XI 



PHYLUM MOLLUSCA 



273 



tubes all open dorsally into a supr abranchial chamber, 

 continuous posteriorly with the cloaca and thus opening on 

 the exterior by the exhalant siphon. 





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 



