Editing Onychophora (section)

===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>