STRUCTURE OF THE OYSTER. 67 



of Venice, of the bay of Cumae, and of England, were 

 preferred ; but they especially attached great value to 

 those which, brought from various places, were deposited 

 in the Lucrine lake, where they grew remarkably fat. 

 Sergius Grata, at BaisB, was the first Roman who en- 

 tertained the idea of establishing this sort of oyster- 

 bed. 



If we take one of these animals, and open the 

 valves of the shell, we immediately find each valve to 

 be lined with a membrane, in some species bordered 

 with a fringe of filaments arising from a thickened edge, 

 which is, indeed, of a glandular structure ; this mem- 

 brane is the mantle, and excepting at the back where 

 the two valves are united, it is free, enclosing the body 

 of the mollusk as if between two leaves. It is from 

 the outer margin of the mantle that the outer layers 

 of the shell are secreted, so as to effect its extension, 

 according to the growth of the animal. Between the 

 leaves of this mantle lie four other leaves, composed 

 of delicately fine fibres ; these membranes attached to 

 the centre of the body, are the branchise, gills, or aera- 

 ting organs, for the purpose of preparing the blood 

 for general use. Between the two innermost of these 

 leaves is the mouth. The mass of the body contains 

 the heart ; the stomach, imbedded in a liver of ample 



