X.— A CONTEIBUTION TO THE LIFE-HISTORY OF THE OYSTER. 



{Ostrea virginica, Gmelin, and 0. edulis, Linn.) 



By John A. Etder. 



210. OUTLINE SKETCH OF THE COARSER ANATOMY OF THE OYSTER. 



" The general structure of an Oyster may be roughly represented by a long, narrow memorandum 

 book, with the back at one of the narrow ends instead of at one of the long ones. The covers of 

 such a book represent the two shells of the Oyster, and the back represents the hinge, or the 

 area where the two valves of the shell are fastened together by the hinge ligament. This ligament 

 is an elastic, dark -brown structure, which is placed in such a relation to the valves of the shell 

 that it tends to throw their free ends a little apart. In order to understand its manner of working, 

 open the memorandum book and place between its leaves, close to the back, a small piece of 

 rubber to represent the ligament. If the free ends of the cover are pulled together the rubber 

 will be compressed and will throw the covers apart as soon as they are loosened. The ligament 

 of the oyster-shell tends, by its elasticity, to keep the shell open at all times, and while the 

 Oyster is lying undisturbed upon the bottom, or when its muscle is cut, or when the animal is 

 dying or dead, the edges of the shell are separated a little. 



" The shell is lined by a thin membrane, the mantle, which folds down on each side, and may 

 be compared to the leaf next the cover on each side of the book. The next two leaves of each 

 side roughly represent the four gills, the so-called 'beard' of the Oyster, which hang down like 

 leaves into the space inside the two lobes of the mantle. The remaining leaves maybe compared 

 to the body or visceral mass of the Oyster. 



" Although the Oyster lies upon the bottom, with one shell above and one below, the shells 

 are not upon the top and bottom of the body, but upon the right and left sides. The two shells 

 are symmetrical in the young Oyster, but after it becomes attached the lower or attached side 

 grows faster than the other, and becomes deep and spoon-shaped, while the free valve remains 

 nearly flat. In nearly every case the lower or deep valve is tbe left. As the hinge marks the 

 anterior end of the body, an Oyster which is held on edge, with the hinge away from the observer 

 and the flat valve on the right side, will be placed with its dorsal surface uppermost, its ventral 

 surface below, its anterior end away from the observer, and its posterior end toward him, and its 

 right and left sides on his right and left hands, respectively. 



" In order to examine the soft parts, the Oyster should be opened by gently working a thin, flat 

 knife-blade under the posterior end of the right valve of the shell, and pushing the blade forward 

 until it strikes and cuts the strong adductor muscle, which passes from one shell to another and 

 pulls them together. As soon as this muscle is cut the valves separate a little, and the right 

 valve may be raised up and broken off from the left, thus exposing the right side of the body. 

 The surface of the body is covered by the mantle, a thin membrane which is attached to the body 

 over a great part of its surface, but hangs free like a curtain around nearly the whole circum- 

 ference. By raising its edge, or gently tearing the whole right half away from the body, the gills 



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