Cnidarians

Cnidaria Temporal range: 580–0 Ma PreЄ Є O S D C P T J K Pg N Ediacaran–Recent
Pacific sea nettles, Chrysaora fuscescens
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
(unranked):	Radiata
Phylum:	Cnidaria Hatschek, 1888
Subphyla and classes
Class Anthozoa - inc. corals Ceriantharia Hexacorallia Subphylum Medusozoa—jellyfish: Cubozoa—box jellyfish, sea wasps Hydrozoa—hydroids, hydra-like animals Scyphozoa—true jellyfish Staurozoa—stalked jellyfish Polypodiozoa—parasites Subphylum Myxozoa—parasites Malacosporea Myxosporea

Cnidaria (/naɪˈdɛəriə/) is a phylum containing over 10,000species of animals found exclusively in aquatic (freshwater and marine) environments: they are predominantly marine species. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both (hence they are trimorphic). Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing , and some have simple eyes. Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae. Some, however, omit either the polyp or the medusa stage.