III.

Classification

The first detailed descriptions of protists were made in 1676 by the inventor of the microscope, Dutch naturalist Antoni van Leeuwenhoek, who observed microscopic organisms that he called animalcules. Traditionally all organisms that moved had been considered animals, whereas all photosynthetic organisms had been considered plants. The term Protista was first used in 1862 by the German biologist Ernst Haeckel to describe microscopic organisms that were neither clearly animallike nor plantlike. Haeckel recognized that some organisms are both motile and photosynthetic. For almost a hundred years after Haeckel, scientists informally recognized three kingdoms: plants, animals, and protists. The Protista included all the microbes. In the 1930s, it was formally proposed that all single-celled organisms, including bacteria, be placed in their own kingdom, Kingdom Protista.

In 1959 American biologist R. H. Whittaker described a classification system of five primary kingdoms: plants, animals, fungi, protists, and bacteria. Because the Protista are so diverse in form, classification within the kingdom has proved difficult. The classification of the Protista is currently based largely on the structure and organization of the cell, the presence of organelles, and the pattern of reproduction or life cycles. The five-kingdom classification system divides the Protista into 27 distinct phyla. More recently, however, classifications based on comparisons of cell physiology and DNA sequences suggest that many protist phyla may be sufficiently large and diverse to be classified as kingdoms.

The plantlike protists include the golden algae (phylum Chrysophyta; see Diatom), dinoflagellates (Pyrrophyta), cryptomonads (Cryptophyta), and euglenoids (Euglenophyta). The animallike protists, which are also called protozoa, include the animal flagellates (Zoomastigina), amoeboid forms (Sarcodina; see Foraminifera), ciliates and suctorians (Ciliophora), and the parasitic, spore-producing sporozoans (Sporozoa). Funguslike forms include the hyphochytrids (Hyphochytridiomycota) and the plasmodiophores (Plasmodiophoromycota). The slime molds, which include several disputed phyla here treated as belonging to the Protista, have characteristics of both fungi and protozoans.