The decapods or Decapoda (literally "ten-footed") are an order of crustaceans within the class Malacostraca, including many familiar groups, such as crayfish, crabs, lobsters, prawns and shrimp. Most decapods are scavengers. It is estimated that the order contains nearly 15,000 species in around 2,700 genera, with approximately 3,300 fossil species.Nearly half of these species are crabs, with the shrimp (c.?3000 species) and Anomura (including hermit crabs, porcelain crabs, squat lobsters: c.?2500 species), making up the bulk of the remainder.[1] The earliest fossil decapod is the Devonian Palaeopalaemon.