Amphipoda

:  a large group, usually an order, of malacostracan crustaceans (division Peracarida) comprising the beach fleas and related forms; being mostly of small size with laterally compressed body, four anterior pairs of thoracic limbs directed forward, and three posterior pairs directed backward and upward, the thoracic limbs bearing gills; being usually aquatic in fresh or salt water, and a few (as the whale louse) being parasitic