(phylum Arthropoda, subphylum Crustacea)
Class which contains nearly 75% of known crustaceans, including the larger forms, e.g. shrimps, crabs, and lobsters. The typical malacostracan trunk comprises 14 segments and a telson. The thorax is composed of eight segments, and the remaining six form the abdomen. The first antennae are often biramous, and all the body segments bear appendages. The cutting edge of the mandibles comprises a grinding molar process and a cutting incisor; the mandible usually bears a palp. The carapace is lost in some orders and in most malacostracans one or more of the anterior thoracic limbs is upturned and modified into a maxilliped. Primitively, the biramous thoracic limbs are all similar, with the endopodite better developed than the exopodite, and are used for crawling or grasping. The anterior abdominal limbs are pleopods, and there are usually five pairs. They may be modified for crawling, for creating feeding and respiratory currents, or in the females for egg-carrying. In the males, the first two pairs are usually adapted for use as intromittent organs. The terminal tail fin, used in swimming, comprises the dorsal telson and a pair of ventral uropods. In the female, the genital openings (gonopores) are on the sixth thoracic segment, while those of the male are on the eighth. The nauplius stage is passed within the egg.
Subjects: Zoology and Animal Sciences.