<P> Nautilus is the only genus of cephalopod that has a well - developed external shell . Females of the cephalopod genus Argonauta create a papery egg case which sometimes washes up on tropical beaches and is referred to as a "paper nautilus". </P> <P> The largest group of shelled cephalopods, the ammonites, are extinct, but their shells are very common in certain areas as fossils . </P> <P> Empty molluscan seashells are a sturdy, and usually readily available, "free" resource which is often easily found on beaches, in the intertidal zone, and in the shallow subtidal zone . As such they are sometimes used second - hand by animals other than humans for various purposes, including for protection (as in hermit crabs) and for construction . </P> <Ul> <Li> Carrier shells in the family Xenophoridae are marine shelled gastropods, fairly large sea snails . Most species of xenophorids cement a series of objects to the rim of their shells as they grow . These objects are sometimes small pebbles or other hard detritus . Very often shells of bivalves or smaller gastropods are used, depending on what is available on the particular substrate where the snail itself lives . It is not clear whether these shell attachments serve as camouflage, or whether they are intended to help prevent the shell sinking into a soft substrate . </Li> </Ul>

Animals that have shells and live in water