TRANSFORMATION OF HELIOTROPIC ANIMALS 277 



larvae positive by exposing them to weak light. I must em- 

 phasize the fact that the larvae which I caught during the 

 first week did not become negative in direct sunlight ; while 

 the larvae caught during the second week promptly showed 

 the effect of the sunlight by becoming negative. I do not 

 know what caused this difference. I tried to see now whether 

 the same animals, which I was able to make negatively helio- 

 tropic in a few moments at ordinary temperature by direct 

 sunlight, would still be influenced in the same way as before 

 by low temperature. I found that at a temperature be- 

 low + 7 C. even the strongest sunlight was not able to 

 make the animals negatively heliotropic. (The influence 

 of temperature in this case reminds one of the critical tem- 

 perature of gases.) It will be necessary to describe these 

 experiments in somewhat greater detail. 



Some Polygordius larvae had stood quietly in the north 

 room for three days, and had become positively heliotropic. 

 I took one portion of the animals and put them into direct 

 sunlight. The temperature of the water in the vessel con- 

 taining them was 20 C. In order to keep the sun from 

 heating the water, the vessel containing the animals was set 

 into another large vessel of glass containing sea-water and 

 pieces of ice. The water in the outer vessel was kept in 

 constant circulation by stirring. Even though the tempera- 

 ture in the dish containing the animals, which was exposed 

 to direct sunlight, not only did not rise, but even sank a 

 little it fell during the experiment to 15 C. the animals, 

 nevertheless, became all negatively heliotropic in three 

 minutes under the influence of the direct sunlight. When 

 later I carried the animals back into the north room and 

 kept the temperature constant at 15-16 C., they again 

 became positively heliotropic in the course of twenty 

 minutes. The sunlight must be very intense in order to 

 bring about this effect. If the dish containing the animals 



