238 LESLIE B. AREY AND W. J. CROZIER 



the conditions of existence, no one of experience will be prepared 

 to deny; there are also, of course, modes of reaction under con- 

 ditions artificially imposed which are important from this stand- 

 point. It is rather the harmonious complexity of these indi- 

 vidual responses which presents in reality the difficult problem 

 and the one of importance. The behavior of Chiton is most 

 illuminating in this respect. 



In an earlier chapter we have briefly touched upon the coor- 

 dinations between size, habitat, coloration, and photic orienta- 

 tion in the Chiton population. The main fact appears to be that 

 as Chiton gets older it moves out into more open situations. The 

 other correlations follow automatically in the wake of the chang- 

 ing sense of the animal's phototropism. This change is due to a 

 gradual shifting of the 'indifferent point' of light intensity sepa- 

 rating the region of lower intensities, leading to photopositive 

 behavior, from the region of higher intensities, leading to photo- 

 negative movements. We have shown that the principal factor 

 involved in this alteration is the erosion of the tegmenta of the 

 valves (bringing about the destruction of the photosensitive 

 aesthetes) . 



It remains to discover what it is that produces the erosion of 

 the shell. Comparative studies of differing environments fre- 

 quented by Chiton are now in progress and should ultimately 

 afford a quantitative answer to this question. The erosion of the 

 shell is general among the chitons of large size. Plate ('01 a, 

 pp. 381-3) supposed it to result from 'wave action' and the 

 mechanical effect of sand. In C. tuberculatus it would seem that 

 the periostracum is in part eroded as a result of chemical action 

 of the water, combined with the failure to continue its secretion 

 upon older parts of the shell; as well as to the activity of barnacles, 

 algae, and other organisms which settle upon the valves.^^ In 

 different localities it would seem that these two factors are of 



" Barnacles live upon the valves of a chiton until they have formed two or 

 three distinct growth lines. The firmness with which they are attached to the 

 shell plates of Chiton depends upon the degree of the original erosion. In chitons 

 of medium size the barnacles usually are not very firmly attached; after death 

 they drop off and leave no scar. On older chitons (seven to nine years) the 

 dead remains of barnacles adhere for some time. 



