VERMES, FLY LARVAE, AND ECUINODERMS 173 



intercepted after the larvae are directed toward a point 

 between the two, the intensity on the side facing this beam 

 is decreased more than that on the opposite side, and when 

 this Hght is turned on again after the larvae are directed 

 toward the other, the intensity on the same side is increased 

 more than it is on the opposite side. Under both condi- 

 tions however we find that the larvae turn toward the side 

 most highly illuminated. Under one therefore they turn 

 toward the side on which the intensity is increased, under 

 the other from the side on which it is decreased. It is 

 evident then that if the orienting stimulus is due to change 

 of intensity, it may be due to an increase as well as to a 

 decrease of intensity. And if this is true the organism must 

 in some way perceive the difference between a stimulus due 

 to an increase and one due to a decrease of illumination, 

 for in response to the former it turns toward the point of 

 stimulation whereas in response to the latter it turns from 

 this point. 



It can be definitely stated then that orientation in Areni- 

 cola larvae is due to difference of intensity on opposite sides. 

 Whether it is the result of light acting constantly as a 

 directive stimulation like a constant electric current, or 

 whether it is the result of reactions due to changes of inten- 

 sity on the sensitive structures in the organisms brought 

 about largely by its movements, is a question concerning 

 which our evidence does not warrant a definite conclusion. 

 The facts that the organisms are frequently thrown out of 

 orientation as they proceed toward the source of light, and 

 that the anterior end is almost constantly turned from side 

 to side speak in favor of the latter. The organism is not 

 held definitely on its course as one would expect in case of 

 light acting constantly as a directive stimulation in accord 

 with the definitions of tropisms of Loeb and Verworn. It 

 must however be remembered that even if the organism is 

 under the influence of a constantly acting directive stimu- 

 lus it might turn from side to side frequently owing to the 

 effect of internal processes or other external stimuli. 



