FURTHER INVESTIGATIONS ON HELIOTROPISM 93 
Within the next few days the six first-mentioned animals 
further elevated their heads, so that the animals on Decem- 
ber 22—eight days after being placed in the aquarium— 
were all similarly oriented toward the light. The head was 
directed toward the window, and the axis of symmetry of 
the gills which were exposed to the light lay in the direction 
of the more intense rays of daylight which entered from 
FIG. 8 
without and above. I waited to discover whether this orien- 
tation would last. The aquarium remained undisturbed 
until February 16, 1890; that is, for more than two months, 
The animals also did not change their positions, as indicated 
in Fig. 7. 
3. On the afternoon of February 17, 1890, the aquarium 
was turned 180° about its vertical axis, and the zinc box 
was again inverted over the aquarium so that the movable 
end was directed toward the window. By turning the 
aquarium around in this way, the heads of the animals, 
which had been until then directed toward the source of 
light, were suddenly turned toward the room side of the 
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