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 274 STUDIES IN GENERAL PHYSIOLOGY 



different animals. When other external conditions are the 

 same, we deal in these cases with different degrees or inten- 

 sities of heliotropic irritability. We therefore measure the 

 intensity of the irritability by the (reciprocal) value of the 

 oscillating deviations of the animal from the direction of the 

 rays of light. It will therefore be understood what is meant 

 when I speak of an increase or a decrease in the positive- 

 ness or negativeness of the heliotropism. 



2. I succeeded regularly in making the larvae of Poly- 

 gordius negatively heliotropic through an increase of tem- 

 perature, and positively heliotropic through cooling. 



A large number of freshly caught larvae were distributed 

 into seven glass dishes. Each dish contained thousands of 

 larvae, and they were all without exception negatively helio- 

 tropic. I chose a vessel with such negative animals, and set 

 it into a larger vessel containing ice and salt in order to cool 

 the water containing the animals. The experiment was 

 made before a window facing the north. At the beginning 

 of the experiment, at 2:05 p. M., the temperature in all the 

 vessels was about 16.5 C. In the course of the next seven 

 minutes the temperature in the dish surrounded by the mix- 

 ture of ice and salt fell to 11 C., without a change occur- 

 ring in the behavior of the animals. No matter how often I 

 changed the orientation of the dish toward the window, the 

 animals went in a straight line back to the room side of the 

 vessel. At 2:15 p. M. the temperature had fallen to 8 C. 

 A few of the animals then left the negative side of the dish 

 and moved to the window side. The temperature fell to 

 6 C., and the larvaB went in swarms to the opposite side. At 

 2:33 the temperature of the dish was 5 C C., and only a small 

 proportion of the animals were negatively heliotropic. At 

 2:30 at a temperature of 4 C. only about ten animals 

 remained at the negative side ; while the remainder, practi- 

 cally thousands, were collected at the positive side. In the 



