The members of this order resemble those of the preceding in the nature of the respiratory organs, which consist of membranous vesicles attached to the bases of the thoracic limbs. The first thoracic segment, however, is distinct from the head, and the abdomen is well developed, and is composed of seven segments. There are seven pairs of thoracic limbs, directed partly forwards and partly backwards, the name of the order being derived from this circumstance. As in the Laemodipoda, the heart has the form of a long tube extending through the six segments following the head, and having the blood admitted to its interior by three pairs of valvular fissures. The three posterior pairs of abdominal limbs are bent backwards, and form, with the telson, a natatory or saltatorial tail. The young Amphipod acquires its full number of segments and limbs before its liberation from the egg; and, as a rule, the young undergo little or no metamorphosis in reaching maturity.
Fig. 153. - Laemodipoda. Caprella phasma.
All the Amphipoda are small, the "Sand-hopper" (Talitrus locusta, fig. 154) and the "fresh-water Shrimp" (Gammarus pulex) being two of the commonest forms. The Sand-hoppers and Gammari swim on their side when in the water, and the former leap with great activity on land.
Fig. 154. - Amphipoda. The Sand-hopper, Talitrus locusta, enlarged.