170 The Animal Mind 



light, the medusa on beginning to move again "usually turns 

 in such a way as to move back into the shaded region." This 

 effect Yerkes, who first observed it, thinks due to the contrac- 

 tion of the bell on the more illuminated side ; that is, it is a 

 definite reflex to a localized stimulus. Orientation results 

 from the fact that the greater intensity of stimulus on one side 

 of the bell produces contraction at that point (451, 470, 468). 



We have already seen that planarians offer an illustration 

 of photopathy. Light is not, however, wholly without effect 

 on the direction of the animal's movements. It has been 

 found that when planarians are placed with the head toward 

 the source of light, they have a distinct tendency to turn out 

 of the path, while if their heads are directed straight away 

 from the light, their tendency is to keep in the path (313). 

 It is probable that when the animal turns its head toward the 

 light, its movement is checked by a negative reaction. An 

 attempt has been made to show that photopathy, rather than 

 response to the direction of the light rays as such, governs the 

 responses of the land planarian Bipalium kewense to photic 

 stimuli. The apparatus was arranged so that a shadow was 

 thrown from above upon an area of light coming horizontally 

 from one side. Although the animal is negatively phototropic 

 to a marked degree, it would crawl toward the source of light 

 in order to get into the shadow (81). The explanation for 

 this may very likely correspond to that offered by Jennings for 

 the reverse behavior of Strasburger's swarm spores. That 

 is, the planarian might have obtained a greater diminution 

 of light intensity by moving into the shadow than by turning 

 aside from the path of the rays. The one possibility excluded 

 is that the negative reaction of planarians is a response to the 

 direction of the light rays as such. 



The same evidence for photopathy, as distinct from photo- 

 taxis produced by direction of light, has been obtained for the 



