Editing Onychophora (section)

==Phylogeny==

=== Internal phylogeny ===
[[File:Eoperipatus_totoro.jpg|thumb|''[[Eoperipatus totoro]],'' a basal member of [[Peripatidae]]]]
Living velvet worms are divided into two families: [[Peripatidae]] and [[Peripatopsidae]]. These diverged around 274 million years ago during the [[Late Devonian]] and have since diversified.<ref name=":12">{{Cite journal |last1=Murienne |first1=Jerome |last2=Daniels |first2=Savel R. |last3=Buckley |first3=Thomas R. |last4=Mayer |first4=Georg |last5=Giribet |first5=Gonzalo |date=2014-01-22 |title=A living fossil tale of Pangaean biogeography |journal=Proceedings of the Royal Society B: Biological Sciences |volume=281 |issue=1775 |pages=20132648 |doi=10.1098/rspb.2013.2648 |pmc=3866409 |pmid=24285200}}</ref> Within Peripatidae, the genera [[Eoperipatus]] (found in [[Southeast Asia]]) and [[Mesoperipatus]] (found in [[Gabon]]) were the most basal, while the rest of the group is found in tropical regions of the [[Americas]].<ref name=":12" /> On the other hand, Peritpatopsidae can be divided into two main [[clade]]s. One has members in [[Southern Africa]] and [[Chile]], while the others live in [[Australasia]].<ref name=":02">{{Cite journal |last1=Baker |first1=Caitlin M |last2=Buckman-Young |first2=Rebecca S |last3=Costa |first3=Cristiano S |last4=Giribet |first4=Gonzalo |date=2021-12-01 |title=Phylogenomic Analysis of Velvet Worms (Onychophora) Uncovers an Evolutionary Radiation in the Neotropics |url=https://academic.oup.com/mbe/article/38/12/5391/6357048?login=true |journal=Molecular Biology and Evolution |volume=38 |issue=12 |pages=5391–5404 |doi=10.1093/molbev/msab251 |pmid=34427671 |issn=1537-1719|pmc=8662635 }}</ref><ref name=":12" />

Below is a [[genus]]-level [[cladogram]] of velvet worms. Note that this phylogeny doesn't analyze every species and lumps most peripatids into [[Neopatida]], as many [[Peripatidae]] genera are [[paraphyletic]].<ref name=":05">{{Cite journal |last=Giribet |first=Gonzalo |last2=Buckman-Young |first2=Rebecca S. |last3=Costa |first3=Cristiano Sampaio |last4=Baker |first4=Caitlin M. |last5=Benavides |first5=Ligia R. |last6=Branstetter |first6=Michael G. |last7=Daniels |first7=Savel R. |last8=Pinto-da-Rocha |first8=Ricardo |date=2018 |title=The 'Peripatos' in Eurogondwana? – Lack of evidence that south-east Asian onychophorans walked through Europe |url=https://bioone.org/journals/Invertebrate-Systematics/volume-32/issue-4/IS18007/The-iPeripatos-i-in-Eurogondwana--Lack-of-evidence-that/10.1071/IS18007.full |journal=Invertebrate Systematics |volume=32 |issue=4 |pages=840–863 |doi=10.1071/IS18007}}</ref> As of 2023, there are around 232 total living species, meaning this phylogeny should eventually be updated.<ref name="Oliveira 2023" />{{clade|{{clade
   |label1=[[Peripatopsidae]]
   |1={{clade
      |1={{clade
         |1=''[[Paraperipatus]]''
         |2={{clade
            |1={{clade
               |1={{clade
                  |1=Peripatopsidae sp. MCZ 131371 & MCZ 141416
                  |2={{clade
                     |1=''[[Ooperipatus]]''
                     |2={{clade
                        |1=''[[Phallocephale]]''
                        |2={{clade
                           |1=''[[Aethrikos]]''
                           |2=''[[Euperipatoides]]''
                        }}
                     }}
                  }}
               }}
               |2={{clade
                  |1=''[[Occiperipatoides]]''
                  |2=''[[Kumbadjena]]''
               }}
            }}
            |2={{clade
               |1={{clade
                  |1={{clade
                     |1=''[[Peripatoides]]''
                     |2=''[[Diemenipatus]]''
                  }}
                  |2=''[[Tasmanipatus]]''
               }}
               |2=''[[Ooperipatellus]]''
            }}
         }}
      }}
      |2={{clade
         |1={{clade
            |1=''[[Peripatopsis]]''
            |2=''[[Metaperipatus]]''
         }}
         |2=''[[Opisthopatus]]''
      }}
   }}
   |label2=[[Peripatidae]]
   |2={{clade
      |1=''[[Eoperipatus]]''
      |2={{clade
         |1=''[[Mesoperipatus]]''
         |2='''[[Neopatida]]'''
      }}
   }}
}}|style=font-size:100%;line-height:85%|label1='''Onychophora'''}}

=== External phylogeny ===

In their present forms, the velvet worms are probably very closely related to the arthropods, a very extensive taxon that incorporates, for instance, the crustaceans, insects, and arachnids. They share, among other things, an exoskeleton consisting of α-chitin and non-collagenous proteins; gonads and waste-elimination organs enclosed in true coelom tissue; an open blood system with a tubular heart situated at the rear; an abdominal cavity divided into pericardial and perivisceral cavities; respiration via tracheae; and similar embryonic development. Segmentation, with two body appendages per segment, is also a shared feature.

However, the antennae, mandibles, and oral papillae of velvet worms are probably ''not'' homologous to the corresponding features in arthropods; i.e., they probably developed independently.

Another closely related group are the comparatively obscure water bears ([[Tardigrada]]); however, due to their very small size, water bears have no need for—and hence lack—blood circulation and separate respiratory structures: shared characteristics that support common ancestry of velvet worms and arthropods.

Together, the velvet worms, arthropods, and water bears form a [[monophyletic]] taxon, the [[Panarthropoda]], i.e., the three groups collectively cover all descendants of their last common ancestor. Due to certain similarities of form, the velvet worms were usually grouped with the water bears to form the taxon Protoarthropoda. This designation would imply that both velvet worms and water bears are not yet as highly developed as the arthropods. Modern systematic theories reject such conceptions of "primitive" and "highly developed" organisms and instead consider exclusively the historical relationships among the taxa. These relationships are not as yet fully understood, but it is considered probable that the velvet worms' sister groups form a taxon designated [[Tactopoda]], thus:
{{clade
  |label1=[[Panarthropoda]]
  |1={{clade
    |label1=
    |1='''Velvet worms''' ('''Onychophora''') [[File:Velvet worm.png|80px]]
    |label2=[[Tactopoda]]
    |2={{clade
      |label1=
      |1=Water bears ([[Tardigrada]]) 
[[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|60px]]
      |2=Arthropods ([[Arthropoda]]) [[File:Aptostichus simus Monterey County.jpg|70px]]
    }}
  }}
}}
For a long time, velvet worms were also considered related to the [[annelid]]s. They share, among other things, a worm-like body; a thin and flexible outer skin; a layered musculature; paired waste-elimination organs; as well as a simply constructed brain and simple eyes. Decisive, however, was the existence of segmentation in both groups, with the segments showing only minor specialisation. The parapodia appendages found in annelids therefore correspond to the stump feet of the velvet worms. Within the [[Articulata hypothesis]] developed by [[Georges Cuvier]], the velvet worms therefore formed an evolutionary link between the annelids and the arthropods: worm-like precursors first developed parapodia, which then developed further into stub feet as an intermediate link in the ultimate development of the arthropods' appendages. Due to their structural conservatism, the velvet worms were thus considered "living fossils". This perspective was expressed paradigmatically in the statement by the French zoologist A. Vandel:

:''Onychophorans can be considered highly evolved annelids, adapted to terrestrial life, which announced prophetically the Arthropoda. They are a lateral branch which has endured from ancient times until today, without important modifications.''

Modern taxonomy does not study criteria such as "higher" and "lower" states of development or distinctions between "main" and "side" branches—only family relationships indicated by [[cladistic]] methods are considered relevant. From this point of view, several common characteristics still support the Articulata hypothesis — segmented body; paired appendages on each segment; pairwise arrangement of waste-elimination organs in each segment; and above all, a rope-ladder-like nervous system based on a double nerve strand lying along the belly. An alternative concept, most widely accepted today, is the so-called [[Ecdysozoa]] [[hypothesis]]. This places the annelids and Panarthropoda in two very different groups: the former in the [[Lophotrochozoa]] and the latter in the Ecdysozoa. Mitochondrial gene sequences also provide support for this hypothesis.<ref>{{ cite journal |title= The Complete Mitochondrial Genome of the Onychophoran ''Epiperipatus biolleyi'' Reveals a Unique Transfer RNA Set and Provides Further Support for the Ecdysozoa Hypothesis |author1=Podsiadlowski, L. |author2=Braband. A. |author3=Mayer, G. |journal= Molecular Biology and Evolution |date=January 2008 |volume= 25 |issue= 1 |pages= 42–51 |doi= 10.1093/molbev/msm223 |pmid= 17934206 |doi-access= free }}</ref> Proponents of this hypothesis assume that the aforementioned similarities between annelids and velvet worms either developed convergently or were primitive characteristics passed unchanged from a common ancestor to both the Lophotrochozoa and Ecdysozoa. For example, in the first case, the rope-ladder nervous system would have developed in the two groups independently, while in the second case, it is a very old characteristic, which does not imply a particularly close relationship between the annelids and Panarthropoda. The Ecdysozoa concept divides the taxon into two, the Panarthropoda into which the velvet worms are placed, and the sister group [[Cycloneuralia]], containing the threadworms ([[Nematoda]]), horsehair worms ([[Nematomorpha]]) and three rather obscure groups: the mud dragons ([[Kinorhyncha]]); penis worms ([[Priapulida]]); and brush-heads ([[Loricifera]]).
{{clade
  |label1=[[Protostomia]]
  |1={{clade
    |label1=[[Ecdysozoa]]
    |1={{clade
      |label1=
      |1=[[Panarthropoda]] (arthropods, velvet worms, water bears) [[File:Aptostichus simus Monterey County.jpg|70px]] [[File:Velvet worm.png|80px]] [[File:SEM image of Milnesium tardigradum in active state - journal.pone.0045682.g001-2 (white background).png|60px]]
      |2=[[Cycloneuralia]] (threadworms, horsehair worms, and others) [[File:Adult priapulid 2.jpg|50px]] [[File:CelegansGoldsteinLabUNC 2.jpg|70px]] [[File:Nematomorpha Somiedo (white background).jpg|60px]]
    }}
  |2=[[Lophotrochozoa]]  (annelids, molluscs, and others) [[File:Polychaeta (no) 2.jpg|70px]]  <span style="{{MirrorH}}">[[File:Grapevinesnail 01a.jpg|70px]]</span> [[File:LingulaanatinaAA (cropped).JPG|70px]]
  |3=(other Protostomes)
  }}
}}

Particularly characteristic of the Cycloneuralia is a ring of "circumoral" nerves around the mouth opening, which the proponents of the Ecdysozoa hypothesis also recognise in modified form in the details of the nerve patterns of the Panarthropoda. Both groups also share a common skin-shedding mechanism ([[ecdysis]]) and molecular biological similarities. One problem of the Ecdysozoa hypothesis is the velvet worms' subterminal position of their mouths: Unlike in the Cycloneuralia, the mouth is not at the front end of the body, but lies further back, under the belly. However, investigations into their developmental biology, particularly regarding the development of the head nerves, suggest that this was not always the case, and that the mouth was originally terminal (situated at the tip of the body). This is supported by the [[fossil record]].

The "stem-group arthropod" hypothesis is very widely accepted, but some trees suggest that the onychophorans may occupy a different position; their brain anatomy is more closely related to that of the [[chelicerate]]s than to any other arthropod.<ref>{{cite journal |first1=Nicholas J. |last1=Strausfeld |first2=Camilla Mok |last2=Strausfeld |first3=Rudi |last3=Loesel |first4=David |last4=Rowell |first5=Sally |last5=Stowe |date=August 2006 |title=Arthropod phylogeny: onychophoran brain organization suggests an archaic relationship with a chelicerate stem lineage |journal=Proceedings: Biological Sciences |volume=273 |issue=1596 |pages=1857–1866 |pmid=16822744 |pmc=1634797 |doi=10.1098/rspb.2006.3536}}</ref> The modern velvet worms form a monophyletic group, incorporating all the descendants of their common ancestor. Important common derivative characteristics ([[synapomorphy|synapomorphies]]) include, for example, the mandibles of the second body segment and the oral papillae and associated slime glands of the third; nerve strands extending along the underside with numerous cross-linkages per segment; and the special form of the tracheae. By 2011, some 180&nbsp;modern species, comprising 49&nbsp;[[genus|genera]], had been described;<ref name=Zhang2011>{{ Cite journal |last=Zhang |first=Zhi-Qiang |year=2011 |title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness |journal=Zootaxa |volume=3148 |pages=7–12 |isbn=978-1-86977-849-1 |url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-date=2022-10-10 |url-status=live|doi=10.11646/zootaxa.3148.1.3 }}</ref> the actual number of species is probably about twice this. According to more recent study, 82&nbsp;species of Peripatidae and 115&nbsp;species of Peripatopsidae have been described thus far. However, among the 197&nbsp;species, 20&nbsp;are [[nomen dubium|''nomina dubia'']], due to major taxonomic inconsistencies.<ref name="Oliveira et al.2012">{{cite journal |first1=Ivo de&nbsp;Sena |last1=Oliveira |first2=V. Morley St.&nbsp;J. |last2=Read |first3=Georg |last3=Mayer |year=2012 |title=A world checklist of Onychophora (velvet worms), with notes on nomenclature and status of names |journal=ZooKeys |issue=211 |pages=1–70 |pmid=22930648 |pmc=3426840 |doi=10.3897/zookeys.211.3463 |doi-access=free |bibcode=2012ZooK..211....1O }}</ref> The best-known is the type genus ''[[Peripatus]]'', which was described as early as 1825 and which, in English-speaking countries, stands representative for all velvet worms. All genera are assigned to one of two families, the distribution ranges of which do not overlap but are separated by arid areas or oceans:
* The Peripatopsidae exhibit relatively many characteristics that are perceived as original or "primitive". The number of leg pairs in this family range from 13 (in ''[[Ooperipatellus nanus]]''<ref name=":0"/>) to 29 (in ''[[Paraperipatus papuensis]]''<ref>{{cite journal |last=Monge-Nájera |first=Julián |year=1994 |title=Reproductive trends, habitat type and body characteristcs in velvet worms (Onychophora) |language=en |journal=Revista de Biología Tropical |volume=42 |issue=3 |pages=611–622 |issn=2215-2075 |url=https://revistas.ucr.ac.cr/index.php/rbt/article/view/23266}}</ref><ref>{{cite journal |last=Sedgwick |first=A. |year=1910 |title=Peripatus papuensis |journal=Nature |language=en |volume=83 |issue=2117 |pages=369–370 |doi=10.1038/083369b0 |bibcode=1910Natur..83..369S |s2cid=45711430 |issn=1476-4687 |url=https://www.nature.com/articles/083369b0}}</ref>).<ref name=":1">{{cite book |last1=Giribet |first1=Gonzalo |last2=Edgecombe |first2=Gregory D. |date=2020-03-03 |df=dmy-all |chapter=30.&nbsp;Onychophora |title=The Invertebrate Tree of Life |editor1=Giribet, Gonzalo |editor2=Edgecombe, Gregory D. |publisher=Princeton University Press |isbn=978-0-691-19706-7 |pages= |language=en <!-- chapter doi = 10.1515/9780691197067-032; whole book doi = 10.1515/9780691197067 --> |doi=10.1515/9780691197067-032 |s2cid=240645062 |chapter-url=https://www.degruyter.com/document/doi/10.1515/9780691197067-032/html |url=https://www.degruyter.com/document/doi/10.1515/9780691197067/html |postscript=;}} &thinsp; chapter&nbsp;{{doi|10.1515/9780691197067-032}}&nbsp;; &thinsp; book&nbsp;{{doi|10.1515/9780691197067}}.</ref> Behind or between the last leg pair is the genital opening ([[gonopore]]).<ref name=":1"/><ref name="Mayer 21–37">{{cite journal |last=Mayer |first=Georg |date=2007-04-05 |df=dmy-all |title=Metaperipatus inae sp. nov. (Onychophora: Peripatopsidae) from Chile with a novel ovarian type and dermal insemination |journal=Zootaxa |volume=1440 |issue=1 |pages=21–37 |doi=10.11646/zootaxa.1440.1.2 |issn=1175-5334 |url=https://www.researchgate.net/publication/216267921 |via=ResearchGate}}</ref> Both oviparous and ovoviviparous, as well as genuinely viviparous, species exist, although the peripatopsids essentially lack a placenta. Their distribution is circumaustral, encompassing [[Australasia]], [[South Africa]], and [[Chile]].<ref name="Oliveira et al.2012"/>
* The Peripatidae exhibit a range of derivative features. They are longer, on average, than the Peripatopsidae and also have more legs. The number of leg pairs in this family range from 19 (in ''[[Typhloperipatus williamsoni]]''<ref>{{cite journal |last=Kemp |first=Stanley |year=1914 |title=Onychophora |journal=Records of the Indian Museum |volume=8 |pages=471–492 |doi=10.5962/bhl.part.1194 |s2cid=88237018 |url=https://www.biodiversitylibrary.org/page/11128933#page/637/mode/1up |via=Biodiversity Heritage Library|doi-access=free }}</ref>) to 43 (in ''[[Plicatoperipatus jamaicensis]]''<ref name=":0"/>).<ref name=":1"/><ref name="Mayer 21–37"/> The gonopore is always between the [[wikt:penultimate|penultimate]] leg pair.<ref name=":1"/> None of the peripatid species are oviparous, and the overwhelming majority are viviparous. The females of many viviparous species develop a placenta with which to provide the growing embryo with nutrients. Distribution of the peripatids is restricted to the tropical and subtropical zones; in particular, they inhabit [[Central America]], northern [[South America]], [[Gabon]], [[Northeast India]], and [[Southeast Asia]].<ref name="Oliveira et al.2012"/>

=== Genomics ===
As of February 2025, velvet worms have had only two [[nuclear genome]]s sequenced.<ref>{{Cite web |title=Euperipatoides rowelli genome assembly Erow_1.0 |url=https://www.ncbi.nlm.nih.gov/datasets/genome/GCA_003024985.2/ |access-date=2025-02-23 |website=NCBI |language=en}}</ref><ref>{{Cite web |title=Epiperipatus broadwayi genome assembly ASM2802345v1 |url=https://www.ncbi.nlm.nih.gov/datasets/genome/GCA_028023455.1/ |access-date=2025-02-23 |website=NCBI |language=en}}</ref> These are of ''[[Euperipatoides rowelli]]'' (a [[Peripatopsidae|peripatopsid]]) and ''[[Epiperipatus broadwayi]]'' (a [[Peripatidae|peripatid]]). The first one is highly fragmented, while the second is less so, but still needs improvement.<ref name=":03">{{Cite journal |last1=Sato |first1=Shoyo |last2=Cunha |first2=Tauana J |last3=de Medeiros |first3=Bruno A S |last4=Khost |first4=Danielle E |last5=Sackton |first5=Timothy B |last6=Giribet |first6=Gonzalo |date=2023-03-01 |title=Sizing Up the Onychophoran Genome: Repeats, Introns, and Gene Family Expansion Contribute to Genome Gigantism in Epiperipatus broadwayi |url=https://academic.oup.com/gbe/article/15/3/evad021/7039704?login=true |journal=Genome Biology and Evolution |volume=15 |issue=3 |pages=evad021 |doi=10.1093/gbe/evad021 |pmid=36790097 |issn=1759-6653|pmc=9985152 }}</ref> Velvet worms seem to display [[genome]] gigantism, with the more complete assembly (E. broadwayi) having an size of 5.60 giga-[[base pair]]s.<ref name=":03" /> Around 70.92% of its genome are [[Repeated sequence (DNA)|repeat sequences]], something that contributes to the bulk of its size. While less substantial, it also has very large [[intron]]s, or parts of a gene that do not become [[protein]]s.<ref name=":03" />