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Description of AnthozoaThe Anthozoa is that group of coelenterates (Cnidaria) that contains the sea anemones and corals. Typically they are attached to a substrate, have a columnar body with an opening at the top, and arms radiate around the opening. The name means flower animals, a reference to the floral appearance of their perennial polyp stage. Food, mostly small plankton but some species can catch fish, is captured by nematocysts on the arm, and pushed through the opening to be digested inside the body. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows. Some anthozoans can also reproduce asexually through budding. The gastrovascular cavity that the mouth opens into is subdivided by a number of radiating partitions, or mesenteries. The gonads are also located within the cavity walls. Some species harbour photosynthetic dinoflagellates or zooxanthellae, in a symbiotic relationship; the reef building corals known as hermatypic corals rely on this symbiotic relationship particularly. The zooxanthellae benefit by having access to nitrogenous waste and carbon dioxide produced by the host or , and the cnidarian gains products of photosynthesis and an increased to deposit calcium carbonate. The association created coral reef systems, entirely new and rich ecosystems, the evolution of which reveals the property of emergence - the result is not simply a sum of the parts. Anemones and certain species of coral live in isolation, however most corals form colonies of genetically identical polyps; these polyps closely resemble anemonies in structure, although are generally considerably smaller. Stony coral live in all parts of the underwater world, but corals with symbionts occur in shallow waters around tropical islands and coasts. They provide a rich source of food and provide physical protection for shores. Corals are threatened by increasing acidification of the oceans.