Isopoda (pill bugs, slaters, woodlice; class Malacostraca, superorder Pericarida) Large, diverse order of crustaceans, 5–15 mm long, forming a specialized subdivision of their superorder. Isopods occupy many habitats, from the deep sea to true terrestrial niches. Some are even parasitic. They are almost always dorsoventrally flattened and adapted for crawling, and lack a carapace. The pereon is made of up to seven somites, each bearing one pair of uniramous walking legs. The leathery, flexible exoskeleton may have brown or grey markings, and some species possess chromatophores which to some degree enable the animal to match its colour to the substratum. The head bears unstalked, compound eyes, and uniramous first antennae with sensory setae. Most isopods are scavengers and omnivores, with compact mouth-parts and a well-developed cardiac (or triturating) stomach. The pleopods, unlike those of most other Crustacea, have become developed for gaseous exchange. Burrowing is common in marine forms and some species can damage docks and wooden piling (e.g. Sphaeroma and the gribble, Limnoria). Characters such as internal fertilization, a marsupium (or brood pouch), and walking legs are pre-adaptations for the successful life-style of the terrestrial forms. There are 4000 known species, most of them marine.

pill bugs See ARMADILLIDAE; ISOPODA.