The Reactions of Planar/ niis to Light 



67 



As a result of over 3000 observations on the manner in which 

 the critical line separating the two intensities was passed, three 

 facts become evident. First, responses were considerably more 



TABLE IX 



Kind and percentage of responses of Dendroccelum and Phagocata in passing from one intensity of non- 

 directive light to another 



Character of course 



Going into greater intensity, 

 per cent 



Going into lesser intensity, 

 per cent 



Average responses, per cent. 



50 

 35-5 



frequent when the worms were passing into the lesser intensity 

 than they were when entering the greater intensity. Secondly, 

 lack of response is more frequent than a visible response of 

 any kind since 64.5 per cent of the crossings made over the 

 critical line were of the type A. Thirdly, the responses at the 

 critical line were more frequent when the worm was upside down, 

 /. e., moving on the surface film, than when it was on the floor 

 of the aquarium. This latter point was illustrated most fully 

 by Phagocata, which, being an active worm, takes quite readily 

 to the surface film, so that it was possible with this species to get 

 a series of observations in which the behavior when crossing the 

 critical line on the bottom of the aquarium could be compared 

 with that when the same line was encountered at the surface film. 

 Table X contains the results of these observations. 



The doubling of responses when the worm is on the surface 

 film is probably not due to an unequal receptivity of light stimulus 

 by the dorsal and ventral surfaces of the planarian as might at 

 first thought seem possible. As will be shown further on, the 

 worm's rate of locomotion on the bottom of the aquarium is nearly 

 the same whether the light comes from below or from above, pro- 



