In zoology, any of a group of freshwater polyps, belonging among the 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 paralysed by stinging cells on the tentacles. (Genus Hydra, family Hydridae, phylum Coelenterata, subphylum Cnidaria.)

Hydras reproduce asexually in the summer and sexually in the winter. They have no specialized organs except those of reproduction.