A large crustacean superorder generally classified within the phylum Arthropoda, subphylum Crustacea, class Malacostraca, subclass Eumalacostraca. The Peracarida are defined as having the following characteristics: a telson (posteriormost body segment) without appendages; one pair (rarely two or three pairs) of maxillipeds (thoracic appendages modified as mouthparts); mandibles (primary jaws) with articulated accessory processes in adults, between the molar and incisor teeth, called the lacinia mobilis; a carapace, when present, that is not fused with posterior thoracic segments and that is usually reduced in size; basalmost leg segments of the thorax with unique, thin, flattened ventral plates (oöstegites) that enclose a ventral brood pouch (marsupium) in all orders except the Thermosbaenacea (which use the carapace to brood embryos); and having the young hatch as a prejuvenile stage called a manca, lacking the last pair of legs (that is, no free-living larvae occur in Peracarida). The roughly 21,500 species of peracarids are divided among nine orders. The orders Mysida and Lophogastrida were formerly combined (as the Mysidacea), but most authorities now treat them separately.