LYMNZIDZ 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.* 
A number of interesting experiments bearing on this subject were 
carried on by Walter® 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 larve 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 Lymneids, 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 Lymnzeus 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 Lymnzus 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 
“Walter, Cold Spring Harbor Mon., VI., p. 19. 
SO CLE ee Lilie 
