ABSTRACT
Protozoa are single-celled eukaryotic microorganisms that do not form a natural group but are placed together essentially for convenience. They are ubiquitous in natural waters and moist soil, and are found as symbionts, endocommensals, or parasites of most animals and many plants. They are separated into major taxa based primarily on microscopic features such as flagella, cilia, and pseudopods, and then further defined by ultrastructural morphology, uniqueness of life cycle, host specificity, biochemistry, and molecular genetics.
