Animal: Difference between revisions

Revision as of 03:28, 22 May 2007

Latest revision as of 23:22, 25 August 2025

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{{Short description|Kingdom of living things}}

{{otheruses}}

{{hatnote group|

{{redirect5|Animalia|the book|Animalia (book)}}

{{Other uses}}

{{Taxobox

{{redirect|Animalia|other uses}}

| color = pink

| name = Animal

| fossil_range = [[Ediacaran]] - Recent

| image = Animalia diversity.jpg

| image_width = 250px

| image_caption = Clockwise from top-left: ''[[European Squid|Loligo vulgaris]]'' (a [[Mollusca|mollusc]]), ''[[sea nettle|Chrysaora quinquecirrha]]'' (a [[cnidaria]]n), ''[[Aphthona flava]]'' (an [[arthropod]]), ''[[Nereis|Eunereis longissima]]'' (an [[annelid]]), and ''[[tiger|Panthera tigris]]'' (a [[chordate]]).

| ___domain = [[Eukaryote|Eukaryota]]

| unranked_regnum = [[Opisthokont]]a

| regnum = '''Animalia'''

| regnum_authority = [[Carolus Linnaeus|Linnaeus]], [[Systema Naturae|1758]]

| subdivision_ranks = [[Phylum|Phyla]]

| subdivision =

'''''Subregnum [[Parazoa]]'''''

* [[Sponge|Porifera]]

(alternatively)

* [[Calcarea]]

* [[Silicarea]]

'''''Subregnum [[Eumetazoa]]'''''

* [[Trichoplax|Placozoa]]

* '''''[[Radiata]] (unranked)'''''

** [[Ctenophora]]

** [[Cnidaria]]

** [[Trilobozoa]] [[extinction|†]]

* '''''[[Bilateria]] (unranked)'''''

** [[Orthonectida]]

** [[Rhombozoa]]

** [[Acoelomorpha]]

** [[Myxozoa]]

** [[Chaetognatha]]

** '''Superphylum [[Deuterostome|Deuterostomia]]'''

*** [[Chordate|Chordata]]

*** [[Hemichordata]]

*** [[Echinoderm]]ata

*** [[Xenoturbellida]]

*** [[Vetulicolia]] [[extinction|†]]

** '''Superphylum [[Ecdysozoa]]'''

*** [[Kinorhyncha]]

*** [[Loricifera]]

*** [[Priapulida]]

*** [[Nematoda]]

*** [[Nematomorpha]]

*** [[Lobopodia]] [[extinction|†]]

*** [[Onychophora]]

*** [[Tardigrada]]

*** [[Arthropoda]]

** '''Superphylum [[Platyzoa]]'''

*** [[Platyhelminthes]]

*** [[Gastrotricha]]

*** [[Rotifera]]

*** [[Acanthocephala]]

*** [[Gnathostomulida]]

*** [[Micrognathozoa]]

*** [[Cycliophora]]

** '''Superphylum [[Lophotrochozoa]]'''

*** [[Sipuncula]]

*** [[Hyolitha]] [[extinction|†]]

*** [[Nemertea]]

*** [[Phoronida]]

*** [[Ectoprocta]]

*** [[Bryozoa]]

*** [[Entoprocta]]

*** [[Brachiopoda]]

*** [[Mollusca]]

*** [[Annelida]]

*** [[Echiura]]

}}

{{good article}}

[[Image:Elephant-ear-sponge.jpg|thumb|Orange elephant ear sponge, ''Agelas clathrodes'', in foreground. Two corals in the background: a [[sea fan]], ''Iciligorgia schrammi'', and a sea rod, ''Plexaurella nutans''.]]

{{pp-semi-indef}}

{{pp-move}}

{{Use British English|date=November 2024}}

{{Use dmy dates|date=August 2024}}

{{automatic taxobox

| name = Animals

| display_parents = 7

| taxon = Animalia

| authority = [[Linnaeus]], [[10th edition of Systema Naturae|1758]]

| fossil_range = [[Cryogenian]] – present, {{Long fossil range |665|0}}<!--please do not attempt to change this without providing citations to reliable evidence in the body of this article; the infobox is to reflect what the article says, not to introduce anything "new"-->

| image = <imagemap>

File:Animal diversity b.png |300px

rect 0 0 118 86 [[Echinoderm]]

'''Animals''' are a major group of [[organism]]s, classified as the [[Kingdom (biology)|kingdom]] '''Animalia''' or '''Metazoa'''. In general they are [[multicellular|multi&shy;cellular]], responsive to their environment, and feed by consuming other organisms or parts of them. Their [[body plan]] becomes fixed as they develop, usually early on in their [[ontogeny|development]] as [[embryo]]s, although some undergo a process of [[metamorphosis (biology)|metamorphosis]] later on.

rect 0 86 118 172 [[Cnidaria]]

rect 0 172 118 258 [[Tardigrade]]

rect 0 258 118 344 [[Flatworm]]

rect 118 0 236 86 [[Sponge]]

The word "animal" comes from the [[Latin]] word ''animal'', of which ''animalia'' is the plural, and is derived from ''anima'', meaning vital breath or soul. In everyday colloquial usage, the word usually refers to non-[[human]] animals. The biological definition of the word refers to all members of the Kingdom Animalia. Therefore, when the word "animal" is used in a biological context, humans are included.

rect 118 86 236 172 [[Arthropod]]

rect 118 172 236 258 [[Bryozoa]]

rect 118 258 236 344 [[Acanthocephala]]n

rect 236 0 354 86 [[Mollusc]]

==Characteristics==

rect 236 86 354 172 [[Annelid]]

Animals have several characteristics that set them apart from other living things. Animals are [[eukaryote|eukaryotic]] and usually [[multicellular]] (although see [[Myxozoa]]), which separates them from [[bacteria]] and most [[protist]]s. They are [[heterotroph]]ic, generally digesting food in an internal chamber, which separates them from [[plant]]s and [[alga]]e. They are also distinguished from plants, algae, and [[fungus|fungi]] by lacking [[cell wall]]s. All animals are motile, if only at certain life stages. Embryos pass through a [[Blastula|blastula stage]], which is a characteristic exclusive to animals. HELLO

rect 236 172 354 258 [[Vertebrate]]

rect 236 258 354 344 [[Tunicate]]

</imagemap>

===Structure===

| image_upright = 1.3

With a few exceptions, most notably the [[sponge]]s (Phylum Porifera), animals have bodies differentiated into separate [[biological tissue|tissues]]. These include [[muscle]]s, which are able to contract and control locomotion, and [[nervous system|nerve tissue]], which sends and processes signals. There is also typically an internal [[digestion|digestive]] chamber, with one or two openings. Animals with this sort of organization are called metazoans, or [[eumetazoan]]s when the former is used for animals in general.

| subdivision_ranks = Major groups

| subdivision = *[[Bilateria]] (~30 phyla)

* [[Cnidaria]]

* [[Ctenophora]]

* [[Placozoa]]

* [[Porifera]]

| synonyms = * Metazoa {{au|Haeckel 1874}}<ref name="Phylonyms Metazoa">{{cite book |chapter=Metazoa E. Haeckel 1874 [J. R. Garey and K. M. Halanych], converted clade name |title=Phylonyms: A Companion to the PhyloCode |edition=1st |date=2020 |publisher=[[CRC Press]] |doi=10.1201/9780429446276 |pages=1352 |isbn=9780429446276 |s2cid=242704712 |editor-first1=Kevin |editor-last1=de Queiroz |editor-first2=Philip |editor-last2=Cantino |editor-first3=Jacques |editor-last3=Gauthier}}</ref>

* Choanoblastaea {{au|Nielsen 2008}}<ref name="Choanoblastaea">{{cite journal |title=Six major steps in animal evolution: are we derived sponge larvae? |journal=Evolution & Development |volume=10 |issue=2 |date=2008 |pages=241–257 |first1=Claus |last1=Nielsen |doi=10.1111/j.1525-142X.2008.00231.x |pmid=18315817 |s2cid=8531859 |issn=1520-541X}}</ref>

* Gastrobionta {{au|Rothm. 1948}}<ref name="Rothmaler">{{cite journal |first1=Werner |last1=Rothmaler |date=1951 |title=Die Abteilungen und Klassen der Pflanzen |journal=Feddes Repertorium |doi=10.1002/fedr.19510540208 |volume=54 |issue=2–3 |pages=256–266}}</ref>

* Zooaea {{au|Barkley 1939}}<ref name="Rothmaler"/>

* Euanimalia {{au|Barkley 1939}}<ref name="Rothmaler"/>

}}

'''Animals''' are [[multicellular]], [[eukaryotic]] [[organism]]s comprising the [[Biology|biological]] [[Kingdom (biology)|kingdom]] '''Animalia''' ({{IPAc-en|ˌ|æ|n|ɪ|ˈ|m|eɪ|l|i|ə}}<ref>{{Cite Merriam-Webster|animalia|accessdate=12 May 2024}}</ref>). With few exceptions, animals [[heterotroph|consume organic material]], [[Cellular respiration#Aerobic respiration|breathe oxygen]], have [[myocyte]]s and are [[motility|able to move]], can [[reproduce sexually]], and grow from a hollow sphere of [[Cell (biology)|cells]], the [[blastula]], during [[embryonic development]]. Animals form a [[clade]], meaning that they arose from a single [[common ancestor]]. Over 1.5 million<!--<ref name="Zhang2013"/>--> [[extant taxon|living]] animal [[species]] have been [[species description|described]], of which around 1.05 million are [[insect]]s, over 85,000 are [[mollusc]]s, and around 65,000 are [[vertebrate]]s. It has been estimated there are as many as 7.77&nbsp;million animal species on Earth. Animal body lengths range from {{cvt|8.5|μm|in}} to {{cvt|33.6|m|ft}}. They have complex [[ecologies]] and [[biological interaction|interaction]]s with each other and their environments, forming intricate [[food web]]s. The scientific study of animals is known as [[zoology]], and the study of animal behaviour is known as [[ethology]].<!--note: British English -->

The animal kingdom is divided into five major clades, namely [[Porifera]], [[Ctenophora]], [[Placozoa]], [[Cnidaria]] and [[Bilateria]]. Most living animal species belong to the clade Bilateria, a highly proliferative clade whose members have a [[bilaterally symmetric]] and significantly [[cephalization|cephalised]] [[body plan]], and the vast majority of bilaterians belong to two large clades: the [[protostome]]s, which includes organisms such as [[arthropod]]s, [[mollusc]]s, [[flatworm]]s, [[annelid]]s and [[nematode]]s; and the [[deuterostome]]s, which include [[echinoderm]]s, [[hemichordate]]s and [[chordate]]s, the latter of which contains the [[vertebrate]]s. The much smaller [[basal (phylogenetics)|basal]] [[phylum]] [[Xenacoelomorpha]] have an uncertain position within Bilateria.

All animals have [[eukaryotic]] cells, surrounded by a characteristic extracellular matrix composed of [[collagen]] and elastic [[glycoprotein]]s. This may be calcified to form structures like [[animal shell|shell]]s, [[bone]]s, and [[spicule]]s. During development it forms a relatively flexible framework upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms like plants and fungi have cells held in place by cell walls, and so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: [[tight junction]]s, [[gap junction]]s, and [[desmosome]]s.

Animals first appeared in the fossil record in the late [[Cryogenian]] period and diversified in the subsequent [[Ediacaran]] period in what is known as the [[Avalon explosion]]. Earlier evidence of animals is still controversial; the [[sponge]]-like organism ''[[Otavia]]'' has been dated back to the [[Tonian]] period at the start of the [[Neoproterozoic]], but its identity as an animal is heavily contested.<ref>{{cite journal |last1=Antcliffe |first1=Jonathan B. |last2=Callow |first2=Richard H. T. |last3=Brasier |first3=Martin D. |title=Giving the early fossil record of sponges a squeeze |journal=Biological Reviews |date=November 2014 |volume=89 |issue=4 |pages=972–1004 |doi=10.1111/brv.12090 |pmid=24779547 |s2cid=22630754}}</ref> Nearly all modern animal phyla first appeared in the fossil record as [[marine species]] during the [[Cambrian explosion]], which began around 539&nbsp;[[million years ago]] (Mya), and most [[class (biology)|class]]es during the [[Ordovician radiation]] 485.4&nbsp;Mya. Common to all living animals, 6,331 groups of [[gene]]s have been identified that may have arisen from a single [[#Phylogeny|common ancestor]] that lived about 650&nbsp;Mya during the [[Cryogenian]] period.

===Reproduction and development===

Nearly all animals undergo some form of sexual reproduction. Adults are [[diploid]] or [[polyploid]]. They have a few specialized reproductive cells, which undergo [[meiosis]] to produce smaller motile [[spermatozoon|spermatozoa]] or larger non-motile [[ovum|ova]]. These fuse to form [[zygote]]s, which develop into new individuals.

Historically, [[Aristotle]] divided animals [[Aristotle's biology|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 [[Synonym (taxonomy)|synonymous]] with 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 [[evolution]]ary relationships between [[taxa]].

Many animals are also capable of [[asexual reproduction]]. This may take place through [[parthenogenesis]], where fertile eggs are produced without mating, or in some cases through [[fragmentation_(biology)|fragmentation]].

[[Human]]s make [[human uses of animals|use of]] many other animal species for [[Human food|food]] (including [[meat]]<!--no list please-->, [[eggs as food|egg]]s, and [[dairy product]]s), for [[animal product|material]]s (such as [[leather]], [[fur]], and [[wool]]), as [[pet]]s and as [[working animal]]s for [[transportation]], and [[service (economics)|service]]s. [[Dog]]s, the first [[domesticate]]d animal, have been used [[hunting dog|in hunting]], [[guard dog|in security]] and [[dogs in warfare|in warfare]], as have [[equestrianism|horse]]s, [[pigeon post|pigeon]]s and [[falconry|birds of prey]]; while other [[terrestrial animal|terrestrial]] and [[aquatic animal]]s are [[hunt]]ed for sports, trophies or profits. Non-human animals are also an important [[cultural]] element of [[human evolution]], having appeared in [[cave art]]s and [[totem]]s since the earliest times, and are frequently featured in [[mythology]], [[religion]], [[art]]s, [[literature]], [[heraldry]], [[politics]], and [[sport]]s.

[[Image:Mitosis-flourescent.jpg|thumb|left|200px|A [[newt]] [[lung]] [[cell (biology)|cell]] [[staining (biology)|stained]] with [[fluorescent]] [[dye]]s undergoing [[mitosis]], specifically early [[anaphase]].]]

A [[zygote]] initially develops into a hollow sphere, called a [[blastula]], which undergoes rearrangement and differentiation. 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 [[invagination|invaginates]] to form a [[gastrula]] with a digestive chamber, and two separate [[germ layer]]s - 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.

==Etymology==

Most animals grow by indirectly using the energy of [[sunlight]]. Plants use this [[energy]] to convert sunlight into simple [[sugars]] using a process known as [[photosynthesis]]. Starting with the molecules [[carbon dioxide]] (CO₂) and [[water]] (H₂O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of [[glucose]] (C₆H₁₂O₆) and releases [[oxygen]] (O₂). These sugars are then used as the building blocks which allow the plant to grow. When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal. They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion. This process is known as [[glycolysis]].

<!--intentional blank line after all headings-->

The word ''animal'' comes from the Latin noun {{Wikt-lang|la|animal}} of the same meaning, which is itself derived from Latin {{Wikt-lang|la|animalis}} 'having breath or soul'.<ref>{{cite dictionary |last=Cresswell |first=Julia |dictionary=The Oxford Dictionary of Word Origins |year=2010 |publisher=[[Oxford University Press]] |___location=New York |edition=2nd |isbn=978-0-19-954793-7 |entry=Animal |quote='having the breath of life', from anima 'air, breath, life'.}}</ref> The biological definition includes all members of the kingdom Animalia.<ref name=americanheritage_animal>{{cite dictionary |title=Animal |dictionary=The American Heritage Dictionary |publisher=[[Houghton Mifflin]] |year=2006 |edition=4th}}</ref> In colloquial usage, the term ''animal'' is often used to refer only to nonhuman animals.<ref>{{cite dictionary |dictionary=English Oxford Living Dictionaries |title=Animal |url=https://en.oxforddictionaries.com/definition/animal |access-date=26 July 2018 |archive-url=https://web.archive.org/web/20180726233938/https://en.oxforddictionaries.com/definition/animal |archive-date=26 July 2018 |url-status=dead}}</ref><ref>{{cite journal |last1=Boly |first1=Melanie |last2=Seth |first2=Anil K. |last3=Wilke |first3=Melanie |last4=Ingmundson |first4=Paul |last5=Baars |first5=Bernard |last6=Laureys |first6=Steven |last7=Edelman |first7=David |last8=Tsuchiya |first8=Naotsugu |display-authors=5 |year=2013 |title=Consciousness in humans and non-human animals: recent advances and future directions |journal=[[Frontiers in Psychology]] |volume=4 |page=625 |doi=10.3389/fpsyg.2013.00625 |pmc=3814086 |pmid=24198791 |doi-access=free}}</ref><ref>{{Cite web |website=[[Royal Society]] |url=https://royalsociety.org/topics-policy/publications/2004/non-human-animals/ |title=The use of non-human animals in research |access-date=7 June 2018 |archive-url=https://web.archive.org/web/20180612140908/https://royalsociety.org/topics-policy/publications/2004/non-human-animals/ |archive-date=12 June 2018 |url-status=live}}</ref><ref>{{Cite dictionary |url=https://www.collinsdictionary.com/dictionary/english/nonhuman |title=Nonhuman |dictionary=Collins English Dictionary |access-date=7 June 2018 |archive-url=https://web.archive.org/web/20180612142932/https://www.collinsdictionary.com/dictionary/english/nonhuman |archive-date=12 June 2018 |url-status=live}}</ref> The term ''metazoa'' is derived from Ancient Greek {{lang|grc|μετα}} {{tlit|grc|meta}} 'after' (in biology, the prefix ''meta-'' stands for 'later') and {{lang|grc|ζῷᾰ}} {{tlit|grc|zōia}} 'animals', plural of {{lang|grc|ζῷον}} {{tlit|grc|zōion}} 'animal'.<ref>{{cite dictionary |title=Metazoa |dictionary=Collins |url=https://www.collinsdictionary.com/ko/dictionary/english/metazoa |access-date=6 July 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730091429/https://www.collinsdictionary.com/ko/dictionary/english/metazoa |url-status=live}} and further [https://www.collinsdictionary.com/ko/dictionary/english/meta meta- (sense 1)] {{Webarchive|url=https://web.archive.org/web/20220730091429/https://www.collinsdictionary.com/ko/dictionary/english/meta |date=30 July 2022 }} and [https://www.collinsdictionary.com/ko/dictionary/english/zoa -zoa] {{Webarchive|url=https://web.archive.org/web/20220730091429/https://www.collinsdictionary.com/ko/dictionary/english/zoa |date=30 July 2022 }}.</ref> A '''metazoan''' is any member of the group ''Metazoa''.<ref>{{cite dictionary |title=Metazoan |dictionary=Merriam-Webster |url=https://www.merriam-webster.com/dictionary/metazoan |access-date=6 July 2022 |archive-date=6 July 2022 |archive-url=https://web.archive.org/web/20220706115538/https://www.merriam-webster.com/dictionary/metazoan |url-status=live}}</ref>

Animals who live close to [[hydrothermal vent]]s and [[cold seep]]s on the ocean floor are not dependent on the energy of sunlight. Instead, [[chemosynthesis|chemosynthetic]] [[archaea]] and [[eubacteria]] form the base of the food chain.

== Characteristics ==

==Origin and fossil record==

Animals are generally considered to have [[Evolution|evolved]] from a [[flagellate]]d eukaryote. Their closest known living relatives are the [[choanoflagellate]]s, collared flagellates that have a morphology similar to the choanocytes of certain sponges. Molecular studies place animals in a supergroup called the [[opisthokont]]s, which also include the [[ choanoflagellates]], [[fungus|fungi]] and a few small parasitic [[protist]]s. 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.

[[File:Blastulation.png|thumb|Animals are unique in having the ball of cells of the early [[embryo]] (1) develop into a hollow ball or [[blastula]] (2).]]

The first fossils that might represent animals appear towards the end of the [[Precambrian]], around 575 million years ago, and are known as the [[Ediacaran biota|Ediacaran or Vendian biota]]. These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all. Aside from them, most known animal phyla make a more or less simultaneous appearance during the [[Cambrian]] period, about 542 million years ago. It is still disputed whether this event, called the [[Cambrian explosion]], represents a rapid divergence between different groups or a change in conditions that made fossilization possible.

Animals have several characteristics that they share with other living things. Animals are [[eukaryotic]], [[multicellular]], and [[aerobic respiration|aerobic]], as are [[plants]] and [[fungi]].<ref name="Avila1995">{{cite book |last=Avila |first=Vernon L. |title=Biology: Investigating Life on Earth |url={{GBurl |id=B_OOazzGefEC |p=767}} |year=1995 |publisher=Jones & Bartlett |isbn=978-0-86720-942-6 |page=767}}</ref> Unlike plants and [[alga]]e, which [[Autotroph|produce their own food]],<ref name=AnimalCells>{{cite web |last=Davidson |first=Michael W. |title=Animal Cell Structure |url=https://micro.magnet.fsu.edu/cells/animalcell.html |access-date=20 September 2007 |archive-url=https://web.archive.org/web/20070920235924/https://micro.magnet.fsu.edu/cells/animalcell.html |archive-date=20 September 2007 |url-status=live}}</ref> animals [[heterotroph|cannot produce their own food]],<ref name="palaeos">{{cite web |title=Palaeos:Metazoa |url=https://palaeos.com/metazoa/metazoa.html |website=Palaeos |access-date=25 February 2018 |archive-url=https://web.archive.org/web/20180228005641/https://palaeos.com/metazoa/metazoa.html |archive-date=28 February 2018 |url-status=dead}}</ref><ref name=Windows>{{cite web |last=Bergman |first=Jennifer |title=Heterotrophs |url=https://www.windows.ucar.edu/tour/link=/earth/Life/heterotrophs.html&edu=high |access-date=30 September 2007 |archive-url=https://web.archive.org/web/20070829051950/https://www.windows.ucar.edu/tour/link%3D/earth/Life/heterotrophs.html%26edu%3Dhigh |archive-date=29 August 2007 |url-status=dead}}</ref> a feature they share with fungi. Animals ingest organic material and digest it internally.<ref>{{cite journal |last1=Douglas |first1=Angela E. |last2=Raven |first2=John A. |title=Genomes at the interface between bacteria and organelles |journal=[[Philosophical Transactions of the Royal Society B]] |volume=358 |issue=1429 |pages=5–17 |date=January 2003 |pmid=12594915 |pmc=1693093 |doi=10.1098/rstb.2002.1188}}</ref>

==Groups of animals==

The sponges ([[Porifera]]) diverged from other animals early. As mentioned above, they lack the complex organization found in most other phyla. Their cells are differentiated, but in most cases not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through pores. [[Archaeocyatha]], which have fused skeletons, may represent sponges or a separate phylum.

=== Structural features ===

Among the eumetazoan phyla, two are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus. These are the [[Cnidaria]], which include [[sea anemone]]s, [[coral]]s, and [[jellyfish]], and the [[Ctenophora]] or comb jellies. Both have distinct tissues, but they are not organized into [[organ (anatomy)|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 [[Placozoa]]ns are similar, but they do not have a permanent digestive chamber.

Animals have structural characteristics that set them apart from all other living things:

The remaining animals form a monophyletic group called 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 [[echinoderm]]s are radially symmetric, and certain parasitic worms have extremely simplified body structures.

* cells surrounded by an [[extracellular matrix]]<ref name="Heino"/> composed of

Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to four major lineages:

** [[collagen]]<ref>{{cite book |last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |title=Molecular Biology of the Cell |edition=4th |year=2002 |publisher=[[Garland Science]] |url=https://www.ncbi.nlm.nih.gov/books/NBK26810/ |isbn=978-0-8153-3218-3 |access-date=29 August 2017 |archive-url=https://web.archive.org/web/20161223074013/https://www.ncbi.nlm.nih.gov/books/NBK26810/ |archive-date=23 December 2016 |url-status=live}}</ref> and

# [[Deuterostomes]]

** elastic [[glycoprotein]]s<ref name="Heino">{{cite journal |last1=Heino |first1=Jyrki |last2=Huhtala |first2=Mikko |last3=Käpylä |first3=Jarmo |last4=Johnson |first4=Mark S. |title=Evolution of collagen-based adhesion systems |journal=The International Journal of Biochemistry & Cell Biology |date=February 2009 |volume=41 |issue=2 |pages=341–348 |doi=10.1016/j.biocel.2008.08.021|pmid=18790075 }}</ref><ref>{{cite book |last1=Alberts |first1=Bruce |last2=Johnson |first2=Alexander |last3=Lewis |first3=Julian |last4=Raff |first4=Martin |last5=Roberts |first5=Keith |last6=Walter |first6=Peter |title=Molecular Biology of the Cell |edition=4th |year=2002 |publisher=[[Garland Science]] |url=https://www.ncbi.nlm.nih.gov/books/NBK26810/ |isbn=978-0-8153-3218-3 |access-date=29 August 2017 |archive-url=https://web.archive.org/web/20161223074013/https://www.ncbi.nlm.nih.gov/books/NBK26810/ |archive-date=23 December 2016 |url-status=live}}</ref>

# [[Ecdysozoa]]

# [[Platyzoa]]

# [[Lophotrochozoa]]

* [[motile|motility]]<ref name=Concepts>{{cite web |url=https://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm |last=Saupe |first=S. G. |title=Concepts of Biology |access-date=30 September 2007 |archive-url=https://web.archive.org/web/20071121084100/https://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm |archive-date=21 November 2007 |url-status=live}}</ref> i.e. able to spontaneously move their bodies during at least part of their [[Biological life cycle|life cycle]].

In addition to these, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the [[Acoelomorpha]], [[Rhombozoa]], and [[Orthonectida]]. The [[Myxozoa]], single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Bilateria as well.

* a [[blastula]] stage during [[Embryogenesis|embryonic development]]<ref>{{cite book |last=Minkoff |first=Eli C. |title=Barron's EZ-101 Study Keys Series: Biology |year=2008 |publisher=Barron's Educational Series |isbn=978-0-7641-3920-8 |edition=2nd, revised |page=48}}</ref>

===Deuterostomes===

[[Image:Caerulea3 crop.jpg|thumb|[[White's Tree Frog]], ''Litoria caerulea''.]]

[[Deuterostome]]s differ from the other Bilateria, called [[protostome]]s, in several ways. In both cases there is a complete digestive tract. However, in protostomes the initial opening (the [[archenteron]]) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. 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. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.

Typically, there is an internal [[Digestion|digestive]] chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).<ref>{{cite book |last1=Hillmer |first1=Gero |last2=Lehmann |first2=Ulrich |translator-first=J. |translator-last=Lettau |title=Fossil Invertebrates |year=1983 |publisher=[[Cambridge University Press]] Archive |isbn=978-0-521-27028-1 |page=54 |url={{GBurl |id=9jE4AAAAIAAJ |p=54}} |access-date=8 January 2016}}</ref>

All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the [[Echinodermata]] and [[Chordate|Chordata]]. The former are radially symmetric and exclusively marine, such as [[starfish]], [[sea urchin]]s, and [[sea cucumber]]s. The latter are dominated by the [[vertebrate]]s, animals with backbones. These include [[fish]], [[amphibian]]s, [[reptile]]s, [[bird]]s, and [[mammal]]s.

=== Development ===

In addition to these, the deuterostomes also include the [[Hemichordata]] or acorn worms. Although they are not especially prominent today, the important fossil [[graptolite]]s may belong to this group.

Animal development is controlled by [[Hox genes]], which signal the times and places to develop structures such as body segments and limbs.<ref name="Ryan Mazza Pang 2007">{{cite journal |last1=Ryan |first1=Joseph F. |last2=Mazza |first2=Maureen E. |last3=Pang |first3=Kevin |last4=Matus |first4=David Q. |last5=Baxevanis |first5=Andreas D. |last6=Martindale |first6=Mark Q. |last7=Finnerty |first7=John R. |title=Pre-Bilaterian Origins of the Hox Cluster and the Hox Code: Evidence from the Sea Anemone, Nematostella vectensis |journal=PLOS ONE |volume=2 |issue=1 |date=24 January 2007 |doi=10.1371/journal.pone.0000153 |doi-access=free |page=e153|pmid=17252055 |pmc=1779807 |bibcode=2007PLoSO...2..153R }}</ref><ref name="de Rosa Grenier Andreeva 1999">{{cite journal |last1=de Rosa |first1=Renaud |last2=Grenier |first2=Jennifer K. |last3=Andreeva |first3=Tatiana |last4=Cook |first4=Charles E. |last5=Adoutte |first5=André |last6=Akam |first6=Michael |last7=Carroll |first7=Sean B. |last8=Balavoine |first8=Guillaume |title=Hox genes in brachiopods and priapulids and protostome evolution |journal=Nature |volume=399 |issue=6738 |date=1999 |issn=0028-0836 |doi=10.1038/21631 |pages=772–776|pmid=10391241 |bibcode=1999Natur.399..772D }}</ref>

The [[Chaetognatha]] or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.

During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised into specialised tissues and organs, making the formation of complex structures possible, and allowing [[Cellular differentiation|cells to be differentiated]].<ref>{{cite book |last=Minkoff |first=Eli C. |title=Barron's EZ-101 Study Keys Series: Biology |year=2008 |publisher=Barron's Educational Series |isbn=978-0-7641-3920-8 |edition=2nd, revised |page=48}}</ref> The extracellular matrix may be calcified, forming structures such as [[Exoskeleton|shells]], [[bone]]s, and [[spicule (sponge)|spicules]].<ref>{{cite book |last=Sangwal |first=Keshra |title=Additives and crystallization processes: from fundamentals to applications |url=https://archive.org/details/additivescrystal00sang |url-access=limited |year=2007 |publisher=John Wiley & Sons |isbn=978-0-470-06153-4 |page=[https://archive.org/details/additivescrystal00sang/page/n228 212]}}</ref> 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.<ref>{{cite book |last=Becker |first=Wayne M. |title=The world of the cell |year=1991 |publisher=[[Benjamin Cummings]] |isbn=978-0-8053-0870-9 |url=https://archive.org/details/worldofcell00beck_0}}</ref>

===Ecdysozoa===

[[Image:Sympetrum flaveolum - side (aka).jpg|thumb|[[Yellow-winged Darter]], ''Sympetrum flaveolum'']]

The [[Ecdysozoa]] are protostomes, named after the common trait of growth by moulting or [[ecdysis]]. The largest animal phylum belongs here, the [[Arthropoda]], including [[insect]]s, [[spider]]s, [[crab]]s, 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.

=== Reproduction ===

The ecdysozoans also include the [[Nematoda]] or roundworms, 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, which are invisible to the unaided eye, and the [[Kinorhyncha]], [[Priapulida]], and [[Loricifera]]. These groups have a reduced coelom, called a pseudocoelom.did you know that your mum said hello to me

{{see also|Sexual reproduction#Animals|Asexual reproduction#Examples in animals}}

The remaining two groups of protostomes are sometimes grouped together as the [[Spiralia]], since in both embryos develop with spiral cleavage.

[[File:Odonata copulation.jpg|thumb|[[Sexual reproduction]] is nearly universal in animals, such as these [[dragonflies]].]]

===Platyzoa===

[[Image:Bedford's Flatworm.jpg|thumb|Bedford's flatworm, ''Pseudobiceros bedfordi'']]

The [[Platyzoa]] include the phylum [[Platyhelminthes]], the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.

Nearly all animals make use of some form of sexual reproduction.<ref>{{cite book |last=Knobil |first=Ernst |title=Encyclopedia of reproduction |volume=1 |year=1998 |publisher=Academic |isbn=978-0-12-227020-8 |page=[https://archive.org/details/encyclopediaofre0000unse_f1r2/page/315 315] |url=https://archive.org/details/encyclopediaofre0000unse_f1r2/page/315}}</ref> They produce [[haploid]] [[gamete]]s by [[meiosis]]; the smaller, motile gametes are [[spermatozoa]] and the larger, non-motile gametes are [[ova]].<ref>{{cite book |last=Schwartz |first=Jill |title=Master the GED 2011 |year=2010 |publisher=Peterson's |isbn=978-0-7689-2885-3 |page=[https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371 371] |url=https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371}}</ref> These fuse to form [[zygote]]s,<ref>{{cite book |last=Hamilton |first=Matthew B. |title=Population genetics |url=https://archive.org/details/populationgeneti00hami |url-access=limited |year=2009 |publisher=[[Wiley-Blackwell]] |isbn=978-1-4051-3277-0 |page=[https://archive.org/details/populationgeneti00hami/page/n69 55]}}</ref> 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.<ref>{{cite book |last1=Ville |first1=Claude Alvin |last2=Walker |first2=Warren Franklin |last3=Barnes |first3=Robert D. |title=General zoology |year=1984 |publisher=Saunders College |isbn=978-0-03-062451-3 |page=467}}</ref> In most other groups, the blastula undergoes more complicated rearrangement.<ref>{{cite book |last1=Hamilton |first1=William James |last2=Boyd |first2=James Dixon |last3=Mossman |first3=Harland Winfield |title=Human embryology: (prenatal development of form and function) |year=1945 |publisher=[[Williams & Wilkins]] |page=330}}</ref> It first [[Invagination|invaginates]] to form a [[gastrula]] with a digestive chamber and two separate [[germ layer]]s, an external [[ectoderm]] and an internal [[endoderm]].<ref>{{cite book |last=Philips |first=Joy B. |title=Development of vertebrate anatomy |year=1975 |publisher=Mosby |isbn=978-0-8016-3927-2 |page=[https://archive.org/details/developmentofver0000phil/page/176 176] |url=https://archive.org/details/developmentofver0000phil/page/176}}</ref> In most cases, a third germ layer, the [[mesoderm]], also develops between them.<ref>{{cite book |title=The Encyclopedia Americana |volume=10 |year=1918 |page=281}}</ref> These germ layers then differentiate to form tissues and organs.<ref>{{cite book |last1=Romoser |first1=William S. |author-link1=William S. Romoser |last2=Stoffolano |first2=J. G. |title=The science of entomology |year=1998 |publisher=WCB McGraw-Hill |isbn=978-0-697-22848-2 |page=156}}</ref>

A number of parasites are included in this group, such as the [[fluke]]s and [[tapeworm]]s. Flatworms lack a coelom, as do their closest relatives, the microscopic [[Gastrotricha]].

Repeated instances of [[inbreeding|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.<ref name="pmid19834483">{{cite journal |last1=Charlesworth |first1=D. |last2=Willis |first2=J. H. |title=The genetics of inbreeding depression |journal=[[Nature Reviews Genetics]] |volume=10 |issue=11 |pages=783–796 |year=2009 |pmid=19834483 |doi=10.1038/nrg2664 |s2cid=771357}}</ref><ref name="pmid3324702">{{cite book |last1=Bernstein |first1=H. |last2=Hopf |first2=F. A. |last3=Michod |first3=R. E. |chapter=The Molecular Basis of the Evolution of Sex |title=Molecular Genetics of Development |series=Advances in Genetics |volume=24 |pages=323–370 |year=1987 |pmid=3324702 |doi=10.1016/s0065-2660(08)60012-7 |isbn=978-0-12-017624-3}}</ref> Animals have evolved numerous mechanisms for [[inbreeding avoidance|avoiding close inbreeding]].<ref name=Pusey>{{cite journal |last1=Pusey |first1=Anne |last2=Wolf |first2=Marisa |title=Inbreeding avoidance in animals |journal=Trends Ecol. Evol. |volume=11 |issue=5 |pages=201–206 |year=1996 |pmid=21237809 |doi=10.1016/0169-5347(96)10028-8 |bibcode=1996TEcoE..11..201P}}</ref>

The other platyzoan phyla are microscopic and pseudocoelomate. The most prominent are the [[Rotifera]] or rotifers, which are common in aqueous environments. They also include the [[Acanthocephala]] or spiny-headed worms, the [[Gnathostomulida]], [[Micrognathozoa]], and possibly the [[Cycliophora]]. These groups share the presence of complex jaws, from which they are called the [[Gnathifera]].

Some animals are capable of [[asexual reproduction]], which often results in a genetic clone of the parent. This may take place through [[Fragmentation (reproduction)|fragmentation]]; [[budding]], such as in [[Hydra (genus)|''Hydra'']] and other [[cnidaria]]ns; or [[parthenogenesis]], where fertile eggs are produced without [[mating]], such as in [[aphid]]s.<ref>{{cite book |last1=Adiyodi |first1=K. G. |title=Reproductive Biology of Invertebrates|last2=Hughes |first2=Roger N. |last3=Adiyodi |first3=Rita G. |date=July 2002 |publisher=Wiley |isbn=978-0-471-48968-9 |volume=11, Progress in Asexual Reproduction|page=116}}</ref><ref>{{cite web |last1=Schatz |first1=Phil |title=Concepts of Biology: How Animals Reproduce |url=https://philschatz.com/biology-concepts-book/contents/m45547.html |publisher=OpenStax College |access-date=5 March 2018 |archive-url=https://web.archive.org/web/20180306022745/https://philschatz.com/biology-concepts-book/contents/m45547.html |archive-date=6 March 2018 |url-status=live}}</ref>

===Lophotrochozoa===

[[Image:Reef2063.jpg|thumb|Big blue octopus, ''Octopus cyanea'']]

The [[Lophotrochozoa]] include two of the most successful animal phyla, the [[Mollusca]] and [[Annelida]]. The former includes animals such as [[snail]]s, [[clam]]s, and [[squid]]s, and the latter comprises the segmented worms, such as [[earthworm]]s and [[leech]]es. These two groups have long been considered close relatives because of the common presence of [[trochophore]] larvae, but the annelids were considered closer to the arthropods, because they are both segmented. Now this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.

== Ecology ==

The Lophotrochozoa also include the [[Nemertea]] or ribbon worms, the [[Sipuncula]], and several phyla that have a fan of cilia around the mouth, called a [[lophophore]]. These were traditionally grouped together as the lophophorates, but it now appears they are [[paraphyletic]], some closer to the Nemertea and some to the Mollusca and Annelida. They include the [[Brachiopoda]] or lamp shells, which are prominent in the fossil record, the [[Entoprocta]], the [[Phoronida]], and possibly the [[Bryozoa]] or moss animals.

[[File:Ultramarine Flycatcher (Ficedula superciliaris) Naggar, Himachal Pradesh, 2013 (cropped).JPG|thumb|upright|left|[[Predator]]s, such as this [[ultramarine flycatcher]] (''Ficedula superciliaris''), feed on other animals.]]

==History of classification==

[[Image:Carl von Linné.jpg|thumb|[[Carolus Linnaeus]], known as the "father of modern [[taxonomy]]".]]

[[Aristotle]] divided the living world between animals and [[plant]]s, and this was followed by [[Carolus Linnaeus]] in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic [[protozoa]] were originally considered animals because they move, but are now treated separately.

Animals are categorised into ecological groups depending on their [[trophic level]]s and [[feeding behaviour|how they consume organic material]]. Such groupings include [[carnivore]]s (further divided into subcategories such as [[piscivore]]s, [[insectivore]]s, [[ovivore]]s, etc.), [[herbivore]]s (subcategorised into [[folivore]]s, [[graminivore]]s, [[frugivore]]s, [[granivore]]s, [[nectarivore]]s, [[algivore]]s, etc.), [[omnivore]]s, [[fungivore]]s, [[scavenger]]s/[[detritivore]]s,<ref>{{cite book |last1=Marchetti |first1=Mauro |last2=Rivas |first2=Victoria |title=Geomorphology and environmental impact assessment |year=2001 |publisher=Taylor & Francis |isbn=978-90-5809-344-8 |page=84}}</ref> and [[parasite]]s.<ref>{{cite book |last=Levy |first=Charles K. |title=Elements of Biology |year=1973 |publisher=[[Appleton-Century-Crofts]] |isbn=978-0-390-55627-1 |page=108}}</ref> [[Biological interaction|Interaction]]s between animals of each [[biome]] form complex [[food web]]s within that [[ecosystem]]. In carnivorous or omnivorous species, [[predation]] is a [[consumer-resource systems|consumer–resource interaction]] where the predator feeds on another organism, its [[prey]],<ref name=Ecology>{{cite book |last1=Begon |first1=M. |last2=Townsend |first2=C. |last3=Harper |first3=J. |date=1996 |title=Ecology: Individuals, populations and communities |edition=3rd |publisher=Blackwell |isbn=978-0-86542-845-4 |url=https://archive.org/details/ecology00mich}}</ref> who often evolves [[anti-predator adaptation]]s to avoid being fed upon. [[Selective pressure]]s imposed on one another lead to an [[evolutionary arms race]] between predator and prey, resulting in various antagonistic/[[competition (biology)|competitive]] [[coevolution]]s.<ref>{{cite book |last1=Allen |first1=Larry Glen |last2=Pondella |first2=Daniel J. |last3=Horn |first3=Michael H. |title=Ecology of marine fishes: California and adjacent waters |year=2006 |publisher=[[University of California Press]] |isbn=978-0-520-24653-9 |page=428}}</ref><ref>{{cite book |last=Caro |first=Tim |author-link=Tim Caro |title=Antipredator Defenses in Birds and Mammals |year=2005 |publisher=[[University of Chicago Press]] |pages=1–6 and passim}}</ref> Almost all multicellular predators are animals.<ref name="SimpsonCB">{{cite journal |last1=Simpson |first1=Alastair G. B. |last2=Roger |first2=Andrew J. |doi=10.1016/j.cub.2004.08.038 |pmid=15341755 |title=The real 'kingdoms' of eukaryotes |journal=Current Biology |volume=14 |issue=17 |pages=R693–R696 |year=2004 |s2cid=207051421 |doi-access=free |bibcode=2004CBio...14.R693S}}</ref> Some [[consumer (food chain)|consumers]] use multiple methods; for example, in [[parasitoid wasp]]s, the larvae feed on the hosts' living tissues, killing them in the process,<ref>{{cite journal |last=Stevens |first=Alison N. P. |title=Predation, Herbivory, and Parasitism |journal=Nature Education Knowledge |year=2010 |volume=3 |issue=10 |page=36 |url=https://www.nature.com/scitable/knowledge/library/predation-herbivory-and-parasitism-13261134 |access-date=12 February 2018 |archive-url=https://web.archive.org/web/20170930230324/https://www.nature.com/scitable/knowledge/library/predation-herbivory-and-parasitism-13261134 |archive-date=30 September 2017 |url-status=live}}</ref> but the adults primarily consume nectar from flowers.<ref>{{Cite journal |last1=Jervis |first1=M. A. |last2=Kidd |first2=N. A. C. |date=November 1986 |title=Host-Feeding Strategies in Hymenopteran Parasitoids |journal=Biological Reviews |volume=61 |issue=4 |pages=395–434 |doi=10.1111/j.1469-185x.1986.tb00660.x |s2cid=84430254}}</ref> Other animals may have very specific [[feeding behaviour]]s, such as [[hawksbill sea turtle]]s which mainly [[Spongivore|eat sponges]].<ref name="ScienceSpongi">{{cite journal |last=Meylan |first=Anne |title=Spongivory in Hawksbill Turtles: A Diet of Glass |journal=Science |volume=239 |issue=4838 |pages=393–395 |date=22 January 1988 |doi=10.1126/science.239.4838.393 |pmid=17836872 |jstor=1700236 |bibcode=1988Sci...239..393M |s2cid=22971831}}</ref>

In [[Carolus Linnaeus|Linnaeus']] original scheme, the animals were one of three kingdoms, divided into the classes of [[Vermes]], [[Insect]]a, [[Fish|Pisces]], [[Amphibia]], [[bird|Aves]], and [[Mammal]]ia. Since then the last four have all been subsumed into a single phylum, the [[chordate|Chordata]], whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.

[[File:Expl0072 - Flickr - NOAA Photo Library.jpg|thumb|[[Hydrothermal vent]] mussels and shrimps]]

Most animals rely on [[biomass]] and [[biological energy|bioenergy]] produced by [[plant]]s and [[phytoplankton]]s (collectively called [[producer (biology)|producer]]s) through [[photosynthesis]]. Herbivores, as [[consumer (food chain)#Levels of the food chain|primary consumer]]s, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher [[trophic level]]s indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidise [[carbohydrate]]s, [[lipid]]s, [[protein]]s and other biomolecules, which allows the animal to grow and to sustain [[basal metabolism]] and fuel other biological processes such as [[animal locomotion|locomotion]].<ref>{{cite book |title=Understanding Science: Upper Primary |last=Clutterbuck |first=Peter |year=2000 |publisher=Blake |isbn=978-1-86509-170-9 |page=9}}</ref><ref>{{cite book |last1=Garrett |first1=Reginald |last2=Grisham |first2=Charles M. |title=Biochemistry |url=https://archive.org/details/biochemistry00rhga |url-access=limited |year=2010 |publisher=Cengage |isbn=978-0-495-10935-8 |page=[https://archive.org/details/biochemistry00rhga/page/n572 535]}}</ref> Some [[benthic]] animals living close to [[hydrothermal vent]]s and [[cold seep]]s on the dark [[sea floor]] consume organic matter produced through [[chemosynthesis]] (via [[oxidising]] [[inorganic compound]]s such as [[hydrogen sulfide]]) by [[archaea]] and [[bacteria]].<ref>{{cite book |last1=Castro |first1=Peter |last2=Huber |first2=Michael E. |title=Marine Biology |publisher=McGraw Hill |year=2007 |edition=7th |page=376 |isbn=978-0-07-722124-9}}</ref>

Animals evolved in the sea. Lineages of arthropods colonised land around the same time as [[land plant]]s, probably between 510 and 471 million years ago during the [[Late Cambrian]] or Early [[Ordovician]].<ref name="Rota-Stabelli2013">{{cite journal |last1=Rota-Stabelli |first1=Omar |last2=Daley |first2=Allison C. |last3=Pisani |first3=Davide |title=Molecular Timetrees Reveal a Cambrian Colonization of Land and a New Scenario for Ecdysozoan Evolution |journal=Current Biology |volume=23 |issue=5 |pages=392–398 |year=2013 |doi-access=free |doi=10.1016/j.cub.2013.01.026 |pmid=23375891 |bibcode=2013CBio...23..392R}}</ref> [[Vertebrate]]s such as the [[lobe-finned fish]] ''[[Tiktaalik]]'' started to move on to land in the late [[Devonian]], about 375 million years ago.<ref>{{cite journal |last1=Daeschler |first1=Edward B. |last2=Shubin |first2=Neil H. |last3=Jenkins |first3=Farish A. Jr. |title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan |journal=[[Nature (journal)|Nature]] |volume=440 |pages=757–763 |date=6 April 2006 |doi=10.1038/nature04639 |pmid=16598249 |issue=7085 |bibcode=2006Natur.440..757D |doi-access=free}}</ref><ref>{{cite journal |author-link=Jennifer A. Clack |last=Clack |first=Jennifer A. |journal=[[Scientific American]] |volume=293 |issue=6 |pages=100–107 |title=Getting a Leg Up on Land |date=21 November 2005 |bibcode=2005SciAm.293f.100C |doi=10.1038/scientificamerican1205-100 |pmid=16323697}}</ref> Animals occupy virtually all of earth's [[habitat]]s and microhabitats, with [[fauna]]s adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms.<ref name="Margulis">{{cite book |last=Margulis |first=Lynn |author1-link=Lynn Margulis |author2=Schwartz, Karlene V. |author3=Dolan, Michael |title=Diversity of Life: The Illustrated Guide to the Five Kingdoms |url={{GBurl |id=8wJXWBMsEOkC |p=115}} |year=1999 |publisher=Jones & Bartlett |isbn=978-0-7637-0862-7 |pages=115–116}}</ref> Animals are however not particularly [[Thermophile|heat tolerant]]; very few of them can survive at constant temperatures above {{convert|50|°C|0|abbr=on}}<ref name="Clarke2014">{{cite journal |last=Clarke |first=Andrew |title=The thermal limits to life on Earth |journal=International Journal of Astrobiology |volume=13 |issue=2 |pages=141–154 |year=2014 |bibcode=2014IJAsB..13..141C |url=https://nora.nerc.ac.uk/id/eprint/507274/1/Clarke.pdf |archive-url=https://web.archive.org/web/20190424155004/https://nora.nerc.ac.uk/id/eprint/507274/1/Clarke.pdf |archive-date=24 April 2019 |url-status=live |doi=10.1017/S1473550413000438 |doi-access=free}}</ref> or in the most extreme cold deserts of continental [[Antarctica]].<ref name="bas-land">{{cite web |title=Land animals |url=https://www.bas.ac.uk/about/antarctica/wildlife/land-animals/ |publisher=[[British Antarctic Survey]] |access-date=7 March 2018 |archive-url=https://web.archive.org/web/20181106225451/https://www.bas.ac.uk/about/antarctica/wildlife/land-animals/ |archive-date=6 November 2018 |url-status=live}}</ref>

The collective global geomorphic influence of animals on the processes shaping the Earth's surface remains largely understudied, with most studies limited to individual species and well-known exemplars.<ref>{{Cite journal |last1=Harvey |first1=Gemma L. |last2=Khan |first2=Zareena |last3=Albertson |first3=Lindsey K. |last4=Coombes |first4=Martin |last5=Johnson |first5=Matthew F. |last6=Rice |first6=Stephen P. |last7=Viles |first7=Heather A. |date=2025-02-25 |title=Global diversity and energy of animals shaping the Earth's surface |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=122 |issue=8 |pages=e2415104122 |doi=10.1073/pnas.2415104122 |issn=1091-6490 |pmid=39964729|pmc=11874378 |bibcode=2025PNAS..12215104H }}</ref>

== Diversity ==

=== Size ===

{{further|Largest organisms|Smallest organisms}}

The [[blue whale]] (''Balaenoptera musculus'') is the largest animal that has ever lived, weighing up to 190 [[tonne]]s and measuring up to {{convert|33.6|m|ft}} long.<ref name=Wood>{{cite book |last=Wood |first=Gerald |title=The Guinness Book of Animal Facts and Feats |year=1983 |isbn=978-0-85112-235-9 |url=https://archive.org/details/guinnessbookofan00wood |publisher=Guinness Superlatives |___location=Enfield, Middlesex}}</ref><ref>{{cite news |last1=Davies |first1=Ella |title=The longest animal alive may be one you never thought of |url=https://www.bbc.com/earth/story/20160420-the-longest-animal-alive-may-not-be-the-blue-whale |work=BBC Earth |access-date=1 March 2018 |date=20 April 2016 |archive-url=https://web.archive.org/web/20180319073808/https://www.bbc.com/earth/story/20160420-the-longest-animal-alive-may-not-be-the-blue-whale |archive-date=19 March 2018 |url-status=live}}</ref> The largest extant terrestrial animal is the [[African bush elephant]] (''Loxodonta africana''), weighing up to 12.25 tonnes<ref name="Wood"/> and measuring up to {{convert|10.67|m|ft}} long.<ref name=Wood/> The largest terrestrial animals that ever lived were [[titanosaur]] [[Sauropoda|sauropod dinosaurs]] such as ''[[Argentinosaurus]]'', which may have weighed as much as 73 tonnes, and ''Supersaurus'' which may have reached 39 metres.<ref name="Mazzettaetal2004">{{cite journal |last1=Mazzetta |first1=Gerardo V. |last2=Christiansen |first2=Per |last3=Fariña |first3=Richard A. |year=2004 |title=Giants and Bizarres: Body Size of Some Southern South American Cretaceous Dinosaurs |journal=Historical Biology |volume=16 |issue=2–4 |pages=71–83 |doi=10.1080/08912960410001715132 |bibcode=2004HBio...16...71M |citeseerx=10.1.1.694.1650 |s2cid=56028251}}</ref><ref>{{Cite conference |last=Curtice |first=Brian |year=2020 |conference=Society of Vertebrate Paleontology |url=https://vertpaleo.org/wp-content/uploads/2021/10/SVP_2021_VirtualBook_final.pdf#page=92 |title=Dinosaur Systematics, Diversity, & Biology |access-date=30 December 2022 |archive-date=19 October 2021 |archive-url=https://web.archive.org/web/20211019192436/https://vertpaleo.org/wp-content/uploads/2021/10/SVP_2021_VirtualBook_final.pdf#page=92 |url-status=live |page=92}}</ref> Several animals are microscopic; some [[Myxozoa]] ([[obligate parasite]]s within the Cnidaria) never grow larger than 20 [[μm]],<ref>{{cite web |url=https://tolweb.org/Myxozoa/2460/2008.07.10 |title=Myxozoa |last=Fiala |first=Ivan |date=10 July 2008 |publisher=Tree of Life Web Project |access-date=4 March 2018 |archive-url=https://web.archive.org/web/20180301225416/https://tolweb.org/Myxozoa/2460/2008.07.10 |archive-date=1 March 2018 |url-status=live}}</ref> and one of the smallest species (''Myxobolus shekel'') is no more than 8.5&nbsp;μm when fully grown.<ref>{{cite journal |last1=Kaur |first1=H. |last2=Singh |first2=R. |title=Two new species of Myxobolus (Myxozoa: Myxosporea: Bivalvulida) infecting an Indian major carp and a cat fish in wetlands of Punjab, India |pmc=3235390 |pmid=23024499 |doi=10.1007/s12639-011-0061-4 |volume=35 |issue=2 |year=2011 |journal=Journal of Parasitic Diseases |pages=169–176}}</ref>

<gallery class="center" mode="nolines" widths="220" heights="160">

File:Anim1754 - Flickr - NOAA Photo Library (1).jpg|The [[blue whale]] is the largest animal that has ever lived; it can be up to {{convert|33.6|m|ft}} long.

File:Fdl17-9-grey.jpg|[[Myxozoa]]ns such as ''[[Myxobolus cerebralis]]'' are single-celled parasites, never more than 20 [[μm]] across.

</gallery>

=== Numbers and habitats of major phyla ===

<!--We are NOT trying to be comprehensive here, there are plenty of lower-level Wikipedia articles for that-->

The following table lists estimated numbers of described extant species for the major animal phyla,<ref name="Zhang2013">{{cite journal |last=Zhang |first=Zhi-Qiang |title=Animal biodiversity: An update of classification and diversity in 2013 |publisher=[[Magnolia Press]] |journal=Zootaxa |volume=3703 |issue=1 |date=30 August 2013 |doi=10.11646/zootaxa.3703.1.3 |url=https://biotaxa.org/Zootaxa/article/download/zootaxa.3703.1.3/4273 |page=5 |access-date=2 March 2018 |archive-url=https://web.archive.org/web/20190424154926/https://biotaxa.org/Zootaxa/article/download/zootaxa.3703.1.3/4273 |archive-date=24 April 2019 |url-status=live |doi-access=free}}</ref> along with their principal habitats (terrestrial, fresh water,<ref name=Balian2008>{{cite book |last1=Balian |first1=E. V. |last2=Lévêque |first2=C. |last3=Segers |first3=H. |first4=K. |last4=Martens |title=Freshwater Animal Diversity Assessment |url={{GBurl |id=Dw4H6DBHnAgC |p=628}} |year=2008 |publisher=Springer |isbn=978-1-4020-8259-7 |page=628}}</ref> and marine),<ref name="Hogenboom2016">{{cite web |last1=Hogenboom |first1=Melissa |title=There are only 35 kinds of animal and most are really weird |url=https://www.bbc.co.uk/earth/story/20150325-all-animal-life-in-35-photos |publisher=BBC Earth |access-date=2 March 2018 |archive-url=https://web.archive.org/web/20180810141811/https://www.bbc.co.uk/earth/story/20150325-all-animal-life-in-35-photos |archive-date=10 August 2018 |url-status=live}}</ref> and free-living or parasitic ways of life.<ref name=Poulin2007>{{cite book |last=Poulin |first=Robert |author-link=Robert Poulin (zoologist) |title=Evolutionary Ecology of Parasites |publisher=[[Princeton University Press]] |year=2007 |isbn=978-0-691-12085-0 |page=[https://archive.org/details/evolutionaryecol0000poul/page/6 6] |url=https://archive.org/details/evolutionaryecol0000poul/page/6}}</ref> 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.<ref name=Felder2009>{{cite book |last1=Felder |first1=Darryl L. |last2=Camp |first2=David K. |title=Gulf of Mexico Origin, Waters, and Biota: Biodiversity |url={{GBurl |id=CphA8hiwaFIC |pg=RA1-PA1111}} |year=2009 |publisher=Texas A&M University Press |isbn=978-1-60344-269-5 |page=1111}}</ref> Using patterns within the [[taxonomy (biology)|taxonomic]] hierarchy, the total number of animal species—including those not yet described—was calculated to be about 7.77 million in 2011.<ref>{{cite web |title=How many species on Earth? About 8.7 million, new estimate says |url=https://www.sciencedaily.com/releases/2011/08/110823180459.htm |access-date=2 March 2018 |date=24 August 2011 |archive-url=https://web.archive.org/web/20180701164954/https://www.sciencedaily.com/releases/2011/08/110823180459.htm |archive-date=1 July 2018 |url-status=live}}</ref><ref name="Mora2011">{{cite journal |last1=Mora |first1=Camilo |last2=Tittensor |first2=Derek P. |last3=Adl |first3=Sina |last4=Simpson |first4=Alastair G. B. |last5=Worm |first5=Boris |editor-last=Mace |editor-first=Georgina M. |title=How Many Species Are There on Earth and in the Ocean? |journal=PLOS Biology |volume=9 |issue=8 |date=23 August 2011 |doi=10.1371/journal.pbio.1001127 |page=e1001127 |pmid=21886479 |pmc=3160336 |doi-access=free}}</ref>{{efn|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]]).<ref>{{cite journal |last1=Hebert |first1=Paul D. N. |last2=Ratnasingham |first2=Sujeevan |last3=Zakharov |first3=Evgeny V. |last4=Telfer |first4=Angela C. |last5=Levesque-Beaudin |first5=Valerie |last6=Milton |first6=Megan A. |last7=Pedersen |first7=Stephanie |last8=Jannetta |first8=Paul |last9=deWaard |first9=Jeremy R. |display-authors=5 |title=Counting animal species with DNA barcodes: Canadian insects |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=1 August 2016 |volume=371 |issue=1702 |pages=20150333 |doi=10.1098/rstb.2015.0333 |pmid=27481785 |pmc=4971185}}</ref>}}

{| class="wikitable sortable plainrowheaders"

|-

! scope="col" | [[Phylum]]

! scope="col" class="unsortable" | Example<!--image-->

! scope="col" | Species

! scope="col" | [[Terrestrial animal|Land]]

! scope="col" | [[Marine animals|Sea]]

! scope="col" | [[Fresh water#Aquatic organisms|Freshwater]]

! scope="col" | Free-living

! scope="col" | [[Parasitic]]

|-

! scope="row" | '''[[Arthropoda]]'''

|[[File:European wasp white bg02.jpg|alt=wasp|100px]]

|align=right data-sort-value="1257000"|1,257,000<ref name="Zhang2013"/>

|Yes 1,000,000<br />([[insect]]s)<ref name="Stork2018">{{cite journal |last=Stork |first=Nigel E. |s2cid=23755007 |title=How Many Species of Insects and Other Terrestrial Arthropods Are There on Earth? |journal=Annual Review of Entomology |volume=63 |issue=1 |date=January 2018 |doi=10.1146/annurev-ento-020117-043348 |pmid=28938083 |pages=31–45 |doi-access=free}} Stork notes that 1m insects have been named, making much larger predicted estimates.</ref>

|Yes >40,000<br />([[Malacostraca|Malac-<br />ostraca]])<ref>{{cite book |year=2002 |series=Zoological catalogue of Australia |volume=19.2A |title=Crustacea: Malacostraca |publisher=[[CSIRO Publishing]] |isbn=978-0-643-06901-5 |chapter=Introduction |last=Poore |first=Hugh F. |pages=1–7 |chapter-url={{GBurl |id=ww6RzBz42-4C |p=1}}}}</ref>

|Yes 94,000<ref name=Balian2008/>

|Yes<ref name="Hogenboom2016"/>

|Yes >45,000{{efn|Not including [[parasitoid]]s.<ref name=Poulin2007/>}}<ref name=Poulin2007/>

|-

! scope="row" | '''[[Mollusca]]'''

|[[File:Grapevinesnail 01.jpg|alt=snail|100px]]

|align=right data-sort-value="85000" |85,000<ref name="Zhang2013"/><br />107,000<ref name=Nicol1969/>

| 35,000<ref name=Nicol1969>{{cite journal |last=Nicol |first=David |title=The Number of Living Species of Molluscs |journal=Systematic Zoology |volume=18 |issue=2 |date=June 1969 |pages=251–254 |doi=10.2307/2412618 |jstor=2412618 |jstor-access=free}}</ref>

| 60,000<ref name=Nicol1969/>

| 5,000<ref name=Balian2008/><br />12,000<ref name=Nicol1969/>

| Yes<ref name="Hogenboom2016"/>

| >5,600<ref name=Poulin2007/>

|-

! scope="row" | '''[[Chordata]]'''

|[[File:Lithobates pipiens.jpg|alt=green spotted frog facing right|100px]]

|align=right data-sort-value="70000"|>70,000<ref name="Zhang2013"/><ref>{{Cite journal |last=Uetz |first=P. |title=A Quarter Century of Reptile and Amphibian Databases |url=https://www.researchgate.net/publication/352462027 |journal=Herpetological Review |volume=52 |pages=246–255 |via=ResearchGate |access-date=2 October 2021 |archive-date=21 February 2022 |archive-url=https://web.archive.org/web/20220221154655/https://www.researchgate.net/publication/352462027_A_Quarter_Century_of_Reptile_and_Amphibian_Databases |url-status=live}}</ref>

| 23,000<ref name="Reaka-Kudla1996">{{cite book |last1=Reaka-Kudla |first1=Marjorie L. |last2=Wilson |first2=Don E. |last3=Wilson |first3=Edward O. |author3-link=E. O. Wilson |title=Biodiversity II: Understanding and Protecting Our Biological Resources |url={{GBurl |id=-X5OAgAAQBAJ |p=90}} |year=1996 |publisher=Joseph Henry |isbn=978-0-309-52075-1 |page=90}}</ref>

| 13,000<ref name="Reaka-Kudla1996"/>

| 18,000<ref name=Balian2008/><br />9,000<ref name="Reaka-Kudla1996"/>

|Yes

| 40<br />([[catfish]])<ref>{{cite book |last1=Burton |first1=Derek |last2=Burton |first2=Margaret |title=Essential Fish Biology: Diversity, Structure and Function |url={{GBurl |id=U0o4DwAAQBAJ |p=281}} |year=2017 |publisher=Oxford University Press |isbn=978-0-19-878555-2 |pages=281–282 |quote=[[Trichomycteridae]] ... includes obligate parasitic fish. Thus 17 genera from 2 subfamilies, [[Vandelliinae]]; 4 genera, 9spp. and [[Stegophilinae]]; 13 genera, 31 spp. are parasites on gills (Vandelliinae) or skin (stegophilines) of fish.}}</ref><ref name=Poulin2007/>

|-

! scope="row" | '''[[Platyhelminthes]]'''

|[[File:Pseudoceros dimidiatus.jpg|100px]]

|align=right data-sort-value="29500"|29,500<ref name="Zhang2013"/>

|Yes<ref>{{Cite journal |last=Sluys |first=R. |title=Global diversity of land planarians (Platyhelminthes, Tricladida, Terricola): a new indicator-taxon in biodiversity and conservation studies |journal=Biodiversity and Conservation |volume=8 |issue=12 |pages=1663–1681 |doi=10.1023/A:1008994925673 |year=1999 |bibcode=1999BiCon...8.1663S |s2cid=38784755}}</ref>

| Yes<ref name="Hogenboom2016"/>

| 1,300<!--Turbellaria--><ref name=Balian2008/>

| Yes<ref name="Hogenboom2016"/><br />

3,000–6,500<ref name=Pandian>{{cite book |last=Pandian |first=T. J. |title=Reproduction and Development in Platyhelminthes |publisher=CRC Press |year=2020 |isbn=978-1-000-05490-3 |pages=13–14 |url={{GBurl |id=l6rMDwAAQBAJ |pg=PT14}} |access-date=19 May 2020}}</ref>

| >40,000<ref name=Poulin2007/><br />

4,000–25,000<ref name=Pandian/>

|-

! scope="row" | '''[[Nematoda]]'''

|[[File:CelegansGoldsteinLabUNC.jpg|100px]]

|align=right data-sort-value="25000"|25,000<ref name="Zhang2013"/>

| Yes (soil)<ref name="Hogenboom2016"/>

| 4,000<ref name=Felder2009/>

| 2,000<ref name=Balian2008/>

| 11,000<ref name=Felder2009/>

| 14,000<ref name=Felder2009/>

|-

! scope="row" | '''[[Annelida]]'''

|[[File:Nerr0328.jpg|100px]]

|align=right |17,000<ref name="Zhang2013"/>

| Yes (soil)<ref name="Hogenboom2016"/>

| Yes<ref name="Hogenboom2016"/>

| 1,750<ref name=Balian2008/>

| Yes

| 400<ref name=Poulin2007/>

|-

! scope="row" | '''[[Cnidaria]]'''

|[[File:FFS Table bottom.jpg|alt=Table coral|100px]]

|align=right data-sort-value="16000"|16,000<ref name="Zhang2013"/>

|

| Yes<ref name="Hogenboom2016"/>

| Few<ref name="Hogenboom2016"/>

| Yes<ref name="Hogenboom2016"/>

| >1,350<br />([[Myxozoa]])<ref name=Poulin2007/>

|-

! scope="row" | '''[[Porifera]]'''

|[[File:A colourful Sponge on the Fathom.jpg|100px]]

|align=right data-sort-value="10800"|10,800<ref name="Zhang2013"/>

|

| Yes<ref name="Hogenboom2016"/>

| 200–300<ref name=Balian2008/>

| Yes

| Yes<ref>{{cite book |last1=Morand |first1=Serge |last2=Krasnov |first2=Boris R. |last3=Littlewood |first3=D. Timothy J. |title=Parasite Diversity and Diversification |url={{GBurl |id=o2t2BgAAQBAJ |p=44}} |year=2015 |publisher=Cambridge University Press |isbn=978-1-107-03765-6 |page=44 |access-date=2 March 2018}}</ref>

|-

! scope="row" | '''[[Echinodermata]]'''

|[[File:Starfish, Caswell Bay - geograph.org.uk - 409413.jpg|100px]]

|align=right data-sort-value="7500"|7,500<ref name="Zhang2013"/>

|

| 7,500<ref name="Zhang2013"/>

|

| Yes<ref name="Hogenboom2016"/>

|

|-

! scope="row" | '''[[Bryozoa]]'''

|[[File:Bryozoan at Ponta do Ouro, Mozambique (6654415783).jpg|100px]]

|align=right data-sort-value="6000" |6,000<ref name="Zhang2013"/>

|

| Yes<ref name="Hogenboom2016"/>

| 60–80<ref name=Balian2008/>

| Yes

|

|-

! scope="row" | '''[[Rotifera]]'''

|[[File:20090730 020239 Rotifer.jpg|100px]]

|align=right data-sort-value="2000"|2,000<ref name="Zhang2013"/>

|

| >400<ref>{{cite web |last=Fontaneto |first=Diego |title=Marine Rotifers: An Unexplored World of Richness |url=https://ukmarinesac.org.uk/PDF/rotifers.pdf |publisher=JMBA Global Marine Environment |access-date=2 March 2018 |pages=4–5 |archive-url=https://web.archive.org/web/20180302225409/https://ukmarinesac.org.uk/PDF/rotifers.pdf |archive-date=2 March 2018 |url-status=live}}</ref>

| 2,000<ref name=Balian2008/>

| Yes

| Yes<ref>{{cite conference |last=May |first=Linda |title=Epizoic and parasitic rotifers |conference=Rotifer Symposium V: Proceedings of the Fifth Rotifer Symposium, held in Gargnano, Italy, September 11–18, 1988 |publisher=Springer |year=1989}}</ref>

|-

! scope="row" | '''[[Nemertea]]'''

|[[File:Némerte.jpg|100px]]

|align=right data-sort-value="1350"|1,350<ref name="Chernyshev_2021">{{cite journal |last1=Chernyshev |first1=A. V. |title=An updated classification of the phylum Nemertea |journal=Invertebrate Zoology |date=September 2021 |volume=18 |issue=3 |pages=188–196 |doi=10.15298/invertzool.18.3.01 |s2cid=239872311 |url=https://www.researchgate.net/publication/354810461 |access-date=18 January 2023 |doi-access=free}}</ref><ref name="Hookabe_2022">{{cite journal |last1=Hookabe |first1=Natsumi |last2=Kajihara |first2=Hiroshi |last3=Chernyshev |first3=Alexei V. |last4=Jimi |first4=Naoto |last5=Hasegawa |first5=Naohiro |last6=Kohtsuka |first6=Hisanori |last7=Okanishi |first7=Masanori |last8=Tani |first8=Kenichiro |last9=Fujiwara |first9=Yoshihiro |last10=Tsuchida |first10=Shinji |last11=Ueshima |first11=Rei |title=Molecular Phylogeny of the Genus Nipponnemertes (Nemertea: Monostilifera: Cratenemertidae) and Descriptions of 10 New Species, With Notes on Small Body Size in a Newly Discovered Clade |journal=Frontiers in Marine Science |year=2022 |volume=9 |article-number=906383 |doi=10.3389/fmars.2022.906383 |url=https://www.researchgate.net/publication/362813258 |access-date=18 January 2023 |doi-access=free|bibcode=2022FrMaS...906383H }}</ref>

|

| Yes

| Yes

| Yes

|

|-

! scope="row" | '''[[Tardigrada]]'''

|[[File:Tardigrade (50594282802).jpg|100px]]

|align=right data-sort-value="1335"|1,335<ref name="Zhang2013"/>

| Yes<ref name="Hickman Diversity 2018">{{cite book |title=Animal Diversity |first1=Cleveland P. |last1=Hickman |first2=Susan L. |last2=Keen |first3=Allan |last3=Larson |first4=David J. |last4=Eisenhour |edition=8th |publisher=McGraw Hill |year=2018 |isbn=978-1-260-08427-6}}</ref><br />(moist plants)

| Yes

| Yes

| Yes

|

|}

== Evolutionary origin ==

{{Further|Urmetazoan}}

Evidence of animals is found as long ago as the [[Cryogenian]] period. [[24-Isopropylcholestane]] (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges and [[pelagophyte]] algae. Its likely origin is from sponges based on [[molecular clock]] estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a [[Phanerozoic]] origin, while analyses of sponges recover a [[Neoproterozoic]] origin, consistent with the appearance of 24-ipc in the fossil record.<ref>{{cite journal |last1=Gold |first1=David |display-authors=et al. |title=Sterol and genomic analyses validate the sponge biomarker hypothesis |journal=PNAS |date=22 February 2016 |volume=113 |issue=10 |pages=2684–2689 |doi=10.1073/pnas.1512614113 |doi-access=free |pmid=26903629 |pmc=4790988 |bibcode=2016PNAS..113.2684G}}</ref><ref>{{cite journal |last1=Love |first1=Gordon |display-authors=et al. |title=Fossil steroids record the appearance of Demospongiae during the Cryogenian period |journal=Nature |date=5 February 2009 |volume=457 |issue=7230 |pages=718–721 |doi=10.1038/nature07673 |pmid=19194449 |bibcode=2009Natur.457..718L|url=https://resolver.caltech.edu/CaltechAUTHORS:20090807-101203081 }}</ref>

The first body fossils of animals appear in the [[Ediacaran]], represented by forms such as ''[[Charnia]]'' and ''[[Spriggina]]''. It had long been doubted whether these fossils truly represented animals,<ref>{{cite journal |last1=Shen |first1=Bing |last2=Dong |first2=Lin |last3=Xiao |first3=Shuhai |last4=Kowalewski |first4=Michał |year=2008 |title=The Avalon Explosion: Evolution of Ediacara Morphospace |journal=Science |volume=319 |issue=5859 |pages=81–84 |doi=10.1126/science.1150279 |pmid=18174439 |bibcode=2008Sci...319...81S |s2cid=206509488}}</ref><ref>{{cite journal |title=Late Ediacaran trackways produced by bilaterian animals with paired appendages |first1=Zhe |last1=Chen |first2=Xiang |last2=Chen |first3=Chuanming |last3=Zhou |first4=Xunlai |last4=Yuan |first5=Shuhai |last5=Xiao |date=1 June 2018 |journal=Science Advances |volume=4 |issue=6 |pages=eaao6691 |doi=10.1126/sciadv.aao6691 |pmid=29881773 |pmc=5990303 |bibcode=2018SciA....4.6691C}}</ref><ref>{{cite book |last=Schopf |first=J. William |title=Evolution!: facts and fallacies |year=1999 |publisher=Academic Press |isbn=978-0-12-628860-5 |page=[https://archive.org/details/evolutionfactsfa0000unse/page/7 7] |url=https://archive.org/details/evolutionfactsfa0000unse/page/7}}</ref> but the discovery of the animal lipid [[cholesterol]] in fossils of ''[[Dickinsonia]]'' establishes their nature.<ref name="Bobrovskiy Hope Ivantsov Nettersheim pp. 1246–1249"/> 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 specialised for aerobic metabolism they became fully dependent on oxygen in their environments.<ref name="Zimorski2019">{{cite journal |last1=Zimorski |first1=Verena |last2=Mentel |first2=Marek |last3=Tielens |first3=Aloysius G. M. |last4=Martin |first4=William F. |title=Energy metabolism in anaerobic eukaryotes and Earth's late oxygenation |journal=Free Radical Biology and Medicine |volume=140 |pages=279–294 |year=2019 |doi=10.1016/j.freeradbiomed.2019.03.030 |pmid=30935869 |pmc=6856725}}</ref>

Many animal phyla first appear in the [[fossil]] record during the [[Cambrian explosion]], starting about 539 million years ago, in beds such as the [[Burgess shale]].<ref>{{cite web |title=Stratigraphic Chart 2022 |url=https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |publisher=International Stratigraphic Commission |date=February 2022 |access-date=25 April 2022 |archive-date=2 April 2022 |archive-url=https://web.archive.org/web/20220402100018/https://stratigraphy.org/ICSchart/ChronostratChart2022-02.pdf |url-status=live}}</ref> Extant phyla in these rocks include [[mollusc]]s, [[brachiopod]]s, [[onychophora]]ns, [[tardigrade]]s, [[arthropod]]s, [[echinoderm]]s and [[hemichordate]]s, along with numerous now-extinct forms such as the [[predator]]y ''[[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.<ref>{{cite journal |last1=Maloof |first1=A. C. |last2=Porter |first2=S. M. |last3=Moore |first3=J. L. |last4=Dudas |first4=F. O. |last5=Bowring |first5=S. A. |last6=Higgins |first6=J. A. |last7=Fike |first7=D. A. |last8=Eddy |first8=M. P. |s2cid=6694681 |title=The earliest Cambrian record of animals and ocean geochemical change |journal=Geological Society of America Bulletin |year=2010 |volume=122 |issue=11–12 |pages=1731–1774 |doi=10.1130/B30346.1 |bibcode=2010GSAB..122.1731M}}</ref><ref>{{cite web |title=New Timeline for Appearances of Skeletal Animals in Fossil Record Developed by UCSB Researchers |url=https://www.ia.ucsb.edu/pa/display.aspx?pkey=2364 |publisher=The Regents of the University of California |access-date=1 September 2014 |date=10 November 2010 |archive-url=https://web.archive.org/web/20140903062054/https://www.ia.ucsb.edu/pa/display.aspx?pkey=2364 |archive-date=3 September 2014 |url-status=live}}</ref><ref>{{cite journal |last=Conway-Morris |first=Simon |author-link=Simon Conway Morris |title=The Cambrian "explosion" of metazoans and molecular biology: would Darwin be satisfied? |journal=The International Journal of Developmental Biology |year=2003 |volume=47 |issue=7–8 |pages=505–515 |pmid=14756326 |url=https://ijdb.ehu.eus/article/pdf/14756326 |access-date=28 September 2024 |archive-url=https://web.archive.org/web/20231114204853if_/https://ijdb.ehu.eus/article/pdf/14756326 |archive-date=14 November 2023 |url-status=live}}</ref><ref>{{cite journal |last1=Morris |first1=Simon Conway |author-link=Simon Conway Morris |title=Darwin's dilemma: the realities of the Cambrian 'explosion'. |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |date=29 June 2006 |volume=361 |issue=1470 |pages=1069–83 |doi=10.1098/rstb.2006.1846 |pmid=16754615 |pmc=1578734}}</ref><ref name="Royal Ontario Museum">{{cite web |title=The Tree of Life |url=https://burgess-shale.rom.on.ca/en/science/origin/01-life-tree.php |website=The Burgess Shale |publisher=[[Royal Ontario Museum]] |access-date=28 February 2018 |archive-url=https://web.archive.org/web/20180216054845/https://burgess-shale.rom.on.ca/en/science/origin/01-life-tree.php |archive-date=16 February 2018 |url-status=dead |date=10 June 2011}}</ref> That view is supported by the discovery of ''[[Auroralumina attenboroughii]]'', the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from [[Charnwood Forest]], England. It is thought to be one of the earliest [[predator]]s, catching small prey with its [[nematocyst]]s as modern cnidarians do.<ref name="Dunn Kenchington Parry Clark 2022">{{cite journal |last1=Dunn |first1=F. S. |last2=Kenchington |first2=C. G. |last3=Parry |first3=L. A. |last4=Clark |first4=J. W. |last5=Kendall |first5=R. S. |last6=Wilby |first6=P. R. |title=A crown-group cnidarian from the Ediacaran of Charnwood Forest, UK |journal=Nature Ecology & Evolution |date=25 July 2022 |volume=6 |issue=8 |pages=1095–1104 |doi=10.1038/s41559-022-01807-x |pmid=35879540 |pmc=9349040 |bibcode=2022NatEE...6.1095D}}</ref>

Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.<ref>{{cite book |last1=Campbell |first1=Neil A. |last2=Reece |first2=Jane B. |title=Biology |year=2005 |publisher=Pearson, Benjamin Cummings |isbn=978-0-8053-7171-0 |edition=7th |page=526}}</ref> Early fossils that might represent animals appear for example in the 665-million-year-old rocks of the [[Trezona Formation]] of [[South Australia]]. These fossils are interpreted as most probably being early [[sponges]].<ref name=roseMaloof>{{cite journal |title=Possible animal-body fossils in pre-Marinoan limestones from South Australia |journal=Nature Geoscience |volume=3 |pages=653–659 |date=17 August 2010 |doi=10.1038/ngeo934 |issue=9 |bibcode=2010NatGe...3..653M |last1=Maloof |first1=Adam C. |last2=Rose |first2=Catherine V. |last3=Beach |first3=Robert |last4=Samuels |first4=Bradley M. |last5=Calmet |first5=Claire C. |last6=Erwin |first6=Douglas H. |last7=Poirier |first7=Gerald R. |last8=Yao |first8=Nan |last9=Simons |first9=Frederik J.}}</ref>

[[Trace fossil]]s such as tracks and burrows found in the [[Tonian]] period (from 1 gya) may indicate the presence of [[triploblastic]] worm-like animals, roughly as large (about 5&nbsp;mm wide) and complex as earthworms.<ref name=Seilacher1998>{{cite journal |last1=Seilacher |first1=Adolf |author1-link=Adolf Seilacher |last2=Bose |first2=Pradip K. |last3=Pfluger |first3=Friedrich |title=Triploblastic animals more than 1 billion years ago: trace fossil evidence from india |journal=Science |volume=282 |pages=80–83 |date=2 October 1998 |doi=10.1126/science.282.5386.80 |pmid=9756480 |issue=5386 |bibcode=1998Sci...282...80S}}</ref> However, similar tracks are produced by the giant single-celled protist ''[[Gromia sphaerica]]'', so the Tonian trace fossils may not indicate early animal evolution.<ref name=Matz2008>{{cite journal |last1=Matz |first1=Mikhail V. |last2=Frank |first2=Tamara M. |last3=Marshall |first3=N. Justin |last4=Widder |first4=Edith A. |last5=Johnsen |first5=Sönke |title=Giant Deep-Sea Protist Produces Bilaterian-like Traces |journal=Current Biology |volume=18 |issue=23 |pages=1849–54 |date=9 December 2008 |doi=10.1016/j.cub.2008.10.028 |pmid=19026540 |s2cid=8819675 |doi-access=free |bibcode=2008CBio...18.1849M}}</ref><ref name=MSNBC200811>{{cite news |last=Reilly |first=Michael |title=Single-celled giant upends early evolution |newspaper=NBC News |date=20 November 2008 |url=https://www.nbcnews.com/id/wbna27827279 |access-date=5 December 2008 |archive-url=https://web.archive.org/web/20130329062924/https://www.nbcnews.com/id/27827279/ |archive-date=29 March 2013 |url-status=live}}</ref> Around the same time, the layered mats of [[microorganism]]s called [[stromatolite]]s decreased in diversity, perhaps due to grazing by newly evolved animals.<ref name="Bengtson2002OriginsOfPredation">{{Cite encyclopedia |last=Bengtson |first=S. |year=2002 |chapter=Origins and early evolution of predation |encyclopedia=The Paleontological Society Papers |volume=8 |title=The fossil record of predation |editor-last=Kowalewski |editor-first=M. |editor-last2=Kelley |editor-first2=P. H. |pages=289–317 |publisher=[[The Paleontological Society]] |chapter-url=https://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf |access-date=3 March 2018 |archive-date=30 October 2019 |archive-url=https://web.archive.org/web/20191030140248/https://www.nrm.se/download/18.4e32c81078a8d9249800021552/Bengtson2002predation.pdf |url-status=live}}</ref> Objects such as sediment-filled tubes that resemble trace fossils of the burrows of wormlike animals have been found in 1.2 gya rocks in North America, in 1.5 gya rocks in Australia and North America, and in 1.7 gya rocks in Australia. Their interpretation as having an animal origin is disputed, as they might be water-escape or other structures.<ref name="Seilacher 20072">{{cite book |last=Seilacher |first=Adolf |author-link=Adolf Seilacher |title=Trace fossil analysis |date=2007 |publisher=Springer |isbn=978-3-540-47226-1 |publication-place=Berlin |pages=176–177 |oclc=191467085}}</ref><ref>{{cite journal |last=Breyer |first=J. A. |year=1995 |title=Possible new evidence for the origin of metazoans prior to 1 Ga: Sediment-filled tubes from the Mesoproterozoic Allamoore Formation, Trans-Pecos Texas |journal=[[Geology (journal)|Geology]] |volume=23 |issue=3 |pages=269–272 |doi=10.1130/0091-7613(1995)023<0269:PNEFTO>2.3.CO;2 |bibcode=1995Geo....23..269B}}</ref>

<gallery class="center" mode="nolines" widths="220" heights="160">

File:DickinsoniaCostata.jpg|''[[Dickinsonia costata]]'' from the [[Ediacaran biota]] (c. 635–542 mya) is one of the earliest animal species known.<ref name="Bobrovskiy Hope Ivantsov Nettersheim pp. 1246–1249">{{cite journal |last1=Bobrovskiy |first1=Ilya |last2=Hope |first2=Janet M. |last3=Ivantsov |first3=Andrey |last4=Nettersheim |first4=Benjamin J. |last5=Hallmann |first5=Christian |last6=Brocks |first6=Jochen J. |title=Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals |journal=Science |volume=361 |issue=6408 |date=20 September 2018 |doi=10.1126/science.aat7228 |pmid=30237355 |pages=1246–1249 |bibcode=2018Sci...361.1246B |doi-access=free}}</ref>

File:Auroralumina attenboroughii reconstruction.jpg|''[[Auroralumina attenboroughii]]'', an Ediacaran predator (c.&nbsp;560 mya)<ref name="Dunn Kenchington Parry Clark 2022" />

File:20191203 Anomalocaris canadensis.png|''[[Anomalocaris canadensis]]'' is one of the many animal species that emerged in the [[Cambrian explosion]], starting some 539 mya, and found in the fossil beds of the [[Burgess shale]].

</gallery>

== Phylogeny ==

{{further|Lists of animals}}

=== External phylogeny ===

Animals are [[monophyletic]], meaning they are derived from a common ancestor. Animals are the [[sister group]] to the [[choanoflagellate]]s, with which they form the [[Choanozoa]].<ref name="Budd2015">{{cite journal |doi=10.1111/brv.12239 |pmid=26588818 |title=The origin of the animals and a 'Savannah' hypothesis for early bilaterian evolution |journal=[[Biological Reviews]] |volume=92 |issue=1 |pages=446–473 |year=2017 |last1=Budd |first1=Graham E. |last2=Jensen |first2=Sören |doi-access=free|hdl=10662/8091 |hdl-access=free }}</ref>

Ros-Rocher and colleagues (2021) trace the origins of animals to unicellular ancestors, providing the external phylogeny shown in the cladogram. Uncertainty of relationships is indicated with dashed lines. The animal clade had certainly originated by 650 mya, and may have come into being as much as 800 mya, based on [[molecular clock]] evidence for different phyla.<ref name="Ros-Rocher Pérez-Posada Leger Ruiz-Trillo 2021">{{cite journal |last1=Ros-Rocher |first1=Núria |last2=Pérez-Posada |first2=Alberto |last3=Leger |first3=Michelle M. |last4=Ruiz-Trillo |first4=Iñaki |title=The origin of animals: an ancestral reconstruction of the unicellular-to-multicellular transition |journal=Open Biology |publisher=The Royal Society |volume=11 |issue=2 |year=2021 |page=200359 |doi=10.1098/rsob.200359 |pmid=33622103 |pmc=8061703}}</ref>

{{Clade |style=font-size:100%; line-height:100%

|label1=[[Opisthokonta]] |sublabel1=

|1={{Clade

|1=[[Holomycota]] (inc. fungi) [[File:Asco1013.jpg|60 px]]

|label2=[[Holozoa]] |sublabel2=

|2={{Clade |state=dashed

|1=[[Ichthyosporea]] [[File:Abeoforma whisleri-2.jpg|50 px]]

|2=[[Pluriformea]] [[File:Corallochytrium limacisporum.png|50 px]]

|label3=[[Filozoa]]

|3={{Clade

|1=[[Filasterea]] [[File:Ministeria vibrans.jpeg|60 px]]

|label2=[[Choanozoa]] |sublabel2=

|2={{Clade

|label1=[[Choanoflagellate]]a |1=[[File:Desmarella moniliformis.jpg|60 px]]

|label2 ='''Animalia''' |sublabel2=over 650 mya |2= [[File:Polychaeta (no) 2.jpg|60 px]]

}}

}}

}}

}}

}}

=== Internal phylogeny ===

{{anchor|basal}}

The relationships at the base of the animal tree have been debated.<ref>{{Cite bioRxiv |last1=Erives |first1=Albert |last2=Fritzsch |first2=Bernd |date=2019-07-17 |title=A screen for gene paralogies delineating evolutionary branching order of early Metazoa |biorxiv=10.1101/704551}}</ref><ref name="Kapli_2020">{{cite journal |last1=Kapli |first1=Paschalia |last2=Telford |first2=Maximilian J. |title=Topology-dependent asymmetry in systematic errors affects phylogenetic placement of Ctenophora and Xenacoelomorpha |journal=Science Advances |date=11 December 2020 |volume=6 |issue=10 |pages=eabc5162 |doi=10.1126/sciadv.abc5162 |pmid=33310849 |pmc=7732190 |bibcode=2020SciA....6.5162K |doi-access=free}}</ref> Other than Ctenophora, the Bilateria and Cnidaria are the only groups with symmetry, and other evidence shows they are closely related.<ref name="Giribet Edgecombe 2020">{{cite book |last1=Giribet |first1=G. |last2=Edgecombe |first2=G.D. |title=The Invertebrate Tree of Life |year=2020 |publisher=[[Princeton University Press]] |url={{GBurl |id=YHetDwAAQBAJ |p=21}} |page=21 |isbn=978-0-6911-7025-1 |access-date=27 May 2023}}</ref> In addition to sponges, Placozoa has no symmetry and was often considered a "missing link" between protists and multicellular animals. The presence of [[hox gene]]s in Placozoa shows that they were once more complex.<ref name="Wanninger 2024">{{cite journal |last=Wanninger |first=Andreas |date=2024 |title=Hox, homology, and parsimony: An organismal perspective |journal=Seminars in Cell & Developmental Biology |volume=152-153 |pages=16–23 |doi=10.1016/j.semcdb.2023.01.007 |doi-access=free |pmid=36670036 }}</ref>

The [[Porifera]] (sponges) have long been assumed to be sister to the rest of the animals, but there is evidence that the [[Ctenophora]] may be in that position. Molecular phylogenetics has supported both the sponge-sister and ctenophore-sister hypotheses. In 2017, Roberto Feuda and colleagues, using [[amino acid]] differences, presented both, with the following cladogram for the sponge-sister view that they supported (their ctenophore-sister tree simply interchanging the places of ctenophores and sponges):<ref name="Feuda Dohrmann 2017">{{Cite journal |last1=Feuda |first1=Roberto |last2=Dohrmann |first2=Martin |last3=Pett |first3=Walker |last4=Philippe |first4=Hervé |last5=Rota-Stabelli |first5=Omar |last6=Lartillot |first6=Nicolas |last7=Wörheide |first7=Gert |last8=Pisani |first8=Davide |display-authors=5 |title=Improved Modeling of Compositional Heterogeneity Supports Sponges as Sister to All Other Animals |journal=Current Biology |volume=27 |issue=24 |pages=3864–3870.e4 |doi=10.1016/j.cub.2017.11.008 |pmid=29199080 |year=2017 |doi-access=free |bibcode=2017CBio...27E3864F |hdl=10449/43929 |hdl-access=free}}</ref>

{{clade |style = font-size:100%; line-height:100%

|label1='''Animalia''' |sublabel1=''[[multicellular]]''

|1={{clade

|1=[[Porifera]] [[File:Callyspongiatransparent.png|50 px]]

|label2=[[Eumetazoa]]

|2={{clade

|1=[[Ctenophora]] [[File:Mnemiopsis leidyi 247259012.png|60 px]]

|label2=[[ParaHoxozoa]] |sublabel2=''[[hox gene]]s''

|2={{clade

|1=[[Placozoa]] [[File:Trichoplax adhaerens photograph (no background).png|60 px]]

|sublabel2=''symmetry''

|2={{clade

|1=[[Cnidaria]] <span style="{{MirrorH}}">[[File:Jellyfish, Shaw Ocean Discovery Centre (7201323966).png|70px]]</span>

|2=[[Bilateria]] [[File:Saccoglossus kowalevskii by Spengel 1893.png|50 px]]

}}

}}

}}

}}

}}

Conversely, a 2023 study by Darrin Schultz and colleagues uses ancient [[gene linkage]]s to construct the following ctenophore-sister phylogeny:<ref name="Schultz Haddock 2023">{{Cite journal |last1=Schultz |first1=Darrin T. |last2=Haddock |first2=Steven H. D. |author2-link=Steven Haddock |last3=Bredeson |first3=Jessen V. |last4=Green |first4=Richard E. |last5=Simakov |first5=Oleg |last6=Rokhsar |first6=Daniel S. |date=2023-05-17 |title=Ancient gene linkages support ctenophores as sister to other animals |url=https://rdcu.be/dcJSY |journal=Nature |volume=618 |issue=7963 |pages=110–117 |doi=10.1038/s41586-023-05936-6 |pmid=37198475 |pmc=10232365 |bibcode=2023Natur.618..110S}}</ref>

{{clade |style = font-size:100%; line-height:100%

|label1='''Animalia''' |sublabel1=''[[multicellular]]''

|1={{clade

|1=[[Ctenophora]] [[File:Mnemiopsis leidyi 247259012.png|60 px]]

|label2=[[Myriazoa]]

|2={{clade

|1=[[Porifera]] [[File:Callyspongiatransparent.png|50 px]]

|label2=[[ParaHoxozoa]] |sublabel2=''[[hox gene]]s''

|2={{clade

|1={{clade

|1=[[Placozoa]] [[File:Trichoplax adhaerens photograph (no background).png|60 px]]

|2=[[Cnidaria]] <span style="{{MirrorH}}">[[File:Jellyfish, Shaw Ocean Discovery Centre (7201323966).png|70px]]</span>

}}

|label2= |sublabel2=''[[bilateral symmetry|symmetry]]''

|2=[[Bilateria]] [[File:Saccoglossus kowalevskii by Spengel 1893.png|50 px]]

}}

}}

}}

}}

=== Non-bilaterians ===

[[File:Elephant-ear-sponge.jpg|thumb|upright|Non-bilaterians include sponges (centre) and corals (background).]]

Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a sister clade to all other animals.<ref>{{cite book |last1=Bhamrah |first1=H. S. |last2=Juneja |first2=Kavita |title=An Introduction to Porifera |year=2003 |publisher=Anmol Publications |isbn=978-81-261-0675-2 |page=58}}</ref> Despite their morphological dissimilarity with all other animals, genetic evidence suggests sponges may be more closely related to other animals than the comb jellies are.<ref name="Schultz-2023">{{Cite journal |last1=Schultz |first1=Darrin T. |last2=Haddock |first2=Steven H. D. |last3=Bredeson |first3=Jessen V. |last4=Green |first4=Richard E. |last5=Simakov |first5=Oleg |last6=Rokhsar |first6=Daniel S. |date=17 May 2023 |title=Ancient gene linkages support ctenophores as sister to other animals |url=https://rdcu.be/dcJSY |journal=Nature |volume=618 |issue=7963 |pages=110–117 |doi=10.1038/s41586-023-05936-6 |pmid=37198475 |s2cid=258765122 |pmc=10232365 |bibcode=2023Natur.618..110S}}</ref><ref>{{Cite journal |last1=Whelan |first1=Nathan V. |last2=Kocot |first2=Kevin M. |last3=Moroz |first3=Tatiana P. |last4=Mukherjee |first4=Krishanu |last5=Williams |first5=Peter |last6=Paulay |first6=Gustav |last7=Moroz |first7=Leonid L. |last8=Halanych |first8=Kenneth M. |display-authors=5 |date=9 October 2017 |title=Ctenophore relationships and their placement as the sister group to all other animals |url=https://rdcu.be/dcJS3 |journal=Nature Ecology & Evolution |volume=1 |issue=11 |pages=1737–1746 |doi=10.1038/s41559-017-0331-3 |pmid=28993654 |pmc=5664179 |bibcode=2017NatEE...1.1737W}}</ref> Sponges lack the complex organisation found in most other animal phyla;<ref>{{cite book |last=Sumich |first=James L. |title=Laboratory and Field Investigations in Marine Life |year=2008 |publisher=Jones & Bartlett Learning |isbn=978-0-7637-5730-4 |page=67}}</ref> their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals.<ref>{{cite book |last=Jessop |first=Nancy Meyer |title=Biosphere; a study of life |year=1970 |publisher=[[Prentice-Hall]] |page=428}}</ref> They typically feed by drawing in water through pores, filtering out small particles of food.<ref>{{cite book |last=Sharma |first=N. S. |title=Continuity And Evolution Of Animals |year=2005 |publisher=Mittal Publications |isbn=978-81-8293-018-6 |page=106}}</ref>

The Ctenophora and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus.<ref>{{cite book |title=A Living Bay: The Underwater World of Monterey Bay |year=2000 |publisher=University of California Press |isbn=978-0-520-22149-9 |last1=Langstroth |first1=Lovell |last2=Langstroth |first2=Libby |editor-last=Newberry |editor-first=Todd |page=[https://archive.org/details/livingbayunderwa0000lang/page/244 244] |url=https://archive.org/details/livingbayunderwa0000lang/page/244}}</ref> Animals in both phyla have distinct tissues, but these are not organised into discrete [[organs]].<ref>{{cite book |last=Safra |first=Jacob E. |title=The New Encyclopædia Britannica |volume=16 |year=2003 |publisher=Encyclopædia Britannica |isbn=978-0-85229-961-6 |page=523}}</ref> They are [[diploblastic]], having only two main germ layers, ectoderm and endoderm.<ref>{{cite book |last=Kotpal |first=R.L. |title=Modern Text Book of Zoology: Invertebrates |publisher=Rastogi Publications |isbn=978-81-7133-903-7 |page=184 |year=2012}}</ref>

The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae.<ref>{{cite book |last=Barnes |first=Robert D. |year=1982 |title=Invertebrate Zoology |publisher=Holt-Saunders International |pages=84–85 |isbn=978-0-03-056747-6}}</ref><ref>{{cite web |author=<!--"A.G.C."--> |title=Introduction to Placozoa |url=https://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html |publisher=UCMP Berkeley |access-date=10 March 2018 |archive-url=https://web.archive.org/web/20180325202849/https://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html |archive-date=25 March 2018 |url-status=live}}</ref> Their phylogeny is poorly defined, and under active research.<ref name="Schultz-2023" /><ref>{{Cite journal |last1=Srivastava |first1=Mansi |last2=Begovic |first2=Emina |last3=Chapman |first3=Jarrod |last4=Putnam |first4=Nicholas H. |last5=Hellsten |first5=Uffe |last6=Kawashima |first6=Takeshi |last7=Kuo |first7=Alan |last8=Mitros |first8=Therese |last9=Salamov |first9=Asaf |last10=Carpenter |first10=Meredith L. |last11=Signorovitch |first11=Ana Y. |last12=Moreno |first12=Maria A. |last13=Kamm |first13=Kai |last14=Grimwood |first14=Jane |last15=Schmutz |first15=Jeremy |display-authors=5 |date=1 August 2008 |title=The Trichoplax genome and the nature of placozoans |journal=Nature |volume=454 |issue=7207 |pages=955–960 |doi=10.1038/nature07191 |pmid=18719581 |bibcode=2008Natur.454..955S |s2cid=4415492 |doi-access=free}}</ref>

=== Bilateria ===

{{main|Bilateria|Symmetry (biology)#Bilateral symmetry}}

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form the [[Bilateria]] [[clade]], which have a bilaterally symmetric [[body plan]]. The Bilateria are [[Triploblasty|<!--this is the primary ___location for this wikilink-->triploblastic]], with three well-developed germ layers, and their tissues [[Organogenesis|form distinct organs]]. The digestive chamber has two openings, a mouth and an anus, and in the [[Nephrozoa]] there is an internal body cavity, a [[coelom]] or pseudocoelom. These animals have a head end (anterior) and a tail end (posterior), a back (dorsal) surface and a belly (ventral) surface, and a left and a right side.<ref name="Minelli2009"/><ref name=Brusca2016/> A modern consensus [[phylogenetic tree]] for the Bilateria<!--, from a 2014 review by Casey Dunn and colleagues,--> is shown below.<ref name="Dunn Giribet Edgecombe Hejnol 2014">{{cite journal |last1=Dunn |first1=Casey W. |last2=Giribet |first2=Gonzalo |last3=Edgecombe |first3=Gregory D. |last4=Hejnol |first4=Andreas |title=Animal Phylogeny and Its Evolutionary Implications |journal=Annual Review of Ecology, Evolution, and Systematics |volume=45 |issue=1 |date=23 November 2014 |doi=10.1146/annurev-ecolsys-120213-091627 |pages=371–395}}</ref>

{{clade |style=font-size:85%;line-height:85%

|label1='''Bilateria'''

|1={{clade

|1=[[Xenacoelomorpha]] [[File:Xenoturbella japonica.jpg|70 px]]

|label2= [[Nephrozoa]]

|sublabel2=650 Mya

|2={{clade

|label1=[[Deuterostomia]]

|1={{clade

|1=[[Ambulacraria]] [[File:Portugal 20140812-DSC01434 (21371237591).jpg|60 px]]

|2=[[Chordata]] [[File:Blueback herring fish (white background).jpg|60 px]]

}}

|label2= [[Protostomia]]

|sublabel2=610 mya

|2={{clade

|1=[[Ecdysozoa]] [[File:Long nosed weevil edit.jpg|60 px]]

|2=[[Spiralia]] [[File:Grapevinesnail 01.jpg|60 px]]

}}

}}

}}

}}

[[File:Bilaterian body plan.svg|thumb|upright=1.35|Idealised [[nephrozoa]]n body plan.{{efn|Compare [[:File:Annelid redone w white background.svg]] for a more specific and detailed model of a particular phylum with this general body plan.}} With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the [[cephalisation|basis of the head]]. Opposed circular and longitudinal muscles enable [[peristaltic motion]].]]

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 organ]]s and a mouth. Many bilaterians have a combination of circular [[muscle]]s that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body;<ref name=Brusca2016/> these enable soft-bodied animals with a [[hydrostatic skeleton]] to move by [[peristalsis]].<ref name=Quillin>{{cite journal |last=Quillin |first=K. J. |title=Ontogenetic scaling of hydrostatic skeletons: geometric, static stress and dynamic stress scaling of the earthworm lumbricus terrestris |journal=[[Journal of Experimental Biology]] |volume=201 |issue=12 |pages=1871–1883 |date=May 1998 |doi=10.1242/jeb.201.12.1871 |pmid=9600869 |url=https://jeb.biologists.org/cgi/pmidlookup?view=long&pmid=9600869 |doi-access=free |bibcode=1998JExpB.201.1871Q |access-date=4 March 2018 |archive-date=17 June 2020 |archive-url=https://web.archive.org/web/20200617135617/https://jeb.biologists.org/content/201/12/1871.long |url-status=live}}</ref> They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary [[larva]]e which swim with [[cilia]] and have an apical organ containing sensory cells. However, over evolutionary time, descendant spaces have evolved which have lost one or more of each of these characteristics. For example, adult echinoderms are radially symmetric (unlike their larvae), while some [[parasitic worms]] have extremely simplified body structures.<ref name="Minelli2009">{{cite book |last=Minelli |first=Alessandro |title=Perspectives in Animal Phylogeny and Evolution |url={{GBurl |id=jIASDAAAQBAJ |p=53}} |year=2009 |publisher=[[Oxford University Press]] |isbn=978-0-19-856620-5 |page=53}}</ref><ref name=Brusca2016>{{Cite book |url=https://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |chapter=Introduction to the Bilateria and the Phylum Xenacoelomorpha {{!}} Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation |title=Invertebrates |last=Brusca |first=Richard C. |date=2016 |publisher=[[Sinauer Associates]] |pages=345–372 |isbn=978-1-60535-375-3 |access-date=4 March 2018 |archive-url=https://web.archive.org/web/20190424155137/https://www.sinauer.com/media/wysiwyg/samples/Brusca3e_Chapter_9.pdf |archive-date=24 April 2019 |url-status=live}}</ref>

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]].<ref name="Telford 2008 pp. 457–459">{{cite journal |last=Telford |first=Maximilian J. |title=Resolving Animal Phylogeny: A Sledgehammer for a Tough Nut? |journal=Developmental Cell |volume=14 |issue=4 |year=2008 |doi=10.1016/j.devcel.2008.03.016 |pages=457–459 |pmid=18410719 |doi-access=free}}</ref> It is often suggested that the basalmost bilaterians are the [[Xenacoelomorpha]], with all other bilaterians belonging to the subclade [[Nephrozoa]].<ref name="Philippe2011">{{cite journal |last1=Philippe |first1=H. |last2=Brinkmann |first2=H. |last3=Copley |first3=R. R. |last4=Moroz |first4=L. L. |last5=Nakano |first5=H. |last6=Poustka |first6=A. J. |last7=Wallberg |first7=A. |last8=Peterson |first8=K. J. |last9=Telford |first9=M. J. |title=Acoelomorph flatworms are deuterostomes related to ''Xenoturbella'' |journal=[[Nature (journal)|Nature]] |volume=470 |pages=255–258 |year=2011 |pmid=21307940 |doi=10.1038/nature09676 |bibcode=2011Natur.470..255P |issue=7333 |pmc=4025995}}</ref><ref name="Perseke2007">{{cite journal |last1=Perseke |first1=M. |last2=Hankeln |first2=T. |last3=Weich |first3=B. |last4=Fritzsch |first4=G. |last5=Stadler |first5=P. F. |last6=Israelsson |first6=O. |last7=Bernhard |first7=D. |last8=Schlegel |first8=M. |title=The mitochondrial DNA of Xenoturbella bocki: genomic architecture and phylogenetic analysis |journal=Theory Biosci |volume=126 |issue=1 |date=August 2007 |pages=35–42 |url=https://www.bioinf.uni-leipzig.de/Publications/PREPRINTS/07-009.pdf |pmid=18087755 |doi=10.1007/s12064-007-0007-7 |citeseerx=10.1.1.177.8060 |s2cid=17065867 |access-date=4 March 2018 |archive-url=https://web.archive.org/web/20190424154927/https://www.bioinf.uni-leipzig.de/Publications/PREPRINTS/07-009.pdf |archive-date=24 April 2019 |url-status=live}}</ref><ref name="Cannon 2016">{{cite journal |last1=Cannon |first1=Johanna T. |last2=Vellutini |first2=Bruno C. |last3=Smith III |first3=Julian. |last4=Ronquist |first4=Frederik |last5=Jondelius |first5=Ulf |last6=Hejnol |first6=Andreas |date=3 February 2016 |title=Xenacoelomorpha is the sister group to Nephrozoa |journal=[[Nature (journal)|Nature]] |volume=530 |issue=7588 |pages=89–93 |doi=10.1038/nature16520 |pmid=26842059 |bibcode=2016Natur.530...89C |s2cid=205247296 |url=https://urn.kb.se/resolve?urn=urn:nbn:se:nrm:diva-1844 |access-date=21 February 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730091447/http://nrm.diva-portal.org/smash/record.jsf?pid=diva2%3A1037430&dswid=-7165 |url-status=live}}</ref> However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to [[Ambulacraria]] than to other bilaterians.<ref name="Kapli 2021">{{Cite journal |last1=Kapli |first1=Paschalia |last2=Natsidis |first2=Paschalis |last3=Leite |first3=Daniel J. |last4=Fursman |first4=Maximilian |last5=Jeffrie |first5=Nadia |last6=Rahman |first6=Imran A. |last7=Philippe |first7=Hervé |last8=Copley |first8=Richard R. |last9=Telford |first9=Maximilian J. |date=19 March 2021 |title=Lack of support for Deuterostomia prompts reinterpretation of the first Bilateria |journal=Science Advances |volume=7 |issue=12 |pages=eabe2741 |doi=10.1126/sciadv.abe2741 |pmc=7978419 |pmid=33741592 |bibcode=2021SciA....7.2741K}}</ref>

{{clear}}

==== Protostomes and deuterostomes ====

{{further|Embryological origins of the mouth and anus}}

{{main|Protostome|Deuterostome}}

[[File:Protovsdeuterostomes.svg|thumb|The bilaterian gut develops in two ways. In many [[protostome]]s, the blastopore develops into the mouth, while in [[deuterostome]]s it becomes the anus.]]

Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial [[Cleavage (embryo)|cleavage]] during cell division, while many protostomes (the [[Spiralia]]) undergo spiral cleavage.<ref>{{cite journal |last=Valentine |first=James W. |date=July 1997 |title=Cleavage patterns and the topology of the metazoan tree of life |journal=PNAS |volume=94 |pages=8001–8005 |bibcode=1997PNAS...94.8001V |doi=10.1073/pnas.94.15.8001 |pmid=9223303 |pmc=21545 |issue=15 |doi-access=free}}</ref>

Animals from both groups possess a complete digestive tract, but in protostomes the first opening of the [[archenteron|embryonic gut]] develops into the mouth, and the anus forms secondarily. In deuterostomes, the anus forms first while the mouth develops secondarily.<ref>{{cite book |last1=Peters |first1=Kenneth E. |last2=Walters |first2=Clifford C. |last3=Moldowan |first3=J. Michael |title=The Biomarker Guide: Biomarkers and isotopes in petroleum systems and Earth history |volume=2 |year=2005 |publisher=Cambridge University Press |isbn=978-0-521-83762-0 |page=717}}</ref><ref name=Hejnol2009>{{cite book |last1=Hejnol |first1=A. |last2=Martindale |first2=M. Q. |url=https://www.researchgate.net/publication/230766195 |chapter=The mouth, the anus, and the blastopore – open questions about questionable openings |title=Animal Evolution – Genomes, Fossils, and Trees |editor-last1=Telford |editor-first1=M. J. |editor-last2=Littlewood |editor-first2=D. J. |year=2009 |publisher=Oxford University Press |isbn=978-0-19-957030-0 |pages=33–40 |access-date=1 March 2018 |archive-url=https://web.archive.org/web/20181028190247/https://www.researchgate.net/publication/230766195 |archive-date=28 October 2018 |url-status=live}}</ref> Most protostomes have [[Schizocoely|schizocoelous development]], where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by [[Enterocoely|enterocoelic pouching]], through invagination of the endoderm.<ref>{{cite book |last=Safra |first=Jacob E. |title=The New Encyclopædia Britannica, Volume 1; Volume 3 |year=2003 |publisher=Encyclopædia Britannica |isbn=978-0-85229-961-6 |page=767}}</ref>

The main deuterostome taxa are the Ambulacraria and the Chordata.<ref>{{cite book |last=Hyde |first=Kenneth |title=Zoology: An Inside View of Animals |year=2004 |publisher=[[Kendall Hunt]] |isbn=978-0-7575-0997-1 |page=345}}</ref> Ambulacraria are exclusively marine and include [[acorn worm]]s, [[starfish]], [[sea urchin]]s, and [[sea cucumber]]s.<ref>{{cite book |last=Alcamo |first=Edward |title=Biology Coloring Workbook |year=1998 |publisher=[[The Princeton Review]] |isbn=978-0-679-77884-4 |page=220}}</ref> The chordates are dominated by the [[vertebrates]] (animals with [[Vertebral column|backbones]]),<ref>{{cite book |last=Holmes |first=Thom |title=The First Vertebrates |publisher=Infobase |year=2008 |isbn=978-0-8160-5958-4 |page=64}}</ref> which consist of [[fish]]es, [[amphibia]]ns, [[reptile]]s, [[bird]]s, and [[mammal<!--1st usage in main text-->]]s.<ref>{{cite book |last=Rice |first=Stanley A. |title=Encyclopedia of evolution |publisher=Infobase |year=2007 |page=[https://archive.org/details/encyclopediaofev0000rice/page/75 75] |isbn=978-0-8160-5515-9 |url=https://archive.org/details/encyclopediaofev0000rice/page/75}}</ref><ref>{{cite book |last1=Tobin |first1=Allan J. |last2=Dusheck |first2=Jennie |title=Asking about life |year=2005 |publisher=Cengage |isbn=978-0-534-40653-0 |page=497}}</ref><ref>{{cite journal |title=Hemichordate genomes and deuterostome origins |journal=[[Nature (journal)|Nature]] |date=26 November 2015 |pages=459–465 |volume=527 |issue=7579 |doi=10.1038/nature16150 |first1=Oleg |last1=Simakov |first2=Takeshi |last2=Kawashima |first3=Ferdinand |last3=Marlétaz |first4=Jerry |last4=Jenkins |first5=Ryo |last5=Koyanagi |first6=Therese |last6=Mitros |first7=Kanako |last7=Hisata |first8=Jessen |last8=Bredeson |first9=Eiichi |last9=Shoguchi |display-authors=5 |pmid=26580012 |pmc=4729200 |bibcode=2015Natur.527..459S}}</ref>

[[File:Spiral cleavage in Trochus.png|thumb|upright=1.5|The [[Spiralia]] develop with [[spiral cleavage]] in the embryo, as here in a sea snail.]]

The protostomes include the [[Ecdysozoa]], named after their shared [[Phenotypic trait|trait]] of [[ecdysis]], growth by moulting,<ref>{{cite book |last=Dawkins |first=Richard |author-link=Richard Dawkins |title=The Ancestor's Tale: A Pilgrimage to the Dawn of Evolution |year=2005 |publisher=[[Houghton Mifflin Harcourt]] |isbn=978-0-618-61916-0 |page=[https://archive.org/details/ancestorstale00rich_0/page/381 381] |url=https://archive.org/details/ancestorstale00rich_0/page/381}}</ref> Among the largest ecdysozoan phyla are the [[arthropod]]s and the [[nematode]]s.<ref>{{cite book |last1=Prewitt |first1=Nancy L. |last2=Underwood |first2=Larry S. |last3=Surver |first3=William |title=BioInquiry: making connections in biology |year=2003 |publisher=John Wiley |isbn=978-0-471-20228-8 |page=[https://archive.org/details/bioinquiry00nanc_0/page/289 289] |url=https://archive.org/details/bioinquiry00nanc_0/page/289}}</ref> The rest of the protostomes are in the [[Spiralia]], named for their pattern of developing by spiral cleavage in the early embryo. Major spiralian phyla include the [[annelid]]s and [[Mollusca|mollusc]]s.<ref name=Shankland>{{cite journal |pmid=10781038 |pmc=34316 |jstor=122407 |bibcode=2000PNAS...97.4434S |doi=10.1073/pnas.97.9.4434 |title=Evolution of the bilaterian body plan: What have we learned from annelids? |journal=Proceedings of the National Academy of Sciences |volume=97 |issue=9 |pages=4434–4437 |year=2000 |last1=Shankland |first1=M. |last2=Seaver |first2=E.C. |doi-access=free}}</ref>

== History of classification ==

{{further|Taxonomy (biology)|History of zoology through 1859|History of zoology since 1859}}

[[File:Jean-Baptiste de Lamarck.jpg|thumb|left|upright|[[Jean-Baptiste de Lamarck]] led the creation of a modern classification of [[invertebrates]], breaking up Linnaeus's "Vermes" into 9 phyla by 1809.<ref name=Gould2011/>]]

In the [[classical era]], Aristotle [[Aristotle's biology|divided animals]],{{efn|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 [[Scala naturae|arranged on a scale]] from man (with blood, two legs, rational soul) down through the live-bearing tetrapods (with blood, four 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.<ref>{{cite book |last=Leroi |first=Armand Marie |author-link=Armand Marie Leroi |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |publisher=Bloomsbury |date=2014 |isbn=978-1-4088-3622-4 |pages=111–119, 270–271}}</ref>

In 1758, [[Carl Linnaeus<!--1st usage in main text-->]] created the first [[hierarchical]] classification in his ''[[Systema Naturae]]''.<ref name=Linn1758>{{cite book |last=Linnaeus |first=Carl |author-link=Carl Linnaeus |title=Systema naturae per regna tria naturae :secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. |edition=[[10th edition of Systema Naturae|10th]] |publisher=Holmiae (Laurentii Salvii) |year=1758 |url=https://www.biodiversitylibrary.org/bibliography/542 |access-date=22 September 2008 |language=la |trans-title=The System of Nature through the Three Kingdoms of Nature |archive-url=https://web.archive.org/web/20081010032456/https://www.biodiversitylibrary.org/bibliography/542 |archive-date=10 October 2008 |url-status=live}}</ref> In his original scheme, the animals were one of three kingdoms, divided into the classes of [[Vermes in the 10th edition of Systema Naturae|Vermes]], [[Insecta in the 10th edition of Systema Naturae|Insecta]], [[Pisces in the 10th edition of Systema Naturae|Pisces]], [[Amphibia in the 10th edition of Systema Naturae|Amphibia]], [[Aves in the 10th edition of Systema Naturae|Aves]], and [[Mammalia in the 10th edition of Systema Naturae|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<!--1st usage in main text-->]], who called the Vermes {{lang|fr|une espèce de chaos}} ('a chaotic mess'){{efn|The French prefix {{lang|fr|une espèce de}} is pejorative.<ref>{{cite dictionary |title=Espèce de |url=https://dictionnaire.reverso.net/francais-anglais/esp%C3%A8ce%20de%20cr%C3%A9tin |dictionary=Reverso dictionnnaire |access-date=1 March 2018 |archive-url=https://web.archive.org/web/20130728151210/https://dictionnaire.reverso.net/francais-anglais/esp%C3%A8ce%20de%20cr%C3%A9tin |archive-date=28 July 2013 |url-status=live |language=fr,en}}</ref>}} 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 nine phyla apart from vertebrates (where he still had four phyla: mammals, birds, reptiles, and fish) and molluscs, namely [[cirripede]]s, annelids, crustaceans, arachnids, insects, worms, [[radiates]], polyps, and [[infusoria]]ns.<ref name=Gould2011>{{cite book |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |title=The Lying Stones of Marrakech |url={{GBurl |id=wApMpVmi-5gC |p=130}} |year=2011 |publisher=Harvard University Press |isbn=978-0-674-06167-5 |pages=130–134}}</ref>

In his 1817 ''{{lang|fr|[[Le Règne Animal]]}}'', [[Georges Cuvier<!--1st usage in main text-->]] used [[comparative anatomy]] to group the animals into four {{lang|fr|embranchements}} ('branches' with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and [[zoophytes|zoophytes (radiata)]] (echinoderms, cnidaria and other forms).<ref>{{cite book |last=de Wit |first=Hendrik C. D. |title=Histoire du développement de la biologie |volume=III |publisher=Presses polytechniques et universitaires Romandes |year=1994 |pages=94–96 |isbn=978-2-88074-264-5 |language=fr}}</ref> 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.<ref name=Valentine2004/>

In 1874, [[Ernst Haeckel<!--1st usage in main text-->]] 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.<ref>{{cite book |last1=Haeckel |first1=Ernst |author-link=Ernst Haeckel |title=Anthropogenie oder Entwickelungsgeschichte des menschen |url=https://archive.org/details/anthropogenieod05haecgoog |year=1874 |page=202 |language=de |trans-title=Anthropogeny or the Development story of Humans |publisher=W. Engelmann}}</ref><ref name=Valentine2004>{{cite book |last=Valentine |first=James W. |title=On the Origin of Phyla |url={{GBurl |id=DMBkmHm5fe4C |p=8}} |year=2004 |publisher=University of Chicago Press |isbn=978-0-226-84548-7 |pages=7–8}}</ref> The protozoa were later moved to the former kingdom [[Protista]], leaving only the Metazoa as a synonym of Animalia.<ref>{{cite book |last1=Hutchins |first1=Michael |title=Grzimek's Animal Life Encyclopedia |url=https://archive.org/details/animallifeprotos02mich_714 |url-access=limited |year=2003 |edition=2nd |publisher=Gale |isbn=978-0-7876-5777-2 |page=[https://archive.org/details/animallifeprotos02mich_714/page/n21 3]}}</ref>

== In human culture ==

=== Practical uses ===

{{main|Human uses of animals}}

[[File:Carni bovine macellate.JPG|thumb|upright|left|Sides of [[beef]] in a [[slaughterhouse]]]]

The human population exploits a large number of other animal species for food, both of [[domestication of animals|domesticated]] livestock species in [[animal husbandry]] and, mainly at sea, by hunting wild species.<ref name=FAOFish>{{cite web |url=https://www.fao.org/fishery/ |title=Fisheries and Aquaculture |publisher=[[Food and Agriculture Organization]] |access-date=8 July 2016 |archive-url=https://web.archive.org/web/20090519173740/https://www.fao.org/fishery |archive-date=19 May 2009 |url-status=live}}</ref><ref name=Economist/> Marine fish of many species are [[fishing|caught commercially]] for food. A smaller number of species are [[fish farming|farmed commercially]].<ref name=FAOFish/><ref>{{cite book |last=Helfman |first=Gene S. |title=Fish Conservation: A Guide to Understanding and Restoring Global Aquatic Biodiversity and Fishery Resources |url=https://archive.org/details/fishconservation00helf |url-access=limited |date=2007 |publisher=Island |isbn=978-1-59726-760-1 |page=[https://archive.org/details/fishconservation00helf/page/n25 11]}}</ref><ref>{{cite web |title=World Review of Fisheries and Aquaculture |url=https://www.fao.org/docrep/016/i2727e/i2727e01.pdf |publisher=FAO |access-date=13 August 2015 |archive-url=https://web.archive.org/web/20150828131307/https://www.fao.org/docrep/016/i2727e/i2727e01.pdf |archive-date=28 August 2015 |url-status=live}}</ref> Humans and their [[livestock]] make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.<ref name="Eggleton 2020">{{cite journal |last=Eggleton |first=Paul |title=The State of the World's Insects |journal=Annual Review of Environment and Resources |date=17 October 2020 |volume=45 |issue=1 |pages=61–82 |doi=10.1146/annurev-environ-012420-050035 |doi-access=free}}</ref>

[[Invertebrates]] including [[cephalopod]]s, [[crustacea]]ns, [[insect]]s—principally [[Western honey bee|bees]] and [[silkworms]]—and [[bivalve]] or [[gastropod]] molluscs are hunted or farmed for food, fibres.<ref>{{cite journal |title=Shellfish climbs up the popularity ladder |journal=Seafood Business |url=https://www.highbeam.com/doc/1G1-85675992.html |archive-url=https://web.archive.org/web/20121105143157/https://www.highbeam.com/doc/1G1-85675992.html |url-status=dead |archive-date=5 November 2012 |access-date=8 July 2016 |date=January 2002}}</ref><ref>{{Cite encyclopedia |date=2024-09-17 |title=Western honeybee |encyclopedia=Encyclopædia Britannica |url=https://www.britannica.com/animal/western-honeybee |access-date=2024-10-20}}</ref> [[Chicken]]s, [[cattle]], [[sheep]], [[pig]]s, and other animals are raised as livestock for meat across the world.<ref name=Economist>{{cite news |title=Graphic detail Charts, maps and infographics. Counting chickens |newspaper=The Economist |url=https://www.economist.com/blogs/dailychart/2011/07/global-livestock-counts |access-date=23 June 2016 |date=27 July 2011 |archive-url=https://web.archive.org/web/20160715181213/https://www.economist.com/blogs/dailychart/2011/07/global-livestock-counts |archive-date=15 July 2016 |url-status=live}}</ref><ref>{{cite web |url=https://cattle-today.com/ |title=Breeds of Cattle at Cattle Today |publisher=Cattle-today.com |access-date=15 October 2013 |archive-url=https://web.archive.org/web/20110715234745/https://cattle-today.com/ |archive-date=15 July 2011 |url-status=live}}</ref><ref>{{cite web |last1=Lukefahr |first1=S. D. |last2=Cheeke |first2=P. R. |title=Rabbit project development strategies in subsistence farming systems |url=https://www.fao.org/docrep/U4900T/u4900T0m.htm |publisher=[[Food and Agriculture Organization]] |access-date=23 June 2016 |archive-url=https://web.archive.org/web/20160506105314/https://www.fao.org/docrep/U4900T/u4900T0m.htm |archive-date=6 May 2016 |url-status=live}}</ref> Animal fibres such as wool and silk are used to make textiles, while animal [[sinew]]s 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.<ref>{{cite web |title=Ancient fabrics, high-tech geotextiles |url=https://www.naturalfibres2009.org/en/fibres/ |publisher=Natural Fibres |access-date=8 July 2016 |archive-url=https://web.archive.org/web/20160720093749/https://www.naturalfibres2009.org/en/fibres/ |archive-date=20 July 2016 |url-status=dead}}</ref> Dyestuffs including [[carmine]] ([[cochineal]]),<ref>{{cite book |url=https://www.fao.org/docrep/v8879e/v8879e09.htm |chapter=Cochineal and Carmine |title=Major colourants and dyestuffs, mainly produced in horticultural systems |publisher=FAO |access-date=16 June 2015 |archive-url=https://web.archive.org/web/20180306060330/https://www.fao.org/docrep/v8879e/V8879e09.htm |archive-date=6 March 2018 |url-status=live}}</ref><ref>{{cite web |url=https://www.fda.gov/ForIndustry/ColorAdditives/GuidanceComplianceRegulatoryInformation/ucm153038.htm |title=Guidance for Industry: Cochineal Extract and Carmine |publisher=FDA |access-date=6 July 2016 |archive-url=https://web.archive.org/web/20160713100106/https://www.fda.gov/ForIndustry/ColorAdditives/GuidanceComplianceRegulatoryInformation/ucm153038.htm |archive-date=13 July 2016 |url-status=dead}}</ref> [[shellac]],<ref>{{cite news |title=How Shellac Is Manufactured |url=https://nla.gov.au/nla.news-article55073762 |access-date=17 July 2015 |newspaper=The Mail |___location=Adelaide |date=18 December 1937 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730091433/https://trove.nla.gov.au/newspaper/article/55073762 |url-status=live}}</ref><ref>{{cite journal |last1=Pearnchob |first1=N. |last2=Siepmann |first2=J. |last3=Bodmeier |first3=R. |year=2003 |title=Pharmaceutical applications of shellac: moisture-protective and taste-masking coatings and extended-release matrix tablets |journal=Drug Development and Industrial Pharmacy |volume=29 |issue=8 |pages=925–938 |pmid=14570313 |doi=10.1081/ddc-120024188 |s2cid=13150932}}</ref> and [[Kermes (dye)|kermes]]<ref>{{cite book |last=Barber |first=E. J. W. |title=Prehistoric Textiles |year=1991 |publisher=Princeton University Press |isbn=978-0-691-00224-8 |pages=230–231}}</ref><ref name="Munro214">{{cite book |last=Munro |first=John H. |title=The Cambridge History of Western Textiles |chapter=Medieval Woollens: Textiles, Technology, and Organisation |editor-last1=Jenkins |editor-first1=David |year=2003 |publisher=Cambridge University Press |isbn=978-0-521-34107-3 |pages=214–215}}</ref> 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.<ref name="Pond2004">{{cite book |last=Pond |first=Wilson G. |title=Encyclopedia of Animal Science |url={{GBurl |id=1SQl7Ao3mHoC |p=248}} |year=2004 |publisher=CRC Press |isbn=978-0-8247-5496-9 |pages=248–250 |access-date=22 February 2018}}</ref>

Animals such as the fruit fly ''[[Drosophila melanogaster]]'' serve a major role in science as [[model organism|experimental models]].<ref>{{cite web |title=Genetics Research |url=https://www.aht.org.uk/cms-display/genetics.html |publisher=Animal Health Trust |access-date=24 June 2016 |archive-url=https://web.archive.org/web/20171212193051/https://www.aht.org.uk/cms-display/genetics.html |archive-date=12 December 2017 |url-status=dead}}</ref><ref>{{cite web |title=Drug Development |url=https://www.animalresearch.info/en/drug-development/ |publisher=Animal Research.info |access-date=24 June 2016 |archive-url=https://web.archive.org/web/20160608124406/https://www.animalresearch.info/en/drug-development/ |archive-date=8 June 2016 |url-status=live}}</ref><ref>{{cite web |title=Animal Experimentation |url=https://www.bbc.co.uk/ethics/animals/using/experiments_1.shtml |publisher=BBC |access-date=8 July 2016 |archive-url=https://web.archive.org/web/20160701220536/https://www.bbc.co.uk/ethics/animals/using/experiments_1.shtml |archive-date=1 July 2016 |url-status=live}}</ref><ref name="EUstatistics2013">{{cite web |title=EU statistics show decline in animal research numbers |url=https://speakingofresearch.com/2013/12/12/eu-statistics-show-decline-in-animal-research-numbers/ |publisher=Speaking of Research |year=2013 |access-date=24 January 2016 |archive-url=https://web.archive.org/web/20171006162448/https://speakingofresearch.com/2013/12/12/eu-statistics-show-decline-in-animal-research-numbers/ |archive-date=6 October 2017 |url-status=live}}</ref> Animals have been used to create [[vaccine]]s since their discovery in the 18th century.<ref>{{cite web |title=Vaccines and animal cell technology |date=10 June 2013 |url=https://www.actip.org/library/vaccines-and-animal-cell-technology/ |publisher=Animal Cell Technology Industrial Platform |access-date=9 July 2016 |archive-url=https://web.archive.org/web/20160713184805/https://www.actip.org/library/vaccines-and-animal-cell-technology/ |archive-date=13 July 2016 |url-status=live}}</ref> Some medicines such as the cancer drug [[trabectedin]] are based on [[toxin]]s or other molecules of animal origin.<ref>{{cite web |title=Medicines by Design |url=https://publications.nigms.nih.gov/medbydesign/chapter3.html |publisher=National Institute of Health |access-date=9 July 2016 |archive-url=https://web.archive.org/web/20160604214644/https://publications.nigms.nih.gov/medbydesign/chapter3.html |archive-date=4 June 2016 |url-status=dead}}</ref>

[[File:Hebbuz.JPG|thumb|A [[gun dog]] retrieving a duck during a hunt]]

People have used [[hunting dog]]s to help chase down and retrieve animals,<ref>{{cite book |last=Fergus |first=Charles |title=Gun Dog Breeds, A Guide to Spaniels, Retrievers, and Pointing Dogs |publisher=The Lyons Press |date=2002 |isbn=978-1-58574-618-7}}</ref> and [[birds of prey]] to catch birds and mammals,<ref>{{cite web |title=History of Falconry |url=https://www.thefalconrycentre.co.uk/bird-info/conservation/nocturnal-raptors/history-falconry/ |publisher=The Falconry Centre |access-date=22 April 2016 |archive-url=https://web.archive.org/web/20160529023926/https://thefalconrycentre.co.uk/bird-info/conservation/nocturnal-raptors/history-falconry/ |archive-date=29 May 2016 |url-status=live}}</ref> while tethered [[cormorant]]s have been [[Cormorant fishing|used to catch fish]].<ref name="King2013">{{cite book |last=King |first=Richard J. |title=The Devil's Cormorant: A Natural History |url={{GBurl |id=ucGyAAAAQBAJ |p=9}} |date=2013 |publisher=University of New Hampshire Press |isbn=978-1-61168-225-0 |page=9}}</ref> [[Poison dart frog]]s have been used to poison the tips of [[blowdart|blowpipe darts]].<ref name="amphibiaweb1">{{cite web |url=https://amphibiaweb.org/lists/Dendrobatidae.shtml |title=Dendrobatidae |publisher=AmphibiaWeb |access-date=10 October 2008 |archive-url=https://web.archive.org/web/20110810090554/https://amphibiaweb.org/lists/Dendrobatidae.shtml |archive-date=10 August 2011 |url-status=live}}</ref><ref>{{cite web |url=https://animaldiversity.ummz.umich.edu/site/accounts/information/Dendrobatidae.html |title=Dendrobatidae |access-date=9 July 2016 |last=Heying |first=H. |year=2003 |publisher=Animal Diversity Web |archive-url=https://web.archive.org/web/20110212005358/https://animaldiversity.ummz.umich.edu/site/accounts/information/Dendrobatidae.html |archive-date=12 February 2011 |url-status=live}}</ref>

A wide variety of animals are kept as pets, from invertebrates such as tarantulas, octopuses, and [[praying mantis]]es,<ref>{{cite web |title=Other bugs |date=18 February 2011 |url=https://www.keepinginsects.com/cockroaches-locusts-ants/ |publisher=Keeping Insects |access-date=8 July 2016 |archive-url=https://web.archive.org/web/20160707170022/https://www.keepinginsects.com/cockroaches-locusts-ants/ |archive-date=7 July 2016 |url-status=live}}</ref> reptiles such as [[snake]]s and [[chameleon]]s,<ref>{{cite web |last=Kaplan |first=Melissa |title=So, you think you want a reptile? |url=https://www.anapsid.org/parent.html |publisher=Anapsid.org |access-date=8 July 2016 |archive-url=https://web.archive.org/web/20160703115141/https://www.anapsid.org/parent.html |archive-date=3 July 2016 |url-status=live}}</ref> and birds including [[Domestic canary|canaries]], [[parakeet]]s, and [[parrot]]s<ref>{{cite web |title=Pet Birds |url=https://www.humanesociety.org/animals/pet_birds/ |publisher=PDSA |access-date=8 July 2016 |archive-url=https://web.archive.org/web/20160707053516/https://www.humanesociety.org/animals/pet_birds/ |archive-date=7 July 2016 |url-status=live}}</ref> all finding a place. However, the most kept pet species are mammals, namely [[dog]]s, [[cat]]s, and [[rabbit]]s.<ref>{{cite web |url=https://www.shea-online.org/Portals/0/PDFs/Animals%20in%20Healthcare%20Facilities.pdf |title=Animals in Healthcare Facilities |year=2012 |url-status=dead |archive-url=https://web.archive.org/web/20160304102728/https://www.shea-online.org/Portals/0/PDFs/Animals%20in%20Healthcare%20Facilities.pdf |archive-date=4 March 2016}}</ref><ref>{{cite web |last=[[Humane World for Animals|The Humane Society of the United States]] |title=U.S. Pet Ownership Statistics |url=https://www.humanesociety.org/issues/pet_overpopulation/facts/pet_ownership_statistics.html |access-date=27 April 2012 |archive-url=https://web.archive.org/web/20120407193941/https://www.humanesociety.org/issues/pet_overpopulation/facts/pet_ownership_statistics.html |archive-date=7 April 2012 |url-status=live}}</ref><ref>{{cite web |title=U.S. Rabbit Industry profile |publisher=[[United States Department of Agriculture]] |url=https://www.aphis.usda.gov/animal_health/emergingissues/downloads/RabbitReport1.pdf |access-date=10 July 2013 |url-status=dead |archive-url=https://web.archive.org/web/20131020161216/https://www.aphis.usda.gov/animal_health/emergingissues/downloads/RabbitReport1.pdf |archive-date=20 October 2013}}</ref> There is a tension between the role of animals as companions to humans, and their existence as [[animal rights|individuals with rights]] of their own.<ref>{{cite journal |last=Plous |first=S. |title=The Role of Animals in Human Society |date=1993 |doi=10.1111/j.1540-4560.1993.tb00906.x |journal=Journal of Social Issues |volume=49 |issue=1 |pages=1–9}}</ref>

A wide variety of terrestrial and aquatic animals are hunted [[Animals in sport|for sport]].<ref>{{cite book |last=Hummel |first=Richard |title=Hunting and Fishing for Sport: Commerce, Controversy, Popular Culture |date=1994 |publisher=Popular Press |isbn=978-0-87972-646-1 |url-access=registration |url=https://archive.org/details/huntingfishingfo0000humm}}</ref>

=== Symbolic uses ===

The [[signs of the zodiac|signs of the Western]] and [[Chinese zodiac]]s are based on animals.<ref>{{cite book |last=Lau |first=Theodora |title=The Handbook of Chinese Horoscopes |pages=2–8, 30–35, 60–64, 88–94, 118–124, 148–153, 178–184, 208–213, 238–244, 270–278, 306–312, 338–344 |publisher=Souvenir |year=2005}}</ref><ref name="Tester1987">{{cite book |last=Tester |first=S. Jim |title=A History of Western Astrology |url={{GBurl |id=L0HSvH96alIC |p=31}} |year=1987 |publisher=Boydell & Brewer |isbn=978-0-85115-446-6 |pages=31–33 and passim}}</ref> In China and Japan, the [[butterfly]] has been seen as the [[personification]] of a person's [[soul]],<ref name=Hearn>{{cite book |last=Hearn |first=Lafcadio |author-link=Lafcadio Hearn |year=1904 |title=Kwaidan: Stories and Studies of Strange Things |title-link=Kwaidan: Stories and Studies of Strange Things |publisher=Dover |isbn=978-0-486-21901-1}}</ref> and in classical representation the butterfly is also the symbol of the soul.<ref>{{Cite journal |url=https://quod.lib.umich.edu/cgi/t/text/text-idx?c=did;cc=did;rgn=main;view=text;idno=did2222.0001.694 |title=Butterfly |journal=Encyclopedia of Diderot and d'Alembert |access-date=16 December 2023 |date=January 2011 |first=Louis |last=De Jaucourt |archive-url=https://web.archive.org/web/20160811042437/https://quod.lib.umich.edu/cgi/t/text/text-idx?c=did;cc=did;rgn=main;view=text;idno=did2222.0001.694 |archive-date=11 August 2016 |url-status=live}}</ref><ref>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.</ref>

[[File:Alexander Coosemans - Still Life with Lobster and Oysters.jpg|thumb|Artistic vision: ''[[Still Life]] with [[Lobster]] and [[Oyster]]s'' by [[Alexander Coosemans]], {{Circa|1660}}]]

Animals have been the [[Animal style|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 ''[[Dürer's Rhinoceros|The Rhinoceros]]'', and [[George Stubbs]]'s {{Circa|1762}} horse portrait ''[[Whistlejacket]]''.<ref name="Jones">{{cite news |last1=Jones |first1=Jonathan |title=The top 10 animal portraits in art |url=https://www.theguardian.com/artanddesign/jonathanjonesblog/2014/jun/27/top-10-animal-portraits-in-art |access-date=24 June 2016 |newspaper=[[The Guardian]] |date=27 June 2014 |archive-url=https://web.archive.org/web/20160518105922/https://www.theguardian.com/artanddesign/jonathanjonesblog/2014/jun/27/top-10-animal-portraits-in-art |archive-date=18 May 2016 |url-status=live}}</ref> [[Arthropods in film|Insects]], birds and mammals play roles in literature and film,<ref>{{Cite journal |last1=Paterson |first1=Jennifer |title=Animals in Film and Media |url=https://www.oxfordbibliographies.com/view/document/obo-9780199791286/obo-9780199791286-0044.xml |journal=Oxford Bibliographies |access-date=24 June 2016 |date=29 October 2013 |doi=10.1093/obo/9780199791286-0044 |archive-url=https://web.archive.org/web/20160614200642/https://www.oxfordbibliographies.com/view/document/obo-9780199791286/obo-9780199791286-0044.xml |archive-date=14 June 2016 |url-status=live|url-access=subscription }}</ref> such as in [[Big bug movie|giant bug movies]].<ref name="GregersdotterHöglund2016">{{cite book |last1=Gregersdotter |first1=Katarina |last2=Höglund |first2=Johan |last3=Hållén |first3=Nicklas |title=Animal Horror Cinema: Genre, History and Criticism |url={{GBurl |id=hV-hCwAAQBAJ |p=147}} |date=2016 |publisher=Springer |isbn=978-1-137-49639-3 |page=147}}</ref><ref name="WarrenThomas2009">{{cite book |last1=Warren |first1=Bill |last2=Thomas |first2=Bill |title=Keep Watching the Skies!: American Science Fiction Movies of the Fifties, The 21st Century Edition |url={{GBurl |id=B7kUCwAAQBAJ |pg=PT32}} |date=2009 |publisher=[[McFarland & Company]] |isbn=978-1-4766-2505-8 |page=32}}</ref><ref name="Crouse2008">{{cite book |last=Crouse |first=Richard |title=Son of the 100 Best Movies You've Never Seen |url={{GBurl |id=B5alnowvF3sC |pg=PT200}} |year=2008 |publisher=ECW Press |isbn=978-1-55490-330-6 |page=200}}</ref>

Animals including [[insects in mythology|insects]]<ref name=Hearn/> and mammals<ref name=TFL/> feature in mythology and religion. The [[Scarab (artifact)|scarab beetle]] was sacred in [[ancient Egypt]],<ref>{{cite book |last=Ben-Tor |first=Daphna |title=Scarabs, A Reflection of Ancient Egypt |___location=Jerusalem |publisher=Israel Museum |date=1989 |isbn=978-965-278-083-6 |page=8}}</ref> and the [[Cattle in religion and mythology#Hinduism|cow is sacred in Hinduism]].<ref>{{Cite news |last=Biswas |first=Soutik |title=Why the humble cow is India's most polarising animal |url=https://www.bbc.co.uk/news/world-asia-india-34513185 |publisher=BBC |access-date=9 July 2016 |date=15 October 2015 |archive-url=https://web.archive.org/web/20161122205058/https://www.bbc.co.uk/news/world-asia-india-34513185 |archive-date=22 November 2016 |url-status=live}}</ref> Among other mammals, [[deer in mythology|deer]],<ref name=TFL>{{cite web |title=Deer |url=https://treesforlife.org.uk/forest/mythology-folklore/deer/ |publisher=[[Trees for Life (Scotland)|Trees for Life]] |access-date=23 June 2016 |archive-url=https://web.archive.org/web/20160614200842/https://treesforlife.org.uk/forest/mythology-folklore/deer/ |archive-date=14 June 2016 |url-status=live}}</ref> [[Horse worship|horses]],<ref>{{cite book |title=Hayagrīva: The Mantrayānic Aspect of Horse-cult in China and Japan |publisher=Brill Archive |page=9 |last=van Gulik |first=Robert Hans}}</ref> [[Cultural depictions of lions|lions]],<ref>{{cite web |last1=Grainger |first1=Richard |title=Lion Depiction across Ancient and Modern Religions |url=https://lionalert.org/page/Lion_Depiction_Across_Ancient_and_Modern_Religions |publisher=Alert |access-date=6 July 2016 |date=24 June 2012 |url-status=dead |archive-url=https://web.archive.org/web/20160923134807/https://lionalert.org/page/Lion_Depiction_Across_Ancient_and_Modern_Religions |archive-date=23 September 2016}}</ref> [[Bat#Cultural significance|bats]],<ref name=ReadGonzalez>{{cite book |last1=Read |first1=Kay Almere |last2=Gonzalez |first2=Jason J. |year=2000 |title=Mesoamerican Mythology |publisher=[[Oxford University Press]] |pages=132–134}}</ref> [[bear worship|bears]],<ref>{{Cite journal |last=Wunn |first=Ina |s2cid=53595088 |date=January 2000 |title=Beginning of Religion |journal=Numen |volume=47 |issue=4 |pages=417–452 |doi=10.1163/156852700511612}}</ref> and [[Wolves in folklore, religion and mythology|wolves]]<ref>{{cite book |last=McCone |first=Kim R. |title=Studien zum indogermanischen Wortschatz |publisher=Institut für Sprachwissenschaft der Universität Innsbruck |year=1987 |editor-last=Meid |editor-first=W. |___location=Innsbruck |pages=101–154 |language=de |chapter=Hund, Wolf, und Krieger bei den Indogermanen}}</ref> are the subjects of myths and worship.

== See also ==

* [[Fauna]]

* [[List of animal names]]

* [[List of songs about animals]]

* [[Animal behavior]]

* [[Animal rights]]

* [[List of animals by number of neurons]]

* [[Animal coloration]]

==References==

* [[Ethology]]

* Klaus Nielsen. ''Animal Evolution: Interrelationships of the Living Phyla'' (2nd edition). Oxford Univ. Press, 2001.

* [[Lists of organisms by population]]

* Knut Schmidt-Nielsen. ''Animal Physiology: Adaptation and Environment''. (5th edition). Cambridge Univ. Press, 1997.

* [[World Animal Day]], observed on 4 October

== Notes ==

{{notelist}}

== References ==

{{reflist}}

== External links ==

* {{Commons category-inline}}

{{Sisterlinks|Animalia}}

* {{Wikispecies-inline}}

{{wikispecies| Animalia}}

* [httphttps://tolweb.org/ Tree of Life Project]. {{Webarchive|url=https://web.archive.org/web/20110612121424/http://www.tolweb.org//|date=12 June 2011}}

* [httphttps://animaldiversity.ummz.umich.eduorg/site/index.html Animal Diversity Web] -– [[University of Michigan]]'s database of animals, showing taxonomic classification, images, and other information.

* [httphttps://archive.today/20160426231847/https://www.arkive.org/ Wildscreen ARKiveArkive] -– multimedia database of worldwide endangered/protected species and common species of UK.

* [http://www.georgetown.edu/faculty/ballc/animals/animals.html Sounds of the World's Animals] - animal sounds in many languages

* [http://www.sciam.com/article.cfm?chanID=sa006&articleID=000DC8B8-EA15-137C-AA1583414B7F0000 Scientific American Magazine (December 2005 Issue) - Getting a Leg Up on Land] About the evolution of four-limbed animals from fish.

* [http://www.packmass.com Animal Wallpapers]

* [http://animals.timduru.org/ The animal photo archive.] - animals photos submitted and collected from all over the web.

* [http://www.animalpicturesarchive.com/ The animal pictures archive.] - comprehensive database of pictures and information on animals.

{{Nature}}

[[Category:Animals| ]]

{{Organisms et al.}}

{{Life on Earth}}

{{Eukaryota}}

{{Animalia}}

{{Taxonbar |from=Q729}}

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