Most bivalves absorb nutrition by filter feeding, which involves drawing particles of food into their gills through their mouths and into their stomachs. Some bivalves trap food with mucus-covered tentacles. Others are carnivorous, using siphons to draw in worms, larvae and crustaceans that are then pushed into their mouths.
Bivalves are aquatic mollusks that have hinged shells but no head or jaws. The large gills in most bivalves draw oxygen and food from the surrounding water. Mucus traps the food, and cilia move the mucus-bound food to the mouth. From there, it moves through the esophagus into the stomach, where it is digested. Some bivalves that live beneath sand or sediment extend long siphons out to open water to draw nourishment into the gills. Parasitic bivalves that live within the esophagi of sea cucumbers extract food particles from the sediment the sea cucumbers suck in. Shipworms not only draw nourishment from the sea but also digest the wood they bore through.
Bivalves that are not filter feeders have smaller gills. The bivalves, called protobranchia, use tentacles to scrape particles into their mouths. Cilia transport them to appendages that sort the particles and move suitable food particles to the mouth. Carnivorous bivalves have stomachs similar to other bivalves with the addition of gizzards that grind the food up before it is digested.