Crabs are crustaceans, and along with shrimps and lobsters, they are called "decapods" - crustaceans with five pairs of legs. There are two major groups of animals called "crabs"- the brachyurans or "true crabs" and the anomurans or "false crabs". In general, true crabs have four pairs of legs used for walking, while false crabs have just two or three. True crabs have a short abdomen which is curled underneath their bodies; false crabs have a longer abdominal segment that is less well-protected. Hermit crabs are false crabs which use borrowed mollusk shells to house and protect their abdomens.