46 MOLLUSC A. 



Many of these breathe the air, though they Hve in the 

 water. 



Those which breathe the air (Lymnea) come to the sur- 

 face for that purpose, and may be observed opening the 

 orifice described above, in order to fill the air sac or lung. 

 This serves them while under water until they rise again 

 for a new supply. 



Among these true air-breathing snails, there is no spe- 

 cial provision for closing the shell. The animal can double 

 up and withdraw its foot inside ; and in winter the land 

 Pulmonates, as part of the air-breathers are called, close 

 the aperture often with five successive partitions formed 

 of mucous. These dry into a hard, dense membrane, 

 and keep out the cold and marauding insects, etc. The 

 foot destroys these on the return of spring. Snails pro- 

 tect themselves in this way from excessive dryness also, 

 and in the tropics during the extreme heats of summer. 

 The water-breathers, which have branchiae for that pur- 

 pose, must not be confounded with these, and can always 

 be distinguished by their remaining below the surface ; and 

 they also have an operculum, or little plate, on the foot, 

 which they use to close the aperture of the shell when they 

 retire within ; and the mouth on a sort of proboscis, as in 

 most of the marine forms, to which they are more closely 

 allied than to the true air-breathers with which they live. 

 Fig. 30 shows a specimen of the genus Paludina expanded, 

 and Fig. 32 one contracted.* 



The foot itself is a most interesting region for gen- 

 eral examination. 



The peculiar motions of the land snails as they 

 crawl, — the head part always in front ; the feelers 

 stretching but to touch everything, the pair above look- 



* See also, for other figures and a fuller account, Morse's 

 First Book in Zoology. 



