162 GENERAL ZOOLOGY 



to the nephridia of simpler animals. Each nephridium of the 

 clam opens by one end into the pericardium. The other end 

 of each nephridium opens into the mantle-cavity just posterior 

 to the digestive gland. 



As the blood-vessels in the nephridia divide into capilla- 

 ries, the nitrogenous waste of the body (uric acid) passes into 

 the nephridial tube and is carried out into the mantle-cavity. 

 The small blood-vessels, reuniting into large vessels, convey 

 the partially purified blood into a vessel that runs along the 

 line of attachment of -the gills. 



There are four of these gills and they hang like double 

 curtains along the right and left sides of the body (Fig. 82, 20). 

 The oxygen coming into the mantle-cavity with the water, 

 through the incurrent tube of the siphon, passes over the 

 floating gills. They are thin and soft, arid are thus adapted 

 for the ready passage of oxygen to the blood-vessels inside. 



In the blood the oxygen combines with Immocyanin, a 

 substance analogous to hemoglobin (see p. 203). At the same 

 time the waste carbon dioxide in the blood is given off to 

 the water in the mantle-cavity. The mantle-folds, as well 

 as the gills, take part in respiration. It is possible for them 

 to do so because of their rich supply of superficial blood- 

 vessels. 



Returning from the gills and the mantle, the blood freed 

 of carbon dioxide is carried to the right (Fig. 82, 21) and left 

 auricles of the heart. These thin-walled, sac-like reservoirs 

 force the blood into the ventricle (Fig. 82, 22) of the heart 

 through valve-guarded openings on either side (Fig. 82, 23). 

 The heart contracts and forces the blood both forward and 

 backward through arteries. The anterior artery lies above 

 the intestine (Fig. 82, 24), and the posterior artery lies below 

 the rectum (Fig. 82, 25). Both arteries branch into smaller 

 arteries in all parts of the body. The blood flows from the 

 open ends of the smallest arteries into blood-spaces, from 



