THE GEOTROPISM OF FRESHWATER SNAILS. 93 



and was placed on a "rack" specially made for angle determina- 

 tion. The whole arrangement was covered as soon as possible 

 with a dark box. Curiously enough, darkness seemed not to 

 interfere very much with Physa s activity, if the experiment was 

 conducted in the air, though it did interfere in water, as already 

 shown. Physa oriented itself in the line of the force of gravity 

 and crawled in that direction. The relative weight of Physa 

 is about a thousand times as great in the air as in the water. This 

 probably made a difference in its activity in the air even in total 

 darkness. 



The results given in Table VI. show that from the angle of 

 io% of inclination to that of 563/4, positive geotropism in- 

 creases as the degree of the angle increases. It should be added 

 that this was not because the lung was full of air. On the con- 

 trary, about two thirds of these positive animals sank, when they 

 were tested in water. This means that their lungs were empty. 

 Negative geotropism and horizontal crawling, on the other hand, 

 decrease in reverse proportion as the angle of inclination is 

 increased. This was not because the lung was empty. On the 

 contrary, about half of these negative animals floated, when they 

 were tested in water. 



But there was a limit to the degree of inclination of the support, 

 beyond which Physa could not actively move on the plain glass 

 plate on account of the force of gravity. This is significant. At 

 the angle of 67^ positive geotropism suddenly decreases, and 

 negative geotropism and horizontal crawling increase. Gravity, 

 of course, is constant and always exerted vertically. But the 

 effective force exerted on the animals depends upon the inclina- 

 tion of the surface on which the animals crawl. The exertion 

 required to enable the animals to move on a horizontal surface is 

 least; that required on a vertical surface is greatest. At the 

 angle of 67^, therefore, the effective force of gravity was so 

 great that some individuals of Physa could not actively move 

 against it (in air). An apparent increase of negative geotropism, 

 therefore, was the result. This becomes clear, when one con- 

 siders the failure of the experiments at the angle of 78^, which 

 was due to the fact that nearly all the animals passively slid down 

 the plate, mostly with their heads up. The optimum inclination 



