The Reactions of Planarians to Light 



85 



photic stimuli, due to the worm's lowered physiological state and 

 a chance turn toward the light. In fact the final position taken 

 by 49 fatigued worms with reference to the source of light, showed 

 that only five of them, or 10+ per cent, pointed away from the 

 light while 15 (30+ per cent) were headed toward the light and 29 

 (59+ per cent) stopped indifferently at right angles to it. It is 

 quite probable that among the external factors that influence a 

 worm to come to a halt, light plays an exceedingly insignificant role, 

 as compared with the stimulus of contact or some stimulus, prob- 

 ably chemical, given out by other worms in close proximity. 



One curious instance was observed, however, in which light 

 was apparently of more importance than contact or other stimuli 

 in determining the place of coming to rest. A large crystallizing 

 dish half full of water was left over night with a few planarians in 





Fig. 9 



it. Floating on the surface of the water in this dish was a small 

 Petri dish, in which a few more planarians were isolated. In the 

 morning the worms in both vessels were found grouped at the 

 same region on the outside and inside of the smaller dish, as shown 

 in Fig. 9. 



This curious distribution on both surfaces of the Petri dish 

 could not be due to chemical stimulus exerted by one group of 

 worms on the other, and there seems to be no particular reason 

 why a thigmotactic reaction should have caused them to assemble 

 in such a way. The locality chanced to be one, however, where 

 the intensity of the light was considerably reduced; this seems 

 to offer a reasonable explanation of the observation. 



Bdelloura in coming to rest shows an entirely different behavior. 

 When left over night free to wander in an aquarium half of which 



