Crustaceans (Crustacea) are a very large group of arthropods, usually treated as a subphylum . The group comprises almost 52,000 described species, although the number of undescribed species may be 10-100 times higher . They include various familiar animals, such as crabs, lobsters, crayfish, shrimp, krill and barnacles. The majority of them are aquatic, living in either marine or fresh water environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs and woodlice. Crustaceans are among the most successful animals, and are as abundant in the oceans as insects are on land. The majority of crustaceans are also motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts (including sea lice, fish lice, whale lice, tongue worms, and Cymothoa exigua, all of which may be referred to as "crustacean lice"), and adult barnacles live a sessile life-they are attached headfirst to the substrate and cannot move independently.