THE CLAM AND OTHER BIVALVES 193 



processes of digestion and also aids in keeping the food well 

 mixed in the stomach. 



The intestine coils and twists in many planes from the 

 posterior end of the stomach to the point where it penetrates 

 the heart. The penetration of the heart by the intestine is 

 of common occurrence in the class to which Mya belongs, 

 but it occurs in no other class of animals. The part of the 

 alimentary canal from the heart to the anus is called the 

 rectum. The rectum is inclosed in a large, spindle-shaped 

 organ of unassigned name and unknown function. 



The Circulatory, Respiratory, and Excretory Systems. When 

 the food is absorbed by the wall of the intestine it passes 

 into small blood spaces filled with colorless blood. The 

 blood with the contained food then passes into the open ends 

 of small blood vessels. These blood vessels lead (in cer- 

 tain near relatives of Mya which have been studied more 

 fully) to a large blood space below the pericardium, the sac 

 which incloses the heart. From the blood space (not shown 

 in the figure) blood passes by vessels to the nephridia (kid- 

 neys). The nephridia in Mya arenaria lie one on either side 

 near the heart. The anterior end of the left nephridium is 

 indicated in Fig. 101. The rest of the organ could not be 

 shown in the drawing. 



The nephridia are spongy, brownish organs of great com- 

 plexity. Each nephridium of the clam opens at one end 

 into the pericardium. The other end of each nephridium 

 opens into the mantle cavity just posterior to the digestive 

 gland. 



The blood vessels in the nephridia divide into capillaries. 

 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 convey the partially purified blood 

 into a vessel that runs along the line of attachment of the 

 gills. 



