Closed Type of Circulatory Systems
Some non-chordates, like cephalopods (octopuses, squids), earthworms, polychactes and all vertebrates have a closed circulation along with blood flowing in a closed circuit of tubes from arteries to veins by capillaries. There is better separation of functions in closed circulatory system as compared to in open system. In a closed circulation the heart is the main propulsive organ, pumping blood into arterial system and keeps a high blood pressure in arteries. The arterial system, in turn, works as a pressure reservoir forcing blood through the capillaries. The capillary walls are thin, so permitting high rates of transfer of material between blood and tissues. A blood returning from the systemic circulation that is directed to the gills.
Much of the returning blood flows through the two lateral vena cava through the kidneys before arriving at the bases of the gills. Near the base of each gill is a bulbous accessory pulsatile organ, the branchial heart that receives the systemic venous blood. This heart pumps the blood into an afferent branchial vessel, through which it passes through capillaries in the gill to the efferent branchial vessel and return to the systemic heart. Physiologically the circulatory systems of cephalopods (squid and Octopus) resemble those of chordates far much more than to those of the other molluscan groups. As for example, both cephalopods and chordates have closed circulatory systems; the blood volume of octopus is comparatively small, about 6% of the body weight, well within the range of that of vertebrates however radically lower than in non-cephalopod molluscs.