TRANSFORMATION OF HELIOTROPIC ANIMALS 281 
an animal is dependent, not only upon the concentration of 
the salt solution in which it lives, but also upon the amount 
of water which is lost by secretion. It is therefore clear 
that in time an adaptation of the animal to the altered con- 
centration of the sea-water may occur. The temperature in 
all these experiments was usually about 20° C. 
5. I tried to see whether it is possible to make animals 
positively heliotropic in a diluted salt solution by lowering 
the temperature. Positively heliotropic larvee were intro- 
duced into sea-water to which 72 per cent. of fresh water, 
by volume, had been added. At the beginning of the ex- 
periment the temperature was 19.5° C. The animals at 
once became negatively heliotropic. I then began to lower 
the temperature. At the same time as a control I subjected 
a number of negatively heliotropic animals contained in 
normal sea-water to the same lowering of temperature. 
When the temperature reached 11° C., a few of the animals 
in normal sea-water became positively heliotropic, and at 
7° C. the majority of the animals in normal sea-water 
became positively heliotropic. In the dilute salt solution, 
on the other hand, all the animals remained negatively 
heliotropic. Only they reacted more slowly than the ani- 
mals kept in the normal sea-water. At + 4° C. they also 
remained negatively heliotropic, if they reacted at all; while 
those kept in the normal sea-water at this temperature were 
very energetically positively heliotropic. When the tem- 
perature again rose, the reactions of the animals in the dilute 
sea-water again became lively, but they, of course, remained 
negatively heliotropic. I have mentioned the fact that ani- 
mals which have been kept several days in dilute sea-water 
may again become positively heliotropic. In such animals 
it was also possible by lowering the temperature to make at 
least a few positively heliotropic. 
6. It is worthy of note that I found the same dependence 
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