The shell of Nautilus prevents the mantle from playing any part in creating the ventilatory stream. This is generated instead by movements of the collar and funnel folds which fuse to form flaps (the ‘wings’), overlapping below and joined to the head above. The gills lie horizontally, dividing the space enclosed by the wings into three cavities, two lateral and prebranchial, on either side of the head, and a common ventral postbranchial space. Water is drawn in above, behind the eyes, and expelled forward through the siphon of the funnel, which is used both for ventilation and jet propulsion. The pressures driving the ventilatory stream are small (of the order of 0.1 kPa), but the complex movement of the wings, described below, is such that there is (very nearly) always a pressure differential and a water flow across the gills, despite a pulsed intake and outward jet. Oxygen extraction is low by the standards of other cephalopods, only 5–10%, falling during jet propulsion and rising (exceptionally to 40%) at rest after exercise. Ventilation frequency, 35 min−1 at 16°C, rises with temperature. Ventilation stroke volumes ranged from 5 to 22 ml for an animal of 395 g. At 17 °C Nautilus can regulate its oxygen uptake down to a Po2 of about 75 mmHg. Uptake at rest ranged from 0.22 to 0.46 ml kg−1 min−1 (exceptionally 0.75 ml kg−1 min−1 after feeding) for animals of 351–395 g. In terms of flesh weight, this yields an average of 0.50mlkg−1 min−1, half to one-third of the uptake one would expect from coleoids of similar weights and at similar temperatures.

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