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 marsupium (the "brood pouch"), 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 can be quite sizeable, such as the giant isopod Bathynomus giganteus which can reach 76 cm (30 in) in length, and the giant amphipod Alicella gigantea (34 cm (13 in) long). The earliest known peracaridian was Oxyuropoda ligioides, a fossil taxon dated to the Late Devonian of Ireland (more than 360 mya).