The mesoglœa 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 amœboid 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.
A more detailed account of the anatomy of Hydra will be found in any biological text-book, for instance in Parker's Elementary Biology; but it is necessary here to say something more as regards the nettle-cells, which are of great biological and systematic importance.
A nettle-cell of the most perfect type and the structures necessary to it consist of the following parts:—
(1) A true cell (the cnidoblast), which contains—
(2) a delicate capsule full of liquid;
(3) a long thread coiled up in the capsule; and
(4) a cnidocil or sensory bristle, which projects from the external surface of the cnidoblast.
A nerve-cell is associated with each cnidoblast.