Oysters and Scallops like all bivalves are filter feeders and have a complete digestive system. To filter feed they beat their cilia and make water enter their mantle via their in current siphon, and expel the water out their excurrent siphon. While the water is in the mantle both the cilia and mucus on the gills catch food particles and move them to the mouth.  Once the food enters the mouth it passes through the esophagus into the stomach where extracellular digestion takes place. Then from the stomach the food moves into the animals intestine, which passes through the heart, and is where food is intracellularly digested. The waste then moves from the intestine to the anus where it is removed from the body. These animals also have digestive cecum which is located near the mouth and acts as a liver.