Although members of the kingdom Animalia are almost exclusively multicellular, there are several unicellular organisms classified as animals, including species of bacteria, protozoa, fungi, algae and archaea. Bacteria are unicellular organisms of microscopic size that contain no nucleus. They are hardy life forms that can survive in extreme environmental conditions.
Protozoa are typically unicellular organisms of microscopic size that are larger than bacteria and contain a nucleus. Protozoa are found in freshwater, salt water and soil. Protozoa move through their environments using cilia or a flagellum. Cilia are tiny, hair-like structures that propel some protozoa using wave-like motion. A flagellum is a long, whip-like protrusion that propels some protozoa forward through their environment. Some parasitic species of protozoa can cause severe illnesses such as malaria and sleeping sickness.
Fungi are unicellular or multicellular organisms that form the basis of mushrooms, yeasts and molds. A single fungus cell is called a spore.
Algae are unicellular or multicellular organisms that are typically found in aquatic environments. Unicellular examples include chlorella and diatoms.
Archaea are unicellular organisms of microscopic size that contain no nucleus. Like bacteria, archaea can survive in environmental extremes. Archaea were not classified as a distinct group of organisms until the 1970s.