638 



ZOOLOGY 



SECT. 



filaments, and bounded above and below by the inter-filamentar 

 junctions, are minute apertures, or ostia (as.), 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. 529, D), of which produce a current 



FIG. 529. Anodonta cygnea. A, transverse section of outer ; B of inner gill; C, diagram 

 of gill-structure ; D, transverse section of gill filament ; b. c. blood-corpuscle ; b. r. blood- 

 vessels ; c/i. chitin ; /. branchial filaments ; ep. epithelium ; i. f. j. inter-filamentar junction ; 

 /'. /. inner lamella ; i. 1. j. inter-lamellar junction ; o. L outer lamella ; os. external ostiurn ; 

 os'. internal ostium ; r, chitinous rods ; ic. t. water tubes. (A, B, and D, after Peck.) 



running from the exterior through the ostia into the water-tubes, 

 and finally escaping by the wide dorsal apertures of the latter, 

 The whole organ is traversed by bloodvessels (b. v.}. 



The mode of attachment of the gills presents certain features of 

 importance. The outer lamella of the outer gill is attached along 

 its whole length to the mantle (Fig. 530) : the inner lamella of the 

 outer, and the outer lamella of the inner gill are attached 



