The protozoa are defined as a diverse group of unicellular eukaryotic organisms. Historically, protozoa were defined as single-celled animals or organisms with animal-like behaviors, such as motility and predation. But the modern ultrastructural, biochemical, and genetic techniques have shown that protozoa, as traditionally defined, belong to widely divergent lineages, and can no longer be regarded as “primitive animals”. For this reason, the terms “protists”, “Protista” or “Protoctista” are sometimes preferred for the high-level classification of eukaryotic microbes.
Protozoa, are mainly microscopic organisms, ranging in size from 10 to 52 micrometers. Some, however, are significantly larger. Organisms traditionally classified as protozoa are abundant in aqueous environments and soil, occupying a range of trophic levels. The group includes flagellates (which move with the help of whip-like structures called flagella), ciliates (which move by using hair-like structures called cilia) and amoebae (which move by the use of foot-like structures called pseudopodia). Some protozoa are sessile, and do not move at all.
Many protozoa alternate between a free-living vegetative form known as atrophozoite and a resting form called a cyst. The protozoal cyst is somewhat analogous to the bacterial spore, since it resists harsh conditions in the environment. Many protozoal parasites are taken into the body in the cyst form.
All protozoal species are assigned to the kingdom Protista in the Whittaker classification. The protozoa are then placed into various groups primarily on the basis of how they move. The groups are called phyla (singular, phylum) by some microbiologists, and classes by others.
Recent Protozoan Classification
- Phylum Chlorophyta
Examples: Chlamydomonas, volvox
- Phylum Atostlata
Examples: Dientamoeba, Trichomonas
- Phylum Euglenozoa
Example: Euglena, Leishmania, Trypansoma
- Phylum Retortamonada
Examples: Giardia, Entermonas, Spironucleus, Trigonomas
- Phylum Apicomplexa (formerly sporozoa)
Examples: Gregarina, Monosystis , Babesia, Cyclospora, Cryptospordidium, Emeria, Toxoplasma, Plasmodium
- Phylum Ciliophora
Examples: Balantidium, Paramecium, Stentor, Tetrahyymena, Trichodina, Vorticella
- Phylum Dinozoa (formerly dinoflagellata)
Examples: Noticiluca, Zooxanthella, Peridinium, Ceratium, Gymodinium
- Phylum Granuloreticulosa
Examples: Globogernia, Vetebranlia
- Phylum Radiozoa
Examples: Actinophyrys. Clatrulina