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. [Illustration: 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 coelenterates, viz., (i) an outer cellular layer of comparatively small cells, the ectoderm; (ii) an intermediate, structureless or apparently structureless layer, the mesogloea 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. The mesogloea is very thin. The endoderm consists mainly of comparatively large cells with polygonal bases which can be seen from the external surface of the column in colourless individuals. Their inner surface is amoeboid and in certain conditions bears one or more vibratile cilia or protoplasmic lashes. Nettle-cells are occasionally found in the endoderm, but apparently do not originate in this layer. The walls of the tentacles do not differ in general structure from those of the column, but the cells of the endoderm are smaller and the nematocysts of the ectoderm more numerous, and there are other minor differences.