BIOLOGY OF PHYSA 113 



chance, trial and error, or later by the return of the sense of 

 gravity which would cause them to crawl to the highest point 

 of the sloping shore. 



The time which it takes Physa to adjust itself to a new experi- 

 ence depends, within certain limits, upon the frequency with 

 which the experiment is repeated. Snails that had expelled 

 the air from their lungs when lifted gently from the pond were 

 placed in a beaker. They were left undisturbed for six hours, 

 with only an occasional moving of the dish. The first time that 

 the beaker was gently touched, the snails expelled the air from 

 their lungs and fell to the bottom. As time went on, they grew 

 less sensitive and would crawl upon the film without expelling 

 their air while the beaker was moved about. This experiment 

 was repeated with other snails from the pond. The beaker 

 was carried about continually with but short intervals of quiet. 

 The snails adjusted themselves to the new experience in 

 less than half the time required for the first lot. 



V. Physa forms memories of its habitat. 



A number of tamed Physa, after being compelled to expel 

 the air from their lungs, were dropped back into the aquarium 

 in which they had been living for some weeks, and the aquarium 

 was then filled and covered so as to exclude all atmospheric air. 

 The snails came to the top almost immediately and most of 

 them crawled upon the glass cover. Some, however, that were 

 moving at a faster rate than others, swung the shell about and 

 elevated the siphon as though to take air but, upon touching 

 the glass, withdrew the siphon and crawled upon the glass cover. 

 The head did not receive even a semblance of a film stimulus, 

 yet the shell was swung about and the siphon projected as though 

 it did. It would, seem that the snail was acting from force of 

 habit and that this was likewise the explanation of its behavior 

 to the film formed by the carbonic acid and water in a pre- 

 vious experiment (p. 98). The snails were not upon the cover 

 long, before they projected their siphons and began to tap here 

 and there on the glass. They lingered upon the cover sometimes 

 for hours, but most of the tapping was done in the early part of 

 their stay. This experiment was repeated, and the snails, after 

 they had lingered tapping with their siphons about the cover 

 for a while, were placed upon the bottom and the air excluded 



