6 



98 JEAN DAWSON 



lung is relatively empty is put into chemically pure distilled 

 water which has not taken up air from the room during the 

 process of condensation, it dies before it can reach the surface 

 of the water. If, however, it is allowed to take up more or less 

 air, or is shaken, the snail lives for some time but remains near 

 the surface film. If the water is aerated by passing atmos- 



Cpheric air through it, the snail lives as in tap water. It is thus 

 seen that Physa dies if air is lacking or carbonic acid is in excess 

 y I in the water and that it collects near the surface film if the 



water is foul, if it contains little air, or has carbonic acid to a 

 I certain amount. 



3. Physa reacts positively to oxygen and negatively to car- 

 bonic a.cid. An inverted bottle was partly filled with oxygen 

 and a number of Physa were placed in the neck and allowed 

 to crawl up to the oxygen above. Each snail ascended until its 

 tentacles touched the surface film, when it opened its siphon 

 and filled its lung with oxygen. The reaction was most positive, 

 the snail even resting its head and foot against the film and 

 crawled upon its under surface. They were allowed to breathe 

 the oxygen for twenty-four hours, and the only difference noticed 

 between them and the control snails breathing air, was that 

 ' ihe former w^ere more lively and seemed to be very hungry. 

 This same experiment was tried with carbonic acid instead 

 of oxygen. The reaction of the snails differed according to 

 the velocity with which they came up the side of the bottle 

 to the surface film. If the snail chanced to be moving very 

 slowly, it would upon touching the film stop, withdraw its 

 tentacles, then slowly protrude them again and touch the film 

 again, withdraw the tentacles, turn and go down into the water 

 for about an inch and then turn back to repeat the former pro- 

 cedure. The snail that chanced to be moving upward at a lively 

 rate when it touched the film with its tentacles immediately 

 sv\^ung the shell about and touched the film with the siphon. 

 The moment the siphon touched the film, however, it was mth- 

 drawn and the snail turned immediateh^ and went down. One 

 snail moving more rapidly than the others reacted toward the 

 carbonic acid as though it was air; opened its siphon and took 

 in some of the gas before the negative reaction occurred. The 

 snails that touched their siphons to the film, and especially 

 the one that took in carbonic acid into the lung, subsequently 



