LIGHT-ADAPTATION IN THE PLANARIAN 245 



continuous movement in the opposite direction. Pearl seemed 

 to think that the positive response to continued strong stimula- 

 tion was the result of the stimulation itself rather than of the 

 movement caused by the stimulation. If Pearl's interpretation 

 were correct, it should be possible after long exposure to direc- 

 tive light to cause the planarian to turn always toward the 

 stimulus. Upon a theory of compensation the rotation could 

 be brought only to neutrality; the sign of the reaction could 

 not be reversed. In the subjects studied the sign of the reac- 

 tion was certainly not reversed; yet there is no positive evidence 

 that it could not have been. The theory of compensation is 

 offered merely because it is simpler and more comprehensible 

 and, at the same time, it is adequate to all the facts so far 

 observed . 



FATIGUE 



The effect of general fatigue upon the rate of movement can 

 be observed in the series with both B and E (Table II). Here 

 the rate under either normal or intense directive stimulation 

 decreases almost uniformly ; but the rate under intense illumina- 

 tion is almost invariably less than that under normal illumination 

 at the same part of the series. This latter fact appears with 

 all the individuals studied, the average normal rate before and 

 after directive illumination being 9.5 and 10.0 cm. per min., 

 respectively; and during directive illumination, 8.4 cm. per 

 min. This difference agrees with that observed by Parker and 

 Burnett, 10 who found that a rate of 6.7 cm. per min. under 

 vertical illumination was reduced to 6.24, when the planarian 

 was started directly toward the light and continued turning 

 away throughout its course. 



CONSCIOUSNESS 



Of interest to the comparative psychologist is this question : 

 If we impute consciousness to the planarian in connection with 

 the negative reaction, what is this consciousness like? It is 

 apparent that the response of the animal is not to the absolute 

 intensity of a stimulus, but to some resultant of the relative 

 intensity of two stimuli; for, under the general distribution of 

 light with the intense illumination, the animal moves toward 



10 Loc. cit., p. 384. Cf. also Walter, H. E., loc. cit., pp. 56ff., 75ff. 



