112 



Descriptive Zoology. 



the inner wall of the outer gill joining the upper edge of the 

 outer wall of the inner gill, while the upper edge of the inner 

 wall of the inner gill is sometimes attached to the body or 

 sometimes free. Back of the body the upper edges of the 

 inner gills of the two sides unite with each other, thus sepa- 

 rating the lower, or gill, cavity, into which the water first 



FIG. 68. CLAM, SIDE VIEW. 



Water currents to the mouth and through the gills. 



enters, through the lower incurrent siphon, from the upper, 

 or cloacal, chamber, from which the water passes out. The 

 question presents itself, " How does the water pass from 

 the one cavity into the other ? " 



The sides of the gills are perforated, so much so that they 

 are compared to a sieve or trelliswork. The vibrations of 

 the myriads of cilia with which the gills are covered drive 



