An integrated description of the respiratory system of the abalone Haliotis iris is presented. These animals are believed to be inherently primitive and still bear the ancestral gastropod gill arrangement, thus allowing physiological examination of a 'living fossil'. Ventilation, gaseous diffusion, blood transport and the anatomical arrangement of the vascular system are examined under a range of conditions. Resting H. iris consume an average of 0.47 µmol 0₂.g live weight⁻¹ .h⁻¹, 87% of which is taken up across the gills, the remainder diffuses directly into the foot and epipodium. A 300g abalone ventilates its gills at a rate of 28mL.min⁻¹, a rate which, due to low resistance to diffusion (diffusion limitation index = 0.47) and a well matched ventilation/perfusion conductance ratio, is adequate to support the quiescent animal. Increased oxygen demand is accommodated by an increase in cardiac stroke volume, elevating output from 9.1 to 24.4 µL.g⁻¹.min⁻¹. At rest the right gill is the predominant gas exchanger, receiving 95.7% of the branchial blood flow, when cardiac output is elevated the left gill becomes equally perfused, effectively doubling the diffusing surface. Ventilation does not increase, and an increased reliance on assistance from external water currents is seen. Previously undescribed components of the vascular system, notably an extensive sinus of mixed venous and arterial blood surrounding the gut and a large vessel that offers a bypass to the right kidney, provide a low resistance circuit between the heart and gills, bypassing the major organs and muscles. The low resistance circuit allows haemolymph to pass from the aorta to the base of the gills with minimal loss of pressure and no phase shift in the pulse, allowing blood to cross the gills with maximal inertia and instantaneous pressure gradient. Haliotis iris therefore appears to have exploited its limited physiological resources to the maximum in the routine operation of its gas exchange system. It is concluded that further improvement could not occur without substantial remodeling of the body plan, which may account for the abandonment of the system by higher gastropods.