Animal
Animals are multicellular eukaryotic organisms that form the biological kingdom Animalia. With few exceptions, animals can move, consume organic material, breathe oxygen, reproduce sexually, and develop from an embryonic stage made of a hollow ball of cells, the blastula. There are perhaps 2 million living animal species, of which around 1 million are insects. The study of animals is called zoology.

Animals were traditionally divided by body plan into vertebrates, those with a spine, and invertebrates, all the rest. In modern classification, most animals are in the Bilateria, a clade whose members are characterized by a bilaterally symmetric body plan. The Bilateria include the protostomes and the deuterostomes. The protostomes are divided into two major groups: the Ecdysozoa (animals which grow by moulting), such as the nematodes and the arthropods; and the Spiralia (animals with spiral cleavage in the early embryo), such as the molluscs and annelids. The deuterostomes include the Echinoderms and the Chordates (among them, the vertebrates). Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Most modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, about 542 million years ago.

Zoologists starting with Aristotle have studied and compared animals to classify them; Aristotle divided them into those with blood and those without. In 1758, Carl Linnaeus created the first hierarchical classification, dividing the animals into Vermes, Insecta, Pisces, Amphibia, Aves, and Mammalia. This system was broken up by Jean-Baptiste de Lamarck, who by 1809 had identified 14 animal phyla. In 1817 Georges Cuvier created four major branches with different body plans, namely vertebrates, molluscs, articulated animals, and zoophytes. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa and the single-celled Protozoa (no longer considered to be animals). Today the classification of animals relies on advanced techniques such as molecular phylogenetics, which continue to unveil the evolutionary relationships between animal groups. 

Humans make use of many other animal species for food including meat, milk, and eggs, for materials such as leather and wool, as pets, and as working animals for power and transport. Dogs especially have been used in hunting, while many terrestrial and aquatic animals are hunted for sport. Animals have appeared in art from the earliest times, and feature also in mythology and religion.

Etymology
The word "animal" comes from the Latin , meaning having breath, having soul or living being. The biological definition of the word refers to all members of the kingdom Animalia, whereas in colloquial use, animal often refers to non-human animals. Sometimes the word may refer to only other vertebrates, or just mammals.

Characteristics
thumb|upright=1.3|Animals are unique in having the ball of cells of the early embryo (1) develop into a hollow ball or blastula (2).

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular, which separates them from bacteria and most protists, which are prokaryotic and unicellular. Unlike plants and algae, which produce their own nutrients (and, with fungi, have rigid cell walls), animals are heterotrophic, consuming organic material and digesting it internally. All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle, but some animals later become sessile, such as corals, mussels, and barnacles. The blastula is a stage in embryonic development that is unique to most animals, allowing cells to be differentiated into specialized tissues and organs.

Structure
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganized, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, and spicules. 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. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.

With few exceptions—most notably the two phyla consisting of sponges and placozoans—animals have bodies that are differentiated into tissues. These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (as in flatworms) or two openings (as in deuterostomes).

Reproduction and development

thumb|A newt lung cell in the early anaphase stage of mitosis, cell division

Nearly all animals undergo some form of sexual reproduction. They produce haploid gametes by meiosis (see Origin and function of meiosis). The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a mesoderm also develops between them. These germ layers then differentiate to form tissues and organs.

Repeated instances of mating with a close relative (inbreeding) during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding. In some species, such as the splendid fairywren, females benefit by mating with multiple males, thus producing more offspring of higher genetic quality.

Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding (such as in Hydra and other cnidarians); or parthenogenesis, where fertile eggs are produced without mating, as in some life-cycle stages of aphids.

Food and energy sourcing
thumb|upright|left|Animals, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other organisms.

With very few exceptions, animals breathe oxygen and respire aerobically. All animals are heterotrophs, meaning that they feed directly or indirectly on other living things. They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.

Predation is a consumer-resource interaction where a predator feeds on another organism, nearly always resulting in its death. Almost all multicellular predators are animals. The other main category of consumption is detritivory, the consumption of dead organic matter. Some of these feeding behaviours grade into each other, for example, where parasites feed on a host organism and then lay their eggs on it for their offspring to feed on its decaying corpse, or where parasitoid wasps paralyse their host, for their larvae to feed on it while alive and inevitably kill it. Selective pressures imposed on one another lead to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.

Most animals indirectly use the energy of sunlight by eating plants or plant-eating animals. Most plants photosynthesise, using light to convert inorganic molecules in their environment into carbohydrates, lipids, proteins and other biomolecules, characteristically containing reduced carbon in the form of carbon-hydrogen bonds. When an animal eats plants (or eats other animals which have eaten plants), the reduced carbon compounds in the food become a source of energy and building materials for the animal. They are either used directly to help the animal grow, or broken down, releasing energy for the animal's biological processes, such as locomotion.

Animals living close to hydrothermal vents and cold seeps on the dark ocean floor are not dependent on the energy of sunlight. Instead chemosynthetic archaea and bacteria form the base of the food web there.

Habitat

While originally evolved in the sea, animals have soon colonized the land, possibly since the late Cambrian. Animals are not particularly heat tolerant: very rarely they can function and survive at constant temperatures beyond 50 °C.

Diversity

The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species,The World Conservation Union. 2014. IUCN Red List of Threatened Species, 2014.3. Summary Statistics for Globally Threatened Species. Table 1: Numbers of threatened species by major groups of organisms (1996–2014). along with their principal habitats, and whether species are free-living, parasitic, or both.
 Phylum Class Example No. ofSpecies Land Sea Freshwater Free-living Para-sitic Chordates alt=green spotted frog facing right|100px 75,000 X X X X X few Arthropods Insects alt=wasp|100px 1,000,000 X X X X many Arthropods Arachnids alt=black spider|100px 100,000 X X few X few X X Arthropods Malacostracans alt=Tasmanian giant crab|100px 40,000 X X many X X X Molluscs  alt=snail|100px 90,000 X X X X X Flatworms 100px 25,000 X X X X many Nematodes 100px 25,000–1,000,000  X soil X X X X many Annelids 100px 17,000  X soil X X X Echinoderms 100px 7,000  X X Cnidaria alt=Table coral|100px 11,000 X X Sponges 100px 7,700  X X Moss animals 100px 5,000  X X X Rotifers 100px 2,000 X few X X 

Evolutionary origin

thumb|A colony of choanoflagellates, the closest living relatives of animals.

Animals are generally considered to have emerged within flagellated eukaryota. Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists. The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.

thumb|left|If Dickinsonia costata from the Ediacaran biota, (c. 635–542 Mya), is indeed an animal, then it is one of the earliest animal species known. 

The first fossils that might represent animals appear in the Trezona Formation of South Australia. These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.

The next oldest possible animal fossils are found in the Ediacaran biota, towards the end of the Precambrian, around 610 million years ago. These are difficult to relate to later fossils. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.

Most known animal phyla make a more or less simultaneous appearance during the Cambrian explosion, starting about 542 million years ago, in beds such as the Burgess shale. Extant phyla seen as Cambrian fossils include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, while numerous now-extinct forms are also found. However, the suddenness of the event is disputed.

thumb|Anomalocaris canadensis is one of the many animal species that emerged in the Cambrian explosion, starting some 542 million years ago.

Some palaeontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. Trace fossils such as tracks and burrows found in the Tonian period indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms. Around 1 billion years ago, there was a decrease in stromatolite diversity, which may indicate the appearance of grazing animals, since stromatolite diversity later increased when grazing animals became extinct at the End Permian and End Ordovician extinction events, and decreased shortly after the grazer populations recovered. However, similar tracks are produced today by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution.

Phylogeny

Cladistics classifies animals according to their evolutionary tree, by assigning species descending from a most recent common ancestor to groups called clades. Animals are monophyletic, forming a single clade within the Apoikozoa, with the Choanoflagellata as their sister clade. 
The most basal animals (Ctenophora, Porifera, Cnidaria and Placozoa) lack a bilaterally symmetric body plan, with their relationships still disputed, though in 2017, the Porifera are considered basalmost animals. The remainder of the animals form the Eumetazoa. An alternative scenario is the Ctenophora-sister hypothesis. Some of the issues are the rapid evolutionary rate within Ctenophora, insufficient sampling, and the recent internal divergence date of Ctenophora. The other animals are the Bilateria, whose bodies display bilateral symmetry. The Bilateria are split into deuterostomes and protostomes.
In the following cladogram (of major lineages only), numbers indicate approximately how many million years ago (Mya) the lineages split.

 Non-bilaterian animals 
[[File:Elephant-ear-sponge.jpg|thumb|upright|alt=Orange elephant ear sponge under water with sea fan in background|Non-bilaterians include sponges and coral: the orange animal midfield is an elephant ear sponge, Agelas clathrodes. In the background are two corals: upper center is a sea fan Iciligorgia schrammi, and upper right  is a sea rod, Plexaurella nutans.]]

Several animal phyla lack of bilateral symmetry, diverging from other animals early in evolution. Among these, the sponges (Porifera) probably diverged first, representing the oldest animal phylum; they lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues; they typically feed by drawing in water through pores. 

The Ctenophora and the sponges are unique among the animals in lacking true hox genes.Evolution and Development. page 38  The presence of a Hox/Parahox gene in the Placozoa suggests that either the Porifera or the Ctenophora are the most basal animal clades.Jakob W, Sagasser S, Dellaporta S, Holland P, Kuhn K, Schierwater B (2004) The Trox-2 Hox/ParaHox gene of Trichoplax (Placozoa) marks an epithelial boundary. Dev Genes Evol 214(4): 170–175 Another DNA based study suggests that the Ctenophora are the earliest branching animals.Arcila D, Ortí G, Vari R, Armbruster JW, Stiassny MLJ, Ko KD, Sabaj MH, Lundberg J, Revell LJ, Betancur-R R (2017) Genome-wide interrogation advances resolution of recalcitrant groups in the tree of life. Nat Ecol Evol 1(2): 20.  Another study also suggests that this group are a sister group to other animals. The gene classes SINE, LIM, POU, NKL-ANTP and Q50-PRD are present in Porifera and Diploblasts but not in other organisms.

Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. Both have distinct tissues, but they are not organized into organs. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic. The tiny placozoans are similar, but they do not have a permanent digestive chamber. The Myxozoa, microscopic parasites originally considered Protozoa, are now believed to have evolved within Cnidaria.

Bilaterian animals

The remaining animals form a clade, the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however—for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.

Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the protostomes, which includes the Ecdysozoa and the Spiralia, and the deuterostomes.

In addition, there are a few small groups of bilaterians with relatively cryptic morphology whose relationships with other animals are not well-established. For example, recent molecular studies have identified Acoelomorpha and Xenoturbella as forming a monophyletic group, but studies disagree as to whether this group evolved from within deuterostomes, or whether it represents the sister group to all other bilaterian animals (Nephrozoa). The Chaetognatha or arrow worms have traditionally been classified as deuterostomes, but recent molecular studies have identified this group as a basal protostome lineage.

 Deuterostomes and protostomes 
thumb|A seagull eating a starfish. Both animals are deuterostomes.

Deuterostomes differ from protostomes in several ways. Animals from both groups possess a complete digestive tract. However, in protostomes, the first opening of the gut to appear in embryological development (the archenteron) develops into the mouth, with the anus forming secondarily. In deuterostomes the anus forms first, with the mouth developing secondarily. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostome embryos undergo radial cleavage during cell division, while protostomes undergo spiral cleavage.

All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as starfish, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.

The deuterostomes also include the Hemichordata, or acorn worms, which are thought to be closely related to Echinodermata forming a group known as Ambulacraria. Although they are not especially prominent today, the important fossil graptolites may belong to this group.

Ecdysozoa
thumb|alt=A multi-colored dragonfly on branch facing left |Ecdysozoa like the yellow-winged darter, Sympetrum flaveolum, grow by moulting (ecdysis).

The Ecdysozoa are protostomes, named after the common trait of growth by moulting (ecdysis). The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, 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. A number are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.

 Spiralia 
thumb|Spiralia include molluscs like the Roman snail, Helix pomatia, as well as the annelids and flatworms.

The Spiralia is a large group of protostomes. Its phylogeny has been disputed, but it contains a large clade, the superphylum Lophotrochozoa, and smaller groups of phyla such as the Rouphozoa which includes the gastrotrichs and the flatworms. All of these are grouped as the Platytrochozoa, which has a sister group, the Gnathifera.

The Lophotrochozoa includes several phyla, namely the molluscs, annelids, brachiopods, nemerteans, bryozoa and entoprocts. The molluscs, the second-largest animal phylum by number of described species, includes snails, clams, and squids, while the annelids are the segmented worms, such as earthworms, lugworms, and leeches. These two groups have long been considered close relatives because they share trochophore larvae.

History of classification
thumb|upright|left|Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla.

In the classical era, Aristotle divided animals in his History of Animals and Parts of Animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then arranged on a scale 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 animals like sponges (no blood, no legs, vegetable soul).

In 1758, Carl Linnaeus created the first hierarchical classification in his Systema Naturae. 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 Chordata, while his Insecta (which included the crustaceans and arachnids) and Vermes have been broken up. The process was begun in 1793 by Jean-Baptiste de Lamarck, who called the Vermes une espèce de chaos (a sort of chaos) 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 cirripedes, annelids, crustaceans, arachnids, insects, worms, radiates, polyps, and infusorians.

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 zoophytes (cnidaria and other phyla).

In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals) and Protozoa (single-celled animals). The protozoa were later moved to the former kingdom Protista, leaving only the  Metazoa as a synonym of Animalia.

In human culture
thumb|upright|Sides of beef in a slaughterhouse

The human population exploits a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially.
Invertebrates including cephalopods, crustaceans, and bivalve or gastropod 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 sinews 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 carmine (cochineal), shellac, and kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the origins of agriculture.

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

thumb|left|A gun dog retrieving a duck during a hunt

People have used hunting dogs to help chase down and retrieve animals, and birds of prey to catch birds and mammals, while tethered cormorants have been used to catch fish. Poison dart frogs have been used to poison the tips of blowpipe darts.
A wide variety of animals are kept as pets, from invertebrates such as tarantulas and octopuses, insects including praying mantises, reptiles such as snakes and chameleons, and birds including canaries, parakeets and parrots all finding a place. However, the most kept pet species are mammals, namely dogs, cats, and rabbits. There is a tension between the role of animals as companions to humans, and their existence as individuals with rights of their own.
A wide variety of terrestrial and aquatic animals are hunted for sport.

thumb|Artistic vision: Still Life with Lobster and Oysters by Alexander Coosemans, c. 1660

Animals have been the subjects of art from the earliest times, both historical, as in Ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's c. 1762 horse portrait Whistlejacket''.
Insects, birds and mammals play roles in literature and film, such as in giant bug movies. Animals including insects and mammals feature in mythology and religion. In both Japan and Europe, a butterfly was seen as the personification of a person's soul,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. Among the mammals, cattle, deer, horses, lions, bats and wolves are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are based on animals.Lau, Theodora, The Handbook of Chinese Horoscopes, pp. 2–8, 30–5, 60–4, 88–94, 118–24, 148–53, 178–84, 208–13, 238–44, 270–78, 306–12, 338–44, Souvenir Press, New York, 2005

See also

 Animal attacks
 Animal coloration
 Ethology
 Fauna
 List of animal names
 Lists of animals
 Lists of organisms by population
 Taxonomy (biology)

References

Bibliography

 
 
 
 

External links

 
 
 Tree of Life Project
 Animal Diversity Web – University of Michigan's database of animals
 ARKive – multimedia database of endangered/protected species 

 
Animals
Category:Cryogenian first appearances