Bivalvia

This class of molluscs have two saucer-shaped shells, or valves, which enclose the soft body of the animal. They generally burrow in sediment though some bore into harder substrates such as wood or even rock. Some species have adapted to living anchored to harder substrates, remaining attached by secreting a series of fine threads, known as 'byssus threads'.

They are filter feeders extracting food particles either from the water or the substrate in which they live. Water containing the food particles is pumped over the gills which are specially adapted to extract any food particles and then passed to the mouth by flap-like palps. The group has completely lost the molluscan radula.

Locomotion is generally quite restricted and most species are only capable of burrowing into sediment and moving up an down within it. This movement is controlled by a muscular organ known as the 'foot'. However, a few exceptional species have evolved the ability to 'swim' by repeatedly flapping their valves together.

The sexes are usually separate with eggs and sperm being released freely into the water where fertilisation occurs. Once fertilised the egg develops into a trochophore larvae and later a veliger larvae. The freshwater Unionoida are slightly different, developing in a glonchidium that attaches to any firm surface to avoid being swept downstream.