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A nautilid lives in a compressed spiral shell which is subdivided into chambers by partitions. The animal itself occupies the outer body chamber. All the other chambers are filled with liquid and gas with a higher nitrogen content than the atmosphere. The right liquid/gas balance allows nautilids to float at various depths in the water column. The propulsive force they need for movement is generated by a funnel-shaped tube which protrudes from the shell. By contracting the mantle cavity and closing the inlet, water is forced out, causing the animal to move backwards. This means of propulsion is common to all squids, living or extinct: they have always swum backwards.