Caridoid facies
| Malacostraca Temporal range: Cambrian–Recent |
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| Scientific classification | |
| Kingdom: | Animalia |
| Clade: | Euarthropoda |
| Subphylum: | Crustacea |
| Class: |
Malacostraca Latreille, 1802 |
| Subclasses | |
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See text for orders. |
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See text for orders.
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 (malakós, meaning "soft") and (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.
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