LIGHT-ADAPTATION IN THE PLANARIAN 247 



turning away from a stimulus and presumably of equal benefit 

 to the organism. If we postulate a " get-away " process for the 

 first reaction, we may assume a " stretch " process, different in 

 quality, for the second. Consciousness would then consist of 

 the mere alternation between two qualitatively different pro- 

 cesses, paralleling the alternation between the two kinds of 

 reaction, or possibly merely the flashing in of the processes at 

 the moment of change. 13 



SUMMARY 



...... 



Planarians, it subjected to continued, intensive, directive 



light, continue turning from the source of light. If they are 

 immediately placed in non-directive light after being in directive 

 light, they turn consistently in the direction from which the 

 light at first came. This behavior indicates that the planarian 

 becomes light-adapted on one side during the exposure to direc- 

 tive light, and later, under a uniformly applied stimulus, turns 

 toward the light-adapted side, because that side, on account 

 of its decreased sensitivity, is excited to a less degree than the 

 other, just as would be the case were the animal subjected to 

 directive light from the other side. 



After continued turning in one direction under directive 

 light, the planarian tends to make frequent abrupt changes of 

 direction toward the light. These abrupt turns increase in 

 frequency for a while, but, if the stimulation is very long con- 

 tinued, may finally disappear altogether. They may be ex- 

 plained as a form of compensatory muscular movement, initiated 



12 An attempt was made to duplicate planarian conditions upon two human 

 observers, who, with only diffuse light admitted to the eyes through a tracing- 

 cloth blind, the left eye in addition very darkly screened, an electric light fastened 

 to the body in front of the right shoulder, and the arms and legs bound so as to 

 admit only of very limited movement, were instructed " to leave the mind as 

 blank as possible, to move in response to any strong impulse that might be felt, 

 and, when not strongly impelled to do otherwise, to turn from the light." The 

 result was a slow shuffling forward to the left with a tendency, when interrupted 

 by an object, to move backward (compensatory movement?) for a little before 

 going forward again. The introspections showed a scant consciousness, with the 

 left-forward movement carried throughout in unclear kinesthetic terms. At the 

 pauses, there was a heightened consciousness, with an alternation between the 

 left-forward and backward impulses, both carried kinesthetically, in which the latter 

 often supplanted the former, with accompanying backward movement. Although 

 the kind of muscular movement, involved is not exactly analogous to that in the 

 planarian, it is believed that the similarity of the introspective account to that 

 derived for the planarian argues in favor of the latter. Cj. a preliminary report 

 on The Use of the Maze in Comparative Psychology, by L. M. Day and the writer, 

 Psych. Bull, 9, 1912, p. 60. 



