A T}rpical Bivalve MoUusk 



oxygen and carbonic acid gas goes on over an area greater many- 

 fold than the visible surface of the gills. 



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 ven- 

 tricle. 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, colour- 

 less 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 cham- 

 ber, whence they pass out through the excurrent siphon. Fer- 

 tiUsation 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 in 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, 



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