226 THE LAMELLIBRANCHIA 



specialised forms they reach to the anterior extremity of the body 

 (Fig. 221). Each ctenidium consists of a hollow vascular axis 

 bearing on each face a row of more or less flattened hollow filaments, 

 which are nothing more than simple expansions of the axis. In the 

 Protobranchia the filaments are broad, simple, and free (Fig. 230, g), 

 and the two rows are situated on opposite sides of the axis (Fig. 

 206, A, B). In all other Lamellibranchia the filaments are more or 

 less narrow, and the two rows are normally parallel to one another 



B 



FIG. 206. 



Diagrammatic sections taken transversely to the axes of the gills of various Lamellibranchs, 

 A, Nucula ; B, Solenomya ; C, Dimya ; D, the majority of the Lamellibranchs ; E, Donax faba ; 

 F, Donax variabilis, Tapes, Venus G, Lasaea ; H, Tellinn ; J, Lyonsia ; K, Lucina, Montacuta. 

 a, axis ; 6, direct (usually descending) lamella of the outer gill-plate ; c, reflected (usually 

 ascending) lamella of the outer gill-plate ; d, direct or descending lamella of the inner gill-plate ; 

 e, reflected or ascending lamella of the inner gill-plate ; /, leaflets of the outer gill-plate ; g, 

 leaflets of the inner gill-plate. (After Ridewood.) 



and directed towards the ventral surface (Fig. 206, C-K). The distal 

 moieties of the filament^ are, however, reflected ectaxially and 

 dorsalwards, in such a manner that each row forms a double 

 lamina, that is to say, consists of two leaves or lamellae (Fig. 205, 

 br, br) between which there is an interlamellar space or cavity, 

 serving, in some species, for the incubation of the ova. 



In the Filibranchia the successive filaments of each ctenidial 

 row are locked together by ciliary junctions, sometimes specialised 

 to form ciliated discs whose cilia interdigitate closely with one 

 another (Fig. 210, A, cj). The direct and reflected limbs of each 



