The function of hemoglobin-containing red blood cells in O₂ transport and storage was examined in a tube-dwelling marine animal, Phoronis architecta. The maximal rate at which O₂ could be transported by the circulation in one animal (≈ 0.014 g) was 0.0018 μp mol O₂ · h⁻¹ in physical solution and 0.054 μmol O₂ · h⁻¹ by hemoglobin (both computed at Po₂ = 150 mmHg). Oxygen consumption rates of an average animal ranged from 0.005 to 0.049μmol O₂ · h⁻¹ at external O₂ pressuresfrom 2 to 150 mmHg, respectively. These data indicate that augmented transport of O₂ by hemoglobin is necessary under all conditions for the animal to utilize its aerobic metabolic capacity. Although the animal is surrounded by anoxic sediment and some O₂ loss is inevitable, the geometric arrangement of the O₂ source (red blood cells) relative to the O₂, sink (consuming tissue plus anoxic sediment) and the O₂-unloading properties of the hemoglobin allow the animal to utilize a substantial quantity of the O₂ transported by the hemoglobin. ...