The mussel’s external shell is composed of two hinged halves or “valves”.  The valves are joined together on the outside by a ligament, and are closed when necessary by strong internal muscles.  Mussel shells carry out a variety of functions, including support for soft tissues, protection from predators and protection against desiccation.

The shell has three layers.  In the pearly mussels there is an inner iridescent layer of nacre (mother-of-pearl) composed of calcium carbonate, which is continuously secreted by the mantle; the prismatic layer, a middle layer of chalky white crystals of calcium carbonate in a protein matrix; and the periostracum, an outer pigmented layer resembling a skin.  The periostracum is composed of a protein called conchin, and its function is to protect the prismatic layer from abrasion and dissolution by acids (especially important in freshwater forms where the decay of leaf materials produces acids).

Like most bivalves, mussels have a large organ called a foot.  In freshwater mussels, the foot is large, muscular, and generally hatchet-shaped.  It is used to pull the animal through the sand, gravel, or silt in which it lies partially buried.  It does this by repeatedly advancing the foot through the sand, gravel or silt, expanding the end so it serves as an anchor, and then pulling the rest of the animal with its shell forward.  It also serves as a fleshy anchor when the animal is stationary.

In marine mussels, the foot is smaller, tongue-like in shape, with a groove on the ventral surface which is continuous with the byssus pit.  In this pit, a viscous secretion is exuded, entering the groove and hardening gradually upon contact with sea water.  This forms extremely tough, strong, elastic, byssus threads that secure the mussel to the ground.  The byssus thread is also sometimes used by mussels as a defensive measure, to tether predatory molluscs, such as dog whelks, that invade mussel beds, immobilizing them and thus starving them to death.

In cooking, the byssus of the mussel is known as the “beard” and is removed before the mussels are prepared.