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The Scyphozoa are an exclusively marine class of the phylum Cnidaria, sometimes referred to as the "jellyfish" (or sea jellies - as they are not, in fact, fish). The class name Scyphozoa comes from the Greek word ''skyphos'' (σκύφος), denoting a kind of drinking cup and alluding to the cup shape of the organism. Scyphozoans have existed, in geological time, from the Ediacaran period to the present. ==Biology== Most species of Scyphozoa have two life history phases, including the planktonic medusa or jellyfish form, which is most evident in the warm summer months, and an inconspicuous, but longer-lived, bottom-dwelling polyp, which seasonally gives rise to new medusae. Most of the large, often colorful, and conspicuous jellyfish found in coastal waters throughout the world are Scyphozoa. They typically range from in diameter, but the largest species, ''Cyanea capillata'' can reach across. Scyphomedusae are found throughout the world's oceans, from the surface to great depths; no Scyphozoa occur in freshwater (or on land). As medusae, they eat a variety of crustaceans and fish, which they capture using stinging cells called nematocysts. The nematocysts are located throughout the tentacles that radiate downward from the edge of the umbrella dome, and also cover the four or eight oral arms that hang down from the central mouth. Some species, however, are instead filter feeders, using their tentacles to strain plankton from the water. ==Anatomy== Scyphozoans usually display a four-part symmetry and have an internal gelatinous material called mesoglea, which provides the same structural integrity as a skeleton. The mesoglea includes mobile amoeboid cells originating from the epidermis. Scyphozoans have no durable hard parts, including no head, no skeleton, and no specialized organs for respiration or excretion.〔Cartwright, P., Halgedahl, S.L., Hendriks, J.R., Jarrad, R.D., Marques, A.C., Collins, A.G., and Lieberman, B.S., 2007, (Exceptionally preserved jellyfishes from the Middle Cambrian ). PLOSONE Issue 10: e1121, p.1-7.〕〔Richards, H.G., 1947, Preservation of fossil jellyfish: Bulletin of the Geological Society of America, v. 58, p. 1221.〕 Marine jellyfish can consist of as much as 98% water, so are rarely found in fossil form. Unlike the hydrozoan jellyfish, Hydromedusae, Scyphomedusae lack a vellum, which is a circular membrane beneath the umbrella that helps propels the (usually smaller) Hydromedusae through the water. However, a ring of muscle fibres is present within the mesoglea around the rim of the dome, and the jellyfish swims by alternately contracting and relaxing these muscles.〔Morris, M., and Fautin, D., 2001, Animal Diversity Web: University of Michigan Museum of Zoology, "(Scyphozoa )., Accessed: September 28, 2008.〕 The periodic contracting and relaxing propels the jellyfish through the water, allowing it to escape predation or catch its prey. The mouth opens into a central stomach, from which four interconnected diverticula radiate outwards. In many species, this is further elaborated by a system of radial canals, with or without an additional ring canal towards the edge of the dome. Some genera, such as ''Cassiopea'', even have additional, smaller mouths in the oral arms. The lining of the digestive system includes further stinging nematocysts, along with cells that secrete digestive enzymes.〔 The nervous system usually consists of a distributed net of cells, although some species possess more organised nerve rings. In species lacking nerve rings, the nerve cells are instead concentrated into small structures called ''rhopalia''. There are between four and sixteen of these small lobes arranged around the rim of the umbrella, where they coordinate the muscular action allowing the animal to move. Each rhopalium is typically associated with a pair of sensory pits, a statocyst, and sometimes a pigment-cup ocellus.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Scyphozoa」の詳細全文を読む スポンサード リンク
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