Malacostraca (from New Latin; from Ancient Greek μαλακός (malakós) 'soft', and όστρακον (óstrakon) 'shell') 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, prawns, woodlice , amphipods, mantis shrimp, tongue-eating lice 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.[1] The name comes from the Greek roots μαλακός (malakós, meaning "soft") and ὄστρακον (óstrakon, meaning "shell").[2] The name is misleading, since the shell is soft only immediately after moulting, and is usually hard.[3] The word was used by Aristotle, who contrasted them with oysters, in comparison with which their shells are pliable. Malacostracans are sometimes contrasted with entomostracans, a name applied to all crustaceans outside the Malacostraca, and named after the obsolete taxon Entomostraca.[4]
The class Malacostraca includes about 40,000 species,[5] and "arguably ... contains a greater diversity of body forms than any other class in the animal kingdom".[6] 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.[6] Malacostracans have abdominal appendages, a fact that differentiates them from all other major crustacean taxa except Remipedia.[7] Each body segment bears a pair of jointed appendages, although these may be lost secondarily.[8]
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.[7] The mouthparts consist of pairs each of mandibles, maxillules (second pair of mouthparts) and maxillae. Except for fairy shrimps, malacostracans are the only extant arthropods with compound eyes placed on moveable stalks,[9][10] although in some taxa the eyes are unstalked, reduced or lost.[11][12]
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).[7] 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.[6] It is variable in form and may be fused dorsally with some of the thoracic segments or occasionally be in two parts, hinged dorsally.[11] 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.[7]