Mollusca are triploblastic animals that usually have shells and are not always hermaphroditic. They have an open circulatory system except for cephalopods. Scaphopoda have tubular shells shaped like tusks. Bivalves like oysters and clams have two shells connected by adductor muscles and ligaments. Powdered mollusk shells are used to treat gastritis. Nautiluses are the only cephalopod that has an external shell. Gastropods use their foot to move. Bivalves' two shells are called valves. Mollusks can change color due to chromatophores in their skin.