48 THE ECONOMIC.; MOLLUSCA OF ACADIA. 



minute mouth, guarded by four large membranous flaps, minutely 

 striated on tbeir inner faces. From among tiiem, there run backwards 

 four other larger folds, transversely striated and with crenulate edges, 

 -which follow the most curved or ventral side to the posterior margin, 

 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<5S which contains the liver (a 

 very dark brown organ), the intestine and the generative organs. At 

 the breeding season the latter will be found ver}^ large, and filled with 

 minute eggs or with a milky fiuid (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 individual in the Euroi^ean species,* 

 and there are about as many of one se.x 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 the}' grow 

 to a great size. Specimens a foot or more in length, and about four 

 inches in breadth being found sometimes in deep watei". 



The breeding habits of the American Oyster have been very carefully 

 investigated by Professors W. K. Brooks ;>nd John A. Ryder, and their 



*Yet it is never self-fertilized, for the eggs aucl the sperniatazoa come to 

 imturity in each individual at different times. 



