ASL STEM

Definition

Malacostraca is a large and diverse taxon (generally class, but sometimes subclass or order) of marine, freshwater, and terrestrial crustaceans, including many of the most familiar crustaceans, such as crabs, lobsters, shrimps, which are characterized by a maximum of 19 pairs of appendages, as well as trunk limbs that are sharply differentiated into a thoracic series and an abdominal series. Other familiar members of the Malacostraca are the stomatopods (mantis shrimp) and euphausiids (krill), as well as 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.

Source: http://www.newworldencyclopedia.org/entry/Malacostraca (New World Encyclopedia)