decapod (Gr.,=10 feet), name for invertebrate animals of the crustacean order Decapoda (phylum Arthropoda) including the crabs, the lobsters and crayfish, and the true shrimps, all having five pairs of legs. The name Decapoda was also formerly applied to a very different group of animals, a cephalopod order including the cuttlefish and squids, characterized by two long and eight short tentacles. The cuttlefish are now classified in the order Sepioidea and the squids in the order Teuthoidea of the class Cephalopoda in the phylum Mollusca.

Any of more than 8,000 species (order Decapoda) of crustaceans having five pairs of legs attached to the thorax. The shrimps and shrimplike species, which can be as small as 0.5 in. (12 mm) long, have a slender body with a long abdomen, a well-developed fantail, and, often, long, slender legs. The crabs and crablike species, whose claw span can measure 13 ft (4 m), have a flattened body and, frequently, stout short legs and a reduced tail fan. Decapods are primarily marine and are most abundant in shallow tropical waters, but they are commercially valuable throughout the world. Some species (e.g., hermit and fiddler crabs) are adapted to terrestrial environments. Seealso crab; crayfish; lobster; shrimp.

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