Malacostraca

The Malacostraca (Greek: "soft shell") are the largest subgroup of crustaceans. They include decapods (such as crabs,  lobsters and shrimp), stomatopods (mantis shrimp) and krill. There are 22,000 members in this group. It represents two thirds of all crustacean species. The first malacostracans appeared in the Cambrian period.

The classification of crustaceans is currently being debated. Some think Malacostraca is a class and others think it is a subclass.

 Morphology 

Malacostraca have heads with six segments. They have a pair of antennules and a pair of antennae. They also have mouthparts. There are appendages near the mouthparts, called maxillipeds. They have five pairs of walking legs. The first pair is formed like a pincer. There are eight thoracic segments. There are six abdominal segments. They are used for swimming. Members of Malacostraca have compound stalked or sessile eyes. They have a two-chambered stomach and a centralized nervous system.

 Classification 
200px|thumb|Lysiosquilla maculata, a mantis shrimp
200px|thumb|Armadillidium vulgare, an isopod
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)