INTRODUCTION. XV11 



last eleven are complete and perfectly distinct, and each 

 of them, without exception, bears its appropriate pair of 

 members. 



Amongst the higher forms of Crustacea, it is in the 

 Urachyura, where the nervous system is found in the most 

 concentrated condition, that the condensation of the rings 

 of which the body is composed, is carried to the greatest 

 extent. It is indeed somewhat difficult, at first sight, to 

 determine the homologies of the segments of which the 

 carapace, as it is termed, is theoretically composed. This 

 large enveloping buckler in fact covers the whole of the 

 thorax, and even the abdomen itself is folded underneath 

 it, so that the whole animal is hidden, when viewed from 

 above, by this extraordinary development of two of the 

 cephalic segments ; and although in the Brachyura the 

 first two segments, the ophthalmic and the antennary, are 

 soldered to the carapace, yet, as we find that in some 

 other forms these two are entirely distinct, it would appear 

 that the carapace is essentially composed of the third and 

 fourth rings, composing what Dr. Milne Edwards terms 

 the antenno-maxillary segment. 



This remarkable portion of the tegumentary system, 

 covering, as it does, the whole of the viscera, is found to 

 be more or less distinctly divided into regions, which are 

 indicated by elevations, separated from each other by 

 grooves ; and to these regions have been given names 

 derived from the different organs which are immediately 

 covered by them. As reference is frequently made to 

 these regions in generic and specific descriptions, I here 

 give an illustration of them.* 



* The gastric or stomachal region is marked r s ; the branchial, r b ; the hepatic, 

 rh ; the genital, rg ; the cardiac r c ; the intestinal, r i. 



