Crabs' eyes are mounted on movable stalks that can be drawn into the carapace. Crabs that live in water breathe by means of gills. Land crabs have modified gills that function as lungs. The smallest crabs are pea crabs that live within the shells of sandollars, oysters or mussels. Japanese spider crabs are the largest crustaceans and may measure up to 12 feet from tip to tip of their outstretched claws.
Hermit crabs hide their soft abdomens in spiral shells cast off by sea snails or other mollusks.
With two pairs of walking legs, the crabs drag their shells about with them. The other two pairs of walking legs are reduced and act as struts against the shell's inner wall. As the crab grows, it leaves its old shell and moves into a larger one. They occupy abandoned snail shells by thrusting the abdomen into the shell and holding onto it with the hooks on a pair of leglike organs. They do not kill the original owner of the shell, though they will fight other hermit crabs to determine occupancy of a shell.