Classification of Organisms
The classification system given here divides living organisms into five kingdoms—Monera, Protista, Fungi, Plantae, and Animalia. Although the five-kingdom classification is widely accepted, some scientists disagree on the placement of certain groups of organisms. In the following scheme, for example, slime molds are placed in the protist kingdom, and algae are placed in the monera and protist kingdoms; some scientists, however, place slime molds in the fungus kingdom and place the red, brown, and green algae in the plant kingdom. Formerly, all organisms were considered to be either plants or animals and placed in the plant or animal kingdom.
Scientists also disagree on the number of phyla or divisions in each kingdom. For example, the system given in this article places organisms of the monera kingdom into 4 phyla and organisms of the protist kingdom into 11 phyla; some scientists, however, recognize as many as 17 phyla of monera and 27 of protists.
All organisms in the five kingdoms described here are composed of one or more cells. The members of the monera kingdom are prokaryotes. (A prokaryote is a single-celled organism having a nucleoid instead of a true nucleus; in a nucleoid, the hereditary material is not bound by a membrane.) The members of the other four kingdoms are eukaryotes. (A eukaryote is a single-celled or multicelled organism having cells with true nuclei—that is, nuclei that are bound by membranes.)
Viruses, which do not have a cell structure, are not considered living things by most scientists and are not listed in the classification system given in this article. Those scientists who consider viruses to be living things place them in the monera kingdom, even though they are not cellular.