Velvet worm

Velvet worms
A species of Peripatoides, a member of the Peripatopsidae family
Scientific classification
Kingdom:	Animalia
Superphylum:	Ecdysozoa
(unranked):	Panarthropoda
Phylum:	Onychophora
Families and genera
Peripatidae Peripatopsidae †Antennacanthopodia †Antennipatus †Helenodora? †Tritonychus? †Tertiapatus? †Succinipatopsis?

The velvet worms (Onychophora — literally "claw bearers") are a minor panarthropod phylum.

There are two families and about 200 species. All living species live on land, in moist or wet tropical areas.

The two living families are the Peripatidae and the Peripatopsidae. They show a peculiar distribution, with the peripatids being mainly equatorial and tropical, while the peripatopsids are all found in what used to be Gondwana.^[1]

The segmented worm-like organisms have tiny eyes, antennae, multiple pairs of legs, slime glands, and bumpy skin that feels like velvet. The group is thought to be related to arthropods and tardigrades. They prey on smaller animals such as insects, which they catch by squirting a sticky slime.

Today, velvet worms are found in the tropics and also the temperate zone of the Southern hemisphere. Members of the family Peripatidae live in tropical regions of South America, Central America, the Caribbean islands, Gabon, Northeast India, and Southeast Asia. Members of the family Peripatopsidae are found in areas of what used to be Gondwana (Chile, Australia, Southern Africa, New Guinea, and New Zealand).^[2]

One exception to this is Peripatopsis capensis from South Africa. Peripatopsis capensis is a Peripatopsid velvet worm that arrived at Santa Cruz Island in the Galápagos Islands by accident. It now lives with native species from the family Peripatidae.^[3] For all the velvet worms currently alive, most species live in either Australia or South America.^[2]

Fossil velvet worms have been discovered in parts of the Northern Hemisphere where they currently do not live.^[4][5][6] Succinipatopsis and Helenodora might not be velvet worms,^[6][7] but Antennipatus (which was found in France) is at least a close relative of the two living families (Peripatidae and Peripatopsidae).^[6][8][9] This shows that velvet worms were far more widespread in the past.^[10]

Compared to other phyla, Onychophora (the formal name for velvet worms) is organized very strangely. This is because it still follows an old way of classifying things from 2000 which no longer works.^[5] To show how weird things are, the two main clades of velvet worm split around 274 million years ago during the Late Devonian (a long time ago), but are grouped as families rather than a class or order.^[11]

As hinted at above, velvet worms are divided into two main families (Peripatidae and Peripatopsidae).^[11][8] The group also contains two extinct genera (Antennipatus and Antennacanthopodia).^[6][12]

Within Peripatidae, the genera Eoperipatus (found in Southeast Asia) and Mesoperipatus (found in Gabon) split from the others first, while the rest of the group is found in tropical places of the Americas.^[11] On the other hand, Peritpatopsidae can be divided into two main clades. One has members in Southern Africa and Chile, while the others live in Australasia.^[13][11]

This is a combined genus-level cladogram of velvet worms and their close relatives.^{[6][8][14][15][16]} Not every genus is included, and most members of Peripatidae are into a clade named Neopatida (since so many in that group are paraphyletic).^[8] As of 2023, there are around 232 total living species of velvet worm, meaning this diagram should eventually be updated.^[17]

Velvet worms' closest living relatives are the arthropods and tardigrades (sometimes called water bears). These three types of animal form a larger clade called Panarthropoda. All three groups evolved from a broad category of legged, worm-like animals called lobopodians. Below is a simplified phylogeny showing Panarthropoda's major groups (both living and dead) and where velvet worms fit within it.^[16]

Certain fossils from the Early Cambrian look very similar to velvet worms. These fossils, known as lobopodians, were an evolutionary grade that gave rise to arthropods, tardigrades, velvet worms, and the extinct radiodonts.^[18]

It is still unsure how all the lobopodians are related. However, other than a single paper,^[19] Antennacanthopodia is the only animal confidently seen as a close relative of velvet worms.^{[18][20][21][7][22][16]} Antennacanthopodia lived during the Cambrian Stage 3 and possessed many onychophoran traits: stubby legs, spiny foot pads, a body with ringed skin, and small eyes behind the main antenna. Still, unlike living velvet worms, Antennacanthopodia had not one, but two pairs of antennae.^[12] These secondary antennae were shorter than the first pair and thought to be homologous with a velvet worm's slime glands^[12] or jaws.^[23]