APPENDIX II. 



A SKETCH OF THE LIFE-HISTORY OF THE OYSTER. 

 By John A. Ryder. 



The oyster always presents a definite right and left side; a dorsal or 

 upper, and a ventral or lower part of the body, and an anterior or head 

 end to which the hinder extremity is opposed. Thus it will be seen 

 that it resembles greatly many common animals, not only in the respects 

 already noted, but also in that the right and left halves of the soft parts 

 are, with the exception of the alimentary canal, repetitions of each other, 

 so that, as in man and the higher animals, there is apparent in the oys- 

 ter that likeness of opposite sides of the body which has been termed 

 bilateral symmetry. While this symmetry of the soft parts is so evi- 

 dent, it is less palpable when we compare together the two valves or 

 shells which inclose and protect the animal. In the natural beds the 

 left valve is usually undermost or inclined to be so, but in the crowded 

 banks the shells, as growth proceeds, tend to assume a vertical position. 

 The left valve is also more concave or hollowed out internally than the 

 right one, which is often very nearly flat. In the European oyster 

 iOntrea edulis) both valves are much flatter than in the American and 

 Portuguese (0. virgtnka and 0. angulata); in the former the muscular 

 impressions are also very nearly pyriform and colorless, while in the 

 two latter they are usually more nearly kidney-shaped and deep purple 

 in color. The average size of the American and Portuguese is also much 

 greater than that of the common European species, and both the former 

 grow much more rapidly than the latter. 



Fig. 1, Plate LXXIII, represents an American oyster which has had the 

 right valve and the most of the mantle of the right side removed, in order 

 to show the soft parts in position as they lie on the left one. The head end 

 of the animal lies close against the hinge A, or the point where the two 

 valves are firmly joined to each other by a dark-brown, crescent shaped, 

 elastic body, I, known as the ligament. This ligament, while it serves 

 to attach, also tends, because of its elastic properties, to separate the 

 valves fi'om each other at their broader, free extremities. In life, this 

 separation of the valves at their wider free borders admits of the ready 

 passage of water inwards to the gills g, and of food to the mouth »«, while 

 it also allows the water which has passed through the gills to escape by 

 way of the wide cloacal space cl, carrying with it in its current the faeces 

 from the vent or anus v. The tendency to separate the valves, inherent 

 in the ligament, is balanced by the adductor muscle M, which upon the 



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