INTRODUCTION. xlvii 



system in these two orders is that of a typical crustacean. 

 A ganglion corresponds to every segment of the animal ; 

 those belonging to the organs purely of sensation being 

 amalgamated together into a cephalic lobe. This is very 

 beautifully shown by HH. Loven and Bruzelius (Bidrag 

 till Kannedomen om Amphipodernas inre byggnad*). 



Every ganglion of the several segments after the head 

 is united to the others by two parallel cords in the 

 Amphipoda, and one in the Isopoda, although in the 

 genus Ligia we distinctly made out two, as in the Amphi- 

 poda : from each ganglion, on the right and left, is given 

 off two main branches, and in Ligia we observed two 

 other less important threads. These supply the legs and 

 internal viscera. From the cords, about midway between 

 each ganglion, branches off, on the external side of each, 

 a single branch, which in the Oniscida M. Lereboullet 

 places nearer to the preceding ganglion. In the Amphi- 

 poda, we found it rather nearer to the succeeding ganglion. 

 In Ligia, it appears to be just midway between the two, 

 from the base of which, both before and behind, spring 

 other thread-like branches. 



The diagrams of the arrangement of the caudal supply 

 of nerves, given in the memoir of Lereboullet, differ from 

 that given by M. Milne Edwards in his " Histoire des 

 Crustaces." The latter author figures a distinct ganglion 

 to each of the caudal segments, illustrating his view from 

 observations on Cymothoe, in which the six segments are 

 separate, while Lereboullet illustrates the caudal ganglia 

 as being consolidated into a single mass, from which 

 numerous threads are sent back to the extremity of the 

 animal. Moreover, this author only figures six separate 

 ganglia after the cephalic mass, which would make (even 

 allowing the oral appendages to be supplied with small 

 filaments of nerves, instead of branches springing from 



* Ofversigt af Kongl. Yetenskaps-Akademiens Forhandlingar, Jan. 1859. 



