42 



junctions, and so the various filaments can be separated 

 easily in the living animal. Organic iuterfilamentar 

 junctions occur in P. tenuicostatus (1), which makes with 

 Avicula argentea and Margaritifera vulgaris (9) a third 

 exceptional member of the Eleutherorhabda. 



The histology of the gills has been worked out from 

 serial sections cut chiefly parallel to the ctenidial axis, 

 that is transverse!}^ through the filaments, but in addition 

 sections have been cut parallel to the filaments in two 

 directions. 



Perennyi, or Mann's Fluid, can be used for fixing the 

 gills, and then Methyl-blue-eosin differentiates the 

 chitinous framework well, staining it blue, the cytoplasm 

 purple and the cilia bright red. The principal filaments 

 should be dissected out, stained, cleared and mounted in 

 Canada Balsam in order to make out the various parts. 



The filaments are tubes bounded by a very delicate 

 epithelial wall, the cells of which differ in shape and size 

 at various points. 



The ordinary filaments (fig. 2-3, Fil. o.) have deeper 

 cells round the margin further away from the inter- 

 lamellar space, and these cells are deepest in three places 

 forming longitudinal ridges, the cells of which bear cilia. 

 Thus in transverse sections of the filaments there will be 

 seen three sets of cilia on the filament, one down the 

 frontal surface — the frontal cilia (fig. 23, C. fr.) — and 

 one on each side of the filament — the lateral cilia (fig. 23, 

 C I.), a narrow space separates the frontal from the lateral 

 cilia. This disposition of cilia is characteristic of most 

 of the Lamellibranchiata. These cilia, unlike those of 

 the ciliated discs, are, in life, in constant action producing 

 the currents of water on which the animal depends for its 

 food supply and oxygen. 



The ciliated discs are patches of elongated cells 



