OBSERVATIONS ON UNICELLULAR FORMS 85 



self has drawn, with text description, leads to quite a 

 different conclusion from his. The figure indicates that 

 Euglena is both unterschiedsempfindlich and heliotropic. 

 At a (Fig. 12) the reversal in the direction of the light, 

 which has been coming from the direction in which the 

 creature has been swimming, produces a sudden change in 

 intensity of stimulation, a shock which results in the swerv- 

 ing from the previous course, as indicated between a and c. 

 The organism recovers rapidly, only to be subjected to the 

 constant stimulus of a steady light from one direction to 

 the end of the experiment. The result of the action of the 

 constant stimulus is a path, from c to 5, so perfectly in 

 harmony with the tropic schema, that, in spite of Jennings' 

 descriptions and elucidations, I can only wonder at his 

 running so boldly and so far into the enemy's camp. . . . 

 In heliotropism . . . the oriented organism is in a condi- 

 tion of physiological stimulation, and . . . the response to 

 stimulation is local ; finally, . . . the interpretation of the 

 behavior of heliotropic organisms on the basis of general 

 changes concerning the whole organism, not only does not 

 accord with the main facts, but is rather psychical than 

 physiological in character." 



It is thus evident that while Torrey recognizes that 

 Euglena responds to change of light intensity, he considers 

 that orientation is due to the local effect of unequal stimu- 

 lation of symmetrically situated points on the body, and 

 that after the organism is oriented it is held upon its course 

 by constantly acting directive stimulation. He does not, 

 however, explain where the symmetrically located points 

 which are subject to local stimulation are situated in Euglena. 

 They might be conceived to be in the flagellum or in the 

 body. In the former case It would imply direct action of 

 the point stimulated, In the latter a reaction in harmony 

 with the location of the stimulus, i.e., if the stimulus is 

 applied to the left side of the body the flagellum would 

 strike toward the left; if applied to the right side. It would 

 strike toward the right, etc. 



