Typical amphipod showing the main divisions of the body: head (red), thorax (yellow), and abdomen (brown). The first two pairs of thoracic legs (purple), the gnathopods, assist feeding and are used by males during copulation. The first three pairs of abdominal legs (blue) create a water current which aerates the gills for respiration, and provide the main propulsive force in swimming. The rear three abdominal appendages are known as uropods, and two of these (blue) are usually modified to act as rudders.
An amphipod is a member of the order Malacostraca of crustaceans including marine and freshwater species, characteristics of which are a body flattened from side to side and the absence of a carapace. Among the few which have a common name are the shrimp-like beach-fleas. Some amphipods are nonpigmented cave-dwellers. About 3,600 species are known.