any of an order (Amphipoda) of malacostracan crustaceans with a vertically thin body and one set of legs for jumping or walking and another set for swimming, as the sand hopper
See amphipod in American Heritage Dictionary 4
(ămˈfə-pŏdˌ)
noun
A small crustacean of the order Amphipoda, such as the beach flea, having a laterally compressed body with no carapace.