Crustaceans are part of the phylum Arthropoda ('jointed appendages') with contains many land based relatives - the insects. The marine species also have a similar body set-up with a hard exoskeleton and come in a variety of body forms. Many crab and lobster skeletons that you may see are not dead animals, they are in fact the shell of the growing crustacean.
Hermit crabs are extremely common on rocky reefs (anyone who have done some night diving or diving in a cave may see these tiny shells running to hide). They hide under rocks and graze on a variety of food items that may come their way. When they grow up they have to find a new shell that has been vacated by and old gastropod mollusc.