e, inner cell layer.
m, middle jelly-like layer.
z, outer cell layer.
a, digestive cavity.
i, perforations in the wall.
The other great group of primitive multicellular animals is that of the Cœlenterata, and as an example of the most primitive of these we may take the common freshwater Hydra. The Hydra reaches a length of nearly half an inch, and is to be found attached to water-weed and the like in streams. It consists of a hollow tube-shaped body which is fixed by the so-called 'foot.' Two layers of cells form the wall of this tube, these being separated by a thin membrane of gelatinous material. At the upper end is the mouth, which leads immediately into the internal cavity or stomach. The mouth is surrounded by a ring of from six to eight tentacles, which are outgrowths of both cell layers. The cells of the inner layer are large, and bear cilia that protrude into the internal cavity. Their functions are those of digestion and absorption. Part of the protoplasm of the outer cells is modified into a fibrous, contractile substance, which represents the beginnings of muscle tissue. The outer layer also forms a protective skin-like covering. In the outer layer also occur a large number of stinging cells, each of which has a complex mechanism for injecting a fluid poison into any creature which should happen to come in contact with them. These 'nettle cells' occur in much greater numbers in the tentacles than elsewhere, and here they are brought into play against the animals, such as minute Crustaceans, which form the Hydra's prey. Coming in contact with the tentacles, such creatures are caught, paralysed by means of the stinging cells, and are gradually transferred into the mouth by a slow contraction of the tentacles. The Hydra reproduces, for the most part, by a simple process of budding. Small lateral outgrowths are formed, which gradually develop mouth and tentacles of their own. Ultimately these separate and are carried off by the water, later to settle down and become attached to some fixed object. Sometimes, however, sexual reproduction occurs. The reproductive cells are produced, male and female on the same individual, among the ordinary cells of the outer layer. These are set free, fertilisation occurs in the water, and the egg develops in the same manner as that of the coral. The Hydra is able, by means of the fibrous protoplasm of its outer cells, to show well-marked movements. It can bend its body in this direction or that, can contract its whole body into a small oval mass, and is even able, by performing a number of slow somersaults, to change its position. The structure and the methods of reproduction in Hydra will be readily understood from the illustrations of the creature on Figs. 33 and 34.
Fig. 33.—Specimens of Hydra on green water-weed.
A, Contracted; B, extended; C, specimen with vegetative buds; D, specimen with sex cells; sp, sperm cells; e, egg.
Fig. 34.—Diagrammatic section of Hydra.
en, Inner cell layer; ec, outer cell layer; c, nettle cell.
If now we make a brief general survey of the group to which the Hydra belongs, we find in it two somewhat strikingly different types. On the one hand are sedentary forms that resemble, in a general way, the Hydra; that consist of a tube-shaped body, with the mouth, surrounded by a ring of tentacles, at the upper end. The sea-anemones and corals are examples of this type, in which, however, the structure shows various complexities as compared with that of the Hydra, which complexities we cannot here pause to describe. On the other hand is the well-known Medusa form, of which the common jelly-fish is a typical example. This creature, as is well known, is mushroom shaped, with tentacles round the edge. The mouth is in the middle of the lower aspect, at the end of a short 'stalk.' This type is very different in general appearance from the Hydra or sea-anemone, yet the one form may be somewhat easily derived from the other; we have only to imagine that a Hydra is turned upside down, that it is squashed, vertically, until the internal cavity is greatly reduced, and the circumference, especially in the region of the tentacles, greatly increased, and we should have something resembling a Medusa. That the two types are actually closely related is shown by the fact that there is in the life-history of one group of Cœlenterates a regular alternation between the one and the other.
