Bivalve molluscs are completely enclosed by a shell made of two valves hinged at the top. A hinge ligament made of elastic protein joins the two halves of the shell together, and large adductor muscles between the two valves hold them closed. When the shell is closed, the top part of the hinge ligament is stretched and the lower part is compressed. This means that when the adductor muscles relax, the ligament acts like a spring and the shell opens. The adductor muscles contain both smooth and striated fibres, enabling sustained (‘catch muscle’) and rapid shell closure.

The hinge ligament is usually external to the shell, but in many species has shifted to an internal position between the hinge teeth. The hinge teeth (consisting of cardinal and lateral teeth) interlock on the two valves preventing slippage. Differences in the nature of the hinge teeth are important in the classification of bivalves.

Venus clam Callista erycina showing the hinge ligament that joins the two halves of the shell.

Two species with very different hinge structure. Top: Arca senilis Bottom: Lucina exasperata

Bivalve molluscs are completely enclosed by a shell Shown here are two species of bivalves with very different hinge structures. Arca senilis has a taxodont hinge, with many small, similarly sized interlocking teeth. Lucina exasperata has a heterodont hinge, with fewer but larger and differently sized hinge teeth.

Rising above the hinge line on each valve is a swelling called the umbo, the oldest part of the shell.

The freshwater mussel Mytilopsis leucophaeata showing the incurrent and excurrent siphons.

Inside the shell, the animal has a simple head without eyes or radula (the rasping, ribbon-like tongue used for feeding in other mollusc groups). The muscular foot typically has no sole, is flattened laterally and is used for burrowing and general locomotion. Bivalves have just a single pair of gills (‘*ctenidia*’). However, these are large and fold back on themselves providing a highly efficient surface for filter feeding. The mantle (the sheet-like organ that produces the shell) is large and often fused at the edges to form inhalent and exhalent siphons. These structures circulate currents of water needed for feeding and breathing.

The soft parts of a bivalve often leave impressions on the inner surface of the shell