TRANSFORMATION OF HELTOTROPIO ANIMALS 275 



other vessels in which the temperature had not been changed, 

 all the animals had remained, without exception, negatively 

 heliotropic. I took pains to keep the temperature in the 

 vessel itself uniform throughout. 



Later, when I allowed the temperature in the cooled dish 

 to rise again, the animals gradually became negatively 

 heliotropic as soon as the temperature reached 6 C. and 

 above. 



It could be shown that the absolute Iteii/ltl of the tempera- 

 ture, and not the sudden fall in the temperature alone 

 made the animals positively heliotropic. When I removed 

 negatively heliotropic animals from water having a tempera- 

 ture of 23 C., and brought them suddenly into water hav- 

 ing a temperature of 13 C., they remained negative even 

 when I waited as long as an hour ; while, when the tempera- 

 ture sank as low as 7 C., the animals became positively 

 heliotropic in a few minutes. When the temperature w T as 

 lower than 6 C., the animals remained positively helio- 

 tropic as long as the temperature remained as low as this (in 

 some experiments this was for two hours). The temperature 

 at which the animals become positive is, of course, not ab- 

 solutely the same in all experiments. I have repeated the 

 experiments with many modifications, and have always found 

 that when cooled below +7 C. all, or almost all, the animals 

 became positively heliotropic. 



A few observations on the behavior of these animals at 

 low temperatures may perhaps be of interest. The positive- 

 ness of the animals at +-4 C. was greater than the positive- 

 ness of the animals at +1 C., the oscillations from the 

 straight line were smaller, and their movements more ener- 

 getic a fact which I had not anticipated. I still observed 

 positively heliotropic reactions at a temperature of +0.4 C. 

 Tin; reaction to light ceased at 0.5 C., although the 

 animals still moved, and at --2 C. the anim.-ils passed into 



