22 THE OYSTER. 



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, 

 so that each new layer is a little larger in area than 

 the last one. The soft mantle readily conforms to the 

 shape of the body to which the oyster is fastened, 

 and however irregular this may be, the new shell takes 

 its shape and closely adheres to it, because the new 

 deposits are laid down directly upon it. 



This shows the error of the current belief that an old 

 oyster cannot fasten itself. Since the adhesion takes 

 place around the growing edge, an oyster may fasten 

 itself at any time ; and clusters of oysters are often 

 found with their shells soldered together near their tips. 

 Of course this can occur only after they are well 

 grown. 



Oysters are able to close up broken places in their 

 shells, and most molluscs sometimes absorb and rebuild 

 parts of their shells. If any foreign body gets in be- 

 tween the shell and the mantle, shelly matter is 

 deposited upon it. The pearls of the pearl oyster 

 are formed in this way. Some small particle works 

 its way in, and forms a nucleus which is gradually 



