 Unlike many animals that live in tubes, phoronids do not ventilate their trunks with oxygenated water, but rely on respiration by the lophophore, which extends above hypoxic sediments.The blood has hemocytes containing hemoglobin, which unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments.The blood of "Phoronis architecta" carries as much oxygen per cm3 as that of most vertebrates; the blood's volume in cm3 per gm of body weight is twice that of a human.