GEOTROPISM OF THE MARINE SNAIL, LITTORINA LITTOREA. 65 



excluded as usual. The results given in Table III. show an 

 interesting difference compared with those of Table II., which 

 must not be overlooked. The number in the negative geotropism 

 column in Table III. is always higher than the corresponding one 

 in Table II., and that in the positive column in the former is 

 always lower than that in the latter. A possible explanation of 

 these differences is found in the buoyancy effect of the water, 

 which decreases the weight of the animals. In consequence, 

 the pull of gravity is more effective on the animals in air. It 

 would seem therefore that the pull of gravity on the animal 

 as a whole must be one factor in its orientation. 



TABLE III. 



GEOTROPISM OF SNAILS AT THE DIFFERENT ANGLES OF INCLINATION OF A GLASS 

 PLATE IN AIR IN TOTAL DARKNESS. 



At beginning of experiments each head pointing downward. Table shows results 

 after one minute. 



The above fact reminds the writer of the "mechanical theory" 

 (6, pp. 2-13) and, also, the "resistance or weight theory" (6, pp. 

 14-20) of geotropism in Paramecium. If the greater weight .of 

 snails in air causes the quicker orientation of the animals, and 

 also if the posterior region of the animals is heavier than the 

 anterior region (as has been proven to be the case), the theories 

 above mentioned are favored to some extent. Nevertheless, 

 no mechanical theory can satisfactorily explain why "positive 

 geotropism" increases with decrease in the angle of inclination. 

 This, if a real gravity response, must be explained on the basis of 

 physiological conditions, as Loeb pointed out many years ago. 



