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A Typical Bivalve Mollusk — Part 2

Microscopic, transparent cilia, like the pile of velvet, stand erect on every part of the gill that comes in contact with the water. These hairs have the property of rhythmic wave motion, stroking strongly inward at the mouth of the incurrent siphon, and urging the water steadily through the complex channels of the gill substance. The stream, polluted by waste, makes its exit through the other siphon.

All the food the clam gets accumulates in the groove between the gill plates, and, too coarse to get through the pores into the network of the gills, it is worked along by the cilia to the palpi, and enters the mouth. Small infusorians, diatoms, the eggs and young of many sea creatures, decaying organic matter-all is grist for this mill. The clam does not choose its diet, but takes what comes its way. No need for teeth nor jaw. The digestive fluids prepare the food for use.

The heart lies in a loose bag, the pericardium, just anterior to the posterior adductor muscle. The single, muscular ventricle receives the pure blood from the lateral auricles, to which it flows from the gills. The intestine passes directly through the ventricle. The blood is distributed through closed tubes, arteries, which branch in the body tissues, and it is gathered into veins for return to the gills. The kidneys are complex organs that remove nitrogenous wastes. The mantle surface is richly supplied with capillaries. It supplements the work of the gills. Cold, colourless blood is the rule among mollusks.

In bivalve mollusks the sexes are usually separate. The reproductive glands lie near the kidneys, and discharge their products when mature into the posterior part of the mantle chamber, whence they pass out through the excurrent siphon. Fertilisation occurs in the water. In females the gills become brood chambers, distended with eggs in process of incubation. The young clams are free-swimming at first, but soon settle down, and take on the sedentary habits of the parent. Subsequent travel is accomplished by burrowing mud or sand with the muscular foot.

The nervous system of the clam consists of widely separated ganglia, (little brains) connected by commissures, and sending out nerve ends to the surrounding tissues. There are two white, rounded ganglia the size of pin heads, on guard at the sides of the oesophagus. They are joined by a short cord, and send out two pairs of long cords. One pair pass to the pedal ganglion in the fleshy muscle of the food. The other pair go to the visceral ganglion, in the posterior dorsal region. All the organs of the body receive nerves from ganglia or connecting commissures. The bivalve ear, when present, is in the foot, at the end of a nerve branch of the pedal ganglion.

Feeling is the most important sense to the bivalve mollusk. It is not centralised, but the mantle border, and the palpi, especially, are sensitive to touch. Eyes are wanting, except in rare instances. Taste and smell are probably not differentiated from the generalised sense of feeling.