Gaseous exchange is take place on by both mantle and the gills. The gills of most bivalves are highly changed for filter feeding. They are derived from the primitive ctenidial by lengthening of the filaments on every side of filamentous axis. As the ends of long filaments became folded back towards the central axis; the ctenidial filaments took the shape of a long slender 'W'. The filaments lying beside each other became joined through ciliary junctions or tissue fusions, forming plate like lamellae along with many vertical water tubes inside. Thus water enters the incurrent siphon, propelled by ciliary action, then enters the water tubes by pores between the filaments in the lamellae proceeds dorsally into a common suprabranchial chamber and then out of the ex-current aperture.