any of various crabs (esp. family Paguridae) that have asymmetrical, soft abdomens and live in the empty shells of certain mollusks, as snails
See hermit crab in American Heritage Dictionary 4
noun
Any of various crabs belonging to a group within the order Decapoda that protect their soft unarmored abdomens by occupying and carrying about the empty shells of snails or other univalve mollusks.