OBSERVATIONS ON UNICELLULAR FORMS 97 



more strongly illuminated the organism responds by bend- 

 ing, and when it becomes shaded the creature gradually 

 straightens out and resumes its normal form again; thus 

 the anterior end becomes directed more and more nearly 

 toward the source of light until the organism reaches an 

 axial position in which the eye-spot is no longer exposed to 

 sufficient changes in illumination during the process of ro- 

 tation to cause a bending reaction. The organism therefore 

 continues in this direction, i.e., more or less nearly toward 

 the source of light. Orientation is frequently brought about 

 in two or three rotations. It is clear that during this pro- 

 cess light does not act continuously as an orienting stimulus. 

 Euglena responds with reactions leading to orientation only 

 when the dorsal side is turned toward the source of illu- 

 mination, not when the ventral side is exposed. And it 

 should be emphasized that the first movement in the re- 

 sponse is a bending away from the source of light, toward 

 which it later becomes oriented. 



It is evident from the above description that turning into 

 such a position that the eye-spot faces the source of light 

 produces a stimulation which results in a definite reaction. 

 In this reaction the organism always bends the anterior 

 end toward the ventral surface. It appears at first thought 

 as though this reaction were due to the illumination of the 

 eye-spot. It will however be demonstrated that this is 

 not true. 



If the light from the two sources arranged as described 

 above is not equal, and the tw^o beams which reach the 

 aquarium are alternately intercepted, it is evident that the 

 organism will be subjected simultaneously to a change in 

 the direction of the light rays and a change of light intensity. 

 If the stronger light is thrown upon the Euglenae after 

 they are oriented in the weaker, they orient just as de- 

 scribed above, but if the weaker is turned on after they are 

 oriented in the stronger there is an immediate reaction, 

 no matter which surface, the ventral or the dorsal, happens 

 to be exposed at the time. If it is the dorsal and the 



