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Malacostraca
Eophreatoicus, a fresh water isopod from Australia.
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Subphylum: Crustacea Class: Malacostraca Latreille, 1802
Subclasses
Eumalacostraca Hoplocarida Phyllocarida See text for orders.

The Malacostraca (Greek: "soft shell") are the largest subgroup of crustaceans and include most of the animals that non-experts recognise as crustaceans, including decapods (such as crabs, lobsters and shrimp), stomatopods (mantis shrimp) and krill. They also include the amphipods and the only substantial group of land-based crustaceans, the isopods (woodlice and related species). With more than 22,000 members, this group represents two thirds of all crustacean species and contains all the larger forms. The first malacostracans appeared in the Cambrian.

The classification of crustaceans is currently being debated, and the Malacostraca are regarded by some authors as a class and by others as a subclass.

Their characteristics include:

• The head has 6 segments, with a pair of antennules and a pair of antennae, as well as mouthparts.
• They have 5 pairs of walking legs, with the first often modified to form pincers.
• There are 8 thoracic segments. The cephalothorax is covered by a carapace form via fusion of 3 of them, letting the 5 other uncovered.
• The abdomen is behind and often used for swimming. There are 6 abdominal segments.
• There are appendages near the mouthparts, these are called maxillipeds.
• They have compound stalked or sessile eyes.
• They have a two-chambered stomach.
• They have a centralised nervous system.

Martin and Davis^[1] present the following classification of living malacostracans into orders, to which extinct orders have been added, indicated by †.

Class Malacostraca Latreille, 1802

• Subclass Phyllocarida Packard, 1879

• †Order Archaeostraca
• †Order Hoplostraca
• †Order Canadaspidida
• Order Leptostraca Claus, 1880

• Subclass Hoplocarida Calman, 1904

• Order Stomatopoda Latreille, 1817 (mantis shrimp)

• Subclass Eumalacostraca Grobben, 1892
  • Superorder Syncarida Packard, 1885
    • †Order Palaeocaridacea
    • Order Bathynellacea Chappuis, 1915
    • Order Anaspidacea Calman, 1904
  • Superorder Peracarida Calman, 1904
    • Order Spelaeogriphacea Gordon, 1957
    • Order Thermosbaenacea Monod, 1927
    • Order Lophogastrida Sars, 1870
    • Order Mysida Haworth, 1825
    • Order Mictacea Bowman, Garner, Hessler, Iliffe & Sanders, 1985
    • Order Amphipoda Latreille, 1816
    • Order Isopoda Latreille, 1817 (woodlice, slaters)
    • Order Tanaidacea Dana, 1849
    • Order Cumacea Krøyer, 1846
  • Superorder Eucarida Calman, 1904
    • Order Euphausiacea Dana, 1852
    • Order Amphionidacea Williamson, 1973
    • Order Decapoda Latreille, 1802 (crabs, lobsters, shrimp)

1. ^ Joel W. Martin and George E. Davis (2001). An Updated Classification of the Recent Crustacea. Natural History Museum of Los Angeles County.