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. 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.
