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 



f\ 



2 I 



74 



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 



