Protozoans are microscopic in size however some are large enough to be seen with naked eye. Microscopic organisms such as these have numerous advantages, one of which is that they have a high surface-to-volume ratio. That's an advantage since diffusion is an important process for many of their functions. Don't let their small size fool you; these are very complex organisms. Everything they do, and how they do it, is packed into a single cell. In case of animals, organs and tissues become specialized for specific functions; for protozoans, it's their organelles that take on the same functions.