50 LAND AND FRESH-WATER SHELLS. 



gill thus becomes the outer lamella, and the outer lamella of the 

 inner gill the inner lamella, — i.e., of course, of their respective 

 gill-plates. The outer lamella of the outer gill-plate becomes 

 united by concrescence with the mantle, the inner lamella of the 

 inner gill remains free as a slit through which the infra-commu- 

 nicates with the supra-branchial chamber. Thus, " regarding, 

 then " — as Professor Lloyd Morgan in that most practical of all 

 recent text-books, " Animal Biology," says, — " the gills as com- 

 posed of a greater number of filaments with descending and 

 ascending moieties, and we are justified by comparative morphology 

 and development in so regarding them, we have to note the large 

 amount of fusion or concrescence of parts. 



I. The inner surfaces 'of adjoining filaments have become 

 united by a membranous expansion, the lamellar membrane, 



Fig. 17.— The Organ of Bojanus Fig. 18.— Diagrammatic Section 



in diagrammatic section. of a Mussel. 



perforated with oval fenestrce or windows. Thus the ascending 

 and descending filaments respectively come to form ascending and 

 descending lamellae. 



2. The two lamellae of each gill become fused by interlamellar 

 junctions, occurring in the inner gill at intervals of about twenty 

 filaments, and in the outer gill at intervals of seven or eight 

 filaments. 



3. The ascending lamella of the outer gill is concrescent with 

 the mantle along its whole length. 



