Like all cephalopods, cuttlefish have three hearts. Two of its three hearts are used to pump blood to the cuttlefish's large gills, and the third is used to circulate oxygenated blood to the rest of its body. The circulatory system of cuttlefish is closed, unlike other mollusks, but consistent with other cephalopods and vertebrates. The blood pumped through the cuttlefish's heart is blue-green in color because, like its cephalopod relatives, it contains a copper-based protein, hemocyanin.