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, scuds (
Amphipoda),
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.