BIOLOGY OF PHYSA 93 



the air from their lungs. All three bottles were then inverted. 

 The snails in the first two bottles immediately began to crawl 

 upward away from the earth, while those in the third bottle 

 seemed indifferent to the change in position. They crawled 

 about as before and did not cluster at the upper end of the bottle, 

 as did the others. The snails in the first two bottles had empty 

 lungs. They were so sensitive that upon tilting the bottle 

 even to the least observable angle, they would immediately 

 turn and go to the highest point. The response to the stim- 

 ulus of gravity was very quick and unhesitating when the' 

 animal was in need of air, and it varied with the amount of air 

 in the lung. The snails seemed indifferent to the stimulus 

 when the lung was full. 



From the above experiments the conclusion was drawn that 

 snails are negatively geotactic when their lungs are empty, 

 but when they have sufficient air they become indifferent to 

 gravity and crawl in all directions. It can readily be seen 

 that the relation that the snail bears to gravity is of great bio- 

 logical advantage to the animal. If the snail were at all times 

 indifferent to gravity, it might at times lack air if its contact 

 with the surface film were left to mere chance. On the other 

 hand, if the snail were always negatively geotactic it would 

 seek its food at as high a level as its habitat would allow. This 

 would cut off the food supply greatly and indeed would cause 

 many places that otherwise furnish optimum conditions to the 

 snail, to be abandoned. 



Physa is seen to be less sensitive to gravity after it has been 

 deprived of air for some time, since it then moves about in the 

 bottle, but is much less active. This behavior is adaptive 

 in that it would be disastrous to the species if the animals re- 

 mained at the highest points in their habitats when their lungs 

 were empty, in as much as lung respiration is often interfered 

 with by currents or waves and even prevented altogether. 

 The habitat in which the snail lives has a great controlling 

 influence upon the amount of air to which the animal has access. 

 If it is living in swiftly flowing water it has little or no oppor- 

 tunity of reaching the air and even if it could do so it would 

 find great difficulty in opening its siphon for air upon such a 

 disturbed surface. In the creek-bed habitat, for instance, 

 the snail could not reach the air unless it crawled to the side 



