The hemocyanin-like molecule found in the blood of the most primitive bivalves (protobranchs) reversibly binds O2. Its respiratory properties and its sedimentation behavior are both distinctive. Although its electron-dense image looks like that of the gastropod hemocyanins, its molecular weight differs from those of all other molluscan Hcs and is more consistent with the concept of bivalve hemocyanin as a pair of octopod hemocyanins. Bivalve hemocyanin occurs in the solemyoids as well as the nuculoids, which argues for the integrity of the Protobranchia as a natural taxon. The ancestral bivalve O2 carrier was previously believed to be a simple intracellular hemoglobin, which is found in the less primitive Pteriomorpha. The most obvious interpretation of the present results, however, is that hemocyanin is the primitive bivalve O2 cannier and that it was replaced by the red blood cell, which originated at least twice: once in the pteriomorph bivalves and at least once in other taxa.