The Reactions of Planarians to Light 137 



It will be seen that when Planaria gonocephala was first intro- 

 duced into a field of contrasted intensities, it made the wigwag 

 response at the critical line marking a change of light intensity, in 

 84 per cent of the first 25 crossings, while during the second, third 

 and fourth sets of 25 crossings, the per cents uniformly decreased 

 until at the fourth 25 crossings the number of wigwag responses 

 fell to 32 per cent. It may be objected that the instances thus far 

 cited in this section find a more reasonable explanation upon the 

 hypoth sis of fatigue, but the same surely cannot be said of the fol- 

 lowing case. 



It was found that Planaria maculata oriented itself to directive 

 light at successively shorter intervals when the position of the 

 light was suddenly changed. To produce such a series of re- 

 sponses there was placed in the dark room a shallow aquarium 

 with an electric lamp at either end, under the control of the right 

 and left hand, respectively, of the experimenter. A planarian was 

 placed in the middle of the aquarium and the right-hand light 

 turned on. As soon as the worm was fairly oriented to this light 

 and gliding away from it, the right-hand light was turned off and 

 at the same instant the left-hand light turned on. The time in 

 seconds required for the worm to orient to the new light; that is, 

 to turn 180° and begin to glide away, was recorded. On p. 89 

 a typical series of records of such responses is given, in which the 

 number of seconds required for re-orientation when the source of 

 light was reversed, varied from 260 seconds, at first, irregularly 

 down to 35 upon the sixteenth trial. It will be seen from this series 

 that the worm acquired by experience some degree of facility in 

 adapting itself to certain variations in its environment which it 

 would never be liable to encounter in nature, and that this adapta- 

 tion cannot be explained as due to fatigue. Davenport and Cannon 

 ('97, p. 32) found similarly that "Daphnias respond more quickly 

 and accurately to light after having made several trips to it." 



It is quite certain, however, that any educative attainment 

 which a planarian may experience, or which a planarian may 

 acquire, is exceedingly evanescent and also that there is no evi- 

 dence that the worm emerges from reflex behavior into responses 

 connected with consciousness. 



