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 l ; F indicates the plane of 

 the window. The portion DB 

 of the tube had described the 

 surface DBB V about the point 

 D as center. A similar change 



in the orientation of all the 

 FIG. 9 . . . , T -, 



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 abed was left quite low, so that almost nothing but 

 horizontal rays entered the aquarium (Fig. 8). I wished 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. 



