The Reactions of Plauarians to Light 63 



will be discussed here only in its relation to non-directive light, the 

 effects of sudden changes in directive light coming more properly 

 in a later section. 



a Abrupt Changes of Light Intensity in Time 



Whenever worms were left over night in the experimental aqua- 

 rium completely shut off from light, a large proportion of them 

 would be found at rest in the morning when the light in the hood 

 was again turned on. By removing the diaphragm (Fig. i, H), 

 under the edge of which near the paraffine wall the worms w^ere 

 usually collected, it was possible without any mechanical disturb- 

 ance to subject resting worms to sudden non-directive light after 

 a prolonged period of complete darkness. This sudden stim- 

 ulus rarely had an instantaneous effect. The interval of response 

 was often several minutes and frequently non-directive light alone 

 proved insufficient to start the worms into activity. 



No sudden increase of intensity ever proved powerful enough 

 to throw a gliding worm into the more rapid method of crawling. 

 Pearl ('03, p. 551 ) stated the same fact after subjecting planarians 

 to much stronger intensities of light than were employed in the 

 present experiments. 



It was found that P. gonocephala showed a decided response — 

 either some change in course or a wigwag motion of the anterior 

 end — more frequently when suddenly subjected to dark than 

 to light. By inserting a key into the electric circuit it was 

 possible to control the light in the hood to a fraction of a second. 

 Worms in complete darkness were by this means subjected to 

 various intervals of sudden light and worms in light to intervals of 

 sudden dark, the results being at once noted. While the worms 

 were in the dark their behavior could not, of course, be directly 

 observed, but by watching them closely just before the light was 

 turned off and also the instant it was turned on again there was 

 no great difficulty in determining whether a response had occurred 

 during the interval. The results obtained from nearly a thousand 

 trials are indicated in Table VIII. 



It will be seen from this table that there are more responses 

 than failures to respond and that the responses occur more fre- 



