XII 



PHYLUM MOLLUSCA 



087 



minute apertures, or ostia (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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F 1 !!!. j07. 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; 

 h. r. blood-vessels ; fh. chitin ; /. branchial filaments ; ep. epithelium ; /. /'. j. inter- 

 filamentar junction ; /. /. inner lamella ; i. I. j. inter-lamellar junction ; o. I. outer lamella ; 

 os. external ostium ; os' internal ostium ; r, chitinous rods ; w. t. 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 (&. v.). 



The 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 



