SHELL OF THE OYSTER. 69 



obtained. The action of the cilia seems to be incessant, 

 and goes on, when the shell is closed, acting on the 

 water previously taken in ; nay, even when a portion 

 of the branchioe is cut away, they continue their move- 

 ments in the detached piece so long as vitality remains, 

 and row it rapidly through the water, as if indeed it 

 had independent existence. The branchial filaments 

 consist of minute vessels, running a parallel course, 

 and enveloped in a most delicate tissue ; they com- 

 municate with each other, and ultimately merge into 

 two principal trunks, conveying the pale blood to the 

 heart. 



Our wonder is not limited, however, to the internal 

 structure ; the shell of the oyster is not a little re- 

 markable, consisting as it does of concentric layers 

 of membrane and carbonate of lime, and the marks 

 of the successive layers appearing in its rough outer 

 surface. That which now forms the centre and utmost 

 convexity of the shell was, at an earlier stage, sufficient 

 to cover the whole animal. But as the oyster grows, it 

 throws out from its surface a new secretion, composed 

 of animal matter and carbonate of lime, which is at- 

 tached to the shell already formed, and projects farther 

 at its edges. Thus the animal is not only defended 

 by this covering, but as it grows, the shell becomes 



