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animal,any member of the animal kingdom (kingdom Animalia), as distinguished from organisms of the plantplant,
any organism of the plant kingdom, as opposed to one of the animal kingdom or of the kingdoms Fungi, Protista, or Monera in the five-kingdom system of classification.
..... Click the link for more information. kingdom (kingdom Plantae) and the kingdoms FungiFungi
, kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte).
..... Click the link for more information. , ProtistaProtista
, in the five-kingdom system of classification, a kingdom comprising a variety of unicellular and some simple multinuclear and multicellular eukaryotic organisms.
..... Click the link for more information. , and MoneraMonera,
taxonomic kingdom that comprises the prokaryotes (bacteria and cyanobacteria). Prokaryotes are single-celled organisms that lack a membrane-bound nucleus and usually lack membrane-bound organelles (mitochondria, chloroplasts; see cell, in biology).
..... Click the link for more information. in the five-kingdom system of classification. (Another classification system, suggested by genetic sequencing studies, places animals with plants and some other forms in a larger taxonomic unit called the eukarya to distinguish them from the prokaryotic bacteria and archaea, or ancient bacteria.)
Essentially, animals are many-celled heterotrophic organisms. Plants and algae characteristically manufacture their food from inorganic substances (usually by photosynthesis); animals must secure food already organized into organic substances. They are dependent upon photosynthetic organisms, which provide oxygen as a byproduct and are the ultimate source of all their food. Animals (as well as plants) provide carbon dioxide through respiration and the decomposition of their dead bodies (see carbon cyclecarbon cycle,
in biology, the exchange of carbon between living organisms and the nonliving environment. Inorganic carbon dioxide in the atmosphere is converted by plants into simple carbohydrates, which are then used to produce more complex substances.
..... Click the link for more information. ; nitrogen cyclenitrogen cycle,
the continuous flow of nitrogen through the biosphere by the processes of nitrogen fixation, ammonification (decay), nitrification, and denitrification. Nitrogen is vital to all living matter, both plant and animal; it is an essential constituent of amino acids,
..... Click the link for more information. ). In addition, most animals have specialized means of locomotion, generally involving muscle cells, and possess nervous systems and sense organs—all adaptations for securing food. In most forms there is a distinct alimentary canal or digestive system. Animal cells do not have cell walls. Almost all animals, unlike most plants, possess a limited scheme of growth; that is, the adults of a given species are nearly identical in their characteristic form and are similar in maximum size. Most animals reproduce sexually, but some are capable of asexual reproduction under certain circumstances.
With the advent of electron microscopy and advanced biochemical analyses, intricate differences between simple and microscopic organisms were better understood, and many that were previously fit into the animal or plant kingdom were then placed into separate kingdoms (i.e., Monera for the bacteria, Protista for the algae and protozoans, and so forth). In zoological classificationclassification,
in biology, the systematic categorization of organisms into a coherent scheme. The original purpose of biological classification, or systematics, was to organize the vast number of known plants and animals into categories that could be named, remembered, and
..... Click the link for more information. the animal kingdom has been divided into the three subkingdoms of Parazoa (the sponges), Mezozoa (wormlike parasites), and Eumetazoa. Eumetazoa comprises numerous invertebrateinvertebrate
, any animal lacking a backbone. The invertebrates include the tunicates and lancelets of phylum Chordata, as well as all animal phyla other than Chordata. The major invertebrate phyla include: the sponges (Porifera), coelenterates (Cnidaria), echinoderms
..... Click the link for more information. phyla and the phylum ChordataChordata
, phylum of animals having a notochord, or dorsal stiffening rod, as the chief internal skeletal support at some stage of their development. Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.
..... Click the link for more information. . The chordates include two primitive subphyla of a few species each and the subphylum Vertebrata (see vertebratevertebrate,
any animal having a backbone or spinal column. Verbrates can be traced back to the Silurian period. In the adults of nearly all forms the backbone consists of a series of vertebrae. All vertebrates belong to the subphylum Vertebrata of the phylum Chordata.
..... Click the link for more information. ). There are at least 1.5 million animal species; approximately 95% of these are invertebrates.
The scientific study of animals is called zoologyzoology,
branch of biology concerned with the study of animal life. From earliest times animals have been vitally important to man; cave art demonstrates the practical and mystical significance animals held for prehistoric man.
..... Click the link for more information. ; the study of their relation to their environment and of their distribution is animal ecologyecology,
study of the relationships of organisms to their physical environment and to one another. The study of an individual organism or a single species is termed autecology; the study of groups of organisms is called synecology.
..... Click the link for more information. . For specific approaches to the study of living things, see biologybiology,
the science that deals with living things. It is broadly divided into zoology, the study of animal life, and botany, the study of plant life. Subdivisions of each of these sciences include cytology (the study of cells), histology (the study of tissues), anatomy or
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Any living organism which possesses certain characteristics that distinguish it from plants. There is no single criterion that can be used to distinguish all animals from all plants. Animals usually lack chlorophyll and the ability to manufacture foods from raw materials available in the soil, water, and atmosphere. Animal cells are usually delimited by a flexible plasma or cell membrane rather than a cell wall. Animals generally are limited in their growth and most have the ability to move in their environment at some stage in their life history, whereas plants are usually not restricted in their growth and the majority are stationary.
The presence or lack of chlorophyll in an organism does not determine its affinity to the plant or animal kingdom. Among the protozoa, the class Phytamastigophora includes animals, such as the euglenids, which have chromatophores containing chlorophyll. These organisms are considered to be animals by zoologists and plants by phycologists. Higher parasitic plants and the large plant group Fungi also lack chlorophyll. Another borderline group is the slime molds: the Mycetozoa of zoologists and the Myxomycophyta of the botanists; these organisms exhibit both plant and animal characteristics during their life history. Movement is not a characteristic restricted to the animal kingdom; many of the thallophytes such as Oscillatoria, numerous bacteria, and colonial chlorophytes are motile.
Classifying organisms as plants or animals is difficult. Today biologists recognize up to five kingdoms. Most place the one-celled animals and plants, sometimes along with algae and certain other groups, into the Protista. Other kingdoms are the Monera for the bacteria and blue-green algae, and the Fungi for the slime molds and true fungi. These schemes for recognizing additional kingdoms have the practical advantage of eliminating the difficulties of delimiting and describing the kingdoms of multicellular animals and plants. See Animal kingdom, Plant, Plant kingdom