The basic taxonomic character for this group is the absence/presence and type of motile organelles
Sarcodina e.g. Entamoeba
Matsigophora e.g. Trypanosoma, Trichomonas, Leishmania, Opalina, Giardia
Ciliophorans e.g. Balantidium
Apicomplexa e.g. Toxoplasma, Plasmodium, and Eimeria.
Biology (Characteristic Features)
Protozoans are generally unicellular, microscopic eukaryotic organisms. They are the first animals with membrane-bound nucleus, as well as heterotrophism. A considerable number of these organisms have motile organelles; while majority have at least one stage of their life cycle being motile.
Parasitic Protozoology ( Protozoans of parasitic importance)
These include Entamoeba spp (E. histolytica, E. coli, E. gingivalis), Opalina ranarum, Trypanosoma (T. brucei brucei, T. brucei rhodesiense, T. brucei gambiense, and T. cruzi), Trichomonas (T. vaginalis), Leishmania (L. donovani, L. aethiopica), Giardia lamblia, Balantidium coli, Toxoplasma gondii, Plasmodium (P. falciparum, malariae, ovale, and vivax), and Eimeria.
This is generally asexual, involving binary fission, multiple fissions (schizogony, Merogony, Sporogony, Polyembryony), budding, and sexual (syngamy, autogamy, and conjugation). Many of these organisms have direct life cycle (wherein parasites undergo complete development within the host), while some others have part of their life cycle completed outside the host; this could be a substratum, intermediate host and/or vectors.
Many of these parasites are found in nutrient-rich sites such as the GIT (especially the small intestine) liver, blood plasma, blood cells, and other tissues Epidemiology, Control and Management.