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Animals (also called Metazoa) are organisms that form the biological kingdom Animalia. With few exceptions, animals , breathe oxygen, are , can reproduce sexually, and grow from a hollow sphere of cells, the , during . Over 1.5 million animal have been described—of which around 1 million are —but it has been estimated there are over 7 million animal species in total. Animals range in length from to . They have with each other and their environments, forming intricate . The scientific study of animals is known as .

Most living animal species are in , a whose members have a bilaterally symmetric body plan. The Bilateria include the —in which many groups of are found, such as , , and —and the , containing both the as well as the , the latter containing the . Life forms interpreted as early animals were present in the of the late . Many modern animal became clearly established in the as during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived .

Historically, Aristotle divided animals into those with blood and those without. created the first hierarchical biological classification for animals in 1758 with his , which Jean-Baptiste Lamarck expanded into 14 by 1809. In 1874, divided the animal kingdom into the multicellular Metazoa (now synonymous for Animalia) and the , single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as molecular phylogenetics, which are effective at demonstrating the relationships between .

make use of many other animal species, such as for food (including , , and ), for materials (such as and ), as , and as including for transport. Dogs have been , as have , while many terrestrial and aquatic animals were hunted for sports. Nonhuman animals have appeared in art from the earliest times and are featured in mythology and religion.

The word "animal" comes from the Latin , meaning having breath, having soul or living being.
(2021). 9780199547937, Oxford University Press.
The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term animal is often used to refer only to nonhuman animals.

Animals have several characteristics that set them apart from other living things. Animals are and .
(1995). 9780867209426, Jones & Bartlett Learning. .
Unlike plants and , which animals are heterotrophic, feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically. All animals are (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as , , , and , later become sessile. The blastula is a stage in that is unique to most animals,
(2021). 9780764139208, Barron's Educational Series.
allowing cells to be differentiated into specialised tissues and organs.

All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of and elastic .
(2021). 9780815332183, Garland Science. .
During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as , , and spicules.
(2021). 9780470061534, John Wiley and Sons. .
In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth.
(1991). 9780805308709, Benjamin/Cummings. .
Animal cells uniquely possess the called , , and .
(2021). 9780375763939, The Princeton Review. .

With few exceptions—in particular, the sponges and —animal bodies are differentiated into tissues.

(2021). 9780495381501, Cengage Learning. .
These include , which enable locomotion, and , which transmit signals and coordinate the body. Typically, there is also an internal chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).
(1983). 9780521270281, CUP Archive. .

Reproduction and development
Nearly all animals make use of some form of sexual reproduction.
(1998). 9780122270208, Academic Press. .
They produce by ; the smaller, motile gametes are and the larger, non-motile gametes are .
(2021). 9780768928853, Peterson's. .
These fuse to form ,
(2021). 9781405132770, Wiley-Blackwell. .
which develop via into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.
(1984). 9780030624513, Saunders College Pub.
In most other groups, the blastula undergoes more complicated rearrangement. It first to form a with a digestive chamber and two separate , an external and an internal .
(1975). 9780801639272, Mosby. .
In most cases, a third germ layer, the , also develops between them. These germ layers then differentiate to form tissues and organs.
(1998). 9780697228482, WCB McGraw-Hill.

Repeated instances of during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits.

(1987). 9780120176243
Animals have evolved numerous mechanisms for avoiding close inbreeding.

Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; , such as in Hydra and other ; or , where fertile eggs are produced without , such as in .

(2002). 9780471489689, Wiley.

Animals are categorised into groups depending on how they obtain or consume organic material, including , , , ,
(2021). 9789058093448, Taylor & Francis.
and .
(1973). 9780390556271, Appleton-Century-Crofts.
Interactions between animals form complex . In carnivorous or omnivorous species, is a consumer-resource interaction where a predator feeds on another organism (called its prey).
(1996). 9780865428454, Blackwell Science. .
Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations.
(2021). 9780520246539, University of California Press.
Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in , the larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours, such as hawksbill sea turtles primarily .

Most animals rely on the biomass and energy produced by plants through . Herbivores eat plant material directly, while carnivores, and other animals on higher typically acquire it indirectly by eating other animals. Animals oxidize , , , and other biomolecules to unlock the chemical energy of molecular oxygen, which allows the animal to grow and to sustain biological processes such as locomotion.

(2021). 9781865091709, Blake Education.
(2021). 9788171338962, Rastogi Publications.
(2021). 9780495109358, Cengage Learning. .
Animals living close to hydrothermal vents and on the dark consume organic matter of and bacteria produced in these locations through (by oxidizing inorganic compounds, such as ).
(2021). 9780077221249, McGraw-Hill.

Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as , probably between 510 and 471 million years ago during the or Early . such as the started to move on to land in the late , about 375 million years ago. Animals occupy virtually all of earth's and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks.

(1999). 9780763708627, Jones & Bartlett Learning. .
Animals are however not particularly ; very few of them can survive at constant temperatures above . Only very few species of animals (mostly ) inhabit the most extreme cold deserts of continental .


The ( Balaenoptera musculus) is the largest animal that has ever lived, weighing up to at least 190 and measuring up to long.
(1983). 9780851122359, Enfield, Middlesex : Guinness Superlatives. .
The largest extant terrestrial animal is the African bush elephant ( Loxodonta africana), weighing up to 12.25 tonnes and measuring up to long. The largest terrestrial animals that ever lived were such as , which may have weighed as much as 73 tonnes. Several animals are microscopic; some (obligate parasites within the Cnidaria) never grow larger than 20 , and one of the smallest species ( Myxobolus shekel) is no more than 8.5 µm when fully grown.

Numbers and habitats
The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species, along with their principal habitats (terrestrial, fresh water,
(2021). 9781402082597, Springer. .
and marine), and free-living or parasitic ways of life.
(2021). 9780691120850, Princeton University Press. .
Species estimates shown here are based on numbers described scientifically; much larger estimates have been calculated based on various means of prediction, and these can vary wildly. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million.
(2021). 9781603442695, Texas A&M University Press. .
Using patterns within the taxonomic hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.

17,000Yes (soil)Yes1,750Yes400
(insects) Stork notes that 1m insects have been named, making much larger predicted estimates.
(2021). 9780643069015, .
6,000 Yes60–80Yes

(1996). 9780309520751, Joseph Henry Press. .

(2021). 9780198785552, Oxford University Press. .
16,000 YesYes (few)Yes>1,350
7,500 7,500 Yes


25,000Yes (soil)4,0002,00011,00014,000
(2021). 9781000054903, CRC Press. .
2,000 >4002,000Yes
10,800 Yes200-300YesYes
(2021). 9781107037656, Cambridge University Press. .

Evolutionary origin
The first that might represent animals appear in the 665-million-year-old rocks of the Trezona Formation of . These fossils are interpreted as most probably being early .

The oldest animals are found in the , towards the end of the Precambrian, around 610 million years ago. It had long been doubtful whether these included animals,

(1999). 9780126288605, Academic Press. .
but the discovery of the animal lipid in fossils of establishes that these were indeed animals. Animals are thought to have originated under low-oxygen conditions, suggesting that they were capable of living entirely by anaerobic respiration, but as they became specialized for aerobic metabolism they became fully dependent on oxygen in their environments.

Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 542 million years ago, in beds such as the . Extant phyla in these rocks include , , , , , and , along with numerous now-extinct forms such as the . The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously.

Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.

(2021). 9780805371710, Pearson, Benjamin Cummings.
such as tracks and burrows found in the period may indicate the presence of worm-like animals, roughly as large (about 5 mm wide) and complex as earthworms. However, similar tracks are produced today by the giant single-celled protist , so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of called decreased in diversity, perhaps due to grazing by newly evolved animals.

Animals are , meaning they are derived from a common ancestor. Animals are sister to the , with which they form the . The most basal animals, the , , , and , have body plans that lack bilateral symmetry. Their relationships are still disputed; the sister group to all other animals could be the Porifera or the Ctenophora, both of which lack , important in body plan development.

These genes are found in the Placozoa and the higher animals, the Bilateria. 6,331 groups of common to all living animals have been identified; these may have arisen from a single common ancestor that lived in the . 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and signalling pathways which may have enabled animals to become multicellular by providing a pattern for the body's system of axes (in three dimensions), and another 7 are for transcription factors including proteins involved in the control of development.

The phylogenetic tree (of major lineages only) indicates approximately how many millions of years ago () the lineages split.

Several animal phyla lack bilateral symmetry. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum.
(2021). 9788126106752, Anmol Publications.
Sponges lack the complex organization found in most other animal phyla;
(2021). 9780763757304, Jones & Bartlett Learning.
their cells are differentiated, but in most cases not organised into distinct tissues. They typically feed by drawing in water through pores.
(2021). 9788182930186, Mittal Publications.

The Ctenophora (comb jellies) and Cnidaria (which includes , , and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.

(2021). 9780520221499, University of California Press. .
Animals in both phyla have distinct tissues, but these are not organised into organs.
(2021). 9780852299616, Encyclopædia Britannica.
They are , having only two main germ layers, ectoderm and endoderm.
(2021). 9788171339037, Rastogi Publications.
The tiny are similar, but they do not have a permanent digestive chamber.
(1982). 9780030567476, Holt-Saunders International.

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a , the Bilateria. The body is , with three well-developed germ layers, and their tissues . The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a or pseudocoelom. Animals with this bilaterally symmetric and a tendency to move in one direction have a head end (anterior) and a tail end (posterior) as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.

Having a front end means that this part of the body encounters stimuli, such as food, favouring , the development of a head with and a mouth. Many bilaterians have a combination of circular that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by . They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary which swim with and have an apical organ containing sensory cells. However, there are exceptions to each of these characteristics; for example, adult echinoderms are radially symmetric (unlike their larvae), while some have extremely simplified body structures.

(2021). 9780198566205, Oxford University Press. .
(2021). 9781605353753, Sinauer Associates. .

Genetic studies have considerably changed zoologists' understanding of the relationships within the Bilateria. Most appear to belong to two major lineages, the and the . The basalmost bilaterians are the .

Protostomes and deuterostomes
Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the ) undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.
(2021). 9780521837620, Cambridge University Press.
(2021). 9780199570300, Oxford University Press. .
Most protostomes have , where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by , through invagination of the endoderm.
(2021). 9780852299616, Encyclopædia Britannica.

The main deuterostome phyla are the Echinodermata and the Chordata.

(2021). 9780757509971, Kendall Hunt.
Echinoderms are exclusively marine and include , , and .
(1998). 9780679778844, The Princeton Review.
The chordates are dominated by the (animals with ),
(2021). 9780816059584, Infobase Publishing.
which consist of , , , , and mammals.
(2021). 9780816055159, Infobase Publishing. .
The deuterostomes also include the (acorn worms).
(2021). 9780534406530, Cengage Learning.

The Ecdysozoa are protostomes, named after their shared of , growth by moulting.
(2021). 9780618619160, Houghton Mifflin Harcourt. .
They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the and , are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water;
(2021). 9780471202288, John Wiley. .
some are important parasites.
(1998). 9780691059242, Princeton University Press.
Smaller phyla related to them are the or horsehair worms, and the , , and . These groups have a reduced coelom, called a pseudocoelom.
(2021). 9780070636828, McGraw-Hill. .

The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo. The Spiralia's phylogeny has been disputed, but it contains a large clade, the superphylum , and smaller groups of phyla such as the which includes the and the . All of these are grouped as the , which has a sister group, the Gnathifera, which includes the .

The Lophotrochozoa includes the , , , , and . The molluscs, the second-largest animal phylum by number of described species, includes , , and , while the annelids are the segmented worms, such as , , and . These two groups have long been considered close relatives because they share larvae.

History of classification
In the , Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then from man (with blood, 2 legs, rational soul) down through the live-bearing tetrapods (with blood, 4 legs, sensitive soul) and other groups such as crustaceans (no blood, many legs, sensitive soul) down to spontaneously-generating creatures like sponges (no blood, no legs, vegetable soul). was uncertain whether sponges were animals, which in his system ought to have sensation, appetite, and locomotion, or plants, which did not: he knew that sponges could sense touch, and would contract if about to be pulled off their rocks, but that they were rooted like plants and never moved about.
(2021). 9781408836224, Bloomsbury.

In 1758, Carl Linnaeus created the first classification in his . In his original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. Since then the last four have all been subsumed into a single phylum, the , while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos (a chaotic mess) and split the group into three new phyla, worms, echinoderms, and polyps (which contained corals and jellyfish). By 1809, in his Philosophie Zoologique, Lamarck had created 9 phyla apart from vertebrates (where he still had 4 phyla: mammals, birds, reptiles, and fish) and molluscs, namely , annelids, crustaceans, arachnids, insects, worms, , polyps, and .

(2021). 9780674061675, Harvard University Press. .

In his 1817 Le Règne Animal, Georges Cuvier used comparative anatomy to group the animals into four embranchements ("branches" with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and (echinoderms, cnidaria and other forms).

(1994). 9782880742645, Presses Polytechniques et Universitaires Romandes.
This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist in 1857, and the comparative anatomist in 1860.

In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges.

(2021). 9780226845487, University of Chicago Press. .
The protozoa were later moved to the former kingdom , leaving only the Metazoa as a synonym of Animalia.
(2021). 9780787657772, Gale. .

In human culture
The human population exploits a large number of other animal species for food, both of domesticated species in and, mainly at sea, by hunting wild species. Marine fish of many species are for food. A smaller number of species are .
(2021). 9781597267601, Island Press. .
Invertebrates including , , and or molluscs are hunted or farmed for food. Chickens, cattle, sheep, pigs and other animals are raised as livestock for meat across the world. Animal fibres such as wool are used to make textiles, while animal have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats. Dyestuffs including (), , and kermes
(1991). 9780691002248, Princeton University Press.
(2021). 9780521341073, Cambridge University Press.
have been made from the bodies of insects. including cattle and horses have been used for work and transport from the first days of agriculture.
(2021). 9780824754969, CRC Press. .

Animals such as the fruit fly Drosophila melanogaster serve a major role in science as . Animals have been used to create since their discovery in the 18th century. Some medicines such as the cancer drug are based on or other molecules of animal origin.

People have used to help chase down and retrieve animals,

(2021). 9781585746187, The Lyons Press.
and birds of prey to catch birds and mammals, while tethered have been used to catch fish.
(2021). 9781611682250, University of New Hampshire Press. .
Poison dart frogs have been used to poison the tips of . A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including , reptiles such as and , and birds including canaries, , and all finding a place. However, the most kept pet species are mammals, namely , , and . There is a tension between the role of animals as companions to humans, and their existence as of their own. A wide variety of terrestrial and aquatic animals are hunted for sport.
(1994). 9780879726461, Popular Press. .

Animals have been the from the earliest times, both historical, as in , and prehistoric, as in the . Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and 's c. 1762 horse portrait . Insects, birds and mammals play roles in literature and film, such as in giant bug movies.

(2021). 9781137496393, Springer. .
(2021). 9781476625058, McFarland. .
(2021). 9781554903306, ECW Press. .
Animals including insects and mammals feature in mythology and religion. In both Japan and Europe, a was seen as the personification of a person's soul,
(2021). 9780486219011, Dover.
Hutchins, M., Arthur V. Evans, Rosser W. Garrison and Neil Schlager (Eds) (2003) Grzimek's Animal Life Encyclopedia, 2nd edition. Volume 3, Insects. Gale, 2003. while the scarab beetle was sacred in ancient Egypt.
(1989). 9789652780836, Israel Museum.
Among the mammals, cattle, deer, , lions, bats, , and wolves are the subjects of myths and worship. The signs of the Western and are based on animals.
(1987). 9780851154466, Boydell & Brewer. .

See also


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