292, STUDIES IN GENERAL PHYSIOLOGY 
with a cap impermeable to light, the animals which are very 
energetically positively heliotropic should not go under the 
dark cap. The latter does, however, actually occur. Hence 
another and more powerful circumstance is at work besides 
the positive heliotropism, and this might be negative geot- 
ropism. The temperature in the 
tube was everywhere the same in 
: these experiments. The need for 
oxygen does not compel the animals 
to come to the top in these experi- 
ea ments, for when the eudiometer tube 
. 32 8 filled entirely with water, and the 
FIG. 68 open end is turned downward into 
a larger vessel filled with water, the animals nevertheless move 
from this vessel to the cap of the eudiometer, and remain here 
even though fresh oxygen can reach the animals at this point 
only through diffusion from below. The significance of this 
negative geotropism (which I believe it to be) for the upward 
movement of the Loligo larve is shown still more beautifully 
in the following experiment: AB (Fig. 68) represents a 
vertical section through the plane of the window; CD is 
a eudiometer tube filled with sea-water and closed with a 
cork. If the larve at the beginning of the experiment are 
collected at the room side C of the tube, they all move to 
the window side D in consequence of their positive heliot- 
ropism, and remain there. If, on the other hand, the tube 
is so placed that it is at an angle with the horizontal —for 
example, in the position C,D—the larve gradually but 
steadily rise by active swimming movements away from the 
window to the elevated end C, and remain there. They 
therefore leave the window side and go to the room side, in 
spite of their positive heliotropism. To accomplish this the 
angle C\DC must in this case be not smaller than about 20°. 
1 The papers of Wolfgang Ostwald seem, however, to indicate the possibility of 
a purely physical explanation of this experiment. [1903] 
Ay 
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