The three orders of crustaceans described here, all belong to the subclass Malacostraca, and are small, but unrelated groups: is: the euphausiids (with 16 British species), the stomatopods (2 British species) and the leptostracans (4 British species). Each group is placed in a separate subclass of the Malacostraca in many classifications. This subclass, however, contains nearly three-quarters of all the known species of crustaceans and includes animals such as the mysids, cumaceans, isopods, amphipods and decapods.