a diverse and abundant group, whose members have a carapace which is fused to the thoracic segments to form a cephalothorax. The Eucarida is divided into three orders, the Euphausiacea, the Decapoda and the Amphionidacea. The members of the Euphausiacea are commonly called krill and are all marine shrimp-like species whose pleopods (abdominal appendages) function as swimmerets, they swarm and mostly feed on plankton, this group is composed of only 90 species, but some of these are one of the most abundant species on the planet, in fact, it is estimated that the biomass of the Antarctic krill Euphausia superba is 500 million tons. The Decapoda is a group with 18,000 species which have 5 pairs of thoracopods and a well-developed carapace that covers the gills (which are exposed in krill). Many of these species have common names and are often eaten. The decapods are further subdivided on the basis of the gill structure into two suborders Dendrobranchiata (prawns) and Pleocyemata, which is further subdivided into several infraorders, such as the Caridea ("true shrimp"), the Stenopodidea (boxer shrimp) and the Anomura and the Brachyura (crabs) and so forth. In addition, there is an enigmatic eucarid species, Amphionides reynaudii, which is the sole representative of its order, but due to the loss of several features resulting from its small size, its classification has been unclear. Peracarida The other major malacostracan superorder, the Peracarida, is highly diverse in habit, size and shape and contains 21,500 species, but this number is a gross underestimate as the number of described species has tripled in the past 20 years. Most authors studying morphological characters propose a monophyletic Peracarida which forms a well supported subtree that is sister to a Eucarida subtree, one paper is an exception and proposes that the Peracarida is derived from a polyphyletic Eucarida. With the exception of thermosbaenacean species, a characteristic of the members of this group is that they brood their young in a marsupium formed by branches (endites) of their thoracopods, called oostegites. The Peracarida is divided into nine orders (Isopoda, Amphipoda, Mysida, Lophogastrida, Cumacea, Tanaidacea, Mictacea, Thermosbaenacea and Spelaeogriphacea), although some authors prefer to unite two pairs of orders with similar organisms, which are the Mysidacea, formed by the Mysida and the Lophogastrida, and the Edriophthalma, formed by the Isopoda and the Amphipoda. The members of the Mysida and the Lophogastrida have several common features: they are shrimp-like