112 HELIOTROPISM 



kind have drawn from some writers the curious comment that "these 

 reactions are non-adaptive only under artificial conditions" (Mast, 

 1911), with the further assumption that "the reactions were inherited 

 from ancestors in which they were adaptive." The normal inhi- 

 bition of heliotropism in Onchidium is adaptive, if you like, but the 

 further difficulty of finding some adaptive excuse for the existence of 

 hehotropism at all remains all the more insoluble. The behavior of 

 Onchidium is more curious than that of the crustaceans cited by Loeb, 

 for there is physical opportunity to "make use" of their photic sensi- 

 tivity, since they are exposed to Ught. Whether the mechanism for 

 such a response is merely an "inheritance" from ancestors in which it 

 was adaptive, one need not say, but the mechanism is fully present, 

 though seemingly not used, and of obscure, if any, functional value — 

 as far as concerns the movements of Onchidium on the rock surfaces. 



BIBLIOGRAPHY. 



Arey, L. B., and Crozier, W. J., 1918, The 'homing habits' of the pulmonale 

 moUusk Onchidium, Proc. Nat. Acad. Sc, iv, 319; 1919, The nervous 

 organization of a nudib ranch, Ibid., in press. 



Crozier, W. J., and Arey, L. B., 1918, On the significance of the reaction to shad- 

 ing in Chiton, Am. J. Physiol., :idvi, 487; 1919, Onchidium and the question of 

 adaptive coloration. Am. Nat., liii, 415. 



Cushny, A., 1919, Note on strychnine tetanus, Quart. J. Exp. Physiol., xii, 153. 



Hirasaka, K., 1912, On the structure of the dorsal eyes of Onchidium, [Japanese], 

 Zool. Mag., Tokyo, xxiv, 20. 



Loeb, J., 1916, The organism as a whole from a physicochemical viewpoint, New 

 York. 



Mast, S. O., 1911, Light and the behavior of organisms, New York, 1st edition. 



Semper, C, 1877, Ueber Schneckenaugen vom Wirbelthiertypus, u. s. w.. Arch. 

 Mikr. Anat., xiv, 117. 



Stantschinsky, W., 1908, t)ber den Bau der Riickenaugen und die Histologie der 

 Ruckenregion der Oncidiiden, Z. wiss. Zool., xc, 137. 



