282 MOTOR RESPONSES 



tensity and aggregate in a spot of relatively weak light in the field 

 (Mast, 1911). 



Orientation. — If euglenae are exposed in a beam of light, they usually 

 swim toward or away from the source of light, i.e., they may be either 

 photopositive or photonegative. 



Verworn (1895) postulated that if the euglenae are not directed to- 

 ward or away from the light, so that one side is more strongly illuminated 

 than the other, the flagellum beats more effectively in one direction 

 than in the opposite, that this causes the euglenae to turn until both sides 

 are equally illuminated, and that the flagellum then beats equally in op- 

 posite directions and the organism moves directly toward or away from 

 the source of light. 



The above hypothesis is, in principle, essentially the same as that 

 formulated by Ray (1693), in reference to orientation in plants, and 

 later accepted by de Candolle (1832). According to this idea, the effect 

 of light on the activity of the motor mechanism, or upon the photo- 

 receptors connected with it, is dependent upon the intensity (not upon 

 change of intensity) of the illumination. The light acts continuously 

 after orientation has been attained, as well as during the process of 

 orientation. During the process of orientation, the illumination on op- 

 posite sides is unequal, which results in quantitatively unequal action 

 in the motor mechanism; but after orientation, it is equal on opposite 

 sides, and consequently the action of the mechanism on opposite sides 

 is equalized. Verworn applied this theory to ciliates as well as flagellates. 

 In his earlier work, Loeb (1890) strenuously opposed the theory out- 

 lined above, accepting Sachs's "ray-direction theory" as the alternative. 

 He adopted it later (1906), however, and applied it to higher animals, 

 introducing the idea that the action of the locomotor appendages is 

 quantitatively proportional to the intensity of the light on the photo- 

 receptors connected with them. He maintained that this is due to the 

 effect of light on muscle tonus. This theory has been designated the 

 "difference of intensity theory," the "continuous-action theory," the 

 "tropism theory," and "Loeb's muscle-tonus theory" (Mast, 1923). 



Engelmann (1882) demonstrated that only the anterior end of 

 Euglena is sensitive to changes in luminous intensity. Jennings (1904) 

 contends that because of this, all turning from the light results in a 

 reduction of illumination, whereas all turning toward the light results 



