What is diffusion? Name the animals in which excretion takes place by diffusion.

Diffusion is the movement of molecules from a region of high concentration to lower concentration leading finally to the uniform concentration everywhere. In some unicellular organisms like Chlamydomonas-(algae), Amoeba and Paramoecium excretion takes place by diffusion. In these unicellular organisms, elimination of metabolic wastes takes place by diffusion from the body surface into the surrounding water.