Why size decides the system
In a very small or thin organism, every cell lies close to the surface. Materials such as oxygen, glucose and wastes move fast enough by diffusion alone, so no transport system is needed. A single Amoeba or a flat Planaria works this way. As an animal grows larger and more layered, inner cells are too far from the surface, and diffusion becomes too slow. Such animals need a circulatory system to carry substances quickly over long distances.
Open and closed systems
In an open circulatory system, a fluid called haemolymph is pumped out of the heart into large body spaces (the haemocoel) where it bathes the tissues directly, then drains back to the heart. Blood and tissue fluid are not separated, and the pressure is low. Insects, prawns and other arthropods use this design. In a closed circulatory system, blood stays inside blood vessels the whole time, exchange happens across thin capillary walls, and the pressure is higher and better controlled. Earthworms, fish and all vertebrates use closed systems.
Example
A grasshopper's haemolymph does not carry oxygen — its tissues get oxygen from a separate tube (tracheal) system. This shows that an open system can be efficient for a small, active insect even without transporting respiratory gases.
Single versus double circulation
Among closed systems, the path through the heart differs. In single circulation, blood passes through the heart once in each complete circuit. A fish has a two-chambered heart; blood flows heart → gills → body → heart. Pressure drops after the gills, so flow to the body is slower. In double circulation, blood passes through the heart twice per circuit through two loops: the pulmonary circulation (heart ↔ lungs) and the systemic circulation (heart ↔ body). Mammals and birds have a four-chambered heart that keeps oxygenated and deoxygenated blood fully separate and restores high pressure before sending blood to the body.
Key idea
Diffusion suits tiny bodies; open systems suit many invertebrates; double circulation with a four-chambered heart gives active mammals fast, high-pressure delivery of oxygen.