PHOTOTAXIS 47 



While some exceptions to the above rule have been pointed 

 out by Minkiewicz and Bohn, it is one of very general 

 validity and affords a striking parallelism to the phototactic 

 movements of plants. 



The phototactic response in animals may be modified or 

 reversed by a variety of agents. Whether animals are^ 

 positive or negative often depends upon the intensity of 

 the light. This is well illustrated by the reactions of Volvox. 

 This form consists of an almost spherical colony of cells 

 each of which is furnished with a pair of flagella which serve 

 as organs of locomotion. One axis of the colony is some- 

 what longer than the others and upon this axis Volvox 

 commonly rotates while swimming. When exposed to a 

 moderate light the colony swims toward it in a nearly 

 straight line; when it happens to get out of orientation it 

 turns directly back into line again. If it reaches a point of 

 too great intensity its movements become slower, its orien- 

 tation less precise and it may stop or swim about slowly hi 

 various directions. If exposed to light above the optimum 

 it orients itself in the reverse direction and swims away. 

 Many flagellate protozoa and the swarm spores of many 

 algse are similarly positive in weak light and negative in 

 strong. Among higher organisms reversals following change 

 of light intensity are less common although there are several 

 cases. The earthworm Allolobophora Joetida which is com- 

 monly negative shows a slight positive reaction in exceed- 

 ingly weak light (Adams). A great many positive forms 

 remain positive in as strong light as has been brought to 

 bear upon them and most negative species remain negative 

 hi as weak light as they respond to in any definite way. 



The sense of the phototactic response is sometimes 

 changed by exposure to light or darkness. Groom and 

 Loeb found that the larva? of Balanus are positive in the 



