An order of monocotyledonous angiosperms composed of a single genus, Acorus (sweet flag or sweet calamus), with two species widespread in the Northern Hemisphere. Formerly, Acorus was included in the aroid family, Araceae, but several lines of evidence, including deoxyribonucleic acid (DNA) sequences, have firmly established it as the first-branching lineage of the monocotyledons. These species are emergent aquatics with peculiar inflorescences, and they are unusual among the monocots in having characters otherwise confined to the magnoliid dicots, such as the anther formation and the presence of ethereal oils. These oils are the basis of the species' frequent use as a medicine to treat toothache and dysentery, and as an ointment in religious ceremonies. See also: Magnoliophyta

The processes of biological and organic change in animals by which descendants come to differ from their ancestors. Animal evolution encompasses the theory that modern animals are the modified descendants of animals that formerly existed and that these earlier forms descended from still earlier and different organisms. The concepts of evolutionary biology and their relationship to phylogeny (the genealogical history of organisms, both living and extinct), adaptation, and speciation were formulated by Charles Darwin. Much of Darwin's conclusions emanated from his investigative study of animals. Animals are multicellular organisms that feed by ingestion of other organisms or their products, being unable to derive energy through photosynthesis or chemosynthesis. Animals are currently classed into approximately 36 phyla, and each of these phyla has evolved a distinctive body plan or architecture. Representatives of the major animal groups are depicted in Fig. 1. See also: Adaptation (biology); Animal; Animal kingdom; Charles Darwin and his theory of evolution; Evolution; Macroevolution; Phylogeny; Speciation; Species concept

The classification rank comprising the animals. Animals are taxonomically assigned to the classification kingdom known as Animalia. (The other kingdoms for classification of organisms include Plantae, Fungi, Protista, and Monera; in certain systematic schemes, the Monera kingdom is replaced by the Archaea and Bacteria.) Animals are eukaryotic multicellular organisms that take food into their bodies and that develop from blastula embryos. The animal kingdom includes more than 1.2 million described species. Animal species are organized into phyla (see illustration) that are defined according to comparative patterns of development, body structures, behavior, biochemical pathways, modes of nutrition, and ancestry. See also: Biological classification; Eukaryota; Metazoa; Systematics; Taxonomic categories; Taxonomy; Zoology

A group of diverse microscopic prokaryotic organisms that constitute one of the three domains of life. Taxonomists recognize three domains of cellular life: Archaea, Bacteria, and Eukaryota [(also termed Eukarya or Eucarya), Fig. 1]. The members of the Archaea and Bacteria comprise the prokaryotes and look identical under a microscope; however, they differ in their chemical composition and metabolic processes. Archaea microorganisms are often found in some of the most extreme environments on Earth, including anaerobic, hypersaline, extremely cold, and extremely high temperature environments. For example, they comprise about one-third of the prokaryotic biomass in coastal Antarctic waters, providing food for many marine animals. In addition, some are symbionts in the digestive system of animals. Notably, the members of the Archaea are not known to cause disease in humans or animals. See also: Bacteria; Eukaryota; Microbiology; Prokaryote; Symbiosis

The classification, nomenclature, and identification of bacteria. Bacterial taxonomy is concerned with the naming of bacterial organisms and with organizing these names according to various criteria. Biologically, bacteria (Fig. 1) are prokaryotic organisms, which are defined in terms of the unique structural and biochemical properties of their cells—more specifically, the organization of the deoxyribonucleic acid (DNA) in the nucleoid region, the lack of a nuclear membrane, the lack of independent membrane-bounded organelles, the lack of endocytosis and exocytosis, and the chemical nature of some components of the plasma membrane and cell walls. In addition, bacteria inhabit a great diversity of ecologic niches, and the individual kinds of bacteria within these bacterial populations exhibit a variety of physiologic capabilities and accomplish remarkable chemical transformations. Therefore, the systematic study of bacterial populations demands recognition of these properties, as well as a taxonomic classification of those bacteria so that they may be assigned to already named species or be described as a new organism to be classified and named. Overall, classification involves the recognition of similarities and relationships as a basis for the arrangement of the bacteria into taxonomic groups or taxa. The basic taxon is the species. Identification also involves the recognition of a bacterium as a member of one of the established taxa, appropriately named, by the comparison of a number of characters with those in the description. With the advent of molecular biology and genomics, there has been a shift towards classifications that reflect evolutionary relationships. See also: Bacteria; Bacterial physiology and metabolism; Bacteriology; Genomics; Microbiology; Prokaryote; Speciation; Systematics; Taxonomic categories; Taxonomy

A major division of the animal kingdom comprising all forms with a bilaterally symmetrical body plan (that is, their left and right sides are mirror images, or nearly so) and organ systems that develop from three tissue layers. The animal kingdom, Metazoa, is subdivided into about 30 major animal groups, or phyla, each constructed on a different architectural plan. Most of these phyla are grouped as the Bilateria, comprising a major early branch on the family tree of animals that may have originated nearly 600 million years ago. All bilaterians possess some features that are not perfectly symmetrical (for example, the human heart and liver), but all have descended from an essentially bilateral ancestor. The nonbilaterian phyla include sponges (which lack muscles and nerves), jellyfish, and sea anemones and their allies, which have quasiradial symmetry and organ systems that develop from only two tissue layers (see illustration). See also: Animal kingdom; Animal symmetry; Metazoa

The biological diversity of different plant and animal species living on the island of Borneo. Borneo is the third largest island in the world. Located in Southeast Asia and straddling the Equator, it is divided among three countries: Indonesia, Malaysia, and Brunei. The largest part of Borneo, comprising the southern and eastern parts of the island, is Indonesian territory and often referred to as Kalimantan, whereas the northern part belongs to Malaysia (the states of Sarawak and Sabah) and Brunei (Fig. 1). Ecologically, Borneo has a profusion of tropical rainforests and is one of the most important centers of biodiversity in the world, with numerous plant and animal species having evolved on and inhabiting the island. As such, the natural richness and biological diversity of Borneo have attracted scientists, and dozens of new species of plants and animals are being discovered annually. However, it takes much effort to uncover the natural secrets of the island because the interior of the island is a remote area that is difficult to reach. Most of it is very mountainous, and few inland roads exist. See also: Animal evolution; Biodiversity; East Indies; Ecological communities; Island biogeography; Oceanic islands as evolutionary laboratories; Plant evolution; Plant geography; Rainforest; Speciation; Zoogeography

Arrangements of the names of species and groups of species meant to convey how these organisms are related to one another. The purposes of biological classifications are to organize the known biodiversity, provide a framework that will incorporate newly discovered biodiversity, and provide a predictive framework for studying biodiversity. Systematics focuses on studies of biodiversity and relationships among organisms. Taxonomy is a subdiscipline of systematics that focuses on formal classifications of organisms. See also: Biodiversity; Systematics; Taxonomy

The process by which unrelated or distantly related organisms independently evolve similar traits and features as they adapt to comparable environments. Convergent evolution, or evolutionary convergence, is a term used to describe instances in which species from different taxonomic groups have evolved a similar form or body plan (see illustration). In these cases, separate and unrelated organisms have independently acquired almost identical adaptations and characteristics under similar environmental conditions or selection pressures. Convergent evolution thus stands in contrast to resemblances among related species, which may share similar adapted features because of their shared origins. See also: Adaptation (biology); Evolution; Macroevolution; Species concept; Taxonomic categories

A taxonomic domain consisting of the vast array of living and fossil organisms with complex cells in which the genetic material is organized into chromosomes [linear pieces of deoxyribonucleic acid (DNA) bound with histone proteins and visible as compact structures during mitosis and meiosis] and contained within a membrane-bound nucleus. Taxonomists recognize three domains of cellular life: Eukaryota, Bacteria, and Archaea. Members of the Eukaryota (or Eukarya, which is sometimes spelled as Eucarya; also formerly referred to as Eukaryotae), commonly known as eukaryotes (see illustration), have a common origin from organisms with prokaryotic cells (Bacteria and Archaea) that do not have their DNA in chromosomes and lack membrane-bound organelles. The eukaryotes thus include all multicellular plants, animals, and fungi, as well as a collection of the unicellular protists (including protozoa). See also: Animal; Animal kingdom; Archaea; Bacteria; Cell membrane; Cell nucleus; Chromosome; Fungi; Plant; Plant kingdom; Prokaryote; Protista; Protozoa