A TYPICAL BIVALVE MOLLUSK. The hard shell clam of the east coast of the United States exhibits the structural plan of bivalve mollusks.
The Shell.— It is an oval box, somewhat flattened, com posed of two symmetrical concave valves, joined by a ligament and locked hinge teeth at the back, and internally by two strong muscular cross bands, one at each end of the shell. The oval outline broadens at the anterior, or front end. The posterior, or hind end, is narrowed. The dorsal margins of the valves unite on the hinge line. The ventral margins are free and open. A swelling, the umbo, or beak, is seen on each valve well forward on the dorsal margin. Each umbo (pl. umbones) is more or less pointed. Lines of growth, concentric, and parallel with the outer, free shell margins, are seen on the outside. The surface has more or less of the horny epidermis overlying the lines of growth. The left valve is in your left hand when you hold the clam between your palms with the hinge line uppermost and the breaks pointing toward you. A heart-shaped imprint, the lunule,
is below the beaks.
Examining the inside of an empty valve, note the anterior and posterior adductor muscle scars, two prominent patches at the ends of the expanse of enamel lining. Dorsal to each is the scar of a small foot muscle. The curved pallial line, parallel with the ventral margin, joins the two large muscle scars. It dips in at the posterior end, forming the pallial sinus. The thick edge at the hinge shows teeth that lock together. The ligament, a narrow, blackish, rubbery band, is fastened externally to both valves, from the hinge backward. It springs the valves slightly apart at the ventral margin. In order to close the valves tightly together the clam must contract the adductor muscles, and stretch the dorsal ligament. The normal position of the valves is slightly open. Closing them puts a strain upon muscles and ligament.