256 LESLIE B. AREY AND W. J. CROZIER 



depends upon the intact condition of the pallial nerve strand on 

 that side. The responses of isolated portions of an animal sec- 

 tioned transversely are such as to show the absence of any strong 

 centralization. This is in agreement with the known occurrence 

 of ganglion cells throughout the whole length of the nerve strands. 

 In Chiton nervous centralization is relatively at an incipient stage. 



d. Alteration in the behavior of Chiton toward light with ad- 

 vancing age of the animal is the primary variable determining 

 the exhibition of a very complex series of environmental inter- 

 relations. The young Chiton is photonegative, the old Chiton 

 photopositive, to sunhght. Chitons of intermediate age are 

 positive to weak light, negative to strong. Photic orientation is 

 direct, and is determined by the constant intensity of light, 

 not by change of intensity. The progressive alteration in the 

 sense of phototropism is determined by the erosive destruction 

 of the photosensitive aesthetes, conditioning in older Chitons a 

 lower specific stimulating power of the light. The erosion of the 

 shell is in turn produced, in part, by, 1) normal growth effects; 

 2) the activity of organisms settling upon the shell plates. 



The homochromic coloration of Chiton is determined by the 

 nature of its algal food and by the organisms living upon its 

 dorsal surface. The older chitons are relatively stationary; 

 therefore specific local environmental influences have oppor- 

 tunity to affect the appearance of the chitons. The animals 

 associate in groups, commonly of a certain average size and con- 

 taining numbers of both sexes. Certain seemingly 'adaptive' 

 consequences may reasonably be attributed to this mode of 

 occurrence. A homochromically colored isopod is characteristi- 

 cally associated with Chiton tuberculatus. 



These and other harmonious correlations, of which mention 

 is made in the body of this report, follow automatically in the 

 wake of the changing phototropism of Chiton. The animal's 

 habits determine the environment in which it dwells. The pre- 

 cise and intricate bionomic correlations here briefly mentioned 

 are an automatic consequence of its modes of reaction. 



Dyer Island, Bermuda, 

 May, 1918 



