HELIOTBOPISM 49 



the latter is placed at right angles against a window, all 

 the animals begin to move to the window in as straight 

 a line as the imperfections of their locomotion and col- 

 lisions permit. As soon as they reach the window side 

 of the test tube they remain there permanently, unless 

 the test tube is turned around. If we turn the test tube 

 around an angle of 180 the animals go at once to the 

 window again. They react in this way whether the source 

 of light is sunlight, diffused daylight, or lamp light ; and 

 this can be repeated indefinitely. The animals are slaves 

 of the light. These experiments are typical for posi- 

 tively heliotropic motile animals. 



When the animals have reached the window end of 

 the test tubes they remain there, since the light prevents 

 them from going back. But in staying there they may 

 assume any kind of orientation, thus proving that the light 

 orients them only as long as they are in motion. The light 

 affects the tension of the muscles and we shall see later 

 that when, the animals are not moving, the change in the 

 tension of the muscles manifests itself by changes in the 

 position of the legs, w r hich is noticeable in organisms with 

 comparatively large appendages. 



That these animals do not go to the light because 

 they prefer light to darkness but because the light orients 

 them is proved by the fact that they will go from light 

 into the shade if by so doing they remain oriented with 

 their heads toward the source of light. 287 Let direct 

 sunlight S fall upon a table through the upper half of a 

 window (W, Fig. 16), the diffused daylight D through 

 the lower half. A test tube ac is placed on the table in such 

 a way that its long axis is at right angles with the plane 

 of the window; and one-half ab is in the direct sunlight, 

 the other half in the shade. If at the beginning of the 

 4 



