712 NATURAL HISTORY OF AQUATIC ANIMALS. 



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 ihe oval, firm adductor muscle, 

 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 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 repro- 

 ductive organs and the dark-colored digestive organs, packed together in one continuous mass. 



" If the Oyster has been opened very carefully, a transparent, crescent-shaped space will be 

 seen between the muscle and 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 without 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 one a minute, 

 or even less, so the heart must be watched attentively for some time in order to see one of 

 the contractions." 1 



The dark-purple scars near the centers of both valves are simply the areas covered by the 

 attachments of the adductor, which is composed of a vast number of extremely fine muscular 

 fibers, which collectively pass straight across the space between the inside of the valves, being 

 firmly fixed at either end of the latter. The tendency to separate the valves at their free borders, 

 inherent in the 1 igament, is balanced or counteracted by the muscle. The head end of the animal 

 lies close against the hinge, the point where, as previously described, the two valves are firmly 

 fixed to each other by a dark-brown, crescent-shaped body, the ligament, which, while it serves to 

 attach, also tends, by reason of its elastic properties, to cause the valves to separate at their free 

 borders in order to allow the passage of the water inward to the gills, and of food to the mouth, 

 while it also allows the water which has passed through the gills to escape by way of the cavity 

 above the gills which is prolonged into the cloaca, carrying along with it, in its outward passage, 

 the faeces from the vent. The foregoing lines fairly describe the mechanism of the shell and in 

 part the physiological significance of the same. 



The structure of the shell is laminar, or, in other words, it is composed of numerous layers of 

 a material identical in composition with chalk, deposited one on the other by the mantle, the organ 

 which builds the whole shell in this way, the chalky substance being derived from the fluids of the 

 animal, which in turn derives it from its food. These layers, deposited as they are internally, in 

 a horny organic matrix, as growth proceeds project in succession pa st each other at the free edges 

 of the valves and external surface of the shell, so that the successive deposits may readily be 

 distinguished on its external surface, giving rise to a very rough imbricated appearance of the 

 edges of the layers on the outside. Attempts which I have made to determine the age of Oysters 

 from a supposed periodic deposition of the shelly material, corresponding to the years of its age, I 

 find to be impracticable. 



The structure in the layers of the shell of the chalk or calcic carbonate is minutely prismatic ( 

 Nathusius-Konig.sborii lias found that certain portions of the shell of the European Oyster contain 

 very minute air-spaces. Both native and foreign species are found to have hollow cavities in the 

 valves, usually containing water. 



1 W. K. BROOKS : Development of the American Oyster. Studies from the Biological Laboratory of Johns Hopkins 

 University, No. IV, 1880, pp. 5-7. 



