"Probably in no other species of marine aquatic animal is this mode of behavior as easy to study as the hermit crab. Because they are ceaselessly active, hermits make good aquarium additions. They are easy to maintain. Feed them brine shrimp and an occasional bit of seaweed. Forget hunks of met that might entice other crabs; hermits eat only small fragments and will not tear big pieces apart.
Unlike other critters of the order Decapoda (prawns, shrimp, crabs, and lobsters), hermit crabs do not come equipped with a full suit of armor. They lack any hard parts aft of their cephalothorax, which leaves their soft and defenseless hindquarters exposed to easy assault.
Their evolutionary response to nature providing only plated gloves and jacket but no pants has been to employ empty snail shells as leggings. Since hermits of various species come in different sizes, each must habituate the haunts of a similarly sized snail host to find a custom home". -- Dave Bulloch