Mussels are filter feeds. When mussels are in the adult phase they live non-mobile lifes. Larva's legs turn into feathering appendages when they enter adult stage. These beat rhithmically to draw plankton, bacteria, detritus... from the water and move towards the "mouth". So water is pumped into their body by a cilary motion of their gills. Digestible particules are transported and pass through the degistive track. Unwanted particles, those aren't food are wrap in their mucus.