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) including the cuttlefishes (Sepia), the bobtail squids (Sepiola), and the ram's-horn squid (Spirula). The group is characterized by an internal shell that is calcareous and broad with closely packed laminate chambers (the cuttlebones of cuttlefishes); calcareous, cylindrical, and coiled with about 40 subspherical chambers (the ram's-horn shell of Spirula); chitinous and rod- or bladelike (the gladius of bobtail squids); or absent. The mouth is surrounded by ten appendages (eight arms and two longer tentacles) that bear suckers with chitinous rings. The tentacles are contractile and retractile into pockets at their bases (see illustration).
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