proboscis or rostrum. [See Fig. 5.) The shells, too, are, as a general thing, more solid.
Thus far we have examined those snails which live in fresh water. Some of these were air-breathers, and came to the surface of the water
Fig. 5.—Snail with Operculum.
o, Operculnm; e, Eye; r, Rostrum; g, Entrance to Gill-Cavity.
at intervals to breathe air. We have studied other fresh-water snails which did not breathe air directly, but performed this function by means of an organ called the gill. And these snails were operculated, that is, they all possessed a little scale called the operculum, which closed the aperture tightly when the snail contracted within the shell.
Fig. 6.—Aperture of Shell closed by Operculum, o.
We have also learned that the shells grow in size by successive additions of limy matter deposited around the free border of the aperture, and that the delicate lines which mark the surface of the shell, and which run parallel to the outer edge of the aperture, are lines of accretion, or lines of growth.
Looking over our fresh-water shells again, we find many that are known as muscles, or clams. These shells are common everywhere along the margins of brooks, rivers, and lakes. The muskrats feed upon the soft parts of the muscles, and the remains of their feasts may be found in piles of mussel-shells all along the shores of certain lakes. The shell is composed of two pieces, or valves, as they are called.
