 In the Filibranchia and Eulamellibranchia , water is drawn into the shell from the posterior ventral surface of the animal , passes upwards through the gills , and doubles back to be expelled just above the intake . In burrowing species , there may be two elongated , retractable siphons reaching up to the seabed , one each for the inhalant and exhalant streams of water . The gills of filter @-@ feeding bivalves are known as ctenidia and have become highly modified to increase their ability to capture food . For example , the cilia on the gills , which originally served to remove unwanted sediment , have become adapted to capture food particles , and transport them in a steady stream of mucus to the mouth . The filaments of the gills are also much longer than those in more primitive bivalves , and are folded over to create a groove through which food can be transported . The structure of the gills varies considerably , and can serve as a useful means for classifying bivalves into groups . 
