Roper, Clyde F. E. Division of Molluscs, Smithsonian Institution, U.S. National Museum, Washington, DC.
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An order of the class Cephalopoda (subclass Coleoidea, superorder Decapodiformes) commonly known as squids. The true squids arose in the early Mesozoic Period (Permian/Triassic) and have proliferated from the Jurassic to the Recent (Holocene). They are now represented by well over 300 species of living forms (see illustration). Two main groups, the Myopsida and the Oegopsida, are included in the Teuthoidea (also known as Teuthida), and they occur in various marine habitats of the world. Characteristics include 10 circumoral (surrounding the mouth) appendages (eight arms and two tentacles); suckers with chitinous rings or hooks; a buccal membrane (lips); an internal, simple, rod- or featherlike chitinous shell (pen or gladius); and eyes that are covered by a transparent membrane with a minute pore (myopsids) or eyes without any membrane, completely open to the sea (oegopsids). The teuthoids inhabit virtually all marine and estuarine habitats of the world, from surface waters to abyssal depths of 5000 m (16,400 ft) or more. They are foundation members of many ecosystems, as both prey and predator. Currently, in aggregate, more than 4.22 million metric tons (4.65 million tons) of squid are harvested annually in the worldwide squid fisheries. See also: Cephalopoda; Coleoidea; Deep-sea fauna; Marine ecology
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