Calder, Dale R. Department of Natural History, Royal Ontario Museum, Ontario, Canada.
- Defining characteristics
- Reproduction and development
- Fossil record and evolution
- Links to Primary Literature
- Additional Readings
A class of the aquatic animal phylum Cnidaria, comprising hydroids and hydropolyps (for example, Hydra), veiled and mostly small jellyfishes or hydromedusae (for example, freshwater jellyfish, Craspedacusta), and siphonophores (for example, Portuguese man-of-war, Physalia). Hydrozoans occur as polypoids or medusoids, or both, with life cycles of a given species often including an alternation between these two stages. Polypoids are usually bottom-dwelling, sedentary, colonial, and polymorphic (as in most hydroids), whereas medusoids are usually planktonic and solitary (as in most hydromedusae). Siphonophores are colonial, highly polymorphic, predominantly planktonic hydrozoans with both polypoid and medusoid components. Most hydroids, hydropolyps, and hydromedusae, and all siphonophores, are marine. Approximately 3800 species are currently recognized worldwide. See also: Cnidaria
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