326 NATURAL HISTORY. 
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- 
mgl. ‘They are.-of 
the size of a small 
pea, and are depos- 
ited in the ground 
» about two inches 
oa below the surface. 
ad ae Se 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 are terrestrial, the Slugs and the 
Snails, are included in an order by themselves, as having 
lungs—the order Pulmonifera. 
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 
rater 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 always to be observed 
in the shell where this siphon 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 example of the Turbinidee, or W hor] fam- 
