The first on our list is a class of single cell organisms called Protozoa. Chances are you haven’t actually seen one of these with your naked eyes, they are far too small. Protozoa are typically between 10 and 52 micrometers in length. There is some debate in scientific circles whether these primitive organisms can actually be classified as animals but they are some of the strangest creatures around. They sustain themselves using chemical processes such as photosynthesis or even old fashioned predatory techniques for those that prey on other tiny organisms.