An excellent general account of the cœlenterates will be found in the Cambridge Natural History, vol. i (by Prof. Hickson).
STRUCTURE OF HYDRA.
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 cœlenterate 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.
The tentacles vary in number, but are never very numerous. They are disposed in a single circle round the oral disk and are hollow, each containing a prolongation of the central cavity of the column. Like the column but to an even greater degree they are contractile, and in some forms they are capable of great elongation. They cannot seize any object between them, but are able to move in all directions.
The disk that surrounds the mouth, which is a circular aperture, is narrow and can to some extent assume the form of a conical proboscis, although this feature is never so marked as it is in some hydroids. The basal disk is even narrower and is not splayed out round the edges.
Fig. 27.—Nettle-cells of Hydra.
A=capsules from nettle-cells of a single specimen of the summer phase of H. vulgaris from Calcutta, × 480: figures marked with a dash represent capsules with barbed threads. B=a capsule with the thread discharged, from the same specimen, × 480. C=capsule with barbed thread, from a specimen of H. oligactis from Lahore. D=undischarged nettle-cell of H. vulgaris from Europe (after Nussbaum, highly magnified). E=discharged capsule of the same (after the same author). a=cnidoblast; b=capsule; c=thread; d=cnidocil. Only the base of the thread is shown in E.
A section through the body-wall shows it to consist of the three typical layers of the cœlenterates, viz., (i) an outer cellular layer of comparatively small cells, the ectoderm; (ii) an intermediate, structureless or apparently structureless layer, the mesoglœa or "central jelly"; and (iii) an internal layer or endoderm consisting of relatively large cells. The cells of the ectoderm are not homogeneous. Some of them possess at their base narrow and highly contractile prolongations that exercise the functions of muscles. Others are gland-cells and secrete mucus; others have round their margins delicate ramifying prolongations and act as nerve-cells. Sense-cells, each of which bears on its external surface a minute projecting bristle, are found in connection with the nerve-cells, and also nettle-cells of more than one type.