HaemacyaninH[ae]m`a*cy"a*nin\, n. [H[ae]ma- + Gr.? a dark blue substance.] (Physiol. Chem.) A substance found in the blood of the octopus, which gives to it its blue color. Note: When deprived of oxygen it is colorless, but becomes quickly blue in contact with oxygen, and is then generally called oxyh[ae]macyanin. A similar blue coloring matter has been detected in small quantity in the blood of other animals and in the bile.