Chlorocruorin is a dichroic red-green respiratory protein. It is chemically similar to haemoglobin, and is only found dissolved in the blood of certain marine annelid worms. Chlorocruorin is the characteristic blood pigment of the Serpulimorpha (serpulids and sabellids), but in the genus Serpula both chlorocruorin and haemoglobin are present together in the blood. This is the first time that two respiratory pigments have been found in the blood of one animal. Young individuals have relatively more haemoglobin, older ones more chlorocruorin. Within the serpulid genus Spirorbis, one species has chlorocruorin in its blood, another has haemoglobin, while a third has neither pigment. As their habitats are similar, no functional explanation for these differences suggests itself. The oxygen affinity of all chlorocruorins tested is considerably lower than that of most haemoglobins. But in Serpula the oxygen affinities of the chlorocruorin and haemoglobin are the same as one another. The carbon monoxide affinity of chlorocruorin (in Branchiomma) is higher than that of any haemoglobin. Although Serpulimorpha have chlorocruorin in their blood, the haem present in their tissues (muscles, eggs, sperm) is protohaem, not chlorocruorohaem. One genus, Potamilla, with chlorocruorin in its blood, has haemoglobin in the muscles. Chlorocruorin is known only from blood, and from the mucous tube of Myxicola; none has been found in cells. Coelomic fluid contains none. Protohaem is secreted into the protective tubes of both serpulids and sabellids. A protohaemochromogen is present in the gut fluid of serpulids, recalling that found in crustaceans and molluscs.