MOVEMENT TOWARD LIGHT IN COELENTERATES 159 



stated above, in light there is no evidence whatever of direct 

 bending toward or from the source of stimulation. In a 

 constant electric current however it bends directly until 

 the long axis is in line with the direction of the current, as 

 represented in Fig. 25. The electric current in the process 

 of orientation acts constantly as a directive stimulation; 

 the reaction is tropic, according to Loeb's definition. Light 

 does not act constantly as a directive stimulation; orienta- 

 tion is the result of " selection of random movements; " the 

 reactions are not in accord with Loeb's definition of 

 tropism. 



2. Eudendrium Planulae 



The planulae of Eudendrium are set free during the latter 

 part of July and the first part of August. In the labora- 

 tory they are usually liberated early in the forenoon, after 

 which they immediately begin to travel toward the light. 

 These organisms are cone-shaped, about i mm. in length 

 and about 0.2 mm. in diameter at the larger end when 

 expanded; when contracted they are shorter and consider- 

 ably wider at the larger end. They are light reddish in 

 color and consist of numerous similar cells so arranged as 

 to inclose a cavity. All the cells are well covered with 

 comparatively short cilia on the outer surface. 



Eudendrium planulae are always in contact with the 

 substratum. They move along something like planaria. 

 Locomotion seems to be due entirely to the action of the 

 cilia, but the planulae are never found swimming freely 

 through the water like infusoria. There is no evidence of 

 constant rotation on the long axis. It may be, however, 

 since all sides appear the same, that they move with dif- 

 ferent sides in contact with the substratum at different 

 times. Locomotion is very slow, the average rate being 

 only about i cm. in 15 minutes. Every movement of this 

 creature can therefore be easily followed even under high 

 magnification. The larger end is always ahead, and this 

 end is constantly turned from side to side very slowly, and 



