LIGHT-ADAPTATION IN THE PLANARIAN 235 



Table IV shows the excess percentages of both linear and 

 angular distances for the entire series on individual B. The 

 periods of non-directive illumination are given as before (Table 

 II) ; the periods of directive illumination are fractionated. 



ADAPTATION TO LIGHT 



In this connection it is desired to show that the planarian, 

 after a continued period of turning under directive light, exhibits 

 a marked tendency to turn in the opposite direction under non- 

 directive light. 



As a measure of turning there have been taken the excess 

 percentages of distance passed through while turning, as given 

 in Table I. 



It might appear at first thought that the proper measure of 

 the predominance of movement toward one side or the other 

 would be the number of degrees turned through. It must be 

 remembered, however, that the planarian under a strong direc- 

 tive light sometimes exhibits a tendency to turn abruptly toward 

 the light. A similar, although less marked, reversal of direction 

 occurs frequently in the period immediately following that under 

 directive illumination. In this manner the planarian may turn 

 rapidly around tw T o or three times, thus passing in a few seconds 

 through as large an angle toward the light as it has turned 

 through away from the light in several minutes of steady glid- 

 ing. Since it is most probable that these abrupt turns are not 

 the result of the ordinary directive influences under which the 

 planarian turns, but a specialized response to a continued inten- 

 sive stimulation, it seems fairer to measure the continuous ten- 

 dency of the animal to rotate, not by the number of degrees of 

 rotation, but by the distance passed through while turning. 

 The reliability of this substitution of linear for angular measure 

 is attested in a few courses where the abrupt turns were almost 

 entirely absent and the percentages obtained in both ways were 

 very nearly equal. 7 



7 For example, with individual F the average curvature toward the light was 

 only 1.07 that away from the light; the excess per cent, based on linear measure 

 was 66%; on angular measure, 65%. 



All percentages were computed upon both bases, and both, in general, gave 

 the same result, although those from linear measure were the more striking. The 

 average values for initial non-directive, directive, and final non-directive perform- 

 ance are, respectively, as follows: For linear measure, 5% L, 59% D, and 67% L; 

 for angular measure, 11% L, 32% D, and 69% L. 



