Cnidaria (pronounced /naɪˈdɛəriə/ with a silent c) is a phylum containing over 9,000 species of animals found exclusively in aquatic, mostly marine, environments. 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. Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess balance-sensing statocysts, and some have simple eyes. All cnidarians reproduce sexually. Many have complex lifecycles with asexual polyp stages and sexual medusae, but some omit either the polyp or the medusa stage.
Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: sessile Anthozoa (sea anemones, corals, sea pens); swimming Scyphozoa (jellyfish); Cubozoa (box jellies); and Hydrozoa, a diverse group that includes all the freshwater cnidarians as well as many marine forms, and has both sessile members such as Hydra and colonial swimmers such as the Portuguese Man o' War. Staurozoa have recently been recognised as a class in their own right rather than a sub-group of Scyphozoa, and there is debate about whether Myxozoa and Polypodiozoa are cnidarians or closer to bilaterians (more complex animals).
Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from endosymbiotic algae, and a few are parasites. Many are preyed upon by other animals including starfish, sea slugs, fish and turtles. Coral reefs, whose polyps are rich in endosymbiotic algae, support some of the world's most productive ecosystems, and protect vegetation in tidal zones and on shorelines from strong currents and tides. While corals are restricted to warm, shallow marine waters, other cnidarians live in the depths, in polar seas and in freshwater.
Fossil cnidarians have been found in rocks formed about 580 million years ago, and other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. Fossils of cnidarians that do not build mineralized structures are very rare. Scientists currently think that cnidarians, ctenophores and bilaterians are more closely related to calcareous sponges than these are to other sponges, and that anthozoans are the evolutionary "aunts" or "sisters" of other cnidarians, and the most closely related to bilaterians. Recent analyses have concluded that cnidarians, although considered more "primitive" than bilaterians, have a wider range of genes.
Jellyfish stings killed several hundred people in the 20th century, and cubozoans are particularly dangerous. On the other hand, some large jellyfish are considered a delicacy in eastern and southern Asia. Coral reefs have long been economically important as providers of fishing grounds, protectors of shore buildings against currents and tides, and more recently as centers of tourism. However, they are vulnerable to over-fishing, mining for construction materials, pollution, and damage caused by tourism.
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