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]

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.

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.

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 CŒLENTERATA