Protozoa means first animals, and these creatures were indeed the first animals to impact upon human awareness in the pioneering applications of the microscope over three hundred years ago. The description "first" also of course refers to our belief that the protozoa are the present day descendants of the earliest forms of life -- the ancestors of the modern animals.

Today, the traditional protozoa, along with all the algae and slime moulds, have been absorbed into the kingdom Protoctista -- a much larger group containing all the microscopic unicells which possess a nucleus.

The protozoa of the freshwater environment range in size from about 1mm in the case of Stentor and some of the multinucleate amoebae, down to 5µm or so in the case of the smaller flagellates, and they are extremely varied in both appearance and lifestyle.

Apart from the very smallest, which are apparently able to absorb nutrients directly through their cell wall, they feed upon other micro organisms. The amoebae feed mainly upon bacteria and algal unicells, as do most of the ciliates. Whereas the amoebae ingest food particles by absorbing them with their pseudopodia, the ciliates feed by either actively capturing and engulfing prey organisms, or by using their cilia to create currents in the surrounding water which bring the food organisms to their mouths. The captured organism is enclosed in a food vacuole, a membrane-bound vesicle which moves through the cytoplasm as digestion occurs. Undigested remains are discharged into the surrounding water, usually at a definite location in the organism's outer pellicle.

The freshwater protozoa regulate the water content of their bodies by the expansion and periodic collapse of their contractile vacuole, a vesicle which increases in size as it extracts water from the interior of the cell and collapses to nothing as it expels the extracted water.
The contractile vacuole is not observed in the marine protozoa.

Both in organic pollution of the natural environment and in the biological processing of human and domestic animal sewage, the ceaseless activity of the protozoa -- particularly the colonial ciliates -- in the extraction and digestion of bacteria and other suspended particles is the main element of the natural process by which the water supply is rendered once again fit for consumption by humans and other creatures.
Any change in our environment which threatens the life of a balanced community of protozoans threatens also the continuity of a clean water supply for humans. This is particularly relevant in the light of our current over-use of kitchen and lavatory disinfectants and their effect upon the ciliates at sewage processing plants and in the waterways beyond.

The enormous diversity of the Protozoa and the large volume of related knowledge which has accumulated has made the task of classifying them a complex one.
The fact that they are now classified on the basis of many characteristics which have nothing to do with their appearance poses a problem to the presenter of this material to an internet community. Many visitors to this site will have, as their only knowledge of a microscopic creature, the memory of (or pictures of) what they saw under their microscopes, and will require a system of identification which is based upon appearance. Such were the systems used by biologists in the nineteenth century and up to fairly recent times, and such is the system presented here. For more detail on Micrographia's position on this issue, the visitor is directed to the notes on taxonomy included in the pages on site policy.

The Protozoa (Protoctists) dealt with in these galleries are:

Further notes on the relationship between this presentation system and the taxonomic systems in present use will be added as necessary.