1 1 6 Descriptive Zoology. 



clear material having the consistency of a gumdrop. This 

 is the " crystalline rod," and is thought by some to be a 

 store of food material. Others regard it as a secretion to 

 protect the lining of the intestine from injury. 



The Circulatory System of the Clam. From the gills 

 and mantle of each side the blood passes up into the cor- 

 responding auricle (Figs. 67 and 69). The auricles are 

 wide at the base, where they arise from the upper margins 

 of the gills, but narrow as they approach the ventricle, so 

 that the lateral view gives a triangular appearance. The 

 auricles are thin-walled, delicate structures. They open 

 into the sides of the median ventricle. From the ventricle 

 arise two arteries, one carrying blood forward above the 

 intestine, the other extending backward beneath the intes- 

 tine (Fig. 67). After leaving the arteries, the blood 

 passes into irregular and ill-defined channels, supplying all 

 parts except the shell. The blood collects in a caval vein 

 under the floor of the pericardium, then passes through 

 the kidneys, and to the gills once more. In the gills and 

 mantle the blood loses carbon dioxid and gains oxygen. 

 As it passes through the kidneys it loses nitrogenous waste 

 matter, and from the digestive tube it absorbs new food 

 material for the support of the life processes. 



The Kidneys. The kidneys are ill-defined, dark-colored 

 organs, lying just beneath the floor of the pericardium and 

 in front of the posterior adductor muscle. Each kidney 

 consists of a tube doubled on itself, the bend being near 

 the adductor muscle. One end of the tube communicates 

 with the bottom of the anterior part of the pericardium, 

 the other end opens on the side of the abdomen, near the 

 upper edge of the inner wall of the inner gill, and above 

 the tip of the corresponding palp. Here the excretion 

 is poured out, and is carried away by the water current. 



