Decapoda

Decapoda is a large order of crustaceans within the class Malacostraca, whose name, from Greek for ten feet, refers to the ten walking legs, or pereiopods, borne on the last five thoracic segments that define the group; some decapods carry as many as 38 total appendages arranged in pairs along the body. The order contains roughly 15,000 living species across about 2,700 genera, including crabs, which make up nearly half of all decapod species, some 3,000 species of shrimp, the roughly 2,500 species of the Anomura, hermit crabs, king crabs, and squat lobsters, as well as lobsters and crayfish. Many decapods carry enlarged pincer-like claws called chelae, and most are scavengers. The earliest confirmed decapod fossils date to the Devonian, though molecular evidence suggests the order arose earlier still, in the Late Ordovician, roughly 455 million years ago.