Abstract
Members of the subkingdom Protozoa represent a heterogenous group of protists that belong to the kingdom Protoctista (Margulis et al., 1990). Comparative analysis of the ribosomal RNA sequences of protozoa has placed them in an evolutionary branch between the plant and animal kingdoms. These unicellular organisms are an ancient group that arose well before animals, helminths, and yeasts. Protozoa are not monophyletic and diverged very early in evolutionary time and on a scale greater than that estimated for either the plant or animal kingdoms. Protozoa possess at least one motile stage powered by cilia, flagella, or pseudopodia. The vegetative cells of most protozoa are membrane-bound and lack the rigid cell wall structure common to both fungi and plants. The structure of the cell membrane of protozoa is similar to that found in other eukaryotes and generally fits the fluid mosaic model. In many cases the outer membrane surface may be coated with a thin pellicle or glycocalyx.
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