INTRODUCTION. 



XV 11 



Medical Times and Gazette, vol. xxxvi. p. 467, 7th Novem- 

 ber, 1857, named the Endoiihragmal arch (Fig. 3, En.). 



Pig. 3. 



The seven segments which succeed the cephalon, or 

 head, are, in the higher orders, protected by the carapace. 

 This becomes gradually smaller in the descending series, 

 until, in the Sessile-eyed Crustacea, each segment is 

 exposed and developed into a perfect ring, analogous 

 in appearance to the segments of the pleon in the 

 Macrura. The several appendages that belong to the 

 segments of the pereion are locomotive in their charac- 

 ter, some being perfectly natatorial or ambulatory, others 

 adapted for climbing and grasping. In this respect the 

 two anterior pairs in the Amphipoda are most constant 

 in their adaptation. The probability is, that these last 

 are never in the Amphipoda used, except for carrying food 

 to the mouth, or more rarely for climbing, or occasionally 

 grasping the female. In this they are found to possess 



b 



