(538 



ZOOLOGY 



SECT. 



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

 junctions, are 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. 529, D), of which produce a current 



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1 !' - "'-".i. Anodonta cygnea. A, transverse section of outer; B of inner gill; C, diagram 

 "f gill-structure j 1), transverse section of gill filament; I,, c. blood-corpuscle : >>. ,-. blood- 

 ve?sels ; ch. chitin ; /. branchial filaments ; ".;). epithelium ; ;. ;'./'. inter-filamentar junction : 

 '. I. inner lamella; <'. /. /'. inter-lamellar juiK-tion ; o. I. outer lamella; fix. external ostium : 

 os. internal ostium ; ;, chitinous rods ; tt: 1. water tubes. (A, 13, 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. r.). 



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 



