A Typical Univalve MoUusk 



kidney discharges by a narrow slit easily found at the back of 

 the mantle cavity. 



The gills are thin and their tubular passages are lined with 

 cilia which continually wave, creating an inward current of 

 water from the mantle chamber. The siphon is always lifted 

 into clear water, no matter if the foot is in the mud, for the water 

 supply of the gills must be clean and constant. In the network 

 of gill passages the oxygen in the water passes into the blood 

 and the carbonic acid gas passes out in the stream of foul water 

 discharged. 



The Circulation of Blood. — A vein which brings blood back 

 pure to the heart is found along the left side of the gill. It turns 

 downward at the posterior end of the gill and empties into the 

 auricle. Another vein brings blood from the kidney. The gill 

 receives blood from a vessel that borders its right side. This 

 blood is gathered by smaller vessels from the mantle and from the 

 glandular part of the kidney. 



The blood is sent from the ventricle through a short trunk, 

 the aorta, which gives oflF a large branch, the visceral artery. 

 This branches and distributes blood to the visceral dome. Now 

 the aorta turns downward and forward, enlarges to form the 

 "secondary heart," close to the oesophagus. From this trunk 

 arise several vessels that carry the blood to the foot, the head, 

 the siphon and other organs. 



Course of the blood: 



1 . Heart to the system. 



2. System to the kidney. 



3. Kidney to gill. 



4. Gill to heart. 



The auricle is the receiver of the blood. The ventricle is 

 the pump. The pericardium is the loose bag containing the heart. 

 The arteries distribute pure blood throughout the living tissues; 

 the veins collect it impure from these tissues. In the kidney 

 the blood is relieved of its urea. In the gills it receives oxygen 

 and gives out carbonic acid gas. The kidney and the gill are the two 

 organs that relieve the blood of the impurities collected in the liv- 

 ing tissues. The blood of mollusks is cold and usually colourless. 



The Alimentary Canal. — ^The mouth is at the tip of the long 

 proboscis; behind it is the straight oesophagus, or gullet, which 

 leads to the stomach. The odontophore has been mentioned 



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