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).

Characteristics

The most obvious characteristic of the group is the marsupium in females. This brood pouch is enclosed by the large, flexible oostergites, bristly flaps which extend from the basal segments of the thoracic appendages, which form the floor of a chamber roofed by the animal's sternum. This chamber is where the eggs are brooded, development being direct in most cases. Other characteristics include the possession of a single pair of maxillipeds (rarely 2–3), of mandibles with an articulated accessory process between the molar and incisor teeth in the adults (called the lacinia mobilis), and of a carapace which is often reduced in size and is not fused with the posterior thoracic somites. In some orders, the young hatch at a post-larval, prejuvenile stage called a manca which lacks the last pair of legs. In the underground order Thermosbaenacea, there are no oostergites and the carapace of the female is expanded to form a dorsal marsupium.