OYSTERS, CLAMS, MUSSELS 217 



short distance from the edge. The crease on the shell indicat- 

 ing this line of attachment is called the pallial line. Near the 

 anterior end of the inner surface of the valve is a rather large 

 distinct impression. This is the point of attachment of the 

 anterior adductor muscle. Just behind and above this is the 

 smaller impression of the anterior retractor muscle, and behind 

 and below it is the impression of the protractor muscle. At 

 the other end of the valve is the large impression of the poste- 

 rior adductor muscle and the small impression of the posterior 

 retractor muscle. The anterior and posterior adductor muscles 

 extend from one valve to the other and when they contract 

 the edges of the two sides of the shell are held close together. 

 When these muscles are relaxed the valves are opened slightly 

 by the strong hinge ligament which is stretched tightly when 

 the valves are closed. The retractor and protractor muscles 

 govern the movement of the foot. 



Where the mantle covers the body it is a thin delicate mem- 

 brane, but the free part, below the pallial line, is somewhat 

 heavier, and the edges are thicker. At the posterior end the 

 thickened fringed portions of the two mantle lobes form two 

 short tubes, the inhalant and exhalant siphons. The cilia on 

 the fringes of the mantle cause a current of water to flow in 

 through the lower or inhalant siphon into the mantle cavity, 

 the space inclosed by the mantle lobes. Here the water bathes 

 the inner surface of the mantle and the gills, and passes 

 through the gills to a space just above them known as the 

 supra-branchial cavity. In this space it passes backward again 

 and out through the upper or exhalant siphon. The mantle 

 is an important organ of respiration, for as in the gills of fishes, 

 oxygen is taken from the water and carbon dioxide passed out 

 through its thin delicate walls. The shell is the product of the 

 secretions of the mantle. Over its whole surface the mantle is 

 constantly secreting a thin layer of carbonate of lime which 

 serves to thicken the older parts of the shell and to extend and 

 harden the thin soft margins. 



When the mantle is removed from one side of the body the 

 large muscular foot, the thin delicate leaf-like gills and the soft 

 visceral mass of the body are exposed. The foot is large and 



