vii MOLLUSC A RESPIRATORY ORGANS 93 



points between the ascending and descending portions of the branchial filaments, 

 but no blood-vessels run into them. 



In Atwmia, the dorsal ends of the ascending portions of the outer lamella are 

 free, but in the Arcidce united, although their internal cavities are not in communi- 

 cation. In such cases, the interior of each filament is divided by a longitudinal 

 septum into two canals. In one of these the blood flows from base to tip, and in the 

 other back from tip to base, i.e. to the axis. In the Mytilidce, the dorsal ends of the 

 recurved portions of the filaments of each branch have grown together, and their 

 blood-vessels communicate at the points of junction, i.e. along the upper edge of the 

 ascending lamella. 



(c) Pseudolamellibranchia. Each leaf of the gill is here folded, to secure increase 

 of surface. The plications run longitudinally with regard to the filaments, and are 

 thus almost dorso-ventral. There are, therefore, distinct alternate ridges and furrows 

 on each leaf, the ridges on the one surface corresponding with those on the other, and 

 the furrows corresponding with furrows. Each ridge or furrow is formed by one 

 filament ; the filament forming the furrow is in some way, such as greater breadth, 

 distinguished from the others. The two lamellse of each leaf of the gill are united 

 here and there by trabeculse, which may or may not contain blood-vessels. They occur 

 either between the opposite furrows or between the opposite ridges, i.e. between the 

 ascending and descending portions of the filaments which lie either in the furrows 

 or ridges. The upper edge of the ascending lamella of the outer leaf may unite with 

 the mantle. The consecutive filaments of the same leaf are only connected by means 

 of tufts of cilia. 



(d) Eulamellibranchia (Figs. 89-91). The branchial leaves are either smooth or 

 folded, but there is always organic connection, by means of numerous vascularised 

 junctions, not only between the ascending and descending lamellse, but between the 

 successive filaments. The junctions are therefore both interfoliar and interfilamentar. 

 This leads to the entire disappearance of the original filamentous structure of each 

 leaf, which now becomes an actual leaf or lamella with perforations or slits, the 

 remains of the spaces between the original filaments, leading into an internal system 

 of sinuses or canals, which in their turn are the remains of the spaces between the 

 ascending and descending lamellse. This peculiar arrangement was formerly con- 

 sidered typical of the Lamellibranchia, and was the origin of their name. It was 

 supposed that the animals of this class had two leaf-like gills on each side of the 

 mantle cavity, i.e. four altogether, but we now know how the two branchial leaves on 

 each side arose, that they are in fact the two, modified, rows of leaflets of the original 

 plumose gill of the Protobrancliia. The Lamellibranchia in reality possess only one 

 gill on each side in the mantle cavity. 



The blood now no longer flows through the primitive filaments of the lamellae 

 of the gills and back again, but the afferent and efferent channels lie in the trabecular 

 network between the two lamellae of a branchial leaf. 



Instead of the two leaves of a gill hanging down into the mantle cavity parallel 

 to one another, the outer leaf may stand up dorsally in the cavity, so that the two 

 come to lie in the same plane (Tcllinidce and Anatinacca). 



The ascending lamella of the outer leaf may be wanting (Anatinacea, Lascea), 

 and in fact the entire outer leaf may be absent (Lucina, Corbis, Montacuta, 

 Cryptodoti). 



In all Lamellibranchia, with the exception of the Protobranchia, and further, of 

 the Arcidce, Trigonidce, and Pectinidce, the gill and mantle unite, the dorsal edge of 

 the ascending (outer) lamella or, where this is wanting, the free edge of the single 

 lamella of the outer leaf becoming fused with the mantle. In the same way, the 

 dorsal edge of the ascending (inner) lamella of the inner leaf may become fused with 

 the upper part of the foot (Fig. 88 C). If the two gills, which have fused with the foot, 



