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Animal

For other uses, see Animal (disambiguation).
"Animalia" redirects here. For other uses, see Animalia (disambiguation).

Animals (also called Metazoa) are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in which their body consists of a hollow sphere of cells, the blastula, during embryonic development. Over 1.5 million living animal species have been described—of which around 1 million are insects—but it has been estimated there are over 7 million animal species in total. Animals range in length from 8.5 micrometres (0.00033 in) to 33.6 metres (110 ft). They have complex interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology.

Animals
Temporal range: Cryogenian – present,
Scientific classification
Domain: Eukaryota
(unranked): Unikonta
(unranked): Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Linnaeus, 1758
Major divisions

see text

Synonyms
  • Metazoa
  • Choanoblastaea
  • Gastrobionta
  • Zooaea
  • Euanimalia
  • Animalae

Most living animal species are in Bilateria, a clade whose members have a bilaterally symmetric body plan. The Bilateria include the protostomes, containing invertebrates such as nematodes, arthropods, and molluscs, and the deuterostomes, containing the echinoderms and the chordates, the latter including the vertebrates. Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian. Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 542 million years ago. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago.

Historically, Aristotle divided animals into those with blood and those without. Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae, which Jean-Baptiste Lamarck expanded into 14 phyla by 1809. In 1874, Ernst Haeckel divided the animal kingdom into the multicellular Metazoa (now synonymous for Animalia) and the Protozoa, 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 evolutionary relationships between taxa.

Humans make use of many animal species, such as for food (including meat, milk, and eggs), for materials (such as leather and wool), as pets, and as working animals including for transport. Dogs have been used in hunting, as have birds of prey, 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.

Contents

The word animal comes from the Latin animalis, meaning 'having breath', 'having soul' or 'living being'. 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 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. Unlike plants and algae, which produce their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically. All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. The blastula is a stage in embryonic development that is unique to animals, (though it has been lost in some) allowing cells to be differentiated into specialised 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 reorganised, 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—in particular, the sponges and placozoans—animal bodies 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 (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).

Reproduction and development

Sexual reproduction is nearly universal in animals, such as these dragonflies.

Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by 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, attach to the seabed, 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 third germ layer, the mesoderm, also develops between them. These germ layers then differentiate to form tissues and organs.

Repeated instances of mating with a close relative 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.

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, such as in aphids.

Predators, such as this ultramarine flycatcher (Ficedula superciliaris), feed on other animals.

Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores, and parasites. Interactions between animals form complex food webs. In carnivorous or omnivorous species, predation is a consumer-resource interaction where a predator feeds on another organism (called its prey). Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations. Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps, 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 eating sponges.

Hydrothermal vent mussels and shrimps

Most animals rely on the biomass and energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher trophic levels typically acquire it indirectly by eating other animals. Animals oxidize carbohydrates, lipids, proteins, 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. Animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter of archaea and bacteria produced in these locations through chemosynthesis (by oxidizing inorganic compounds, such as hydrogen sulfide).

Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician. Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's habitats 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. Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F). Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.

The blue whale is the largest animal that has ever lived.

Size

Further information: Largest organisms and Smallest organisms

The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to at least 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes. Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 µm, 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, and marine), and free-living or parasitic ways of life. 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. 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.

Phylum Example No. of
Species
Land Sea Fresh
water
Free-
living
Parasitic
Annelids 17,000 Yes (soil) Yes 1,750 Yes 400
Arthropods 1,257,000 1,000,000
(insects)
>40,000
(Malac-
ostraca)
94,000 Yes >45,000
Bryozoa 6,000 Yes 60–80 Yes
Chordates >70,000
23,000

13,000
18,000
9,000
Yes 40
(catfish)
Cnidaria 16,000 Yes Yes (few) Yes >1,350
(Myxozoa)
Echinoderms 7,500 7,500 Yes
Molluscs 85,000
107,000

35,000

60,000
5,000
12,000
Yes >5,600
Nematodes 25,000 Yes (soil) 4,000 2,000 11,000 14,000
Platyhelminthes 29,500 Yes Yes 1,300 Yes

3,000–6,500

>40,000

4,000–25,000

Rotifers 2,000 >400 2,000 Yes
Sponges 10,800 Yes 200-300 Yes Yes
Total number of described extant species as of 2013[update]: 1,525,728
Further information: Urmetazoan
Dickinsonia costata from the Ediacaran biota (c. 635–542 MYA) is one of the earliest animal species known.

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

The oldest animals are found in the Ediacaran biota, towards the end of the Precambrian, around 610 million years ago.[failed verificationsee discussion] It had long been doubtful whether these included animals, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia 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.

Anomalocaris canadensis is one of the many animal species that emerged in the Cambrian explosion, starting some 542 million years ago, and found in the fossil beds of the Burgess shale.

Many animal phyla first appear in the fossil record during the Cambrian explosion, starting about 542 million years ago, in beds such as the Burgess shale. Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory Anomalocaris. 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. Trace fossils such as tracks and burrows found in the Tonian period may indicate the presence of triploblastic 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 Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals.

Further information: Lists of animals

Animals are monophyletic, meaning they are derived from a common ancestor. Animals are sister to the Choanoflagellata, with which they form the Choanozoa. The most basal animals, the Porifera, Ctenophora, Cnidaria, and Placozoa, 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 hox genes, important in body plan development.

These genes are found in the Placozoa and the higher animals, the Bilateria. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian. 25 of these are novel core gene groups, found only in animals; of those, 8 are for essential components of the Wnt and TGF-beta 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 homeodomain proteins involved in the control of development.

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

Non-bilateria

Non-bilaterians include sponges (centre) and corals (background).

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

The Ctenophora (comb jellies) and Cnidaria (which includes jellyfish, sea anemones, and corals) are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm. The tiny placozoans are similar, but they do not have a permanent digestive chamber.

Bilateria

Idealised bilaterian body plan. With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form a clade, the Bilateria. The body is triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is an internal body cavity, a coelom or pseudocoelom. Animals with this bilaterally symmetric body plan 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 cephalisation, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles 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 peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia 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 parasitic worms have extremely simplified body structures.

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

Protostomes and deuterostomes

The bilaterian gut develops in two ways. In many protostomes, the blastopore develops into the mouth, while in deuterostomes it becomes the anus.
Main articles: Protostome and Deuterostome

Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial cleavage during cell division, while many protostomes (the Spiralia) undergo spiral cleavage. Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the embryonic gut develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily. Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm.

The main deuterostome phyla are the Echinodermata and the Chordata. Echinoderms are exclusively marine and include starfish, sea urchins, and sea cucumbers. The chordates are dominated by the vertebrates (animals with backbones), which consist of fishes, amphibians, reptiles, birds, and mammals. The deuterostomes also include the Hemichordata (acorn worms).

Ecdysozoa
Ecdysis: a dragonfly has emerged from its dry exuviae and is expanding its wings. Like other arthropods, its body is divided into segments.
Main article: Ecdysozoa

The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. 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 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; some 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
Main article: Spiralia
Spiral cleavage in a sea snail embryo

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 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, which includes the rotifers.

The Lophotrochozoa includes 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.

Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla by 1809.

In the classical era, Aristotle divided 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 creatures like sponges (no blood, no legs, vegetable soul). Aristotle 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.

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 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 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 (radiata) (echinoderms, cnidaria and other forms). This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen 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. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.

Practical uses

Sides of beef in a slaughterhouse
Main article: Animals in culture

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. Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.

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 first days 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.

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.

In art

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, while the scarab beetle was sacred in ancient Egypt. Among the mammals, cattle, deer, horses, lions, bats, bears, and wolves are the subjects of myths and worship. The signs of the Western and Chinese zodiacs are based on animals.

  1. The application of DNA barcoding to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000 insect species for Canada alone, and extrapolated that the global insect fauna must be in excess of 10 million species, of which nearly 2 million are in a single fly family known as gall midges (Cecidomyiidae).
  2. Not including parasitoids.
  3. Compare for a more specific and detailed model of a particular phylum with this general body plan.
  4. In his History of Animals and Parts of Animals.
  5. The prefix une espèce de is pejorative.
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Animal
Animal Language Watch Edit For other uses see Animal disambiguation Animalia redirects here For other uses see Animalia disambiguation Animals also called Metazoa are multicellular eukaryotic organisms in the biological kingdom Animalia With few exceptions animals consume organic material breathe oxygen are able to move can reproduce sexually and go through an ontogenetic stage in which their body consists of a hollow sphere of cells the blastula during embryonic development Over 1 5 million living animal species have been described of which around 1 million are insects but it has been estimated there are over 7 million animal species in total Animals range in length from 8 5 micrometres 0 00033 in to 33 6 metres 110 ft They have complex interactions with each other and their environments forming intricate food webs The scientific study of animals is known as zoology Animals Temporal range Cryogenian present 665 0 Ma Pha Proterozoic Archean Had nScientific classificationDomain Eukaryota unranked Unikonta unranked Obazoa unranked Opisthokonta unranked Holozoa unranked FilozoaKingdom Animalia Linnaeus 1758Major divisionssee textSynonymsMetazoa Choanoblastaea Gastrobionta Zooaea Euanimalia Animalae Most living animal species are in Bilateria a clade whose members have a bilaterally symmetric body plan The Bilateria include the protostomes containing invertebrates such as nematodes arthropods and molluscs and the deuterostomes containing the echinoderms and the chordates the latter including the vertebrates Life forms interpreted as early animals were present in the Ediacaran biota of the late Precambrian Many modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion which began around 542 million years ago 6 331 groups of genes common to all living animals have been identified these may have arisen from a single common ancestor that lived 650 million years ago Historically Aristotle divided animals into those with blood and those without Carl Linnaeus created the first hierarchical biological classification for animals in 1758 with his Systema Naturae which Jean Baptiste Lamarck expanded into 14 phyla by 1809 In 1874 Ernst Haeckel divided the animal kingdom into the multicellular Metazoa now synonymous for Animalia and the Protozoa 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 evolutionary relationships between taxa Humans make use of many animal species such as for food including meat milk and eggs for materials such as leather and wool as pets and as working animals including for transport Dogs have been used in hunting as have birds of prey 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 Contents 1 Etymology 2 Characteristics 2 1 Structure 2 2 Reproduction and development 3 Ecology 4 Diversity 4 1 Size 4 2 Numbers and habitats 5 Evolutionary origin 6 Phylogeny 6 1 Non bilateria 6 2 Bilateria 6 2 1 Protostomes and deuterostomes 6 2 1 1 Ecdysozoa 6 2 1 2 Spiralia 7 History of classification 8 In human culture 8 1 Practical uses 8 2 In art 9 See also 10 Notes 11 References 12 External linksEtymologyThe word animal comes from the Latin animalis meaning having breath having soul or living being 1 The biological definition includes all members of the kingdom Animalia 2 In colloquial usage the term animal is often used to refer only to nonhuman animals 3 4 5 6 Characteristics 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 7 8 Unlike plants and algae which produce their own nutrients 9 animals are heterotrophic 8 10 feeding on organic material and digesting it internally 11 With very few exceptions animals respire aerobically 12 All animals are motile 13 able to spontaneously move their bodies during at least part of their life cycle but some animals such as sponges corals mussels and barnacles later become sessile The blastula is a stage in embryonic development that is unique to animals 14 though it has been lost in some allowing cells to be differentiated into specialised tissues and organs Structure All animals are composed of cells surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins 15 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 shells bones and spicules 16 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 17 Animal cells uniquely possess the cell junctions called tight junctions gap junctions and desmosomes 18 With few exceptions in particular the sponges and placozoans animal bodies are differentiated into tissues 19 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 in Ctenophora Cnidaria and flatworms or two openings in most bilaterians 20 Reproduction and development Sexual reproduction is nearly universal in animals such as these dragonflies See also Sexual reproduction Animals and Asexual reproduction Examples in animals Nearly all animals make use of some form of sexual reproduction 21 They produce haploid gametes by meiosis the smaller motile gametes are spermatozoa and the larger non motile gametes are ova 22 These fuse to form zygotes 23 which develop via mitosis 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 24 In most other groups the blastula undergoes more complicated rearrangement 25 It first invaginates to form a gastrula with a digestive chamber and two separate germ layers an external ectoderm and an internal endoderm 26 In most cases a third germ layer the mesoderm also develops between them 27 These germ layers then differentiate to form tissues and organs 28 Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits 29 30 Animals have evolved numerous mechanisms for avoiding close inbreeding 31 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 such as in aphids 32 33 Ecology Predators such as this ultramarine flycatcher Ficedula superciliaris feed on other animals Animals are categorised into ecological groups depending on how they obtain or consume organic material including carnivores herbivores omnivores detritivores 34 and parasites 35 Interactions between animals form complex food webs In carnivorous or omnivorous species predation is a consumer resource interaction where a predator feeds on another organism called its prey 36 Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey resulting in various anti predator adaptations 37 38 Almost all multicellular predators are animals 39 Some consumers use multiple methods for example in parasitoid wasps the larvae feed on the hosts living tissues killing them in the process 40 but the adults primarily consume nectar from flowers 41 Other animals may have very specific feeding behaviours such as hawksbill sea turtles primarily eating sponges 42 Hydrothermal vent mussels and shrimps Most animals rely on the biomass and energy produced by plants through photosynthesis Herbivores eat plant material directly while carnivores and other animals on higher trophic levels typically acquire it indirectly by eating other animals Animals oxidize carbohydrates lipids proteins and other biomolecules to unlock the chemical energy of molecular oxygen 43 which allows the animal to grow and to sustain biological processes such as locomotion 44 45 46 Animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter of archaea and bacteria produced in these locations through chemosynthesis by oxidizing inorganic compounds such as hydrogen sulfide 47 Animals originally evolved in the sea Lineages of arthropods colonised land around the same time as land plants probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician 48 Vertebrates such as the lobe finned fish Tiktaalik started to move on to land in the late Devonian about 375 million years ago 49 50 Animals occupy virtually all of earth s habitats 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 51 Animals are however not particularly heat tolerant very few of them can survive at constant temperatures above 50 C 122 F 52 Only very few species of animals mostly nematodes inhabit the most extreme cold deserts of continental Antarctica 53 Diversity The blue whale is the largest animal that has ever lived Size Further information Largest organisms and Smallest organisms The blue whale Balaenoptera musculus is the largest animal that has ever lived weighing up to at least 190 tonnes and measuring up to 33 6 metres 110 ft long 54 55 56 The largest extant terrestrial animal is the African bush elephant Loxodonta africana weighing up to 12 25 tonnes 54 and measuring up to 10 67 metres 35 0 ft long 54 The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus which may have weighed as much as 73 tonnes 57 Several animals are microscopic some Myxozoa obligate parasites within the Cnidaria never grow larger than 20 µm 58 and one of the smallest species Myxobolus shekel is no more than 8 5 µm when fully grown 59 Numbers and habitats The following table lists estimated numbers of described extant species for the animal groups with the largest numbers of species 60 along with their principal habitats terrestrial fresh water 61 and marine 62 and free living or parasitic ways of life 63 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 64 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 65 66 a Phylum Example No of Species Land Sea Fresh water Free living ParasiticAnnelids 17 000 60 Yes soil 62 Yes 62 1 750 61 Yes 400 63 Arthropods 1 257 000 60 1 000 000 insects 68 gt 40 000 Malac ostraca 69 94 000 61 Yes 62 gt 45 000 b 63 Bryozoa 6 000 60 Yes 62 60 80 61 YesChordates gt 70 000 60 70 23 000 71 13 000 71 18 000 61 9 000 71 Yes 40 catfish 72 63 Cnidaria 16 000 60 Yes 62 Yes few 62 Yes 62 gt 1 350 Myxozoa 63 Echinoderms 7 500 60 7 500 60 Yes 62 Molluscs 85 000 60 107 000 73 35 000 73 60 000 73 5 000 61 12 000 73 Yes 62 gt 5 600 63 Nematodes 25 000 60 Yes soil 62 4 000 64 2 000 61 11 000 64 14 000 64 Platyhelminthes 29 500 60 Yes 74 Yes 62 1 300 61 Yes 62 3 000 6 500 75 gt 40 000 63 4 000 25 000 75 Rotifers 2 000 60 gt 400 76 2 000 61 YesSponges 10 800 60 Yes 62 200 300 61 Yes Yes 77 Total number of described extant species as of 2013 update 1 525 728 60 Evolutionary originFurther information Urmetazoan Dickinsonia costata from the Ediacaran biota c 635 542 MYA is one of the earliest animal species known 78 The first fossils that might represent animals appear in the 665 million year old rocks of the Trezona Formation of South Australia These fossils are interpreted as most probably being early sponges 79 The oldest animals are found in the Ediacaran biota towards the end of the Precambrian around 610 million years ago failed verification see discussion It had long been doubtful whether these included animals 80 81 82 but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes that these were indeed animals 78 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 83 Anomalocaris canadensis is one of the many animal species that emerged in the Cambrian explosion starting some 542 million years ago and found in the fossil beds of the Burgess shale Many animal phyla first appear in the fossil record during the Cambrian explosion starting about 542 million years ago in beds such as the Burgess shale Extant phyla in these rocks include molluscs brachiopods onychophorans tardigrades arthropods echinoderms and hemichordates along with numerous now extinct forms such as the predatory Anomalocaris The apparent suddenness of the event may however be an artefact of the fossil record rather than showing that all these animals appeared simultaneously 84 85 86 87 Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion possibly as early as 1 billion years ago 88 Trace fossils such as tracks and burrows found in the Tonian period may indicate the presence of triploblastic worm like animals roughly as large about 5 mm wide and complex as earthworms 89 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 90 91 Around the same time the layered mats of microorganisms called stromatolites decreased in diversity perhaps due to grazing by newly evolved animals 92 PhylogenyFurther information Lists of animals Animals are monophyletic meaning they are derived from a common ancestor Animals are sister to the Choanoflagellata with which they form the Choanozoa 93 The most basal animals the Porifera Ctenophora Cnidaria and Placozoa 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 94 both of which lack hox genes important in body plan development 95 These genes are found in the Placozoa 96 97 and the higher animals the Bilateria 98 99 6 331 groups of genes common to all living animals have been identified these may have arisen from a single common ancestor that lived 650 million years ago in the Precambrian 25 of these are novel core gene groups found only in animals of those 8 are for essential components of the Wnt and TGF beta 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 homeodomain proteins involved in the control of development 100 101 The phylogenetic tree of major lineages only indicates approximately how many millions of years ago mya the lineages split 102 103 104 105 106 Choanozoa Choanoflagellata Animalia Porifera Eumetazoa Ctenophora ParaHoxozoa Placozoa Cnidaria Bilateria Xenacoelomorpha Nephrozoa Deuterostomia Chordata Ambulacraria Protostomia Ecdysozoa Scalidophora Panarthropoda Nematoida gt 529 myaSpiralia Gnathifera Rotifera and allies Chaetognatha Platytrochozoa Platyhelminthes and allies Lophotrochozoa Mollusca and allies Annelida and allies 550 mya580 mya 610 mya650 myaTriploblasts680 mya 760 mya950 mya Non bilateria Non bilaterians include sponges centre and corals background Several animal phyla lack bilateral symmetry Among these the sponges Porifera probably diverged first representing the oldest animal phylum 107 Sponges lack the complex organization found in most other animal phyla 108 their cells are differentiated but in most cases not organised into distinct tissues 109 They typically feed by drawing in water through pores 110 The Ctenophora comb jellies and Cnidaria which includes jellyfish sea anemones and corals are radially symmetric and have digestive chambers with a single opening which serves as both mouth and anus 111 Animals in both phyla have distinct tissues but these are not organised into organs 112 They are diploblastic having only two main germ layers ectoderm and endoderm 113 The tiny placozoans are similar but they do not have a permanent digestive chamber 114 115 Bilateria Main articles Bilateria and Symmetry biology Bilateral symmetry Idealised bilaterian body plan c With an elongated body and a direction of movement the animal has head and tail ends Sense organs and mouth form the basis of the head Opposed circular and longitudinal muscles enable peristaltic motion The remaining animals the great majority comprising some 29 phyla and over a million species form a clade the Bilateria The body is triploblastic with three well developed germ layers and their tissues form distinct organs The digestive chamber has two openings a mouth and an anus and there is an internal body cavity a coelom or pseudocoelom Animals with this bilaterally symmetric body plan 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 116 117 Having a front end means that this part of the body encounters stimuli such as food favouring cephalisation the development of a head with sense organs and a mouth Many bilaterians have a combination of circular muscles that constrict the body making it longer and an opposing set of longitudinal muscles that shorten the body 117 these enable soft bodied animals with a hydrostatic skeleton to move by peristalsis 118 They also have a gut that extends through the basically cylindrical body from mouth to anus Many bilaterian phyla have primary larvae which swim with cilia 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 parasitic worms have extremely simplified body structures 116 117 Genetic studies have considerably changed zoologists understanding of the relationships within the Bilateria Most appear to belong to two major lineages the protostomes and the deuterostomes 119 The basalmost bilaterians are the Xenacoelomorpha 120 121 122 Protostomes and deuterostomes Further information Embryological origins of the mouth and anus The bilaterian gut develops in two ways In many protostomes the blastopore develops into the mouth while in deuterostomes it becomes the anus Main articles Protostome and Deuterostome Protostomes and deuterostomes differ in several ways Early in development deuterostome embryos undergo radial cleavage during cell division while many protostomes the Spiralia undergo spiral cleavage 123 Animals from both groups possess a complete digestive tract but in protostomes the first opening of the embryonic gut develops into the mouth and the anus forms secondarily In deuterostomes the anus forms first while the mouth develops secondarily 124 125 Most protostomes have schizocoelous development where cells simply fill in the interior of the gastrula to form the mesoderm In deuterostomes the mesoderm forms by enterocoelic pouching through invagination of the endoderm 126 The main deuterostome phyla are the Echinodermata and the Chordata 127 Echinoderms are exclusively marine and include starfish sea urchins and sea cucumbers 128 The chordates are dominated by the vertebrates animals with backbones 129 which consist of fishes amphibians reptiles birds and mammals 130 The deuterostomes also include the Hemichordata acorn worms 131 132 Ecdysozoa Ecdysis a dragonfly has emerged from its dry exuviae and is expanding its wings Like other arthropods its body is divided into segments Main article Ecdysozoa The Ecdysozoa are protostomes named after their shared trait of ecdysis growth by moulting 133 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 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 134 some are important parasites 135 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 136 Spiralia Main article Spiralia Spiral cleavage in a sea snail embryo The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo 137 The Spiralia s 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 which includes the rotifers 138 139 The Lophotrochozoa includes the molluscs annelids brachiopods nemerteans bryozoa and entoprocts 138 140 141 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 142 143 History of classificationFurther information Taxonomy biology History of zoology through 1859 and History of zoology since 1859 Jean Baptiste de Lamarck led the creation of a modern classification of invertebrates breaking up Linnaeus s Vermes into 9 phyla by 1809 144 In the classical era Aristotle divided animals d 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 creatures like sponges no blood no legs vegetable soul Aristotle 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 145 In 1758 Carl Linnaeus created the first hierarchical classification in his Systema Naturae 146 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 renamed or broken up The process was begun in 1793 by Jean Baptiste de Lamarck who called the Vermes une espece de chaos a chaotic mess e 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 144 In his 1817 Le Regne 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 radiata echinoderms cnidaria and other forms 148 This division into four was followed by the embryologist Karl Ernst von Baer in 1828 the zoologist Louis Agassiz in 1857 and the comparative anatomist Richard Owen in 1860 149 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 150 149 The protozoa were later moved to the former kingdom Protista leaving only the Metazoa as a synonym of Animalia 151 In human culturePractical uses Sides of beef in a slaughterhouse Main article Animals in culture 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 152 153 Marine fish of many species are caught commercially for food A smaller number of species are farmed commercially 152 154 155 Humans and their livestock make up more than 90 of the biomass of all terrestrial vertebrates and almost as much as all insects combined 156 Invertebrates including cephalopods crustaceans and bivalve or gastropod molluscs are hunted or farmed for food 157 Chickens cattle sheep pigs and other animals are raised as livestock for meat across the world 153 158 159 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 160 Dyestuffs including carmine cochineal 161 162 shellac 163 164 and kermes 165 166 have been made from the bodies of insects Working animals including cattle and horses have been used for work and transport from the first days of agriculture 167 Animals such as the fruit fly Drosophila melanogaster serve a major role in science as experimental models 168 169 170 171 Animals have been used to create vaccines since their discovery in the 18th century 172 Some medicines such as the cancer drug Yondelis are based on toxins or other molecules of animal origin 173 A gun dog retrieving a duck during a hunt People have used hunting dogs to help chase down and retrieve animals 174 and birds of prey to catch birds and mammals 175 while tethered cormorants have been used to catch fish 176 Poison dart frogs have been used to poison the tips of blowpipe darts 177 178 A wide variety of animals are kept as pets from invertebrates such as tarantulas and octopuses insects including praying mantises 179 reptiles such as snakes and chameleons 180 and birds including canaries parakeets and parrots 181 all finding a place However the most kept pet species are mammals namely dogs cats and rabbits 182 183 184 There is a tension between the role of animals as companions to humans and their existence as individuals with rights of their own 185 A wide variety of terrestrial and aquatic animals are hunted for sport 186 In art 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 Durer s 1515 The Rhinoceros and George Stubbs s c 1762 horse portrait Whistlejacket 187 Insects birds and mammals play roles in literature and film 188 such as in giant bug movies 189 190 191 Animals including insects 192 and mammals 193 feature in mythology and religion In both Japan and Europe a butterfly was seen as the personification of a person s soul 192 194 195 while the scarab beetle was sacred in ancient Egypt 196 Among the mammals cattle 197 deer 193 horses 198 lions 199 bats 200 bears 201 and wolves 202 are the subjects of myths and worship The signs of the Western and Chinese zodiacs are based on animals 203 204 See alsoAnimal attacks Animal coloration Ethology Fauna List of animal names Lists of organisms by populationNotes The application of DNA barcoding to taxonomy further complicates this a 2016 barcoding analysis estimated a total count of nearly 100 000 insect species for Canada alone and extrapolated that the global insect fauna must be in excess of 10 million species of which nearly 2 million are in a single fly family known as gall midges Cecidomyiidae 67 Not including parasitoids 63 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