48 
BULLETIN OF THE NATURAL HISTORY SOCIETY. 
minute mouth, guarded by four large membranous flaps, minutely 
striated on their inner faces. From among them, there run backwards 
four other larger folds, transversely striated and with crenulate edges, 
which follow the most curved or ventral side to the posterior margiu, 
and around it to end near the straighter or dorsal side. These are 
generally known to oystermen as the “ beard.” They are the gills, and 
serve both for respiration as they do in fishes, and also to help collect 
the food and guide it to the mouth. Just in front of the muscle, in a 
cavity of its own, lies the heart, which, in a freshly-opened, healthy 
Oyster, can be seen to beat very slowly. It has three chambers, two 
auricles and a ventricle, and receiving the aerated blood from the mantle 
and gills, forces it through the various organs of the body, then a part 
of it through the gills for aeration again, and so on. The nervous 
system is quite simple, consisting of two ganglia or nerve-knots near the 
mouth which control the internal organs, and two others near the great 
muscle which control the latter and the mantle. 
In front of the heart lies a large ma«s which contains the liver (a 
very dark brown organ), the intestine and the generative organs. At 
the breeding season the latter will be found very large, and filled with 
minute eggs or with a milky fluid (the spermatazoa), according as the 
animal is a female or a male. The sexes are distinct in the American 
Oyster, though united in the same undividual in the European species,* 
and there are about as many of one sex as of the other. The posterior 
end of the intestine is on the dorsal side of the great muscle. The water 
forced along by minute vibrating rods passes into the animal along the 
ventral margin, bathes the gills and palps and gives up to them its 
oxygen and food, and passes out by the dorsal margin taking with it the 
waste matters. The food of the Oyster consists of minute animals and 
plants, principally that group of the latter known as Diatoms, 
It is on account of the abundance of these Diatoms on muddy 
bottoms in brackish waters that Oysters flourish and fatten better in 
such situations than in any others. They never burrow but lie upon 
the surface, and if accidentally covered with mud must perish. They 
are found also upon rocky and even sand bottoms, but they grow less 
rapidly and have more enemies in such places. When lying undisturbed, 
they are firmly attached to some support by the most convex valve; this 
is nearly always the left. The upper valve usually is a little lifted by 
the elastic hinge, and allows of the constant circulation of the water 
through the animal as described above. If left to themselves they grow 
to a great size. Specimens a foot or more in length, and about four 
inches in breadth being found sometimes in deep water. 
The breeding habits of the American Oyster have been very carefully 
investigated by Professors W. K. Brooks and John A. R} T der, and their 
*Yet it is never self-fertilized, for the eggs and the spermatazoa come to 
maturity in each individual at different times. 
