A HARD-SHELL CLAM 99 



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 lacunae 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 interfilarnentary 

 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 semidiagrammatic view representing the 

 pericardial cavity. 



Exercise 9. Draw a diagram showing the kidney and its relation 

 to the pericardium. 



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^ 



