VERMES, FLY LARVAE, AND ECHINODERMS 195 



unable to obtain any evidence of the power of differential 

 response to localized stimulation. The orienting reactions 

 could readily be explained by assuming the area sensitive 

 to light to be restricted to a small mass of substance located 

 in the middle of the very tip of the anterior end. 



The idea that the stimulations leading to orientation are 

 due to changes of intensity (in fly larvae an increase only) 

 on the sensitive surface seems to fit the facts as far as known. 

 Stimulations thus produced cause an increase in the lateral 

 head movements somewhat similar to the avoiding reac- 

 tions and shock movements in the lower forms. Owing to 

 the difference in exposure of the anterior end, the move- 

 ments from the source of light are increased more than 

 those toward the light. This continues until the organism 

 is directed away from the source of light and the change of 

 intensity on the anterior end is no longer sufficient to cause 

 a response. 



The process of orientation in the fly larva is strikingly 

 similar in principle to that in Euglena and Stentor. Sten- 

 tor, e.g., is most sensitive when the oral side is exposed; the 

 fly larvae when the anterior end is exposed. When Stentor is 

 not oriented the highly sensitive oral side is alternately fully 

 illuminated and shaded by means of rotation on the long 

 axis. In the fly larvae the alternate illuminating and shad- 

 ing of the sensitive anterior end is brought about by the 

 swinging of the head from side to side. If the intensity is 

 not high, Stentor never turns toward the light; it responds 

 only after the oral side is turned toward the light. This 

 response consists in a rapid swerving from the source of 

 light and eventually results in orientation. Likewise the 

 fly larva under similar conditions responds only after the 

 anterior end is exposed, and the response consists in sharp 

 turning from the source of light, which on repetition results 

 in orientation. Stentor makes no mistakes in the pro- 

 cess of orientation under these conditions. It never turns 

 toward the source of light, but still there are constant trial 

 movements during the process of orientation. The same 



