92 THE FRESHWATER MUSSEL 



is continuous with, and inseparable from, the floor of 

 the pericardium ; and its floor is continuous with the 

 roof of the kidney. On its inner side it is separated 

 from the ureter of the other side by the vena cava. 

 The two ureters communicate with each other 

 through a large oval opening near their anterior ends. 



3. The kidney appears as a black sausage -shaped body 



lying beneath the ureter. Posteriorly it enlarges 

 considerably, and is in close contact with the posterior 

 adductor muscle, which it partially embraces. 

 Lay open the kidney longitudinally with scissors. 



Its walls are thrown into thick spongy folds, 

 clothed with a black glandular epithelium. The 

 enlarged posterior end opens into the hinder end of 

 the ureter. 



4. The reno-pericardial openings are a pair of crescentic 



slits at the anterior end of the pericardial cavity, in 

 the angle between the rectum and the floor of the 

 pericardium ; they lead from the pericardial cavity 

 into the anterior ends of the right and left kidneys 

 respectively. 



Find the reno-pericardial opening on the right side, and 

 pass a seeker through it, and backwards along the kidney. 

 Slit open the passage, using the seeker as a guide. 



The pericardial cavity is developmentally a part 

 of the body-cavity or coelom. Hence in the mussel, 

 as in the earthworm and in other animals generally, 

 the kidneys are tubular organs with glandular walls, 

 leading from the body-cavity to the exterior. 



From the external apertures the excretory pro- 

 ducts are carried back by the exhalent respiratory 

 stream along the supra-branchial passages to the 

 cloacal cavity, and so out. 



E. The Nervous System. 



The nervous system of the mussel consists of three main 

 pairs of ganglia widely distant from one another, but united 



