Although much study of oxygen transport and utilization in animals has centered on the hemoglobins of vertebrates, a very large number of invertebrate marine species utilize very different proteins, the hemocyanins. Both molluscs and arthropods possess these proteins; in each phylum they utilize binuclear copper centers as oxygen binding sites, rather than an iron heme. The molecular structure, and mechanisms of regulation of oxygen binding appear to be quite different in the two phyla. We have concentrated on the hemocyanin of the cephalopod, Octopus dofleini. The functional molecule in the blood is immense, with molecular weight of about 3.6 × 106; this is comprised of ten identical subunits, each containing 2898 amino acid residues. We have recently cornpleted the c-DNA sequencing of this chain. It is divided into seven neatly equivalent domains, each containing one oxygen binding site. The structure of one such domain has recently been determined in collaboration with the laboratory of Dr. W. Hendrickson. The seventy oxygen binding sites in this molecule make it one of the largest allosteric assemblies known. Exactly how such unusual proteins serve in facilitating oxygen transport to the tissues of the major invertebrates remains a matter of continued study. For a comprehensive review, see van Holde and Miller, 1995.