HELIOTROPISM 137 



where, on account of the refraction of light, the intensity is a relative 



minimum.* 



The fact can be demonstrated still differently: if one part of the 

 aquarium is covered with an opaque body, these organisms gradually 

 gather under the covered part, where they come 

 to rest. If the cover is suddenly removed, they w ~ w 



begin to become restless and creep about. In 

 heliotropism we deal with an automatic orientation 

 of the animal by light, which compels all the 

 animals to move in the same direction. In 

 animals like earthworms or Planarians this 

 orienting effect of light is very slight, and the 

 animals may or do move in every direction. 



Of course, it is possible that sensitiveness to 

 sudden changes in the intensity of light exists 

 also in a heliotropic animal. Serpula uncinata 

 is positively heliotropic, and yet contracts rapidly when the inten- 

 sity of the light is suddenly decreased. In Planarians the sensitive- 

 ness to changes in intensity prevails, while, according to G. H. Parker, 

 they show a slight degree of negative heliotropism. 



* Loeb, Pfliiger's Archiv, Vol. 53, p. 81, 1893. 



