Malacostraca

Malacostraca is the largest of the six classes of crustaceans, containing about 40,000 living species, divided among 16 orders. Its members, the malacostracans, display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, woodlice, amphipods, mantis shrimp and many other, less familiar animals. They are abundant in all marine environments and have colonised freshwater and terrestrial habitats. They are segmented animals, united by a common body plan comprising 20 body segments (rarely 21), and divided into a head, thorax, and abdomen. The name Malacostraca was coined by a French zoologist Pierre André Latreille in 1802. He was curator of the arthropod collection at the National Museum of Natural History in Paris. The name comes from the Greek roots μαλακός (malakós, meaning 'soft') and ὄστρακον (óstrakon, meaning 'shell'). The name is misleading, since the shell is only soft immediately after moulting, and is usually hard. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca. The class Malacostraca includes about 40,000 species, and 'arguably ... contains a greater diversity of body forms than any other class in the animal kingdom'. Its members are characterised by the presence of three tagmata (specialized groupings of multiple segments) – a five-segmented head, an eight-segmented thorax and an abdomen with six segments and a telson, except in the Leptostraca, which retain the ancestral condition of seven abdominal segments. Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa except Remipedia. Each body segment bears a pair of jointed appendages, although these may be lost secondarily. The head bears two pairs of antennae, the first of which is often biramous (branching into two parts) and the second pair bear exopods (outer branches) which are often flattened into antennal scales known as scaphocerites. The mouthparts consist of pairs each of mandibles, maxillules (second pair of mouthparts) and maxillae. Usually a pair of stalked compound eyes is present, although in some taxa the eyes are unstalked, reduced or lost. Up to three thoracic segments may be fused with the head to form a cephalothorax; the associated appendages turn forward and are modified as maxillipeds (accessory mouthparts). A carapace may be absent, present or secondarily lost, and may cover the head, part or all of the thorax and some of the abdomen. It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally. Typically, each of the thoracic appendages is biramous and the endopods are the better developed of the branches, being used for crawling or grasping. Each endopod consist of seven articulating segments; the coxa, basis, ischium, merus, carpus, propodus and dactylus. In decapods, the claw is formed by the articulation of the dactylus against an outgrowth of the propodus. In some taxa, the exopods are lost and the appendages are uniramous. There is a clear demarcation between the thorax and the six or seven-segmented abdomen. In most taxa, each abdominal segment except the last carries a pair of biramous pleopods used for swimming, burrowing, gas exchange, creating a current or brooding eggs. The first and second abdominal pleopods may be modified in the male to form gonopods (accessory copulatory appendages). The appendages of the last segment are typically flattened into uropods, which together with the terminal telson, make up the 'tail fan'. It is the sudden flexion of this tail fan that provides the thrust for the rapid escape response of these crustaceans and the tail fan is also used in steering. In Leptostraca, the appendages on the telson instead form caudal rami (spine-like protrusions). The digestive tract is straight and the foregut consists of a short oesophagus and a two-chambered stomach, the first part of which contains a gizzard-like 'gastric mill' for grinding food. The walls of this have chitinous ridges, teeth and calcareous ossicles. The fine particles and soluble material are then moved into the midgut where chemical processing and absorption takes place in one or more pairs of large digestive caeca. The hindgut is concerned with water reclamation and the formation of faeces and the anus is situated at the base of the telson. Like other crustaceans, malacostracans have an open circulatory system in which the heart pumps blood into the hemocoel (body cavity) where it supplies the needs of the organs for oxygen and nutrients before diffusing back to the heart. The typical respiratory pigment in malacostracans is haemocyanin. Structures that function as kidneys are located near the base of the antennae. A brain exists in the form of ganglia close to the antennae, there are ganglia in each segment and a collection of major ganglia below the oesophagus. Sensory organs include compound eyes (often stalked), ocelli (simple eyes), statocysts and sensory bristles. The naupliar eye is a characteristic of the nauplius larva and consists of four cup-shaped ocelli facing in different directions and able to distinguish between light and darkness.