Onychophora: Difference between revisions - Wikipedia

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'''Onychophora''' {{IPAc-en|Q|n|ᵻ|'|k|Q|f|@|r|@}} (from {{lang-grc|ονυχής}}, {{transliteration|grc|onyches}}, "claws"; and {{lang|grc|φέρειν}}, {{transliteration|grc|pherein}}, "to carry"), commonly known as '''velvet worms''' (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as '''peripatus''' {{IPAc-en|p|ə|ˈ|r|ɪ|p|ə|t|ə|s}} (after the first described genus, ''[[Peripatus]]''), is a [[phylum]] of elongate, soft-bodied, many-legged [[animal]]s.<ref>{{ cite book |last1=Holm |first1=E. |last2=Dippenaar-Schoeman |first2=A. |title=The Arthropods of Southern Africa |year=2010 |isbn=978-0-7993-4689-3}}{{page needed|date=November 2014}}</ref><ref name="Prothero">{{cite book |last1=Prothero |first1=D. R. |first2=C. D. |last2=Buell |title=Evolution: What the Fossils Say and Why It Matters |publisher=Columbia University Press |location=New York |year=2007 |page=[https://archive.org/details/evolutionwhatfos00prot_0/page/193 193] |isbn=978-0-231-13962-5 |url=https://archive.org/details/evolutionwhatfos00prot_0/page/193 }}</ref> In appearance they have variously been compared to worms with legs, caterpillars, and slugs.<ref>{{cite book |last1=Ruppert |first1=E. E. |last2=Fox |first2=R. S. |last3=Barnes |first3=R. D. |year=2004 |title=Invertebrate Zoology: A Functional Evolutionary Approach |edition=7th |location=Belmont |publisher=Thomson-Brooks / Cole |isbn=978-0-03-025982-1 |page=[https://archive.org/details/isbn_9780030259821/page/505 505] |quote=Because they resemble worms with legs ... Superficially they resemble caterpillars, but have also been compared with slugs |url=https://archive.org/details/isbn_9780030259821/page/505 }}</ref> They prey upon other invertebrates, which they catch by ejecting an adhesive slime. Approximately 200 species of velvet worms have been described, although the true number of species is likely greater. The two extant families of velvet worms are [[Peripatidae]] and [[Peripatopsidae]]. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are all found south of the equator. It is the only phylum within [[Animalia]] that is wholly endemic to terrestrial environments, at least among extant members.<ref>{{cite book |author-link=Ross Piper |last=Piper |first=Ross |year=2007 |chapter=Velvet Worms |chapter-url=https://books.google.com/books?id=eqegRf2UstIC&pg=PA109 |pages=[https://archive.org/details/extraordinaryani0000pipe/page/109 109–11] |title=Extraordinary Animals: An Encyclopedia of Curious and Unusual Animals |publisher=Greenwood Press |isbn=978-0-313-33922-6 |url=https://archive.org/details/extraordinaryani0000pipe/page/109 }}</ref><ref name="GarwoodEdgecombe2016" /> Velvet worms are generally considered close relatives of the [[Arthropoda]] and [[Tardigrada]], with which they form the proposed [[taxon]] [[Panarthropoda]].<ref>{{ cite book |last=Fishelson |first=L. |title=Zoology |edition=3rd |year=1978 |publisher=Hakibutz Hameuchad Publishing |location=Israel |volume=1 |page=430 }}{{verify source|date=November 2014}}</ref> This makes them of palaeontological interest, as they can help reconstruct the ancestral arthropod. Only two fossil species are confidently assigned to as onychophorans: ''[[Antennipatus]]'' from the [[Late Carboniferous]], and ''[[Cretoperipatus]]'' from the [[Late Cretaceous]], the latter belonging to Peripatidae.<ref name="GarwoodEdgecombe2016" /><ref name=G18/> In modern [[zoology]], they are particularly renowned for their curious [[mating]] behaviours and the [[viviparity|bearing of live young]] in some species.

==Anatomy and ~~Physiology~~physiology==

Velvet worms are [[segmentation (biology)|segmented]] animals with a flattened [[cylinder (geometry)|cylindrical]] body cross-section and rows of unstructured body [[appendage]]s known as oncopods or lobopods (informally: stub feet). They reach lengths between {{cvt|0.1|and(-)|22|cm|2}} depending on species, with the smallest known being ''[[Ooperipatellus nanus]]'' and the largest known is ''[[Mongeperipatus solorzanoi]]''.<ref name=":4">{{cite journal |last1=Allwood |first1=Julia |last2=Gleeson |first2=Dianne |last3=Mayer |first3=Georg |last4=Daniels |first4=Savel |last5=Beggs |first5=Jacqueline R. |last6=Buckley |first6=Thomas R. |title=Support for vicariant origins of the New Zealand Onychophora |journal=Journal of Biogeography |year=2010 |volume=37 |issue=4 |pages=669–681 |doi=10.1111/j.1365-2699.2009.02233.x |s2cid=55395265 }}</ref><ref>{{cite journal|last1=Morera-Brenes|first1=B.|last2=Monge-Nájera|first2=J.|title=A new giant species of placented worm and the mechanism by which onychophorans weave their nets (Onychophora: Peripatidae)|journal=Revista de Biología Tropical|year=2010|volume=58|issue=4|pages=1127–1142|pmid=21246983|doi=10.15517/rbt.v58i4.5398|arxiv=1511.00983}}</ref> The number of leg pairs ranges from as few as 13 (in ''Ooperipatellus nanus'') to as many as 43 (in ''[[Plicatoperipatus jamaicensis]]'').<ref name=":0">{{Cite journal|last1=Yang|first1=Jie|last2=Ortega-Hernández|first2=Javier|last3=Gerber|first3=Sylvain|last4=Butterfield|first4=Nicholas J.|last5=Hou|first5=Jin-bo|last6=Lan|first6=Tian|last7=Zhang|first7=Xi-guang|date=2015-07-14|title=A superarmored lobopodian from the Cambrian of China and early disparity in the evolution of Onychophora|url=https://www.researchgate.net/publication/279385005|journal=Proceedings of the National Academy of Sciences|language=en|volume=112|issue=28|pages=8678–8683|doi=10.1073/pnas.1505596112|issn=0027-8424|pmid=26124122|pmc=4507230 |bibcode=2015PNAS..112.8678Y |via=ResearchGate|doi-access=free }}</ref> Their [[skin]] consists of numerous, fine transverse rings and is often inconspicuously coloured orange, red or brown, but sometimes also bright green, blue, gold or white, and occasionally patterned with other colours. Segmentation is outwardly inconspicuous, and identifiable by the regular spacing of the pairs of legs and in the regular arrangement of skin pores, [[excretion]] organs and concentrations of [[nerve cell]]s. The individual body sections are largely [[specialization (biology)|unspecialised]]; even the head develops only a little differently from the [[abdomen|abdominal]] segments. Segmentation is apparently specified by the same [[gene]] as in other groups of animals, and is activated in each case, during [[embryo]]nic development, at the rear border of each segment and in the growth zone of the stub feet. Although onychophorans fall within the [[protostome]] group, their early development has a [[deuterostome]] trajectory, (with the mouth and anus forming separately); this trajectory is concealed by the rather sophisticated processes which occur in early development.<ref>{{Cite journal | doi = 10.1098/rspb.2014.2628| title = Fate and nature of the onychophoran mouth-anus furrow and its contribution to the blastopore| journal = Proceedings of the Royal Society B: Biological Sciences| volume = 282| issue = 1805| page = 20142628| date = 18 March 2015| last1 = Janssen | first1 = R.| last2 = Jorgensen | first2 = M.| last3 = Lagebro | first3 = L.| last4 = Budd | first4 = G. E. | pmid=25788603 | pmc=4389607}}</ref>

===Appendages===

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The stub feet that characterise the velvet worms are [[cone (geometry)|conical]], baggy appendages of the body, which are internally hollow and have no joints. Although the number of feet can vary considerably between species, their structure is basically very similar. Rigidity is provided by the [[hydrostatic]] pressure of their [[fluid]] contents, and movement is usually obtained passively by stretching and contraction of the animal's entire body. However, each leg can also be shortened and bent by internal [[muscle]]s.<ref name="Boudreaux">{{Cite book | title = Arthropoda phylogeny with special reference to insects | last = Boudreaux | first = H. Bruce | url = https://archive.org/details/arthropodphyloge0000boud | year = 1979| publisher = Wiley | isbn = 9780471042907 }}</ref> Due to the lack of joints, this bending can take place at any point along the sides of the leg. In some species, two different organs are found within the feet:

*Crural glands are situated at the shoulder of the legs, extending into the body cavity. They open outwards at the crural ~~papillae—small~~papillae – small wart-like bumps on the belly side of the ~~leg—and~~leg – and secrete chemical messenger materials called [[pheromone]]s. Their name comes from the [[Latin]] ''cruralis'' meaning "of the legs".<ref>{{cite web |url=https://www.latin-is-simple.com/en/vocabulary/adjective/3204/ |title=cruralis/crurale, cruralis M |work=Latin is Simple |publisher=Peter Waldert |accessdate=2021-02-21 }}</ref>

*Coxal vesicles are pouches located on the belly side of the leg, which can be everted and probably serve in water absorption. They belong to the family Peripatidae and are named from {{lang|la|coxa}}, the Latin word for "hip".<ref>{{cite web |url=https://www.latin-is-simple.com/en/vocabulary/noun/7204/?h=coxa |title=Cox, coxae, [f]|work=Latin is Simple |publisher=Peter Waldert |accessdate=2021-02-21 }}</ref>

On each foot is a pair of retractable, hardened (sclerotised) [[chitin]] claws, which give the taxon its scientific name: Onychophora is derived from the {{lang-grc|ονυχής}}, {{transliteration|grc|onyches}}, "claws"; and {{lang|grc|φέρειν}}, {{transliteration|grc|pherein}}, "to carry". At the base of the claws are three to six spiny "cushions" on which the leg sits in its resting position and on which the animal walks over smooth substrates. The claws are used mainly to gain a firm foothold on uneven terrain. Each claw is composed of three stacked elements, like [[Russian nesting doll]]s. The outermost is shed during ecdysis, which exposes the next element ~~in —~~, which is fully formed, and so does not need time to harden before it is used.<ref name="doi10.1038/nature13576">{{cite journal |first1=Martin R. |last1=Smith |first2=Javier |last2=Ortega-Hernández |date=October 2014 |title=''Hallucigenia''<nowiki/>'s onychophoran-like claws and the case for Tactopoda |journal=Nature |volume=514 |issue=7522 |pages=363–6 |pmid=25132546 |doi=10.1038/nature13576|bibcode=2014Natur.514..363S |s2cid=205239797 |url=http://dro.dur.ac.uk/19108/1/19108.pdf |archive-url=https://ghostarchive.org/archive/20221010/http://dro.dur.ac.uk/19108/1/19108.pdf |archive-date=2022-10-10 |url-status=live }}</ref> This distinctive construction identifies many early Cambrian fossils as early offshoots of the onychophoran lineage.<ref name="doi10.1038/nature13576" /> Apart from the pairs of legs, there are three further body appendages, which are at the head and comprise three segments:

* On the first head segment is a pair of slender [[antenna (biology)|antennae]], which serve in [[sensory perception]]. They probably do not correspond directly to the antennae of the Arthropoda,<ref>{{cite journal |first1=Bo Joakim |last1=Eriksson |first2=Noel N. |last2=Tait |first3=Graham E. |last3=Budd |first4=Ralf |last4=Janssen |first5=Michael |last5=Akam |date=September 2010 |title=Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem |journal=Development Genes and Evolution |volume=220 |issue=3–4 |pages=117–22 |pmid=20567844 |doi=10.1007/s00427-010-0329-1|s2cid=6755763 |url=http://uu.diva-portal.org/smash/get/diva2:374350/FULLTEXT01 }}</ref> but perhaps rather with their "lips" or [[labrum (arthropod mouthpart)|labrum]]. At their base is ~~found~~ a pair of [[simple eye in invertebrates|simple eyes]], except in a few [[blindness|blind]] species. In front of these, in many [[Australia]]n species, are various dimples, the function of which is not yet clear. It appears that in at least some species, these serve in the transfer of sperm-cell packages ([[spermatophore]]s).{{Citation needed|date=January 2021}}

* On the belly side of the second head segment is the labrum, a mouth opening surrounded by sensitive "lips". In the velvet worms, this structure is a muscular outgrowth of the [[throat]], so, despite its name, it is probably not [[Homology (biology)|homologous]] to the labrum of the Arthropoda and is used for feeding. Deep within the oral cavity lie the sharp, crescent-shaped "jaws", or [[mandible (arthropod mouthpart)|mandibles]], which are strongly hardened and resemble the claws of the feet, with which they are serially homologous;<ref name=Mayer2015/> early in development, the jaw appendages have a position and shape similar to the subsequent legs.<ref name="Eriksson2003">{{cite journal |first1=B. Joakim |last1=Eriksson |first2=Noel N. |last2=Tait |first3=Graham E. |last3=Budd |date=January 2003 |title=Head development in the onychophoran ''Euperipatoides kanangrensis'' with particular reference to the central nervous system |journal=Journal of Morphology |volume=255 |issue=1 |pages=1–23 |pmid=12420318 |doi=10.1002/jmor.10034|s2cid=42865895 |doi-access=free }}</ref> The jaws are divided into internal and external mandibles and their concave surface bears fine denticles. They move backward and forward in a longitudinal direction, tearing apart the prey, apparently moved in one direction by musculature and the other by hydrostatic pressure.<ref name=Mayer2015/> The claws are made of sclerotised α-chitin, reinforced with phenols and quinones, and have a uniform composition –, except that there is a higher concentration of calcium towards the tip, presumably affording greater strength.<ref name=Mayer2015/>

The surface of the mandibles is smooth, with no ornamentation.<ref name="Wright1989">{{cite journal |first1=Jonathan C. |last1=Wright |first2=Barbara M. |last2=Luke |title=Ultrastructural and histochemical investigations of ''peripatus'' integument |journal=Tissue & Cell |volume=21 |issue=4 |pages=605–25 |year=1989 |pmid=18620280 |doi=10.1016/0040-8166(89)90012-8}}</ref> The cuticle in the mandibles (and claws) is distinct from the rest of the body. It has an inner and outer component; the outer component has just two layers (whereas body cuticle has four), and these outer layers (in particular the inner epicuticle) are dehydrated and strongly tanned, affording toughness.<ref name="Wright1989" />

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==Distribution and habitat==

===Distribution===

Velvet worms live in all [[tropical]] habitats and in the [[temperate zone]] of the [[Southern Hemisphere]], showing a [[circumtropical]] and circumaustral distribution. Individual species are found in [[Central America|Central]] and [[South America]]; the [[Caribbean]] islands; [[equator]]ial [[West Africa]] and [[Southern Africa]]; northeastern [[India]];<ref>{{cite journal |author=Kemp, S. |title=Preliminary note on a new genus of Onychophora from the N. E. Frontier of India |journal=Records of the Indian Museum |year=1913 |volume=9 |pages=241–242 |url=https://archive.org/stream/recordsofindianm09indi#page/241/mode/1up}}</ref><ref>{{cite journal |author=Kemp, S. |title=Onychophora. Zoological results of the Abor expedition, 1911–1912 |journal=Records of the Indian Museum |year=1914 |volume=8 |pages=471–492 |url=https://archive.org/stream/recordsofindianm08indi#page/n636/mode/1up |doi=10.5962/bhl.part.1194|s2cid=88237018 |doi-access=free }}</ref> [[Thailand]];<ref>{{cite journal |title=Unexplored character diversity in ''Onychophora'' (velvet worms): A comparative study of three peripatid species |first1=Ivo de Sena |last1=Oliveira |first2=Franziska Anni |last2=Franke |first3=Lars |last3=Hering |first4=Stefan |last4=Schaffer |first5=David M. |last5=Rowell |first6=Andreas |last6=Weck-Heimann |first7=Julián |last7=Monge-Nájera |first8=Bernal |last8=Morera-Brenes |first9=Georg |last9=Mayer |date=17 December 2012 |journal=PLOS ONE |volume=7 |issue=12 |pages=e51220 |doi=10.1371/journal.pone.0051220 |pmid=23284667 |pmc=3524137 |bibcode=2012PLoSO...751220O|doi-access=free }}</ref> [[Indonesia]] and parts of [[Malaysia]]; [[New Guinea]]; [[Australia]]; and [[New Zealand]]<ref name=":4" />.{{Citation needed|date=January 2021}}

Fossils have been found in [[Baltic amber]], indicating that they were formerly more widespread in the [[Northern Hemisphere]] when conditions were more suitable.<ref>{{cite journal |last=Poinar |first=George |title=Fossil velvet worms in Baltic and Dominican amber: Onychophoran evolution and biogeography |journal=Science |date=September 1996 |volume=273 |issue=5280 |pages=1370–1 |doi=10.1126/science.273.5280.1370 |bibcode=1996Sci...273.1370P |jstor=2891411|s2cid=85373762 }}</ref>

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==Slime==

[[File:Peripatopsis overbergiensis 45218773.gif|thumb|''[[Peripatopsis overbergiensis]]'' squirting its slime]]

The Onychophora forcefully [[projectile use by living systems|squirt]] glue-like slime{{efn|

The Onychophora forcefully [[projectile use by living systems|squirt]] glue-like slime<ref>sometimes referred to as 'glue', but termed slime in current scientific literature, e.g. {{Cite journal | last1 = Baer | first1 = A. | last2 = Mayer | first2 = G. | doi = 10.1002/jmor.20044 | title = Comparative anatomy of slime glands in onychophora (velvet worms) | journal = Journal of Morphology | volume = 273 | issue = 10 | pages = 1079–1088 | date = October 2012 | pmid = 22707384| s2cid = 701697 }}, {{Cite journal | doi = 10.1111/jzs.12070| title = Slime protein profiling: A non-invasive tool for species identification in Onychophora (velvet worms)| journal = Journal of Zoological Systematics and Evolutionary Research| volume = 52| issue = 4| pages = 265–272| year = 2014| last1 = Baer | first1 = A. | last2 = De Sena Oliveira | first2 = I. | last3 = Steinhagen | first3 = M. | last4 = Beck-Sickinger | first4 = A. G. | last5 = Mayer | first5 = G. | doi-access = free }}</ref> from their oral papillae; they do so either in defense against predators or to capture prey.<ref name="Baer2012">{{cite journal |first1=Alexander |last1=Baer |first2=Georg |last2=Mayer |title=Comparative anatomy of slime glands in onychophora (velvet worms) |journal=Journal of Morphology |volume=273 |issue=10 |pages=1079–88 |date=October 2012 |pmid=22707384 |doi=10.1002/jmor.20044|s2cid=701697 }}</ref> The openings of the glands that produce the slime are in the papillae, a pair of highly modified limbs on the sides of the head below the antennae. Inside, they have a syringe-like system that, by a geometric amplifier, allows for fast squirt using slow muscular contraction.<ref name="Concha2015">{{cite journal |first1=Andrés |last1=Concha |first2=Paula |last2=Mellado |first3=Bernal |last3=Morera-Brenes |first4=Cristiano |last4=Sampaio-Costa |first5=L. |last5=Mahadevan |first6=Julián |last6=Monge-Nájera |title=Oscillation of the velvet worm slime jet by passive hydrodynamic instability |journal=Nature Communications |volume=6 |date=March 2015 |doi=10.1038/ncomms7292 |pmid=25780995 |pmc=4382676 |page=6292|bibcode=2015NatCo...6.6292C }}</ref> High speed films show the animal expelling two streams of adhesive liquid through a small opening (50 to 200 microns) at a speed of {{convert|3|to|5|m/s|ft/s|abbr=on|sigfig=1}}.<ref name="Concha2015"/> The interplay between the elasticity of oral papillae and the fast unsteady flow produces a passive oscillatory motion (30–60 Hz) of the oral papillae.<ref name="Concha2015"/> The oscillation causes the streams to cross in mid air, weaving a disordered net; the velvet worms can control only the general direction where the net is thrown.<ref name="Morera-Brenes2010">{{cite journal |first1=Bernal |last1=Morera-Brenes |first2=Julián |last2=Monge-Nájera |title=A new giant species of placented worm and the mechanism by which onychophorans weave their nets (onychophora: Peripatidae) |journal=Revista Biologìa Tropical |volume=58 |pages=1127–1142 |date=December 2010|arxiv=1511.00983 }}</ref>▼

The secretion ejected by onychophorans is sometimes referred to as ''glue'', but termed ''slime'' in current scientific literature; e.g. {{harvp|Benkendorff|Beardmore|Gooley|Packer|Tait |1999}},<ref name=Benkendorff1999/> {{harvp|Baer|Mayer|2012}},<ref name=Baer2012/> {{harvp|Concha|Mellado|Morera-Brenes|Sampaio-Costa|Mahadevan|Monge-Nájera|2014}},<ref name=Concha2015/> and others.<ref>{{cite journal |last1 = Baer |first1 = A. |last2 = de Sena Oliveira | first2 = I. |last3 = Steinhagen |first3 = M. |last4 = Beck-Sickinger |first4 = A.G. |last5 = Mayer |first5 = G. |year = 2014 |title = Slime protein profiling: A non-invasive tool for species identification in Onychophora (velvet worms) |journal = Journal of Zoological Systematics and Evolutionary Research| volume = 52 |issue = 4 |pages = 265–272 | doi = 10.1111/jzs.12070 |doi-access = free }}</ref>

}}

▲The Onychophora forcefully [[projectile use by living systems|squirt]] glue-like slime<ref>sometimes referred to as 'glue', but termed slime in current scientific literature, e.g. {{Cite journal | last1 = Baer | first1 = A. | last2 = Mayer | first2 = G. | doi = 10.1002/jmor.20044 | title = Comparative anatomy of slime glands in onychophora (velvet worms) | journal = Journal of Morphology | volume = 273 | issue = 10 | pages = 1079–1088 | date = October 2012 | pmid = 22707384| s2cid = 701697 }}, {{Cite journal | doi = 10.1111/jzs.12070| title = Slime protein profiling: A non-invasive tool for species identification in Onychophora (velvet worms)| journal = Journal of Zoological Systematics and Evolutionary Research| volume = 52| issue = 4| pages = 265–272| year = 2014| last1 = Baer | first1 = A. | last2 = De Sena Oliveira | first2 = I. | last3 = Steinhagen | first3 = M. | last4 = Beck-Sickinger | first4 = A. G. | last5 = Mayer | first5 = G. | doi-access = free }}</ref> from their oral papillae; they do so either in defense against predators or to capture prey.<ref name="Baer2012">{{cite journal |first1=Alexander |last1=Baer |first2=Georg |last2=Mayer |date=October 2012 |title=Comparative anatomy of slime glands in onychophora (velvet worms) |journal=Journal of Morphology |volume=273 |issue=10 |pages=~~1079–88 |date=October 2012~~1079–1088 |pmid=22707384 |doi=10.1002/jmor.20044|s2cid=701697 }}</ref> The openings of the glands that produce the slime are in the papillae, a pair of highly modified limbs on the sides of the head below the antennae. Inside, they have a syringe-like system that, by a geometric amplifier, allows for fast squirt using slow muscular contraction.<ref name="Concha2015">{{cite journal |first1=Andrés |last1=Concha |first2=Paula |last2=Mellado |first3=Bernal |last3=Morera-Brenes |first4=Cristiano |last4=Sampaio-Costa |first5=L. |last5=Mahadevan |first6=Julián |last6=Monge-Nájera |title=Oscillation of the velvet worm slime jet by passive hydrodynamic instability |journal=Nature Communications |volume=6 |date=March 2015 |doi=10.1038/ncomms7292 |pmid=25780995 |pmc=4382676 |page=6292|bibcode=2015NatCo...6.6292C }}</ref> High speed films show the animal expelling two streams of adhesive liquid through a small opening (5050–200 ~~to 200 microns~~[[micron]]s) at a speed of {{convert|3|to|5|m/s|ft/s|abbr=on|sigfig=1}}.<ref name="Concha2015"/> The interplay between the elasticity of oral papillae and the fast unsteady flow produces a passive oscillatory motion (30–60 Hz) of the oral papillae.<ref name="Concha2015"/> The oscillation causes the streams to cross in mid air, weaving a disordered net; the velvet worms can control only the general direction where the net is thrown.<ref name="Morera-Brenes2010">{{cite journal |first1=Bernal |last1=Morera-Brenes |first2=Julián |last2=Monge-Nájera |date=December 2010 |title=A new giant species of placented worm and the mechanism by which onychophorans weave their nets (onychophora: Peripatidae) |journal=Revista Biologìa Tropical |volume=58 |pages=1127–1142 ~~|date=December 2010~~|arxiv=1511.00983 }}</ref>

The slime glands themselves are deep inside the body cavity, each at the end of a tube more than half the length of the body. The tube both conducts the fluid and stores it until it is required. The distance that the animal can propel the slime varies; usually it squirts it about a centimetre,<ref name=Benkendorff1999/> but the maximal range has variously been reported to be ten centimetres,<ref name="ISBN 978-0-7993-4689-3">{{cite book |~~author1~~last1=Holm, |first1=Erik |~~author2~~last2=Dippenaar-Schoeman, |first12=Ansie |year=2010 |title=Goggo Guide |publisher=LAPA publishers |isbn=978-0-7993-4689-3 |url=http://www.LAPA.co.za ~~|year=2010~~}}{{page needed|date=November 2014}}</ref> or even nearly a foot,<ref name="CNH4">{{cite book |~~author1~~last1=Harmer, |first1=Sidney Frederic |~~author2~~last2=Shipley, |first2=Arthur Everett |display-authors=etal |year=1922 |title=Peripatus, Myriapods, Insects |series=The Cambridge Natural ~~Nistory~~History |volume=5 ~~|title=Peripatus, Myriapods, Insects~~ |publisher=Macmillan Company ~~|year=1922~~}}{{page needed|date=November 2014}}</ref> although accuracy drops with range.<ref name=Read1987/> It is not clear to what extent the range varies with the species and other factors. One squirt usually suffices to snare a prey item, although larger prey may be further immobilised by smaller squirts targeted at the limbs; additionally, the fangs of spiders are sometimes targeted.<ref name=Read1987/> Upon ejection, it forms a net of threads about twenty microns in diameter, with evenly spaced droplets of viscous adhesive fluid along their length.<ref name=Benkendorff1999/> It subsequently dries, shrinking, losing its stickiness, and becoming brittle.<ref name=Benkendorff1999/> Onychophora eat their dried slime when they can, which isseems ~~appropriate~~provident, ~~because it takes~~since an onychophoran requires about 24 days to replenish an exhausted slime repository.<ref name=Read1987/>

The slime can account for up to 11% of the organism's dry weight<ref name=Read1987/> and is 90% water; its dry residue consists mainly of proteins — primarily a [[collagen]]-type protein.<ref name=Benkendorff1999/> 1.3% of the slime's dry weight consists of sugars, mainly [[galactosamine]].<ref name=Benkendorff1999/> The slime also contains lipids and the [[surfactant]] [[nonylphenol]]. Onychophora are the only organisms known to produce this latter substance.<ref name=Benkendorff1999>{{cite journal |first1=Kirsten |last1=Benkendorff |first2=Kate |last2=Beardmore |first3=Andrew A. |last3=Gooley |first4=Nicolle H. |last4=Packer |first5=Noel N. |last5=Tait |date=December 1999 |title=Characterisation of the slime gland secretion from the peripatus, ''Euperipatoides kanangrensis'' (Onychophora: Peripatopsidae) |journal=Comparative Biochemistry and Physiology B |volume=124 |issue=4 |pages=457–65 |doi=10.1016/S0305-0491(99)00145-5}}</ref> It tastes "slightly bitter and at the same time somewhat astringent".<ref>{{cite journal |~~last1~~last=Moseley |~~first1~~first=H. N. |year=1874 |title=On the ~~Structure~~structure and ~~Development~~development of ''Peripatus capensis'' |journal=Philosophical Transactions of the Royal Society of London |volume=164 |pages=757–82 |jstor=109116 |bibcode=1874RSPT..164..757M |doi=10.1098/rstl.1874.0022 |doi-access=free |url=https://zenodo.org/record/1432452 ~~|doi-access=free~~ }}</ref> The proteinaceous composition accounts for the slime's high [[tensile strength]] and stretchiness.<ref name=Benkendorff1999/> The lipid and nonylphenol constituents may serve one of two purposes.: They may line the ejection channel, stopping the slime from sticking to the organism when it is secreted; or they may slow the drying process long enough for the slime to reach its target.<ref name=Benkendorff1999/>

==Behaviour==

Line 237 ⟶ 240:

==Evolution==

~~The~~Onychophoran paleontology is plagued by the vagaries of the preservation process ~~can~~that ~~make~~makes fossils difficult to interpret. Experiments on the decay and compaction of onychophora demonstrate difficulties in interpreting fossils; certain parts of living onychophora are visible only in certain conditions:▼

* The mouth may or may not be preserved;▼

* The claws may be re-oriented or lost;▼

* The leg width may increase or decrease; and▼

* The mud may be mistaken for organs.<ref name=Monge2002>{{cite journal |first1=Julián |last1=Monge-Nájera |first2=Hou |last2=Xianguang |date=December 2002 |title=Experimental taphonomy of velvet worms (Onychophora) and implications for the Cambrian 'explosion, disparity and decimation' model |journal=Revista de Biología Tropical |volume=50 |issue=3–4 |pages=1133–1138 |pmid=12947596 |url=http://www.scielo.sa.cr/scielo.php?script=sci_arttext&pid=S0034-77442002000300030&lng=en&nrm=iso }}</ref>▼

More significantly, features seen in fossils may be ~~artefacts~~artifacts of the preservation process:. For instance, "shoulder pads" may simply be the second row of legs coaxially compressed onto the body; branching "antennae" may in fact behave been ~~produced~~created ~~through~~during decay.<ref name=Monge2002/>▼

[[File:NMNH-USNM83935 Hallucigeniasp (cropped).jpg|thumb|left|''[[Hallucigenia]]'' fossil|260x260px]]

Certain fossils from the early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as the [[lobopodian]]s, were marine and represent a grade from which arthropods, tardigrades, and Onychophora arose.<ref name=~~"Cite doi|10.1038/nature13576"~~Smith-OrtegaHz-2014>{{~~Cite~~cite journal | ~~doi~~last1 = ~~10.1038/nature13576~~Smith | ~~pmid~~first1 = ~~25132546~~M.R. | ~~journal~~last2 = ~~Nature~~Ortega ~~| volume = 514~~Hernández | ~~issue~~first2 = ~~7522~~J. | ~~pages~~year = ~~363–366 | last1 = Smith | first1 = M. R.~~ 2014 | title = ''Hallucigenia''<nowiki/>'s onychophoran-like claws and the case for Tactopoda | ~~last2~~journal = ~~Ortega~~Nature ~~Hernández~~| volume = 514 | ~~first2~~issue = J.7522 | ~~url~~pages = ~~http://rdcu~~363–366 | doi = 10.be1038/~~bKoD~~nature13576 | ~~year~~pmid = ~~2014~~25132546 | bibcode = 2014Natur.514..363S | s2cid = 205239797 | url = http://rdcu.be/bKoD }}</ref><ref name="Budd2001">{{cite journal |~~last1~~last=Budd |~~first1~~first=Graham E. |date=September–October 2001 |title=Why are arthropods segmented? |journal=Evolution & Development |volume=3 |issue=5 |pages=332–342 |pmid=11710765 |doi=10.1046/j.1525-142X.2001.01041.x |s2cid=37935884 }}</ref>▼

Possible fossils of onychophorans are found in the [[Cambrian]],<ref name=Bergstrom2001>{{cite journal |first1=Jan |last1=Bergström |first2=Xian-Guang |last2=Hou |year=2001 |title=Cambrian Onychophora or Xenusians |journal=Zoologischer Anzeiger |volume=240 |issue=3–4 |pages=237–245 |doi=10.1078/0044-5231-00031~~|year=2001~~ }}</ref> [[Ordovician]] (possibly),<ref name="Van Roy2010">{{cite journal |first1=Peter |last1=~~Van~~ van Roy |first2=Patrick J. |last2=Orr |first3=Joseph P. |last3=Botting |first4=Lucy A. |last4=Muir |first5=Jakob |last5=Vinther |first6=Bertrand |last6=Lefebvre |first7=Khadija |last7=el  Hariri |first8=Derek E.G. |last8=Briggs |date=May 2010 |title=Ordovician faunas of Burgess Shale type |journal=Nature |volume=465 |issue=7295 |pages=215–218 |pmid=20463737 |bibcode=2010Natur.465..215V |doi=10.1038/nature09038 |s2cid=4313285 }}</ref> [[Silurian]]<ref name=vonBitter2007>{{cite journal |first1=Peter H. |last1=von  Bitter |first2=Mark A. |last2=Purnell |first3=Denis K. |last3=Tetreault |first4=Christopher A. |last4=Stott |year=2007 |title=Eramosa Lagerstätte — Exceptionally preserved soft-bodied biotas with shallow-marine shelly and bioturbating organisms (Silurian, Ontario, Canada) |journal=Geology |volume=35 |issue=10 |pages=879–882 |bibcode=2007Geo....35..879V |doi=10.1130/G23894A.1 }}</ref> and [[Pennsylvanian (geology)|Pennsylvanian]]<ref name="GarwoodEdgecombe2016" /><ref>{{cite journal |first1=Ida |last1=Thompson |first2=Douglas S. |last2=Jones |date=May 1980 |title=A ~~Possible~~possible Onychophoran from the ~~Middle~~middle Pennsylvanian Mazon Creek ~~Beds~~beds of ~~Northern~~northern Illinois |journal=Journal of Paleontology |volume=54 |issue=3 |pages=588–596 |jstor=1304204 }}</ref> periods.▼

Historically, all fossil Onychophora and lobopodians were lumped into the taxon [[Xenusia]], further subdivided by some authors to the Paleozoic Udeonychophora and the Mesozoic/Tertiary Ontonychophora; living Onychophora were termed Euonychophora.<ref name=Grimaldi2002/> Importantly, few of the Cambrian fossils bear features that distinctively unite them with the Onychophora; none can be confidently assigned to the onychophoran crown or even stem group.<ref name="Budd2001a">{{cite journal |~~first1~~first=Graham E. |~~last1~~last=Budd |year=2001 |title=Tardigrades as '~~Stem~~stem-~~Group~~group Arthropods': The ~~Evidence~~evidence from the Cambrian ~~Fauna~~fauna |journal=Zoologischer Anzeiger |volume=240 |issue=3–4 |pages=265–279 |doi=10.1078/0044-5231-00034}}</ref> Possible exceptions are ''[[Hallucigenia]]'' and related taxa such as ''[[Collinsium|Collinsium ciliosum]]'', which bear distinctly onychophoran-like claws.<ref name=~~"Cite doi|10.1038/nature13576"~~Smith-OrtegaHz-2014/> It is not clear when the transition to a terrestrial existence was made, but it is considered plausible that it took place between the Ordovician and late ~~Silurian—approximately~~Silurian – approximately {{Ma|Ordovician|Late Silurian|round=-1}}~~—via~~ – via the intertidal zone.<ref name=Monge1995/>~~[[File:Helenodora.jpg|thumb|Reconstruction of the Carboniferous possible onychophoran ''[[Helenodora]]''|350x350px]]~~▼

▲Certain fossils from the early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as the [[lobopodian]]s, were marine and represent a grade from which arthropods, tardigrades, and Onychophora arose.<ref name="Cite doi|10.1038/nature13576">{{Cite journal | doi = 10.1038/nature13576 | pmid = 25132546 | journal = Nature | volume = 514 | issue = 7522 | pages = 363–366 | last1 = Smith | first1 = M. R. | title = ''Hallucigenia'''s onychophoran-like claws and the case for Tactopoda | last2 = Ortega Hernández | first2 = J. | url = http://rdcu.be/bKoD| year = 2014 | bibcode = 2014Natur.514..363S | s2cid = 205239797 }}</ref><ref name="Budd2001">{{cite journal |last1=Budd |first1=Graham E. |date=September–October 2001 |title=Why are arthropods segmented? |journal=Evolution & Development |volume=3 |issue=5 |pages=332–342 |pmid=11710765 |doi=10.1046/j.1525-142X.2001.01041.x|s2cid=37935884 }}</ref>

[[File:Helenodora.jpg|thumb|Reconstruction of the Carboniferous possible onychophoran ''[[Helenodora]]''|350x350px]]

▲Possible fossils of onychophorans are found in the [[Cambrian]],<ref name=Bergstrom2001>{{cite journal |first1=Jan |last1=Bergström |first2=Xian-Guang |last2=Hou |title=Cambrian Onychophora or Xenusians |journal=Zoologischer Anzeiger |volume=240 |issue=3–4 |pages=237–245 |doi=10.1078/0044-5231-00031|year=2001 }}</ref> [[Ordovician]] (possibly),<ref name="Van Roy2010">{{cite journal |first1=Peter |last1=Van Roy |first2=Patrick J. |last2=Orr |first3=Joseph P. |last3=Botting |first4=Lucy A. |last4=Muir |first5=Jakob |last5=Vinther |first6=Bertrand |last6=Lefebvre |first7=Khadija |last7=el Hariri |first8=Derek E.G. |last8=Briggs |date=May 2010 |title=Ordovician faunas of Burgess Shale type |journal=Nature |volume=465 |issue=7295 |pages=215–218 |pmid=20463737 |bibcode=2010Natur.465..215V |doi=10.1038/nature09038|s2cid=4313285 }}</ref> [[Silurian]]<ref name=vonBitter2007>{{cite journal |first1=Peter H. |last1=von Bitter |first2=Mark A. |last2=Purnell |first3=Denis K. |last3=Tetreault |first4=Christopher A. |last4=Stott |year=2007 |title=Eramosa Lagerstätte — Exceptionally preserved soft-bodied biotas with shallow-marine shelly and bioturbating organisms (Silurian, Ontario, Canada) |journal=Geology |volume=35 |issue=10 |pages=879–882 |bibcode=2007Geo....35..879V |doi=10.1130/G23894A.1}}</ref> and [[Pennsylvanian (geology)|Pennsylvanian]]<ref name="GarwoodEdgecombe2016" /><ref>{{cite journal |first1=Ida |last1=Thompson |first2=Douglas S. |last2=Jones |date=May 1980 |title=A Possible Onychophoran from the Middle Pennsylvanian Mazon Creek Beds of Northern Illinois |journal=Journal of Paleontology |volume=54 |issue=3 |pages=588–596 |jstor=1304204}}</ref> periods.

The low [[preservation potential]] of the non-mineralised onychophorans means that they have a sparse fossil record. The lobopodian ''[[Helenodora]]'' from the Carboniferous of North America has been suggested to be a member of Onychophora,<ref name=Jr2000>{{cite journal |first=George |last=Poinar |date=Winter 2000 |title=Fossil onychophorans from Dominican and Baltic amber: ''Tertiapatus dominicanus'' n.g., n.sp. (Tertiapatidae n. fam.) and ''Succinipatopsis balticus'' n.g., n.sp. (Succinipatopsidae n. fam.) with a Proposed Classification of the Subphylum Onychophora |journal=Invertebrate Biology |volume=119 |issue=1 |pages=104–109 |jstor=3227105 |doi=10.1111/j.1744-7410.2000.tb00178.x |doi-access=free }}</ref> but other studies recover it as more closely related to other lobopodians. A [[Late Carboniferous]] fossil from [[Montceau-les-Mines]], France, ''[[Antennipatus]]'' has been suggested to have clear onychophoran affinities, likely the first terrestrial onychophoran, but its poor preservation prohibits differentiating between its placement on the stem or crown of the two extant families, or on the onychophoran stem-group.<ref name=GarwoodEdgecombe2016>{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |last3=Charbonnier |first3=Sylvain |last4=Chabard |first4=Dominique |last5=Sotty |first5=Daniel |last6=Giribet |first6=Gonzalo |year=2016 |title=Carboniferous Onychophora from Montceau-les-Mines, France, and onychophoran terrestrialization |journal=Invertebrate Biology |volume=135 |issue=3 |pages=179–190 |issn=1077-8306 |pmid=27708504 |pmc=5042098 |doi=10.1111/ivb.12130 |doi-access=free }}</ref>

▲Historically, all fossil Onychophora and lobopodians were lumped into the taxon [[Xenusia]], further subdivided by some authors to the Paleozoic Udeonychophora and the Mesozoic/Tertiary Ontonychophora; living Onychophora were termed Euonychophora.<ref name=Grimaldi2002/> Importantly, few of the Cambrian fossils bear features that distinctively unite them with the Onychophora; none can be confidently assigned to the onychophoran crown or even stem group.<ref name="Budd2001a">{{cite journal |first1=Graham E. |last1=Budd |year=2001 |title=Tardigrades as 'Stem-Group Arthropods': The Evidence from the Cambrian Fauna |journal=Zoologischer Anzeiger |volume=240 |issue=3–4 |pages=265–279 |doi=10.1078/0044-5231-00034}}</ref> Possible exceptions are ''[[Hallucigenia]]'' and related taxa such as ''[[Collinsium|Collinsium ciliosum]]'', which bear distinctly onychophoran-like claws.<ref name="Cite doi|10.1038/nature13576"/> It is not clear when the transition to a terrestrial existence was made, but it is considered plausible that it took place between the Ordovician and late Silurian—approximately {{Ma|Ordovician|Late Silurian|round=-1}}—via the intertidal zone.<ref name=Monge1995/>[[File:Helenodora.jpg|thumb|Reconstruction of the Carboniferous possible onychophoran ''[[Helenodora]]''|350x350px]]

The low [[preservation potential]] of the non-mineralised Onychophora means that they have a sparse fossil record. The lobopodian ''[[Helenodora]]'' from the Carboniferous of North America has been suggested to be a member of Onychophora,<ref name="Jr2000">{{cite journal |first1=George |last1=Poinar |date=Winter 2000 |title=Fossil Onychophorans from Dominican and Baltic Amber: ''Tertiapatus dominicanus'' n.g., n.sp. (Tertiapatidae n. fam.) and ''Succinipatopsis balticus'' n.g., n.sp. (Succinipatopsidae n. fam.) with a Proposed Classification of the Subphylum Onychophora |journal=Invertebrate Biology |volume=119 |issue=1 |pages=104–109 |jstor=3227105 |doi=10.1111/j.1744-7410.2000.tb00178.x|doi-access=free }}</ref> but other studies recover it as more closely related to other lobopodians. A [[Late Carboniferous]] fossil from [[Montceau-les-Mines]], France, ''[[Antennipatus]]'' has been suggested to have clear onychophoran affinities, likely the first terrestrial onychophoran, but its preservation prohibits differentiating between its placement on the stem or crown of the two extant families, or on the onychophoran stem-group.<ref name="GarwoodEdgecombe2016">{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |last3=Charbonnier |first3=Sylvain |last4=Chabard |first4=Dominique |last5=Sotty |first5=Daniel |last6=Giribet |first6=Gonzalo |year=2016 |title=Carboniferous Onychophora from Montceau-les-Mines, France, and onychophoran terrestrialization |journal=Invertebrate Biology |volume=135 |issue=3 |pages=179–190 |issn=1077-8306 |pmid=27708504 |pmc=5042098 |doi=10.1111/ivb.12130 |doi-access=free}}</ref> In 2018, the identification of ''Antennipatus'' as the oldest onychophoran has been ~~agreed~~argued by Giribet and colleagues, who suggested that the minimum age of ''Antennipatus'' would be during the [[Gzhelian]] age around {{Ma|298.75}}, and incorporated the taxon conservatively for the phylogenetic analysis of oncyhophorans based on the uncertainty of its placement within the order.<ref name="G18">{{cite journal |~~author1~~last1=Giribet, |first1=Gonzalo |~~author2~~last2=Buckman-Young, |first2=Rebecca S. |~~author3~~last3=Costa, |first3=Cristiano Sampaio |~~author4~~last4=Baker, |first4=Caitlin M. |~~author5~~last5=Benavides, |first5=Ligia R. |~~author6~~last6=Branstetter, |first6=Michael G. |~~author7~~last7=Daniels, |first7=Savel R. |~~author8~~last8=Pinto-da-Rocha, |first8=Ricardo |display-authors=6 |year=2018 |title=The 'Peripatos' in Eurogondwana? – Lack of evidence that south-east Asian onychophorans walked through Europe |journal=Invertebrate Systematics |volume=32 |issue=4 |pages=840–863 |doi=10.1071/IS18007 }}</ref> In 2021, Baker and colleagues conducted divergence analyses using molecular dating and treating ''Antennipatus'' conservatively as a stem-group onychophoran with a minimum age of {{Ma|300}}, resulting in a divergence date of {{Ma|376}} for the crown group onychophorans.<ref>{{~~Cite~~cite journal |last1=Baker |first1=Caitlin M. |last2=Buckman-Young |first2=Rebecca S. |last3=Costa |first3=Cristiano S. |last4=Giribet |first4=Gonzalo |date=9 December 2021 <!-12-09 |editor-last=Xia |editor-first=Xuhua --> |title=Phylogenomic ~~Analysis~~analysis of ~~Velvet~~velvet ~~Worms~~worms (Onychophora) ~~Uncovers~~uncovers an ~~Evolutionary~~evolutionary ~~Radiation~~radiation in the ~~Neotropics |url=https://academic.oup.com/mbe/article/38/12/5391/6357048~~neotropics |journal=Molecular Biology and Evolution ~~|language=en~~ |volume=38 |issue=12 |pages=5391–5404 |doi=10.1093/molbev/msab251 |issn=1537-1719 |pmc=8662635 |pmid=34427671 |url=https://academic.oup.com/mbe/article/38/12/5391/6357048 |lang=en }}</ref> Crown group representatives are known only from amber, the oldest being ''[[Cretoperipatus]]'' from [[Burmese amber]] during the [[Cenomanian]]-[[Turonian]] stages of the [[Late Cretaceous]], around 100-90  million years old, assigned to the family Peripatidae.<ref name="Grimaldi2002">{{cite journal |first1=David A. |last1=Grimaldi |first2=Michael S. |last2=Engel |first3=Paul C. |last3=Nascimbene |date=March 2002 |title=Fossiliferous Cretaceous ~~Amber~~amber from Myanmar (Burma): Its ~~Rediscovery~~rediscovery, ~~Biotic~~biotic ~~Diversity~~diversity, and ~~Paleontological~~paleontological ~~Significance~~significance |journal=American Museum Novitates |issue=3361 |pages=1–71 |doi=10.1206/0003-0082(2002)361<0001:FCAFMB>2.0.CO;2 |hdl=2246/2914 |s2cid=53645124 |url=https://www.biodiversitylibrary.org/bibliography/178519 }}</ref> The affinity of amber records from the Cenozoic, like ''[[Tertiapatus]],'' and ''[[Succinipatopsis]]'', which form the suggested superfamily termed Tertiapatoidea,<ref name="Jr2000" /> has been considered doubtful by other authors.<ref name="GarwoodEdgecombe2016" />

▲The vagaries of the preservation process can make fossils difficult to interpret. Experiments on the decay and compaction of onychophora demonstrate difficulties in interpreting fossils; certain parts of living onychophora are visible only in certain conditions:

▲* The mouth may or may not be preserved;

▲* The claws may be re-oriented or lost;

▲* The leg width may increase or decrease; and

▲* The mud may be mistaken for organs.<ref name=Monge2002>{{cite journal |first1=Julián |last1=Monge-Nájera |first2=Hou |last2=Xianguang |date=December 2002 |title=Experimental taphonomy of velvet worms (Onychophora) and implications for the Cambrian 'explosion, disparity and decimation' model |journal=Revista de Biología Tropical |volume=50 |issue=3–4 |pages=1133–1138 |pmid=12947596 |url=http://www.scielo.sa.cr/scielo.php?script=sci_arttext&pid=S0034-77442002000300030&lng=en&nrm=iso}}</ref>

==Footnotes==

▲More significantly, features seen in fossils may be artefacts of the preservation process: For instance, "shoulder pads" may simply be the second row of legs coaxially compressed onto the body; branching "antennae" may in fact be produced through decay.<ref name=Monge2002/>

==References==

==External links==