Protozoa are unicellular eukaryotes that seem to live as parasites in all species of multicellular organisms, and some even parasitise other unicellular organisms. Some are intracellular parasites, while others live in extracellular locations such as body fluids, blood or lumen of the host animal. They phylum Protozoa is divided into four classes, of which three are important.
1. Class Mastigophora – consists of flagellates that are equipped with whip-like locomotor structures. The arrangement and number of these flagella are used in classification. Some of the flagellates are classified as plants. Indeed subclass Phytomastigina contains plant like flagellates, which have pigmented bodies called chromoplasts, or chromatophores in which photosynthesis occurs. Like other flagellates, the Phytomastigina move by one or more flagella. Some of them form colonies, thus suggesting an evolutionary trend towards multicellularity.
Subclass Zoomastigina contains animal-like flagellates. They lack chromatophores and are divided into several orders:
Order Rhizomastigida consists of flagellates that move by amoeboid movement, using pseudopodia.
Order Protomastigida is made up of simple flagellates. Included in this group is the family Trypanosomidae that includes disease-causing genera Trypanosoma, Schizotrypanum and Leishmania.
Order Polymastigida contains uninucleate and binucleate species as well as some with polynuclei. There may be as many as five to eight flagella; all must have at least three. This Order includes the family Hexamitidae, to which the intestinal flagellate Giardia, belongs
Order Trichomonadida consists of either uninucleate or multinucleate flagellates with internal rod-like axostyle and a flagella complex of 3 to 6 flagella and a prominent parabasal body. The group contains a number of parasites of man and animals, including Trichomonas vaginalis.
Order Hypermastigida consists of uninucleate flagellates with many flagella. These are symbiotic parasites of wood eating termites
Order Opalinida includes parasites of amphibians.
2. Class Sarcodina- includes free-living and parasitic amoebae of the order Amoebida. Family Endomoebidae contains the genus Amoeba that is parasitic in man. This includes Entamoeba histolytica, E. coli and E. gingivalis.
3. Class Ciliata contains ciliated organisms. The cilia are used for locomotion. Ciliates possess two kinds of nuclei - the macronucleus and the micronucleus. The macronucleus interacts with the cytoplasm in the control of metabolism and growth, while the micronucleus functions in reproduction .The only ciliate parasitic to man is Balantidium coli.
4. Class Sporozoa consists of spore-forming protozoa that are enclosed in tough membranes that are quite resistant to adverse conditions, and can be dispersed by wind and water. They also produce sporozoites that can only survive in the host. Included in this group are malaria parasites and coccidia.