Onychophora /ɒnɪˈkɒfərə/ (from Ancient Greek: ονυχής, onyches, "claws"; and φέρειν, pherein, "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus /pəˈrɪpətəs/ (after the first described genus, Peripatus), is a phylum of elongate, soft-bodied, many-legged animals. In appearance they have variously been compared to worms with legs, caterpillars, and slugs. 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 is likely to be much 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. Velvet worms are generally considered close relatives of the Arthropoda and Tardigrada, with which they form the proposed taxon Panarthropoda. This makes them of palaeontological interest, as they can help reconstruct the ancestral arthropod. Only two fossil species are confidently assigned as onychophorans: Antennipatus from the Late Carboniferous, and Cretoperipatus from the Late Cretaceous, the latter belonging to Peripatidae. In modern zoology, they are known for their mating behaviours and for some species bearing live young.

Velvet worms are segmented animals with a flattened cylindrical body cross-section and rows of unstructured body appendages known as oncopods or lobopods ("stub feet"). They reach lengths between 0.1 and 22 cm (0.04–8.66 in) depending on species, with the smallest known being Ooperipatellus nanus and the largest known Mongeperipatus solorzanoi. The number of leg pairs ranges from as few as 13 (in Ooperipatellus nanus) to as many as 43 (in Plicatoperipatus jamaicensis). 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 cells. The individual body sections are largely unspecialised; even the head develops only a little differently from the abdominal segments. Segmentation is apparently specified by the same gene as in other groups of animals, and is activated in each case, during embryonic 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.

On the first head segment is a pair of slender antennae, which serve in sensory perception. They probably do not correspond directly to the antennae of the Arthropoda, but perhaps rather with their "lips" or labrum. At their base is a pair of simple eyes, except in a few blind species. In front of these, in many Australian species, are various dimples, whose function is not yet clear. It appears that in at least some species, these serve in the transfer of sperm-cell packages (spermatophores).^{[citation needed]}

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 homologous to the labrum of the Arthropoda and is used for feeding. Deep within the oral cavity lie the sharp, crescent-shaped "jaws", or mandibles, which are strongly hardened and resemble the claws of the feet, with which they are serially homologous; early in development, the jaw appendages have a position and shape similar to the subsequent legs. 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. 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.

The surface of the mandibles is smooth, with no ornamentation. 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.

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. 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. 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.

Velvet worm slime glands and oral papilla are likely modified and repurposed limbs. The glands themselves are probably modified crural glands. All three structures correspond to an evolutionary origin in the leg pairs of the other segments.^{[citation needed]}