FISH AND FISHERIES. 113 



The oyster has, properly speaking, no head or taih The portion in 

 which the mouth is situate is called the head. It is nearer to the hinge 

 than the outer edge of the shell, and therefore the hinge side is called 

 the anterior and the free edge of the shell is the posterior side of the 

 body. In the young state the two shells of the oyster are of equal size 

 and alike, but it soon becomes attached by one side, and then the adher- 

 ing side grows much faster and is much larger. This lower valve is 

 invariably the left one, and it is deep and concave, while the right valve is 

 usually flat. If an oyster be held edgewise, so that the right valve is 

 on the right, and the hinge away from the observer, the dorsal surface 

 will be uppermost, and the ventral below. The hinge is provided with 

 an elastic ligament which would always keep the shells a little open 

 were it not for the strong adductor muscle in the middle of the inside 

 of each shell. This has to be cut through before an oyster can be 

 opened. Both valves are lined with a fine transparent membrane called 

 the mantle. Inside this there is a fringe with a narrow dark margin ; 

 this fringe will be found to consist of four laminae, which occupy the 

 ventral half of the valves, and extend from the open edges nearly 

 round to the hinge. Remembering the definition of what is the dorsal 

 and what the ventral edge of the body, it will be found that the gill 

 laminfe are free on the ventral side — that is distinct from each other, 

 but they are joined together and united with the body and mantle on 

 the dorsal side. Round the adductor muscle the great mass of the sub- 

 stance is taken up by the ova or milt. Just by the side of the muscle is 

 a small transparent vesicle like a blister; it will be seen moving from time 

 to time in a freshly opened oyster, but very slowly and at long intervals. 

 This vesicle contains the heart and the movement is its pulsation. The 

 blood of oysters is colourless. The transparent veins and arteries can 

 be easily seen on a close inspection of the tissue. The heart consists of 

 a small compact A\hite ventricle and a loose spongy transparent auricle. 

 The auricle receives the blood from the gills, and the ventricle pumps it 

 to the various organs. The whole process can be easily seen with a very 

 ordinary hand-lens. In front of the gills- — that is between them and 

 the hinge — there are four flaps of flesh, two on each side of the body. 

 These are around the mouth, which is near the hinge, and away from 

 the open end of the shell. The oyster feeds on extremely minute 

 organisms, as the mouth is supplied by the cu.rrents caused by cilia. 

 These are organs only visible with high microscopic powers, and con- 

 stantly moving. They are like little hairs. The intestine is coiled up 

 amidst the ovary, and there is a long slit close to the middle of the 

 posterior face of the adductor muscle, which is the anus. 



As already observed, the sexes are divided in the oyster, but there is no 

 external mark by which a male oj^ster can be distinguished from a female. 

 The only test is the microscope. If a portion of the white stuff" called the 

 fat be taken out and mingled with a drop of sea water, and then covered 

 by thin glass and examined with a good inch objective, it is easy to say at 

 once whether the oyster is a male or female. If a male, the white fluid is 

 clouded with minute masses of granules which can scarcely be distinguished. 

 If a female, the ova are quite distinct as regular granulations. It requires 

 a very high magnifying power to make anything out of the male fluid, 

 but with a good quarter of an inch objective it is seen to be filled with 

 small organisms like tadpoles, with tails which are incessantly moving. 



