They belong to the phylum Mollusca which includes squids, snails and cuttlefish. They belong to the class bivalve.
Common modern bivalves include: cockles, scallops and muscles which may live in brackish or salt water.
Bivalves evolved in the Cambrian and are still alive today.
Bivalves most distinctive feature is the equal size calcareous shells.
There is a plane of symmetry running along the hinge line. The place where the valves are joined is called the hinge plate, where in life an external or internal ligament would be found.
Some bivalves have valves that close completely, some have a gape or opening where soft body parts like feet or siphon may extend from.
Siphons are soft tissues that take in fresh water (inhalant) and filter then remove used water (exhalent). Inside the valves, gills remove particles from the water.
The hinge of a bivalve is made of protruding teeth and sunkenpits. Each shell has both teeth and sockets which help to open and close the valves and keep the structure stable.
The position of soft tissue is marked by the pallial line, either a bend or sinus if the animal had a siphon.
2 muscle scars where the adductor muscles were attached. The muscles would have been used to keep the shells closed. The valves open using the ligament.
Some bivalves are vagrant and move around using a foot made of muscle. The foot can also be used by Infuanal bivalves to burrow into sediment. Some bivalves are sessile.
