326 NATUEAL HISTOET. 



•which usually incloses a small shell. This shield is over 

 the breathing apparatus (§ 557), and the head can be so 

 drawn in as to be under it. The Snails have very much 

 the same shape and arrangement with the Slug, except 

 that they have a shell into which they can withdraw the 

 whole body. The common Snail, Fig. 257, lays eggs, 



which are very large 

 in comparison with 

 the size of the ani- 

 mal. They are of 

 the size of a small 

 pea, and are depos- 

 ited in the ground 

 about two inches 

 below the surface. 

 Fig. csr.-snaiL 560. A few of the 



Gasteropods that, like the Snails and Slugs, breathe with 

 lungs, are yet aquatic in their habits. But, like other 

 aquatic animals that have lungs, as the Whales, they are 

 obliged every now and then to come to the surface to 

 get air. Among these are the Pond Snails, a species of 

 which is represented in Fig. 255, page 324. These ]Mol- 

 lusks, and those which arc terrestrial, the Slugs and the 

 Snails, are included in an order by themselves, as having 

 lungs — the order Pnlmonifera. 



561. The second order of the Gasteropods includes all 

 those which have gills instead of lungs, and also have a 

 shell, usually of a spiral form. This order is much larger 

 than the others, and presents a great variety of beautiful 

 shell-coverings. Some of them have siphons to introduce 

 water into the cavities where the gills are, so that the 

 animal can breathe without putting its body out from 

 the shell. There is a little notch al wa3's to be observed 

 in the shell where this sijihon passes out. 



562. Of the many varieties of the .shells of these Gas- 

 teropods I will notice but a few. In Fig. 249, page 317, 

 on the left, is an e;cample of the Turbinidae, or Whorl fam- 



