232 LESLIE B. AREY AND W. J. CROZIER 



precise responses of the character described when the hght in- 

 tensity is suddenly reduced. 



b. Increased light intensity. Chitons from medium to large size, 

 under water in aquaria placed in direct sunlight, give also a 

 response to increased illumination. This response is not evident 

 except in bright sunhght. It consists in a depression of the girdle 

 similar to that induced by shading. The response is, however, 

 never of such quickness, vigor, or completeness as that to shad- 

 ing. In smaller chitons, or with larger ones in diffuse light, 

 the reaction to increased illumination (if present at all) is so 

 slight as to escape detection. At its maximal development, it 

 comprises a local depression of the girdle to the substratum, and 

 does not involve a 'suction reflex' of the whole animal, such as 

 is induced by a very slight decrease in illumination. The thresh- 

 old of sensitivity for increased illumination is, indeed, very much 

 higher than that for decreased. 



It should, however, be noted that if a large chiton is shaded 

 dorsally the girdle may not be much elevated again for some time, 

 provided the state of lowered intensity is allowed to continue. 

 On removing the shadow, the girdle is again elevated. If the 

 intensity be slightly decreased the girdle of a large chiton may be 

 lowered rather slowly, so that after two seconds the shadow may 

 be removed before the girdle has been completely depressed; 

 the movement of the girdle may then cease abruptly with the 

 incidence of the more intense light. 



A similar response is evidenced upon the ventral surface of the 

 animal. If the light be suddenly increased (to direct sunlight) 

 about two seconds subsequent to the beginning of the 'curling-up' 

 response induced by a shading, this response sometimes ceases 

 abruptly; the chiton may or may not proceed then to straighten 

 out. 



The effects of increased illumination are more conspicuous in 

 lamplight at night. Clear indications have been obtained of a 

 specifically higher photic sensitivity at night as compared with 

 dayhght hours. A discussion of this matter awaits further 

 investigation. 



