As a crab grows, it sheds or molts its shell. The new shell underneath will quickly harden into a new, larger shell. To escape from enemies a crab may shed a leg or claw which can be regenerated during a new molt. Most crabs are scavengers. They live in abandoned snail shells and must periodically change to larger shells as they grow.
Females carry eggs near the abdomen, well inside the shell, for protection.
They are covered with hundreds of radiating spines, which are used for defense, locomotion, and trapping bits of seaweed. They feed on kelp and algae. In between the spines are long tube feet for gripping surfaces and absorbing oxygen. Some urchins have tiny pincers among the spines for keeping their bodies clear of barnacles, sponges and larvae. Feet and pincers can be regenerated.