The Portuguese Man O' War is named for its air bladder, which looks similar to the sails of the Portuguese fighting ship (Man of war) Caravela redonda (an armed 4-sail caravel), of the 14th and 15th centuries. The Portuguese Man O' War has an air bladder, known as the pneumatophore or sail, that allows it to float on the surface of the ocean. It has no means of propulsion and is pushed by the winds and the current. The sail is filled with air, but may build up a high concentration of carbon dioxide (up to 90%). The bladder must stay wet to ensure survival; every so often it may roll slightly to wet the surface of the float. To escape a surface attack, the pneumatophore can be deflated allowing the Man O' War to briefly submerge. The Portuguese Man O' War is often confused with a jellyfish, which is incorrect.