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 in 
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 
alge 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 fetida 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 
in 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 larve of Balanus are positive in the 
