REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 



57 



shall have to keep in mind two questions : First, how is the position 

 of orientation brought about? Second, what is the real stimulus in 

 producing orientation? 



To answer the first question we must observe the movements of the 



dk 



FIG. 24.* 



organism at the time orientation occurs. Observation of the individ- 

 uals as they are becoming oriented shows that orientation is brought 

 about through the same motor reaction that we have already described ; 



* FIG 24. Path followed by Euglena when the direction of the light is 

 changed. From I to 2 the organism swims forward in the usual spiral path. 

 At 2 the position of the source of light is changed, so that it now comes from 

 behind. The organism then begins to swerve farther than usual toward the 



