Il8 LIGHT AND THE BEHAVIOR OF ORGANISMS 



backing slightly at the same time, and goes ahead swerving 

 sharply toward the ventral surface and the oral side. This 

 throws the animal out of orientation; the first method of 

 response does not. This shows that the reaction is due to 

 an increase of intensity on some part or on the whole of 

 the body. It is evident that the intensity of light on the 

 sides of Stentor changes, if it rotates while it is illumi- 

 nated from the side, owing to its own shadow; and since it 

 reacts only when the oral side is carried from a position in 

 which it is shaded to one in which it is illuminated, it is 

 clear that this side must be more sensitive than the aboral, 

 or perhaps better, that the animal is more sensitive when 

 the oral side is exposed than it is when the aboral side is 

 exposed. 



If the light intensity is increased at the same time that 

 the direction of the rays is changed as described above, all 

 the organisms respond with the avoiding reaction at once no 

 matter which side faces the light, just as in the case of Eu- 

 glena. In this response some may be seen to turn upward, 

 some downward, and others to the right or left. They all 

 turn toward the aboral side. The direction in which they 

 turn therefore depends upon the position of this side when 

 the change is made. Stentors frequently respond thus w^hen 

 the direction of the rays is changed without varying the 

 intensity. This takes place when the organisms are highly 

 sensitive. 



It is clear from this description that the orienting reac- 

 tions in Stentor and Euglena are the same in princible. 

 Both organisms can orient in a field of uniform light of 

 constant intensity. The stimuli causing orientation are 

 however due to changes of intensity on the sensitive struc- 

 tures in the body. Such changes of intensity in a field of 

 light of uniform and constant intensity are caused by the 

 shadows produced by one part passing over other parts as 

 the organisms rotate. There is no evidence that the direc- 

 tion of the rays functions in orientation excepting in so 

 far as it may influence changes of intensity; nor is there 



