68 



Herbert Eugene Walter 



vided the amount of light in both cases is equal. Planarians, as 

 Pearl has emphasized, are strongly thigmotactic. Naturally, then, 

 their response to contact is much greater when they are on the 

 glass bottom of the aquarium than when they are suspended on 

 the less resistant surface film. In other words, the less the worm 

 is influenced by the stimulus of contact the freer it is to respond 

 to the stimulus of light. 



TABLE X 



Percentage of the responses made by Phagocata at the critical line separating two intensities of non-direc- 

 tive light either on the bottom of the aquarium or on the surface film 





Number of 

 observations 



No response 

 per cent 



Response 

 per cent 





740 

 1664 





45* 

 76 



54* 

 24 



Total 



2404 





60:} 



39* 



Finally, a series of experiments was tried in which the contrast 

 between two intensities was varied by raising or lowering one of 

 the lights in the hood. It was found that the responses made by 

 Phagocata under these circumstances increased with the increase 

 in contrast between the two intensities as shown on the bottom 

 line of Table XI, where these contrasting intensities are expressed 

 in a ratio between the constant light taken as unity and the mov- 

 able light. 



The fact that responses by no means invariably occur when 

 bright light and complete darkness are suddenly substituted for 

 each other (see Table VIII) rendered a further extension of this 

 series unnecessary. The contrasts here used form probably a 

 much greater range of intensity contrasts than the worms ever 

 encounter in nature. 



Attention to the details presented in Table XI brings to light 

 the fact that, although the number of responses is correlated in a 

 general way with an increase in the contrast between the' two 

 illuminated areas, as shown in the bottom line of the table, yet 

 the percentage of the responses is further influenced by the actual 

 degree of the intensities employed. For example, when the two 



