BIVALVE MOLLUSCA. 373 
The common oyster, Ostrea edulis, is found in many seas. It is 
unequally valved, modified in shape by the form of the submarine 
body to which it happens to be attached. The lower or adherent 
valve is concave, always the largest ; the upper one is thin, usually 
flat; the shell is lamellar, rough externally, and seems to be com- 
posed of broken layers, adhering slightly to each other, as if the 
successive layers had been built up from within, and each succeeding 
one was an enlargement upon its predecessor. The hinge which 
unites the valves is an elastic toothless ligament, placed behind the 
centre, which opens the valves. 
The interior surface of the valves is smooth and white, diapha- 
nous or pearly towards the centre, but near the back an oval or 
rounded impression may be observed, to which a thick and whitish 
fleshy muscle is attached. This is the central muscle which draws 
the valves together, hermetically closing them upon the animal. This 
muscle is cut through in the process of opening the oyster. 
The animal has no power of locomotion ; its foot is very small, 
and often wanting, no siphon, but lies with its mouth open, and 
firmly attached to its shell. The shell itself is always adherent, as 
if soldered to the rock or other submarine body, the point of 
adherence being near the summit of the lower valve. 
Let us suppose the oyster opened by the cutting through of the 
ligament of the central muscle and of the adductor muscles. When 
displayed before our eyes, we see in the bottom of the shell a flattened 
shapeless animal, semi-transparent, greyish, and somewhat oval-shaped. 
The gastronomist, who seldom sees beyond his nose, thinks that, in 
spite of its culinary merits, the oyster belongs to the lowest rank of 
animal existence ; but he deceives himself, and does not know how 
complex and delicate is the organisation of the humble bivalve. The 
animal is enveloped in a sort of smooth, thin, contractile tissue called 
the mantle, which folds round it, presenting two lobes, separated on 
the greatest part of its circumference, and forming a sort of hood, 
the summit of which abuts upon the hinge of the bivalve. The edges 
of this mantle are fringed with very small cilia, which the creature can 
extend and draw back at pleasure, and which seem to be gifted with 
a certain amount of sensibility. It is this mantle which secretes and 
deposits the calcareous matter which forms the shell, each plate of 
which is an enlargement on. the preceding one, until it constitutes a 
pyramid of thin convex lamelle. : 
At the point where the lobes of the mantle meet, near the summit 
of the valve, is the mouth of the animal, with its thin membranous 
lips. This organ is large and dilatable, and is accompanied by four 
