546 THE SENSORIAL FUNCTIONS. 



of the higher Crustacea : thus, in the Lobster, the 

 early condition of the nervous system is that of 

 two separate parallel cords, each having a dis- 

 tinct chain of ganglia, as is the case in the Tali- 

 trus : then the cords are observed gradually to 

 approximate, and the ganglia on each side to 

 coalesce, as represented in the Cymothoa: and 

 at the period when the limbs begin to be deve- 

 loped, the thoracic ganglia approach one ano- 

 ther, unite in clusters, and acquire a rapid en- 

 largement, preparatory to the growth of the 

 extremities from that division of the body, the 

 abdominal ganglia remaining of the same size as 

 before. The cephalic ganglion, which was ori- 

 ginally double, and has coalesced into one, is 

 also greatly developed, in correspondence with 

 the growth of the organs of sense. The next 

 remarkable change is that taking place in the 

 hinder portions of the nervous cords, which are 

 shortened, at the same time that their ganglia 

 are collected into larger masses, preparatory to 

 the growth of the tail and hinder feet ; so that 

 throughout the whole extent of the system the 

 number of ganglia diminishes in the progress of 

 dev elopement, while their size is augmented. 



All Insects have the nervous system con- 

 structed on the same general model as in the 

 last mentioned classes ; and it assumes, as in the 

 Crustacea, various degrees of concentration in 

 the different stages of developement. As an 



