CHAPTER I: 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 (pi. 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 

 beaks pointing toward you. A heart-shaped imprint, the lunula, 

 is seen 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. 



The Soft Parts. — By severing the adductor muscles with 

 a knife blade close to the left valve, and then breaking the liga- 

 ment, the soft parts of a live clam are revealed, lying undisturbed 



305 



