Malacostraca is the largest class (around 25000 species) of crustaceans and includes the decapods (crabs, lobsters, shrimps), stomatopods (mantis shrimp), euphausiids (krill), amphipods and isopods. Most species are found in marine environments, but some are found in fresh water habitats, and some are even found on land. Malacostracans exhibit the hard, calcified exoskeleton typical of crustaceans. They are dioecious, and sex is genetically determined. Several malacostracan taxa are parasitic. Still others are scavengers. Herbivorous malacostracans also exist.