72 



LAMELLIBRANGHIA. 



and even in Area, is very difficult to explain. When isolated filaments for the 

 sake of increase of surface grow in length, they are not likely to retain such 

 a regular arrangement, even if we bear in mind their position in one row, 

 the limited space within the Lamellibranch shell, and the circulation of the 

 water between them. We therefore think ourselves justified in assuming, in 

 the case of those Lamellibranch gills which, while filiform in structure, show 

 such a regular leaf-like shape, a secondary breaking up of a gill which 

 originally consisted of two plates to which allusion has already been made 

 (p. 70). A satisfactory explanation of these obscure points can, however, 

 only be obtained by comprehensive investigation not only of the gills them- 

 selves but also of the whole structure of those Lamellibranchs which may be 

 regarded as transitionary forms. 



Fig 30 -Diagrams illustrating the position of the gills in the Lamellibranchia A 

 ToMia, BTDimya; 0, Amusium Dalli; />, Area noae; K, Aiwdonta;^ toot; 

 in, mantle;' i, inner, e, outer branchial leaf. 



We may regard as the most primitive form of the Lamellibranch gill a 

 ridge [the' ctenidial axis] with two rows of branchial filaments. In place of 

 the filaments, triangular leaflets must originally have been present, with 

 vertically expanded surfaces, placed transversely to the long axis of the ridge, 

 a condition permanently retained in the gills of Nv^ula and Yoldia (Fig. 30 

 A, Mitsukuri). Taking into account the similar form of the gills in the 

 Aspidobranchiate Gastropoda, this latter condition might be regarded as the 

 original condition. It is, indeed, not essentially different from that with the 

 double row of papillae, since the leaflets correspond in every respect to the 

 still unreflected papillae. 



