 Most bivalves are filter feeders , using their gills to capture particulate food such as phytoplankton from the water . The protobranchs feed in a different way , scraping detritus from the seabed , and this may be the original mode of feeding used by all bivalves before the gills became adapted for filter feeding . These primitive bivalves hold on to the substratum with a pair of tentacles at the edge of the mouth , each of which has a single palp , or flap . The tentacles are covered in mucus , which traps the food , and cilia , which transport the particles back to the palps . These then sort the particles , rejecting those that are unsuitable or too large to digest , and conveying others to the mouth . 
