In his classification scheme, Linnaeus recognized only two kingdoms of living things: Animalia and Plantae. At the time, microscopic organisms had not been studied in detail. They were placed either in a separate category called Chaos or, in some cases; they were classified with plants or animals. Then in the 1860s, the German investigator Ernst Haeckel proposed a three‐kingdom system of classification. Haeckel's three kingdoms were Animalia, Plantae, and Protista. Members of the kingdom Protista included the protozoa, fungi, bacteria, and other microorganisms. Haeckel's system was not widely accepted, however, and microorganisms continued to be classified as plants (for example, bacteria and fungi) or animals (for example, protozoa).
Currently, the system of classification widely accepted by biologists is that devised by Robert Whittaker in 1968. Whittaker's classification scheme recognizes five kingdoms: Monera, Protista, Fungi, Plantae, and Animalia. The five-kingdom classification scheme is in general use today.
The kingdom Monera includes the bacteria and the cyanobacteria. These one-celled organisms are prokaryotic. Prokaryotic organisms have neither nucleus nor organelles in their cytoplasm, possess only a single chromosome, have small ribosomes, and reproduce by simple fission. Many of the organisms (calledautotrophic) can synthesize their own foods, and some (called heterotrophic) digest preformed organic matter.
The second kingdom, Protista, includes the protozoa, the one-celled algae, and the slime molds. The cells of these organisms are eukaryotic. They are unicellular, and they may be autotrophic or heterotrophic. Eukaryotic organisms have a nucleus and organelles in their cytoplasm, possess multiple chromosomes, have large ribosomes, and reproduce by mitosis.
The third kingdom, Fungi, includes the yeasts, molds, mildews, mushrooms, and other similar organisms. The cells of this kingdom are eukaryotic and heterotrophic. Some fungal species are unicellular, whereas other species form long chains of cells and are called filamentous fungi. A cell wall containing chitin or cellulose is found in most members. Food is taken in by the absorption of small molecules from the external environment.
The fourth kingdom is Plantae. Classified here are the mosses, ferns, and seed-producing plants. All plant cells are eukaryotic and autotrophic. The organisms synthesize their own foods by photosynthesis, and their cell walls contain cellulose. All the organisms are multicellular.
The final kingdom, Animalia, includes animals. Animals without backbones (invertebrates) and with backbones (vertebrates) are included here. The cells are eukaryotic; the organisms are heterotrophic. All animals are multicellular, and none have cell walls. In the kingdom Animalia, biologists classify such organisms as sponges, hydras, worms, insects, starfish, reptiles, amphibians, birds, and mammals. The feeding form is one in which large molecules from the external environment are consumed, then broken down to usable parts in the animal body.