216 BEHAVIOR OF THE LOWER ORGANISMS 



back into the shade. Yerkes is convinced, from analogy with the effects 

 of other stimuli, that this is due to a stronger contraction on the side 

 most intensely lighted — that farthest from the shadow. This would, 

 of course, turn the medusa back into the shade. 



Thus in the course of time practically all the medusa? in the vessel 

 will be found in the shaded region. 



In the behavior of Gonionemus with relation to light there are evi- 

 dently a number of paradoxical facts. The medusa swims toward the 

 source of light, yet tends to gather in shaded regions. It goes at first 

 toward a source of strong light, later reverses this reaction. It moves 

 toward the source of light when excited, but becomes indifferent when 

 undisturbed. Different individuals react differently to the same con- 

 ditions, and the same individual reacts differently at different times. 

 We have here an excellent illustration of the fact that the reactions of 

 organisms, even to simple agents, depend on a multiplicity of factors. 

 If we could study the medusa in the natural conditions under which it 

 lives, and if we knew thoroughly the physiological processes taking place 

 within it, we should doubtless find all these peculiarities explained, 

 and should probably discover that its reactions are regulatory. When 

 we carry such an animal to the laboratory and experiment upon it there, 

 it is like removing an organ from the body and studying it in a dissect- 

 ing dish. We cannot understand its activities without knowing their 

 relations to the rest of the body — to the environmental conditions. 



9. Behavior of Ccelenterates with Relation to Food 



The behavior of organisms is largely determined by the relation of 

 the environment to their internal physiological processes. In no field 

 is this so striking as in the relation of behavior to the obtaining of ma- 

 terial for carrying on the processes of metabolism. Under this point of 

 view come the reactions of organisms with reference to food, and to the 

 gases necessary for respiration. These reactions in the Ccelenterata we 

 shall take up now. 



A. Food and Respiratory Reactions in Hydra 



Hydras are usually found in the upper parts of a vessel of water, 

 near the surface. This is not due to a reaction to gravity, but rather 

 to the relative quantity of oxygen in different parts of the water. If 

 an experiment is arranged in such a way that the lower surface of the 

 vessel is free and in contact with air, while the upper is not, the Hydras 

 tend to gather near the lower surface (Wilson, 1891). Collecting in 



