102 HELIOTROPIC MECHANISM 



chemical reaction will be unequal in both eyes and .... as a 

 consequence the direction in which the animal moves will change."^ 



The validation of this theory rests upon presenting proof that light 

 acting upon photosensitive organs does affect the tension (tonus) of 

 muscles, and that asymmetrical effects are produced by unequal 

 illumination of the eyes of heliotropic forms. A typical example of 

 the reflex muscle tonus produced by light is noted in the contraction of 

 the sphincter iridis of the vetebrate eye when the retina is illuminated. 

 The degree of contraction is proportional to the intensity of the illumi- 

 nation, it is quite independent of the rate of change in the illumina- 

 tion ; furthermore the contraction is a true tonus which is maintained 

 as long as the light affects the retina. Exactly the same type of reac- 

 tion was described by Loeb for certain sessile heHotropic animals, 

 Spirogniphis, Eudendrium, which respond to unilateral illumination 

 by bending toward the light.'* The muscles contract on the illumi- 

 nated side and like the sphincter of the iris, remain in tonus as 

 long as the light continues to act. Loeb and Ewald and Loeb and 

 Wasteneys^ showed that this reaction of Eudendrium was proportional 

 to the product of the intensity of the Hght into the duration of illumi- 

 nation, thus following the photochemical law of Bunsen and Roscoe. 



In the case of the motile larvae of the marine worm, Arenicola, which 

 are positively heliotropic, Lilhe states that there is an "increase of 

 muscular tone under strong illumination" and that an "inequality of 

 tone on the two sides of the body will result when one side is more 

 strongly illuminated than the other," the muscles of the more 

 strongly illuminated side being in stronger contraction. Thus 

 "heHotropic orientation is a purely muscular phenomenon."" Mast^ 

 showed that this reaction was due to the unequal illumination of the 

 eye spots and Garrey'- showed that the difference in contractile state 

 (tonus) of the muscles of the two sides of the body of Arenicola larvae 

 persists as long as the difference in the illumination of the eye spots is 



^ Loeb, The organism as a whole, New York and London, 1916, 258. 

 * Loeb, Arch. ges. Physiol., 1890, xlvii, 391. 



^ Loeb, J., and Ewald, W. F., Zcnir. Physiol., 1914, xxvii, 1165. Loeb, J., and 

 Wasteneys, H., /. Exp. Zool., 1917, xxii, 187. 

 '"' Lillie, R., Am. J. Physiol., 1901, v, 59, 60. 

 ^ Mast, S. O., Light and the behavior of organisms, London, 1911. 



