490 



THE SENSORIAL FUNCTIONS. 



greater compactness, assumes a more globular 

 form, and has no central perforation. 



These different forms of structure are also exem- 

 plified in the progress of the developement of the 



higher Crustacea : thus, in the Lobster, the early 

 condition of the nervous system is that of two sepa- 

 rate parallel cords, each having a distinct chain of 

 ganglia, as is the case in the Talitrus : 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 developed, the thoracic ganglia approach 

 one another, unite in clusters, and acquire a rapid 

 enlargement, preparatory to the growth of the ex- 

 tremities from that division of the body ; the abdo- 

 minal ganglia remaining of the same size as before. 

 The cephalic ganglion, which was originally double, 

 and has coalesced into one, is also greatly developed, 



