A Typical Bivalve Mollusk — Part 1

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, composed 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 beaks pointing toward you. A heart-shaped imprint, the lunule, 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 ligament, the soft parts of a live clam are revealed, lying undisturbed in the right valve. The mantle spreads over everything. It lines each valve, and is attached to each along the pallial line. Outside this it hangs free as a thickened frill. At the posterior end of the body the two mantle edges unite to form two tubes. Through one, the incurrent siphon, water passes into the mantle chamber. Through the other, the excurrent siphon, water is discharged. When withdrawn these tubes occupy the pallial sinus. The siphons are always behind.

By throwing the mantle back over the dorsal margin of the valve, the left gills, two layers of brown plate-like membrane, of basket-work texture, appear. They are attached along their dorsal margins to each other, to the body underneath, and to the mantle above them. The right gills are like them. The central space between the two valves is occupied by the body of the clam, the visceral mass. Its ventral part is the oblique, muscular foot. Above the foot the kidney, liver, stomach and heart are embedded in the fleshy mass. At the anterior end two flaps, the labial palpi, conceal the mouth.

The bilateral symmetry of this mollusk is evident. The central body is flanked on each side by a pair of gills, a mantle and a hard valve. Right and left, the halves of the body are alike. In this particular the univalve exhibits a marked difference. Clams have no heads, no tentacles, no eyes.

The alimentary canal begins with the mouth, situated between the flapping palpi. There is no jaw nor radula. The muscular oesophagus widens into a pouched stomach, from which the intestine extends downward into the foot, and after several convolutions passes backward, through the heart, ending in the excurrent siphon. The liver surrounds a portion of the stomach.

The inner wall of the intestine is folded to increase its secretory surface. In the stomach and intestine is found a gelatinous, transparent substance called the crystalline stylet, which often fills the space completely. Nobody knows the origin or the use of this strange body. It may be a reserve store of food, partially digested. It dissolves gradually when food is withheld, and reappears when feeding is resumed. It may protect the stomach walls from injury by sharp food particles. This is but one of many unsolved problems in molluscan anatomy. The digested food is absorbed by the veins in the intestinal walls.

Breathing and feeding are closely allied processes, for the inflow of water supplies the gills with oxygen and the stomach with food. The gills are four in number, and each consists of a double fold of delicate membrane. This membrane is composed of parallel gill filaments, united by connective tissue, bent back upon itself and stayed with cross bands between the two walls of the single gill. An intricate system of fine vessels distributes the blood through the gill substance, and an equally efficient system of tubes and pores admits water, so that the interchange of oxygen and carbonic acid gas goes on over an area greater many-fold than the visible surface of the gills.