Rotifer with Paramecium protozoa, light microscopy footage. Rotifers (one at centre) are microscopic aquatic animals in the phylum Rotifera. They are related to roundworms and use hair-like cilia surrounding their mouth to trap food particles. Paramecium (moving in the water) are unicellular ciliate protozoa. They inhabit fresh water, and feed mainly on bacteria and smaller protozoa. They range from about 50 to 350 micrometres in length.