158 



LIGHT AND THE BEHAVIOR OF ORGANISMS 



While the tendency in Hydra to remain oriented may 

 then be the result of stimulation by constant intensity, 

 there is no evidence whatever that light acts constantly as 

 a directive stimulus. Light may however affect locomotion 

 by acting constantly, much as temperature does; but even 

 in this case it is impossible to be certain that such effects 

 are not due to changes of intensity, for the shadows of 

 some parts of the body move almost constantly over other 

 parts, owing to the fact that this animal is quiet only for 

 short periods. 



?o- 



2 



Fig. 25. I. Reaction of an attached Hydra to a constant electric current of 

 moderate intensity. 1-5, successive stages in the reaction. 



II. Successive stages in the reaction of a Hydra to the electric current when 

 the foot is unattached. The foot becomes directed toward the anode. After 

 Pearl (1901). 



The fact that there is no definite relation between the 

 direction of turning and the side illuminated shows that 

 neither the symmetry of the body, nor the angle the rays 

 make with the surface, nor the direction of the rays through 

 the body, nor local response to local stimulation, nor dif- 

 ferential response to localized stimulation, can be of special 

 importance in orientation in light. 



The difference between the orienting reaction of Hydra 

 in light and in a constant electric current is striking. As 



