Protozoa once included corals and jellyfish as the first animals
When the zoologist Georg Goldfuss coined the word protozoa in 1818, meaning first animals, he threw in not just microbes but sponges, corals, jellyfish and rotifers. Two centuries of microscopes and arguments later, the label survives only as a loose nickname for single-celled organisms that eat other things.
Goldfuss translated a German term, Urthiere, or original animals, and treated protozoa as a class of the simplest creatures. The grouping made intuitive sense: textbook examples such as Amoeba and Paramecium move about and hunt, and they lack the cell walls of plants and many algae.
Better microscopes and the cell theory of Theodor Schwann and Matthias Schleiden changed the picture. In 1848 C. T. von Siebold argued that ciliates and amoebae were single cells, comparable to the building blocks of plant and animal tissue, and he removed all multicellular animals from the group, raising it to a phylum. Otto Bütschli, later hailed as the architect of protozoology, cemented the idea of protozoa as one-celled animals, studied in zoology departments while tiny photosynthesisers went to botany.
Trouble came from organisms that refused to pick a side. Euglena and Dinobryon photosynthesise like plants yet swim and feed like animals. In 1860 John Hogg proposed a separate kingdom he called Primigenum, grouping protozoa with single-celled algae as Protoctista and picturing the plant and animal kingdoms as two pyramids merging at their base. Ernst Haeckel suggested a third kingdom, Protista, in 1866. Even so, protozoa stayed the standard home for amoebae and ciliates for more than a century.
By the 1970s biologists insisted that a formal group must include one ancestor and all its descendants. Protozoa, assembled from unrelated lineages, failed that test, and the idea that they are close relatives of animals could no longer be defended. Mid-century figures including Robert Whittaker and Lynn Margulis championed a protist kingdom instead. Today protozoa is an informal term for single-celled eukaryotes that feed on organic matter, free-living or parasitic, such as Trypanosoma.
Source: Protozoa
