Malacostraca is the second largest class of crustaceans, just behind hexapods (which includes insects as its crown group, but not only), and contains about 40,000 living species, divided among 16 orders. Malacostracans display a great diversity of body forms and include crabs, lobsters, crayfish, shrimp, krill, prawns, isopods, amphipods, mantis shrimp, and many other less familiar animals. They are abundant in all marine environments and have colonized 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.





