Among the snails collected, there will probably be found some which have a peculiar scale on the hinder part of the body. When the snail crawls, this scale will be seen just behind the shell, as in
Fig 5, o. This scale is called the operculum, 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.
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