The Chambered Nautilus is the last surviving genus of the nautiloids. The nautiloids were the largest predators in the seas of the Ordovician period, 450 million years ago. The Nautilus is found in the tropical waters of the Indo-Pacific Ocean. Its most familiar characteristic is its smooth, coiled shell, up to 11 inches in diameter. The shell is lined with mother-of-pearl and is separated into a series of progressively larger compartments; the most recent of which is inhabited by the animal. The walls (septa) dividing the chambers are pierced by a tube (siphuncle) connected to the nautilus. Gas and liquid exchange occurs through the siphuncle walls, by means of which the nautilus can regulate its buoyancy. Nautiluses spend most of their time at depths of 600-800 feet, but rise to 200 feet at night to feed.  Interesting Facts: The Nautilus, which is related to the octopus, is the only cephalopod to have a true external shell.  The form of the shell itself has fascinated naturalists, mathematicians and physicists for years.   The spirally coiled, chambered shell is perfectly proportioned mathematically.