The smallest bivalves are only 1 mm long, while the biggest cephalopods can be up to 30 metres long. The bivalves have a right and a left shell surrounding the soft body. Two tubes channel water to and from the mantle cavity, where there is a single pair of large gills. Edible particles are filtered out and brought to the mouth by short cilia. The mouth lacks the rasping tongue.
Bivalves are common both in the sea and in fresh water today.
Bivalves are known as fossils back to the Ordovician. In Norway, they are common fossils in Quaternary sediments. The scaphopods have a tube-shaped shell open at both ends. In the narrow end, water is led into the mantle cavity, where the gills are. They feed on small organisms which they trap with their tentacles and bring to the mouth, which possesses a rasping tongue.