HISTORY OF THE OYSTER. 13 



pendages called siphons (PI. III. fig. 2, b), with which 

 the Cockle is also provided. 



The mantle (PL I. c) is free, and encloses the 

 bronchiae (PI. I. n), or breathing apparatus. The 

 Cockle progresses by means of extending its foot 

 forward, and dragging the shell edgeways after it. 

 On the opposite extremity to the foot project two 

 tubes, scarcely beyond the shell ; one of these is the 

 entrance to the respiratory organs, the other the vent. 

 The same functions that are carried on by these 

 tubes in the Oyster have considerable modifications. 

 While the bronchiae in the Cockle are internal, those 

 in the Oyster are external, being situated between 

 the folds of the mantle ; and freely admit the access 

 of water when the shell is open. 



The mantle, which I have just mentioned (PI. I. c), 

 is an organ of great importance to the animal, as it is 

 by its means that the shell is enlarged : it lines the 

 edge of the internal surface of both shells, and is fur- 

 nished with glands, which deposit carbonate of lime 

 with a glutinous secretion when the shell requires 

 increasing on the edge, and also on the internal 

 surface. The edges of these repeated deposits, slightly 

 projecting one over the other, form the ridges and 

 carinations observable in most bivalve shells. 



Having now explained, I hope intelligibly, the 

 organs of the Oyster as seen externally, I shall en- 

 deavour to point out the internal ones, which can of 



