Why does the Hermit Crab live in a shell?

The hermit Crab is soft-bodied and so would be extremely vulnerable to its enemies if it had no protection at all. For some reason it has become adapted over time to living within the old shells of mollusks, usually whelks but sometimes periwinkles and top shells. Only two pairs of legs are used for walking, the grip the shell and the claws block the entrance if the crab is frightened and withdraws. You may wonder what happens when the crab grows too big for its shell. The crab sheds its skin and then carefully examines the empty shells around to see if they are suitable for its next home. Once satisfied with one it quickly scuttles from the old into the new and takes up residence in the more specious accommodation. It is a worrying time for the crab for it must expose its unprotected body, but it usually makes sure no predators are about before making the move.