I. arthropods and molluscs. In this type

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Water circulation system:

Water is the medium of transportation. Eg. sponges (water canal system), Hydra (gastro-vascular system) and star fish (ambulacral system).

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II. Blood vascular system:

Blood is the medium of transportation in association with pumping organ (heart). It is of two types: open and closed circulatory system. (a) Open circulatory system: E.g.

arthropods and molluscs. In this type of system, there is neither a true heart nor capillaries as are found in humans. Instead of a heart there are blood vessels that act as pumps to force the blood along. Instead of capillaries, blood vessels join directly with open sinuses. “Blood”, actually a combination of blood and interstitial fluid called haemolymph, is forced from the blood vessels into large sinuses, where it actually baths the internal organs.

While the spaces and lacunae are-together called haemocoel. Other vessels receive blood forced from these sinuses and conduct back to the pumping vessels. Blood flows at low pressure. This is a very inefficient system. Insects can get by with this type system because they have numerous openings in their bodies (spiracles) that allow the blood to come into contact with air. (b) Closed circulatory system: E.g.

annelids (earthworm), some molluscs (such as squid) and all vertebrates. It was discovered by William Harvey (1628). In this blood is pumped through a closed system of heart, arteries, veins and capillaries without coming in direct contact of body tissues or body cavity. Capillaries surround the organs, making sure that all cells have an equal opportunity for nourishment and removal of their waste products. Blood flows at high pressure. Circulatory system of human being is of closed type.

It consists of: (i) Heart (pumping organ) (ii) Arteries and arterioles, and venules (bring blood back to heart) (iii) Capillaries (connecting arterioles and venules).


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