REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 55 



course forms a right angle, or a still greater angle, with the original 

 course; if the turning is through 1 80, the course will be squarely 

 reversed. Indeed, sometimes the organism swings around an entire 

 circle or more. When the usual method of swimming is resumed after 

 such reactions as those just described, the course has been completely 

 changed. 



Strasburger (1878, p. 25) noticed that after a decrease in illumina- 

 tion flagellate swarm-spores often turn strongly to one side or even 

 describe circles. But he did not notice that the turning was always 

 toward the same side of the organism,* and did not perceive the relation 

 between this effect and the remainder of the reaction. 



FIG. 23. t 



This method of reaction is particularly striking when the Euglena? 

 are confined to a very thin layer of water between the slide and the 

 cover glass, so that they cannot swerve up or down. When the light 

 is decreased, we will suppose that the dorsal side is to the (observer's) 

 left The Euglena then swings far to the left. At the same time it 



*Naegeli (1860, p. 96) had, however, before Strasburger, observed that in such 

 swarm-spores the same side always faces the outside of the spiral path. This 

 observation, which really contained the germ of a correct understanding of the 

 reactions to stimuli, seems hardly to have been noticed by later writers. 



* FIG. 23. Diagram of the method by which Euglena becomes oriented with 

 anterior end toward the source of light. At i the Euglena is swimming toward 

 the source of light. When it reaches the position 2 the light is changed so as 

 to come in the direction indicated by the arrows at the right. As a consequence 

 of the decrease in illumination of the anterior end thus caused, the organism 



