94. STUDIES IN GENERAL PHYSIOLOGY 
aquarium. My object in turning the aquarium around was 
to see whether a change in the direction of the rays of light 
would cause the animals to reverse their heliotropic curva- 
tures and to turn their heads again toward the source of light. 
There was no change during the course of the afternoon and 
night. But toward noon of 
the following day I found two 
animals, which in the morning 
had still been in the position 
AB (Fig. 9), in the position 
AB, ; F indicates the plane of 
the window. The portion DB 
of the tube had described the 
surface DBB, about the point 
Dascenter. A similar change 
in the orientation of all the 
remaining animals took place 
during that and the following day. In this experiment the 
direction of the rays of light was modified somewhat; the 
wall abcd was left quite low, so that almost nothing but 
horizontal rays entered the aquarium (Fig. 8). Iwished to 
determine whether the animals would continue to follow the 
direction of the rays and so assume an almost. horizontal 
position. This did, indeed, occur. On February 22, 1890, 
five days after reversing the aquarium, the orientation was 
accomplished, as indicated in Fig. 8. The animals had 
turned their heads toward the source of light, and the axes 
of their gills lay almost horizontally in the direction of the 
rays of light. I left the conditions of the experiment 
unchanged until toward the end of March, and during all 
that time the animals maintained their orientation. 
4. If the rays of light fall vertically from above into the 
aquarium, Spirographis directs its tube vertically upward, 
exactly as a stem grows vertically upward into the open air. 
FIG. 9 
Digitized by Microsoft® 
