Fautin, Daphne G. Department of Ecology and Evolutionary Biology, University of Kansas, Lawrence, Kansas.
Stevens, Calvin H. Department of Geology, San Jose State University, San Jose, California.
Last reviewed:November 2019
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- General morphology
- Spirocysts and ptychocysts
- Functional morphology
- Nerves and sensory structures
- Feeding, digestion, and nutrition
- Reproduction and development
- Sexual reproduction
- Asexual reproduction
- Fossils and phylogeny
- Related Primary Literature
- Additional Reading
A diverse phylum of aquatic invertebrate animals, including jellyfish, hydroids, sea anemones, and corals. Members of the phylum Cnidaria (Fig. 1), termed cnidarians, are characterized by the possession of (1) intrinsic nematocysts (stinging capsules), (2) a single body space that opens to the external medium, (3) radial or biradial symmetry, (4) a tissue level of organization, and (5) diploblastic structure. With few exceptions, cnidarians also have tentacles surrounding the mouth (the opening of the body cavity to the exterior). Most definitions of the phylum include the feature of metagenesis (the alternation between medusa and polyp generations); however, although the life cycle of many cnidarians involves this alternation between an asexually reproducing polyp and a sexually reproducing medusa, this is not true of two entire classes (Anthozoa and Staurozoa) and of some members of the class Hydrozoa. The name Coelenterata, which has been used for this phylum, is being supplanted because, as sometimes used, it included the sponges (Porifera) and comb jellies (Ctenophora), as well as the animals now called cnidarians. See also: Ctenophora; Porifera
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