The following description of the reproductive organs of the oyster cannot well be understood without some acquaintance with the general anatomy of the animal. With the help of Pl. XXII, fig. 1, the most important features of its anatomy may readily be understood. It must be remembered that the valves of the oyster's shell lie right and left of the animal; that the concave valve which lies undermost in the natural position of the animal is the left valve, and the flat upper valve is the right valve. The hinge marks the dorsal border of the animal, and the opposite border is the ventral border. That side which is on the observer's right in fig. 1 is the anterior surface, and that on the left hand is the posterior surface. As the animal is compressed from side to side the anterior and posterior surfaces are very narrow. The great adductor muscle, by which the valves of the shell are closed, is seen lying in the centre of the animal in fig. 1.