Modes of Respiration
Many small organisms obtain oxygen by diffusion through their body surfaces. They do not have any specialised respiratory organs nor do they have blood circulation. Larger and more complex organisms, however, need specialised surfaces for gas exchange and a circulatory system that transports oxygen more readily than that possible by simple diffusion. Calculations based on metabolic demands and rate of diffusion in protoplasm show that simple diffusion is sufficient only to meet the demands of organisms not larger than 1 mm in diameter. These calculations appear reasonable when we see that animals like protozoans and flatworms that meet their respiratory requirements through diffusion are either quite small or have very low metabolic rates.
Giant land planarians may be 50 cm long but they are flat with very large surfaces in relation to mass, therefore, diffusion is sufficient to meet their oxygen demand. Coelentrates, corals and sponges often reach very large sizes but have very modest metabolic demands. Sponges and corals maintain a circulation of water by cilia over the surfaces of cells which line their canal systems. Thus sufficient gas exchange takes place without the aid of a circulatory system or respiratory pigments.