The superorder Peracarida is a large group of malacostracan crustaceans found in diverse habitats. It includes well over 20,000 known species divided among around nine orders (including the Mysida and Lophogastrida, which were formerly treated together as Mysidacea). Most peracarids are marine, but many occur on land and in freshwater and several species live in hot springs at 30 to 50 C. This group includes the most diverse and widespread clade of terrestrial crustaceans, the isopods (pillbugs and sowbugs), as well as the amphipods, a few of which are found in moist microhabitats on land. Pericarids range in size from tiny ones, just a few millimeters in length, living between grains of sand to planktonic amphipods exceeding 12 cm (Cystisoma) and benthic isopods growing to 50 cm (Bathynomus giganteus). Feeding strategies are diverse and some species (especially some isopods and amphipods) have symbiotic relationships with other organisms. (Brusca and Brusca 2003)
An excellent online source on the Peracarida is Gary Anderson's Peracarida Taxa and Literature website.