Spatially Determined Reactions 167 



orient with reference to the direction rather than to the 

 intensity of the light. 



Now on the other hand there are some cases where animals 

 apparently seek or avoid light without being oriented at all ; 

 that is, without having their bodies placed in a definite position 

 during the movement. Planar ians are an example of this. 

 Increased intensity of light stimulates them to activity ; they 

 crawl about until they reach a shaded portion, where they come 

 to rest. Their movements are not directed straight away from 

 the light ; in other words, it does not negatively orient them, 

 but it excites them and the shadow brings them to rest (239). 

 To cases such as this, where a certain intensity of light 

 stimulates activity while a different intensity may inhibit it, but 

 where no orientation of the body with reference to the direction 

 of the rays occurs, the term "photopathy" may conveniently 

 be applied. Bohn suggests that the tendency of certain ani- 

 mals to come to rest in shaded portions may really be an 

 expression of fatigue produced by the action of the light (55). 



A second problem arises in connection with the mechanism 

 of phototaxis. How does light orient an animal? Does 

 it exert an effect upon the muscles or locomotor organs of the 

 body that is equivalent to pulling the animal around into the 

 required position? Or does the organism become oriented 

 because in a series of movements those which would bring it 

 out of the oriented position are corrected by negative reactions ? 

 Again, if the effect of light upon the body is direct, producing 

 orientation by bringing the animal at once into line with the 

 light rays, is this effect produced by the direction of the light 

 rays as they pass through the body, or by the fact that in any 

 other than the oriented position two symmetrical points on 

 opposite sides of the body are illuminated with unequal 

 intensity, a theory of phototaxis which would bring it into 

 nearer relation with photopathy? 



