Pelecypoda. 



in 



as food. The blood circulates in the walls of the gills, and 

 thus the water current and the blood current are brought 

 very close to each other. Oxygen passes from the water 

 into the blood ; and from the blood, carbon dioxid and other 

 waste matters pass into the water through the thin layer of 

 the wall of the gill surrounding the blood tubes. There is 

 also an active circulation of blood in the mantle, and a con- 

 siderable share of the work of respiration is undoubtedly 

 accomplished here. 



The Structure of the Gills. Each gill has the appearance 

 of a thin, single-layered membrane ; but in reality each gill 



Artery 



\ 



Anterior adduc- \ 

 tor muscle 



Auricle Ventricle 



i ! 



Posterior adductor 

 muscle 



Gill 



FIG. 67. BODY OF CLAM. 



Left valve removed. 



is double walled, and a cross section is like a letter V. 

 Each gill is a long, narrow, V-shaped pocket or trough, 

 though divided into many compartments by cross partitions. 

 The two gills of each side are united so as to form, in cross 

 section, a W, the upper margin of the outer wall of the 

 outer gill being attached to the mantle, the upper edge of 



