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, 9), 
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 
® C 
F 
d + 
D E F 
(hae ae 
ev A 
Fic. 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 ; EL, Donax faba ; 
F, Donax variabilis, Tapes, Venus; G, Lasaea; H, Tellina; J, Lyonsia; K, Lucina, Montacuta. 
a, axis; b, 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; f, 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 filaments 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 
