LYMNMIDM OF NORTH AMERICA. 49 



same size as those in the smaller, showing that neither volume nor 

 exposed surface, but the presence of oxygen in the water was the 

 important factor in determining growth in this case. 2 



A number of interesting experiments bearing on this subject were 

 carried on by Walter 3 as follows : 



"A number of individuals were confined in a wide-mouthed bot- 

 tle which was stoppered under water to exclude air bubbles. Twenty- 

 four hours later they were all dead. To test whether increase in tem- 

 perature or the exclusion of fresh microscopic food might have in- 

 fluenced this result, a similar number was placed in the same bottle 

 with a mosquito netting covering the mouth. The bottle was then 

 sunk in the 'cement tub' directly under the inflow of fresh water, 

 eliminating these factors as far as possible. The snails thus im- 

 prisoned also died, although a few survived four days. 



"In another experiment a film of kerosene was spread over the 

 surface of an aquarium containing snails. Although able to pierce this 

 film, the following day all were dead. Whether this was due to their 

 inability to get a sufficient supply of air, which is probably the case 

 with mosquito larvae similarly treated with the oil film, or whether the 

 oil itself had a direct effect on them, is not certain. In this connec- 

 tion it is interesting to learn that the 'pump-pool,' in which was found 

 a numerous and thriving community of Lymnaeids, had been treated 

 with the oil film for the three preceding years in the effort to extermi- 

 nate the mosquitoes breeding there. 



"Pond snails when overcrowded in an aquarium visit the surface 

 cftener than when only a few are present. This fact seems to offer 

 evidence that Lymnasus absorbs a certain amount of its air directly 

 from the water. In the case of gill-breathing snails which derive their 

 total air-supply from the water, such a condition would naturally be 

 expected, but if Lymnaeus depends exclusively on the atmospheric air 

 obtained at the surface, why should the scarcity of air in the water 

 exert any influence on its movements? That it cannot take all the air 

 it requires in this manner is shown by the bottle experiments men- 

 tioned above. 



"Continuing this line of observations, two similar vessels were 

 selected, in each of which six snails were placed. One vessel was 

 filled with fresh spring water and the other with water which had been 

 boiled in order to drive off all the free air contained in it, before cool- 

 ing in a sealed flask to prevent the ingress of fresh oxygen from the 



2 Walter, Cold Spring Harbor Mori., VI., p. 19. 

 3 Op. cit., p. 17. 



