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CHAPTER XL. 



AIR AND WATER. — LIMX^US, SPIRAL. PLANORBIS, DISCOIDAL. ANCYLUS, 



PATELLIFORM. 



LIMN^ID^, OR FRESH- WATER SNAILS. 



Although the mollusca of this family Hve chiefly in the 

 water, they belong to the order of Pulmoniferous Gastero- 

 pods; because, like the laud-snails, they breathe air, and are 

 furnished with a pulmonary cavity instead of gills. They 

 swarm in ditches, ponds, and lakes ; sometimes in running 

 water, sometimes in stagnant pools. They may be seen 

 crawling among the mud and stones at the bottom of the 

 water, or clinging to the weeds growing at the sides, creep- 

 ing on the under surface of water-lily leaves, or, with the 

 shell hanging downwards and the flat of the foot upwards, 

 gliding along the surface of the water. They have short 

 muzzles, and two large, sometimes triangular, tentacles, with 

 eyes at their bases. They lay their eggs in gelatinous 



