Phylum Mollusca (1)
Phylum Mollusca ranges from Cambrian to Recent. Members of this phylum exhibit bilateral symmetry. A shell encloses the soft parts of many mollusks. They have a foot adapted for burrowing, movement on the substrate, or food grasping. Three classes important in the fossil record are Bivalvia, Gastropoda, and Cephalopoda.
This class includes clams, oysters, scallops, and mussels. The shells have two valves and are composed of layered calcite and aragonite. In many bivalves, the two valves are mirror images, unlike brachiopod valves, which are different from each other. Muscles hold the valves together, and some bivalves have a flexible ligament that acts as a spring and opens the shells when the muscles relax. One or two muscles are present, and muscle scars are visible on the interior of the shell. An organ called the mantle secretes the shell by adding calcite along its edge. The pallial line marks the mantle attachment to the shell. If siphons are present, the interior of the shell shows an indentation called a pallial sinus. This indentation commonly indicates a burrowing habit for the bivalve. The incurrent siphon takes in seawater, the gills remove food and oxygen and move the food to the mouth. Wastes move out of the body through the excurrent siphon. The shells hinge together in the beak area. Teeth and sockets on either valve fit the shells together.