a green respiratory pigment that replaces hemoglobin in some polychaetes, specifically, the worms of the families Sabellidae, Serpulidae, Chlorhaemidae, and Ampharetidae. The blood of some of the worms contains both chlorocruorins and hemoglobins. Hemoglobins predominate in young individuals of the genus Serpula, while chlorocruorins predominate in the adults. Blood containing chlorocruorins is red when discharged from the blood vessels and green when diluted.

Chlorocruorins differ from hemoglobins in the chemical structure of the protein and nonprotein parts of the molecule. They contain heme, in which the vinyl group (CH=CH) of protoporphyrin in position 2 is replaced by the formyl group (CHO). In the blood plasma the chlorocruorins are in a dissolved state and, like erythrocruorins, have a high molecular weight (2,800,000). Chlorocruorins and hemoglobins have the same affinity for oxygen.