PHYLUM MOLLUSCA 



687 



minute apertures, or ostin {os.), which lead from the mantle-cavity 

 through a more or less irregular series of cavities into the interior 

 of the water-tubes. The filaments themselves are supported by 

 chitinous rods (r.), and are covered with ciliated epithelium, the 

 large cilia (Fig. 567, D) of which produce a current running from 

 the exterior through the ostia into the water-tubes, and finally 



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Fig. 567. — Anodonta cygnea. A, transverse section of outer, and B, of inner gill-lamina ; 

 C, diagram of gill-structure; D, transverse section of gill filament. /'. c. blood-corpuscle; 

 ft. V. blood-vessels ; cli. chitin ; /. branchial filaments ; ep. epithelium ; i. f. j. inter- 

 filamentar junction ; ('. I. inner lamella ; i. I. j. inter-lhmellar junction ; o. I. outer lamella ; 

 08. external ostium ; os' internal ostium ; 7-, chitinous rods ; w. f. water tubes. (A, B, and D, 

 after Peck.) 



escaping by the wide dorsal apertures of the latter. The whole 

 organ is traversed by blood-vessels (6. v.). 



Th« mode of attachment of the gills presents certain features of 

 importance. The outer lamella of the outer lamina is attached 

 along its whole length to the mantle (Fig. 568) : the inner lamella 

 of the outer and the outer lamella of the inner lamina are attached 

 together to the sides of the visceral mass a little below the origin 



