REACTIONS OF INFUSORIA TO LIGHT AND TO GRAVITY 139 



away from it, until by a continuation of this process the anterior end is 

 directed toward the light. In this position it swims forward. The 

 course of Euglena in becoming oriented is shown in Fig. 93. 



This behavior is intelligible when we recall the effect of the spiral 

 course in causing changes in the intensity of the light affecting the an- 

 terior end. The anterior end is, as we have seen, the part most sensi- 

 tive to light ; it may be compared with the eye of a higher animal. In a 



FIG. 95. Diagram of the method by which Euglena becomes oriented with anterior end 

 toward the source of light. At i the organism is swimming toward the source of light. When 

 it reaches the position 2, the light is changed, so as to come from the direction indicated by 

 the arrows at the right. As a consequence of the decrease of illumination thus caused, the 

 organism swerves strongly toward the dorsal side, at the same time continuing to revolve on the 

 long axis. It thus occupies successively the positions 2-6. In passing from 3 to 6 the illumi- 

 nation of the anterior end is increased, hence the swerving nearly ceases. In the next phase 

 of the spiral therefore the organism swerves but a little, from 7 to 8. But this movement 

 causes the anterior end to become partly shaded, and this decrease of illumination again in- 

 duces a strong swerving toward the dorsal side. Hence, in the next phase of the spiral the 

 organism swings far, through 9 and 10, to n. Thus it continually swerves much toward the 

 source of light and a little away from it, till it reaches the position 16. Now it is directed 

 toward the source of light, and such swerving as occurs in the spiral course neither increases 

 nor decreases the illumination of the anterior end. Hence there is no further cause for re- 

 action; the Euglena continues its usual forward movement, which now takes it toward the 

 source of light. 



Euglena swimming obliquely or transversely to the rays of light, as in 

 Fig. 94, the illumination of the anterior end changes greatly with each 

 turn in the spiral. At d the light is shining almost directly upon the 

 anterior end, while at b the organism is nearly tranverse, so that the 

 anterior end is partly shaded. The effect is like that of turning an eye 

 first toward the sun, then away from it ; though the movement is slight, 



