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In the tenth edition of his book Systema Naturae (1758), Swedish botanist Carolus Linnaeus developed a biological classification system (now known as the Linnaean System) that placed all organisms into seven hierarchical groupings. He suggested that all organisms could be classified as belonging to two kingdoms, the Plantae (plants) and the Animalia (animals), and that members of these kingdoms could be distinguished by whether they have the ability to "sense" (both plants and animals grow and live, but only animals sense). This two-kingdom classification system remained virtually unchallenged for over a century.

Today, most biologists group living things into five kingdoms: the Monera, the Protista, Fungi, Plants, and Animals. The Monera includes all prokaryotes . The monerans include the most ancient forms of life and were the only organisms on Earth from around 3.5 to 1.5 billion years ago. The Protista is a diverse group of single-celled eukaryotes that originally derived from the Monera. The protists gave rise to the other three kingdoms: the multicellular fungi, plants, and animals.

Plants are distinguished from fungi and animals because they are autotrophs , meaning that they gain all their nutrients and energy from inorganic materials and from the Sun. Plants take nutrients from the soil through absorption , the passing of molecules through pores in cells; they take energy from the Sun through photosynthesis . Fungi are different from plants and animals because they are heterotrophs (they require preformed organic material, that is, material made by other organisms), which obtain their nutrients through absorption. Animals, on the other hand, are heterotrophs that obtain their nutrients through ingestion, the active intake of other organisms or decomposing organic material. Today's Animalia, also called the Metazoa, differs from the Animalia of Linnaeus in that it does not include any of the animal-like unicellular eukaryotes, commonly termed the Protozoa , which are now grouped in the Protista.

Multicellularity evolved in the Protista a number of times. The commonly held view is that multicellularity derived from colonial unicellular ancestors, which formed loose collections of interconnected cells. The benefit of being part of a colony is that individual cells may become specialized for certain tasks, such as sensation, secretion, or gamete production, and thus raise the efficiency of the colony as a whole. As these colonial organisms become more intimately associated, some members may lose their flagella to become completely dependent on their neighbors. At the point when a colony transports nutrients from cell to cell, eliminating the necessity of all cells to feed individually, the colony becomes a true multicellular organism.

Multicellular eukaryotes arose approximately 700 million years ago in aquatic habitats . One of the first benefits that multicellularity allowed was an increase in organism size. Greater size and greater cell number allowed greater variation in organism shape and structure. Consequently, the diversity of multicellular life exploded and began occupying a variety of new niches . Around 400 million years ago, multicellular eukaryotes living at the edges of lakes and streams colonized land, giving rise to the enormous diversity of terrestrial multicellular life seen today.

The Metazoa contains more species than any other kingdom. While plants are relatively immobile and utilize simple molecules as a food source, animals have become specialized consumers of all types of organisms, including other animals.

Brief Survey of the Diversity of the Animal Kingdom

The sponges are a group of sedentary aquatic animals first classified as plants. Although sponges and other animals evolved from protists, sponges may have arisen independently from the other animals. Sponges obtain their nutrients by drawing water into a central cavity and filtering the water for food. They are able to move water through their bodies through the coordinated beating of flagellated cells that line their pores. Sponges differ from other animals in that they lack distinct body tissues and body symmetry.

A second group of animals that may also have independently evolved from protists contains the cnidarians and the ctenophorans. These animals are radially symmetrical , meaning that their body parts are arranged symmetrically around one main axis. Examples include jellyfish, sea anemones, and corals. Cnidarians differ from ctenophorans in that their mouth and anus form a single opening, whereas ctenophorans have separate openings for the mouth and anus so that food moves in a single direction through the gut.

The remaining metazoans are more closely related to each other than to the other animals. They may be classified into three groups: the acoelomates , the pseudocoelomates , and the coelomates. All these animals are bilaterally symmetrical , meaning that their bodies can be divided into mirror images through only a single plane.

Acoelomates are animals that lack internal body cavities. The acoelomates include the flatworms and the ribbon worms. Flatworms have distinct organs but do not have a fully formed digestive tract or a means for transporting oxygen through the body. Thus all cells must undergo respiration individually, necessitating a flat body whereby all cells have access to oxygen at the body surface. Ribbon worms are similar to flatworms but have a complete digestive tract and a simple circulatory system.

Pseudocoelomates have a simple, fluid-filled body cavity in which many of the internal organs float. The body cavity protects the internal organs from external jarring and allows them to grow somewhat independently from the rest of the body. The pseudocoelomates include the rotifers and the nematodes. Nematodes are probably the most numerous of all animal species, inhabiting virtually every corner of Earth. One of the most well-studied organisms in the world is the nematode Caenorhabditis elegans, a roundworm that lives in soil.

The third group of bilaterally symmetrical metazoans, the coelomates, have a relatively complex body cavity inside which the organs are suspended from the body wall. Most coelomates can be further divided into two groups, the protostomes and the deuterostomes , which are distinguished by the way in which cells of their zygotes divide. Protostomes include mollusks such as snails, annelids such as earthworms, and arthropods such as insects. The deuterostomes include echinoderms such as starfishes and chordates such as humans.

see also Binomial (Linnaean System); Phylogenetic Relationships of Major Groups.

Todd A. Schlenke


Ax, Peter. Multicellular Animals: A New Approach to the Phylogenetic Order in Nature. New York: Springer-Verlag, 1996.

Brusca, Richard C., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.

Hanson, Earl D. The Origin and Early Evolution of Animals. Middletown, CT: Wesleyan University Press, 1977.

Linnaeus, Carolus. Systema Naturae, 10th ed. Stockholm: Laurentius Salvius, 1758.

Lipps, Jere H., and Philip W. Signor, eds. Origin and Early Evolution of the Metazoa. New York: Plenum Press, 1992.

Nielsen, Claus. Animal Evolution, Interrelationships of the Living Phyla. Oxford: Oxford University Press, 1995.

Willson, Mary F. Vertebrate Natural History. Philadelphia: Saunders College Publications, 1984.

Internet Resources

Maddison, David R., and Wayne P. Maddison. The Tree of Life. <>.

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metazoan A multicellular animal. See ANIMALIA.

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metazoan A multicellular animal. See ANIMALIA.