98 THE FRESHWATER MUSSEL 



At the base of each gill is the supra-branchial 

 canal, along which the respiratory stream of water 

 flows backwards to the cloacal cavity. 



5. The circulatory system. 



a. The pericardial cavity is nearly circular in outline ; 



its roof and sides are thick and spongy, forming 

 what is spoken of as Keber's organ. 



b. The anterior aorta Hes on the dorsal surface of 



the rectum, and its cavity is slit-like in section. 



c. The vena cava lies in the median plane between the 



excretory organs and the visceral mass. 



6. The excretory organs are paired, and lie between the 



pericardium and the visceral mass. 



a. The glandular portions, or kidneys proper, are cut 



across close to their anterior ends. They are 

 seen as a pair of small tubes, with glandular 

 longitudinally folded walls, lying side by side just 

 above the visceral mass and opposite the bases of 

 the inner gills'. 



If the section has passed a little in front of 

 the excretory apertures, the passages from the 

 pericardial cavity to the anterior ends of the 

 kidneys are weU seen. 



b. The ureters are cut through at the part where they 



communicate with each other across the median 

 plane : they appear as a wide cavity immediately 

 below the pericardium and above the kidneys. 

 The external openings are to the outer sides of 

 the kidneys, between the two lamellae of each 

 umer gUl. 



7. The reproductive apertures are immediately ventral to 



the excretory apertures ; they may be followed with 

 a seeker into ducts leading from the genital gland. 



B, Transverse Section through the Middle of the Ventricle. 

 1. The mantle-lobes have the same appearance as in the 

 previous section. ■ 



