Crabs of the family paguridae and coenobitidae. Most crabs have a hard shell covering their whole body and four sets of walking legs. Hermit crabs have only three pairs of walking legs, and lack the hard covering on the rear portion of their body, and so use empty shells to protect themselves (although in some areas, hermit crabs don't use shells at all. Rather they will live in plant stems, sections of bamboo, broken coconut shells, hollow pieces of wood, coral or sponge).

Hermit crabs occur all over the world, and live in water, on land (like the variety often kept as a pet), or sometimes even in trees.

The hermit crab has four pairs of legs, the front-most of which form claws. The claws they serve not only to aid in scavenging (hermit crabs are scanvengers and eat almost anything), but also to block the entrance to their shell. The right claw is usually larger and the body of the crab usually curls to the right (to fit the curvature of the shell). The rear-most pair of legs is used by the crab to grip the central column of the shell. Only the middle two pairs of legs are used for walking.

When a hermit crab outgrows a shell, it will leave its home in search of a new one, often removing the current occupant with its claws. Many animals may live upon the shell (such as sponges or sea anemones), feeding on what the hermit crab spreads about. When the hermit crab finds a new home, it is common for the animals on the old shell to move onto the new one, or for the crab to actually transplant the creatures himself. these animals serve as camouflage.

In the wild, hermit crabs are usually found in groups of about 25. If kept as pets, they do best in groups. In the wild, hermit crabs can live up to 25 years and attain huge sizes, but in captivity they will usually live less than a year, though with good care they may live to four years old.