Life of the Clams 3 3 



book and, by its physiological activities, secretes the hard valves of calcium 

 carbonate, which thus come to occupy the position of the covers of the 

 book. In the simplest form of mantle the edges are free except on the back, 

 where the hinge of the shell is located, corresponding to the arrangement of a 

 book. Sea water may enter the cavity enclosed by the mantle at almost any 

 place. In many groups of bivalves, however, the mantle edges may be fused, 

 not only along the back where the valves are joined together but along all 

 or most of the lower margins. Openings are usually present to accommodate 

 the foot and siphons when such organs are developed. 



Figure io. Structure and layers of a clam shell {Tellma tenuis), a, Diagrammatic 



representation of a small piece of shell; b, Cross-section of shell showing the loose 



end of periostracum around the margin of the shell. (After Trueman 1942.) 



From its food supply the clam absorbs minerals into its blood system 

 which are then carried to the mantle. A certain amount of shell deposition 

 takes place along the thickened borders of the mantle, although a small 

 amount, including pearly or nacreous material in some species, is laid down 

 by other parts of this organ. The liquid secretion of lime salts becomes 

 crystallized when mixed in a colloidal albumen which is also produced by 

 the mantle. Several types of shelly material are laid down in definite layers, 

 and the structure and composition may vary depending upon the family or 

 genus of mollusks. The structure of a layer may be prismatic (made up of 

 tiny, individual, closely packed prisms), foliated (layers built up of over- 

 lapping leaves), nacreous (mother-of-pearl), granular (like grains of sugar 

 stuck together), crossed laniellar (a common type in which the long lamellae 

 are rectangular), or it may be homogejieous with no visible structure. The 

 mineral character of these layers may be calcite (2.7 times as heavy as water) 

 or aragonite (2.9 times as heavy as water), both of which are forms of 

 calcium carbonate. 



The shell of the tellin clam (Tellina), for instance, is made up of three 

 layers of calcium carbonate and the horny periostracum. The latter con- 



