114 JEAN DAWSON 



as before. They came up to the top, but they tapped less upon 

 the cover. After this experiment was performed six or eight 

 times they ceased to tap and they did not Hnger so long upon 

 the cover. 



The above experiment was repeated with snails fresh 

 from their natural habitat. They acted as in a previous ex- 

 periment, lingering for a longer or shorter time upon the bottom, 

 tapping and wandering aimlessly about until they finally reached 

 the cover. Most of them upon reaching the top tapped no 

 more, or differently, than they did upon the bottom or sides; 

 a few opened the siphon against the cover. After they had 

 crawled upon the cover, they scattered about the aquarium, tapping 

 with the siphon occasionally. The snails were again placed upon 

 the bottom of [the vessel and the cover was replaced so as to 

 exclude all atmospheric air. They remained much longer at 

 the bottom than before and tapped but little anywhere, neither 

 did they gather in any part of the vessel. This experiment 

 was repeated four or five times. The snails became very in- 

 active and ceased to tap or crawl and behaved as if in a stupor. 

 After they had reached this state, the cover was removed and they 

 were allowed to fill their lungs at the surface. Soon after filling 

 their lungs, they moved about almost as actively as they did at 

 the beginning of the experiment. They were again touched and 

 again they expelled the air from their lungs and dropped to the 

 bottom. All atmospheric air was excluded and the snails' behavior 

 observed. As in the first trial, they lingered and tapped near 

 the bottom and sides and upon reaching the top they lingered 

 and tapped for some moments. This experiment of causing 

 the snails to expel the air and then cutting off all atmospheric 

 air until they had remained tapping upon the cover for a time, 

 was repeated until they had taken air at the. film a number of 

 times. They ceased to wander about aimlessly, tapping upon 

 bottom and sides, but upon reaching the top they remained 

 upon the cover for some time. From these experiments it 

 would seem that Physa remembers the location of the surface 

 film. 



Tamed Physa were transferred from a large rectangular aqua- 

 rium to a cylindrical one having the same depth and temper- 

 ature of water. The snails came up without hesitation but they 

 tapped with their siphons now and again as they went up. After 



