Protozoa

Protozoa are single-celled creatures with nuclei, usually showing some
animal-like characteristics such as heterotrophy and motility and so
originally classified in one kingdom with the animals. There was never
really a sharp dividing line between them and the plant-like algae, though,
so many forms were included in both. Now it is understood that Protozoa and
algae are neither plants nor animals, and they two have been placed in the
kingdom Protista. Some smaller groups of protozoa are also sometimes treated
as separate kingdoms.

The traditional division of protozoans is on the basis of movement, and
while this is no longer believed to represent true relationships, it's still
a useful way of organizing the many different groups:

   * Flagellates
   * Amoeboids
   * Sporozoans: Apicomplexa, Myxozoa, Microsporidia
   * Ciliates

Most protozoans are too small to be seen with the naked eye - most are
around 0.01-0.05 mm, although forms up to 0.5 mm are still fairly common -
but can easily be found under a microscope. Protozoa are ubiquitous
throughout aqueous environments and the soil, and play an important role in
their ecology. A few are also important parasites.

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