REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 135 



amined with the microscope in the neighborhood of a window. Soon 

 all the Euglenas are seen swimming toward the window. Now the 



Fig. 91. — Diagram of the reaction of Euglena when the 

 light is decreased. The organism is swimming forward at 1; 

 when it reaches 2 it is shaded. It thereupon swerves toward 

 the dorsal side, at the same time continuing to revolve on the 

 long axis, so that its anterior end describes a circle, the Eu- 

 glena occupying successively the positions 2-6. From any of 

 these it may start forward in the directions indicated by the 

 arrows. 



light is decreased by placing the hand or a 

 screen between them and the window. At once 

 all give the avoiding reaction; that is, they stop 

 ■or swim backward an instant, then^ swerve 

 strongly toward the dorsal side, so that the ante- 

 rior end swings about a circle (Fig. 91). If the 

 light is decreased strongly, the anterior end de- 

 scribes a wide circle or may even turn through 

 an angle of 180 degrees, so that the direction 

 of movement is reversed. If only a Little of the 

 ■light is cut off, the anterior end describes only a 

 narrow circle. The organisms soon resume the 

 forward movement, but now the axis of the 

 spiral path coincides with one of the directions 

 indicated by the anterior end in swinging about 



Fig. 92. — Change of 

 direction in the spiral path 

 of the Euglena, as a result 

 of a slightly marked reac- 

 tion. At a the illumination 

 is decreased, causing the 

 organism to swerve toward 

 the dorsal side, thus widen- 

 ing the spiral path. At b 

 the ordinary swimming in a 

 narrow spiral is resumed; 

 since at this point the organ- 

 ism was necessarily more 

 inclined to the axis of the 

 spiral than before the reac- 

 tion, the new course lies at 

 an angle to the previous one. 



