258 



SECTION B. EDRIOPHTHALMA. 



ORDER III. AMPHIPODA. 



794. Passing on to the group of EDRIOPHTHALMA, which is 

 distinguished by the absence of peduncles supporting the eyes, 

 and by the union of the respiratory organs with the locomotive 



members, we have to first notice the 

 Order AMPHIPODA, in which the 

 abdomen is well developed, but the 

 respiratory organs are connected with 

 FIG. 481. GAMMARUS PULEX. the thoracic limbs only. These organs 

 consist of membranous vesicles, at- 

 tached to the base of the legs, of which some traces present them- 

 selves in the preceding Order ; the continual renewal of the 

 water in contact with them is accomplished by the constant 

 movements of the first three pairs of abdominal pro-legs. The 

 thoracic legs are fourteen in number, consisting of the ten which 

 are characteristic of the Decapods, with the addition of two pairs 

 of feet-jaws, only one pair of these last organs being here left 

 in its original form. The legs are partly directed forwards, 

 and partly backwards, from which the name of the Order is 

 derived. The abdomen is much developed, and is composed of 

 seven segments, the last, however, being a mere rudiment ; the 

 appendages of the three preceding divisions are often united into 

 a sort of bundle, constituting an organ which is of great service 

 in leaping. These Crustacea are all of small size ; but they 

 frequently present themselves in very large numbers. Some of 

 them are inhabitants of the sea and shores, whilst others are 

 abundant in our streams. The greater number of them are very 

 agile leapers. 



795. The common Talitrus locusta,OT Sand-hopper (Fig. 456), 



