The Reactions of Planarians to Light 143 



light reactions. To isolate intensity by excluding the possibility 

 of directive light; that is, to secure equalized intensity with refer- 

 ence to the organism, is not difficult and the manner in which this 

 was done, with non-directive light falling upon a horizontal field 

 from above, has been sufficiently detailed in the body of the paper. 



It may be briefly recalled that planarians experimented upon 

 by this method showed a certain unmistakable degree of response 

 which could be referable only to differences in equalized intensity. 

 For example, the rate of locomotion was found to be faster in any 

 non-directive intensity up to 431 cm. than in darkness, although 

 light in itself was not always sufficient to start a worm into ac- 

 tivity, nor was its absence sufficient to check an animal already in 

 motion. Again, though no close correlation between behavior 

 and the degree of intensity was found to exist, there appeared cer- 

 tain general results which were plainly referable to intensity 

 differences only. Instances of such results are the behavior of 

 Planaria gonocephala (which was modified in several particulars 

 at 431 cm. as compared with its behavior at lower intensities); 

 the coming to rest in regions of diminished intensity of individuals 

 of all species except Bdelloura; and the increase of wigwag re- 

 sponses corresponding to an increase of intensity differences when a 

 field of contrasted intensities was used. 



It is interesting to observe that increase in the intensity of non- 

 directive light, and continued exposure to non-directive light of 

 constant intensity, both tend to produce the same behavior that 

 would result in directive light. Under any of the three conditions 

 just mentioned there resulted by actual experiment fewer turnings, 

 fewer "indefinite changes" and more nearly straight paths on the 

 part of planarians than occurred when the worms were (i) placed 

 in non-directive light of lower intensity, (2) subjected a short time 

 to non-directive light of constant intensity, or (3) left in darkness. 

 Now, fewer turnings, fewer " indefinite changes," and more nearly 

 straight paths are ordinarily characteristic results of directive 

 light, so that here is a case of reactions, which if resulting from the 

 employment of directive light would be termed phototaxis, occur- 

 ring in non-directive light as the result of intensity alone. Mast 

 ('03) experimenting upon the reactions of planarians to thermal 



