INTRODUCTION. xlvil 
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 Jsopoda, 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 Oniscide 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 
Crustacés.” The latter author figures a distinct ganglion 
to each of the caudal segments, illustrating his view from 
observations on Cymothoé, 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. Vetenskaps-Akademiens Forhandlingar, Jan. 1859. 
