an order of invertebrate animals of the class Hydrozoa, phylum Coelenterata. The body is cylindrical and up to 1 cm in length. There are about ten species; they live in bodies of fresh water, where they are often encountered in summer on aquatic plants. Hydrida attach themselves to the substrate by one end, which looks something like the flat sole of a shoe. The mouth, surrounded by tentacles (from four to 20) with stinging cells, is at the free end of the body.

Despite their sessile mode of life, Hydrida are capable of slow movement. They reproduce sexually and by budding; they usually bud in summer. The buds grow from the middle section of the body, the mouth and tentacles forming on the free end. Subsequently the buds break free from the maternal body. Some Hydrida are dioecious and others are hermaphroditic. Fertilization of the egg cells occurs within the body of the mother. A large number of Hydrida die toward autumn, but the fertilized egg cells, surrounded by a tough membrane, remain in a dormant state until spring, when fully formed young Hydrida emerge.

Species of three genera of Hydrida are found in the USSR. Hydrida have an exceptionally high capacity for regeneration.


Rukovodstvo po zoologii, vol. 1. Edited by L. A. Zenkevich. Moscow-Leningrad, 1937.
Kanaev, I. I. Gidra. Moscow-Leningrad, 1952.
Zhizn’ zhivotnykh, vol. 1. Moscow, 1968.