LIGHT-ADAPTATION IN THE PLANARIAN 237 



(E) to 94% (A) away from the light ; i.e., they have, on the aver- 

 age, been changed by the change of conditions from 5% in the 

 L-direction to 59% in the D-direction. Under normal illumina- 

 tion, after exposure to directive light, the turning varies from 

 21% (D) to 100% (B and C), on the average, 67% , in the oppo- 

 site direction from that preponderant under directive light. 



From these results, it seems safe to conclude that the plan- 

 arian turns from the light because of a difference of intensity of 

 excitation by the light stimulus impressed upon its two sides. 

 As the animal is exposed continuously to the light, the light- 

 recipient organs on the side toward the light source become 

 more nearly exhausted than those on the other side, i.e., the 

 planarian becomes on this side light-adapted. If now the sub- 

 ject is placed under normal, non-directive illumination, the 

 uniformly intense light will excite the light-adapted side less 

 than the other side, thus producing within the organism the 

 conditions that prevail when the opposite side is more intensely 

 stimulated. Hence the animal turns toward the light-adapted 

 side. 



That the turning reactions after and during directive illum- 

 ination are of the same order is also indicated by the fact that 

 the curvature toward the more excited side is in both cases 

 greater, suggesting that the animal reacts in a positive manner 

 toward the side excited by a relatively (although not absolutely) 

 strong and continued stimulus, just as it responds by reacting 

 toward other intensive, continued stimuli. The difference of 

 •curvature, — the ratio equals 1.22, — is, however, perhaps not 

 great enough to be conclusive on this point. 



Table II presents the complete series of individuals B and E. 

 These results are interesting as a supplement to the material 

 •of Table I, although they also raise a question that can be 

 answered only by further experimentation. 



With B, it will be observed that strong illumination on the 

 right side was alternated with normal trials for more than five 

 hours. It will be noticed that the " leftness " under right stim- 

 ulation and the " lightness " under normal stimulation with 

 right-adaptation increases, in general, throughout the first half 

 •of the series and decreases throughout the second half, although 

 the change is not perfectly regular. This fact may mean that at 

 first, while the right-adaptation is in process, the reaction be- 



