In leeches, there is no true blood vascular system, the coelomic space and fluid have been
modified to form the circulatory system. Hence, it is called haemocoelomic system and the coelomic fluid is called haemocoelomic fluid. Blood is red due to the presence of respiratory pigment haemoglobin or erythrocruorin found dissolved in the plasma. Free amoeboid blood corpuscles are present, but there are no red blood corpuscles.