Class Bivalvia (aka Pelecypoda - "hatchet-foot") is a group of molluscs that differ considerably from the ancestral mollusc. Members of this class have a shell that is divided in two along a dorsal midline, and held together by a hinge with interdigitating teeth and a flexible ligament. The two valves close together by contraction of the adductor muscles. The shell encompasses a body that is compressed laterally (from the side) and has an extended dorsi-ventral axis and a reduced lateral axis. This compression of the body is seen in changing the foot from a broad, flat, creeping structure to a tapering, blade-like structure for digging. The foot no longer supports the visceral mass, which is now laterally compressed and suspended from the dorsal axis of the shell. The two shell plates and their underlying mantle enclose the foot and visceral mass, creating a large mantle cavity space on either side. Besides the foot and visceral mass the bivalve's greatly enlarged ctenidia are also suspended in the mantle cavity, effectively dividing it into a lower incurrent chamber and an upper excurrent chamber. The head is greatly reduced, consisting of only the mouth; the radula is missing.

Observe the bivalve shells on display. Familiarize yourself with the dorsal and ventral orientation and muscle scars on the inside of the shells. What are their functions? Notice that species that burrow into substrata have wedge-shaped shells that are more easily pulled into sand by a digging foot. Examine a live clam for the pair of siphonal openings at the posterior end. The siphons are formed from mantle tissue and are continuous with the lining of the mantle cavity. They carry water into and out of the mantle cavity.

(B) Filter-feeding with a ctenidium

Open the shells of a living mussel. Place the mussel in a dish of sea water. Find the large ctenidia, reduced foot (why reduced in the mussel?) and compressed viscera. Bivalve molluscs are filter-feeders. The ciliated ctenidia with its current-producing ability preadapted bivalves to use gills not only for respiration but to trap food suspended in water carried in by the respiratory current. Water enters the incurrent or ventral siphon, passes through the gills where suspended food is trapped on the gill surface, and ultimately exits the dorsal or excurrent siphon. A healthy mussel can filter several liters of water per hour. Imagine what a whole clam bed or mussel clump can do in a day. In a year! No wonder marine animals produce so many larvae - think how many are lost to these filter-feeders.

What happens to the trapped food? The cilia on the mussel's ctenidia are organized in complicated tracks. Obtain some suspended carmine (dye) particles in a dropper and squirt a few drops near the mantle cavity. Wait a few minutes and view the gills with your dissecting microscope. Note the red carmine particles bound in mucous strings and being carried anteriorly along the margins of the gills toward the sorting palps and ultimately into the mouth. Ask your instructor how the bivalve pulls the mucous-food strand into its mouth.

That is how a bivalve mollusc feeds. It has abandoned the radula and motile existence to become a sedentary or sessile filter-feeder. Have they been successful? There are 20,000 extant species of bivalves. Although chiefly marine, there are freshwater members. In the sea bivalves occur in virtually every soft-sediment habitat from intertidal mud flats to deep ocean. They have also invaded rock, wood, coral and shells of other animals. Many also live openly on the surface like mussels and oysters, attached by stout threads or cemented to the hard bottom. Some, like scallops, move about freely using their mantle cavity as a jet-propulsion chamber. Not bad for no head!