MOLLUSCA. 



2I 9 



from here it is brought by a series of blood spaces to the kidneys 

 where the urea is excreted; it next goes to the gills where it loses 

 carbon dioxide and gains oxygen. From the gills it goes directly 

 to the auricles. In the clam some of the blood goes directly to the 

 mantle and returns as directly to the heart. The accompanying 

 diagram (Fig. 68) will assist the student in following the course of 

 the blood. The blood cells are colorless. If there is color in the 

 blood it is in the plasma. 



m 



g - 



FIG. 68. Diagram showing the general course of the blood in a Lamellibranch, 

 Only a short section is shown, a, auricle (right), with a slit into the ventricle; b, 

 the body (region of spaces, tissues, capillaries) ; g, the region of the gills, with capillaries; 

 k, kidneys, with capillaries; m, mantle; v, the ventricle from which arteries pass forward 

 and backward; v.c., "vena cava," in which the blood collects on returning from the 

 tissues of the body. 



Questions on the Figure. Follow the general course of the blood 

 by means of the arrows and letters. How many sets of capillaries 

 are passed by the blood that goes to the mantle before returning 

 to the heart? How many by that which goes to the system ? What 

 change takes place in the capillaries of the various regions? Why 

 does that which goes to the mantle not need to pass through the 

 gills? 



