LIGHT-ADAPTATION IN THE PLANARIAN 241 



reverse direction abruptly during its course under normal illum- 

 ination after directive light. This does not appear in the course 

 under normal illumination before exposure to directive light. 

 Since the preponderance of abrupt turns in one direction in- 

 creases the average curvature of the path in that direction, the 

 difference can be stated numerically as follows: For the eight 

 subjects under initial, non-directive light the curvature in the 

 L-direction was 1.08 times that in the D-direction, — i.e., very 

 nearly equal. Under directive illumination the L-curvature was 

 1.95 times the D-curvature. Afterward, under final, normal 

 light the D-curvature was 1.22 times the L-curvature. Thus 

 we see that the curvature is greatest in the direction of least 

 turning. 



This curious type of behavior appears similar to that de- 

 scribed by Pearl 8 as the response of the animal to repeated 

 strong mechanical stimulation. After repeatedly giving the 

 negative reaction in response to strong mechanical stimulation 

 of the anterior end, it " finally jerks back with a strong longi- 

 tudinal contraction, and turns the anterior end through a con- 

 siderable arc, so that it points in ... an entirely opposite 

 direction to that of the previous reactions." ' This reaction," 

 Pearl continues, " appears as if, after the animal had tried in 

 vain to get away from an uncomfortable stimulus by its ordinary 

 reaction, it finally tries a wild jump in the opposite direction. 

 This curious change in the reaction induced by a repetition of 

 strong stimuli . . . indicates the effect of the organism 

 as a whole upon its reflexes." 



It is not, however, by any means necessary to assume that 

 the animal, failing to get relief by the usual negative response, 

 alters its mode of behavior, implying, as this does, consciousness 

 other than the simplest. It is quite conceivable that the abrupt 

 reversals of direction for brief periods, the " wild jumps," are 

 forms of a compensatory movement, which acts as a relief, not 

 for the continued stimulation, but for the continued movement 

 away from the stimulus. Who has not, after prolonged, unac- 

 customed bending of the body forward in the removal of potatoes 

 from the ground or tacks from the carpet, stood erect and bent 

 the body far backward in order to " take the kink out of his 

 back," before resuming his labors. The planarian directs its 



8 Quar. Jour. Micr. Sci., 46, 1903, p. 580. 



