ETYM Latin hydra, Greek jydra; akin to hydro water. Related to Otter the animal, Water.
Small tubular solitary freshwater hydrozoan polyp.
In zoology, any member of the family Hydridae, or freshwater polyps, of the phylum Cnidaria (coelenterates). The body is a double-layered tube (with six to ten hollow tentacles around the mouth), 1.25 cm/0.5 in long when extended, but capable of contracting to a small knob. Usually fixed to waterweed, hydras feed on minute animals that are caught and paralyzed by stinging cells on the tentacles.
Hydras reproduce asexually in the summer and sexually in the winter. They have no specialized organs except those of reproduction.

(Greek mythology) Monster with nine heads; when struck off each head was replaced by two new ones.
Many-headed monster in Greek mythology; small fresh-water polyp. In Greek mythology, a huge monster with nine heads. If one were cut off, two would grow in its place. One of the 12 labors of Heracles was to kill it.

In astronomy, the largest constellation, winding across more than a quarter of the sky between Cancer and Libra in the southern hemisphere. Hydra is named for the multiheaded monster slain by Heracles. Despite its size, it is not prominent; its brightest star is second-magnitude Alphard.
A long faint constellation near the equator stretching between Virgo and Cancer; Also called: Snake.