Bivalvia (formally named Pelecypoda or Lamellibranchia) are a class of molluscs that first appeared in the Lower Cambrian. They underwent a period of limited diversity during the Palaeozoic and then boomed during the Mesozoic so that they are now the second largest molluscan class. There are roughly 10-15000 species globally today, found in both freshwater and marine settings.
Bivalves are characterized by two oval or elongate valves, which enclose the organism's body. If the valves are a mirror image of each other then they are said to be equivalved. If not, they are inequivalved. The valves are joined dorsally by a toothed hinge, which prevents the shell from being misplaced when opening and closing. An internal elastic ligament automatically opens the valves, but by contracting one of two of its adductor muscles it is able to hold the shell shut. The first part of a bivalve shell to develop is the umbo which is seen as a large bump towards the anterior end of the dorsal side of the shell.
Bivalves are entirely aquatic and rather stationary organisms. To feed they use siphons. One siphon will carry water to the internal cavity and is known as the inhalant siphon, the other takes water away from the internal cavity and is known as the exhalant siphon. In certain cases these siphons can become fused, but the water in the two tubes still remains separate. Once inside the bivalve the water runs past ciliated gills, which sift out any food particles.
Bivalves also possess a muscular foot which they use to attach themselves to the substrate and/or burrow down into sediments.
Bivalves display several modes of life:
Certain limiting factors determine a bivalve's mode of life.
The morphology of a bivalve varies from one species to another and this is reflected in their differing: