The Class Malacostraca is the largest class of crustaceans, including crabs, lobsters, shrimp, krill, and pill bugs. Members typically have a segmented body divided into head, thorax, and abdomen, with five pairs of walking legs and two pairs of antennae. Most are aquatic, living in marine or freshwater environments, though some, like pill bugs, are terrestrial. Malacostracans play important ecological roles as scavengers, prey, and predators, and many are economically significant as seafood.