(a)Annelids are the first animals to have a closed circulatory system i.e., the blood flows in blood vessels.
(b)Some blood vessels are enlarged to act as pumping hearts. The main blood vessels, one dorsal and one ventral, have branches in each body segment to the skin, muscles, intestine and other organs.
(c) The blood plasma contains respiratory pigment haemoglobin or erythrocruorin and colourless amoeboid corpuscles.
(d) In leech, there is no true blood vascular system.
(e) The coelomic space called haejnocoelomic system and red coelomic fluid called haemocoelomic fluid constitute the circulatory system.