The transport of oxygen in the hemolymph of many arth-ropod and mollusc species is mediated by large copper-proteins that are referred to as hemocyanins. Arthropod hemocyanins are composed of hexamers and oligomers of hexamers. Arachnid hemocyanins usually form 4·6-mers consisting of seven distinct subunit types (termed a–g), although in some spider taxa deviations from this standard scheme have been observed.