92 



COMPARATIVE ANATOMY 



CHAP. 



sinus, which is a continuation of the branchial artery. Two rods of connective 

 tissue run along the lower edge of each leaflet from the axis to its tip, and serve for 

 its support. Similar supports are found in almost all Lamellibranchia and in 

 many Gastropods. 



The epithelium of the branchial leaflets is beset with long cilia (1) at the 

 ventral edge ; (2) on both (anterior and posterior) surfaces, near the ventral edge. 



The first-named cilia form, with regard to the whole gill, a longitudinal row along 

 the free ventral edge of each row of leaflets, and bring about a current in the water 

 along this edge from behind forward. The other cilia mentioned above, mingling 

 together like the bristles of two brushes which arc pressed together, form a loose 

 connection between the successive leaflets of the row. 



(6) In the Filibranchia (Fig. 88 B) the leaflets in each of the two rows are very 

 long and filamentous, and hang down Car into the mantle cavity. The branchial 

 filaments of the two rows are recurved and bent back upon themselves, so that in 

 each filament a descending and an ascending portion can lie distinguished. The 

 prolongation of the filaments corresponds with a necessary increase of the respiratory 



FIG. 88. Morphology of the gills of the Lamellibranchia, diagrammatic transverse sections. 

 A, Protobranchia B, Filibranchia. C, Eulamellibranchia. D, Eeptibranchia. 1, Mantle ; 

 _', body (visceral dome); 3, foot ; ?, in A, branchial leaflets of the outer row in the feathered gill, 

 in U, branchial filaments of the -niter r<>\v, in C, outer branchial leaf; i, branchial leaflets or 

 filaments of the inner row or inner branchial leaf; >i, ascending branch of the outer filament, 

 or lamella of the outer leaf; /i, ascending branch of the inner filament, or lamella of the inner leaf; 

 in D, s, signilies the gill which has become transformed into a muscular septum which divides the 

 mantle cavity into an upper (4) and a lower (.">) chamber, the two communicating by means of slits 

 (o) in the septum. Further explanations in the text. 



surface. By this bending back of the filaments, the gills make the most of the 

 limited space afforded by the mantle cavity. Each filament of the outer row is bent 

 outwards, and of the inner row inwards. 



The filaments of each row may be so crowded together that the whole row looks 

 like a leaf or fringe. This branchial leaf consists of two closely contiguous lamellae, 

 one the descending and the other the ascending, the two passing into one another 

 at the lower edge of the leaf. The descending lamella is formed by the descending 

 portions of the filaments, and the ascending by the ascending portions. On the 

 outer leaf, the ascending lamella is the outer one, on the inner leaf the inner. 



In the Filibranchia, the separate branchial filaments retain their independence 

 they are free, i.e. the separate filaments of a series are unconnected with one 

 another, and the descending and ascending portions of one and the same filament 

 are in no way united. There are, however, on both the anterior and posterior sides 

 of the filaments places covered with long cilia closely crowded together. These 

 ciliated tufts on adjoining filaments mingle, and so give rise to a sort of connection 

 between the filaments of each leaf. 



In the Mytilidce, so-called interfoliar junctions or trabeculse occur at certain 



