[Footnote AK: Similar capsules are found in the tissues of certain worms and molluscs, but there is the strongest evidence that these animals, which habitually devour coelenterates, are able to swallow the capsules uninjured and to use them as weapons of defence (see Martin, Q. J. Micro. Sci. London, lii, p. 261, 1908, and Grosvenor, Proc. Roy. Soc. London, lxxii, p. 462, 1903). The "trichocysts" of certain protozoa bear a certain resemblance to the nettle-cells of coelenterates and probably have similar functions.]
The simplest in structure of the coelenterates are those that constitute the class Hydrozoa. In this class the primitive central cavity is not divided up by muscular partitions and there is no folding in of the anterior part of the body to form an oesophagus or stomatodæum such as is found in the sea-anemones and coral polyps. In many species and genera the life-history is complex, illustrating what is called the alternation of generations. That is to say, only alternate generations attain sexual maturity, those that do so being produced as buds from a sexless generation, which itself arises from the fertilized eggs of a previous sexual generation. The sexual forms as a rule differ considerably in structure from the sexless ones; many medusæ are the sexual individuals in a life-cycle in which those of the sexless generation are sedentary.
An excellent general account of the coelenterates will be found in the Cambridge Natural History, vol. i (by Prof. Hickson).
_Hydra_, the freshwater polyp, is one of the simplest of the Hydrozoa both as regards structure and as regards life-history. Indeed, it differs little as regards structure from the ideally simple coelenterate sketched in a former paragraph, while its descent is direct from one polyp to another, every generation laying its own eggs[AL]. The animal may be described as consisting of the following parts:--(1) an upright (or potentially upright) column or body, (2) a circle of contractile tentacles at the upper extremity of the column, (3) an oral disk or peristome surrounding the mouth and surrounded by the tentacles, and (4) a basal or aboral disk at the opposite extremity. The whole animal is soft and naked. The column, when the animal is at rest, is almost cylindrical in some forms but in others has the basal part distinctly narrower than the upper part. It is highly contractile and when contracted sometimes assumes an annulate appearance; but as a rule the external surface is smooth.