230 



THE LAMELLIBRANCHIA 



tion is effected in the latter chamber by the internal surface of 

 the mantle, which is bathed by a strong current of water passed 

 through by the contractions of the muscular septum. 



As regards the structure of the gills, it has been known since 

 1877 (Peck) that in all the Lamellibranchia each constituent fila- 

 ment of the gill is clothed externally by an epithelial layer con- 

 tinuous with the epithelium of the general surface of the body. 



FIG. 210. 



Ctenidial filaments of Mytilus edulis. A, part of four filaments seen from the outer face in 

 order to show the ciliated junctions c.j. B, diagram of the posterior face of a single complete 

 filament with descending (direct) ramus and ascending (reflected) ramus ending in a hook-like 

 process ; ep, the ciliated junctions ; i.l.j, inter-lamellar junction. C, transverse section of a 

 filament taken so as to cut neither a ciliated junction nor an inter- lamellar junction ; b.c, blood 

 corpuscle ; ch, chitinous tubular lining of the filament ; f.e, frontal epithelium ; l.f.e', ciliated 

 edge-cells ; l.f.e", lateral cells with long cilia ; lac, blood lacuna traversed by a few processes of 

 connective tissue cells. (From Lankester, after Holman Peck.) 



At certain points this epithelium is modified and bears powerful 

 cilia, particularly on the two ventral edges of each filament, where 

 the so-called "corner cells" (Fig. 210, l.f.e'), by the action of their 

 cilia, keep up a brisk current of water over the surface of the gills. 

 There are also "lateral" ciliated cells on the two faces of each 

 filament, which ensure the ciliary union between successive fila- 

 ments. Internally each filament presents a supporting structure, 

 formed by paired longitudinal thickenings of the sub-epithelial 



