Editing Onychophora (section)

==Anatomy and 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 |bibcode=2010JBiog..37..669A |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 [[cellular differentiation|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>

=== Antennae ===
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 |last1=Eriksson |first1=Bo Joakim |last2=Tait |first2=Noel N. |last3=Budd |first3=Graham E. |last4=Janssen |first4=Ralf |last5=Akam |first5=Michael |date=September 2010 |title=Head patterning and Hox gene expression in an onychophoran and its implications for the arthropod head problem |url=http://uu.diva-portal.org/smash/get/diva2:374350/FULLTEXT01 |journal=Development Genes and Evolution |volume=220 |issue=3–4 |pages=117–22 |doi=10.1007/s00427-010-0329-1 |pmid=20567844 |s2cid=6755763}}</ref> but perhaps rather with their "lips" or [[labrum (arthropod mouthpart)|labrum]]. At their base is 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}}

=== Mouth and jaws ===
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 |last1=Eriksson |first1=B. Joakim |last2=Tait |first2=Noel N. |last3=Budd |first3=Graham E. |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 |doi=10.1002/jmor.10034 |pmid=12420318 |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 |last1=Wright |first1=Jonathan C. |last2=Luke |first2=Barbara M. |year=1989 |title=Ultrastructural and histochemical investigations of ''peripatus'' integument |journal=Tissue & Cell |volume=21 |issue=4 |pages=605–25 |doi=10.1016/0040-8166(89)90012-8 |pmid=18620280}}</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" />

===Slime papillae===
[[File:Ooperipatellus viridimaculatus 34495501.jpg|thumb|Head region of ''[[Ooperipatellus viridimaculatus]]'', showing its oral papillae and mouthparts|left|220x220px]]
On the third head segment, to the left and right of the mouth, are two openings called "oral papillae", with each containing a large, heavily branched slime gland.<ref name=":2">{{cite journal |last1=Morera-Brenes |first1=B. |last2=Monge-Najera |first2=J. |date=2010 |title=A new giant species of placented worm and the mechanism by which onychophorans weave their nets (onychophora: Peripatidae) |url=http://www.kerwa.ucr.ac.cr/handle/10669/4483 |url-status=dead |journal=Revista de Biología Tropical |volume=58 |issue=4 |pages=1127–1142 |arxiv=1511.00983 |doi=10.15517/rbt.v58i4.5398 |pmid=21246983 |archive-url=https://web.archive.org/web/20150402132847/http://www.kerwa.ucr.ac.cr/handle/10669/4483 |archive-date=2015-04-02}}</ref> These slime glands lie roughly in the center of a velvet worm's body and secrete a sort of milky-white slime. The slime is used to both ensnare [[prey]] and act as a distraction for defensive purposes.<ref name=":2" /> In certain species, an organ connected to the slime gland known as the "slime conductor" is broadened into a reservoir, allowing it to hold pre-produced slime.<ref name=":3">{{cite journal |last1=Concha |first1=Andrés |last2=Mellado |first2=Paula |last3=Morera-Brenes |first3=B. |last4=Sampaio Costa |first4=Cristiano |last5=Mahadevan |first5=L. |last6=Monge-Nájera |first6=Julián |date=17 March 2015 |title=Oscillation of the velvet worm slime jet by passive hydrodynamic instability |journal=Nature Communications |volume=6 |page=6292 |bibcode=2015NatCo...6.6292C |doi=10.1038/ncomms7292 |pmc=4382676 |pmid=25780995}}</ref>

Velvet worm slime glands and oral papilla are likely modified and repurposed limbs. The glands themselves are probably modified crural glands.<ref name=":3" /> All three structures correspond to an [[evolution]]ary origin in the leg pairs of the other segments.{{citation needed|date=February 2016}}

==== 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 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=none}},<ref name=:3/> and others.<ref>{{cite journal |last1 = Baer |first1 = A. |last2 = de&nbsp;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>
}} from their oral papillae; they do so either in defense against predators or to capture prey.<ref name="Baer2012">{{cite journal |last1=Baer |first1=Alexander |last2=Mayer |first2=Georg |date=October 2012 |title=Comparative anatomy of slime glands in onychophora (velvet worms) |journal=Journal of Morphology |volume=273 |issue=10 |pages=1079–1088 |doi=10.1002/jmor.20044 |pmid=22707384 |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=":3"/> High speed films show the animal expelling two streams of adhesive liquid through a small opening (50–200 [[micron]]s) at a speed of {{convert|3|to|5|m/s|ft/s|abbr=on|sigfig=1}}.<ref name=":3" /> The interplay between the elasticity of oral papillae and the fast unsteady flow produces a passive oscillatory motion (30–60&nbsp;Hz) of the oral papillae.<ref name=":3" /> 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 |last1=Morera-Brenes |first1=Bernal |last2=Monge-Nájera |first2=Julián |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 |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">{{cite journal |last1=Benkendorff |first1=Kirsten |last2=Beardmore |first2=Kate |last3=Gooley |first3=Andrew A. |last4=Packer |first4=Nicolle H. |last5=Tait |first5=Noel N. |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> but the maximal range has variously been reported to be ten centimetres,<ref name="ISBN 978-0-7993-4689-3">{{cite book |last1=Holm |first1=Erik |url=http://www.LAPA.co.za |title=Goggo Guide |last2=Dippenaar-Schoeman |publisher=LAPA publishers |year=2010 |isbn=978-0-7993-4689-3 |first12=Ansie}}{{page needed|date=November 2014}}</ref> or even nearly a foot,<ref name="CNH4">{{cite book |last1=Harmer |first1=Sidney Frederic |title=Peripatus, Myriapods, Insects |last2=Shipley |first2=Arthur Everett |publisher=Macmillan Company |year=1922 |series=The Cambridge Natural History |volume=5 |display-authors=etal}}{{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 seems provident, since an onychophoran requires about 24&nbsp;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" /> It tastes "slightly bitter and at the same time somewhat astringent".<ref>{{cite journal |last=Moseley |first=H.N. |year=1874 |title=On the structure and development of ''Peripatus capensis'' |url=https://zenodo.org/record/1432452 |journal=Philosophical Transactions of the Royal Society of London |volume=164 |pages=757–82 |bibcode=1874RSPT..164..757M |doi=10.1098/rstl.1874.0022 |jstor=109116 |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" />

===Lobopods===
[[File:Euperipatoides claw pair.tif|thumb|A pair of claws from ''[[Euperipatoides kanangrensis]]''|260x260px]]
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. [[Hydrostatic skeleton|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 wart-like bumps on the belly side of the 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 |access-date=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 |access-date=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 {{langx|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, 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" /> 

=== Nervous system ===
The entire body, including the stub feet, is littered with numerous [[Dermal papillae (Onychophora)|papillae]]: warty protrusions responsive to touch that carry a [[mechanoreceptor|mechanoreceptive]] bristle at the tip, each of which is also connected to further [[sensory nerve cell]]s lying beneath. The mouth papillae, the exits of the slime glands, probably also have some function in [[sensory perception]]. Sensory cells known as "sensills" on the "lips" or labrum respond to chemical stimuli and are known as [[chemoreceptor]]s. These are also found on the two antennae, which seem to be the velvet worm's most important sensory organs.{{Citation needed|date=January 2021}}

Except in a few (typically [[subterranea (geography)|subterranea]]n) species, one [[simple eye in invertebrates|simply constructed eye]] (ocellus) lies behind each antenna, laterally, just underneath the head.<ref name="Mayer2006">{{cite journal |last=Mayer |first=Georg |date=December 2006 |title=Structure and development of Onychophoran eyes: What is the ancestral visual organ in arthropods? |journal=Arthropod Structure & Development |volume=35 |issue=4 |pages=231–45 |bibcode=2006ArtSD..35..231M |doi=10.1016/j.asd.2006.06.003 |pmid=18089073}}</ref> This consists of a chitinous ball [[lens (anatomy)|lens]], a [[cornea]] and a [[retina]] and is connected to the centre of the [[brain]] via an [[optic nerve]].<ref name="Mayer2006" /> The retina comprises numerous pigment cells and photoreceptors; the latter are easily modified flagellated cells, whose [[flagellum]] membranes carry a photosensitive pigment on their surface.

The [[rhabdom]]eric eyes of the Onychophora are thought to be homologous with the median ocelli of arthropods; this would suggest that the last common ancestor of arthropods may have only had median ocelli.<ref name="Mayer2006" />
However, the innervation shows that the homology is limited: The eyes of Onychophora form behind the antenna, whereas the opposite is true in arthropods.<ref>{{cite book |author=Eriksson, J. |title=Evolution and Development of the Onychophoran Head and Nervous System |publisher=Acta Universitatis Upsaliensis |year=2003 |isbn=978-91-554-5613-9}}{{page needed|date=November 2014}}</ref>

===Skin and muscle===
{{main|Dermal papillae (Onychophora)}}
Unlike the arthropods, velvet worms do not possess a rigid [[exoskeleton]]. Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many distantly related soft-bodied animals that are cylindrically shaped, for example [[sea anemones]] and various [[worm]]s. Pressure of their (near-)[[Compressibility|incompressible]] internal bodily fluid on the body wall provides rigidity, and muscles are able to act against it. The body wall consists of a non-cellular outer skin, the [[cuticula]]; a single layer of [[epidermis (skin)|epidermis]] cells forming an internal skin; and beneath this, usually three layers of muscle, which are embedded in connective tissues. The cuticula is about a [[micrometre|micrometer]] thick and covered with fine [[wiktionary:villi|villi]]. In composition and structure, it resembles the cuticula of the arthropods, consisting of α<!--not 'a'-->-chitin and various [[protein]]s,<ref name=Mayer2015/> although not containing [[collagen]]. It can be divided into an external epicuticula and an internal procuticula, which themselves consist of exo- and endo-cuticula. This multi-level structure is responsible for the high flexibility of the outer skin, which enables the velvet worm to squeeze itself into the narrowest crevices. Although outwardly [[water-repellent]], the cuticula is not able to prevent water loss by [[respiration (physiology)|respiration]], and, as a result, velvet worms can live only in [[microclimate]]s with high [[humidity]] to avoid [[desiccation]]. The surface of the cuticula is scattered with numerous fine [[Papilla (worms)|papilla]], the larger of which carry visible villi-like sensitive bristles. The papillae themselves are covered with tiny [[scale (zoology)|scale]]s, lending the skin a [[velvet]]y appearance (from which the common name is likely derived). It also feels like dry velvet to the touch, for which its water-repellent nature is responsible. Moulting of the skin ([[ecdysis]]) takes place regularly, around every 14 days,<ref>{{cite book |first1=Sylvia |last1=Campiglia |first2=Roger |last2=Lavallard |year=1990 |chapter=On the ecdysis at birth and intermoult period of gravid and young ''Peripatus acacioi'' (Onycophora, Peripatidae) |chapter-url=https://books.google.com/books?id=BJzFnCunVxsC&pg=PA461 |pages=461–4 |editor1-first=Alessandro |editor1-last=Minelli |title=Proceedings of the 7th International Congress of Myriapodology |publisher=BRILL |isbn=978-90-04-08972-3}}</ref> induced by the [[hormone]] [[ecdysone]]. The inner surface of the skin bears a hexagonal pattern.<ref name="Maas2007">{{cite journal |first1=Andreas |last1=Maas |first2=Georg |last2=Mayer |first3=Reinhardt M. |last3=Kristensen |first4=Dieter |last4=Waloszek |date=December 2007 |title=A Cambrian micro-lobopodian and the evolution of arthropod locomotion and reproduction |journal=Chinese Science Bulletin |volume=52 |issue=24 |pages=3385–92 |doi=10.1007/s11434-007-0515-3|bibcode=2007ChSBu..52.3385M |s2cid=83993887 }}</ref> At each moult, the shed skin is replaced by the epidermis, which lies immediately beneath it; unlike the cuticula, this consists of living cells. Beneath this lies a thick layer of connective tissue, which is composed primarily of collagen fibres aligned either [[parallel (geometry)|parallel]] or [[perpendicular]] to the body's longitudinal axis. The colouration of Onychophora is generated by a range of pigments.{{Clarify|date=May 2009}} The solubility of these pigments is a useful diagnostic character: in all arthropods and tardigrades, the body pigment is soluble in ethanol. This is also true for the Peripatidae, but in the case of the Peripatopsidae, the body pigment is insoluble in ethanol.<ref name=Monge1995>{{cite journal |first1=Julian |last1=Monge-Najera |date=May 1995 |title=Phylogeny, biogeography and reproductive trends in the Onychophora |journal=Zoological Journal of the Linnean Society |volume=114 |issue=1 |pages=21–60 |doi=10.1111/j.1096-3642.1995.tb00111.x}}</ref>

Within the connective tissue lie three continuous layers of unspecialised [[smooth muscle|smooth muscular]] tissue. The relatively thick outer layer is composed of annular muscles, and the similarly voluminous inner layer of longitudinal muscles. Between them lie thin diagonal muscles that wind backward and forward along the body axis in a [[spiral]]. Between the annular and diagonal muscles exist fine [[blood vessel]]s, which lie below the superficially recognisable transverse rings of the skin and are responsible for the pseudo-segmented markings.<ref name="Boudreaux" /> Beneath the internal muscle layer lies the body cavity. In cross-section, this is divided into three regions by so-called dorso-ventral muscles, which run from the middle of the underbelly through to the edges of the upper side: a central midsection and on the left and right, two side regions that also include the legs.{{citation needed|date=November 2014}}

===Circulation===
The body cavity is known as a "pseudocoel", or [[haemocoel]]. Unlike a true [[coelom]], a pseudocoel is not fully enclosed by a cell layer derived from the embryonic [[mesoderm]]. A coelom is, however, formed around the [[gonad]]s and the waste-eliminating [[nephridia]].<ref name=Boudreaux/> As the name ''haemocoel'' suggests, the body cavity is filled with a [[blood]]-like liquid in which all the organs are embedded; in this way, they can be easily supplied with [[nutrient]]s circulating in the blood. This liquid is colourless as it does not contain [[pigment]]s; for this reason, it serves only a limited role in [[oxygen]] transport.
Two different types of blood cells (or haemocytes) circulate in the fluid: [[Amoebocyte]]s and nephrocytes. The amoebocytes probably function in protection from [[bacteria]] and other foreign bodies; in some species, they also play a role in [[reproduction]]. Nephrocytes absorb [[toxin]]s or convert them into a form suitable for [[clearance (medicine)|elimination]] by the nephridia.{{citation needed|date=November 2014}}

The haemocoel is divided by a horizontal partition, the diaphragm,<!-- Do '''''not''''' link this to any article referring to any kind of Chordate diaphragm, in particular not the thoracic diaphragm. They have nothing to do with each other. --> into two parts: The [[pericardial sinus]] along the back and the perivisceral sinus along the belly. The former encloses the tube-like heart, and the latter, the other organs. The diaphragm is perforated in many places, enabling the exchange of fluids between the two cavities.{{citation needed|date=November 2014}} The heart itself is a tube of annular muscles consisting of [[epithelium|epithelial]] tissues, with two lateral openings ([[wikt:ostium|ostia]]) per segment. While it is not known whether the rear end is open or closed, from the front, it opens directly into the body cavity.

Since there are no blood vessels, apart from the fine vessels running between the muscle layers of the body wall and a pair of arteries that supply the antennae, this is referred to as an [[open circulation]].{{citation needed|date=November 2014}} The timing of the pumping procedure can be divided into two parts: [[Diastole]] and [[systole (medicine)|systole]]. During diastole, blood flows through the ostia from the pericardial sinus (the cavity containing the heart) into the heart. When the systole begins, the ostia close and the heart muscles contract inwards, reducing the volume of the heart. This pumps the blood from the front end of the heart into the perivisceral sinus containing the organs. In this way, the various organs are supplied with nutrients before the blood finally returns to the pericardial sinus via the perforations in the diaphragm. In addition to the pumping action of the heart, body movements also influence circulation.{{citation needed|date=November 2014}}

===Respiration===
Oxygen uptake occurs to an extent via simple [[diffusion]] through the entire body surface, with the coxal vesicles on the legs possibly being involved in some species. However, of most importance is gas exchange via fine unbranched tubes, the [[invertebrate trachea|trachea]]e, which draw oxygen from the surface deep into the various organs, particularly the heart.

The walls of these structures, which are less than three micrometers thick in their entirety, consist only of an extremely thin [[biological membrane|membrane]] through which oxygen can easily diffuse. The tracheae originate at tiny openings, the [[Spiracle (arthropods)|spiracle]]s, which themselves are clustered together in dent-like recesses of the outer skin, the [[wikt:atrium|atria]]. The number of "tracheae bundles" thus formed is on average around 75&nbsp;bundles per body segment; they accumulate most densely on the back of the organism.{{citation needed|date=November 2014}}

Unlike the arthropods, the velvet worms are unable to control the openings of their tracheae; the tracheae are always open, entailing considerable water loss in [[arid]] conditions. Water is lost twice as fast as in earthworms and forty times faster than in caterpillars.<ref>{{cite journal |first=S.M. |last=Manton |date=June 20, 1949 |title=Studies on the Onychophora. VII. The early embryonic stages of ''Peripatopsis'', and some general considerations concerning the morphology and phylogeny of the Arthropoda |journal=Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences |volume=233 |issue=606 |pages=483–580 |jstor=92441 |bibcode=1949RSPTB.233..483M |doi=10.1098/rstb.1949.0003 |doi-access=free}}</ref> For this reason, velvet worms are dependent upon [[habitat]]s with high air humidity.<ref name="The Australian Museum 2024">{{cite web |title=Velvet worm |url=https://australian.museum/learn/animals/worms/velvet-worm// |access-date=2024-02-21 |website=The Australian Museum}}</ref><ref name="BBC Science Focus Magazine 2022 e781">{{cite web | title=What is a velvet worm? | website=BBC Science Focus Magazine | date=July 5, 2022 | url=https://www.sciencefocus.com/nature/what-is-a-velvet-worm | access-date=February 17, 2024}}</ref>

Oxygen transport is helped by the oxygen carrier [[hemocyanin]].<ref>[https://academic.oup.com/gbe/article/15/3/evad021/7039704?login=false Sizing Up the Onychophoran Genome: Repeats, Introns, and Gene Family Expansion Contribute to Genome Gigantism in Epiperipatus broadwayi]</ref>

===Digestion and excretion===
[[File:Peripatopsis capensis internal anatomy IMG 0781a.JPG|thumb|287x287px|Digestive system of ''[[Peripatopsis capensis]]'']]
The digestive tract begins slightly behind the head, the mouth lying on the underside a little way from the frontmost point of the body. Here, prey can be mechanically dismembered by the mandibles with their covering of fine toothlets. Two [[saliva]]ry glands discharge via a common conductor into the subsequent "throat", which makes up the first part of the front [[intestine]]. The saliva that they produce contains mucus and [[hydrolysis|hydrolytic]] [[enzyme]]s, which initiate [[digestion]] in and outside the mouth.

The throat itself is very muscular, serving to absorb the partially liquified food and to pump it, via the [[oesophagus]], which forms the rear part of the front intestine, into the central intestine. Unlike the front intestine, this is not lined with a cuticula but instead consists only of a single layer of epithelial tissue, which does not exhibit conspicuous indentation as is found in other animals.

On entering the central intestine, food particles are coated with a mucus-based [[peritrophic membrane]], which serves to protect the lining of the intestine from damage by sharp-edged particles. The intestinal epithelium secretes further digestive enzymes and absorbs the released nutrients, although the majority of digestion has already taken place externally or in the mouth. Indigestible remnants arrive in the rear intestine, or [[rectum]], which is once again lined with a cuticula and which opens at the [[anus]], located on the underside near to the rear end.{{citation needed|date=November 2014}}

In almost every segment is a pair of excretory organs called nephridia, which are derived from coelom tissue. Each consists of a small pouch that is connected, via a [[flagellum|flagellated]] conductor called a nephridioduct, to an opening at the base of the nearest leg known as a nephridiopore. The pouch is occupied by special cells called [[podocyte]]s, which facilitate [[ultrafiltration]] of the blood through the partition between haemocoelom and nephridium.

The composition of the [[urine|urinary]] solution is modified in the nephridioduct by selective recovery of nutrients and water and by isolation of poison and waste materials, before it is excreted to the outside world via the nephridiopore. The most important nitrogenous excretion product is the water-insoluble [[uric acid]]; this can be excreted in solid state, with very little water. This so-called [[uricotelic]] excretory mode represents an adjustment to life on land and the associated necessity of dealing economically with water.{{citation needed|date=November 2014}}

A pair of former nephridia in the head were converted secondarily into the salivary glands, while another pair in the final segment of male specimens now serve as glands that apparently play a role in reproduction.<ref>{{cite web |last=Holt |first=Jack |date=2010-02-07 |title=Introduction to the Onychophora |url=http://comenius.susqu.edu/biol/202/animals/protostomes/ecdysozoa/onychophora/onychophora.html |url-status=dead |archive-url=https://web.archive.org/web/20221005050117/http://comenius.susqu.edu/biol/202/animals/protostomes/ecdysozoa/onychophora/onychophora.html |archive-date=Oct 5, 2022 |access-date=2022-11-01 |series=Biol. 202: Systematic Biology |publisher=Susquehanna University |place=Selinsgrove, PA |quote=Excretory system: Pared metanephridia opening by their own ducts at each internal "segment". Anterior nephridia modified to salivary glands; posterior nephridia modified to gonopores. |department=Protostomes > Ecdysozoa > Onychophora}}</ref>

===Reproductive organs===
Both sexes possess pairs of [[gonad]]s, opening via a channel called a gonoduct into a common genital opening, the [[gonopore]], which is located on the rear ventral side. Both the gonads and the gonoduct are derived from true coelom tissue.{{Citation needed|date=January 2021}}

[[File:Dissected Euperipatoides.jpg|thumb|A dissected ''[[Euperipatoides kanangrensis]]''. The two ovaries, full of stage&nbsp;II embryos, are floating to the bottom of the image.|300x300px]]
In females, the two [[ovary|ovaries]] are joined in the middle and to the horizontal diaphragm. The gonoduct appears differently depending on whether the species is live-bearing or [[ovipary|egg-laying]]. In live-bearing species, each exit channel divides into a slender oviduct and a roomy "womb", the [[uterus]], in which the embryos develop. The single [[vagina]], to which both uteri are connected, runs outward to the gonopore. In egg-laying species, whose gonoduct is uniformly constructed, the genital opening lies at the tip of a long egg-laying apparatus, the [[ovipositor]]. The females of many species also possess a sperm repository called the ''receptacle seminis'', in which sperm cells from males can be stored temporarily or for longer periods.{{citation needed|date=November 2014}}

Males possess two separate [[testes]], along with the corresponding sperm vesicle (the [[vesicula seminalis]]) and exit channel (the [[vasa efferentia]]). The two vasa efferentia unite to a common sperm duct, the [[vas deferens]], which in turn widens through the ejaculatory channel to open at the gonopore. Directly beside or behind this lie two pairs of special glands, which probably serve some auxiliary reproductive function; the rearmost glands are also known as anal glands.<ref>{{Cite journal|last1=Oliveira|first1=Ivo de Sena|last2=Franke|first2=Franziska Anni|last3=Hering|first3=Lars|last4=Schaffer|first4=Stefan|last5=Rowell|first5=David M.|last6=Weck-Heimann|first6=Andreas|last7=Monge-Nájera|first7=Julián|last8=Morera-Brenes|first8=Bernal|last9=Mayer|first9=Georg|date=2012-12-17|title=Unexplored Character Diversity in Onychophora (Velvet Worms): A Comparative Study of Three Peripatid Species|journal=PLOS ONE|language=en|volume=7|issue=12|pages=e51220|doi=10.1371/journal.pone.0051220|issn=1932-6203|pmc=3524137|pmid=23284667|bibcode=2012PLoSO...751220O|doi-access=free}}</ref> A [[penis]]-like structure has so far been found only in males of the genus ''Paraperipatus'' but has not yet been observed in action.{{citation needed|date=November 2014}}

There are different mating procedures: in some species males deposit their [[spermatophore]] directly into the female's genital opening, while others deposit it on the female's body, where the cuticle will collapse, allowing the sperm cells to migrate into the female. There are also Australian species where the male place their spermatophore on top of their head, which is then pressed against the female's genitals. In these species the head have elaborate structures like spikes, spines, hollow stylets, pits, and depressions, whose purpose is to either hold the sperm and / or assist in the sperm transfer to the female. The males of most species also secrete a pheromone from glands on the underside of the legs to attract females.<ref name="The Australian Museum 2024"/>