Spiral-shelled mollusks are a type of aquatic invertebrate that includes two main species: Nautili and Snails. Nautili are cephalopods, meaning they have a head and tentacles, and their shells are coiled in a spiral shape. Snails are gastropods, meaning they have a single shell and a muscular foot, and their shells are also coiled in a spiral shape. Both species are found in oceans and other bodies of water around the world.