The Hydrozoa are defined as Coelenterata in which the walls 0/ the digestive sac are not separated from that of the general body-cavity, the two coinciding with one another; the reproductive organs are in the form of external processes of the body-wall. (Fig. 37, B.)

Fig. 37.   A, The common Hydra (Hydra vulgaris), carrying young Hydrae which it has produced by budding, considerably magnified (after Hincks). B, Diagrammatic section of the Hydra, showing the mouth surrounded by the tentacles, and the disc of attachment; the dark and light lines indicate the two layers of the integument, and on one side of the body is shown a single large egg.

Fig. 37. - A, The common Hydra (Hydra vulgaris), carrying young Hydrae which it has produced by budding, considerably magnified (after Hincks). B, Diagrammatic section of the Hydra, showing the mouth surrounded by the tentacles, and the disc of attachment; the dark and light lines indicate the two layers of the integument, and on one side of the body is shown a single large egg.

It follows from the above, that, since there is but a single internal cavity, the body of a Hydrozoon on transverse section appears as a single tube, the walls of which are formed by the limits of the combined digestive and somatic cavity.

The Hydrozoa are all aquatic, and the great majority are marine. The class includes both simple and composite organisms, the most familiar examples being the common Fresh-water Polype (Hydra), the Sea - firs (Sertularida), the Jelly - fishes (Medusae), and the Portuguese man-of-war (Physalia).

Owing to the great difficulty which is ordinarily experienced by the student in mastering the details of this class of animals, it has been thought advisable to introduce here a short explanation of some of the technical terms which are in more general use in describing these organisms.