Three iron-containing pigments (hemerythrin, hemoglobin,
and the green pigment chlorocruorin) transport oxygen in blood
vessels and coelomic fluid in most annelids, in corpuscles or in
solution in blood. The annelid dorsal blood vessel is contractile
with unidirectional valves, forcing blood through five aortic
arches (“hearts”) that act as pressure regulators and then into
the ventral blood vessel. From the ventral vessel, blood moves to
the digestive tube wall, the body wall, and the nephridia. Blood
carried to the body wall exchanges oxygen and carbon dioxide
through highly vascularized parapodia (in polychaetes)—sometimes
modified into gills—and through the moist body wall,
even through the protective cuticle (Figure B). The continuous
coelom of the leech lacks septa; most circulatory functions
in leeches are carried out by coelomic fluid that is transported
within the contractile channels and sinuses of the coelom itself.
The closed annelid circulatory system, with contractile heart,
blood vessels, and capillaries, is quite unlike the circulation pattern
in arthropods.