Bivalves Classification PHYLUM: Mollusca CLASS: Bivalvia

Bivalve Morphology 1 Look at page 32 of Black. Copy diagram 18a. The soft body is held between two shells (valves) which are usually made of calcite or aragonite. The valves act as protection from carnivores. Bivalves are bilaterally symmetrical, the plane of symmetry occurring along the line where the two valves meet. Therefore the valves are more or less mirror images.

Bivalve morphology 2 VALVES: The shell is secreted by the animal like in brachiopods from the mantle. The valves are called left and right. What are the two valves called in brachiopods?   UMBONES: The original shell from which the bivalve grew.

CONCENTRIC GROWTH RINGS: Bivalve morphology 3 CONCENTRIC GROWTH RINGS: There are concentric rings increasing in size away from the umbones. These are added as the bivalve grows much in the same way as the growth rings on a tree. If the growth season is good then the rings will be more widely spaced.  RADIATING RIBS:  The shell ornament and it may also have spines.

EQUIVALVE/INEQUIVALVE: Bivalve morphology 4  EQUIVALVE/INEQUIVALVE: In most bivalve species the valves look very similar (equivalve) unlike Brachiopods although sometimes the valves do look different (inequivalve).   DORSAL/VENTRAL: The dorsal side is the hinge area the opposite is the ventral. ANTERIOR/POSTERIOR: The anterior end is the mouth end. The posterior is the anus end.

Bivalve morphology 5 SOFT PARTS: Most soft parts are in the dorsal half, which contains the main organs and muscles. MANTLE:  This is the fleshy part that extended out towards the edge of the valves. It enclosed most of the soft parts and was also responsible for the growth of the valves as the mantle wrapped over the edge of the valve. FOOT:  This is a muscular organ, which can extend out of the shell and is used by the bivalve to move through the sediment.