92 THE OCEAN, 



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 animal grows, it wants a larger shell ; 

 and the mantle affords the means of increasinfr 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 each shell. Instead of a collar, each leaf 

 of the mantle is here fringed with a series of glands 

 which secrete the exterior part of the shell, by being 

 thrust out round the edge ; while the whole surface 

 of the mantle deposits the beautiful, rainbow-tinted, 

 pearly substance with which the interior is coated. 



Instead of the fleshy belly on which the Univalves 

 glide along, the Bivalves are furnished with a pecu- 

 liar organ, which in some species serves the purpose 

 of motion. The Oyster, however, and some other 

 species, have no power of changing their position ; 

 but are, as it were, cemented to the rock on which 

 the spawn first chanced to fall. The Mussel, again, 

 is fastened, but in a different manner, being moored 

 by a cable of silken threads, which it spins from its 

 own body. But the Cockle, which is eaten by the 



