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Animal

Animal

Animals are creatures in the kingdom Animalia, one of the five major divisions of organisms. They are multicelled, eukaryotic (pronounced yookar-ee-AH-tik) organisms, meaning their cells contain nuclei and other structures called organelles, all of which are enclosed by thin membranes. (Eukaryote means "true nucleus.") Unlike plants, their cells do not have cell walls. Animals are capable of moving their bodies, often in response to what they sense in their environment. For food, animals ingest plants and other organisms. The scientific study of animals is called zoology.

Animals have existed for millions of years, but it is not known when they first appeared on Earth. The earliest animals were soft-bodied, multicellular life-forms that did not preserve well as fossils. (A fossil is the remains or print of an organism from long ago that has been preserved in rock.) By the time animal parts became hardened in rock about 640 to 670 million years ago, numerous well-developed multicellular animals already existed. Therefore, the beginnings of the animal kingdom must have occurred earlier.

Most zoologists recognize the existence of 30 to 35 phyla (related groups) of animals, some of which are extinct (no longer exist) and are known only from their fossil record. Animals that live today come in many forms and sizes, the very smallest visible only under a microscope and the very largest, the blue whale, reaching 100 feet (30 meters) in length and weighing up to 300,000 pounds (136,000 kilograms). Animals are classified as vertebrates (having backbones) or invertebrates (without backbones).

Among the most primitive of the animals are the sponges, invertebrates that live in water. Sponges have no nerve cells or muscles. They are shaped like vases, with water flowing in through holes in their sides and leaving through an opening in their top. Sponges are remarkable for their ability to regenerate. Although many invertebrates are capable of growing new body parts, sponges are capable of growing into a new individual from even the tiniest fragment of the original body. It is believed that although sponges have lived successfully for about one billion years, yet it seems that they did not give rise to any other animal forms.

Some of the simplest animals with the oldest ancestral lines are invertebrates such as jellyfish, sea anemones (pronounced uh-NEH-muh-neez), and corals, which also live in water. They have radial body symmetry, which means that their bodies are arranged equally around a central point. This arrangement allows them to sense food and danger approaching from all directions. The simplest animals having bilateral symmetry (meaning that both the left and right sides of their bodies are mirror images of each other) include the invertebrate flatworms and roundworms. These animals live in water, on land, and as parasites in the body fluids of other organisms. Bilateral symmetry was an important evolutionary development because it allowed for forward movement of animals. It also is associated with the development of separate head and tail areas, as well as a distinction between the upper and under portions of an animal's body.

Another major step in the evolution of animals was the development of a body cavity called the coelom (pronounced SEE-luhm). The coelom is a cavity in the body between the gut and the body wall that houses the internal organs, such as the liver, stomach, and heart. Animal groups that have coeloms include Mollusca (snails, clams, octopus, and squid), Annelida (earthworms and leeches), Arthropoda (insects, spiders, and crabs), Echinodermata (sea urchins and starfish), and Chordata (fish, amphibians, reptiles, birds, and mammals). All of the vertebrates are included in the group Chordata.

About one million species of animals have been named. However, biologists estimate that a much larger number of animal species has yet to be discovered; the actual total could be as large as 30 to 50 million species.

Words to Know

Bilateral symmetry: Two-sided symmetry where the left and right sides of the body are identical, with each side containing similar structures.

Coelom: The cavity between the body wall and gut that is lined with specialized cells and that serves to protect the organs within.

Eukaryote: Multicellular organism whose cells contain true nuclei and membrane-bound structures called organelles.

Radial symmetry: Identical or similar body shape around one central point so that any line drawn through the center yields similar right and left halves.

[See also Arthropods; Mollusks ]

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animal

animal, any member of the animal kingdom (kingdom Animalia), as distinguished from organisms of the plant kingdom (kingdom Plantae) and the kingdoms Fungi, Protista, and Monera 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 cycle; nitrogen cycle). 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 classification the animal kingdom has been divided into the three subkingdoms of Parazoa (the sponges), Mezozoa (wormlike parasites), and Eumetazoa. Eumetazoa comprises numerous invertebrate phyla and the phylum Chordata. The chordates include two primitive subphyla of a few species each and the subphylum Vertebrata (see vertebrate). There are at least 1.5 million animal species; approximately 95% of these are invertebrates.

The scientific study of animals is called zoology; the study of their relation to their environment and of their distribution is animal ecology. For specific approaches to the study of living things, see biology.

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Animal

Animal

Animals are multicellular, heterotrophic eukaryotes . Because animals are unable to make their own food, they must have some means of ingesting food. They do this by consuming plants, other animals, or decomposing organic matter, or by absorbing nutrients directly from a host. Animals typically store food reserves in their body as glycogen. Animals have nerve tissues to gain information about the environment and muscle tissue to allow them to move. They have membrane-bound cells that lack rigid walls. Most animals reproduce sexually and spend most of their life cycle as diploid organisms. These are the characteristics that generally separate animals from other groups.

By this definition the first animals appeared on Earth in the Precambrian oceans over 500 million years ago. Since that time animals have evolved into many diverse forms. Some of those forms have become extinct while others continue to thrive. At the start of the twenty-first century, more than one million species of animals are known on Earth, with more being discovered all the time. Animals are grouped into about thirty-five phyla . Over 95 percent of the animal species lack a vertebral column and are called invertebrates.

Animals are found in nearly all environments on Earth. The oceans are home to the largest number of animal phyla. Freshwater environments are home to a large number of phyla, but those environments are not as diverse as the oceans. Terrestrial environments have the smallest number of animal phyla.

Allan B. Cobb

Bibliography

Barnes-Svarney, Patricia, ed. The New York Public Library Desk Reference. New York: Macmillan USA, 1995.

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animal

an·i·mal / ˈanəməl/ • n. a living organism that feeds on organic matter, typically having specialized sense organs and nervous system and able to respond rapidly to stimuli: humans are the only animals who weep. ∎  any such living organism other than a human being: are humans superior to animals, or just different? ∎  a mammal, as opposed to a bird, reptile, fish, or insect: the snowfall seemed to have chased all birds, animals, and men indoors. ∎  a person whose behavior is regarded as devoid of human attributes or civilizing influences, esp. someone who is very cruel, violent, or repulsive. ∎  a particular type of person or thing: a regular party animal | the government that followed the election was a very different animal. • adj. of, relating to, or characteristic of animals: the evolution of animal life animal welfare. ∎  of animals as distinct from plants: tissues of animal and vegetable protein. ∎  characteristic of the physical and instinctive needs of animals; of the flesh rather than the spirit or intellect: a crude surrender to animal lust.

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"animal." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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animal

animal Living organism of the animal kingdom, usually distinguishable from members of the plant kingdom by its power of locomotion (at least during some stage of its existence); a well-defined body shape; limited growth; its feeding exclusively on organic matter; the production of two different kinds of sex cells; and the formation of an embryo or larva during the developmental stage. Higher animals, such as the vertebrates, are easily distinguishable from plants, but the distinction becomes blurred with the lower forms. Some one-celled organisms could easily be assigned to either category. Scientists have classified about a million different kinds of animals in more than twenty phyla. The simplest (least highly evolved) animals include the protozoan, sponges, jellyfish, and worms. Other invertebrate phyla include arthropods (arachnids, crustaceans, and insects), molluscs (shellfish, octopus, and squid) and echinoderms (sea urchins and starfish). Vertebrates belong to the chordata phylum, which includes fish, amphibians, reptiles, birds, and mammals.

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"animal." World Encyclopedia. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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animal

animal Animal Farm a fable (1945) by George Orwell which consists of a satire on Russian Communism as it developed under Stalin. The animals of the farm, led by the pigs, revolt against the cruel farmer, and achieve an apparent life of freedom, but as power corrupts their rulers, they are led to a world in which the slogan is ‘All animals are equal but some animals are more equal than others.’
animal, vegetable, and mineral the three traditional divisions into which natural objects have been classified; the classification (earlier in Latin) is first recorded in English in the early 18th century. From the mid 19th century, animal, vegetable, (or) mineral became the name of a parlour game in which players had to guess the identity of an object, having been told to which of the three groups it belongs; they are traditionally allowed up to twenty questions, to be answered by ‘yes’ or ‘no’.

See also political animal.

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"animal." The Oxford Dictionary of Phrase and Fable. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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"animal." The Oxford Dictionary of Phrase and Fable. . Retrieved August 21, 2017 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/animal

animal

animal Any member of the kingdom Animalia, which comprises multicellular organisms that develop from embryos formed by the fusion of haploid eggs and sperm. Unable to manufacture their own food, they feed on other organisms or organic matter (see heterotrophic nutrition). Animals are therefore typically mobile (to search for food) and have evolved specialized sense organs for detecting changes in the environment; a nervous system coordinates information received by the sense organs and enables rapid responses to environmental stimuli. Animal cells lack the cellulose cells walls of plant cells. For a classification of the animal kingdom, see animal kingdom.

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"animal." A Dictionary of Biology. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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animal

animal A multicellular, heterotrophic organism that develops from an embryo derived from gametes produced in specialized organs or surrounded by somatic cells. Typically, animals are motile, at least during some stage of the life cycle, and have sensory apparatus with which to detect changes in their immediate environment. Protozoa are unicellular but otherwise resemble animals in many ways (although there are plant-like protozoons) and were formerly classified as an animal phylum; they are now more usually classified in the kingdom Protista. See ANIMALIA.

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"animal." A Dictionary of Zoology. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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animal

animal XIV. (i) — (O)F. animal or L. animālis (in medL. bestial), f. anima breath, life; see -AL1. (ii) sb. ult. — L. animal, for animāle, sb. use of n. of the adj..
Hence animalism XIX.

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animal

animal •sewellel •camel, enamel, entrammel, mammal, trammel •miasmal, phantasmal •Carmel •abysmal, baptismal, catechismal, dismal, paroxysmal •animal • minimal • lachrymal •maximal •decimal, infinitesimal •septimal • optimal • primal • Rommel •abnormal, conformal, formal, normal, paranormal, subnormal •chromosomal • Kümmel •Brummell, pommel, pummel •epidermal, geothermal, isothermal, pachydermal, taxidermal, thermal

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