OBSERVATIONS ON UNICELLULAR FORMS 83 



Jennings found that as Euglena swims on its spiral course 

 it rotates on its long axis so as to keep the side containing 

 the eye-spot constantly facing out, and that when it is 

 stimulated it always turns toward this side, which is desig- 

 nated the dorsal side. The process of orientation is de- 

 scribed as follows (1906, p. 138): "The Euglenae are 

 swimming about at random in a diffuse light, when a 

 stronger light is allowed to fall upon them from one side. 

 Thereupon the forward movement becomes slower and 

 the Euglenae begin to swerve farther than usual toward the 

 dorsal side. Thus the spiral path becomes wider and the 

 anterior end swings about in a larger circle and is pointed 

 successively in many different directions. In some part 

 of its swinging in a circle the anterior end of course be- 

 comes directed more nearly toward the light ; thereupon the 

 amount of swinging decreases, so that the Euglena tends 

 to retain a certain position so reached. In other parts 

 of the swinging in a circle the anterior end becomes less 

 exposed to the light; thereupon the swaying increases, so 

 that the organism does not retain this position, but swings 

 to another. The result is that in its spiral course it suc- 

 cessively swerves strongly toward the source of light, then 

 slightly 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. 12. 



Orientation in Euglena is, therefore, according to Jen- 

 nings, indirect. The stimulus resulting in orientation is 

 due to changes in light intensity on the organism. The 

 direction of the rays functions in orientation only in so far 

 as it makes such changes possible. Changes of intensity 

 on the organism may be due to movement from a region 

 of one intensity to that of another, or to a change in the 

 axial position of the organism with reference to the source 

 of light. There Is no evidence that orientation is due to a 

 constantly acting directive stimulus In accord with Loeb's 

 theory of troplsms. Jennings does not deny that the 



