294 STUDIES IN GENERAL PHYSIOLOGY 
Another condition must, however, be taken into con- 
sideration, which may have the effect that animals which 
are constantly positively heliotropic must leave the surface 
of the water at higher temperatures. As is well known, the 
processes of oxidation, and consequently the demand for 
oxygen, rise considerably with an increase in temperature. 
It is natural that when the demand for oxygen exceeds the 
supply, the animal can execute no, or only weak, swimming 
motions, and in consequence falls to the bottom. At any 
rate, the latter can, indeed, be observed at high tempera- 
tures. Loligo larve which hold themselves at the surface 
by swimming motions sink passively as soon as the tempera- 
ture exceeds 30° C. 
In conclusion I wish to add that I made experiments on 
most of the animals mentioned in this paper with colored 
light, and found a universal confirmation of the fact, which 
I discovered before, that the more strongly refrangible rays 
of the visible spectrum are the most active heliotropically, 
as in the case of plants. 
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