hydra [key], common name for freshwater organisms in the phylum Cnidaria, which includes jellyfish, sea anemones, and corals. Hydras are widely distributed in lakes, ponds, and sluggish streams. They are small, cylindrical, solitary organisms, the largest reaching about 1 in. (2.5 cm) in length. They attach themselves temporarily to water plants or submerged objects, using an adhesive pedal disk at the anal end. The simple body consists of an outer layer of epidermis, a middle noncellular layer of mesoglea, and an inner layer of gastrodermis lining the simple gastrovascular cavity, where the final stages of food digestion occur. A whorl of tentacles surrounds the mouth. Hydras feed on small plankton organisms, especially microcrustaceans, stunning them with stinging cells (nematocysts) in the tentacles. Hydras reproduce asexually by budding and sexually by means of gonadal cells formed on the sides of the body. The green hydra, Chlorhydra viridissima, contains green algae living symbiotically in its gastrodermal cells. The gray and brown hydras belong to the genus Pelmatohydra. Several species of hydra also occur in American waters. Hydras are classified in the phylum Cnidaria, class Hydrazoa.
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