They are free-living and thrive where there is water present. They are found in places like soil, ditches, ponds, rivers and even ocean. Some remain attached to plants and rock, while others like to swim about. Protozoa have a membrane, and some have a Pellicle, which is a rigid structure. The membrane encloses to form a food vacuole.
Reproduction plays a major part.
In protozoa it usually occurs by asexual mitosis. Many also have a sexual stage. Whittaker's classification places algae, in the kingdom called Protista. Our microbiology class discussed 13 different species of pathogenic or disease causing protozoa this semester. The 13 species are divided into four classes and the classes are distinguished by the method of locomotion that the protozoa uses.