A FRESHWATER MUSSEL 95 



the ventricle. If the left auricle has been injured in the 

 dissection, the right one is easily seen by looking across the 

 pericardial space. From the ventricle an anterior and a poste- 

 rior artery pass to either end of the body. These arteries lie 

 alongside the rectum, to which the anterior one is dorsal and 

 the posterior one is ventral; they are difficult to distinguish 

 from it, except in specimens in which the heart has been 

 injected. 



The course of the blood is the following : by the contraction 

 of the heart the blood is sent to all parts of the body ; on its 

 return course it is first conveyed, through a system of lacunae, 

 to the kidneys, and thence to the gills; here it circulates in 

 vessels which run through the interlamellar partitions, the gill- 

 filaments, and the interfilamentary connections, and is oxy- 

 genated; it then passes into the auricles. 



The excretory system consists of a pair of kidneys, which are dark- 

 colored organs lying just beneath the pericardium and in front 

 of the posterior adductor muscle. Each kidney consists of two 

 parts, the kidney proper and the ureter. The former is a dark, 

 thick-walled gland which lies beneath the ureter and communi- 

 cates with it at its hinder end. The ureter is a thin-walled 

 vessel lying above the kidney proper, with a small external 

 opening on the side of the visceral mass beneath the anterior 

 end of the kidney and near the base of the inner gill. With 

 fine scissors cut off the gills and look for the opening ; it may 

 be recognized by its white lips. The kidney also possesses at 

 its anterior end a duct leading into the pericardial cavity. Slit 

 open the ureter and kidney proper in clean water and observe 

 their inner structure. 



Exercise 8. Draw a diagram representing the pericardial cavity 

 and the kidney, showing the relation of the two structures 

 to each other. Draw the heart in the pericardial cavity, 

 showing the relation of the auricle to the gills. 



