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{{Short description|Kingdom of living things}}
:''For the ] character, see ]. For the ], see ]''.
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{{Use British English|date=November 2024}}
{{Use dmy dates|date=August 2024}}
{{automatic taxobox
|name = Animals
|display_parents = 6
|taxon = Animalia
|authority = ], ]
|fossil_range = ] – 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


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{{Taxobox_begin | color = pink | name = Animals}}
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{{Taxobox_image | image = ] | caption = ]s, ''Chrysaora quinquecirrha''}}
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{{Taxobox_domain_entry | taxon = ]}}
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{{Taxobox_regnum_entry | taxon = '''Animalia'''}}<br/>{{Taxobox authority | author = ] | date = 1758}}
{{Taxobox_end_placement}}
{{Taxobox_section_subdivision | color = pink | plural_taxon = Phyla}}
] (sponges)<br>
] (comb jellies)<br>
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Subregnum ]<br>
&nbsp;]<br>
&nbsp;]<br>
&nbsp;]<br>
&nbsp;]<br>
&nbsp;Superphylum ]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (vertebrates, etc.)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (acorn worms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;]ata<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (arrow worms)<br>
&nbsp;Superphylum ]<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (roundworms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (horsehair worms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (velvet worms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (water bears)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (insects, etc.)<br>
&nbsp;Superphylum ]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (flatworms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (rotifers)<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (jaw worms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;Superphylum ]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (peanut worms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (ribbon worms)<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (moss animals)<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;]<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (mollusks)<br>
&nbsp;&nbsp;&nbsp;&nbsp;] (segmented worms)
{{Taxobox_end}}


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'''Animals''' are a major group of ]s, classified as the ] '''Animalia''' or '''Metazoa'''. In general they are ], capable of locomotion and responsive to their environment, and feed by consuming other organisms. Their body plan becomes fixed as they develop, usually early on in their ] as ]s, although some undergo a process of ] later on. ] beings are classified as members of the animal kingdom.
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Kingdom Animalia has several characteristics that set it apart from other living things. First, animals are ]. This separates them from the ]. Second, animals are ], which separates them from ]. Third, they are ], setting them apart from ] and several plant-like ]. Finally, Kingdom Animalia consists of organisms without ], which makes it unique compared to ], ], and ].
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</imagemap>
] divided the living world between animals and ]s, and this was followed by ] in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic ] were originally considered animals because they move, but are now treated separately. Colloquially, and in some ]s, ]s are considered separate from animals (on account of their unique abilities of ] and advanced ] or the view that they are the chosen representatives of a deity), but they belong together by current biological and ] standards. The name animal comes from the ] word ''animal'', of which ''animalia'' is the plural, and ultimately from ''anima'', meaning vital breath or soul.
|image_upright = 1.4
|subdivision_ranks = Subdivisions
|subdivision = *] (~30 phyla)
* ]
* ]
* ]
* ]
|synonyms = * Metazoa {{au|Haeckel 1874}}<ref name="Phylonyms Metazoa">{{cite book |chapter=Metazoa E. Haeckel 1874 , converted clade name |title=Phylonyms: A Companion to the PhyloCode |edition=1st |date=2020 |publisher=] |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 ], ] ]s in the ] '''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 ], ], have ]s and are ], can ], and grow from a hollow sphere of cells, the ], during ]. Animals form a ], meaning that they arose from a single ]. Over 1.5 million<!--<ref name="Zhang2013"/>--> ] animal ] have been ], of which around 1.05 million are ]s, over 85,000 are ]s, and around 65,000 are ]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 ] and ]s with each other and their environments, forming intricate ]s. The scientific study of animals is known as ], and the study of animal behaviour is known as ].<!--note: British English -->


The animal kingdom is divided into five ]s/], namely ], ], ], ] and ]. Most living animal species belong to the infrakingdom Bilateria, a highly proliferative ] whose members have a ] and significantly ] ], and the vast majority of bilaterians belong to two large ]: the ]s, which includes organisms such as ]s, ]s, ]s, ]s and ]s; and the ]s, which include ]s, ]s and ]s, the latter of which contains the ]s. The much smaller ] phylum ] have an uncertain position within Bilateria.
== Structure ==


Animals first appear in the fossil record in the late ] period and diversified in the subsequent ] period in what is known as the ]. Earlier evidence of animals is still controversial; the ]-like organism '']'' has been dated back to the ] period at the start of the ], 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 became clearly established in the fossil record as ] during the ], which began around 539&nbsp;] (Mya), and most ]es during the ] 485.4&nbsp;Mya. 6,331 groups of ]s common to all living animals have been identified; these may have arisen from a single ] that lived about 650&nbsp;Mya during the ] period.
With a few exceptions, most notably the ]s (Phylum ]), animals have bodies differentiated into separate ]. These include ]s, which are able to contract and control locomotion, and a ], which sends and processes signals. There is also typically an internal ] chamber, with one or two openings. Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.


Historically, ] divided animals ]. ] created the first hierarchical ] for animals in 1758 with his '']'', which ] expanded into 14 phyla by 1809. In 1874, ] divided the animal kingdom into the multicellular '''Metazoa''' (now ] with Animalia) and the ], single-celled organisms no longer considered animals. In modern times, the biological classification of animals relies on advanced techniques, such as ], which are effective at demonstrating the ]ary relationships between ].
All animals have ] cells, surrounded by a characteristic ] composed of ] and elastic ]s. This may be ] to form structures like shells, bones, and spicules. 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 ]s and ] have cells held in place by ]s, so develop by progressive growth. Also, unique to animal cells are the following intercellular junctions: ]s, ]s, and ]s.


]s make ] many other animal species for ] (including ]<!--no list please-->, ]s, and ]s), for ]s (such as ], ], and ]), as ]s and as ]s for ], and ]s. ]s, the first ]d animal, have been used ], ] and ], as have ]s, ]s and ]; while other ] and ]s are ]ed for sports, trophies or profits. Non-human animals are also an important ] element of ], having appeared in ]s and ]s since the earliest times, and are frequently featured in ], ], ]s, ], ], ], and ]s.
== Reproduction and development ==


==Etymology==
Nearly all animals undergo some form of ]. Adults are ] or occasionally ]. They have a few specialized reproductive cells, which undergo ] to produce smaller motile ] or larger non-motile ]. These fuse to form ]s, which develop into new individuals.
<!--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=] |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=] |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=] |volume=4 |page=625 |doi=10.3389/fpsyg.2013.00625 |pmc=3814086 |pmid=24198791 |doi-access=free}}</ref><ref>{{Cite web |website=] |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=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><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 {{Webarchive|url=https://web.archive.org/web/20220730091429/https://www.collinsdictionary.com/ko/dictionary/english/meta |date=30 July 2022 }} and {{Webarchive|url=https://web.archive.org/web/20220730091429/https://www.collinsdictionary.com/ko/dictionary/english/zoa |date=30 July 2022 }}.</ref>
Many animals are also capable of ]. This may take place through ], where fertile eggs are produced without mating, or in some cases through fragmentation.


== Characteristics ==
A ] initially develops into a hollow sphere, called a ], 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 invaginates to form a gastrula with a digestive chamber, and two separate germ layers - an external ] and an internal ]. In most cases, a ] also develops between them. These germ layers then differentiate to form tissues and organs.


] (1) develop into a hollow ball or ] (2).]]
Animals grow by indirectly using the energy of ]. Plants use this ] to turn air into simple ] using a process known as ]. 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 ].


Animals have several characteristics that set them apart from other living things. Animals are ] and ].<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 ]e, which ],<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 are ]ic,<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> feeding on organic material and digesting 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=] |volume=358 |issue=1429 |pages=5–17 |date=January 2003 |pmid=12594915 |pmc=1693093 |doi=10.1098/rstb.2002.1188}}</ref> With very few exceptions, animals ].{{efn|'']'' does not have mitochondrial DNA or utilise aerobic respiration.<ref name="CNN-20200226">{{Cite news |last=Andrew |first=Scottie |url=https://www.cnn.com/2020/02/26/world/first-animal-doesnt-breathe-oxygen-scn-trnd/index.html |title=Scientists discovered the first animal that doesn't need oxygen to live. It's changing the definition of what an animal can be |work=] |date=26 February 2020 |access-date=28 February 2020 |archive-date=10 January 2022 |archive-url=https://web.archive.org/web/20220110180353/https://www.cnn.com/2020/02/26/world/first-animal-doesnt-breathe-oxygen-scn-trnd/index.html |url-status=live}}</ref>}}<ref name="oxygen">{{cite journal |last1=Mentel |first1=Marek |last2=Martin |first2=William |title=Anaerobic animals from an ancient, anoxic ecological niche |journal=BMC Biology |volume=8 |page=32 |year=2010 |doi=10.1186/1741-7007-8-32 |pmid=20370917 |pmc=2859860 |doi-access=free}}</ref> All animals are ]<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> (able to spontaneously move their bodies) during at least part of their ], but some animals, such as ]s, ]s, ]s, and ]s, later become ]. The ] is a stage in ] that is unique to animals, allowing ] into specialised tissues and organs.<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>
== Origin and fossil record ==


=== Structure ===
Animals are generally considered to have evolved from ] protozoa. Their closest living relatives are the ]s, collared flagellates that have the same structure as certain sponge cells do. Molecular studies place them in a supergroup called the ]s, which also include the ] and a few small parasitic ]s. The name comes from the posterior location of the ] in motile cells, such as most animal sperm, whereas other eukaryotes tend to have anterior flagella.


All animals are composed of cells, surrounded by a characteristic ] composed of ] and elastic ]s.<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=] |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> During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as ], ]s, and ].<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=}}</ref> In contrast, the cells of other multicellular organisms (primarily algae, plants, and ]) 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=] |isbn=978-0-8053-0870-9 |url=https://archive.org/details/worldofcell00beck_0}}</ref> Animal cells uniquely possess the ]s called ]s, ]s, and ]s.<ref>{{cite book |last=Magloire |first=Kim |title=Cracking the AP Biology Exam, 2004–2005 Edition |year=2004 |publisher=] |isbn=978-0-375-76393-9 |page= |url=https://archive.org/details/crackingapbiolog00magl/page/45}}</ref>
The first fossils that might represent animals appear towards the end of the ], around 600 million years ago, and are known as the ]. 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 animal phyla with known phyla make a more or less simultaneous appearance during the ] period, about 570 million years ago. It is still disputed whether this event, called the ], represents a rapid divergence between different groups or a change in conditions that made fossilization possible.


With few exceptions—in particular, the sponges and ]ns—animal bodies are differentiated into ].<ref>{{cite book |last=Starr |first=Cecie |url={{GBurl |id=EXNFwB-O-WUC |p=362}} |title=Biology: Concepts and Applications without Physiology |year=2007 |publisher=Cengage Learning |isbn=978-0-495-38150-1 |pages=362, 365 |access-date=19 May 2020}}</ref> These include ]s, which enable locomotion, and ]s, which transmit signals and coordinate the body. Typically, there is also an internal ] chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).<ref>{{cite book |last1=Hillmer |first1=Gero |last2=Lehmann |first2=Ulrich |translator-first=J. |translator-last=Lettau |title=Fossil Invertebrates |year=1983 |publisher=] Archive |isbn=978-0-521-27028-1 |page=54 |url={{GBurl |id=9jE4AAAAIAAJ |p=54}} |access-date=8 January 2016}}</ref>
== Groups of animals ==


=== Reproduction and development ===
The sponges (]) diverged from other animals early. As mentioned, they lack the complex organization found in most other phyla. Their cells are differentiated, but not organized into distinct tissues. Sponges are sessile and typically feed by drawing in water through ]s all over the body, which is supported by a skeleton typically divided into spicules. The extinct ], which have fused skeletons, may represent sponges or a separate phylum.


{{see also|Sexual reproduction#Animals|Asexual reproduction#Examples in animals}}
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 ], which include ]s, ]s, and ], and the ] or comb jellies. Both have distinct tissues, but they are not organized into ]. There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called ]. The tiny phylum ] is similar, but individuals do not have a permanent digestive chamber.


] is nearly universal in animals, such as these ].]]
The remaining animals form a monophyletic group called the ]. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is ], 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 ]s are radially symmetric, and certain parasitic worms have extremely simplified body structures.


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= |url=https://archive.org/details/encyclopediaofre0000unse_f1r2/page/315}}</ref> They produce ] ]s by ]; the smaller, motile gametes are ] and the larger, non-motile gametes are ].<ref>{{cite book |last=Schwartz |first=Jill |title=Master the GED 2011 |year=2010 |publisher=Peterson's |isbn=978-0-7689-2885-3 |page= |url=https://archive.org/details/petersonsmasterg0000stew_x3f1/page/371}}</ref> These fuse to form ]s,<ref>{{cite book |last=Hamilton |first=Matthew B. |title=Population genetics |url=https://archive.org/details/populationgeneti00hami |url-access=limited |year=2009 |publisher=] |isbn=978-1-4051-3277-0 |page=}}</ref> which develop via ] into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge.<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=] |page=330}}</ref> It first ] to form a ] with a digestive chamber and two separate ]s, an external ] and an internal ].<ref>{{cite book |last=Philips |first=Joy B. |title=Development of vertebrate anatomy |year=1975 |publisher=Mosby |isbn=978-0-8016-3927-2 |page= |url=https://archive.org/details/developmentofver0000phil/page/176}}</ref> In most cases, a third germ layer, the ], 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>
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to four major lineages:
# ]
# ]
# ]
# ]


Repeated instances of ] during sexual reproduction generally leads to ] within a population due to the increased prevalence of harmful ] traits.<ref name="pmid19834483">{{cite journal |last1=Charlesworth |first1=D. |last2=Willis |first2=J. H. |title=The genetics of inbreeding depression |journal=] |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 ].<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>
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 ], ], and ]. The ], single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Bilateria as well.


Some animals are capable of ], which often results in a genetic clone of the parent. This may take place through ]; ], such as in ] and other ]ns; or ], where fertile eggs are produced without ], such as in ]s.<ref>{{cite book |last1=Adiyodi |first1=K. G. |last2=Hughes |first2=Roger N. |last3=Adiyodi |first3=Rita G. |title=Reproductive Biology of Invertebrates, Volume 11, Progress in Asexual Reproduction |date=July 2002 |publisher=Wiley |page=116 |isbn=978-0-471-48968-9}}</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>
=== Deuterostomes ===


== Ecology ==
]s differ from the other Bilateria, called ]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 ] of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.


]s, such as this ] (''Ficedula superciliaris''), feed on other animals.]]
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the ] and ]. The former are radially symmetric and exclusively marine, such as ]s, ]s, and ]s. The latter are dominated by the ]s, animals with backbones. These include ], ]s, ]s, ]s, and ]s.


Animals are categorised into ecological groups depending on their ]s and ]. Such groupings include ]s (further divided into subcategories such as ]s, ]s, ]s, etc.), ]s (subcategorised into ]s, ]s, ]s, ]s, ]s, ]s, etc.), ]s, ]s, ]s/]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 ]s.<ref>{{cite book |last=Levy |first=Charles K. |title=Elements of Biology |year=1973 |publisher=] |isbn=978-0-390-55627-1 |page=108}}</ref> ]s between animals of each ] form complex ]s within that ]. In carnivorous or omnivorous species, ] is a ] where the predator feeds on another organism, its ],<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 ]s to avoid being fed upon. ]s imposed on one another lead to an ] between predator and prey, resulting in various antagonistic/] ]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=] |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=] |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 ] use multiple methods; for example, in ]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 ]s, such as ]s which mainly ].<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 addition to these, the deuterostomes also include the ] or acorn worms. Although they are not especially prominent today, the important fossil ]s may belong to this group. The ] or arrow worms may also be deuterostomes, but this is less certain.


] mussels and shrimps]]
=== Ecdysozoa ===
Most animals rely on ] and ] produced by ]s and ]s (collectively called ]s) through ]. Herbivores, as ]s, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher ]s indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidise ]s, ]s, ]s and other biomolecules, which allows the animal to grow and to sustain ] and fuel other biological processes such as ].<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=}}</ref> Some ] animals living close to ]s and ]s on the dark ] consume organic matter produced through ] (via ] ]s such as ]) by ] and ].<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 ]s, probably between 510 and 471 million years ago during the ] or Early ].<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> ]s such as the ] '']'' started to move on to land in the late ], 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=] |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=] |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 ]s and microhabitats, with ]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 ]; 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 ].<ref name="bas-land">{{cite web |title=Land animals |url=https://www.bas.ac.uk/about/antarctica/wildlife/land-animals/ |publisher=] |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 ] are protostomes, named after the common trait of growth by moulting or ]. The largest animal phylum belongs here, the ], including ]s, ]s, ]s, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the ] and ], are close relatives of the arthropods and share these traits.


== Diversity ==
The ecdysozoans also include the ] 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 ] or horsehair worms, which are visible to the unaided eye, and the ], ], and ], which are all microscopic. These groups have a reduced coelom, called a pseudocoelom.


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


{{further|Largest organisms|Smallest organisms}}
=== Platyzoa ===


The ] (''Balaenoptera musculus'') is the largest animal that has ever lived, weighing up to 190 ]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 ] (''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 ] ] such as '']'', 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 ] (]s within the Cnidaria) never grow larger than 20 ],<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>
The ] include the phylum ], the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors. A number of parasites are included in this group, such as the ]s and ]s. Flatworms lack a coelom, as do their closest relatives, the microscopic ].


<gallery class="center" mode="nolines" widths="220" heights="160">
The other platyzoan phyla are microscopic and pseudocoelomate. The most prominent are the ] or rotifers, which are common in aqueous environments. They also include the ] or spiny-headed worms, the ], ], and possibly the ]. These groups share the presence of complex jaws, from which they are called the ].
File:Anim1754 - Flickr - NOAA Photo Library (1).jpg|The ] is the largest animal that has ever lived; it can be up to {{convert|33.6|m|ft}} long.
File:Fdl17-9-grey.jpg|]ns such as '']'' are single-celled parasites, never more than 20 ] across.
</gallery>


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


<!--We are NOT trying to be comprehensive here, there are plenty of lower-level Misplaced Pages articles for that-->
The ] include two of the most successful animal phyla, the ] and ]. The former includes animals such as ]s, ]s, and ]s, and the latter comprises the segmented worms, such as ]s and ]es. These two groups have long been considered close relatives because of the common presence of ] 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.
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=] |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=] |year=2007 |isbn=978-0-691-12085-0 |page= |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 ] 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 ] to taxonomy further complicates this; a 2016 barcoding analysis estimated a total count of nearly 100,000 ] species for ] 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 (]).<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"
The Lophotrochozoa also include the ] or ribbon worms, the ], and several phyla that have a fan of cilia around the mouth, called a ]. These were traditionally grouped together as the lophophorates, but it now appears they are ], some closer to the Nemertea and some to the Mollusca and Annelida. They include the ] or lamp shells, which are prominent in the fossil record, the ], the ], and possibly the ] or moss animals.
|-
! scope="col" | ]
! scope="col" class="unsortable" | Example<!--image-->
! scope="col" | Described species
! scope="col" | ]
! scope="col" | ]
! scope="col" | ]
! scope="col" | Free-living
! scope="col" | ]
|-
! scope="row" | ''']'''
|]
|align=right data-sort-value="1257000"|1,257,000<ref name="Zhang2013"/>
|Yes 1,000,000<br />(]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 />(])<ref>{{cite book |year=2002 |series=Zoological catalogue of Australia |volume=19.2A |title=Crustacea: Malacostraca |publisher=] |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 ]s.<ref name=Poulin2007/>}}<ref name=Poulin2007/>
|-
! scope="row" | ''']'''
|]
|align=right data-sort-value="85000" |85,000<ref name="Zhang2013"/><br />107,000<ref name=Nicol1969/>
|Yes 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>
|Yes 60,000<ref name=Nicol1969/>
|Yes 5,000<ref name=Balian2008/><br />12,000<ref name=Nicol1969/>
|Yes<ref name="Hogenboom2016"/>
|Yes >5,600<ref name=Poulin2007/>
|-
! scope="row" | ''']'''
|]
|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>
|Yes 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>
|Yes 13,000<ref name="Reaka-Kudla1996"/>
|Yes 18,000<ref name=Balian2008/><br />9,000<ref name="Reaka-Kudla1996"/>
|Yes
|Yes 40<br />(])<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=] ... includes obligate parasitic fish. Thus 17 genera from 2 subfamilies, ]; 4 genera, 9spp. and ]; 13 genera, 31 spp. are parasites on gills (Vandelliinae) or skin (stegophilines) of fish.}}</ref><ref name=Poulin2007/>
|-
! scope="row" | ''']'''
|]
|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"/>
|Yes 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>
|Yes >40,000<ref name=Poulin2007/><br />
4,000–25,000<ref name=Pandian/>
|-
! scope="row" | ''']'''
|]
|align=right data-sort-value="25000"|25,000<ref name="Zhang2013"/>
|Yes (soil)<ref name="Hogenboom2016"/>
|Yes 4,000<ref name=Felder2009/>
|Yes 2,000<ref name=Balian2008/>
|Yes<br />11,000<ref name=Felder2009/>
|Yes 14,000<ref name=Felder2009/>
|-
! scope="row" | ''']'''
|]
|align=right |17,000<ref name="Zhang2013"/>
|Yes (soil)<ref name="Hogenboom2016"/>
|Yes<ref name="Hogenboom2016"/>
|Yes 1,750<ref name=Balian2008/>
|Yes
|Yes 400<ref name=Poulin2007/>
|-
! scope="row" | ''']'''
|]
|align=right data-sort-value="16000"|16,000<ref name="Zhang2013"/>
|
|Yes<ref name="Hogenboom2016"/>
|Yes (few)<ref name="Hogenboom2016"/>
|Yes<ref name="Hogenboom2016"/>
|Yes >1,350<br />(])<ref name=Poulin2007/>
|-
! scope="row" | ''']'''
|]
|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" | ''']'''
|]
|align=right data-sort-value="7500"|7,500<ref name="Zhang2013"/>
|
|Yes 7,500<ref name="Zhang2013"/>
|
|Yes<ref name="Hogenboom2016"/>
|
|-
! scope="row" | ''']'''
|]
|align=right data-sort-value="6000" |6,000<ref name="Zhang2013"/>
|
|Yes<ref name="Hogenboom2016"/>
|Yes 60–80<ref name=Balian2008/>
|Yes
|
|-
! scope="row" | ''']'''
|]
|align=right data-sort-value="2000"|2,000<ref name="Zhang2013"/>
|
|Yes >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>
|Yes 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" | ''']'''
|]
|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 |doi=10.3389/fmars.2022.906383 |url=https://www.researchgate.net/publication/362813258 |access-date=18 January 2023 |doi-access=free}}</ref>
|
|Yes
|Yes
|Yes
|
|-
! scope="row" | ''']'''
|]
|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
|
|-
! colspan=8 |{{centre|Total number of described extant species {{as of|2013|lc=y}}: 1,525,728<ref name="Zhang2013"/>}}
|}

== Evolutionary origin ==

{{Further|Urmetazoan}}

Evidence of animals is found as long ago as the ] period. ] (24-ipc) has been found in rocks from roughly 650 million years ago; it is only produced by sponges and ] algae. Its likely origin is from sponges based on ] estimates for the origin of 24-ipc production in both groups. Analyses of pelagophyte algae consistently recover a ] origin, while analyses of sponges recover a ] 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}}</ref>

The first body fossils of animals appear in the ], represented by forms such as '']'' and '']''. 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= |url=https://archive.org/details/evolutionfactsfa0000unse/page/7}}</ref> but the discovery of the animal lipid ] in fossils of '']'' 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 ], 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 ] record during the ], starting about 539 million years ago, in beds such as the ].<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 ]s, ]s, ]ns, ]s, ]s, ]s and ]s, along with numerous now-extinct forms such as the ]y '']''. 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=] |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 '']'', the earliest known Ediacaran crown-group cnidarian (557–562 mya, some 20 million years before the Cambrian explosion) from ], England. It is thought to be one of the earliest ]s, catching small prey with its ]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 ] of ]. These fossils are interpreted as most probably being early ].<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>
]s such as tracks and burrows found in the ] period (from 1 gya) may indicate the presence of ] 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 '']'', 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 ]s called ]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=] |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=] |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|'']'' from the ] (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|'']'', an Ediacaran predator (c.&nbsp;560 mya)<ref name="Dunn Kenchington Parry Clark 2022" />
File:20191203 Anomalocaris canadensis.png|'']'' is one of the many animal species that emerged in the ], starting some 539 mya, and found in the fossil beds of the ].
</gallery>

== Phylogeny ==

{{further|Lists of animals}}

=== External phylogeny ===

Animals are ], meaning they are derived from a common ancestor. Animals are the sister group to the ]s, with which they form the ].<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=] |volume=92 |issue=1 |pages=446–473 |year=2017 |last1=Budd |first1=Graham E. |last2=Jensen |first2=Sören |doi-access=free}}</ref> The dates on the ] indicate approximately how many millions of years ago ({{em|mya}}) the lineages split.<ref>{{cite journal |last1=Peterson |first1=Kevin J. |last2=Cotton |first2=James A. |last3=Gehling |first3=James G. |last4=Pisani |first4=Davide |date=27 April 2008 |title=The Ediacaran emergence of bilaterians: congruence between the genetic and the geological fossil records |journal=Philosophical Transactions of the Royal Society of London B: Biological Sciences |volume=363 |issue=1496 |pages=1435–1443 |doi=10.1098/rstb.2007.2233 |pmid=18192191 |pmc=2614224}}</ref><ref>{{cite journal |author-link1=Laura Wegener Parfrey |last1=Parfrey |first1=Laura Wegener |last2=Lahr |first2=Daniel J. G. |last3=Knoll |first3=Andrew H. |author-link4=Laura A. Katz |last4=Katz |first4=Laura A. |date=16 August 2011 |title=Estimating the timing of early eukaryotic diversification with multigene molecular clocks |journal=] |volume=108 |issue=33 |pages=13624–13629 |doi=10.1073/pnas.1110633108 |pmid=21810989 |bibcode=2011PNAS..10813624P |pmc=3158185 |doi-access=free}}</ref><ref>{{cite web |title=Raising the Standard in Fossil Calibration |url=https://fossilcalibrations.org/ |website=Fossil Calibration Database |access-date=3 March 2018 |archive-url=https://web.archive.org/web/20180307054141/https://fossilcalibrations.org/ |archive-date=7 March 2018 |url-status=live}}</ref><ref>{{Cite journal |last1=Laumer |first1=Christopher E. |last2=Gruber-Vodicka |first2=Harald |last3=Hadfield |first3=Michael G. |last4=Pearse |first4=Vicki B. |last5=Riesgo |first5=Ana |last6=Marioni |first6=John C. |last7=Giribet |first7=Gonzalo |year=2018 |title=Support for a clade of Placozoa and Cnidaria in genes with minimal compositional bias |journal=eLife |volume=2018;7 |pages=e36278 |doi=10.7554/eLife.36278 |pmid=30373720 |pmc=6277202 |doi-access=free}}</ref><ref>{{cite journal |last1=Adl |first1=Sina M. |last2=Bass |first2=David |last3=Lane |first3=Christopher E. |last4=Lukeš |first4=Julius |last5=Schoch |first5=Conrad L. |last6=Smirnov |first6=Alexey |last7=Agatha |first7=Sabine |last8=Berney |first8=Cedric |last9=Brown |first9=Matthew W. |display-authors=5 |date=2018 |title=Revisions to the Classification, Nomenclature, and Diversity of Eukaryotes |journal=Journal of Eukaryotic Microbiology |volume=66 |issue=1 |pages=4–119 |doi=10.1111/jeu.12691 |pmid=30257078 |pmc=6492006}}</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.<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=] |sublabel1=1300 mya
|1={{Clade
|1=] (inc. fungi) ]
|label2=] |sublabel2=1100 mya
|2={{Clade |state=dashed
|1=] ]
|2=] ]
|label3=]
|3={{Clade
|1=] ]
|label2=] |sublabel2=950 mya
|2={{Clade
|label1=]a |1=]
|label2 ='''Animalia''' |sublabel2=760 mya |2= ]
}}
}}
}}
}}
}}

=== Internal phylogeny ===
{{anchor|basal}}

The relationships at the base of the animal tree have been debated.<ref>{{Cite journal |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 |url=https://www.biorxiv.org/content/10.1101/704551v1 |journal=bioRxiv |pages=704551 |doi=10.1101/704551 |doi-access=free}}</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=] |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 ]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 ] (sponges) have long been assumed to be sister to the rest of the animals, but there is evidence that the ] may be in that position. Molecular phylogenetics has supported both the sponge-sister and ctenophore-sister hypotheses. In 2017, Roberto Feuda and colleagues, using ] 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='']''
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|label2=] |sublabel2='']s''
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Conversely, a 2023 study by Darrin Schultz and colleagues uses ancient ]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='']''
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=== Non-bilaterians ===

]

Sponges are physically very distinct from other animals, and were long thought to have diverged first, representing the oldest animal phylum and forming a ] 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=] |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= |url=https://archive.org/details/livingbayunderwa0000lang/page/244}}</ref> Animals in both phyla have distinct tissues, but these are not organised into discrete ].<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 ], 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}}

]n body plan.{{efn|Compare ] 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 ]. Opposed circular and longitudinal muscles enable ].]]

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form the ] ], which have a bilaterally symmetric ]. The Bilateria are ], with three well-developed germ layers, and their tissues ]. The digestive chamber has two openings, a mouth and an anus, and in the Nephrozoa there is an internal body cavity, a ] 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/>

Having a front end means that this part of the body encounters stimuli, such as food, favouring ], the development of a head with ]s and a mouth. Many bilaterians have a combination of circular ]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 ] to move by ].<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=] |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 ]e which swim with ] 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 ] 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=] |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=] |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 ] and the ].<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 ], with all other bilaterians belonging to the subclade ].<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=] |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=] |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 ] 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}}

]s, the blastopore develops into the mouth, while in ]s it becomes the anus.]]

Protostomes and deuterostomes differ in several ways. Early in development, deuterostome embryos undergo radial ] during cell division, while many protostomes (the ]) 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 ] 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 ], where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by ], through invagination of the endoderm.<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 phyla are the Echinodermata and the Chordata.<ref>{{cite book |last=Hyde |first=Kenneth |title=Zoology: An Inside View of Animals |year=2004 |publisher=] |isbn=978-0-7575-0997-1 |page=345}}</ref> Echinoderms are exclusively marine and include ], ]s, and ]s.<ref>{{cite book |last=Alcamo |first=Edward |title=Biology Coloring Workbook |year=1998 |publisher=] |isbn=978-0-679-77884-4 |page=220}}</ref> The chordates are dominated by the ] (animals with ]),<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 ]es, ]ns, ]s, ]s, and ]s.<ref>{{cite book |last=Rice |first=Stanley A. |title=Encyclopedia of evolution |publisher=Infobase |year=2007 |page= |isbn=978-0-8160-5515-9 |url=https://archive.org/details/encyclopediaofev0000rice/page/75}}</ref> The deuterostomes also include the ] (acorn worms).<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=] |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>

===== Ecdysozoa =====

{{main|Ecdysozoa}}

]: a ] has emerged from its dry ] and is expanding its wings. Like other ], its body is ].]]

The Ecdysozoa are protostomes, named after their shared ] of ], 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=] |isbn=978-0-618-61916-0 |page= |url=https://archive.org/details/ancestorstale00rich_0/page/381}}</ref> They include the largest animal phylum, the ]a, which contains insects, spiders, crabs, and their kin. All of these have a body divided into ], typically with paired appendages. Two smaller phyla, the ] and ], are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic and occur in nearly every environment where there is water;<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= |url=https://archive.org/details/bioinquiry00nanc_0/page/289}}</ref> some are important parasites.<ref>{{cite book |title=Parasites in social insects |year=1998 |publisher=] |isbn=978-0-691-05924-2 |last=Schmid-Hempel |first=Paul |page=75}}</ref> Smaller phyla related to them are the ] or horsehair worms, and the ], ], and ]. These groups have a reduced coelom, called a pseudocoelom.<ref>{{cite book |last1=Miller |first1=Stephen A. |last2=Harley |first2=John P. |title=Zoology |url={{GBurl |id=BWZFAQAAIAAJ}} |year=2006 |publisher=McGraw Hill |page=173 |isbn=978-0-07-063682-8}}</ref>

===== Spiralia =====

{{main|Spiralia}}

] in a sea snail embryo]]

The Spiralia are a large group of protostomes that develop by spiral cleavage in the early embryo.<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> The Spiralia's phylogeny has been disputed, but it contains a large clade, the superphylum ], and smaller groups of phyla such as the ] which includes the ]s and the ]s. All of these are grouped as the ], which has a sister group, the ], which includes the ]s.<ref name=Struck2014>{{cite journal |last1=Struck |first1=Torsten H. |last2=Wey-Fabrizius |first2=Alexandra R. |last3=Golombek |first3=Anja |last4=Hering |first4=Lars |last5=Weigert |first5=Anne |last6=Bleidorn |first6=Christoph |last7=Klebow |first7=Sabrina |last8=Iakovenko |first8=Nataliia |last9=Hausdorf |first9=Bernhard |last10=Petersen |first10=Malte |last11=Kück |first11=Patrick |last12=Herlyn |first12=Holger |last13=Hankeln |first13=Thomas |title=Platyzoan Paraphyly Based on Phylogenomic Data Supports a Noncoelomate Ancestry of Spiralia |journal=Molecular Biology and Evolution |volume=31 |issue=7 |date=2014 |doi=10.1093/molbev/msu143 |pages=1833–1849 |pmid=24748651 |doi-access=free}}</ref><ref>{{Cite journal |last1=Fröbius |first1=Andreas C. |last2=Funch |first2=Peter |date=April 2017 |title=Rotiferan Hox genes give new insights into the evolution of metazoan bodyplans |journal=Nature Communications |volume=8 |issue=1 |pages=9 |doi=10.1038/s41467-017-00020-w |pmid=28377584 |pmc=5431905 |bibcode=2017NatCo...8....9F}}</ref>

The Lophotrochozoa includes the ]s, ]s, ]s, ]ns, ] and ].<ref name=Struck2014/><ref>{{cite journal |last1=Hervé |first1=Philippe |last2=Lartillot |first2=Nicolas |last3=Brinkmann |first3=Henner |date=May 2005 |title=Multigene Analyses of Bilaterian Animals Corroborate the Monophyly of Ecdysozoa, Lophotrochozoa, and Protostomia |journal=Molecular Biology and Evolution |volume=22 |issue=5 |pages=1246–1253 |doi=10.1093/molbev/msi111 |pmid=15703236 |doi-access=free}}</ref><ref>{{cite web |title=Introduction to the Lophotrochozoa: Of molluscs, worms, and lophophores... |url=https://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html |publisher=UCMP Berkeley |access-date=28 February 2018 |archive-url=https://web.archive.org/web/20000816183847/https://www.ucmp.berkeley.edu/phyla/lophotrochozoa.html |archive-date=16 August 2000 |url-status=dead |last=Speer |first=Brian R. |date=2000}}</ref> The molluscs, the second-largest animal phylum by number of described species, includes ]s, ]s, and ]s, while the annelids are the segmented worms, such as ]s, ]s, and ]es. These two groups have long been considered close relatives because they share ] larvae.<ref name=Giribet2000>{{cite journal |last1=Giribet |first1=G. |last2=Distel |first2=D. L. |last3=Polz |first3=M. |last4=Sterrer |first4=W. |last5=Wheeler |first5=W. C. |year=2000 |title=Triploblastic relationships with emphasis on the acoelomates and the position of Gnathostomulida, Cycliophora, Plathelminthes, and Chaetognatha: a combined approach of 18S rDNA sequences and morphology |journal=Syst Biol |volume=49 |issue=3 |pages=539–562 |doi=10.1080/10635159950127385 |pmid=12116426 |doi-access=free}}</ref><ref>{{cite journal |title=Phylogenetic Relationships of Annelids, Molluscs, and Arthropods Evidenced from Molecules and Morphology |journal=] |volume=43 |issue=3 |pages=207–215 |date=September 1996 |doi=10.1007/PL00006079 |pmid=8703086 |last1=Kim |first1=Chang Bae |last2=Moon |first2=Seung Yeo |last3=Gelder |first3=Stuart R. |last4=Kim |first4=Won |bibcode=1996JMolE..43..207K}}</ref>


== History of classification == == History of classification ==
In ]' original scheme, the animals were one of three kingdoms, divided into the classes of ], ]a, ], ], ], and ]ia. Since then the last four have all been subsumed into a single phylum, the ], 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.


{{further|Taxonomy (biology)|History of zoology through 1859|History of zoology since 1859}}
== Usage of the word ''animal'' ==

In modern vulgar usage, ''animal'' is abused to refer to beasts.
] led the creation of a modern classification of ], breaking up Linnaeus's "Vermes" into 9 phyla by 1809.<ref name=Gould2011/>]]

In the ], Aristotle ],{{efn|In his '']'' and '']''.}} based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then ] 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). ] 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, ] created the first ] classification in his '']''.<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=] |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 ], ], ], ], ], and ]. Since then, the last four have all been subsumed into a single phylum, the ], while his Insecta (which included the crustaceans and arachnids) and Vermes have been renamed or broken up. The process was begun in 1793 by ], 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 '']'', Lamarck had created nine phyla apart from vertebrates (where he still had four phyla: mammals, birds, reptiles, and fish) and molluscs, namely ]s, annelids, crustaceans, arachnids, insects, worms, ], polyps, and ]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|]}}'', ] used ] 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 ] (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 ] in 1828, the zoologist ] in 1857, and the comparative anatomist ] in 1860.<ref name=Valentine2004/>

In 1874, ] 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 ], 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=}}</ref>

== In human culture ==

=== Practical uses ===

{{main|Human uses of animals}}

] in a ]]]

The human population exploits a large number of other animal species for food, both of ] livestock species in ] and, mainly at sea, by hunting wild species.<ref name=FAOFish>{{cite web |url=https://www.fao.org/fishery/ |title=Fisheries and Aquaculture |publisher=] |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 ] for food. A smaller number of species are ].<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=}}</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 ] 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>

] including ]s, ]ns, ]s—principally ] and ]—and ] or ] 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> ]s, ], ], ]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=] |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 ]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 ] (]),<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=live}}</ref> ],<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 ]<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. ] 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 '']'' serve a major role in science as ].<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 ]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 ] are based on ]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=live}}</ref>

] retrieving a duck during a hunt]]

People have used ]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 ] 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 ]s have been ].<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> ]s have been used to poison the tips of ].<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 ]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 ]s and ]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 ], ]s, and ]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 ]s, ]s, and ]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=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=] |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 ] 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 ].<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 ] and ]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 ] has been seen as the ] of a person's ],<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>

] with ] and ]s'' by ], {{Circa|1660}}]]

Animals have been the ] from the earliest times, both historical, as in ancient Egypt, and prehistoric, as in the ]. Major animal paintings include ]'s 1515 '']'', and ]'s {{Circa|1762}} horse portrait '']''.<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=] |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> ], 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}}</ref> such as in ].<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=] |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 ]<ref name=Hearn/> and mammals<ref name=TFL/> feature in mythology and religion. The ] was sacred in ],<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 ].<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, ],<ref name=TFL>{{cite web |title=Deer |url=https://treesforlife.org.uk/forest/mythology-folklore/deer/ |publisher=] |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> ],<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> ],<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> ],<ref name=ReadGonzalez>{{cite book |last1=Read |first1=Kay Almere |last2=Gonzalez |first2=Jason J. |year=2000 |title=Mesoamerican Mythology |publisher=] |pages=132–134}}</ref> ],<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 ]<ref>{{cite book |last=McCone |first=Kim R. |chapter=Hund, Wolf, und Krieger bei den Indogermanen |editor-last=Meid |editor-first=W. |title=Studien zum indogermanischen Wortschatz |location=Innsbruck |year=1987 |pages=101–154 |language=de}}</ref> are the subjects of myths and worship.

== See also ==

* ]
* ]
* ]
* ], observed on 4 October

== Notes ==
{{notelist}}


== Examples == == References ==
{{reflist}}
Some well-known types of animals, listed by their common names:
*], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ], ]


==See also== == External links ==
* {{Commons category-inline}}
*]
* {{Wikispecies-inline}}
*]
* . {{Webarchive|url=https://web.archive.org/web/20110612121424/http://www.tolweb.org//|date=12 June 2011}}
*]
* – ]'s database of animals
*]
* – multimedia database of endangered/protected species
*]
*] terms
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{{Animalia}}
==External links==
{{Wiktionary}} {{Eukaryota}}
{{wikiquote}} {{Nature}}
{{Life on Earth}}
{{Commons2|Animalia}}
{{Organisms et al.}}
{{Wikispecies|Animalia}}
{{dichotomouskey|Animalia|Animalia}}
*
*
*
* - Large table of words: animal, collective, male, female, young, & home
*
* - animal sounds in many languages
* - sound files including animal sound files
* - animals reviewed and evaluated
* - Photos of animals
*
*


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{{Authority control}}
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Latest revision as of 11:10, 10 December 2024

Kingdom of living things For other uses, see Animal (disambiguation). "Animalia" redirects here. For other uses, see Animalia (disambiguation).

Animals
Temporal range: Cryogenian – present, 665–0 Ma Pha. Proterozoic Archean Had.
EchinodermCnidariaTardigradeCrustaceanArachnidSpongeInsectBryozoaAcanthocephalaFlatwormMolluscAnnelidVertebrateTunicatePhoronid
Scientific classification Edit this classification
Domain: Eukaryota
Clade: Amorphea
Clade: Obazoa
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Clade: Choanozoa
Kingdom: Animalia
Linnaeus, 1758
Subdivisions
Synonyms
  • Metazoa Haeckel 1874
  • Choanoblastaea Nielsen 2008
  • Gastrobionta Rothm. 1948
  • Zooaea Barkley 1939
  • Euanimalia Barkley 1939

Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (/ˌænɪˈmeɪliə/). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor. Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology, and the study of animal behaviour is known as ethology.

The animal kingdom is divided into five infrakingdoms/superphyla, namely Porifera, Ctenophora, Placozoa, Cnidaria and Bilateria. Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric and significantly cephalised body plan, and the vast majority of bilaterians belong to two large superphyla: the protostomes, which includes organisms such as arthropods, molluscs, flatworms, annelids and nematodes; and the deuterostomes, which include echinoderms, hemichordates and chordates, the latter of which contains the vertebrates. The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria.

Animals first appear 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. Nearly all modern animal phyla became clearly established in the fossil record as marine species during the Cambrian explosion, which began around 539 million years ago (Mya), and most classes during the Ordovician radiation 485.4 Mya. 6,331 groups of genes common to all living animals have been identified; these may have arisen from a single common ancestor that lived about 650 Mya during the Cryogenian period.

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

Humans make use of many other animal species for food (including meat, eggs, and dairy products), for materials (such as leather, fur, and wool), as pets and as working animals for transportation, and services. Dogs, the first domesticated animal, have been used in hunting, in security and in warfare, as have horses, pigeons and birds of prey; while other terrestrial and aquatic animals are hunted for sports, trophies or profits. Non-human animals are also an important cultural element of human evolution, having appeared in cave arts and totems since the earliest times, and are frequently featured in mythology, religion, arts, literature, heraldry, politics, and sports.

Etymology

The word animal comes from the Latin noun animal of the same meaning, which is itself derived from Latin animalis 'having breath or soul'. The biological definition includes all members of the kingdom Animalia. In colloquial usage, the term animal is often used to refer only to nonhuman animals. The term metazoa is derived from Ancient Greek μετα meta 'after' (in biology, the prefix meta- stands for 'later') and ζῷᾰ zōia 'animals', plural of ζῷον zōion 'animal'.

Characteristics

Animals are unique in having the ball of cells of the early embryo (1) develop into a hollow ball or blastula (2).

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and multicellular. Unlike plants and algae, which produce their own nutrients, animals are heterotrophic, feeding on organic material and digesting it internally. With very few exceptions, animals respire aerobically. All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. The blastula is a stage in embryonic development that is unique to animals, allowing cells to be differentiated into specialised tissues and organs.

Structure

All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as shells, bones, and spicules. In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.

With few exceptions—in particular, the sponges and placozoans—animal bodies are differentiated into tissues. These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (in Ctenophora, Cnidaria, and flatworms) or two openings (in most bilaterians).

Reproduction and development

See also: Sexual reproduction § Animals, and Asexual reproduction § Examples in animals
Sexual reproduction is nearly universal in animals, such as these dragonflies.

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

Repeated instances of mating with a close relative during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for avoiding close inbreeding.

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

Ecology

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

Animals are categorised into ecological groups depending on their trophic levels and how they consume organic material. Such groupings include carnivores (further divided into subcategories such as piscivores, insectivores, ovivores, etc.), herbivores (subcategorised into folivores, graminivores, frugivores, granivores, nectarivores, algivores, etc.), omnivores, fungivores, scavengers/detritivores, and parasites. Interactions between animals of each biome form complex food webs within that ecosystem. In carnivorous or omnivorous species, predation is a consumer–resource interaction where the predator feeds on another organism, its prey, who often evolves anti-predator adaptations to avoid being fed upon. Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various antagonistic/competitive coevolutions. Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours, such as hawksbill sea turtles which mainly eat sponges.

Hydrothermal vent mussels and shrimps

Most animals rely on biomass and bioenergy produced by plants and phytoplanktons (collectively called producers) through photosynthesis. Herbivores, as primary consumers, eat the plant material directly to digest and absorb the nutrients, while carnivores and other animals on higher trophic levels indirectly acquire the nutrients by eating the herbivores or other animals that have eaten the herbivores. Animals oxidise carbohydrates, lipids, proteins and other biomolecules, which allows the animal to grow and to sustain basal metabolism and fuel other biological processes such as locomotion. Some benthic animals living close to hydrothermal vents and cold seeps on the dark sea floor consume organic matter produced through chemosynthesis (via oxidising inorganic compounds such as hydrogen sulfide) by archaea and bacteria.

Animals evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510 and 471 million years ago during the Late Cambrian or Early Ordovician. Vertebrates such as the lobe-finned fish Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's habitats and microhabitats, with faunas adapted to salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of other organisms. Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 °C (122 °F) or in the most extreme cold deserts of continental Antarctica.

Diversity

Size

Further information: Largest organisms and Smallest organisms

The blue whale (Balaenoptera musculus) is the largest animal that has ever lived, weighing up to 190 tonnes and measuring up to 33.6 metres (110 ft) long. The largest extant terrestrial animal is the African bush elephant (Loxodonta africana), weighing up to 12.25 tonnes and measuring up to 10.67 metres (35.0 ft) long. The largest terrestrial animals that ever lived were titanosaur sauropod dinosaurs such as Argentinosaurus, which may have weighed as much as 73 tonnes, and Supersaurus which may have reached 39 metres. Several animals are microscopic; some Myxozoa (obligate parasites within the Cnidaria) never grow larger than 20 μm, and one of the smallest species (Myxobolus shekel) is no more than 8.5 μm when fully grown.

Numbers and habitats of major phyla

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

Phylum Example Described species Land Sea Freshwater Free-living Parasitic
Arthropoda wasp 1,257,000 Yes 1,000,000
(insects)
Yes >40,000
(Malac-
ostraca
)
Yes 94,000 Yes Yes >45,000
Mollusca snail 85,000
107,000
Yes 35,000 Yes 60,000 Yes 5,000
12,000
Yes Yes >5,600
Chordata green spotted frog facing right >70,000 Yes 23,000 Yes 13,000 Yes 18,000
9,000
Yes Yes 40
(catfish)
Platyhelminthes 29,500 Yes Yes Yes 1,300 Yes

3,000–6,500

Yes >40,000

4,000–25,000

Nematoda 25,000 Yes (soil) Yes 4,000 Yes 2,000 Yes
11,000
Yes 14,000
Annelida 17,000 Yes (soil) Yes Yes 1,750 Yes Yes 400
Cnidaria Table coral 16,000 Yes Yes (few) Yes Yes >1,350
(Myxozoa)
Porifera 10,800 Yes 200–300 Yes Yes
Echinodermata 7,500 Yes 7,500 Yes
Bryozoa 6,000 Yes Yes 60–80 Yes
Rotifera 2,000 Yes >400 Yes 2,000 Yes Yes
Nemertea 1,350 Yes Yes Yes
Tardigrada 1,335 Yes
(moist plants)
Yes Yes Yes
Total number of described extant species as of 2013: 1,525,728

Evolutionary origin

Further information: 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.

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, but the discovery of the animal lipid cholesterol in fossils of Dickinsonia establishes their nature. 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.

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. Extant phyla in these rocks include molluscs, brachiopods, onychophorans, tardigrades, arthropods, echinoderms and hemichordates, along with numerous now-extinct forms such as the predatory Anomalocaris. The apparent suddenness of the event may however be an artefact of the fossil record, rather than showing that all these animals appeared simultaneously. 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 predators, catching small prey with its nematocysts as modern cnidarians do.

Some palaeontologists have suggested that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago. 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. Trace fossils 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 mm wide) and complex as earthworms. However, similar tracks are produced by the giant single-celled protist Gromia sphaerica, so the Tonian trace fossils may not indicate early animal evolution. Around the same time, the layered mats of microorganisms called stromatolites decreased in diversity, perhaps due to grazing by newly evolved animals. 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.

Phylogeny

Further information: Lists of animals

External phylogeny

Animals are monophyletic, meaning they are derived from a common ancestor. Animals are the sister group to the choanoflagellates, with which they form the Choanozoa. The dates on the phylogenetic tree indicate approximately how many millions of years ago (mya) the lineages split.

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.

Opisthokonta

Holomycota (inc. fungi)

Holozoa

Ichthyosporea

Pluriformea

Filozoa

Filasterea

Choanozoa
Choanoflagellatea

Animalia

760 mya
950 mya
1100 mya
1300 mya

Internal phylogeny

The relationships at the base of the animal tree have been debated. Other than Ctenophora, the Bilateria and Cnidaria are the only groups with symmetry, and other evidence shows they are closely related. In addition to sponges, Placozoa has no symmetry and was often considered a "missing link" between protists and multicellular animals. The presence of hox genes in Placozoa shows that they were once more complex.

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):

Animalia

Porifera

Ctenophora

ParaHoxozoa

Placozoa

Cnidaria

Bilateria

symmetry
hox genes
multicellular

Conversely, a 2023 study by Darrin Schultz and colleagues uses ancient gene linkages to construct the following ctenophore-sister phylogeny:

Animalia

Ctenophora

Porifera

ParaHoxozoa

Placozoa

Cnidaria

Bilateria

symmetry
hox genes
multicellular

Non-bilaterians

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. 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. Sponges lack the complex organisation found in most other animal phyla; their cells are differentiated, but in most cases not organised into distinct tissues, unlike all other animals. They typically feed by drawing in water through pores, filtering out small particles of food.

The Ctenophora and Cnidaria are radially symmetric and have digestive chambers with a single opening, which serves as both mouth and anus. Animals in both phyla have distinct tissues, but these are not organised into discrete organs. They are diploblastic, having only two main germ layers, ectoderm and endoderm.

The tiny placozoans have no permanent digestive chamber and no symmetry; they superficially resemble amoebae. Their phylogeny is poorly defined, and under active research.

Bilateria

Main articles: Bilateria and Symmetry (biology) § Bilateral symmetry
Idealised nephrozoan body plan. With an elongated body and a direction of movement the animal has head and tail ends. Sense organs and mouth form the basis of the head. Opposed circular and longitudinal muscles enable peristaltic motion.

The remaining animals, the great majority—comprising some 29 phyla and over a million species—form the Bilateria clade, which have a bilaterally symmetric body plan. The Bilateria are triploblastic, with three well-developed germ layers, and their tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and 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.

Having a front end means that this part of the body encounters stimuli, such as food, favouring cephalisation, the development of a head with sense organs and a mouth. Many bilaterians have a combination of circular muscles that constrict the body, making it longer, and an opposing set of longitudinal muscles, that shorten the body; these enable soft-bodied animals with a hydrostatic skeleton to move by peristalsis. They also have a gut that extends through the basically cylindrical body from mouth to anus. Many bilaterian phyla have primary larvae which swim with cilia and have an apical organ containing sensory cells. However, 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.

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. It is often suggested that the basalmost bilaterians are the Xenacoelomorpha, with all other bilaterians belonging to the subclade Nephrozoa. However, this suggestion has been contested, with other studies finding that xenacoelomorphs are more closely related to Ambulacraria than to other bilaterians.

Protostomes and deuterostomes

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

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

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

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

The Ecdysozoa are protostomes, named after their shared trait of ecdysis, growth by moulting. They include the largest animal phylum, the Arthropoda, which contains insects, spiders, crabs, and their kin. All of these have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits. The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic and occur in nearly every environment where there is water; some are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.

Spiralia
Main article: Spiralia
Spiral cleavage in a sea snail embryo

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

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

History of classification

Further information: Taxonomy (biology), History of zoology through 1859, and History of zoology since 1859
Jean-Baptiste de Lamarck led the creation of a modern classification of invertebrates, breaking up Linnaeus's "Vermes" into 9 phyla by 1809.

In the classical era, Aristotle divided animals, based on his own observations, into those with blood (roughly, the vertebrates) and those without. The animals were then arranged on a scale from man (with blood, 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.

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

In his 1817 Le Règne Animal, Georges Cuvier used comparative anatomy to group the animals into four embranchements ('branches' with different body plans, roughly corresponding to phyla), namely vertebrates, molluscs, articulated animals (arthropods and annelids), and zoophytes (radiata) (echinoderms, cnidaria and other forms). This division into four was followed by the embryologist Karl Ernst von Baer in 1828, the zoologist Louis Agassiz in 1857, and the comparative anatomist Richard Owen in 1860.

In 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms: Metazoa (multicellular animals, with five phyla: coelenterates, echinoderms, articulates, molluscs, and vertebrates) and Protozoa (single-celled animals), including a sixth animal phylum, sponges. The protozoa were later moved to the former kingdom Protista, leaving only the Metazoa as a synonym of Animalia.

In human culture

Practical uses

Main article: Human uses of animals
Sides of beef in a slaughterhouse

The human population exploits a large number of other animal species for food, both of domesticated livestock species in animal husbandry and, mainly at sea, by hunting wild species. Marine fish of many species are caught commercially for food. A smaller number of species are farmed commercially. Humans and their livestock make up more than 90% of the biomass of all terrestrial vertebrates, and almost as much as all insects combined.

Invertebrates including cephalopods, crustaceans, insects—principally bees and silkworms—and bivalve or gastropod molluscs are hunted or farmed for food, fibres. Chickens, cattle, sheep, pigs, and other animals are raised as livestock for meat across the world. Animal fibres such as wool and silk are used to make textiles, while animal sinews have been used as lashings and bindings, and leather is widely used to make shoes and other items. Animals have been hunted and farmed for their fur to make items such as coats and hats. Dyestuffs including carmine (cochineal), shellac, and kermes have been made from the bodies of insects. Working animals including cattle and horses have been used for work and transport from the first days of agriculture.

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

A gun dog retrieving a duck during a hunt

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

A wide variety of terrestrial and aquatic animals are hunted for sport.

Symbolic uses

The signs of the Western and Chinese zodiacs are based on animals. In China and Japan, the butterfly has been seen as the personification of a person's soul, and in classical representation the butterfly is also the symbol of the soul.

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

Animals have been the subjects of art from the earliest times, both historical, as in ancient Egypt, and prehistoric, as in the cave paintings at Lascaux. Major animal paintings include Albrecht Dürer's 1515 The Rhinoceros, and George Stubbs's c. 1762 horse portrait Whistlejacket. Insects, birds and mammals play roles in literature and film, such as in giant bug movies.

Animals including insects and mammals feature in mythology and religion. The scarab beetle was sacred in ancient Egypt, and the cow is sacred in Hinduism. Among other mammals, deer, horses, lions, bats, bears, and wolves are the subjects of myths and worship.

See also

Notes

  1. Henneguya zschokkei does not have mitochondrial DNA or utilise aerobic respiration.
  2. 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).
  3. Not including parasitoids.
  4. Compare File:Annelid redone w white background.svg for a more specific and detailed model of a particular phylum with this general body plan.
  5. In his History of Animals and Parts of Animals.
  6. The French prefix une espèce de is pejorative.

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