isopod (Īˈsəpŏdˌ) [key], common name for crustaceans belonging to the order Isopoda and in the same subclass as lobsters and crayfish. Isopods are characterized by their flattened bodies, lack of a carapace, and gills located on the abdominal appendages. About 4,000 species are known. Most are aquatic; they are bottom-dwellers or are associated with water plants in freshwater or marine habitats. Some live under rocks on the shore, and some are terrestrial, living inconspicuously in surface litter or under logs. These, the pill bugs, or sow bugs, are the only large group of terrestrial crustaceans. Some isopods roll up like armadillos when disturbed. Some are parasitic, living on other crustaceans or in the mouths or on the gills of fishes. Most isopods are small, less than 1/2 in. (1.27 cm) long, but Bathynomus gigantea, a deep-sea species, may be over 1 ft (30 cm) long. Isopods are classified in the phylum Arthropoda, subphylum Crustacea, order Isopoda.