. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. M Vertical section of shell and mantle of a mollusk; C, periostracum; P, prismatic layer; L, laminated layer; S, shell; M, mantle. (After Claus.) up of material much like that which forms a cow's horn, or chitin; a middle layer composed of tiny prisms of lime held in by the horny material (this layer is called prismatic layer); and an inner layer (the lami- nated layer), made up of layers of lime and horn alternating parallel to the surface of the shell. The inner layer is formed by the act
. Elements of biology; a practical text-book correlating botany, zoology, and human physiology. Biology. M Vertical section of shell and mantle of a mollusk; C, periostracum; P, prismatic layer; L, laminated layer; S, shell; M, mantle. (After Claus.) up of material much like that which forms a cow's horn, or chitin; a middle layer composed of tiny prisms of lime held in by the horny material (this layer is called prismatic layer); and an inner layer (the lami- nated layer), made up of layers of lime and horn alternating parallel to the surface of the shell. The inner layer is formed by the action of the whole sur- face of the mantle. The two outer layers are made by the edge of the mantle only. So a shell grows in thickness largely from the inner surface of the mantle, while it grows in diam- eter from the edge of the mantle only. The Open Shell. —'PuU the shells completely open. Find on the dorsal side projections and grooves which fit into each other when the shell is closed. These are the hinge teeth. Compare the number in each shell. How might they be of use to the animal? Find the marks on the shell where the adductor muscles were fastened. What was the use of the adductor muscles ? Why do dead mussels always have the shell partly open ? Draw one opened valve showing all above parts. Label the anterior and posterior adductor muscle scars, according to position. Body and Mantle Cavity. — In one valve lies the body of the clam. If we remove the mantle, we shall find under a roundish soft mass, the body, or visceral mass. Surrounding the visceral mass but ventral to it is a cav- ity bounded on the outside by the inner surface of the mantle. This is the mantle cavity. In life this cavity is full of water. See if you can discover how and where water gets in. In a living mussel the posterior edge of the mantle on the right side is folded so as to fit with the adjoining edge of the mantle on the left side. The funnel-like openings thus formed are called siphons. S
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