- Protozoa are unicellular organism’s that are animal-like in being heterotrophic and motile. In all other respects, protozoa are a very diverse assemblage, and the major groups are now commonly treated as separate phyla of eukaryote protistans. The old phylum name Protozoa can be used as a convenient term of reference for any member of these phyla.
- Most protozoa inhabit the sea or fresh water, but there are m any parasitic, commensal, and mutualistic species.
- In contrast to metazoans, complexity in protozoa has proceeded through development and specialization of organelles or skeletal structures. Although a protozoon is a single cell, it is also a complete organism .
- Digestion occurs intracellularly within a food vacuole, and food reaches the vacuole through a cell mouth or by engulfment.
- Excess water is usually eliminated by a contractile vacuole.
- Most of the members of the protozoan phyla are distinguished, in part, by their type of locomotor organelles: flagella, pseudopodia, or cilia.
- Reproduction by fission occurs at sometime in the life history of almost all protozoa. Meiosis, gamete formation, and fertilization have been observed in many species, but the nature of these events and their occurrence in the life cycle of the organism is highly variable. Encystment is common.