paper nautilus or argonaut, pelagic, surface-dwelling cephalopod mollusk of the genus Argonauta. Like the closely related octopus, the paper nautilus has a rounded body, eight tentacles, and no fins. It is so named for the beautiful papery shell, up to 8 in. (20 cm) long, that surrounds the female while she broods her eggs. This structure, actually a calcareous egg case, is secreted by the tips of the female's two greatly expanded dorsal tentacles prior to egg laying. After she deposits her eggs in the floating egg case, the female takes shelter in it herself; she is usually found with her head and tentacles protruding from the opening, but she retreats deeper inside if disturbed. Air trapped in the egg case is used by the female to maintain neutral buoyancy. The much smaller male, which lacks the modified dorsal tentacles, often shares the shell of a female. It was once believed that the paper nautilus used the expanded tentacles, extended from the shell, as a sail. The true nautilus (genus Nautilus ) belongs to a different cephalopod order. The paper nautilus is classified in the phylum Mollusca, class Cephalopoda, order Octopoda.