General Characteristics of Protozoans

The protozoans, are eukaryotes that form a group of about 80,000 single-celled organisms under the kingdom Protista. The unicellular level of organization is the only character which is common to all members of Protozoa. In all other respects they display extreme diversity. Protozoans exhibit all types of symmetry and great range of complexity in their microanatomical structure.

The great majority of protozoans are microscopic. They range in size from one micron as in the case of the planktonic Micromonus to a few millimeters like some Amoeba species and ciliates. Most protozoans occur as solitary individuals but there are numerous colonial forms, for example Volvox. This is the earliest indication of division of labour among cells.

Protozoans are found wherever life exists. Free living protozoa occur in the sea, in various types of freshwater bodies and in the soil. There are also commensals, mutualistic and many parasitic species. Nutrition may be autotrophic, heterotrophic or saprozoic. Reproduction may be asexual by mitotic division through budding, fission and cyst formation. Sexual reproduction by conjugation or zygote formation (syngamy) is found in some species. Various means of locomotion through pseudopodia, flagella and cilia and direct cell movement have evolved in this group. A protozoan, although unicellular, must be recognized as being a complete organism, carrying out all functions found in any multicellular animal. Apart from the usual intracellular structures common to all cells, protozoans possess specialised organelles differentiated for performing specific functions.