hermit crab, a crustacean crustacean (krŭstā`shən), primarily aquatic arthropod of the subphylum Crustacea.
..... Click the link for more information. distinguished from true crabs by its long, soft, spirally coiled abdomen terminating in an asymmetrically hooked tail. Most hermit crabs protect this vulnerable portion of their bodies by occupying the empty shells of periwinkles, whelks, and other gastropod mollusks. A few find other homes; for example, a species that inhabits the Indian Ocean lives in sections of old bamboo cane. When the hermit crab grows out of one shell it seeks a larger one, fighting for it if challenged. Sea anemones often attach themselves to these shells, obtaining free transportation and scraps of food in return for protecting their hosts. Hermit crabs are common beach scavengers in most parts of the world. Most species are marine, but some tropical forms, such as the robber crab, Birgus latro, are largely terrestrial. This species, the largest hermit crab, generally reaches over 1 ft (30 cm) in length. It becomes increasingly terrestrial and develops heavy armor as it matures into an adult, at which stage it is able to completely discard its adopted shell. With its great pincers the robber crab has been known to crack coconuts, which it obtains by climbing palm trees. Hermit crabs are classified in the phylum Arthropoda Arthropoda (ärthrŏp`ədə) [Gr.,=jointed feet], largest and most diverse animal phylum.
..... Click the link for more information. , subphylum Crustacea, order Decapoda.

hermit crab

Hermit crab (Pagurus samuelis).

(credit: Russ Kinne—Photo Researchers)

Any crab (families Paguridae and Coenobitidae) that uses empty shells or other hollow objects as a shelter for partial containment and protection of the body. They are found worldwide in sandy- or muddy-bottomed waters and occasionally on land and in trees. They have two pairs of antennae and four pairs of legs; the first pair of legs is modified to form pincers, shaped to cover the shell entrance when the animal is inside. As the crab grows, it periodically leaves its shell and finds a larger one to occupy. The reddish brown large hermit crab (Pagurus pollicaris; 4–5 in., or 10–12 cm, long) and the small hermit crab (P. longicarpus) are found in North American Atlantic coastal waters.