CHAPTER VIII 
THE NERVOUS SYSTEM IN RELATION TO 
SEGMENTATION 
The bodies of the annelids and arthropods among the 
invertebrates and the bodies of all vertebrates are made 
up of segments, metameres, or somites, and the problem 
of the origin and the evolution of this segmentation or 
metamerism has ranked as one of the great problems of 
speculative morphology, while its physiological aspects 
have received but little attention. 
Without attempting a survey of the various theories 
of segmentation it may be noted that they involve two 
different conceptions of the segment and of the process 
of segmentation. According to the one conception the 
process of segmentation is essentially a process of agamic 
reproduction, the segment being equivalent to a new 
zooid, but instead of becoming a complete new individual 
and undergoing separation, it is co-ordinated or inte- 
grated with the head region and other segments into a 
new individuality. In short this theory conceives the 
process of segmentation as a modification of certain 
processes of agamic reproduction, such as the repeated 
transverse fissions which occur in certain Turbellaria 
and give rise to temporary chains of zooids. 
The other conception regards segmentation as 
originating in the reduplication of certain internal 
organs, and most of the theories based on this conception 
hold that the process is primarily localized in the 
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