PHYLUM MOLLUSCA 



273 



tubes all open dorsally into a suprabranchial chamber, 

 continuous posteriorly with the cloaca and thus opening on 

 the exterior by the exhalant siphon. 



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The physiological importance of the gills will now be 

 obvious. By the action of their cilia a current is produced 

 which sets in through the inhalant siphon into the pallial 



