Rigidity is provided by the hydrostatic pressure of their fluid contents, and movement is usually obtained passively by stretching and contraction of the animal's entire body.

Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many distantly related soft-bodied animals that are cylindrically shaped, for example sea anemones and various worms.

Although outwardly water-repellent, the cuticula is not able to prevent water loss by respiration, and, as a result, velvet worms can live only in microclimates with high humidity to avoid desiccation.

Beneath this lies a thick layer of connective tissue, which is composed primarily of collagen fibres aligned either parallel or perpendicular to the body's longitudinal axis.

Between the annular and diagonal muscles exist fine blood vessels, which lie below the superficially recognisable transverse rings of the skin and are responsible for the pseudo-segmented markings.

[citation needed] 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.

Unlike the front intestine, this is not lined with a cuticula but instead consists only of a single layer of epithelial tissue, which does not exhibit conspicuous indentation as is found in other animals.

The composition of the urinary solution is modified in the nephridioduct by selective recovery of nutrients and water and by isolation of poison and waste materials, before it is excreted to the outside world via the nephridiopore.

[29] The entire body, including the stub feet, is littered with numerous papillae: warty protrusions responsive to touch that carry a mechanoreceptive bristle at the tip, each of which is also connected to further sensory nerve cells lying beneath.

[citation needed] Except in a few (typically subterranean) species, one simply constructed eye (ocellus) lies behind each antenna, laterally, just underneath the head.

[citation needed] Males possess two separate testes, along with the corresponding sperm vesicle (the vesicula seminalis) and exit channel (the vasa efferentia).

They are found particularly in the rainforests of the tropics and temperate zones, where they live among moss cushions and leaf litter, under tree trunks and stones, in rotting wood or in termite tunnels.

If the annular muscles are contracted, the body cross-section is reduced, and the corresponding segment lengthens; this is the usual mode of operation of the hydrostatic skeleton as also employed by other worms.

[53] They feed on almost any small invertebrates, including woodlice (Isopoda), termites (Isoptera), crickets (Gryllidae), book/bark lice (Psocoptera), cockroaches (Blattidae), millipedes and centipedes (Myriapoda), spiders (Araneae),[53] various worms, and even large snails (Gastropoda).

[48] The jaws operate by moving backwards and forwards along the axis of the body (not in a side-to-side clipping motion as in arthropods), conceivably using a pairing of musculature and hydrostatic pressure.

[17] The pharynx is specially adapted for sucking, to extract the liquefied tissue; the arrangement of the jaws about the tongue and lip papillae ensures a tight seal and the establishment of suction.

In many Australian species, there exist dimples or special dagger- or axe-shaped structures on the head; the male of Florelliceps stutchburyae presses a long spine against the female's genital opening and probably positions its spermatophore there in this way.

In a recent peer-reviewed paper published in the "Journal of Zoology," researchers discovered that certain species of Peripatus exhibit a unique form of parental care.

Unlike most invertebrates, where parental involvement is minimal, female Peripatus were observed actively guarding their eggs and even providing protection to their offspring after hatching.

The velvet worm's important predators are primarily various spiders and centipedes, along with rodents and birds, such as, in Central America, the clay-coloured thrush (Turdus grayi).

The collection of reliable data is also hindered by low population densities, their typically nocturnal behaviour and possibly also as-yet undocumented seasonal influences and sexual dimorphism.

To date, the only onychophorans evaluated by the IUCN are: The primary threat comes from destruction and fragmentation of velvet worm habitat due to industrialisation, draining of wetlands, and slash-and-burn agriculture.

[60] In their present forms, the velvet worms are probably very closely related to the arthropods, a very extensive taxon that incorporates, for instance, the crustaceans, insects, and arachnids.

They share, among other things, a worm-like body; a thin and flexible outer skin; a layered musculature; paired waste-elimination organs; as well as a simply constructed brain and simple eyes.

This perspective was expressed paradigmatically in the statement by the French zoologist A. Vandel: Modern taxonomy does not study criteria such as "higher" and "lower" states of development or distinctions between "main" and "side" branches—only family relationships indicated by cladistic methods are considered relevant.

[61] Proponents of this hypothesis assume that the aforementioned similarities between annelids and velvet worms either developed convergently or were primitive characteristics passed unchanged from a common ancestor to both the Lophotrochozoa and Ecdysozoa.

All genera are assigned to one of two families, the distribution ranges of which do not overlap but are separated by arid areas or oceans: Onychophoran paleontology is plagued by the vagaries of the preservation process that makes fossils difficult to interpret.

For instance, "shoulder pads" may simply be the second row of legs coaxially compressed onto the body; branching "antennae" may in fact have been created during decay.

[77] Importantly, few of the Cambrian fossils bear features that distinctively unite them with the Onychophora; none can be confidently assigned to the onychophoran crown or even stem group.

[71] It is not clear when the transition to a terrestrial existence was made, but it is considered plausible that it took place between the Ordovician and late Silurian – approximately 490 to 430 million years ago – via the intertidal zone.