MOTOR RESPONSES IN INVERTEBRATES 579 



orientation it is equal on opposite sides and consequently results in equal 

 action in this mechanism on opposite sides. Verworn applied this theory 

 to ciliates as well as to flagellates. Loeb, in his earlier work, strenuously 

 opposed it, accepting Sachs' "ray-direction" theory, ^ but he adopted it 

 later (126), applied it to higher animals, and introduced the idea that the 

 action of the locomotor appendages is quantitatively proportional to the 

 intensity of the light on the photoreceptors connected with them and 

 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 Ray-Verworn theory," "the tropism theory," 

 "Loeb's muscle-tonus theory," etc. (Mast 148). 



Engelmann (58) maintains that only the anterior end of Euglena is 

 sensitive to change in knninous intensity. Jennings (113) contends that 

 owing to this, all turning from the light results in reduction in illumina- 

 tion and all turning toward the light in increase in illumination of the 

 sensitive substance, that photopositive specimens consequently turn 

 until they face the light and photonegative specimens until they face the 

 opposite direction, and that the stimulus which induces turning then 

 ceases and the organisms consequently continue either directly toward or 

 from the light. 



Mast (140) concludes that the photosensitive substance is confined to 

 the concavity in the opaque portion of the eyespot, that rotation on the 

 longitudinal axis therefore results in alternate shading and exposing of 

 this substance, if the organisms are not directed toward or from the 

 light, that this induces shock-reactions which result in orientation, and 

 that the organisms remain oriented and proceed directly toward or from 

 the light because, after they have attained either of these two axial 

 positions, rotation no longer produces changes in the illumination of the 

 sensitive substance in the eyespot, and they therefore continue in the 

 direction assumed. He holds that the orienting stimulus ceases after 

 the organism has become oriented and that the organism then continues 

 directly toward or from the light because, owing to internal factors, it 

 tends to take a straight course and because, if for any reason it is turned 

 from this course, the orienting stimulus immediately acts, and induces 

 shock-reactions which bring it back on its course. He consequently 

 does not accept the Ray-Verworn theory, according to which the organism 

 is held on its course by the continuous action of the light. 



Bancroft (9) presented evidence against the contention that photic 

 orientation in Euglena is due to shock-reactions and concluded that it is 

 due to tonus effects brought about by "the continuous action of the 

 light" (in accord with his conception of Loeb's tropism theory). Mast 



1 According to this theory orientation is correlated with the direction in which the 

 light passes through the organisms, not with the relation of its intensity on opposite 

 sides. 



