276 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. 



ANATOMY. 



The following popular description of the anatomy of the oyster is 

 extracted from the writings of Professors Brooks and Ryder: 



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 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. (Plate i, lig. 11.) 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 (plate i, fig. 1, mt), which folds 

 down on each side, and may be compared to the leaf next the cover on each side of 

 the book. The nest two leaves of each side roughly represent the four gills, g, the 

 so-called "beard" of the oyster, which hang down like leaves into the 8j)ace inside 

 the two lobes of the mantle. The remaining leaves may be compared to the body or 

 vi8cti7-al 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 (plate viii, fig. 2), 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 the 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, M, 

 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 oft' 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 

 circumference. By raising its edge, or gently tearing the whole right half away 

 from the body, the gills, g, will be exposed. These are four parallel plates which 

 occupy the ventral half of the mantle cavity and extend from the posterior nearly 

 to the anterior end of the body. Their ventral edges are free, but their dorsal edges 

 are united to each other, to the mantle, and to the body. The space above, or dorsal 

 to the posterior ends of the gills, is occupied by the oval, firm adductor muscle, M, 

 the so-called "heart." For some time I was at a loss to know how the muscle 

 came to be called the "heart," but a friend told me that he had always supposed 

 that this was the heart, since the oyster dies when it is injured. The sujiposed 

 "death" is simply the opening of the shell, when the animal loses the power to 

 keep it shut. Between this muscle and the hinge the space above the gills is occupied 



