The Reactions of Phuwriaus to Light 89 



attempt to illuminate one side of the worm alone because the ani- 

 mal invariably turned faster than it was possible to regulate the 



light- 



The quickness with which this delicate response to any change 

 in the direction of the light occurred was found to increase upon suc- 

 cessive trials. A square aquarium was arranged so that it could be 

 illuminated instantly at either end, in a room otherwise dark. 

 With one light on, a planarian was allowed to move until it had 

 assumed the characteristic negative direction, whereupon the 

 source ot illumination was instantly changed 180° by turning this 

 light off and the one at the other end of the aquarium on. The 

 time required for the worm to become headed about was noted and 

 then a reversal of lights repeated and the interval necessary for re- 

 adjustment again recorded. In a typical experiment of this kind 

 the number of seconds required by the worm, Planaria maculata, 

 to accomplish re-orientation were for 16 successive orientations as 

 follows: 260, 70, 100, 60, 65, no, 60, 85, 70, 105, 80, 60, 50, 40, 

 45' 35- The sum of the first eight is 810 sec, that of the last 

 eight, only 485 sec. 



Summary. Planarians show a greater response to sudden 

 change of intensity than to gradual change. This response is 

 more pronounced when the intensity is lowered than when it is 

 raised. 



Bdelloura is particularly affected by sudden changes of in- 

 tensity. 



Planarians respond with great precision to changes in the direc- 

 tion of the light, and as promptly when the change is gradual 

 as when it is abrupt. 



The period required for re-orientation to changes in the direc- 

 tion of light, diminishes upon repetition. 



C In Combination with Other Responses 



It is impossible to subject planarians to the influence of light 

 alone. The best that can be done is to render extraneous factors 

 as uniform as possible. For example, so long as a moving worm 

 is kept upon a horizontal surface there can be no directive geotactic 

 stimulation, because the worm is moving in a plane at right angles 



