The Reactions of Planarians to Light 



83 



Manner of Coming to Rest. Loeb ('93b) and others have shown 

 that planarians under the influence of directive light generally 

 come to rest in regions of lessened intensity. A few experiments 

 were made bearing on this point. By means of screens and back- 

 grounds, both black and white, a rectangular glass aquarium was 

 arranged so that the area of least intensity was plainly localized 

 and could be varied in different ways. In Fig. 8 are shown (1) 

 the places where worms (P. gonocephala) which had been started 

 together in the middle of the dish finally came to rest; (2) the num- 

 ber of worms in each locality; and (3) the different combinations of 

 backgrounds and screens used in each of the experiments. 



TABLE XVII 



Relative effect of fatigue (at right of table) and change in intensity of light (at left of table) as shown by 

 the average number of seconds required for individuals of various species of planarians to leave a circle 

 10 cm. in diameter 



Intensity 



Species 



Groups of trials 



3.3 cm. 



27.0 cm. 



53 



.0 cm. 



First 



Second 



Third 



seconds 



seconds 



seconds 





seconds 



seconds 



seconds 



63 



65 





64 



Dendrocoelum lacteum 



54-5 



63 



72-5 



40 



43 





4i 



Planaria maculata 



37 



42 



45-5 



40 



38 





46 



Phagocata gracilis 



38 



44 



42 



5 2 -5 



47 





49 



Planaria gonocephala 



46 



5 2 -5 



5° 



65 



64 





67 



Average 



58-5 



67 



70 



Wherever shaded borders are indicated the aquarium was 

 surrounded on five sides by black screens and likewise on the sixth 

 side except for a narrow space admitting the light, the direction 

 of which is indicated by arrows; in a similar fashion, where 

 unshaded borders appear, light-reflecting screens enclosed five 

 sides. 



It will be seen at a glance that the great majority of the worms 

 placed in directive light come to rest as far from the light as pos- 

 sible. That this is due to the directive power of light is at once 

 apparent by comparing A, B and C with D, where the light was 

 non-directive. The darkened area was selected whenever the 

 directive force of the light did not prevent, as in A, C and D. 



