I 4 6 THE ORIGIN OF THE NERVOUS SYSTEM 



always present. The physiological basis of nervous cen- 

 tralization exists as soon as axiate pattern is established, 

 that is, more or less definite axiate excitation-transmission 

 relations are determined before the appearance of special- 

 ized nervous structure, and when such structure appears 

 its localization and general pattern are determined by 

 these still more fundamental features of pattern. 



PROGRESSIVE CENTRALIZATION AND CEPHALIZATION 



IN VARIOUS GROUPS 



Starting from the slight degree of centralization or 

 axiation of the nervous system which is present in its 

 most primitive clearly recognizable forms, we find many 

 indications of progressive centralization and cephaliza- 

 tion, which is centralization with respect to the longi- 

 tudinal axis. In most of the great phyla or in some of 

 their groups the evolutionary process appears more or 

 less clearly. Even within the triclad Turbellaria differ- 

 ent degrees of cephalization appear and, as already 

 noted (p. 142), the rhabdocoeles and polyclads show a 

 higher degree of cephalization than the triclads. In the 

 mollusks again conditions range from the "stepladder" 

 nervous system of the amphineura through the gang- 

 lionic system of pelecypods and gastropods to the highly 

 cephalized system of the cephalopods. The annelids, 

 on the other hand, show but little progressive change in 

 these respects, although certain data of regeneration 

 indicate that a number of anterior segments definite 

 for the species, but differing in different species, are 

 more or less integrated with the prostomium to form a 

 head region (Hyman, 1916). In various groups of the 

 Crustacea, in the insects and to some extent in the 



