A HARD-SHELL CLAM 109 
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, whence it 
is conveyed through lacune to the kidneys and thence to the 
gills; here it circulates in vessels which run through the inter- 
lamellar partitions, the gill-filaments, and the interfilamentary 
connections, and is purified; it then passes into the auricles. 
The excretory system consists of a pair of kidneys which lie 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 communicates with it at its hinder 
end. The ureter is a thin-walled vessel lying above the kidney 
proper, with a small external opening in the side of the visceral 
mass near the base of the inner gill. Cut off the gills and look 
for the external opening; it may be recognized by its white 
lips. The kidney also possesses at its anterior end a duct lead- 
ing into the pericardial cavity. Slit open the ureter and 
kidney proper 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. 
The digestive system. Find the mouth between its two pairs of 
palps and place a bristle in it; note the upper and the lower lips, 
which connect the upper and the lower pair of palps, respectively. 
The mouth is seen to the greatest advantage in a specimen 
which has been taken out of both shells. Trace the rectum 
from the anus through the heart to the point where it meets 
