Describe how they feed. The class is defined by having a shell in two parts, or two valves. One distinguishing characteristic of the bivalves is that they have lost their heads. Lacking this basic landmark, many people find bivalve anatomy rather confusing. The siphons stick out of the posterior end.
Some bivalves live on their sides.
You'll do this by examining both dissected specimens and the shells. Study the bivalve dissections on display. Feel free to use a list, table, or paragraph. In the ancestral molluscan condition, the ctenidia function in gas exchange, and cilia on the ctenidia move water through the mantle cavity. A correlated evolutionary change in the bivalves has been the loss of the radula.