Cnidaria

Article By:

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

DOI:https://doi.org/10.1036/1097-8542.145900

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• Cnidaria, published June 2014:Download PDF Get Adobe Acrobat Reader

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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