Crabs have flatter, broader bodies than do lobsters, crayfish, and shrimp.
The small abdomen is folded into a groove under the cephalothorax and does not extend behind the carapace. The first pair of legs have large pinching claws. In crabs that swim, the last pair of legs are broad and flattened, serving as paddles. Crabs' eyes are mounted on movable stalks that can be drawn into the carapace. Crabs that live in water breathe by means of gills.