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{{Paraphyletic group | {{Paraphyletic group | ||
| name = Invertebrates | | name = Invertebrates | ||
| image = Invertebrate montage (2022).jpg | | image = Invertebrate montage (2022).jpg | ||
| image_upright = 1.2 | | image_upright = 1.2 | ||
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| auto = yes | | auto = yes | ||
| parent = Animalia | | parent = Animalia | ||
| includes = *All animal groups not in subphylum [[Vertebrata]] | | includes = *All animal groups not in the subphylum [[Vertebrate|Vertebrata]] | ||
}} | }} | ||
'''Invertebrates''' are [[animal]]s that neither develop nor retain a [[vertebral column]] (commonly known as a ''spine'' or ''backbone''), which evolved from the [[notochord]]. It is a [[paraphyletic]] grouping including all animals excluding the [[chordata|chordate]] [[subphylum]] [[Vertebrata]], i.e. [[vertebrate]]s. | |||
'''Invertebrates''' are [[animal]]s that neither develop nor retain a [[vertebral column]] (commonly known as a ''spine'' or ''backbone''), which evolved from the [[notochord]]. It is a [[paraphyletic]] grouping including all animals excluding the [[chordata|chordate]] [[subphylum]] [[Vertebrata]], i.e. [[vertebrate]]s. Nonetheless, almost all invertebrates belong to the [[monophyly|monophyletic]] clade [[Protostomia]], including [[arthropod]]s, [[mollusc]]s, [[annelid]]s, and [[flatworm]]s. Some of the well-known remaining invertebrates are [[Deuterostomia|deuterostomes]] such as [[echinoderm]]s and [[tunicate]]s, or animals outside the protostome/deuterostome split, including [[cnidarian]]s and [[sponge]]s. | |||
The majority of animal species are invertebrates; one estimate puts the figure at 97%.<ref>{{cite journal|first=Robert M.|last=May|date=16 September 1988|title=How Many Species Are There on Earth?|journal=Science|volume=241|issue=4872|pages=1441–9|url=http://www.ciesin.org/docs/002-253/002-253.html|jstor=1702670|pmid=17790039|doi=10.1126/science.241.4872.1441|bibcode=1988Sci...241.1441M|s2cid=34992724|access-date=17 June 2014|archive-url=https://web.archive.org/web/20161115011411/http://www.ciesin.org/docs/002-253/002-253.html|archive-date=15 November 2016|url-status=dead|url-access=subscription}}</ref> Many invertebrate [[taxon|taxa]] have a greater number and diversity of species than the entire subphylum of Vertebrata.<ref name="isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=978-0-412-61390-6 }}</ref> Invertebrates vary widely in size, from 10 [[Micrometre|μm]] (0.0004 in)<ref>{{cite book |last1=Canning |first1=Elizabeth U. |last2=Okamura |first2=Beth |chapter=Biodiversity and Evolution of the Myxozoa |date=2003-01-01 |title=Advances in Parasitology |volume=56 |pages=43–131 |publisher=Academic Press |doi=10.1016/S0065-308X(03)56002-X |pmid=14710996 |isbn=978-0-12-031756-1}}</ref> [[myxozoa]]ns to the 9–10 m (30–33 ft) [[colossal squid]].<ref>Roper, C.F.E. & P. Jereb (2010). Family Cranchiidae. In: P. Jereb & C.F.E. Roper (eds.) ''[http://www.fao.org/docrep/014/i1920e/i1920e00.htm Cephalopods of the world. An annotated and illustrated catalogue of species known to date. Volume 2. Myopsid and Oegopsid Squids] {{Webarchive|url=https://web.archive.org/web/20190129054613/http://www.fao.org/docrep/014/i1920e/i1920e00.htm |date=29 January 2019 }}''. FAO Species Catalogue for Fishery Purposes No. 4, Vol. 2. FAO, Rome. pp. 148–178.</ref> | The majority of animal species are invertebrates; one estimate puts the figure at 97%.<ref>{{cite journal|first=Robert M.|last=May|date=16 September 1988|title=How Many Species Are There on Earth?|journal=Science|volume=241|issue=4872|pages=1441–9|url=http://www.ciesin.org/docs/002-253/002-253.html|jstor=1702670|pmid=17790039|doi=10.1126/science.241.4872.1441|bibcode=1988Sci...241.1441M|s2cid=34992724|access-date=17 June 2014|archive-url=https://web.archive.org/web/20161115011411/http://www.ciesin.org/docs/002-253/002-253.html|archive-date=15 November 2016|url-status=dead|url-access=subscription}}</ref> Many invertebrate [[taxon|taxa]] have a greater number and diversity of species than the entire subphylum of Vertebrata.<ref name="isbn0-412-61390-5">{{cite book |author1=Richards, O. W. |author2=Davies, R.G. |title=Imms' General Textbook of Entomology: Volume 1: Structure, Physiology and Development Volume 2: Classification and Biology |publisher=Springer |location=Berlin |year=1977 |isbn=978-0-412-61390-6 }}</ref> Invertebrates vary widely in size, from 10 [[Micrometre|μm]] (0.0004 in)<ref>{{cite book |last1=Canning |first1=Elizabeth U. |last2=Okamura |first2=Beth |chapter=Biodiversity and Evolution of the Myxozoa |date=2003-01-01 |title=Advances in Parasitology |volume=56 |pages=43–131 |publisher=Academic Press |doi=10.1016/S0065-308X(03)56002-X |pmid=14710996 |isbn=978-0-12-031756-1}}</ref> [[myxozoa]]ns to the 9–10 m (30–33 ft) [[colossal squid]].<ref>Roper, C.F.E. & P. Jereb (2010). Family Cranchiidae. In: P. Jereb & C.F.E. Roper (eds.) ''[http://www.fao.org/docrep/014/i1920e/i1920e00.htm Cephalopods of the world. An annotated and illustrated catalogue of species known to date. Volume 2. Myopsid and Oegopsid Squids] {{Webarchive|url=https://web.archive.org/web/20190129054613/http://www.fao.org/docrep/014/i1920e/i1920e00.htm |date=29 January 2019 }}''. FAO Species Catalogue for Fishery Purposes No. 4, Vol. 2. FAO, Rome. pp. 148–178.</ref> | ||
Some so-called invertebrates, such as the [[Tunicata]] and [[Cephalochordata]], are actually sister chordate subphyla to Vertebrata, being more closely related to vertebrates than to other invertebrates. This makes the "invertebrates" | Some so-called invertebrates, such as the [[Tunicata]] and [[Cephalochordata]], are actually sister chordate subphyla to Vertebrata, being more closely related to vertebrates than to other invertebrates. This makes the "invertebrates" paraphyletic, so the term has no significance in [[taxonomy (biology)|taxonomy]]. | ||
== Etymology == | == Etymology == | ||
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== Taxonomic significance == | == Taxonomic significance == | ||
The term ''invertebrates'' does not describe a [[taxon]] in the same way that [[Arthropod]]a, [[Vertebrata]] or [[Manidae]] do. Each of those terms describes a valid taxon, [[phylum]], [[subphylum]] or [[Family ( | The term ''invertebrates'' does not describe a [[taxon]] in the same way that [[Arthropod]]a, [[Vertebrata]] or [[Manidae]] do. Each of those terms describes a valid taxon, [[phylum]], [[subphylum]] or [[Family (taxonomy)|family]]. "Invertebrata" is a term of convenience, not a taxon; it has very little [[Circumscription (taxonomy)|circumscriptional]] significance except within the [[Chordate|Chordata]]. The Vertebrata as a [[Taxonomic rank|subphylum]] comprises such a small proportion of the [[Metazoa]] that to speak of the [[Kingdom (biology)|kingdom]] [[Animal]]ia in terms of "Vertebrata" and "Invertebrata" has limited practicality. In the more formal taxonomy of Animalia other attributes that logically should precede the presence or absence of the vertebral column in constructing a [[cladogram]], for example, the presence of a [[notochord]]. That would at least circumscribe the Chordata. However, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry<ref name=" Pech">{{cite book | last = Pechenik | first = Jan | title = Biology of the Invertebrates | publisher = Wm. C. Brown Publishers | location = Dubuque | year = 1996 | isbn = 978-0-697-13712-8 }}</ref> or perhaps [[Body plan|Bauplan]].<ref>{{cite book |last1=Brusca |first1=Richard C. |last2=Brusca |first2=Gary J. | title = Invertebrates |url=https://archive.org/details/invertebrates0000brus |url-access=registration | publisher = Sinauer Associates | location = Sunderland | year = 1990 | isbn = 978-0-87893-098-2 }}</ref> | ||
Despite this, the concept of ''invertebrates'' as a taxon of animals has persisted for over a century among the [[laity]],<ref name="isbn0-19-861271-0">{{cite book |author=Brown, Lesley |title=The New shorter Oxford English dictionary on historical principles |publisher=Clarendon |location=Oxford [Eng.] |year=1993 |isbn=978-0-19-861271-1 |url=https://archive.org/details/newshorteroxford00lesl }}</ref> and within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata.<ref name="Agassiz2013">{{cite book |first=Louis |last=Agassiz |title=Essay on Classification |url=https://books.google.com/books?id=O6fDAgAAQBAJ&pg=PT115 |date=21 March 2013 |publisher=Courier Corporation |isbn=978-0-486-15135-9 |pages=115–}}</ref> The following text reflects earlier scientific understanding of the term and of those animals which have constituted it. According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely varied [[body plan]]s. Many have fluid-filled, hydrostatic skeletons, like [[jellyfish]] or worms. Others have hard [[exoskeleton]]s, outer shells like those of [[insect]]s and [[crustacean]]s. The most familiar invertebrates include the [[Protozoa]], [[Porifera]], [[Coelenterata]], [[Platyhelminthes]], [[Nematoda]], [[Annelida]], [[Echinodermata]], [[Mollusca]] and [[Arthropoda]]. Arthropoda include | Despite this, the concept of ''invertebrates'' as a taxon of animals has persisted for over a century among the [[laity]],<ref name="isbn0-19-861271-0">{{cite book |author=Brown, Lesley |title=The New shorter Oxford English dictionary on historical principles |publisher=Clarendon |location=Oxford [Eng.] |year=1993 |isbn=978-0-19-861271-1 |url=https://archive.org/details/newshorteroxford00lesl }}</ref> and within the zoological community and in its literature it remains in use as a term of convenience for animals that are not members of the Vertebrata.<ref name="Agassiz2013">{{cite book |first=Louis |last=Agassiz |title=Essay on Classification |url=https://books.google.com/books?id=O6fDAgAAQBAJ&pg=PT115 |date=21 March 2013 |publisher=Courier Corporation |isbn=978-0-486-15135-9 |pages=115–}}</ref> The following text reflects earlier scientific understanding of the term and of those animals which have constituted it. According to this understanding, invertebrates do not possess a skeleton of bone, either internal or external. They include hugely varied [[body plan]]s. Many have fluid-filled, hydrostatic skeletons, like [[jellyfish]] or worms. Others have hard [[exoskeleton]]s, outer shells like those of [[insect]]s and [[crustacean]]s. The most familiar invertebrates include the [[Protozoa]], [[Porifera]], [[Coelenterata]], [[Platyhelminthes]], [[Nematoda]], [[Annelida]], [[Echinodermata]], [[Mollusca]] and [[Arthropoda]]. Arthropoda include insects, crustaceans and [[arachnid]]s. | ||
== Number of extant species == | == Number of extant species == | ||
| Line 55: | Line 55: | ||
|- | |- | ||
| [[Crustacean]]s | | [[Crustacean]]s | ||
| [[ | | [[Arthropoda]] | ||
| [[File:J J Wild Pseudocarcinus cropped.jpg|100px]] | | [[File:J J Wild Pseudocarcinus cropped.jpg|100px]] | ||
| align=right | 47,000 | | align=right | 47,000 | ||
| Line 80: | Line 80: | ||
|- | |- | ||
| [[Horseshoe crabs]] | | [[Horseshoe crabs]] | ||
| [[ | | [[Arthropoda]] | ||
| [[File:Carcinoscorpius rotundicauda (mangrove horseshoe crab).jpg|100px]] | | [[File:Carcinoscorpius rotundicauda (mangrove horseshoe crab).jpg|100px]] | ||
| align=right | 4 | | align=right | 4 | ||
| Line 185: | Line 185: | ||
* [[Tunicates|Salps, pyrosomes, doliolids, larvaceans and sea squirts]] (Tunicata) | * [[Tunicates|Salps, pyrosomes, doliolids, larvaceans and sea squirts]] (Tunicata) | ||
* [[Symbion|Cycliophora]] | * [[Symbion|Cycliophora]] | ||
An informal classification of invertabrates between macroinvertebrates, which can be seen withe the naked eye, and microinvertebrates which cannot be seen by the unaided eye is used for convenience. The line is arbitrarily drawn at a length of about 1mm.<ref>Michaluk, Sonja. "macroinvertebrate". Encyclopedia Britannica, 27 Nov. 2025, https://www.britannica.com/animal/macroinvertebrate. Accessed 4 December 2025.</ref><ref> microinvertebrate. Oxford Reference. Retrieved 4 Dec. 2025, from https://www.oxfordreference.com/view/10.1093/oi/authority.20110810105410141. </ref> | |||
== History == | == History == | ||
| Line 202: | Line 204: | ||
== In research == | == In research == | ||
For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" or [[charismatic species]].<ref name="Aristotle"/> Invertebrate biology was not a major field of study until the work of [[Carl Linnaeus|Linnaeus]] and [[Lamarck]] in the 18th century.<ref name="Aristotle">{{cite web |url=http://nmnh.typepad.com/no_bones/2015/06/why-study-invertebrates-a-philosophical-argument-from-aristotle.html |title=Why study invertebrates? A philosophical argument from Aristotle |last1=Ducarme |first1=Frédéric |date=2015 |website=No Bones (Smithsonian Institution website) |access-date=15 July 2015 |archive-date=15 July 2015 |archive-url=https://web.archive.org/web/20150715175131/http://nmnh.typepad.com/no_bones/2015/06/why-study-invertebrates-a-philosophical-argument-from-aristotle.html |url-status=live }}</ref> During the 20th century, invertebrate zoology became one of the major fields of natural sciences, with prominent discoveries in the fields of medicine, genetics, palaeontology, and ecology.<ref name="Aristotle"/> The study of invertebrates has also benefited law enforcement, as arthropods, and especially insects, were discovered to be a source of information for forensic investigators.<ref name=forensic /> | For many centuries, invertebrates were neglected by biologists, in favor of big vertebrates and "useful" or [[charismatic species]].<ref name="Aristotle"/> Invertebrate biology was not a major field of study until the work of [[Carl Linnaeus|Linnaeus]] and [[Jean-Baptiste Lamarck|Lamarck]] in the 18th century.<ref name="Aristotle">{{cite web |url=http://nmnh.typepad.com/no_bones/2015/06/why-study-invertebrates-a-philosophical-argument-from-aristotle.html |title=Why study invertebrates? A philosophical argument from Aristotle |last1=Ducarme |first1=Frédéric |date=2015 |website=No Bones (Smithsonian Institution website) |access-date=15 July 2015 |archive-date=15 July 2015 |archive-url=https://web.archive.org/web/20150715175131/http://nmnh.typepad.com/no_bones/2015/06/why-study-invertebrates-a-philosophical-argument-from-aristotle.html |url-status=live }}</ref> During the 20th century, invertebrate zoology became one of the major fields of natural sciences, with prominent discoveries in the fields of medicine, genetics, palaeontology, and ecology.<ref name="Aristotle"/> The study of invertebrates has also benefited law enforcement, as arthropods, and especially insects, were discovered to be a source of information for forensic investigators.<ref name=forensic /> | ||
Two of the most commonly studied model organisms nowadays are invertebrates: the fruit fly ''[[Drosophila melanogaster]]'' and the nematode ''[[Caenorhabditis elegans]]''. They have long been the most intensively studied [[model organism]]s, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their [[genome]]s, but many [[gene]]s, [[intron]]s, and [[genetic linkage|linkages]] have been lost. Analysis of the [[starlet sea anemone]] genome has emphasised the importance of sponges, placozoans, and [[choanoflagellate]]s, also being sequenced, in explaining the arrival of 1,500 ancestral genes unique to animals.<ref>{{cite journal |author=N.H. Putnam |date=July 2007 |title=Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization |journal=[[Science (journal)|Science]]|volume=31 |issue=5834 |pages=86–94 |doi=10.1126/science.1139158 |pmid=17615350|first1=N.H.|last2=Srivastava |first2=M.|last3=Hellsten |first3=U. |last4=Dirks |first4=B.|last5=Chapman|first5=J.|last6=Salamov |first6=A.|last7=Terry |first7=A.|last8=Shapiro |first8=H. |last9=Lindquist |first9=E.|last10=Kapitonov|first11=J.|last12=Genikhovich|first12=G. |last13=Grigoriev|first13=I.V. |last14=Lucas|first14=S.M.|last15=Steele |first15=R.E.|last16=Finnerty |first16=J.R.|last17=Technau |first17=U.|last18=Martindale |first18=M.Q.|last19=Rokhsar |first19=D.S. |first10=V. V. |last11=Jurka |bibcode=2007Sci...317...86P |osti=910591 |s2cid=9868191 |display-authors=3 |url=https://digital.library.unt.edu/ark:/67531/metadc884638/ |access-date=21 January 2019 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801213038/https://digital.library.unt.edu/ark:/67531/metadc884638/ |url-status=live}}</ref> Invertebrates are also used by scientists in the field of [[aquatic biomonitoring]] to evaluate the effects of water pollution and [[climate change]].<ref>{{cite journal | last1=Lawrence|first1=J.E. |first2=K.B.|last2=Lunde |first3=R.D. |last3=Mazor |first4=L.A.|last4=Bêche |first5=E.P.|last5=McElravy |first6=V.H.|last6=Resh | title = Long-Term Macroinvertebrate Responses to Climate Change: Implications for Biological Assessment in Mediterranean-Climate Streams | journal = Journal of the North American Benthological Society | volume =29 | pages =1424–40 | year = 2010| doi= 10.1899/09-178.1 | issue =4 |s2cid=84679634 }}</ref> | Two of the most commonly studied model organisms nowadays are invertebrates: the fruit fly ''[[Drosophila melanogaster]]'' and the nematode ''[[Caenorhabditis elegans]]''. They have long been the most intensively studied [[model organism]]s, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their [[genome]]s, but many [[gene]]s, [[intron]]s, and [[genetic linkage|linkages]] have been lost. Analysis of the [[starlet sea anemone]] genome has emphasised the importance of sponges, placozoans, and [[choanoflagellate]]s, also being sequenced, in explaining the arrival of 1,500 ancestral genes unique to animals.<ref>{{cite journal |author=N.H. Putnam |date=July 2007 |title=Sea anemone genome reveals ancestral eumetazoan gene repertoire and genomic organization |journal=[[Science (journal)|Science]]|volume=31 |issue=5834 |pages=86–94 |doi=10.1126/science.1139158 |pmid=17615350|first1=N.H.|last2=Srivastava |first2=M.|last3=Hellsten |first3=U. |last4=Dirks |first4=B.|last5=Chapman|first5=J.|last6=Salamov |first6=A.|last7=Terry |first7=A.|last8=Shapiro |first8=H. |last9=Lindquist |first9=E.|last10=Kapitonov|first11=J.|last12=Genikhovich|first12=G. |last13=Grigoriev|first13=I.V. |last14=Lucas|first14=S.M.|last15=Steele |first15=R.E.|last16=Finnerty |first16=J.R.|last17=Technau |first17=U.|last18=Martindale |first18=M.Q.|last19=Rokhsar |first19=D.S. |first10=V. V. |last11=Jurka |bibcode=2007Sci...317...86P |osti=910591 |s2cid=9868191 |display-authors=3 |url=https://digital.library.unt.edu/ark:/67531/metadc884638/ |access-date=21 January 2019 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801213038/https://digital.library.unt.edu/ark:/67531/metadc884638/ |url-status=live}}</ref> Invertebrates are also used by scientists in the field of [[aquatic biomonitoring]] to evaluate the effects of water pollution and [[climate change]].<ref>{{cite journal | last1=Lawrence|first1=J.E. |first2=K.B.|last2=Lunde |first3=R.D. |last3=Mazor |first4=L.A.|last4=Bêche |first5=E.P.|last5=McElravy |first6=V.H.|last6=Resh | title = Long-Term Macroinvertebrate Responses to Climate Change: Implications for Biological Assessment in Mediterranean-Climate Streams | journal = Journal of the North American Benthological Society | volume =29 | pages =1424–40 | year = 2010| doi= 10.1899/09-178.1 | issue =4 |s2cid=84679634 }}</ref> | ||