96 SAKYO KANDA. 



IV. SUMMARY AND CONCLUSION. 



1. Physa gyrina Say is negatively heliotropic. It becomes 

 sluggish in its activity in darkness, particularly in water. 



2. Physa gyrina Say, Planorbis trivolvis, Limncea stagnates, 

 and Limncea columella, are negatively geotropic when their lungs 

 are empty; and positively geotropic when their lungs are full of 

 air. Physa often comes near the surface and crawls down again 

 without filling its lung with air. 



3. Physa, when put w T ith green algae, does not often crawl to 

 the top. 



4. At different angles of inclination of a plain glass plate in the 

 air and in total darkness, Physa is positively geotropic. There 

 is a certain limit of inclination beyond which the animal can not 

 actively move on account of the force of gravity. 



(a) Many of the individuals in question are positive to gravity, 

 even though their lungs are empty. 



(&) The optimum inclination of the plain glass plate on which 

 Physa may crawl is an angle of 56}/ . 



(c) At the angle of 67^/2, positive geotropism decreases as 

 negative geotropism and horizontal crawling increase. The 

 negative geotropism is not necessarily the result of lack of oxygen. 



(d) At the angle of 78% no experiments are successful. 



5. At different angles of inclination of a ground-glass plate in 

 the air and in total darkness, Physa reacts to gravity in a similar 

 manner as though on the plain glass plate, although the limit of 

 inclination is a little higher in the former than in the latter. The 

 optimum inclination of the ground-glass plate is between the 

 angles of 67^ and 78%. 



6. Contact stimuli seem to interfere slightly with the geo- 

 tropism of Physa. 



7. The combination of gravity and light (both the glass support 

 and the rays of light being inclined 11% to the horizontal) 

 accelerates positive geotropism of Physa. 



From the data here given the writer is inclined to draw the 

 conclusion that Physa is naturally positively geotropic. It is 

 little wonder, therefore, that Physa becomes positive to gravity 

 when its lung is filled with air. 



As to the organ of geotropic orientation of Physa and other 

 snails, no direct experimental evidence is yet furnished. But 



