The blood of a cuttlefish is an unusual shade of green-blue because it uses the copper-containing protein haemocyanin to carry oxygen instead of the red iron-containing protein haemoglobin that is found in vertebrates. This is similar to the blood of arthropods. Haemocyanin is not so good at carrying oxygen as is haemoglobin. The blood is pumped by three separate 'hearts'. Two of these are used for pumping blood to the cuttlefish's pair of gills (one heart for each gill), and the third for pumping blood around the rest of the body.