212 BEHAVIOR OF THE LOWER ORGANISMS 



parts of the body, or may shift the weight of the body to. parts unaccus- 

 tomed to bearing it; and these effects might serve as stimuli to cause 

 the animal to take another position. This possibility will be vividly 

 realized by any one who undertakes to rest with a limb doubled in some 

 unusual position beneath him. Again, certain movements with refer- 

 ence to gravity may produce results involving a change of the conditions 

 affecting the organism, and since it is a well-established fact that the 

 results of behavior partly determine future behavior, this fact may 

 determine movements with reference to gravity. There seems to be 

 no a priori reason why each of the relations above mentioned, as well 

 as various others, may not induce reaction in one organism or another, 

 and it seems not difficult to find probable examples of all. We have 

 been assured by various writers that the reaction to gravity must be 

 explained in the same way in all cases, but this is evidently said rather 

 in the capacity of a seer or prophet, than in the capacity of a man of 

 science whose conclusions are inductions from observation and experi- 

 ment. 



C. Reactions to Light 



Many of the sea anemones and medusae do not react to light, so far 

 as known. In other cases a reaction to light is very marked. The 

 relation of the behavior to light is in certain cases exceedingly complex, 

 and very instructive, as showing the numerous factors on which behavior 

 depends. We shall take up especially the reactions of Hydra, and of 

 the medusa Gonionemus. 



(i) Reaction to Light in Hydra 



The behavior of Hydra with relation to light has been studied es- 

 pecially by Wilson (1891). Both the green and the brown Hydra are 

 usually found at the lighted side of the vessel containing them. If they 

 are at first scattered, they will in a day or two be found to have moved 

 to the lighted side. If at the side of the dish next the window there are 

 attached light and dark strips of glass, the Hydras collect in the light 

 strips. If different colored lights are used, by placing strips of glass 

 of different colors on the lighted side of the vessel, the Hydras collect 

 in the blue light, while all other colors (except perhaps green, which 

 seems slightly effective) act like darkness. The animals gather in the 

 blue even in preference to the white light, which of course contains all 

 the blue rays. As to the way in which the reaction to light takes place, 

 the following facts were brought out by Wilson. A change from light 

 to dark, or from blue or white light to one of the colors which acts like 



