Q.Distinguish between: Open and closed circulation.

Every organism has to move material — nutrients in, wastes out — between its cells and its surroundings, and the solution scales with body complexity. Where the body's surface area is large relative to its volume, simple diffusion and active transport between cells suffice, helped along within single cells by cyclosis, the streaming movement of cytoplasm (seen in Paramoecium, Amoeba, root-hair cells, and animal WBCs) — this is intracellular transport. Slightly more complex organisms use extracellular transport without a full circulatory system: sponges and coelenterates circulate surrounding water through body cavities directly, flatworms have parenchymal circulation through their tissue, and roundworms move body fluid around their organs purely by contracting their body wall muscles, without any blood vessels.

Larger, more active animals need a dedicated blood vascular system — a heart pumping blood (and, separately, lymph) through vessels — which comes in two layouts. In open circulation (arthropods like the cockroach, and molluscs), blood empties into open body cavities (haemocoels) and bathes organs directly, flowing at low pressure and usually without a respiratory pigment, so it exchanges material with tissues directly but doesn't transport respiratory gases efficiently. In closed circulation (all vertebrates, higher molluscs and annelids), blood stays confined to a continuous network of vessels, flows at high pressure, carries a respiratory pigment like haemoglobin, and exchanges material with tissues only indirectly, via an intermediate fluid, lymph.

Closed circulation itself splits into single circulation, found in fish, where blood passes through the two-chambered heart only once per cycle (deoxygenated blood to the gills for oxygenation, then out to the body and back to the heart deoxygenated — hence the fish heart is called a 'venous heart'), and double circulation, found in birds and mammals including humans, where blood passes through the four-chambered heart twice per cycle: once through the pulmonary circulation (deoxygenated blood to the lungs and back oxygenated) and once through the systemic circulation (oxygenated blood to the rest of the body and back deoxygenated).