TRANSFORMATION OF HELIOTROPIC ANIMALS 277 
larvee positive by exposing them to weak light. I must em- 
phasize the fact that the larvee which I caught during the 
first week did not become negative in direct sunlight; while 
the larvee 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 larve 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 
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