BIVALVE MOLLUSCA. 373 



The common oyster, Ostrea edu/is, 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 slighdy 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 lamellae. 



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 



