BEHAVIOR OF CCELENTERATA 213 



darkness, causes the animal to become restless and move about. The 

 motion seems undirected, but as soon as the animal comes into the blue 

 or white light, it becomes less restless, and remains. The behavior 

 is thus far, then, like the reaction to heat ; the animal when not lighted 

 simply moves about in various directions, till one of its movements 

 brings it into light. Whether the animal when moving draws back or 

 stops on coming to the boundary of the light, where it would pass into 

 the darkness, as Euglena does, has not been determined. But when the 

 vessel is lighted from one side, the animal moves toward the source 

 of light, and the movement is no longer an irregular wandering, but 

 according to Wilson (1891, p. 432) is fairly direct. This is like the re- 

 action of Euglena, and it seems possible that in Hydra the reaction is 

 produced in the same manner as in that organism. If this is true, 

 there is a tendency for the moving animal to keep its anterior end directed 

 toward the light, due to the fact that when it turns this end away, the 

 change to relative obscurity at the anterior end causes further move- 

 ment, till the light again falls on the anterior end. The movements 

 should be studied further to determine this point. Fixed Hydras do 

 not maintain any particular orientation with reference to the light rays, 

 but change their position frequently, in the way illustrated in Fig. 114. 

 The green Hydra moves to the lighted side of the vessel more rapidly 

 than the yellow Hydra. This is probably due to the generally more 

 rapid movements of the green species. 



In a powerful light the reaction of Hydra, like that of most other 

 positive organisms, becomes reversed. The animals collect in the 

 shadow of leaves or on the bottom. They have not been observed to 

 move directly away from the source of light (Wilson, 1891), so that the 

 reaction is probably an irregular wandering based on the method of 

 trial. 



Hertel (1904) found that both the green and the colorless Hydra 

 react by contraction when subjected to powerful ultra-violet light. These 

 rays killed the colorless Hydra in about one minute, while Hydra viridis 

 resisted their action for six to eight minutes. 



The gathering of Hydras in lighted areas and the movement toward 

 a source of moderate light are of much benefit to the animals in obtain- 

 ing food. Hydra preys upon small Crustacea and other minute animals, 

 and these gather as a rule at the lighted side of the vessel. By taking 

 a position on this side, the Hydras find themselves in the midst of a dense 

 swarm of organisms and are able to capture much food. When in such 

 situations one frequently finds them gorged with prey. In other parts 

 of the vessel they would have almost no opportunity of obtaining food 

 (Wilson, 1891). 



