The nautilus (from the ancient Greek word meaning ‘sailor’) is a pelagic marine mollusk – a distant cousin to squids, octopus, and cuttlefish – and the only member of the family Cephalopoda to retain an external shell. Nautiluses are considered living fossils, who have been around for about 500 million years – long before the dinosaurs.
The “tentacles” of the nautiluses are called cirri, composed of long, soft, flexible appendages that have no suckers, and are retractable into corresponding hardened sheaths. Unlike the 8-10 arms of octopus and cuttlefish, nautili can have anywhere between 60 to 90 cirri. But what makes these animals unique i s undoubtedly their shell! Nautilus shells are complex floating homes made up of a series of chambers connected by the siphuncle – a tube made of tissue. A newborn nautilus starts life with four chambers, adding more and more as it grows (adults have 30 on average). The animal lives in the largest chamber, while the other chambers function like the ballast tanks of a submarine (in fact, this is what inspired its design!): the chambers are filled with gas to stay afloat – to sink, the nautilus simply floods them with water.