SEX OP THE AMERICAN AND EUROPEAN OYSTERS. 719 



through six great branchiocardiac vessels, three of which are arranged on each side; two pairs of 

 these are anterior in position and one pair posterior. 



The circulation of the Oyster is quite different in character from that observed ia a vertebrated 

 animal. In the latter the heart pnrups the purified blood to and through the gills before it passes 

 to all parts of the body; in the Oyster, on the other hand, the fresh, pure blood is pumped by the 

 heart from the gills before it passes to all parts of the body. 



A curious and interesting point which I think it desirable to mention, because I have not 

 noticed that attention has hitherto been especially called to it, is the metamorphosis of the larval 

 Oyster into the adult. A. de Quatrefages 1 has alluded to it, but not in explicit terms. I have 

 shown in my sketch on the growth of the animal that the larval shell was quite different from that 

 of the adult, in fact, more like a very diminutive pisidium than anything else. The metamorphosis 

 of the larval shell, or rather its passage into that of the spat, is abrupt. Not so with the soft parts ; 

 the oldest larva? yet studied by any competent biologist show that the mouth of the larva is 

 placed on the ventral side of the embryo, and that the hinge is situated on almost exactly the 

 dorsal or opposite side. The ventral position of the mouth of the larvae and its anterior or cephalic 

 position in the adult show that a very important series of changes in the position of the viscera 

 must take place between the time when the larva loses its principal embryonic features and 

 acquires the adult arrangement and relations of its hard and soft parts. In other words, we are 

 made aware, after instituting the foregoing comparison, that the Oyster actually undergoes a 

 metamorphosis. 



If an Oyster be carefully opened it will be found that the animal adheres to the shell at four 

 points, or at two points on either valve. The principal points of attachment are of course the 

 insertions of the great compound adductor muscle, made up of two portions which may be 

 distinguished by the color of the cut ends of the component fibers. The great shield-shaped 

 purple areas on either valve mark the points of insertion of the great adductor in the American 

 Oyster, and also in the Portuguese form, which resembles it considerably. In Ostrea edulis, or the 

 European species, the insertion of the adductor muscle is very rarely colored, so rarely indeed that 

 we may regard this feature as one of the specific marks of this form. But in both the American 

 and the European species there is a second muscular attachment, as implied above, which appears 

 to have been very generally overlooked. It is situated nearer to the hinge than to the great 

 adductor, and is sometimes marked by a slight depression not over an eighth of an inch in its 

 greatest transverse diameter. It gives attachment to a feeble muscular bundle which springs out 

 of the mantle on either side of the visceral mass, and when the animal is torn loose a slight 

 whitish scar on the soft part marks its position on the surface of the mantle. I have been 

 informed that Mr. W. H. Dall, who has investigated the matter, has identified this muscle with 

 the pedal muscle of some other acephalous mollusks. 



212. SEX, SEXUAL PRODUCTS, AND DIFFERENCE OF THE SEXUAL HABITS OF THE 



AMERICAN AND EUROPEAN OYSTERS. 



"The number of male cells which a single male will yield is great beyond all power of 

 expression, but the number of eggs which an average female will furnish may be estimated with 

 sufficient exactness. A single ripe egg measures about one five-hundredth of an inch in diameter, 

 or five hundred laid in a row, touching each other, would make one inch ; and a square inch would 

 contain five hundred such rows, or 500 x 500=250,000 eggs. Nearly all the eggs of a perfectly 



'Metamorphoses of Man and the Lower Animals. Translated by H. Lawson, M. D., pp. 104-109. London, 1864. 



