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 Pseudolamellibranchia in which the gill-lamellae are vertically
folded, the inter-filamentar and interlamellar junctions being vascular
or non-vascular; the Eulamellibranchia in which the inter-filamentar and
interlamellar junctions are vascular; and lastly the Septibranchia in
which the gills are reduced to a horizontal partition. The
Pseudolamellibranchia included the oyster, scallop and their allies
which formerly constituted the order Monomyaria, having only a single
large adductor muscle or in addition a very small anterior adductor. The
researches of W. G. 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. The four orders now retained exhibit successive stages in the
modification of the ctenidia by reflection and concrescence of the
filament, but other organs, such as the heart, adductors, renal organs,
may not show corresponding stages. On the contrary considerable
differences in these organs may occur within any single order. The
Protobranchia, however, possess several primitive characters besides
that of the branchiae. In them the foot has a flat ventral surface used
for creeping, as in Gastropods, the byssus gland is but slightly
developed, the pleural ganglia are distinct, there is a relic of the
pharyngeal cavity, in some forms with a pair of glandular sacs, the
gonads retain their primitive connexion with the renal cavities, and the
otocysts are open.