Onychophora Online is a source of information on onychophorans (also known as velvet worms; the name "peripatus" is not recommended because it favors confusion with a taxonomic genus).

Onychophorans are "living fossils" in the sense that they have changed little in their body shape for about 500 million years.

Probably they were the first animals ever to walk, according to a phylogeny (a sort of family tree) constructed by a technique called cladistics.

Fossils indicate that they lived in shallow marine environments in the Cambrian, in tropical habitats from many parts of the world. The first onychophorans often had long legs, spines, head shields and body plates thought to have played a defensive role. They probably hunted smaller animals for food.

With time, they colonized dry land and those that may have remained in the sea became extinct. Terrestrial species have short legs, lack spines and armouring, and hunt their prey with the help of an adhesive that is expelled in liquid form from head organs (this includes specimens preserved in ambar).Apparently, onychophorans evolved from polychaete worms and have intermediate characteristics betweenannelids and arthropods, which results in periodical proposals for incorporating them into any of these two groups, depending on the characters that the proponents happen to chose (for example, biochemical traits).

Onychophorans are found un moist, dark places like rotten logs, leaf litter and soil crevices and normally become active at night, when the danger of dessication is less. The family Peripatopsidae is found in Chile, South Africa, Australia and adjacent islands, some members are oviparous, others ovoviviparous. The family Peripatidae is found in tropical parts of Latin America, Africa and Asia; it has viviparous species: females bear live offspring that develop in association with a placenta, an extraordinary characteristic for a tiny worm labeled as "living fossil".

These worms can reach 20 cm in length (although most are less than 5 cm long, and males are smaller in all species) and often are brown, but can also be red, blue or golden.

Although textbooks tend to present much "general" information about onychophorans, the truth is that we know close to nothing about the great majority of the about 90 described species (a similar number is thought to be unknown to science). Onychophorans are exceedingly rare and probably endangered in many habitats.