10 FRIENDS WORTH KNOWING. 
in that they are provided with lungs, and thereby are fitted 
to live in air‘ instead of water. Hence all true snails are 
terrestrial. As the snail crawls upon a cabbage-leaf, all 
that you can see of the body is the square head bearing 
two long and two short horns, with the muscular base ta- 
pering behind. There is an oily skin, and on the back is 
_ borne a shell containing the rest of the body, twisted up in 
its spiral chamber. Extending along the whole under sur- 
face of the body is the tough corrugated disk upon which 
the animal creeps. This foot is the last part of the body to 
be withdrawn into the shell, and to its end, in a large divi- 
sion of pulmonate as well as marine mollusks, is attached a 
little horny valve which just fits the aperture of the shell 
and completely stops it up when the animal is within. This 
is called the operculum. The foot secretes a viscid fluid 
which greatly facilitates exertion by lubricating the path, 
and snails may often be traced to their hiding-places by a 
silvery trail of dried slime. So tenacious is this exudation 
that some species can hang in mid-air by spinning out a 
mucous thread ; but, unlike the spider, have not the power 
to retrace their way by reeling in the gossamer cable. The 
slime also serves the naked species as a protection, birds 
and animals disliking the sticky, disgusting fluid; and it 
serves others as a weapon, seeming to benumb whatever 
