52 THE ORIGIN OF VERTEBRATES 



This is the great characteristic of the advancement of the central 

 nervous system among the Invertebrata, its concentration in the 

 region of the head. It may be called the principle of cephalization, 

 and is characteristic not only of higher organization in a group, but 

 also of the adult as distinguished from the larval form. Thus in the 

 imago greater concentration is found than in the caterpillar. 



The segmented annelid type of nervous system consists of a 

 supra-oesophageal ganglion, composed of the fused ganglia belonging 

 to the pre-oral segments, and an infra-cesophageal chain of separate 

 ganglia. With the concentration and modification around the 

 mouth of the most anterior locomotor appendages to form organs 

 for prehension and mastication of food, a corresponding concentra- 

 tion and fusion of the ganglia belonging to these segments takes 

 place, so that finally, in the higher annelids, and in most of the great 

 arthropod group, a fusion of a number of the most anterior ganglia 

 has taken place to form the infra-cesophageal ganglion-mass. 



The infra-cesophageal ganglia which are the first to fuse are 

 those which supply the most anterior portion of the animal with 

 nerves, and include always those anterior appendages which are 

 modified for mastication purposes. To this part the name pivsoma 

 has been given ; in many cases it forms a well-defined, distinct 

 portion of the animal. 



Succeeding this prosoma or masticatory region, there occurs in 

 all gill-bearing arthropods a respiratory region, in many cases more 

 or less distinctly defined, which has received the name of mcsosoma. 

 The rest of the body is called the metasoma. 



In accordance with this nomenclature the central nervous system 

 of many of the Arthropoda may be divided as follows : — 



1. Pre-oral, or supra-oesophageal ganglia. 



2. Infra-oral, or infra-cesophageal ganglia and ventral chain, 

 which consist of three groups : prosomatic, mesosomatic, and meta- 

 somatic ganglia. 



The infra-cesophageal ganglion- mass, then, in most of the Arthro- 

 poda may be spoken of as formed by the fusion of the prosomatic or 

 mouth-ganglia, the mesosomatic and metasomatic remaining separate 

 and distinct. The number of ganglia which have fused may be 

 observed by examination of the embryo, in which it is easy to see 

 indications of the individual ganglia or ncuromercs, although all 

 such indication has disappeared in the adult ; thus the infra-ceso- 



