When the oral valve opens in their mouth, the water draws into the buccal cavity. The opercular cavity, which contains the gills, then closes. When the oral valve is closed, the operculum (gill cover) opens the gills to force the water out. The blood in the capillaries flows into the gill filament in a direction contrary to the flow of water. This opposite flow allows countercurrent movement, the movement of materials moving in opposite directions between two fluids.