The larger, more useful and more familiar Crustacea belong to the subclass Malacostraca. It comprises such animals as the shrimps, crayfish, lobsters, crabs, and a number of other forms. The simplist of these are the opossum-shrimps (Mysis americana). In general appearance they resemle crayfishes or prawns. They are readily distinguished by the two-branched thoracic feet.