Roper, Clyde F. E. Division of Molluscs, Smithsonian Institution, U.S. National Museum, Washington, DC.
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An order of coleoid cephalopods, characterized by eight appendages (which encircle the mouth), a saclike body, and an internal shell that is much modified or reduced from that of its ancestors. Members of the order Octopoda (subclass Coleoidea, class Cephalopoda, phylum Mollusca) are known commonly as octopods or octopuses. The origins of the order Octopoda can be traced back to the Mesozoic period, and modern-day forms are divided into two octopod suborders (Fig. 1)—Cirrata (Cirrina) and Incirrata (Incirrina). Cirrate octopuses have paddle-shaped fins on the body and tendril-like cirri (cilia-like strands) on the arms, whereas incirrates lack fins or cirri. The octopods are a speciose (species-rich) group, with almost 300 known species; in addition, many others are recognized, but have yet to be scientifically described and named. The living octopods are characterized by having the following features: eight appendages (arms); radially symmetrical suckers without horny (chitinous) rings; suckers without muscular stalks; head fused to the body (mantle) dorsally; fins present in some forms; an internal shell, in the form of a cartilage-like fin support (in cirrates), or a pair of stylets (but can be absent); and a crop usually present. Octopods are exclusively marine animals, with species occupying virtually every marine habitat in the world's oceans and seas, from the North Pole to the South Pole, and from the surface shallows to depths of more than 5000 m (16,400 ft). Some species of intertidal octopuses may tolerate lower or higher salinities for short periods. See also: Cephalopoda; Coleoidea; Marine ecology; Mollusca
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