A class of arthropods in the subphylum Crustacea. The more than 20,000 described species of Malacostraca can be divided into two groups, the Phyllocarida, and the Eumalacostraca. Phyllocarida contains the oldest crustacean known and includes only one living group.The Eumalacostraca consists of all Malacostracan groups other than the Phyllocarida. Eumalacostracans generally possess a well-developed carapace and a long, muscular abdomen. It is the group that contains most of the animals the general public recognize as crustaceans, such as shrimp, crabs, lobsters .