Orders: Protobranchia, Filibranchia, Eulamellibranchia, Septibranchia.
In the filaments of the gill of Protobranchia and many Filibranchia the tubular cavity is divided by a more or less complete fibrous septum into two channels, for an afferent and efferent blood-current.
This investment, which occurs also in many Filibranchia, forms the pericardial glands, comparable to the pericardial accessory glandular growths of Cephalopoda.
In Filibranchia and many Protobranchia the otocyst (or statocyst) contains numerous particles (otoconia).
Classification Of Lamellibranchia The classification originally based on the structure of the gills by P. Pelseneer included five orders, viz.: the Protobranchia in which the gill-filaments are flattened and not reflected; the Filibranchia in which the filaments are long and reflected, with non-vascular junctions; the Pseudo-lamellibranchia in which the gill-lamellae are vertically folded, the interfilamentar and interlamellar junctions being vascular or non-vascular; the Eulamellibranchia in which the interfilamentar and interlamellar junctions are vascular; and lastly the Septibranchia in which the gills are reduced to a horizontal paltition.
Ridewood have shown that in gill-structure the Pectinacea agree with the Filibranchia and the Ostraeacea with the Eulamellibranchia, and accordingly the order Pseudolamellibranchia is now suppressed and its members divided between the two other orders mentioned.
Order Filibranchia Gill-filament ventrally directed and reflected, connected by ciliated junctions.
The sea mussel (Mytilus edulis) belongs to the second order of the class Lamellibranchia, namely the Filibranchia, distinguished by the comparatively free condition of the gillfilaments, which, whilst adhering to one another to form gillplates, are yet not fused to one another by concrescence.