Editing Onychophora (section)

=== 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>