104 THE OCEAN. 
only to a small extent during the life of the animal. 
Yet we must not for a moment suppose that these 
creatures are unhappy, or that the meanest occupant 
even of a bivalve shell is not supplied with every- 
thing that could conduce to its welfare. It is sIN 
alone that is the source of unhappiness. I will just 
point out one or two particulars in which the Divine 
care for these creatures is manifest. All of them 
have the vital parts of the body protected by a thick 
fleshy coat, somewhat projecting at the edges, called 
the mantle: the surface of this organ has the power 
of forming the shell, by depositing stony matter in 
a sort of glutinous cement, which soon hardens into 
a thin layer of shell. Ifa little piece were broken off 
the edge of the shell of a Whelk, when alive, the 
animal would press the surface of the mantle against 
the fracture, and pass it several times over the place; 
a very thin transparent film would then be seen to 
fill up the space, which in the same way it would 
increase in thickness, until in a few days we could 
scarcely distinguish the renewed part from the 
other, or tell that the shell had been broken, except, 
perhaps, by a slight variation in colour. As the ani- 
mal grows, it wants a-larger shell; and the mantle 
affords the means of increasing its size: the front 
edge of this organ is thicker than the rest, and is 
called the collar; and it is by thrusting this round 
the edge of the shell, while stony matter is poured 
out from its surface, that an addition is made to it. 
In the Bivalves, or those whose shells open and shut 
like the covers of a book, as the Oyster, the mantle 
is twofold, covering the body on each side, just. within 
