What is a Protozoan?

Abstract

The question as stated by the chapter heading is far from trivial. Many standard textbooks define protozoa as “unicellular animals,” but this is not entirely satisfactory. The idea that a protozoan is unicellular in the sense that it corresponds to a single cell of a multicellular organism, was first conceived and accepted about 170 years after the discovery of protozoa by Leeuwenhoek in 1674. The term “Protozoa” was coined by Goldfuss in 1817 to mean “original animals” and it included the coelenterates. The title of Ehrenberg’s memoir: Die Infusionsthierchen als vollkommene Organismen (1838), which also included small multicellular creatures such as rotifers, alluded to the idea that protozoa were quite comparable to higher animals, the feeding vacuole to a stomach, and so on. D’Orbigny, who coined the name Foraminifera in 1826, considered them to be a type of cephalopod because of the resemblance between the test of (some) foraminifera and the shell of the mollusc Nautilus. It was only in the middle of the last century, after the cell was recognized as a building unit of animals and plants, that the idea of protozoa as single cells comparable to cells of multicellular organisms became generally accepted. This idea then led to the theory that multicellular organisms originated as protozoan cell colonies.