CHAPTER IX 



NERVOUS CENTRALIZATION AND CEPHALIZATION 



IN EVOLUTION 



The terms "centralization" and "central nervous 

 system" possess of course a morphological and a physio- 

 logical significance. Morphologically, centralization is 

 the process, both ontogenetic and phylogenetic, of 

 localization and aggregation of the chief mass of nervous 

 tissue in a particular region of the body. Speaking 

 physiologically, centralization represents integration, 

 and the central nervous system is really the integrative 

 nervous system, as Sherrington (1906) has so beauti- 

 fully demonstrated. The term "cephalization" refers 

 to one particular aspect of centralization, viz., the con- 

 dition or process of integration with respect to the 

 longitudinal axis. The point to which I wish to call 

 particular attention is that these terms refer not merely 

 to a morphological localization or aggregation, but to a 

 physiological condition and process, and also to an 

 evolutionary process. Morphological centralization and 

 cephalization in any particular case are at best only the 

 indication or record in structure of the degree of integra- 

 tion attained. Moreover, a high degree of nervous 

 centralization or cephalization may exist in compara- 

 tively simple forms with relatively slight degree of 

 integration as compared with the higher animals, e.g., 

 ascidians, rhabdocoele and polyclad Turbellaria in 

 which the central nervous system is entirely in the head. 



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