Hermit Crabs . Link to Intertidal British Crabs . Crustacea Taxa . Moulting Cycle . Because that's the way their legs bend.
Muscles work in pairs.
The muscle blocks in crabs are attached to the inner surface of the exoskeleton, including the ten tubular legs, including the legs with claws, or chelae , as well as other appendages. Each joint moves in a different plane, and so together they allow the crab to move in all directions like our shoulder and hip joints. However, many crabs have joints in their legs moving in a restricted number of planes so that they can only move sideways. In some crabs, the rear legs are shaped like paddles for limited swimming. Many crabs like the Shore Crab need to scamper sideways to avoid the legs getting tangled up with each other.