A Caribbean hermit crab.
hermit crab definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
any of various crabs (esp. family Paguridae) that have asymmetrical, soft abdomens and live in the empty shells of certain mollusks, as snails
hermit crab definition by American Heritage Dictionary
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.