One of the more highly specialized orders of the subphylum (or class) Crustacea. This order includes the shrimps, lobsters, hermit crabs, and true crabs. The order is so diverse that satisfactory definition is difficult, but a few characters are common to nearly all decapods. The most obvious is a head shield, or carapace, which covers and coalesces with all of the thoracic somites and which overhangs the gills on each side. The first three of the eight pairs of thoracic appendages are specialized as maxillipeds and closely associated with the true mouthparts. The last five pairs are walking legs or chelae (claws); this feature is the basis of the name Decapoda (= ten feet). The gills are usually well developed and arranged in several series.