Editing Onychophora (section)

===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}}