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A class of the phylum Coelenterata which includes the fresh-water hydras, the marine hydroids, many of the smaller jellyfish, a few special corals, and the Portuguese man-of-war. The Hydrozoa may be divided into six orders: the Hydroida, Milleporina, Stylasterina, Trachylina, Siphonophora, and Spongiomorphida. See separate article on each order.

The form of the body varies greatly among the hydrozoans. This diversity is due in part to the existence of two body types, the polyp and the medusa. A specimen may be a polyp, a medusa, a colony of polyps, or even a composite of the first two. Polyps are somewhat cylindrical, attached at one end, and have a mouth surrounded by tentacles at the free end. Medusae are free-swimming jellyfish with tentacles around the margin of the discoidal body.

In a representative life cycle, the fertilized egg develops into a swimming larva which soon attaches itself and transforms into a polyp. The polyp develops stolons (which fasten to substrates), stems, and other polyps to make up a colony of interconnected polyps. Medusae are produced by budding and liberated to feed, grow, and produce eggs and sperm.

Most hydrozoans are carnivorous and capture animals which come in contact with their tentacles. The prey is immobilized by poison injected by stinging capsules, the nematocysts. Most animals of appropriate size can be captured, but small crustaceans are probably the most common food. See Coelenterata



a class of aquatic invertebrates of the phylum Coelenterata. Alternation of generations is characteristic of the majority of Hydrozoa. Polyps alternate with medusae, the sexual generation.

In the majority of Hydrozoa the asexual generation forms colonies consisting of many individuals. The colony is attached by its base to a solid substratum. The vertically rising stem branches out, and individual members of the colony, hydranths, are situated on its branches. Each individual’s mouth is surrounded by long tentacles. Limy skeletons are secreted by some Hydrozoa; a large accumulation of such Hydrozoa forms lime reefs. A colony forms as the result of budding. In contrast to hydras, new individuals of the colonial forms of Hydrozoa, which develop from buds, do not become separated from the common stem but remain on it. Some buds develop into medusae that produce sexual products. Many Hydrozoa have medusae that separate from the colony and lead a free-swimming existence; they are dioecious. Their fertilized eggs develop into planulae, larvae characteristic of all Coelenterata. Among the Hydrozoa, however, many species are known to have medusae that remain incompletely developed and do not become separated from the colony but still produce sex cells. There are some Hydrozoa that have only medusae; their larvae develop directly into new medusae. All Hydrozoa feed on animal food, including planktonic crustaceans, aquatic insect larvae, and even fish fry, by catching them with their tentacles. Some medusae may also be dangerous to man by causing fairly strong burns (for example, Gonionemus).

There are seven orders of Hydrozoa: Hydrida, Leptolida, Limnomedusae, Trachymedusae, Nareomedusae, Disconantae, and Siphonophora. More than 2,500 species are known. Hydrozoa are usually found in the seas. The exceptions include the hydra, which lives in fresh water; some medusae that are found in the lakes of Africa and the rivers of North America, Europe, and Asia; and the colonial hydrozoan Moerisia pallasi, which is found in the Caspian Sea and has penetrated into some rivers. More than 300 species are found in the USSR. The majority of Hydrozoa live in the littoral zone, and only a few are deep-sea forms (for example, Bran-chiocerianthus, which is found in the Pacific Ocean and attains a height of 1 m). Hydrozoa from as early as the Cretaceous period have been found in fossil form, but there are indications that hydromedusae have been located even in Lower Cambrian deposits.


Rukovodstvo po zoologii, vol. 1. Moscow-Leningrad, 1937.
Naumov, D. V. Gidroidy i gidromeduzy morskikh, solonovatovodnykh ipresnovodnykh basseinov SSSR. Moscow-Leningrad, 1960.
Zhizn’ zhivotnykh, vol. 1. Edited by L. A. Zenkevich. Moscow, 1968.



(invertebrate zoology)
A class of the phylum Cnidaria which includes the fresh-water hydras, the marine hydroids, many small jellyfish, a few corals, and the Portuguese man-of-war.
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