REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 



53 



When oriented and swimming toward the source of light the swerv- 

 ing toward the dorsal side is comparatively slight. As seen from 

 above, the organisms seem merely to oscillate a very little from side to 

 side as they revolve on the long axis. Careful examination shows that 

 the swerving is always toward the dorsal side, as in Fig. 20, the alter- 

 nations in direction being due to the alternations of position of the 

 dorsal side. Now, when the illumination is suddenly decreased, the 

 Euglenae at once swing much farther than usual toward the side to 



FIG. 21.* 



which they are already swerving, that is, toward the dorsal side. If 

 the decrease in illumination is not very great, so that the stimulus is 

 not a strong one, the swerving is not very great, and the organism at 

 the same time continues to revolve on its long axis ; thus the anterior 

 end describes a circle and the whole body describes the surface of a 



*FiG. 21. Diagram to illustrate reaction of Euglena when the illumination 

 is decreased. The Euglena is swimming forward at i ; when it reaches the 

 position 2 the illumination is decreased. Thereupon the organism swerves 

 strongly toward the dorsal side. This swerving, combined with the revolution 

 on the long axis, causes the anterior end to swing about a circle, so that the 

 Euglena occupies successively the positions 2, 3, 4, 5. 6, etc. From any of these 

 positions it may start forward, as indicated by the arrows, if the condition 

 causing the reaction ceases to act. In the figure the Euglena is represented as 

 swimming forward from the position 6. 



