Clams are an example of a bivalve mollusk. Oysters, mussels, and scallops are also bivalve mollusks. All bivalve mollusks have a two-piece hinged shell as a hard, external covering for the invertebrate located inside. The animal breaths through gills, and they are filter feeders, which means they collect food through their gills. Bivalves make their own shells by secreting calcium carbonate, allowing them to build “additions” to their own coverings when they need larger ones. The ridges on the shell, from which you can how many layers have been added, will allow you to determine the clam’s age. The United States harvests over $1 billion of bivalve mollusks each year.