382 



THE SENSORIAL FUNCTIONS. 



cords, which run in a longitudinal direction, are here per- 

 fectly distinct from one another, and even separated by a 

 small interval: they present a series of ganglia, which are 

 nearly of equal size, and equidistant from one another, one 

 pair corresponding to each segment of the body,* and united 

 Joy transverse threads: and other filaments, diverging late- 

 rally, proceed from each ganglion. During the progress of 

 growth, the longitudinal cords approach somewhat nearer to 

 each other, but still remain perfectly distinct. The first 



439 



440 



pair of ganglia, or the cephalic, have been considered, though 

 improperly, as the brain of the animal. 



The next step in the gradation occurs in the Phyllosoma 

 (Leach,) where the ganglia composing each pair in the ab- 

 domen and in the head, are united into single masses, while 

 those in the thoracic region are still double. In the Cymo- 

 thoa, (Fab.,) which belongs to the family of Oniscus, there 

 is the appearance of a single chain of ganglia, those on the 

 one side having coalesced with those on the other; each pair 

 composing a single ganglion, situated in the middle line; 

 while the longitudinal cords which connect them still re- 



* These segments are numbered in this and the following figure in their 

 proper order, beginning with that near the head. A is the external antenna; 

 a, the internal antenna; and E, the eye. 



