IIKLIOTROPISM 



53 



can easily move to the source of light on its right side but 

 with difficulty or not at all in the opposite direction. 



When the light is placed behind the animal, the body 

 is raised up in front and the head held high in the air 

 (Fig. 18). The opposite attitude is assumed, when the 

 light is placed in front, the body being lowered and the 

 head bent down (Fig. 18). These effects resemble the 



Fio. 17. — Position of the water scorpion Ranalra when the right eye is toward the light. 



(After Holmes.) 



galvanotropic effects observed in the position of the head 

 of Amblystoma when the current goes forward or back- 

 ward through the animal. 



These latter observations of Holmes explain, as 

 already mentioned, why a positively heliotropic animal 

 cannot move away from the light and why a negatively 

 heliotropic animal cannot move to a source of light. The 

 progressive motions of the negatively heliotropic animal 

 will be stopped when the light strikes it in front; while 



