12 FIRST BOOK OF ZOOLOGY. 



the body. When the snail crawls, this scale will be seen just 

 behind the shell, as in Fig. 16, o. 



This scale is called the oj)e7'culum, and when the snail 

 has contracted, or drawn within the shell, the operculum is 

 seen to fit the aperture of the shell, closing the shell as a 

 stopper closes the mouth of a bottle. 



Fig. 15. — Snail with Opeechlum. — o, Operculum; e, Eye; r, Eostram; ff, Entrance to Gill- 

 Cavity. 



Nearly all sea-snails, that is, snails which live in salt-water, 

 and many species of fresh-water snails, and also many snails 

 which live in damp places on the land, and which are called 

 land snails, have an operculum. 



When the snail has retired within the shell, the operculum 

 will look like this iti the aperture of the shell (Fig. 16) : 



A series of concentric lines will be seen marking the 

 operculum, and these are the lines of growth, the operculum 

 growing around the outer edge by successive additions, 

 just as the shell grows by successive additions to its outer 



