THE COELENTERATA
Next after the animals that consist of one cell only we have to consider
the group of animals among which the lower kinds, at any rate, consist
of a number of cells arranged in two layers. The representative of this
group that the reader is most likely to meet with is the Sea-Anemone,
the Coral animal probably he will be content to know from pictures.
Everybody who has been accustomed to take a little interest in natural
history, remembers the use of the old-fashioned term "Zoophyte." It was
a name given to animals like those named above, which have a flower-like
appearance, due to the possession of a set of petal-like arms or
tentacles, placed all round the mouth; its literal meaning was animal
plant, in allusion to the flower-like form. The great French zoologist,
Cuvier, gave the group name Radiata to animals of this kind. This name
is now not much used, because we have learnt to emphasize other
peculiarities possessed by these animals, as well as that of radial
symmetry, viz., their two-layered body-wall and simple digestive space
(see p. 36). The group called Radiata by Cuvier, included, too, a number
of animals which are widely separated from the "Zoophytes" in modern
systems of classification.
Sea-Anemones may be found on almost every rocky part of the English
shores. Look for them in pools towards low-tide mark; if uncovered by
the water, they will be found with the arms drawn in, so that the animal
looks merely like a small round knob of shiny opaque coloured jelly; if
covered by the water, they will usually be found open, that is to say,
with the arms (often called Tentacles) spread out. In the middle of the
circle of arms is the mouth; and the apparent "flower" possesses an
excellent appetite, as will readily be seen if any unfortunate little
shrimp or sea-snail should come within reach of the arms. The latter
will then at once contract upon it, and draw it into the mouth. Touch
any of the common Sea-Anemones, and you will find that it is firmly
fixed to the rock; at an early period of life it becomes fixed, and
practically it remains always in one place, although a slight movement
of the base is sometimes possible. Hence the advantage of the "radial"
structure, for the arms reach equally in all directions round that most
important centre of activity, the mouth. The most common kind of
Sea-Anemone is of a dull dark red colour, and small in size; but others
are large and brilliant in colouring. No uncoloured drawing would convey
much idea of their beauty: the reader should consult the works of the
late P. Gosse, an authority on Sea-Anemones, in whose books many
beautiful illustrations will be found.
A much smaller animal than the Sea-Anemone is found in fresh water and
is called _Hydra_. Its arms or tentacles are longer in proportion to its
body, especially in one species, than is the case in the Sea-Anemones.
Hence its name, fancifully derived from the seven-headed serpent of
Greek Mythology, the Hydra killed by Hercules, which may be supposed to
have presented a similar straggling appearance. The diagram on page 36
represents a section through the middle of the body, only without the
arms.
Unlike the Sea-Anemone, the Hydra can walk about. This it does in a very
awkward manner, much in the same way as the Caterpillar known as the
"Looper," clinging first with the front and then with the back extremity
of the body (for head and tail they can hardly be called in so simple an
animal as the _Hydra_, although the Looper caterpillar boasts both head
and tail).
The _Hydra_ is so small an animal that it appears to the unaided eye
merely as a tiny speck. It may be found anywhere in British ponds and
ditches, standing on water-weeds. Like the Sea-Anemone it preys on
animals smaller than itself. Nature has provided it with minute stinging
cells, which benumb its prey; and in this all the animals of the
Coelenterate group resemble it.
One of the most curious things about the Hydra is, that it often throws
out buds. It can, of course, produce eggs which are fertilized and
hatched in the usual way of eggs; the buds are an additional way of
multiplying itself.[C]
[C] We may recall in comparison the way trees may be propagated by slips
independently of flowers producing the seeds of the trees.
These buds are at first merely swellings, in which both of the layers
of the body join: they grow larger; become provided with tentacles and
a mouth, like the parent, and finally are cast off as independent
animals.
For this reason the group to which _Hydra_ belongs has received the name
of Eleutheroblasteæ, the animals with free buds. But Hydra has many near
relations in which these buds are not so cast off, but remain attached
to the parent; and they in turn may produce others which also remain
attached.
In this way, groups or colonies are formed, consisting of large numbers
of individuals, and possessing a common stalk or stock which is formed
by degrees as the process of multiplication goes on. The corals and the
corallines are familiar examples of this.
The matter is complicated by the fact that either the separate animals
or the flesh of the stock, or both, may secrete within themselves a hard
supporting structure forming what is known as Corals. This may be
developed in such a complicated manner, that instead of the coral
appearing to be the product of the animal, the animal seems to be
inserted in the coral, into which indeed it can retract itself for
shelter.
The Corallines, on the contrary, secrete a leathery coating or sheath
outside themselves and the stock. The leathery case is fairly
transparent, so that on magnifying the creature the flesh of the common
stock, as well as of the stalks of individual animals, may be seen
inside. The "heads" of the animals poke out at the end of each branch
(see Fig. 9).
The _Hydra_, with which we started, had always the power of producing
eggs; each animal could do so, besides producing buds. But in our
Colonial Coralline this is not necessarily so. Some individuals lose the
power of producing eggs. Others can do nothing else, and become greatly
altered in structure, often losing the power of developing tentacles,
and exhibiting other changes. So much are they altered sometimes that
they seem to be mere buds, not separate animals at all.
In other cases a still more surprising thing happens. The bud that is
destined to produce eggs falls off, and becomes quite independent of the
colony; more than this, it becomes quite different in appearance from
the members of the colony: and instead of being a Hydra-like animal it
becomes a jelly-fish. But the eggs of this jelly-fish do not produce
jelly-fishes: they produce a more or less Hydra-like animal which gives
rise by budding to a fresh colony. This is what is known to Zoologists
as "alternation of generations."
Now comes a puzzling question--Which part of this family group shall we
select and call it an "animal"? Is each Hydroid of the colony an animal,
and the jelly-fish another animal? Zoologists say "No": from the
development of one egg, to the production of another, is the cycle that
constitutes an individual animal. So we have the puzzling result in
nomenclature, that an "individual" consists of a very large colony of
creatures in one place, together with a perfect shoal of creatures quite
unlike it, floating miles away from it on the ocean. What name must we
give to the units, so curiously connected with one another? Zoologists
call them "Zooids" (animal-like parts) or "persons."
This is the story of the jelly-fish as originally told. But there are
innumerable variations upon it. There are kinds of jelly-fish that
produce jelly-fish and have no Hydroid stage at all. Sometimes the
"persons" of the colony present many varieties, each taking up some
different task for the community. Some may be "nutritive persons,"
_i.e._ commonplace Zooids that have mouths and eat food; some
"protective persons," reduced to mere folds or sheathing processes to
guard the others; some are "stinging persons" armed with enormous
quantities of thread cells. Then the whole colony may be like the
jelly-fish, a floating affair, and not fixed at all.
[Illustration: FIG. 9.--An example of the Hydrozoa. A, branch of a
Coralline, _Sertularia Ellisii_, magnified. B, the same, more highly
magnified.]
We have several times above referred to the animals known as corallines.
It may almost be assumed that the ordinary reader knows what these are;
if not, a little search among the treasures of the sea-shore will almost
certainly reveal some of them, living or dead. The texture and
appearance of the dead stems remind one of soft horn or dried gelatine;
the branching arrangement of the stems and the little cells disposed at
the ends of the branches will easily be shown under slight
magnification. Most people will remember the rage for dyed corallines,
by which all the fancy shops and florists were possessed a few years
ago. The corallines, dyed a bright emerald green, or a dull red, which
were used for decorations at that time, were usually a variety of the
Bottle-brush Coralline, found on English shores; but sometimes commoner
kinds were employed.
Fig. 9 shows an example of a coralline, slightly magnified in A, and in
B much more highly magnified, so as to show the individual hydra-like
zooids, each with its circle of tentacula.
The Sea-Anemone and the Hydra respectively represent the two great
groups of the Coelenterata, named after them, the Anthozoa
(Flower-animals), and the Hydrozoa (Hydra-animals). The corals are
forms of the Anthozoa, single or colonial, which possess a skeleton.
[Illustration: FIG. 10.--_Gorgonia verrucosa_, from Guernsey, nearly
one-third of the natural size.]
[Illustration: FIG. 11.--Corals. _A_, _Acanthoporia horrida_.
_B_, _Meandrina strigosa_. _C_, _Madrepora divaricata_. _D_, _Fungia
papillosa_. _E_, Red Coral, _Corallium rubrum_. _F_, _Stylaster
sanguineus_.]
The above diagram shows examples of the Anthozoa. Fig. 10 is _Gorgonia_,
the Sea-Fan; while Fig. 11 represents corals of six different kinds.
Besides the two great groups we have named, the Hydra-like animals
and the Sea-Anemone-like animals, the Coelenterata contain a third
group, the Ctenophora, or Comb-bearers, so called on account of their
possessing bands of cilia, fancifully compared to the teeth of a comb.
At first sight most of them somewhat resemble jelly-fishes, being
transparent forms swimming near the surface of the sea. They are
carnivorous, and some of them highly phosphorescent at night. The
gastric cavity is divided up into branches. The representatives of the
Ctenophores, most often seen on our own coasts, are small rounded forms.
Two remarks must be added before quitting the subject of the
Coelenterata.
Firstly, the description of them as two-layered Animals is one that only
applies typically and to the simpler forms. In others, such as the
jelly-fishes, there is an intermediate layer of jelly, which appears to
acquire a cellular structure by the immigration of cells derived from
the primary layers. Thus we see, within the group of the Coelenterata,
the gradual establishment of that third body-layer, which is found in
all animals of higher structure. Scarcely indicated in _Hydra_, as a
faint trace of a boundary-line (lamella) between the ectoderm and
endoderm, it attains a good thickness in the Jelly-fish and Ctenophora.
In animals of higher structure the third body-layer, being now fully
established, is cellular from its beginning in the embryo; in the
Coelenterata its gradual formation is to be traced.
Secondly, it must be remarked that the colonial structure and the
arrangement sometimes concomitant with it of "alternation of
generations," is by no means confined to the Coelenterata. Both are seen
in other forms of life, in which the units, or zooids, differ greatly in
structure from those of this group.
TABLE SHOWING THE CLASSIFICATION OF THE COELENTERATA
{ HYDROZOA, or
{ HYDRA-LIKE
{ ANIMALS.
=Grade II.= {
{
THE TWO-LAYERED =COELENTERATA.= { ACTINOZOA, or
ANIMALS. { SEA-ANEMONE-LIKE
{ ANIMALS.
{
{
{ CTENOPHORA.