CRUSTACEA. 



49 



three anterior pairs are directed forwards, and the 

 remaining lour pairs backwards. The [sopods move 

 with the head in advance. 



Amphipods. 



The largest and finest specimens of the typical Am- 

 >hipods are those known as beach-fleas, the Gammarus 



irnatus (Fig. 21). and they are found under stones 



Fig. 21. 



Cammnrns ornatus Edw. 



After Emerton.) 



The line shows natural size of spei ies. 



and sea-weed at low-water mark along all our principal 

 beaches. The body of Gammarus is composed of a 

 succession of rings, which remind the pupils of the 

 abdominal rings of the lobster, being not unlike these 

 in form, and freely movable upon each other. The 

 seven posterior segments, constituting the abdomen, 

 bear the six pairs of swimmerets. In Fig. 2T, five of 

 the swimmerets are represented on the left side, and 

 one on the right. 



