HELIOTROPISM OF ANIMALS 



25 



process is repeated, the animals creeping to the window side 

 of the glass just as before. If, however, the position of the 

 glass remains unchanged, the animals remain permanently 

 crowded together on the window side of the test-tube. 



Experiment 2. If the test-tube is laid on the table with 

 the longitudinal axis parallel to the 

 plane of the window, the animals 

 gradually scatter uniformly over the 

 whole of the upper part of the tube. 

 The lower portion of the vessel is in ] 

 consequence again free from animals. 

 If the longitudinal axis of the test- 

 tube lies at even a slight angle with 

 the plane of the window, the animals 

 move to the end of the tube nearest 

 the window, and remain there in their 

 customary position. 



Experiment 3. The test-tube is placed perpendicular to 

 the plane F of the window, and at the beginning of the 

 experiment the animals are collected at the window side B 

 of the test-tube (Fig. 1). That half of the vessel which lies 

 nearest the window is now covered with an opaque paste- 

 board box, K. The following then occurs : The animals 

 soon appear at A on the room side of the pasteboard box ; 

 as soon, however, as they emerge from the box K into A, 

 they turn about, direct their heads toward the window, move 

 to the edge of the pasteboard, and remain at the boundary 

 between the covered and the uncovered portions of the tube, 

 at A and especially at the top of the test-tube. The remark- 

 able thing is that they are not distributed evenly over the 

 whole brightly illuminated part of the test-tube. The 

 explanation is as follows : As soon as the animals near the 

 window at B are covered by the pasteboard, the weak rays 

 of light reflected from the walls of the room fall upon them. 



