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Protozoa Classification System
Committee on Systematics and Evolution of the Society of Protozoologists have revised the either systems of protozoa classification and published a new system in 1980.
In this system, seven phyla are recognised : Sarcomastigophora, Labyrinthomorpha, Apicomlexa, Microspora, Acetospora, Myxozoa and Ciliophora.
In this arrangement, the earlier Sarcodina (Rhizopoda) and Mastigophora phyla are included as subphyla in one phylum, the Sarcomastigophora.
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The other major change is the division of earlier Sporozoa
phylum into four separate phyla, namely, Apicomplexa, Microspora, Acetospora, an Myxospora.
Ciliophora is created which encompasses such slime molds whose trophic (vegetative) phase consists of spindle-shaped cells that glide through channels of an ectoplasmic network.
Mastigophora (Flagellates)
This phylum consists of flagellate protozoa (few produce pseudopodia in addition to flagella) many of which are plant-like (dinoflagellates, euglenoids and some other algal forms) and others animal-like (zooflagellates).
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These protists are generally regarded as the most primitive of all protozoa. The zooflagellates now in existence are largely free-living and phagotrophic, and they generally resemble colourless flagellate algal protists.
Free-living zooflagellates undoubtedly gave rise to many symbiotic forms. They are highly specialized and adapted to specific hosts.
For example, Trichonympha, a wood digesting symbiont living in the gut of termites; Trichomonas, a commensal in the gut of man and other vertebrates;
Trypanosoma, different species of which live parasitically in the bloods of various vertebrates and are transmitted from host through insects; and Astasia.
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Rhizopoda (Amoeboid Protozoa)
These protists, also called Sarcodina, have an amoeboid locomotion generally by pseudopodia, although some may possess flagella.
It is considered that their likely ancestors probably include free-living zooflagellates and algal groups such as Chrysophyta and Dinophyceae.
Since these protozoans generally move by pseudopodia ("false legs"), their body contour, therefore, is ever changing in movement. Most of the rhizopods are free-living whereas some are parasitic. Amoeba is a typical representative of this class.
Entamoeba histolytica, the causative organism of amoebic dysentry in man, is an example of parasitic rhizopod. Some rhizopods develop a protective covering, called 'shell', composed of calcium carbonate.
Many deposits of 'chalk' in nature are said to be due to the deposition of these shells. Arcella, Foraminiferans are other examples.
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Ciliophora (Ciliate Protozoa)
These are undoubtedly the most advanced and structurally, the complex protozoan protists. This large class consists of aquatic phagotrophic protozoans that are characterized by the presence of short hair-like structures called cilia, the organs of locomotion.
The cilia originate from kinetosomes which are components of a complex system of intracellular conductile fibrils.
All ciliates are also uniquely characterized by the possession of two kinds of nuclei, a small 'micronucleus' which functions principally in sexual processes, and a large 'macronucleus' that controls metabolism, development and all other cellular processes.
Paramecium is a typical representative of this class whereas Tetrahymena is one of the simplest ciliate. There are other examples like Stentor, Vorticella, etc. of this phylum which inhabit fresh-water.
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Sporozoa (Spore-forming Protozoa)
This class includes parasitic Protozoan protists, which are generally immotile, develop spores, and reproduce by multiple fission. Plasmodium is undoubtedly the most familiar sporozoan genus.
Four species of Plasmodium, namely, P. vivax, P. malariae, P. falciparum, and P. ovate, cause human malaria;
the female Anapheles mosquito is the specific intermediate host of the protozoan and transfers the latter, the pathogen, to the humans. Other example is Leishmania that causes disease in humans.
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