INTRODUCTION. XVll 
Medical Times and Gazette, vol. xxxvi. p. 467, 7th Novem- 
ber, 1857, named the Endophragmal arch (Fig. 3, En.). 





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 
