An order of invertebrate crustaceans related to crabs, shrimps, and lobsters, all members of the class Malacostraca. The closest relatives of Isopoda are the amphipods, mysids (opposum shrimps), cumaceans, and tanaids, all of which are placed in the superorder Peracarida. Isopods are generally small but very common, highly diversified, and occurring in marine, freshwater, and terrestrial habitats. Sow bugs, pill bugs, and woodlice, as well as their marine relatives (such as gribbles and sea slaters), belong to this group.