252 COMPARATIVE ANATOMY. 



Nervous System. 

 § 195. 



The nervous system of the Arthropoda resembles that of the 

 Annelides, with which it completely agrees in its fundamental 

 characters. A ganglion placed above the oesophagus is the cephalic 

 ganglion or cerebrum; two commissures from it embrace the 

 oesophagus, and form a nervous oesophageal ring, by being 

 connected with a ventral ganglion. From this inferior ganglion a 

 series of ganglia, connected by long commissures, extend along the 

 ventral internal surface of the body, forming the ventral gangli- 

 onic chain. The greater size of the cephalic ganglion, compared 

 with that of the ventral ganglia, has been already seen in many of 

 the Annulata ; in the Arthropoda it is ordinarily still more distinct ; 

 this condition may be partly explained by its relations to the more 

 highly developed organs of sense, if we recognise in the dorsal 

 oesophageal ganglion something similar to the brain of the Verte- 

 brata. Led by an idea of this kind, some have compared even the 

 ventral ganglia, or ventral medulla, with the dorsal medulla of the 

 Vertebrata, and have striven to carry the comparison still further; 

 these attempts ignore the complete difference between the type of 

 structure of the Arthropoda and of the Vertebrata. 



The increase in size of the cerebrum is, as has been above 

 indicated, in direct connection with the development of the 

 higher sensory organs, especially that of sight ; and its modifications 

 are in great part dependent on these. The ventral chain of 

 ganglia also undergoes essential modifications, although it is not 

 always possible to make out in them a regular dependence on 

 the metaineric condition of the body. The repetition of similar 

 metameres gives rise to similarity of character in the ganglia of the 

 ventral chord, and to a regular series of them. Where some meta- 

 meres are especially developed, their ganglia also are considerably 

 increased in size, while where there is concrescence of the metameres 

 an approximation of several groups of ganglia can also be noted ; 

 this not unfrequently leads to a complete fusion into several larger 

 ganglia, or the formation of a single large ventral ganglionic mass. 



The ganglia of the ventral ganglionic chain are primitively 

 paired, and connected by a transverse commissure, as in the 

 Annulata. By the shortening of these transverse commissures, an 

 approximation, and finally a more complete fusion, takes place. 



The peripheral nervous system arises from the ganglia of the 

 central system, that is, of the cerebrum and ventral chain, which are 

 distinguished by the possession of ganglionic cells. The nerves either 

 arise directly from the ganglionic portions, or run for some distance 

 along the longitudinal commissures, and then pass off from them. 



The higher sensory nerves arise, as a rule, from the cerebral gan- 

 glion. This holds especially for the nerves of the eyes and antennoe. 



