Protozoa are unicellular eukaryotic microorganisms lacking a cell wall and belonging to the Kingdom Protista. The vegetative, reproducing, feeding form of a protozoan is called a trophozoite. Under certain conditions, some protozoa produce a protective form called a cyst that enables them to survive harsh environments. Cysts allow some pathogens to survive outside their host.
Thumbnail: A "Giant Amoeba", Chaos carolinense. Image used with permission (CC BY-SA 2.5; Dr.Tsukii Yuuji).
Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)