64 



Herbert Eugene Walter 



quently when the worms are suddenly subjected td dark than to 

 light. 



It may be further noted that the excess of the responses in the 

 dark over those in the light increases with the interval of exposure, 

 indicating that the worm's adjustment to a change in the light 

 stimulus affecting it is not in all cases immediate. 



The effect of previous exposure, whether to several hours of 

 dark or light, is a factor in these results which will be considered 

 more properly later on. 



TABLE VIII 



Percentage of the responses of P. gonocephala in various intervals of time when suddenly subjected to 



dark and to light of J9 cm. 



Number of seconds exposed. 



5 



10 



5 1 



59 



63 



66 



12 



7 



■5 



2 5 



30 Average 



Percentage of responses in light. 

 Percentage of responses in dark. 

 Excess of responses in dark 



54 

 73 

 *9 



54 

 75 



71 

 23 



46 

 71 

 2 5 



5 2 

 70 



It should be added that Bdelloura gives a remarkable response 

 when enveloped in sudden darkness. It will frequently forsake 

 its attachment under these circumstances and unattached in the 

 water go through violent contortions. This striking response can 

 be called forth by an exceedingly brief interval of dark, namely, 

 the shortest time required to turn the electric light off and on. 

 Nagel ('94, p. 387) speaks of animals thus affected by sudden 

 shadow as "skioptic." 



The relation of Bdelloura to light falls into a somewhat different 

 category, however, than that of the fresh-water planarians, since 

 Bdelloura is positive to light, while fresh-water flat-worms are 

 negative. 



b Abrupt Changes of Light Intensity in Space 



Several devices were employed to test the behavior of planarians 

 passing abruptly from an area of one intensity of non-directive 

 light into another. The most successful device tried was that in 

 which two lights of different intensities were mounted overhead 



