6 
disturbed upon the bottom, or when its muscle is cut, or when- 
the animal is dying or dead, the edges of the shell are sepa- 
rated a little. 
The shell is lined by a thin membrane, the mantle, which 
folds down on each side, and may be compared to the leaf next 
the cover on each side of the book. The next two leaves of' 
each side roughly represent the four gills, the so-called “ beard ” 
of the oyster, which hang down like leaves into the space 
inside the two lobes of the mantle. The remaining leaves 
may be compared to the body or visceral mass of the oyster. 
xllthough the oyster lies upon the bottom with one shell 
above and one below, the shells are not upon the top and bot- 
tom of the body, but upon the right and the left sides. The 
two shells are symmetrical in the young oyster, but after it 
becomes attached the lower or attached side grows faster than 
the other, and becomes deep and spoon-shaped, while the free 
valve remains nearly flat. In nearly every case, the lower or 
deep valve is the left. As the hinge marks the anterior end 
of the body, an oyster which is held on edge with the hinge^ 
away from the observer and the flat valve on the right side,., 
will be placed with its dorsal surface uppermost, its ventral 
surface below, its anterior end away from the observer, and its- 
posterior end towards him, and its right and left sides on his 
right and left hands respectively. 
In order to examine the soft parts, the oyster should be 
opened by gently working a thin flat knife blade under the 
posterior end of the right valve of the shell, and pushing the 
blade forwards until it strikes and cuts the strong adductor 
muscle, which passes from one shell to another and pulls them 
together. As soon as this muscle is cut the valves separate a~ 
little, and the right valve may be raised up and broken off from 
the left, thus exposing the right side of the body. The sur- 
face of the body is covered by the mantle, a thin membrane 
which is attached to the body over a great part of its surface, 
but hangs free like a curtain around nearly the whole circum- 
ference. By raising its edge, or gently tearing the whole 
right half away from the body, the gills will be exposed. 
