MOLLUSCA 157 



In land molluscs gills are absent, but the walls of the 

 mantle chamber are moist and lined with fine blood 

 vessels. They act like lungs. As air circulates freely 



Fig. 88.— Clam with foot and siphons extended. (From Kingsley.) 



through the mantle chamber, oxygen can pass into the 

 blood and carbon dioxide from the blood into the air. 



Circulatory System. — The body cavity is very small. 

 It is practically reduced to the small chamber in which 

 the heart is situated. The heart is divided into a ventri- 

 cle which sends the blood over the body and an auricle 

 which receives the blood from the gills. The system 

 is not closed, the vessels open into large spaces or 

 sinuses. In the squid an extra set of contractile organs 

 called branchial hearts situated at the base of the gills 

 force the blood to pass through the gills. 



Alimentary Canal. — The alimentary canal is a well- 

 developed tube. On its way through the body it passes 



