THE CLAM AND OTHER BIVALVES 161 



of a certain fresh-water mussel (An'odon) is the condensed 

 secretion of the digestive gland ; its function is to convert 

 starch into sugar. After a period of feeding and subsequent 

 digestion the style disappears and is later formed anew. 



The intestine (Fig. 82, 13) 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 (Fig. 82, 16) 

 is called the rectum (Fig. 82, 15). The rectum is inclosed 

 in a large, spindle-shaped organ (Fig. 82, 17) 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 certain near relatives of Mya which have been studied 

 more fully) to a large blood-space below the pericardium 

 (Fig. 82, 18), the sac which incloses the heart. From the 

 blood-space (not shown in the figure) blood passes by vessels 

 to the nephridia (kidneys). The nephridia in Mya arenaria 

 lie one on either side near the heart. The anterior end of 

 the left nephridium is indicated at Fig. 82, 19. The rest of 

 the organ could not be shown in the drawing. 



The nephridia are spongy, brownish organs of great com- 

 plexity. They are sometimes referred to under the name 

 "organs ofBojanus." The old-time anatomist, Bojanus, dis- 

 covered the structures,, but he made a most natural mistake 

 in thinking that they were the lungs of the animal, since they 

 look like lungs and lie very near the heart. Other anatomists 

 gave the name " organs of Bojanus," and later investigators 

 found that the organs were alike in structure and in function 



