BIOLOGY OF PHYSA 117 



in a beaker of water of the same depth and temperature as 

 the water from which they were taken. As the snails crawled 

 up and about the beaker they did not tap with their siphons 

 as much as the older ones. The younger snails, observed through 

 a lens, were not seen to tap with a siphon at all. Snails of 

 about five mm. diameter and larger tap occasionally but not as 

 often as snails several times this size. There is a noteworthy 

 difference in the manner of tapping with the siphon in the 

 young and in the adult Physa. The adult snail when it taps 

 with its siphon does so by projecting it between the shell and 

 the foot, touching the glass at one or several points without 

 noticeably swinging the shell or stopping. When it takes 

 air at the film while upon a vertical surface, it swings its shell 

 about horizontally; when it takes air upon the film, the siphon 

 side of the body is slightly elevated ; but the shell is not swung 

 about in the least. The little snail, on the other hand, stops 

 in its upward progress, swings its shell about in a horizontal 

 position and projects its siphon upward in exactly the same 

 manner that the adult snail does when it takes air while on a 

 vertical surface. It is very amusing to see the little snail stop 

 every now and then on its way to the film, swing its shell about, 

 and elevate its siphon as far as it can reach. If the beaker 

 is full of water and all atmospheric air excluded, the young 

 snail swings its shell about upon the glass cover in the same 

 way that it does upon the side of the dish. All gradations 

 of swinging the shell about and reaching up with the siphon 

 are seen in snails of different ages. The younger ones swing 

 their shells about horizontally but with increasing age there 

 is less and less movement of the shell, until in the adult it dis- 

 appears altogether. When the young snail first begins to tap, 

 it projects its siphon straight up in one direction, while in older 

 snails there is more and more of a tendency to feel about. 



VIII. Summary and discussion. 



Physa is sensitive to contact stimuli and its response to a 

 general disturbance of the environment is so delicate at times 

 that it rivals that observed in the leech by Whitman (1898) 

 and the planarian by Pearl (1903). 



The character of the response of Physa to both localized 

 mechanical stimuli and to general disturbances of the environ- 



