20 THE OYSTER. 



not tell how an oyster makes his shell. We can now 

 give a satisfactory answer to what will not, I hope, be 

 thought a fool's question : '' Canst tell how an oyster 

 makes his shell ?" 



The shell, on each side of the body, is lined by a 

 thin, delicate, fleshy fold, the mantle ; which may be 

 compared to the outer leaf on each side of the check- 

 book, next the cover. It lies close against the inside of 

 the shell, and forms a delicate living lining to protect 

 the body and the gills, and it is also the gland which 

 makes the shell. 



At all times, while the animal is alive, it is laying 

 down new layers of pearl over its whole inner surface, 

 and as each successive layer is a little larger in area 

 than the one before, the shell increases in size as well 

 as in thickness, and the hinge, where there are many 

 layers, is very thick, while the edge, which is new, is 

 quite thin and sharp. Each layer is very thin, hardly 

 thicker than a sheet of tissue paper, but the deposition 

 of layer on layer gradually results in a solid box of 

 stone. 



Shells which grow on rough, irregular surfaces con- 

 form to their shape as perfectly as if they had been 

 moulded into the ridges and furrows, like soft clay. 

 An oyster growing in the neck of a bottle takes the 

 smooth, regular curve of the glass, and on the claw of 

 a crab an oyster shell sometimes follows all the angles 

 and ridges and spines, as if it were made of wax instead 

 of inflexible stone. Its apparent plasticity and the 

 mouldings of its surface are due to the flexibility of 

 the soft edge of the mantle. When the oyster is at 

 rest this protrudes a little beyond the edge of the shell. 



