THE GEOTROPISM OF FRESHWATER SNAILS. 



III. EXPERIMENTAL. 



To study the behavior of an animal it is always necessary to 

 discriminate the* force under consideration as much as possible 

 from other forces which may act simultaneously with it, favorably 

 or antagonistically. Oxygen for pulmonate animals such as 

 Physa is, of course, very important. That food is another factor 

 in determining behavior of living animals need hardly be men- 

 tioned. Contact stimuli must also be considered. Light is often 

 important. These with gravity are the chief forces which should 

 be borne in mind. 



The effect of the force of gravity on Physa and others is the 

 problem with which this paper is chiefly concerned. But in 

 considering this the other forces just mentioned must also be 

 considered. Light especially must be taken into account. 



/. Heliotropism of Physa gyrina Say. 



Walter (10, pp. 23-24) has experimentally shown that Physa 

 primeana Tyron and others are generally negatively heliotropic. 

 A series of experiments was made with Physa gyrina Say to 

 compare results with the results obtained by Walter. The 

 experiments were conducted as follows: Five selected individuals 

 were placed on a smooth glass plate with their anterior ends 

 facing direct sunlight. The surface of the plate was carefully 

 moistened. During experiments the angle of the rays of sunlight 

 was about 22.5. The glass plate was horizontally placed in air. 



TABLE I. 



HELIOTROPISM OF Physa ON A HORIZONTAL MOIST GLASS PLATE IN AIR AT 

 THE ANGLE OF 22.5 OF THE RAYS OF SUNLIGHT. 



Table shows results after one minute. Feb. 14, 1914, 9:30 A. M. Tempera- 

 ture, 22 C. 



