192 SHELLS AND MOLLUSCOUS ANIMALS. 



lie there with their flat shells uppermost, that the 

 animal may breathe by opening its shell, and 

 gather in thus its food from the water; and the 

 fishermen who have looked down through the clear 

 waters into these oyster-beds, state that the ani- 

 mals are so sensitive, that if even the shadow of a 

 boat passes near them, they instantaneously close 

 their shells. Designed as they are to be the food 

 of man, as well as of fishes, birds, starfishes, and 

 zoophytes, they are most wonderfully prolific. It 

 is stated by Poli that one of these shell-fish con- 

 tains 1,200,000 eggs, so that a single oyster might 

 furnish enough to fill 12,000 barrels. 



The structure of the bivalve mollusks is beauti- 

 fully fitted to their condition. On gently opening 

 the shell of an oyster, we find within, a membrane, 

 in some species having a delicate fringe of little 

 threads. This membrane is the mantle from which 

 the animal secretes the outer layers of its shell, 

 in such manner as to admit of its being made larger 

 as the shell-fish grows. Between the leaves of 

 this mantle lie four delicately fine membranous 

 leaves, composed of slender fibres. These are the 

 branchiae or gills, the aerating organs, and the 

 mouth is placed between the innermost of these 

 leaves. But these gills have another office to per- 

 form, besides that of preparing the blood for use. 

 They are the organs by which the mollusk procures 

 its food. Shut up in its cell on the sea-rock, 

 or lying, during life, on the bed where it grows, 

 the oyster might seem to be little fitted for making 

 any effort to secure its prey. But the filaments 

 of these gills are thickly covered with minute cilia, 

 or fringe-like hairs. The incessant action of these 

 cilia causes successive currents of water around 



