REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 137 



glenae in the darker portion swim about at random ; many of them thus 

 pass into the lighted region. There is no reaction at passing from the 

 dark to the light. In the lighted region they likewise swim about in all 

 directions. But as soon as an individual reaches the outer boundary of 



FIG. 93. Illustration of the devious path followed by Euglena in becoming oriented 

 when the direction of the light is reversed. From i to a the light comes from above; at a it 

 is reversed. The amount of wandering (a-A) varies in different cases. 



the lighted area, it gives the typical avoiding reaction ; it backs, turns 

 toward the dorsal side, and thus reenters the lighted area. This reac- 

 tion frequently occurs as soon as the anterior tip is pushed into the shade. 

 In other cases the reaction does not occur till the Euglena has passed 



