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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