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-cesophageal 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 prosoma 
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 mesosoma, 
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-cesophageal 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 ganglidn-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 newromeres, although all 
such indication has disappeared in the adult; thus the infra-ceso- 
