Currently reading : Blue Blooded Limulidae
A. Artaud, To Have Done with the Judgement of God in Selected Writings, p.566
Limulidae is the only recent family of the order Xiphosurida and contains all the 4 living species of horseshoe crabs. Unlike humans, horseshoe crabs do not have haemoglobin in their blood, but instead use hemocyanin to carry oxygen. Because of the copper present in hemocyanin, their blood is blue.