GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 83 



10. In the direct sunlight, the snails are apparently more 

 positively geotropic than in darkness, due to the fact that they 

 are negatively heliotropic. 



11. In the direct sunlight, 58 per cent, of the animals orient 

 themselves head downward and crawl in that direction through 

 the surface-film of sea-water into it. 



12. In darkness, the snails, which "hesitated" at the surface- 

 film of sea-water in the daylight, crawl upward through the film. 



13. From the experimental results which the writer has ob- 

 tained, he concludes that neither the mechanical theory, nor the 

 pressure theory, nor the resistance theory is adequate to explain 

 the phenomenon of the negative geotropism of Littorina littorea 

 but a physiological one, that is, the statocyst or statolith theory. 

 This theory is the more likely since these snails have statoliths 

 (17, pp. 119-120). The writer, however, has no direct evidence, 

 at present, in favor of the statolith theory. He is led to accept 

 it largely by the method of exclusion. Furthermore evidence 

 from many sides based on the experiments of the writer causes 

 him to conclude that the surface-film theory is also not correct. 



In conclusion, the writer wishes here to acknowledge his 

 indebtedness to Professors Walter E. Garrey, Ralph S. Lillie, 

 and Elias P. Lyon, for their valuable suggestions and criticism 

 on his experiments at the Marine Biological Laboratory at Woods 

 Hole, Mass., during the summers of 1912 and 1913. His thanks 

 are also due to Professor Frank R. Lillie for the privileges of the 

 Laboratory and to Professor Lyon for criticism and suggestions 

 in the preparation of manuscript. 



VI. BIBLIOGRAPHY. 



1. Bohn, Georges. 



'05 Attractions et Oscillations des animaux marins sous 1'influence de la 

 lumere. Bull. d. FInst. gener. d. Psychol., pp. 171-181. 



2. Davenport, C. B. and Perkins, Helen. 



'97 A Contribution to the Study of Geotaxis in the Higher Animals. Jour, 

 of Physiol., Vol. 22, pp. 99-110. 



3. Frandsen, Peter. 



'01 Studies on the Reactions of Limax maximus to Directive Stimuli. 

 Proc. Am. Acad. Arts and Sci., Vol. 37, pp. 185-227. 



4. Haseman, J. D. 



'n The Rhythmical Movements of Littorina littorea Synchronous with 

 Ocean Tides. BIOL. BULL., Vol. 21, pp. 113-121. 



