XII 



PHYLUM MOLLUSCA 



669 



On each side is a single gill or ctenidium composed of two 

 plates or lamina, an inner and an outer. We have thus right outer 

 and inner gill-laminae, and left outer and inner gill-laminae (Fig. 

 575, 1. ext. gl.y 1. int. gl.). Seen from the surface, each lamina presents 

 a delicate double striation, being marked by faint lines running 

 parallel with, and by more pronounced lines running at right 

 angles to, the long axis of the organ. Moreover, each lamina is 



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FIG. 577. Anodonta cygnea. A, transverse section of outer, and B, of inner gill-lamina ; 

 C, diagram of gill-structure ; Z), transverse section of gill filament, b. c. blood-corpuscle ; 

 6. w. blood-vessels ; ch. chitin ; /. branchial filaments ; ep epithelium ; i. f. j. inter- 

 filamentar junction ; i. 1. inner lamella ; i. 1. j. inter-lamellar junction ; o. I. outer lamella ; 

 os. external ostium ; os' internal ostium ; r, chitinuus rods ; 10. t. water-tubes. (A, B, and D 

 after Peck.) 



double, being formed of two similar plates, the inner and outer 

 lamella, united with one another along the anterior, ventral, and 

 posterior edges of the lamina, but free dorsally. The lamina has 

 thus the form of a long and extremely narrow bag open above 

 (Figs. 576, 577, and 578) : its cavity is subdivided by vertical bars of 

 tissue, the inter-lamellar junctions (i. l.j.), which extend between the 

 two lamellae, and divide the intervening space into distinct compart- 

 VOL. T u u* 



