210 THE FISHERIES OF THE UNITED STATES. 



named Ostrea paiercnla. Ascenrling to the Jurassic, oysters are foiiiid to have been somewhat plentiful, and, in 

 the Cretaceous, the family reached its oulraiuation. Never befoi'e nor since have these mollusks been more 

 abundant in point of species or numbers of individuals, or more widely differentiated in their characteristics. 

 They are of large size, also. In subsequent ages the Ostreadw were abundant, but the kinds were few, many 

 genera, for example Gryphwa and Exogyra, disappearing altogether with the close of the Mesozoic era. The fossil 

 remains of these old oysters are found everywhere throughout the world where the ancient oceans had their 

 margins, and in the United States coextensively with the range of brackish-water formations, from the Cretaceous 

 upward. 



Anatomy of the oyster. — The brief sketch of the anatomy of the oyster which follows, was written by my 

 friend Dr. W. K. Brooks, of the Johns Hopkins University, of Baltimore. It prefaced his account of his successful 

 embryological studies upon the oysters of Chesapeake bay, and is the best and most recent description of this 

 mollusk with which I am acquainted. Therefore I prefer quoting it to writing an imperfect du])licate of the tacts. 

 As Dr. Brooks says, it is hardly possible to write such a description without using a few technical words, such as 

 "anterior", "posterior", "dorsal", and " ventral ", but these can all be found in any dictionary, and will present 

 no difliculty to any ordinary reader, however unaccustomed to scientific terms. "As the end of the body where 

 the mouth is placed is not marked by a head, it must be spoken of as the anterior end, not as the 'head', and the 

 opposite end as the posterior. As the oyster lies on one side, the top and bottom of its body do not corresjwnd to 

 the regions which occupy these positions in an upright mussel or clam, and it is most convenient to speak of that 

 part of the oyster's body which answers to the upper surface of a clam as dorsal, and the opposite as ventral.''^ 



Dv. Brooks' anatomical outline sketch * is as follows : 



The gcueral structure of au oyster miiy be roughly represented liy a loug narrow meruoraudum l)Ook, with the liaek 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 iu 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 plac^e between its leaves, close to the back, a small iiiece 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 tlw cover 

 ou 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 dowu like leaves into the space inside the two lobes of the numtle. The remaining leaves may be compared to the body or risvcml 

 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 

 l)ody, but upon the right and the 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 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 ou the right side, will be placed with its dorsal surface uppermost, its ventral surface 

 below, its auterior end away from the observer, and its posterior end toward him, and its right and left sides on his right and left hands, 

 respectively. 



Iu order to examine the soft parts, the oyster should be opened by gently working a thin, flat kaife-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 seiiarate 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 circumference. By 

 raising its edge, or gently tearing the whole right half away'from the body, the gills will be exposed. These are four parallel )dates 

 which occupy the ventral half of the mantle cavity aiul 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, the so-called " heart". For some time I was at a loss to know how the 

 muscle came to he 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 supposed " 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 by the body, or visceral mass, which is made up mainly of the light-colored 

 reproductive organs and the dark-colored digestive organs, packed together iu one continuous mass. 



If the oyster has been opened very carefully, a transparent, cresceut-shaped space will bo seen between the muscle aud the visceral 

 mass. This space is the pericardium, and if the delicate membrane which forms its sides be carefully cut away, the heart may be found 

 withcmt any difficulty, lying in this cavity, and pulsating slowly. If the oyster has been opened roughly, or if it has been out of water 

 for some time, the rate of beating may be as low as oue a minute, or even less, so the heart must bo watched attentively for some time in 

 order to see one of the contractions. 



The heart is made up of two chambers, a loose, spongy, transparent atirirle, which occupies the lower part of the pericardium, and 

 receives blood from the gills through transjjari^nt blood-vessels, which may usually be seen without difficulty, running from the gills 

 toward the heart, aud a more compact white cenlride, which drives the blood out of the pericardium through transparent arteries, which 

 are usually quite conspicuous. 



The visceral mass is prolonged backward over the pericardium and the adductor muscles, and here contains the rectum, surrounded 

 by prolongations of the white reproductive organs. Still farther back, on the middle of the posterior face of the adductor muscle, is the 

 anus, a long, vertical slit, opening into the space between the lobes of the mantle and above the posterior ends of the gills. 



* lii-poH of ike Commissioners of Fisheries of Manjlmul, January, 18ti0 ; Annapolis, W. T. luglehart & Co., State Printers, 1880, pp. 5-10. 



