ANNULOSA : CRUSTACEA. 243 



entire order is now generally regarded as being merely a section 

 of the Amphipoda. 



ORDER II. AMPHIPODA. The members of this order re- 

 semble 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, how- 

 ever, 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 Lamodipoda, the heart has the 

 form of a long tube extending through the six segments follow- 

 ing 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, 



Fig. 93. Amphipoda. The Sand-hopper, Talitrus locusta, enlarged. 



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. 



All the Amphipoda are small, the "Sand-hopper" (Talitrus 

 locusta, fig. 93) and the "fresh-water Shrimp" (Gammarus 

 puleoc} 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. 



ORDER III. ISOPODA. In this order the head is always 

 distinct from the segment bearing the first pair of feet. The 

 respiratory organs are not thoracic, as in the two preceding 

 orders, but are attached to the inferior surface of the abdomen, 

 and consist of branchiae, which in the terrestrial species are 

 protected by plates which fold over them. The thorax is 



