142 Herbert Eugene Walter 



the two ends of the aquarium, the one opposite and the one next 

 to the source of Hght, with the exception that there was a shght 

 difference at the two ends due to the fact that Hght in its passage 

 through water is partially absorbed. But modification of Hght 

 in any degree results in producing less intensity at the farther end 

 of the aquarium, though this difference is less pronounced when 

 a lens is employed. Therefore, although worms placed in this 

 apparatus went with considerable precision in the direction of the 

 propagation of the Hght, there is no certainty that their behavior 

 was not due simply to differences in intensity. Worms which are 

 thus apparently traveling directly in accordance with the direc- 

 tion of the light, are meantime being subjected to different inten- 

 sities at the anterior and posterior ends of the body, for the reason 

 that the anterior end is more or less shadowed by the rest of the 

 body, since the latter cuts out a certain portion of the light received 

 at the posterior end. 



That direction of light is a factor by no means to be disregarded^ 

 even if it cannot be proven to be the immediate cause of phototaxis, 

 is apparent when it is recalled that slight changes in direction 

 call out corresponding changes in the course of the gliding pla- 

 narian, whereas considerable changes in intensity when the direc- 

 tion remains constant and particularly when such changes are 

 gradually made, may fail entirely to produce corresponding 

 changes in the worm's behavior. This is due to the fact that slight 

 changes in direction may cause considerable changes in the asym- 

 metry of illumination. When a worm, for example, is receiving 

 horizontal light from behind, its head is more or less in shadow, 

 the sides of its body being at the same time equally illuminated. 

 The moment the light is shifted in even a small degree to one side, 

 one entire side of the animal may receive an increase of illu- 

 mination and the opposite side be thrown into shadow. Thus a 

 slight change in position initiates a fundamental change in the 

 distribution of intensity over the planarian's body. 



c Instances of Behavior Due to Intensity Alone 



The effect of intensity as a separate factor from the directive 

 influence of light is clearly demonstrable in certain phases of 



