36 



Our Food Mollusks 



bearing greatly elongated cilia. In the majority of 

 lamellibranchs, are found two such rows, but in some 

 cases there are four, two on each side. Among the cells 

 that bear the small frontal cilia, are certain ones that have 

 become gland cells (g c) producing a large quantity of 

 sticky mucus which they pour out on the surface of the 

 filament. 



The functions performed by these complicated gills 

 are many. They are breathing organs, and were prob- 

 ably developed originally for that 

 purpose alone. Blood, that is con- 

 tinually streaming through the 

 many filaments, is brought so close 

 to the w r ater surrounding the gills 

 that oxygen borne by it enters the 

 fluid by diffusion and is then car- 

 ried to all parts of the body. But 

 it is necessary that water surround- 

 ing the gills should frequently be 

 changed because its oxygen is 

 quickly exhausted. In many aqua- 

 tic animals the gills move back and 

 forth to agitate the water, or 

 special organs are developed to 

 direct a current upon them. Here 

 a very powerful current of the 

 most effective sort is produced by 



Fig. 9. — Transverse sec- 

 tion of a single fila- 

 ment of the gill of 

 Pec ten irradians. 

 Structures designated 

 are : — f c, frontal 

 cilia; (j c, gland cells ; 

 s c, straining cilia ; 

 and b, blood space. 



the gills themselves. The cilia lash 



in such a way as to drive water 

 between filaments on both sides of the gill into its inte- 

 rior. The passageway is indicated by the arrows in the 

 figures of the oyster and scallop gills. The current does 

 not pause here, but continues swiftly upward toward the 



