Malacostraca, SO.Peracarida, O.Isopoda, sO.Oniscoidea, F.Armadillidiidae, Porcellionidae, and Ligiidae Peracarida is a large and ecologically important superorder of malacostracan crustaceans. It includes the amphipods and isopods, each with well over 5000 species as well as several smaller orders including the mysids, cumaceans, and tanaids. Peracarids have the characteristics of their class, Malacostraca, and are further defined by possession of a ventral, thoracic marsupium in which the eggs are brooded.
Development is direct and there is no larva.
Most peracarids are small, less than 2 cm and inhabit marine, freshwater, and terrestrial environments. Isopoda is one of the most successful and the most diverse of the peracarid orders. They are common in marine and freshwater habitats and there are important terrestrial and parasitic suborders. Isopods are usually dorsoventrally flattened and the gills and heart are abdominal. One thoracic segment is fused with the head and there is, correspondingly, one pair of maxillipeds and seven pairs of thoracic walking legs.