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There are more than 10000 species of cnidarians inhabiting exclusively aquatic environments. Cnidae, organelle-like capsules with eversible tubules, are the diagnostic feature of the phylum. All cnidarians possess cnidae; no loss of the feature has been documented. In marine ecosystems cnidarians inhabit both the shallow and deep sea.


Phyllum Cnidaria comprises two monophyletic clades: Anthozoa and Medusozoa. The distinction between clades is well-supported by anatomy and life history, genome structure and DNA sequences. The taxonomic structure of the phyllum Cnidaria mirrors this phylogeny: the class Anthozoa comprises all members of the clade Anthozoa; its sister taxon, Medusozoa, encompass the remaining three classes: Hydrozoa, Scyphozoa and Cubozoa. Only three classes: Anthozoa, Hydrozoa and Scyphozoa are encountered in CCFZ.

Basic anatomy

Cnidarians are diploblastic radially symmetrical animals possessing highly specialized stinging cells or cnidocytes. Members of the phylum have only one orifice, the oral opening, that lead to the gastric cavity. Mouth is often surrended by tentacles. Two life forms of cnidarians are known: benthic polyps and pelagic medusae. Many of cnidarians are colonial.


  • Daly, M., Brugler, M. R., Cartwright, P., Collins, A. G., Dawson, M. N., France, S. C., McFadden, C. S., Opresko, D. M., Rodriguez, E., Romano, S., & Stake, J. 2007. The phylum Cnidaria: A review of phylogenetic patterns and diversity 300 years after Linnaeus. Zootaxa 1668: 127-182 [1]