LIGHT-ADAPTATION IN THE PLANAR IAN 239' 



comes proportionately more pronounced ; but that later, with 

 the right-adaptation complete, the left side, which during hori- 

 zontal illumination by no means remains in complete darkness,, 

 but is on the contrary quite strongly illuminated, also becomes' 

 partially light -adapted, thus decreasing the difference in excita- 

 tion between the two sides, and hence the pronounced char- 

 acter of the turning. 



After planarian E had become adapted to light on the left 

 side, it was exposed to light in three successive trials on the 

 right side. The animal turned from the light consistently, but, 

 instead of reversing direction under normal illumination, it 

 continued to turn decidedly toward the left. It seems as if, at 

 the end of the second trial, the adaptation on the right side 

 could certainly not be less than that on the left, and the animal 

 might be expected to turn to the right or at least to show no 

 preference for one side over the other. Why the leftness persisted 

 is a question that can be decided only by experiments upon a 

 number of individuals. 



The distance percentages in Table III show that during the 

 first total period of lateral stimulation the animal tends to move 

 less and less away from the light, the excess movement from 

 the light being, on the average, 78%, 56%, and 38%, respec- 

 tively, in the three successive fractions. This change is probablv 

 due in the main to increasing light -adaptation, although it may 

 be influenced by compensatory movements and decreased sen- 

 sitivity under fatigue. It seems safe, however, to say that the- 

 effect of light-adaptation reaches a maximum within the first 

 ten minutes' exposure. In planarian E, the effect was very 

 marked at the close of a fifteen -minute period. 



It cannot be said that complete adaptation on both sides 

 was reached at any time, although the absence of the usual 

 response under non-directive illumination in the eighth and 

 tenth trials of B (0% and 6% D) suggests that this condition 

 may have been approximated in the preceding four hours of 

 experimentation. 



The adaptation appears to be true adaptation in that it is 

 not permanent. After twenty-four hours, C, F, G, and H showed 

 practically no preference for one side over another. A, B, D, 

 and E were too fatigued to respond. After forty-eight hours 

 A and E appeared normal and showed practically no preference 



