100 THE FKESHWATEK MUSSEL 



3. The alimentary canal. The intestine is cut across twice 



in the visceral mass ; and the rectum with the 

 typhlosole is cut across as it lies in the pericardial 

 cavity. 



4. The gills. The attachments of the gills are the same 



as in the previous section, except that the inner 

 lamella of the inner gill ends in a free edge above. 



At the base of each gill, between its lamellae, is a 

 wide supra-branchial canal. The inner canal of each 

 side communicates with the branchial cavity through 

 the slit between the unattached inner lamella of the 

 gill and the base of the visceral mass. 



5. The circulatory system. 



a. The pericardial cavity is very much larger and has 



thinner walls than in the previous section, and is 

 triangular in shape. 



b. The ventricle surrounds the rectum : its outer wall 



is thick and muscular, its inner wall thin. 



c. The auricles are a pair of very thin-walled sacs lying 



at the sides of the pericardial cavity. Each is 

 attached along its outer side to the wall of the 

 cavity, opposite the septum formed by the junction 

 of the inner lamella of the outer gill with the 

 outer lamella of the inner gill. Along their inner 

 borders the auricles are attached to the ventricle 

 and open into it by slit-like valved apertures. 



d. The vena cava is a median thin-walled tube lying 



in the floor of the pericardium. 



6. The excretory organs. 



a. The kidneys are two wide tubes with thick spongy 

 walls, whose epithelium is glandular, pigmented, 

 and folded. They lie below the pericardium, above 

 the visceral mass, and opposite the bases of the 

 inner gills. Their inner walls are in contact with 

 each other below ; and are separated dorsally by 

 the vena cava, from which large sinuses enter 

 them. 



