THE OYSTER. 21 



impression on the shell is gradually covered up by the 

 new deposits of lime, and, in an empty shell, it may be 

 traced for some distance up towards the hinge, where 

 it gradually becomes more faintly marked, as the layers 

 of new shell grow thicker. A very good idea of the 

 way the shell grows and keeps pace with the growth of 

 the body, may be gained by the careful examination of 

 the muscular impression on its inner surface. 



Every fool knows why a snail has a house, but the 

 king could 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 deli- 

 cate 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. In the oldest part of the shell, near 

 the hinge, there are many layers, and the shell is 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- 



