The crustaceans (Crustacea) are a large group of arthropods (52,000 described species, usually treated as a subphylum. They include various familiar animals, such as lobsters, crabs, shrimp and barnacles. The majority are aquatic, living in either fresh water or marine environments, but a few groups have adapted to terrestrial life, such as terrestrial crabs, terrestrial hermit crabs and woodlice. The majority are motile, moving about independently, although a few taxa are parasitic and live attached to their hosts, and adult barnacles live a sessile life.