THE SENSORY RESPONSES OF CHITON 235 



4. On the theory of phototropism 



a. The orienting stimulus. In a previous article (Crozier and 

 Arey, '18) we have given some discussion of this matter, and need 

 refer here merely to the main conclusion. In young individuals 

 the simultaneous exhibition of a precise reaction to shading, and 

 no such response to increased illumination, is thoroughly incon- 

 sistent with the idea that negative orientation by light is induced 

 by stimulus derived from an increase of illumination, as such 

 (Crozier, '14, '15 b). The further continued presence of this 

 shading response in older chitons under conditions in which they 

 are photopositive, together with the fact that any reaction to 

 increased light intensity which these older chitons did exhibit 

 was clearly of a negative character, is of the first importance 

 theoretically. It completes the cycle of qualitative proof that 

 the constant action of light, not change of intensity, is the causal 

 stimulating agent in photic orientation. 



Two further types of experiment may be cited in this 

 connection. 



A chiton of medium size orienting away from the light is 

 purposely shaded on the side to which it is turning. Light 

 falling at an angle of about 45° is used in the experiment. The 

 chiton reacts by depressing the girdle, as usual, and swings 

 somewhat away from the shaded side. By repeated successive 

 shadings in this way, the chiton may be made to move for a 

 time at an angle with the line of the incident light, although 

 after it becomes exhausted for shading it completes its orienta- 

 tion in the usual way. 



A chiton completely exhausted by shading orients without 

 delay away from or toward the light, depending on the age of the 

 animal. 



Both types of experiment result in behavior agreeing with the 

 conclusion above stated. 



b. The determination of positive and of negative orientation. In 

 an earlier section of this paper we have called attention to the 

 fact that at a length of about 5.5 cm. (for the Chiton population 

 in Great Sound) the shell begins to show noticeable erosion, 



