1 64 LIGHT AND THE BEHAVIOR OF ORGANISMS 



gested by Loeb (1906, p. 121): " The heliotropic curvature 

 consists here in the stem undergoing a stronger contraction 

 or shortening on the more strongly illuminated side of the 

 polyp than on the opposite side." The fact however that 

 the stem elongates on all sides does not favor this view, 

 although growth might possibly mask shortening due to 

 contraction. Unequal rate in growth may have something 

 to do with the bending, since it takes place only in the region 

 of elongation and is an exceedingly slow process. 



Loeb thinks the orienting reactions in Eudendrium are 

 the same as those in plants. He says (1906, p. 120), '' The 

 same phenomena of heliotropism w^hich we find in plants 

 we find also in sessile animals; and the identity of the 

 heliotropic reactions in these two groups of organisms is so 

 complete that it would be at any time possible to demon- 

 strate the phenomena and laws of plant heliotropism in such 

 animals, and vice versa.'' The identity Loeb maintains 

 exists here is in all probability extremely superficial. 



4. Reactions of Medusae 



Many medusae do not react to light at all ; others respond 

 to changes of intensity by contracting; and still others 

 become more active with change in the illumination. Only 

 a few are know^n to orient. Both Yerkes and Morse have 

 shown that Gonionemus murbachii orients under certain 

 conditions, although very indefinitely. It apparently turns 

 directly toward or away from the source of light without 

 any preliminary movements. These organisms appear to 

 have the power of differential response to localized stimula- 

 tion. As to how light produces the orienting stimulation 

 nothing is known. Many of the light reactions of this form 

 are clearly due directly to change of intensity, while others 

 appear to be due to the effect of constant light intensity. 



The medusae of Bougainvillea superciliaris orient far more 

 accurately than does Gonionemus or any other medusa of 

 which I know, but even in these orientation is not accurate 



