The nautilus is found in some medieval encyclopedias. It is a fish that comes to the surface of the ocean and forces the water out of its body through a tube, then raises a thin membrane between its first two arms to use as a sail, while it uses its other arms to row. It uses its tail like a rudder to control its direction. If it becomes fearful it sucks in water and sinks. The name nautilus is from a Greek word meaning "sailor".

In modern usage the name nautilus refers to a shelled ocean mollusc, but it originally referred to a type of octopus in the Argonauta genus, also known as the paper nautilus, which was thought to use its arms as sails.