They are abundant on sandy shores. During the severe winter of
1894-5, when the Mersey at Liverpool was frozen nearly for one memorable
day, and filled with floating ice for many more, I saw the shore beyond
New Brighton heaped all along with a bank, often two feet across, of the
common Heart-Urchin. These, which afforded a fine feast for the hungry
sea-gulls, had been killed by the intense cold, and afterwards washed
ashore by the tide. The vast numbers of this creature which exist on
that coast were thus unexpectedly brought to light.
These animals are sometimes described as "burrowing" creatures, because
they live covered in sand. The term is rather misleading. Far from
wishing to burrow, they spend their lives in a constant struggle with
sand that closes over them only too readily; and their whole structure
is adapted to prevent their sinking in a quick-sand.
We began our chapter with the Sea-Urchins, because they are the most
important members of the group to which they give their name; but there
are forms belonging to the Echinodermata that are more familiar to the
ordinary observer--the Starfishes. Those who take an interest in the
cultivation of the oyster find them far too familiar--for the starfish
is the oyster's deadliest foe, not even excepting man.
The common Starfish, _Asterias rubens_, may constantly be found among
stones, about low-tide mark. Its manner of walking is peculiar and
characteristic. On the under surface of each ray are rows of white
sucker-like tube-feet, which can either be drawn in or pushed out. By
doing each alternately the animal walks. First the feet are extended to
their full length; then the terminal sucking disc of each catches hold
of the ground. Then the feet are again retracted, while their discs
still cling; the effect of this is, naturally, to pull the ray onwards.
This process is repeated again and again, until some appreciable degree
of movement is effected. The tube-feet are in connection with a system
of vessels filled with fluid, known as the Water-vascular System of the
Starfish. The fluid is driven on by muscular contractions until the feet
are fully extended, and again driven back when the feet are retracted.
The Water-vascular System is a structure common to all Echinoderms; and
vessels of a comparable character are found in some worms.
How does the Starfish know where it is going? Underneath each ray, near
the tip, is a little feeler (or tentacle) and a little eye spot. By
means of these it gets an idea where each ray is going to; and, since it
often moves but one ray at a time, this is sufficient for it. When
necessary, however, the several rays can act in concert with one
another.
The rayed form of the Starfishes led to their being at first included
in the group of Radiate Animals, along with the tentacle-bearing
Coelenterata; but it has long been recognised that they are animals of
much higher structure. Their very larvæ can barely be brought into
comparison with animals so simple as the true "radiates."
The Snake-Stars, or Ophiuroidea, are closely allied to the Starfishes.
In these the arms are thin and sharply defined from the little central
disc, instead of sloping gently out of it, as in the Starfishes. The
rapid wriggling movements of the arms have gained for them their very
appropriate name. They are also called Brittle Stars, because the arms
break off easily, sometimes at the will of the animal. Several kinds of
them are common on our shores, although they are not so common as the
ordinary Starfishes. Fig. 37 shows the general form of a Brittle Star.
[Illustration: FIG. 37.--A Brittle-Star, _Ophiopteris antipodum_.]
[Illustration: FIG. 38. A Sea-Cucumber, _Cucumaria Planci_, from Naples,
natural size.]
The Sea-Cucumbers, Holothuroidea, are another group of Echinodermata
that are represented on our own coasts; by small specimens, however,
while the Pacific Ocean furnishes instances of larger size--the
Trepangs--which are used by the Chinese as articles of food. The name
Sea-Cucumber is given in fanciful comparison to a small Gherkin;
presumably one that has been very badly pickled--for the colour of the
animal is brownish and by no means green. The mouth of a Sea-Cucumber is
surrounded by a circlet of tentacles (partially indicated in the
diagram, Fig. 38). The body is elongated and crawls along: the "star"
shape, so characteristic of the Echinoderms, is scarcely to be
recognised except in cross section, where the longitudinal rows
of tube-feet are seen to outline a pentagon. The skeleton of the
Sea-Cucumber is of a very meagre description. Instead of forming a
rounded case, as in the Sea-Urchin, it consists only of loose pieces
of very small size, situated below the skin. The Starfishes are
intermediate in this respect. Their "skeleton" consists of a vast number
of pieces or "ossicles," which are of fair size, but are not closely
united, as in the Sea-Urchin. They are, however, so numerous and so well
knit, that the skeleton of a dead Starfish presents the complete outward
form of the animal. It must be noted that the ordinary skeleton of the
Sea-Urchin is only _apparently_ exterior. As is the case with the
ossicles of the Starfish and Sea-Cucumber, the skin lies outside, and
the hard particles belong to the middle layer, or mesoderm. In this the
skeleton of Echinoderms differs from the "shell" of a crab or lobster,
which is formed by a hardening of the skin itself.
[Illustration: FIG. 39.--_A_, Head of a Stone Lily or Encrinite,
_Encrinus liliformis_, a fossil from the Muschelkalk of Brunswick,
natural size. _B_, Rock with stalks of encrinites. _C_, Section of a
stalk.]
The Crinoidea, Encrinites or Stone-Lilies, form another group of the
Echinodermata. Though still represented by living forms, they attained
their maximum development in past ages. The English "Mountain Limestone"
of the Carboniferous period is full of their fossilized remains, which
form a marble often used for ornamental purposes. The so-called "Stone
Lily" consists of a "head" comparable with the body of a Star-fish or
other Echinoderm, which is borne at the end of a long fixed stalk. The
marble above named owes its ornamental appearance to the presence of
these stalks, often very long, and cut through at every possible angle.
The Crinoids have their living representative in English Seas,
_Antedon_, the Feather-Star (Fig. 40). On the side opposite the mouth,
where, in the Encrinite, the stalk would be, there are a group of
elongated processes called cirrhi, by means of which the animal can
attach itself to stones or seaweeds. When not thus fixed, it swims
about, by moving its fringed arms, each of which is forked. It will be
seen that when the animal is fixed by its cirrhi, it stands mouth
upwards, so that its position compared with that of the Starfish or
Sea-Urchin is upside down. The young of the Feather-Stars have stalks by
which they are fixed, like the Encrinites; but afterwards the stalk is
lost.
[Illustration: FIG. 40.--A Feather-Star, _Antedon bifida_, British Seas,
three-quarters of the natural size. The short threads in the middle are
the cirrhi.]
Among fossil Echinoderms there are two groups of stalked forms which
have no living representatives. These are the Cystoidea and the
Blastoidea. In both of these the stalk bears, as in Encrinites, a calyx
or head, which is comparable, with the body of the free Echinoderms.
The Sea-Urchins possess a swimming larval stage, which goes through
remarkable changes after passing out of the two-layered (Gastrula)
form. It becomes provided with cilia, which are arranged in bands,
and outgrowths of peculiar form are established in the case of the
Sea-Urchins, while the larvæ of the other groups also present
characteristic shapes. Within the larva the adult form develops, the
outside of the larva being finally thrown off.
In the young Feather-Star, a subsequent stage of the young animal has a
stalk, by which, like the Encrinite, it is fixed. This animal therefore
is at first free-swimming, afterwards fixed, and again free in its final
stage--a remarkable series of changes.
These queer-shaped things, the Sea-Urchins and their allies, are perhaps
the last creatures amongst which we should think of looking for
relations of the Worms. Yet the earliest stages of the larva are
considered to present a certain amount of resemblance to the Wheel-ball
larva, which has been referred to elsewhere (pp. 42 and 72). Still more
startling fact, these larvæ have been compared to that of
_Balanoglossus_, the lowest member of the Chordata, and a relation of
the Vertebrates themselves (see p. 143).
TABLE SHOWING THE CLASSIFICATION OF THE ECHINODERMATA
{ ECHINOIDEA, OR SEA-URCHINS.
{ ASTEROIDEA, OR STAR-FISHES.
=ECHINODERMATA.= { OPHIUROIDEA, OR BRITTLE-STARS.
{ CRINOIDEA, OR FEATHER-STARS AND
{ STONE-LILIES.
{ HOLOTHUROIDEA, OR SEA-CUCUMBERS.
CHAPTER XIII
THE CHORDATA
The older zoologists used to speak of Vertebrata and Invertebrata as
animals with a back-bone and animals without one, and everyone thought
it a very natural way of dividing up the animal kingdom. It never
occurred to anyone that it was possible to bridge the interval between
them and find a link between the two. But now the Vertebrata have been
compelled to give up their aristocratic pretensions, and own that they
have risen from the ranks of the common people of the animal world; in
other words, that they are descended from the Invertebrates. Their
family secrets have been published to the world, and now everybody knows
that they have poor relations. But how many, and how nearly related?
This we do not accurately know; consequently the whole zoological world
for many years has concentrated all its energies on attempts to find out
the truth about the matter.
A great sensation was caused by the first discovery of a poor relation
of the vertebrates among the Ascidians, or Leather-bottle animals. These
are named from their shape and texture, for they have a leathery skin.
Now some of these Ascidians have larvæ with a tail; and in the tail
there is a long cord-like structure, which in many essential particulars
resembles the cord which precedes the back-bone in the vertebrate
embryo. This structure is called the Notochord (a string down the back).
The credit of this great discovery belongs to Russia; for the presence
of the Notochord in the Ascidian larva was discovered by A. Kowalevsky,
in 1866.
To the present generation of zoological students, the Chordate
affinities of Ascidians are part of the ABC of knowledge; and it is
hardly possible for them to realise that it is only thirty years ago
since the idea was so new that Huxley, in his "Text-book of the
Vertebrata," only alluded to it in a footnote. Would-be zoological
critics, at a somewhat later period, met the theory with ridicule, for
want of better argument. For critics include not only "those who have
failed in literature and art," but also those who have failed in
science.
The majority of the Ascidians are sessile animals, which fix themselves,
like Sea-Anemones, to some object when they have passed their earliest
stages of growth; and although there are many forms that swim freely,
most authorities are inclined to believe that these have arisen by
adaptation, and that the kinds that are fixed when adult are the
original type of the group.
Anything more unlike what we should expect to find as a relative of the
vertebrates could not possibly be imagined. What has been written about
these little animals by various observers would make a whole series of
volumes of the size of this one, so many are the puzzles afforded by
their internal structure. The arrangement of their organs is in many
respects very unsymmetrical. Their most striking peculiarity, perhaps,
is the nature of the gills. These form a kind of basket-work, consisting
of minute holes with intermediate supports; and they are associated with
a special cavity outside them called the Atrial chamber. Into this the
gills pass the sea-water which they have breathed.
The group, as a whole, is sometimes considered to present evidence of
having degenerated from a higher type; but whatever else may be doubtful
or obscure in its history, the nature of the larval notochord is quite
clear and certain; zoologists have never had any doubt about its nature
since the first few years after its discovery.
Ascidians are not at all uncommon animals on the English coast. Some of
them may be met with on stones near low-water mark, and I have often
seen them on the shells of oysters sold in the shops--for there the
town-dwelling naturalist may often find a good many interesting things
without much trouble. They are like little lumps of tough jelly; of
various colours, according to the kind, red being the most common, and
of very indefinite shape. You may see some of the colonial kinds forming
pretty star-shaped patterns, attached to various objects, such as stones
and the larger seaweeds.
The place of Ascidians in classification was a puzzle, until their
relationship with Vertebrates was discovered. At one time they were
placed with the Mollusca. Now they are grouped, together with the
Vertebrata and some other creatures that remain to be spoken of, under
the name of Chordata, or animals possessing a Notochord.
Some of the Ascidians present what has been already described in other
types (p. 57) as "alternation of generations." The discovery of this
fact was made by the poet Adelbert von Chamisso. Some of his verses are
known to English readers, for whom they were translated by Mary Howitt,
a poetess whose writings were popular with our grandmothers, and
deserved to be so. This is not the only case in which a poet has been
also a zoologist: Goethe studied the science, and framed a theory
regarding the vertebrate skull, which he regarded as consisting of a
series of vertebræ. In this he was less fortunate than the Italian
poet; for while Chamisso's observations were correct, and were confirmed
by subsequent writers, Goethe's theory of the skull is anything but
correct. It was made worse, too, by the speculations of subsequent
writers, who attempted to follow it into detail, with the result of
demonstrating its absurdity.
CHAPTER XIV
THE VERTEBRATA
We have spoken of the Notochord as a structure which precedes the
formation of the spinal column in Vertebrates. This needs a little more
definite explanation. We all know that the spinal column of vertebrates
is formed to protect the spinal cord. This protection is, however, an
afterthought, so to speak, of the vertebrate structure; the lowest of
all vertebrates is quite without it; and in the lower groups of fishes
we may trace various steps of its formation. But in these cases where
the spinal column is absent or incomplete, there is a large and
well-developed notochord; and in the embryo of higher vertebrates, when
the spinal column has not yet begun to be formed, the notochord is
equally a conspicuous feature. It runs from the region known as the
mid-brain, to the end of the tail, and lies throughout just beneath the
spinal cord. Whatever its original use in the animal body may have
been, it undoubtedly acts now as a support to the spinal cord, and
indeed to the whole body. Bones, we must explain, do not exist either in
the lower vertebrate, or in the early embryo. In the latter they are
formed by degrees. The spinal cord and the notochord each begin to be
surrounded by rings of cartilage or gristle, which by degrees is changed
into bone. The rings surrounding the notochord, however, gradually
encroach upon it and obliterate it. The place where it has been becomes
the Centrum, or most solid part of each vertebra. The notochord at first
is continuous, and has no division into successive parts; but when the
bony spinal column is developed, it consists of a series of successive
vertebræ. Each of them is made up of several parts, which by degrees
become consolidated into the vertebræ.
[Illustration: FIG. 41.--_A_, The Notochord of Vertebrates. Section,
considerably magnified, through the middle of an embryo one inch long,
of Acanthias, one of the Spiny Dog-fishes allied to the sharks. 1,
Section through Spinal Cord; 2, Section through Notochord; below it lies
a bean-shaped space, which is a section through a large blood-vessel;
_sk_, epiblast or skin; _me_, mesoblast or middle layer of the body; the
dots represent the nuclei of its transparent cells. The intestine, _i_,
lined with hypoblast, is traversed by a spiral valve, and surrounded by
the horse-shoe shaped body-cavity. _B_, Diagram indicating the position
of the Notochord in the vertebra of an adult Common Dog-fish (_Scyllium
canicula_). 1, "Neural arch" of the vertebra, consisting of processes of
bone enclosing the central nervous system, or spinal cord; 2, bony
centrum of the vertebra, hollowed out into a cup, in which lies a soft
pad, the remains of the notochord.]
The lowest member of the vertebrate group, separated in fact from the
true vertebrates and placed in a lower division all by itself, is the
little animal called the Lancelet or Amphioxus. It is often spoken of as
a "fish"; but it is only by a stretch of our courtesy that it can
receive that name, being an animal of a much lower form than the fishes.
It was discovered in 1834, in the Mediterranean, and described as a
fish; but it had previously been discovered in 1778, by a German
naturalist who described it as a slug. The latter was misled by its
external shape. He had not the advantage of the modern methods of
preparing animals for examination under the microscope; in these days,
Amphioxus is cut into successive slices along its whole length, and each
of these carefully magnified, so that no detail of structure is lost.
The Amphioxus burrows in the sea-sand; it lies buried in it, with its
mouth just uncovered. Its food consists of microscopic vegetable
organisms. Its distribution is very wide; it is found in both the
Atlantic and Pacific waters. It occurs most abundantly in the salt-water
lakes of Sicily, and in the Gulf of Naples. The specimen first seen, in
1778, came from the coast of Cornwall. There are eight species; the one
which is found in the English Channel is the _Amphioxus lanceolatum_,
also found in the Mediterranean and on the shores of North America.
The classes of the Vertebrata are Fishes, Amphibia, Reptiles, Birds and
Mammals. We used to learn that of these, fishes had gills, and Amphibia
gills for a time; but, to be strictly accurate, we must say that fishes
have gills, and _all_ the rest of the Vertebrata have gills for a
time. There is no exception to this rule, not even among the highest
vertebrates of all. But in those vertebrates which stand higher in the
scale of life than Amphibia, viz., Reptiles, Birds, and Mammals, these
gills are never brought into use. They only exist in the early embryo,
and afterwards disappear, giving rise by their modification to other
structures.
Strange to say, one of these structures is the ear. This takes its
origin from one of the gill-"clefts" or spaces. The Eustachian tube,
which communicates between the ear and the nose, is part of this cleft;
and the little bones which are inside the ear represent the bones
of that gill-cleft. For, in fishes, bones support each gill, and
are connected together to form a complex arrangement. In the higher
vertebrates, which possess gills only in the embryo, this gill-skeleton
is much modified, and persists as a bone, the hyoid bone supporting the
tongue.
The gills of vertebrates, arranged in successive pairs along the throat,
are "perforating gills"; that is to say, they consist essentially of
holes or spaces which pass right through the wall of the throat.
If we were to seek for a general character of the vertebrates, besides
those mentioned above, that they all possess a notochord and gills, we
might also find it in the character of the skin. Fishes, Reptiles, Birds
and Mammals, all agree in this, that they have a special clothing of the
skin--scales, feathers and fur, respectively. These three kinds of
structure, although so widely differing in appearance, are practically
formed all in the same way, viz., by alternate ingrowths and outgrowths
of the skin; the ingrowth forming the root of the scale, hair or
feather, and the outgrowth its projecting part. If these infoldings and
outgrowths of the skin could be straightened out into a plane surface,
the skin of a small vertebrate would cover an enormous area. The above
list excludes the Amphibia: in this class, it should be mentioned, the
scales have been lost, and are only found in one group.
The scales of Fishes were at one time proposed as a basis of
classification: large groups being characterized respectively by the
possession of plain rounded scales (cycloids), scales fringed at the
posterior end (ctenoid, or comb-like); placoid scales, consisting of
bony plates, and ganoid scales, large plates covered with shiny enamel.
These distinctions, however, were not found useful as a guide in
classification. The diagram shows the elaborate scales of the common
sole.
[Illustration: FIG. 42.--Scales of the Common Sole, highly magnified.]
Let us now consider some other creatures that resemble vertebrates in
some ways, and help to form the group of Chordata. Balanoglossus is one
of them, the Acorn-tongue Animal. This odd name is given to it on
account of a structure which is called (like the elephant's trunk) a
Proboscis; this may be compared with a tongue, so far as its use goes,
for it is thrust out to catch prey and again drawn in. It is oval in
shape, and therefore fancifully compared to an acorn. It is highly
sensitive, being richly supplied with nerves. The creature is to all
intents and purposes a kind of worm; and, like many of the higher
worms, it has a larva with bands of cilia. This larva, which is better
represented in some species than in others, was originally described
under the name of _Tornaeria_. It is considered to resemble, in some
degree, the larva of Echinoderms; on this hint, some zoologists have
sought to establish a connection between Vertebrates and Echinoderms,
and have been able to find other points of comparison besides the one
named. It remains to be seen whether this suggestion will lead to
further results. It may be added that the larva of Balanoglossus has
also been compared with that of Phoronis (p. 122), thus assuming a
relationship with the Polyzoa, and through them with the Brachiopoda. It
appears, therefore, that the subject of the possible relationships of
the Vertebrata is one of the greatest complexity. The last named theory,
however, has been adversely criticised by very high authority.
We have not, however, explained yet what is the claim of Balanoglossus
to be grouped with the Chordata. This consists in the fact that a
certain part associated with the interior of the proboscis has been
identified, from its structure, mode of origin, and relations with
the nerves, as a notochord. Balanoglossus also agrees with the true
vertebrates in possessing successive pairs of perforating gills (see p.
142), which are especially noticeable in the young animal. The presence
of this feature is important, in view of the fact that some authorities
have sought to throw doubt on the genuineness of the notochord of
Balanoglossus.
Balanoglossus is not without relations, some of which have been
recently discovered, while others have been known for some time,
although their affinities were not at first recognised. Among these
the most remarkable are sessile forms which have received the names
respectively of _Cephalodiscus_ and _Rhabdopleura_. Both produce buds
and form a colony, and in both a notochord has been distinguished. The
former was procured from the Straits of Magellan, while the latter makes
its dwelling-place in a nearer region, having been found off the
Shetland Islands, and off the Lofoden Islands. Cephalodiscus, which is a
very curious creature, receives its name from a disc placed at the head
end. The use of this structure is believed to be as follows. The units
of the colony live inside a common system of tubes, which they secrete;
each unit, when adult, is independent, and can move about inside the
tubes; the disc is used as a means of attachment to successive spots of
the tube-wall, as the animal wanders from place to place. Above the disc
are twelve plume-like tentacles covered with cilia, which create a
current in the water surrounding the head, and wash food particles into
the mouth.
That these creatures are but distant relations of the true vertebrates
is a fact expressed by the names under which they are grouped in
classification. Those forms which we have just described have received
the name of Hemichordata--that is to say, Chordata which have but half a
notochord, since the notochord is very restricted in extent; while the
Ascidians are grouped under the name of Urochordata, or Chordata which
only possess a notochord in the tail. The name of Adelochorda, "with an
obscure chord," is sometimes applied to the Hemichordata.
TABLE SHOWING THE CLASSIFICATION OF THE CHORDATA
{ HEMICHORDATA, BALANOGLOSSUS, &c.
=CHORDATA.= { UROCHORDATA, THE ASCIDIANS.
{ VERTEBRATA, THE BACK-BONED ANIMALS.
Let us return now to the Vertebrate. A character common to all the
groups of the Vertebrata is the possession of teeth. Readers of the
previous volumes of this series will recollect that, even among birds,
instances of the possession of teeth may be found among fossil forms,
although they are absent in the birds of the present day. In all the
other divisions of the Vertebrata, the presence of teeth is the rule,
their absence an exception so rare that we may easily note the chief
instances of it. Among Amphibia, there are Toads that have no teeth;
among Reptiles, the Tortoises and Turtles have none; among Mammals,
teeth are wanting in _Echidna_, the Spiny Ant-eater; and in the
Ant-eaters and the Whalebone Whales they are absent in the adult,
although present in early embryonic life.
The majority of people, if asked to give a definition of the meaning of
teeth, would reply that they are hard structures that grow in the jaw.
But this is an idea that requires very considerable modification from a
scientific point of view. In the first place, they are found in other
places besides the jaws; and in the second place, they are by rights
structures originally belonging to the skin. Both these important facts
must be illustrated by reference to the Fishes, which exhibit the
primitive types of teeth.
In fishes, not only are teeth found on the jawbone, but sometimes also
on other bones which border upon the cavity of the mouth; they are found
on the palatine bone, or roof-plate of the mouth, and, still more
strange, upon bones which belong to the "hyoid apparatus," or skeleton
of the gills (see above). The latter may form a set of throat-teeth,
which are used for grinders, while the jaw-teeth are used for biting.
Among the Carps, the jaw-teeth are reduced, and the fish depends upon
its throat-teeth only. In the Wrasses, one pair of the bones that bear
throat-teeth (the inferior pharyngeal bones) are fused, so as to form a
stronger apparatus: and from this circumstance, the group of Fishes to
which they belong has been given the name of Pharyngognathi, fishes
possessing throat-jaws. They have, however, biting teeth as well, in the
true jaws. The grinding teeth are apparently used for consuming the food
in a leisurely manner when once it has been taken into the mouth.
A curious circumstance in connection with these "throat-jaws" is, that
they produce musical sounds. Fishes have other means, however, of
producing a voice--usually by means of the swimming-bladder and muscles
in connection with it. Probably they are able, to some extent, to effect
communication with each other in this way.
It has already been stated that teeth, in their primitive form, are to
be regarded as skin-structures. Certain fish, which are looked upon as
ancestral types, have, dispersed throughout the skin, a number of bony
plates, or granules (placoid scales), more or less formidable, and
tipped with a hard enamel-like substance. Teeth are regarded as but a
special form of these. But if they are skin-structures, how come they in
the mouth and throat? Because the mouth and throat are lined by an
ingrowth from the external skin; the origin and growth of this is seen
in the embryo.
In the Mammalia the teeth, though restricted in number, attain the
greatest possible variety of form, so that the jaws of different but
allied species may be distinguished by their teeth.
Let us now return to the lowest vertebrate of all, which has a large
notochord and no bones. This is the _Amphioxus_, the Lancelet. Amphioxus
has no bones whatever, and no head, in the sense in which we usually
employ that term; that is to say, most of the structures which we see in
the vertebrate head are undeveloped. The peculiarities of the structure
of Amphioxus are many. Among them may be named the curious gills: these
form a sort of basket-work along the sides of the throat, which at first
sight bears little resemblance to the gills of fishes, and reminds us of
those of Ascidians. The gills lead also, as in Ascidians, to another
cavity, the Atrial chamber. This basket-work is formed, however, by the
subdivision of the primary pairs of gills. These are very numerous,
ninety pairs being sometimes named as the number. They cut up the wall
of the throat to such an extent, that additional supporting bars are
needed to strengthen it; and, by the formation of these, both in
parallel and in transverse directions to the primary partitions, the
"basket-work" is produced, as the growth of the animal proceeds.
The primitive nature of the notochord is, however, perhaps the most
striking feature of Amphioxus. The chord passes to the front of the
animal's snout--head it can hardly be called--instead of ending in the
middle of the brain, as in true vertebrates, for there is, indeed, no
"brain" of any extent to lie in front of it; and the notochord, together
with the spinal cord itself, have no other protection than a fibrous
sheath. The spinal column is thus entirely absent, except so far as it
may be regarded as represented by this thin sheath.
The Lancelet also differs from the true vertebrates, in that it has no
limbs. There is a fringing fin along the body, but it is not comparable
with the fins of fishes. It differs also in possessing no teeth.
In one respect, however, the Lancelet reminds us of a fish: and that is
in the arrangement of its muscles; these form a successive series of
overlapping masses on each side of the body, as in a fish.
The development of the Lancelet presents us with an instance of
the two-layered larva, or Gastrula. This shows that Amphioxus is a
comparatively primitive type. But it has been suspected that it is less
primitive than it looks, and that it has degenerated from some higher
form, owing to its preferring a dull mode of existence, half-buried in
sand or mud.
There is a huge gap between the Lancelet and the true vertebrates. The
lowest form of the latter is _Ammocoetes_, the larva of the Lamprey
(_Petromyzon_). The latter, even in the adult form, has no true limbs,
though there are fringing fins. The notochord sheath is supplemented,
however, by cartilage bars which are equivalent to the beginnings of the
vertebræ of the back-bone. The gills are very different from those of
other true vertebrates, and it has no jaws. Teeth it has, however, on
the tongue and the lining of the mouth. Probably this creature is
greatly altered by adaptation to its peculiar mode of life, so that no
certain conclusions can be drawn from it regarding the structure of
primitive fishes. It has a sucking mouth, by means of which it hangs on
to fishes, while it rasps away their flesh with its rough tongue. When
not thus engaged, it hangs on to a stone by means of its suctional
mouth, thus fixing itself at rest. The Hag-fish, _Myxine_, in many
respects similar, devours dead fishes chiefly. The Hag-fish is found on
English coasts: so is the Marine Lamprey; while two freshwater forms are
found in streams.
Leaving the Cyclostomata, as the above fishes are called, we reach the
true fishes, which have limbs and scales. Something has already been
said regarding their teeth and gills. The Cartilaginous fishes, in
which most part of the skeleton remains gristle and does not become
transformed into bone, include the Sharks, Rays, and Dog-fishes, all
savage animals with strong teeth. The common spotted Dog-fish of our own
shores is familiar to everybody: fishermen regard it with disgust, as it
is not eatable. The Rays are flattened fishes, which live at the bottom
of rather deep water, and attain enormous size even on our own coasts.
The Thornback Skate is covered with prickles (placoid scales). All these
fishes are grouped under the name of Elasmobranchii, the Strap-gilled,
so called from the structure of the gill-arches.
The majority of familiar fishes, such as the herring, mackerel, cod and
sole, belong to the group of _Teleostei_, or Bony Fishes, in which, by
contradistinction from the last group, as much of the skeleton as
possible becomes bone. Nevertheless, traces of the notochord persist in
the back-bone of these fishes. Break the back-bone across, of a cod or a
sole, and you will find between adjacent sides of the centra, or middle
parts of the vertebræ, a pad of gristly substance. This is the remaining
substance of the notochord, which finds room between the cup-shaped
sides of the centra. When the centrum, instead of being biconcave, is
solid, as in the higher Vertebrata, the notochord is obliterated by its
encroachment.
The Amphibia, familiarly represented by Frogs and Toads, receive their
name, "adapted for both lives," from the fact that they usually divide
their lives between land and water. They are, from one point of view,
the most interesting of the classes of the Vertebrata, for they form a
dividing line between the lower and upper Chordata. Below we have
Hemichordata, Ascidians, Amphioxus, Fishes; all water-dwellers,
breathing by gills. Above, we have Reptiles, Birds, Mammals,
air-breathers, never possessing gills, except for a short time, as
rudiments in the embryo, not brought into use. They are linked by the
Amphibia, in which we see the larva a water-dweller, breathing by gills;
the adult, an air-breather, adapted for life on land, and obliged to
come to the surface to breathe, even when it passes its time in the
water. The individual Amphibian tells us the past history of the higher
groups; once they had gills--but growing older, they lost them.
Fig. 43 shows us an outline sketch of Amphibian larvæ; we should require
an enlarged diagram of an earlier stage, to show the gills, which are
external and projecting at first, but afterwards are overgrown by the
skin with the exception of an orifice on each side. The diagram shows
the gradual change of form. The tails in these tadpoles will presently
be lost, for they belong to the Anura, or tail-less order of Amphibia
(Frogs and Toads). The tailed Amphibians, Urodela, are represented in
Great Britain by the Newts, _Triton_, popularly called Efts. Belonging
to the Tailed Amphibians also, is the Axolotl, a creature found in the
lakes of Mexico, and in those of the Rocky Mountains. It may or may not
retain its gills; and forms with gills, and forms without, may be found
in the same lake, each capable of laying eggs. The two forms were at
first described under two different generic names: but when specimens of
the gill-bearing _Siredon_, kept in confinement, lost their gills, it
was seen that they became _Amblystoma_. There are other cases of larval
forms that produce young, and this curious occurrence is known as
"pædogenesis."
[Illustration: FIG. 43.--Tadpoles, three-quarters of their natural size.
_A_ to _D_, different stages of the Tadpole of the Common Toad, from
Epping Forest, England. _E_, Tadpole of _Pelodytes punctatus_, dorsal
view.]
The Amphibia include the curious creatures called Cæciliæ (blind
animals), or Gymnophiona. They are snake-like in form, and are without
limbs; they burrow underground. Their real place in classification was
not found out at first, but they were classed, by a wrong shot, with the
Reptiles. They are interesting as being the only Amphibians that have
scales. These are very minute, embedded in the skin, and arranged in
transverse rings. The name Gymnophiona, naked serpents, is therefore
doubly inapplicable: for they are not serpents, and not scaleless.