Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat worm-like appearance), is a phylum of long, soft-bodied, many-legged animals. In appearance, they have variously been compared to worms with legs, caterpillars, and slugs. All living onychophorans prey upon other invertebrates, which they catch by ejecting an adhesive slime. Approximately 232 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. Onychophora is the only phylum of animal that is wholly endemic to terrestrial environments, at least among extant members. Velvet worms are considered close relatives of arthropods and tardigrades, with which they form the clade Panarthropoda. This makes them of paleontological interest, as they can help reconstruct the ancestral arthropod body plan. Only three fossil species are confidently assigned as onychophorans or close relatives: Antennacanthopodia from the Early Cambrian, Antennipatus from the Late Carboniferous, and Cretoperipatus from the Late Cretaceous (the latter belonging to Peripatidae). In modern zoology, velvet worms are known for their mating behaviors, and in some species, bearing live young.
Anatomy and physiology
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 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 colored orange, red or brown, but sometimes also bright green, blue, gold or white, and occasionally patterned with other colors. 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 unspecialized; 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.
Antennae 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 arthropods, 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).
Mouth and jaws 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 arthropods 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.
Slime papillae
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.
Slime The Onychophora forcefully squirt glue-like slime from their oral papillae; they do so either in defense against predators or to capture prey. The openings of the glands that produce the slime are in the papillae, a pair of highly modified limbs on the sides of the head below the antennae. Inside, they have a syringe-like system that, by a geometric amplifier, allows for fast squirt using slow muscular contraction. High speed films show the animal expelling two streams of adhesive liquid through a small opening (50–200 microns) at a speed of . The interplay between the elasticity of oral papillae and the fast unsteady flow produces a passive oscillatory motion (30–60 Hz) of the oral papillae. The oscillation causes the streams to cross in mid air, weaving a disordered net; the velvet worms can control only the general direction where the net is thrown.
The slime glands themselves are deep inside the body cavity, each at the end of a tube more than half the length of the body. The tube both conducts the fluid and stores it until it is required. The distance that the animal can propel the slime varies; usually it squirts it about a centimeter, but the maximal range has variously been reported to be ten centimeters, or even nearly a foot, although accuracy drops with range. It is not clear to what extent the range varies with the species and other factors. One squirt usually suffices to snare a prey item, although larger prey may be further immobilized by smaller squirts targeted at the limbs; additionally, the fangs of spiders are sometimes targeted. Upon ejection, it forms a net of threads about twenty microns in diameter, with evenly spaced droplets of viscous adhesive fluid along their length. It subsequently dries, shrinking, losing its stickiness, and becoming brittle. Onychophora eat their dried slime when they can, which seems provident, since an onychophoran requires about 24 days to replenish an exhausted slime repository.
The slime can account for up to 11% of the organism's dry weight and is 90% water; its dry residue consists mainly of proteins—primarily a collagen-type protein. 1.3% of the slime's dry weight consists of sugars, mainly galactosamine. The slime also contains lipids and the surfactant nonylphenol. Onychophora are the only organisms known to produce this latter substance. It tastes "slightly bitter and at the same time somewhat astringent". The proteinaceous composition accounts for the slime's high tensile strength and stretchiness. The lipid and nonylphenol constituents may serve one of two purposes: They may line the ejection channel, stopping the slime from sticking to the organism when it is secreted; or they may slow the drying process long enough for the slime to reach its target.
Lobopods
The stub feet that characterize the velvet worms are conical, baggy appendages of the body, which are internally hollow and have no joints. Although the number of feet can vary considerably between species, their structure is basically very similar. 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. However, each leg can also be shortened and bent by internal muscles. Due to the lack of joints, this bending can take place at any point along the sides of the leg. In some species, two different organs are found within the feet:
Crural glands are situated at the shoulder of the legs, extending into the body cavity. They open outwards at the crural papillae—small wart-like bumps on the belly side of the leg—and secrete chemical messenger materials called pheromones. Their name comes from the Latin cruralis meaning "of the legs". Coxal vesicles are pouches located on the belly side of the leg, which can be everted and probably serve in water absorption. They belong to the family Peripatidae and are named from , the Latin word for "hip". On each foot is a pair of retractable, hardened (sclerotised) chitin claws, which give the taxon its scientific name: Onychophora is derived from the , , "claws"; and , , "to carry". At the base of the claws are three to six spiny "cushions" on which the leg sits in its resting position and on which the animal walks over smooth substrates. The claws are used mainly to gain a firm foothold on uneven terrain. Each claw is composed of three stacked elements, like Russian nesting dolls. The outermost is shed during ecdysis, which exposes the next element, which is fully formed and so does not need time to harden before it is used. This distinctive construction identifies many early Cambrian fossils as early offshoots of the onychophoran lineage. Skin and muscle