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 lacuna, 
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. 
