BIOLOGY OF PHYSA 89 



snail has stopped close to the film, the siphon is extended but 

 little and opens wide upon the film ; but if the snail is not so close 

 to the film, the siphon is elongated into a slender tube which 

 opens with a small aperture. If, when the snail stops, it is 

 too far away from the film for the siphon to reach it, it crawls 

 up a short distance further without changing the relative posi- 

 tion of the shell to the surface. If Physa takes air while crawling 

 upon the film, the siphon is protruded only slightly but opens 

 with a wide aperture; and the shell is raised slightly, but is 

 not swTing about as is the case when the snail comes up through 

 the w'ater to take air. The siphon opens upon the film with 

 a sound like the sudden parting of moistened lips. The time 

 during w^hich it remains open varies from a few seconds to a 

 minute or more, depending, presumably, upon the amount 

 of air in the lung at the beginning and upon the size of the 

 aperture. When the lung is first opened, the air is seen to be 

 gently forced out in a bubble which breaks upon the surface 

 filin; presumably, the lung then expands so as to form a partial 

 vacuum which becomes filled with atmospheric air. The snail 

 may expel the air from its lung whether upon film or solid, 

 bubble by bubble or in a stream of bubbles. The readiness 

 with which Physa gives up the air from its lung w^hen stimulated 

 depends largely upon the strength of the stimulus and whether 

 or not the animal is accustomed to being handled. A snail 

 in this condition may be spoken of as "tamed." Physa tamed 

 in this sense expels the air from its lung with reluctance a few 

 bubbles at a time, unless subjected to a severe mechanical 

 stimulus. An untamed snail, on the other hand, ehipties its 

 lung in a stream of bubbles if the unexpected happens. Physa 

 may start to expel a bubble of air and, when it is projected 

 from the siphon, may slowly take it back, thus showing ability 

 of the lung both to expand and to contract. 



3. Parts of the snail sensitive to air. The jtentacles of Physa 

 are sensitive to the film or to the stimulus of oxygen. As the 

 snail crawis up through the water, its tentacles touch the film 

 when its body is still some distance away. No sooner do they 

 come in contact with the film, than the siphon begins to pro- 

 trude; the snail stops and swings its shell about in the char- 

 acteristic manner above described. The time that it takes 

 a snail to stop after it has received the film stimulus depends 



