Decapods are a group of crustaceans who are named for their tendency to have ten legs. Many members of this group, including crabs, lobsters and ghost shrimp, have limbs which have transformed over time into chelae, or claws, of various sizes. In some other lineages, like the spiny lobsters, the five pairs of legs remain roughly the same size while their antennae grow to be quite large. The segmented bodies of decapods consist of a cephalothorax, formed by the fused head and thorax, and a rear, often segmented, section called an abdomen. A rigid carapace covers the cephalothorax and protects the organs and gills beneath. 

A series of smaller appendages called pleopods which can be found on the lower surfaces of the tails of some decapods are used to help these animals swim. In some forms of shrimp, spiny lobsters and true lobsters, the muscular sections of their abdomens end in tail fans formed by the telson and uropods. These structures are used to help these animals steer themselves through the water. In crabs, the abdomen is generally reduced and folded beneath the cephalothorax. This abdominal reduction is a feature found in true brachyuran crabs as well as several groups of anomuran “false crabs”.