NATURE OF SHELLS. 15 



notice how greatly the operculum varies in the differ- 

 ent species that we are to study ; but for the shells 

 now under consideration the operculum is a somewhat 

 oval, horny plate, just fitting the aperture. If we 

 examine it, we shall notice that it was once very 

 small, and that there are additions around its edge, 

 and fine marks which correspond to the lines of growth 

 on the shell. 



After the operculum, the body of this curious sea- 

 snail begins to appear : first a flat, crawling disk or 

 foot^ by which he clings to the rock, and is able to 

 slowly move along its surface. And now, in the front 

 of this creeping disk, we see the animal's head, with 

 its pair of tentacles or feelers. The eyes are set upon 

 stalks, often at the base of the tentacles ; they are 

 small organs, probably of quite limited vision. 



A considerable portion of the animal, however, 

 never leaves the shell — in fact, the shell is grown to 

 the skin or mantle by which it is secreted. We may 

 often pick up the empty cases of little crabs, and 

 may be pretty sure that they are but the cast-off, 

 hardened outer skins of those crustaceans ; for the 

 crabs shed their shells periodically, somewhat as a 

 snake casts its old skin. 



But if we find the empty shell of any snail, we may 

 be sure that its former occupant is no longer alive. 

 There are land-slugs and sea-slugs which look like 

 snails without shells, and some people believe that 

 they once had such protections ; but the fact is, they 

 belong to species which never had and never will 

 have any shells, except, perhaps, very small and rudi- 

 mentary ones. 



There are many other things which we might 

 notice about our little animal ; such as his teeth, his 



