NERA^OUS SYSTEM OF ARTHROPODA. 



257 



§ 199. 



In tlie lusecta^ we find a form which corresponds to the primi- 

 tively homonomous segmentation of the body at the commencement 

 of the process of development ; and all succeeding arrangements 

 of the nervous system are developed from it. The ventral 

 chord traverses, as a rule, the whole length of the body, the 

 ganglia being separated by equal distances from one another, 

 so that the last ganglion lies in the last segment of the body. 

 This character corresponds to the equivalence of the metameres, 

 which obtains at these stages, and points to its transmission from 

 a lower stage, such as that which we find permanently in the 



*N^_,J7i 



Fig. 131. Nervous system of Insccta. ^ Of Termes (after Lespes). J5 Of a Beetle 

 (Dytiscus). C Of a Fly (.ifter Blanchard). gs Supra-CBSophagoal ganglion 

 (Cerebral ganglion), rji Sub-oesopliageal ganglion, g'r fi g^ Fused ganglia of the 



ventral chord, o Eyes. 



Myriapoda. It is only when the Insect passes out of its larval 

 condition to the perfect one that changes appear. The development 

 of some metameres, the intimate fusion of others to form larger 

 portions of the body, the greater developn^nt of the appendages, 

 which persist on some metameres only, with the consequent increase 

 of the muscular supply, as well as many subordinate arrangements, 

 must be considered as affecting the changes which take place in the 

 nervous system. The decrease in number of the ganglia, by the 

 shortening of the longitudinal commissures, and the consequent 

 fusion of separate ganglia, produces a shoi'tening of the whole 

 ventral chord. Owiug to the independent character of the head of the 

 Insecta, in comparison with the other regions, the first ganglion of 



s 



