FRESH-WATER SNAILS. 



11 



These fresh-water air-breathing snails may be kept under 

 water for many hours before life is extinct. 



FIG. 14. Jar of water, in which is contained a number of species of mollusks, some of which 

 have already been sttidied. Some of them are near the surface, breathing air : A and O 

 are taking in air ; B is just expelling a bubble of air from the lung ; D is crawling on 

 the surface of the water; E, G, and /, are in the act of crawling up, to get a fresh supply 

 of air ; and Jis a, water-breather, having gills, but no lung. 



10. Among the snails collected, there will probably be 

 found some which have a peculiar scale on the hinder part of 



