Freshwater molluscs include freshwater snails and freshwater bivalves. Other mollusks include gastropods, scaphopods and bivalves. Mollusks are mostly represented by gastropods and bivalves. Prey include crustaceans, bivalves, worms and copepods. Other invertebrates include ostracods, isopods, conchostracans, and bivalves. Seashells, namely from bivalves and gastropods, are fundamentally composed of calcium carbonate. They are molluscs, meaning they are related to slugs, snails and bivalves. Nacre is found in some of the most ancient lineages of bivalves, gastropods, and cephalopods. Dense flattened teeth are used to crush prey like bivalves and crustaceans. Black corals around the world provide a unique environment for crustaceans, bivalves, and fish. Most species are generalist predators, eating microalgae, sponges, bivalves, snails and other small animals. Shells of bivalves and aquatic snails are also common. Some annelids and bivalves also have apposition eyes. It cultures marine fish, microalgae, bivalves and Artemia. The generalized mollusc has two paired nerve cords, or three in bivalves.