FRESH- WA TER MOLL XJSI<:S. 



567 



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. Skail with Operculum. 

 0, Operculnm ; e. Eye ; /, Rostrum ; cf, 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 opereu- 

 lated, 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 haA^e also learned that the shells grow in size by successive 

 additions of limy matter deposited around the free border of the aper- 

 ture, 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 musk-rats 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. 



