OF NORTH AMERICA. 39 



cure free air from bubbles which have become entangled in some ob- 

 jects below the surface. Pauly was able to observe a number of indi- 

 viduals actually secure the air from these bubbles, but Walter was not 

 able to verify this observation during his studies on the behavior of the 

 pond snails. (Walter, p. 20.) The author has frequently seen fresh- 

 water pulmonates in the vicinity of air bubbles and there is no reason 

 to doubt the accuracy of Pauly's observations. 



Pauly's third class of abnormal breathing, that of snails artificially 

 deprived of air, bears upon a very interesting subject, that of the 

 ability of Lymnsea to return to the ancestral habit of extracting the 

 oxygen from the water, as observed in those snails living in very deep 

 lakes and also in young Lymnseas. The observations of Moquin-Tan- 

 don, Saint-Simon, Pauly, Walter, etc., all tend to show that Lymnaea 

 is unable to suddenly adapt itself to the water breathing habit. In some 

 of the experiments Lymncea stagnalis and Lymn&a elongata were used, 

 Lymncca stagnalis surviving from 22 to 91 days, but finally succumb- 

 ing. It is interesting to note that the deep water forms easily and 

 readily adapt themselves to the surface-visiting habits of their shal- 

 low-water relatives. 



e. AESTIVATION. 



During seasons of drought when the ponds and streams dry up, 

 the Lymnseas bury themselves in the mud to the depth of several inches, 

 form a rib or varix within the outer lip and cover the aperture with an 

 epiphragm like the hibernating helices. This is especially true of those 

 species or races which inhabit intermittent streams or summer-dry 

 ponds. Cooke 1 records Lymnaa peregra as burying itself to the depth 

 of three inches, "when surprised by a sudden fall of water in the ditch 

 on Coe Fen, behind Peterhouse, Cambridge." Galba truncatula has 

 been found buried in hard dry mud to the depth of 18 inches. 2 



Lymnsea is able to survive for considerable periods when deprived 

 of water. Many of the smaller species (parva, humilis, obrussa) will 

 voluntarily leave the water and remain for a considerable time on 

 stones, sticks and other material on the land. Several of these species 

 are sometimes found quite a distance from water. The European 

 Galba truncatula has been reported by JefTery 4 as living far from 

 water. The large species are not known to possess this habit, although 

 they will live for a long time without water, if placed in wet moss or 

 cotton. Wiedersheim 3 has recorded the instance of Radix auricularia 



1 Mollusca, p. 27. 



2 Op. cit., p. 41. 



Zoologischer Anzeiger. II, p. 573, 1879. 



4 Journ. Conch., Ill, p. 305, 1882. 



