146 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 
