Hermit crabs are arachnids.
The Decapoda or decapods -LRB- literally `` ten-footed '' -RRB- are an order of crustaceans within the class Malacostraca , including many familiar groups , such as crayfish , crabs , lobsters , prawns , and shrimp .. Malacostraca. Malacostraca. crayfish. crayfish. prawns. Dendrobranchiata. shrimp. Caridea. Most decapods are scavengers .. The order is estimated to contain nearly 15,000 species in around 2,700 genera , with around 3,300 fossil species .. fossil. fossil. Nearly half of these species are crabs , with the shrimp -LRB- about 3000 species -RRB- and Anomura -LRB- including hermit crabs , porcelain crabs , squat lobsters -LRB- about 2500 species -RRB- making up the bulk of the remainder .. shrimp. Caridea. Anomura. Anomura. The earliest fossil decapod is the Devonian Palaeopalaemon .. fossil. fossil. Devonian. Devonian. Palaeopalaemon. Palaeopalaemon