Amphipoda, an order of the sessile-eyed Crustacea or Arthrostraca, including the fresh-water shrimp (Gammarus), the sand hopper (Talitrus). and the whale-louse (Cyamus). As in all typical Crustacea, the body consists of three regions, head, thorax, and abdomen, divided into segments, each of which bears a pair of limbs. In this order there are six or seven segments in the thorax, the middle segment of the body; upon the limbs attached to this region are borne three pairs of small, soft, sac-like structures, by which the blood is aerated; these are known as "vesicles." The abdomen, or hindermost region of the body, may be rudimentary as in the sub-order Laemodipoda, or it may consist of seven segments with seven pairs of appendages, of which the first three pairs are adapted for swimming.