Peracarida

The superorder Peracarida is a large group of malacostracan crustaceans, having members in marine, freshwater, and terrestrial habitats. They are chiefly defined by the presence of a brood pouch, or marsupium, formed from thin flattened plates (oostegites) borne on the basalmost segments of the legs. Peracarida is one of the largest crustacean taxa and includes about 12,000 species. Most members are less than 2 cm (0.8 in) in length, but the largest is probably the giant isopod (Bathynomus giganteus) which can reach 76 cm (30 in). The earliest known perecaridian was Oxyuropoda ligioides, a fossil of which has been found dating to the Late Devonian (more than 360 mya) of Ireland.