144 Herbert Eugeiie Walter 



stimuli obtained a similar result. He observed that apparently 

 "negative" as well as "positive" responses resulted when the 

 animals were subjected to non-directive thermal stimuli. 



Another noticeable phenomenon with reference to responses to 

 intensity is, that more wigwag responses occurred at the critical 

 line separating two different intensities when the lesser of the two 

 intensities was i6 cm. than when it was 7,^ cm. (Table XI, p. 69). 

 Similarly responses were more frequent when planarians were 

 subjected suddenly to dark than when they were flooded suddenly 

 by light, and, throughout a large number of series, responses were 

 invariably more frequent when the worms were passing into a 

 region of diminished intensity than when they were entering an 

 area of increased intensity. It is to be inferred that all these 

 phases of behavior are due to the probable fact that the lower inten- 

 sities compared are nearer the worm's optimum as regards light 

 than the higher ones, since the latter apparently have a tendency 

 to inhibit activity. 



Lastly, the relative part played by intensity of light varies 

 decidedly in different species of planarians. The relative inten- 

 sity in different parts of an aquarium, when no lens is used to les- 

 sen the contrast, has comparatively little influence upon Phagocata 

 gracilis, as its extensive wanderings (typically reproduced in 

 Fig. 11) toward and away from the source of light, indicate. 

 Planaria maculata and Planaria gonocephala, on the contrary 

 (Figs. 13 and 14), notwithstanding their ability to come toward the 

 light in the direction of the "rays" throughout the farther half of 

 the dish, seemed invariably to encounter an impassable barrier 

 as soon as they approached within a certain intensity, thereby 

 showing a more delicate responsiveness to intensity differences. 



d The Modifying Effect of Other Factors 



In attempting to analyze the relative bearing of the intensity and 

 of the direction of light upon the behavior of planarians there must 

 be constantly kept in mind two general sources of error which are 

 always present when these factors of light are in operation. These 



