How did the older five-kingdom system classify protozoans?

In the older two-kingdom system, protozoans were classified as "minute animals" or "first animals" because they were motile and lacked photosynthetic pigments and cell walls.1

When the five-kingdom system was later adopted, protozoans were placed within the kingdom Protista. This kingdom was established to encompass eukaryotic microorganisms that did not fit the traditional definitions of plants, animals, or fungi. Under this arrangement, the superkingdom Eukaryota was divided into four kingdoms: Protista, Plantae, Animalia, and Fungi, while the fifth kingdom, Monera, contained the prokaryotes.1

It is important to note that the five-kingdom system, developed by Robert H. Whittaker, was based primarily on morphology and nutrition rather than phylogenetic evolutionary history. Modern biological classification has largely moved away from this hierarchical structure in favor of a three-domain system based on molecular characteristics, such as ribosomal RNA sequences. Consequently, many scientists no longer consider the kingdom Protista to be a valid taxonomic grouping, as current approaches emphasize the phylogenetic relationships between protistan groups and multicellular organisms.21