Hermit crabs have a 'naked' hind body. Only their front end is covered with shell. In order to protect their naked hind end from hungry animals, they hide this part in empty snail shells. The entire back end disappears in the shell; only the head, the legs and the sturdy pincers stick out of the opening. As opposed to the small hermit crab, the right claw of the common hermit crab is larger than the left claw. Because hermit crabs grow but their protective shell doesn't, they must regularly move into large snail shells. Smaller hermit crabs can be found in periwinkles. Larger ones are often found in whelks. Sometimes, you find the shells covered with a rough substance. Although it looks like algae growth, it is the animal polyp sea mat.