BIOLOGY OF PHYSA 103 



it touches it, but remains for some time in contact before taking 

 air into the lung. 



III. Summary and discussion. 



1. Physa may fill its lung with atmospheric air while it is 

 upon a vertical or horizontal surface. The extent to which 

 the siphon is protruded and the size of the aperture depend 

 upon the distance of the snail from the surface film. 



2. The lung has power of contraction and expansion and is 

 presumably filled by the formation of a partial vacuum. The 

 shell is swung about through an arc of ninety degrees if the snail 

 takes air while upon a perpendicular surface; if upon the film 

 the body is merely raised slightly on the left side. 



3. The head (except the ventral side) and the region of 

 the siphon are sensitive to an oxygen stimulus. The snail . 

 can sense and pick up air bubbles under water. If the head 4 

 touches the bubble the shell is s^vung about at just the right 

 angle for the siphon to open into it.- Pauly (1877) observed 

 the Lymnaeidae picking up bubbles of air under the water; he 

 thought that the snails he had observed which did not come 

 to the surface for air received their oxygen supply in this way. 



4. The siphon acts reflexly to a stimulus from oxygen. If 

 exposed continually to the air, the lung opens at more or less 

 regular intervals and resembles the pulmonary reflexes in the 

 higher animals. The head does not respond to the oxygen 

 stimulus as unerringly as the siphon does. If air has been taken 

 into the lungs repeatedly at short intervals, the head and the 

 tentacles may respond with indifference to the stimulus of the 

 film, much as the snail failed to respond to a food stimulus after 

 it is very well fed. 



5. The amount of air taken into the lung at any given time 

 depends upon the amount of ^ir already in the lung , the fre- 

 quency of oxygen stimuli received, the size of the aperture and 

 the length of time the lung remains open. Pauly (1877) be- 

 lieves that the intervals between breathing depend upon the 

 distance over which the snail has traveled to reach the film. 

 Walter (1906) thinks that the amount of atmospheric air which 

 is taken by any individual during the twenty-four hours depends 

 not so much upon the number of times it opens its respiratory 

 tube at the surface, as upon the length of time the tube remains 

 open for inhalation. He noted that some snails kept the siphon 



