hermit crab

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various decapod crustaceans of the superfamily Paguroidea that have a soft asymmetrical abdomen and occupy the empty shells of gastropod mollusks.

from The Century Dictionary.

• noun A crab of the family Paguridæ (which see).

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A type of crab species which salvages empty seashells or other portable objects to permanently shelter and protect their soft abdomens.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun small soft-bodied marine crustaceans living in cast-off shells of gastropods