Hydra. 1 A "water-snake," in Greek mythology a monster which dwelt near Lernee in Argos. It had nine heads, and when it was attacked by Hercules two new ones grew in the place of each which he cut off, but he at last destroyed it by burning the heads instead of cutting them.
2. The common fresh-water Polype, the type of the family Hydrides and order Hydroida. It is a small animal, living in ponds and ditches, attached by its base to the stems and leaves of water-plants. It consists of a small tube formed of two layers, an endoderm and ectoderm (q.v.), between which occurs the slightly-developed mesoglcea
(q.v.); the tube is closed at the base. At the other end it is constructed to form the mouth, which opens on a raised area, or peristome, surrounded by a circle of hollow tentacles. The Hydra catches small organisms by its tentacles: the mouth opens into a cavity which is not separated from the general body cavity, the food is digested here and absorbed by the cells of the endoderm; there is no separate digestive cavity as in higher animals.
It is hermaphrodite. It has long been believed that the Hydra has such a simple organisation that it. can be turned inside out without injury, and its stomach will then act as the skin and vice versa; it was also believed that the animal could also be reproduced indefinitely by cutting it into pieces, each of which was asserted to grow into a fresh individual. Recent attempts to confirm these statements have been unsuccessful, though they have proved the Hydra to possess enormous powers of recovery from injury and reproductions of lost parts. Inversion was, however, always fatal, unless the animal could manage to wriggle back to its normal arrangement.