REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 6l 



usual spiral course, in all phases of which the illumination of the anterior 

 end is equal. If the light came from the rear of Euglena i instead of 

 from the direction indicated by the arrows, the reaction above described 

 would be continued in the same way until the direction of swimming 

 was completely reversed. 



Thus the orientation of Euglena in a continuous light is due to the 

 production of the " motor reaction," with its turning toward the dorsal 

 side, whenever there is a decrease in illumination at the anterior end. 



There is no other explanation of the orientation, so far as I am able 

 to see, that is in agreement with all the facts. At first one is tempted 

 merely to say that the subjection of the anterior end to shadow pro- 

 duces the motor reaction, and that this is continued until the anterior 

 end is no longer shaded. This statement is correct if by "subjection 

 to shadow " we mean an active process, involving a change from a 

 more illuminated condition. But if we mean that darkness as a con- 

 tinuous, static condition is the cause of the reaction, then considera- 

 tion shows that this will not account for all the facts. It leaves out of 

 account the capability of the organism to become acclimatized to cer- 

 tain degrees of light and shade, and certain of the experimental results 

 are crucial against it. Thus, suppose the Euglenae are swimming 

 toward a source of weak light, and a stronger light is then allowed to 

 act upon them from another direction. The anterior end continues to 

 receive the same amount of light as before (since the weak light still 

 persists), yet the organism reacts as usual, becoming oriented toward 

 the stronger light. The motor reaction by which the orientation is 

 brought about cannot therefore be due to darkness or shade (considered 

 statically) at the anterior end. On the other hand, the case just men- 

 tioned is easily understood on applying the explanation given above. 



Again, it might be held that the reaction is due in some way to the 

 relative amount of illumination at the two ends. It might be main- 

 tained, for example, that when the posterior end is more illuminated 

 than the anterior, this difference acts as a stimulus to cause the " motor 

 reaction." There is, of course, no independent evidence in favor of this 

 view, and the experimental results prove it to be incorrect. We have 

 shown that the reaction is produced (i) when both ends are equally 

 stimulated, as when the light comes directly from one side ; (2) when 

 neither end receives light, as when the light is cut off completely. Fur- 

 ther, it might be held that the reaction is produced when the anterior end 

 is not more intensely illuminated than the posterior end. It is, of course, 

 a little difficult to conceive how so indefinite a condition could act as a 

 stimulus to a definite motor reaction, but in any case the experiments show 

 that this is not the real cause of the " motor reaction." Thus certain of 

 the experiments show that the " motor reaction " is produced even when 



