In Octopus and Sepia the inhalant water stream is sucked in to each side of the mantle cavity and passes into the centre of the corresponding gill. From here it runs counter-current to the blood vessels in the secondary gill lamellae and out into a common cavity created by expansion of the posterior part of the mantle. On exhalation the flow runs centrally, bypassing the gills and out through the funnel. This paper deals with the anatomy that ensures this pattern of flow, and with measurement of the POO2 at various points in the mantle in normoxia and hypoxia. Utilization of oxygen from the respiratory stream is often better than 50% and the POO2 of the exhalant water is regularly lower than the likely arterial POO2.

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