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hermit crab, a crustacean 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, subphylum Crustacea, order Decapoda.
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