The Reactions of Planarians to Light 



79 



In the other instance an individual worm, Planaria gonocephala, 

 made the erratic average emergence from a circle of 145 , just 

 35 short of absolute positiveness. This worm was carefully 

 isolated and tested again four days later under identical external 

 conditions when it was found to have returned to a normal nega- 

 tive condition by showing an average record of 56 . 



Accuracy of Orientation. It was found to be frequently the case 

 that when negative worms were subjected to directive light their 

 first movement instead of being directly away from the source of 

 light formed a path in a diagonal direction. This tendency to 



100° 



Light 



Fig. 6 The arrow at the left represents the constant direction of the light. In each of the three sets 

 of trials each worm was headed successively toward o°, the upper (in the diagram) 90 , 180 , and the 

 lower 90 . The point of average emergence for the first set of trials — supposing the records of the 

 lower semicircle to have been transferred to the upper semicircle — was at 45 , of the second set, 

 at ioo°, and of the third set, at 124 . » 



travel diagonally away from the light has also been noted in the 

 case of the earthworm by Smith ('02, p. 469). 



If the negative phototaxis of planarians is to be explained on 

 the theory of tropisms, and if, moreover, the eyes, as Hesse ('97) 

 maintains, are the principal organs which, when unequally illumi- 

 nated, cause the directive response, it may be shown that possi- 

 bly the arrangement of the crescentic pigment shields around the 

 sensory cells of the eyes is such that equal stimulation of both eyes 

 is just as certainly received by the worm when it is in a position 

 diagonal to the light as when it is pointed directly away from the 

 light. 



