FISH AND FISHERIES. 113 
The oyster has, properly speaking, no head. or tail. 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 lamine, 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 
lamine 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 white 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 currents 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 oyster 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 massesof granules which can scarcely be distinguished. 
If a female, the ovaare 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. 
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