Onychophora/ɒnɪˈkɒfərə/ (fromAncient Greek:ονυχής,onyches, "claws"; andφέρειν,pherein, "to carry"), commonly known asvelvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously asperipatus/pəˈrɪpətəs/ (after the first described genus,Peripatus), is aphylum of elongate, soft-bodied, many-leggedanimals.[1][2] In appearance they have variously been compared to worms with legs, caterpillars, and slugs.[3] 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 arePeripatidae andPeripatopsidae. 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 withinAnimalia that is wholly endemic to terrestrial environments, at least among extant members.[4][5] Velvet worms are generally considered close relatives of theArthropoda andTardigrada, with which they form the proposedtaxonPanarthropoda.[6] 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 theLate Carboniferous, andCretoperipatus from theLate Cretaceous, the latter belonging to Peripatidae.[5][7] In modernzoology, they are known for theirmating behaviours and thebearing of live young in some species.
Velvet worms aresegmented animals with a flattenedcylindrical body cross-section and rows of unstructured bodyappendages known as oncopods or lobopods (informally: stub feet). They reach lengths between 0.1 and 22 cm (0.04–8.66 in) depending on species, with the smallest known beingOoperipatellus nanus and the largest known isMongeperipatus solorzanoi.[8][9] The number of leg pairs ranges from as few as 13 (inOoperipatellus nanus) to as many as 43 (inPlicatoperipatus jamaicensis).[10] Theirskin 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 ofnerve cells. The individual body sections are largelyunspecialised; even the head develops only a little differently from theabdominal segments. Segmentation is apparently specified by the samegene as in other groups of animals, and is activated in each case, duringembryonic development, at the rear border of each segment and in the growth zone of the stub feet. Although onychophorans fall within theprotostome group, their early development has adeuterostome trajectory (with the mouth and anus forming separately); this trajectory is concealed by the rather sophisticated processes which occur in early development.[11]
On the first head segment is a pair of slenderantennae, which serve insensory perception. They probably do not correspond directly to the antennae of the Arthropoda,[12] but perhaps rather with their "lips" orlabrum. At their base is a pair ofsimple eyes, except in a fewblind species. In front of these, in manyAustralian species, are various dimples, the function of which 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 thethroat, so, despite its name, it is probably nothomologous to the labrum of the Arthropoda and is used for feeding. Deep within the oral cavity lie the sharp, crescent-shaped "jaws", ormandibles, which are strongly hardened and resemble the claws of the feet, with which they are serially homologous;[13] early in development, the jaw appendages have a position and shape similar to the subsequent legs.[14] 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.[13] 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.[13]
The surface of the mandibles is smooth, with no ornamentation.[15] 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.[15]
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.[16] 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 ensnareprey and act as a distraction for defensive purposes.[16] 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.[17]
Velvet worm slime glands and oral papilla are likely modified and repurposed limbs. The glands themselves are probably modified crural glands.[17] All three structures correspond to anevolutionary origin in the leg pairs of the other segments.[citation needed]
The Onychophora forcefullysquirt glue-like slime[a] from their oral papillae; they do so either in defense against predators or to capture prey.[19] 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.[17] High speed films show the animal expelling two streams of adhesive liquid through a small opening (50–200microns) at a speed of 3 to 5 m/s (10 to 20 ft/s).[17] 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.[17] 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.[21]
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 centimetre,[18] but the maximal range has variously been reported to be ten centimetres,[22] or even nearly a foot,[23] although accuracy drops with range.[24] 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 immobilised by smaller squirts targeted at the limbs; additionally, the fangs of spiders are sometimes targeted.[24] Upon ejection, it forms a net of threads about twenty microns in diameter, with evenly spaced droplets of viscous adhesive fluid along their length.[18] It subsequently dries, shrinking, losing its stickiness, and becoming brittle.[18] Onychophora eat their dried slime when they can, which seems provident, since an onychophoran requires about 24 days to replenish an exhausted slime repository.[24]
The slime can account for up to 11% of the organism's dry weight[24] and is 90% water; its dry residue consists mainly of proteins—primarily acollagen-type protein.[18] 1.3% of the slime's dry weight consists of sugars, mainlygalactosamine.[18] The slime also contains lipids and thesurfactantnonylphenol. Onychophora are the only organisms known to produce this latter substance.[18] It tastes "slightly bitter and at the same time somewhat astringent".[25] The proteinaceous composition accounts for the slime's hightensile strength and stretchiness.[18] 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.[18]
The stub feet that characterise the velvet worms areconical, 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 thehydrostatic pressure of theirfluid 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 internalmuscles.[26] 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 calledpheromones. Their name comes from theLatincruralis meaning "of the legs".[27]
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 fromcoxa, the Latin word for "hip".[28]
On each foot is a pair of retractable, hardened (sclerotised)chitin claws, which give the taxon its scientific name: Onychophora is derived from theAncient Greek:ονυχής,onyches, "claws"; andφέρειν,pherein, "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, likeRussian 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.[29] This distinctive construction identifies many early Cambrian fossils as early offshoots of the onychophoran lineage.[29]
The entire body, including the stub feet, is littered with numerous papillae: warty protrusions responsive to touch that carry amechanoreceptive bristle at the tip, each of which is also connected to furthersensory nerve cells lying beneath. The mouth papillae, the exits of the slime glands, probably also have some function insensory perception. Sensory cells known as "sensills" on the "lips" or labrum respond to chemical stimuli and are known aschemoreceptors. These are also found on the two antennae, which seem to be the velvet worm's most important sensory organs.[citation needed]
Except in a few (typicallysubterranean) species, onesimply constructed eye (ocellus) lies behind each antenna, laterally, just underneath the head.[30] This consists of a chitinous balllens, acornea and aretina and is connected to the centre of thebrain via anoptic nerve.[30] The retina comprises numerous pigment cells and photoreceptors; the latter are easily modified flagellated cells, whoseflagellum membranes carry a photosensitive pigment on their surface.
Therhabdomeric eyes of the Onychophora are thought to be homologous with the median ocelli of arthropods; this would suggest that the last common ancestor of arthropods may have only had median ocelli.[30]However, the innervation shows that the homology is limited: The eyes of Onychophora form behind the antenna, whereas the opposite is true in arthropods.[31]
Unlike the arthropods, velvet worms do not possess a rigidexoskeleton. Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many distantly related soft-bodied animals that are cylindrically shaped, for examplesea anemones and variousworms. Pressure of theirincompressible internal bodily fluid on the body wall provides rigidity, and muscles are able to act against it. The body wall consists of a non-cellular outer skin, thecuticula; a single layer ofepidermis cells forming an internal skin; and beneath this, usually three layers of muscle, which are embedded in connective tissues. The cuticula is about amicrometer thick and covered with finevilli. In composition and structure, it resembles the cuticula of the arthropods, consisting of α-chitin and variousproteins,[13] although not containingcollagen. It can be divided into an external epicuticula and an internal procuticula, which themselves consist of exo- and endo-cuticula. This multi-level structure is responsible for the high flexibility of the outer skin, which enables the velvet worm to squeeze itself into the narrowest crevices. Although outwardlywater-repellent, the cuticula is not able to prevent water loss byrespiration, and, as a result, velvet worms can live only inmicroclimates with highhumidity to avoiddesiccation. The surface of the cuticula is scattered with numerous fine papillae, the larger of which carry visible villi-like sensitive bristles. The papillae themselves are covered with tinyscales, lending the skin avelvety appearance (from which the common name is likely derived). It also feels like dry velvet to the touch, for which its water-repellent nature is responsible. Moulting of the skin (ecdysis) takes place regularly, around every 14 days,[32] induced by thehormoneecdysone. The inner surface of the skin bears a hexagonal pattern.[33] At each moult, the shed skin is replaced by the epidermis, which lies immediately beneath it; unlike the cuticula, this consists of living cells. Beneath this lies a thick layer of connective tissue, which is composed primarily of collagen fibres aligned eitherparallel orperpendicular to the body's longitudinal axis. The colouration of Onychophora is generated by a range of pigments.[clarification needed] The solubility of these pigments is a useful diagnostic character: in all arthropods and tardigrades, the body pigment is soluble in ethanol. This is also true for the Peripatidae, but in the case of the Peripatopsidae, the body pigment is insoluble in ethanol.[34]
Within the connective tissue lie three continuous layers of unspecialisedsmooth muscular tissue. The relatively thick outer layer is composed of annular muscles, and the similarly voluminous inner layer of longitudinal muscles. Between them lie thin diagonal muscles that wind backward and forward along the body axis in aspiral. Between the annular and diagonal muscles exist fineblood vessels, which lie below the superficially recognisable transverse rings of the skin and are responsible for the pseudo-segmented markings.[26] Beneath the internal muscle layer lies the body cavity. In cross-section, this is divided into three regions by so-called dorso-ventral muscles, which run from the middle of the underbelly through to the edges of the upper side: a central midsection and on the left and right, two side regions that also include the legs.[citation needed]
The body cavity is known as a "pseudocoel", orhaemocoel. Unlike a truecoelom, a pseudocoel is not fully enclosed by a cell layer derived from the embryonicmesoderm. A coelom is, however, formed around thegonads and the waste-eliminatingnephridia.[26] As the namehaemocoel suggests, the body cavity is filled with ablood-like liquid in which all the organs are embedded; in this way, they can be easily supplied withnutrients circulating in the blood. This liquid is colourless as it does not containpigments; for this reason, it serves only a limited role inoxygen transport.Two different types of blood cells (or haemocytes) circulate in the fluid:Amoebocytes and nephrocytes. The amoebocytes probably function in protection frombacteria and other foreign bodies; in some species, they also play a role inreproduction. Nephrocytes absorbtoxins or convert them into a form suitable forelimination by the nephridia.[citation needed]
The haemocoel is divided by a horizontal partition, the diaphragm, into two parts: Thepericardial sinus along the back and the perivisceral sinus along the belly. The former encloses the tube-like heart, and the latter, the other organs. The diaphragm is perforated in many places, enabling the exchange of fluids between the two cavities.[citation needed] The heart itself is a tube of annular muscles consisting ofepithelial tissues, with two lateral openings (ostia) per segment. While it is not known whether the rear end is open or closed, from the front, it opens directly into the body cavity.
Since there are no blood vessels, apart from the fine vessels running between the muscle layers of the body wall and a pair of arteries that supply the antennae, this is referred to as anopen circulation.[citation needed] The timing of the pumping procedure can be divided into two parts:Diastole andsystole. During diastole, blood flows through the ostia from the pericardial sinus (the cavity containing the heart) into the heart. When the systole begins, the ostia close and the heart muscles contract inwards, reducing the volume of the heart. This pumps the blood from the front end of the heart into the perivisceral sinus containing the organs. In this way, the various organs are supplied with nutrients before the blood finally returns to the pericardial sinus via the perforations in the diaphragm. In addition to the pumping action of the heart, body movements also influence circulation.[citation needed]
Oxygen uptake occurs to an extent via simplediffusion through the entire body surface, with the coxal vesicles on the legs possibly being involved in some species. However, of most importance is gas exchange via fine unbranched tubes, thetracheae, which draw oxygen from the surface deep into the various organs, particularly the heart.
The walls of these structures, which are less than three micrometers thick in their entirety, consist only of an extremely thinmembrane through which oxygen can easily diffuse. The tracheae originate at tiny openings, thespiracles, which themselves are clustered together in dent-like recesses of the outer skin, theatria. The number of "tracheae bundles" thus formed is on average around 75 bundles per body segment; they accumulate most densely on the back of the organism.[citation needed]
Unlike the arthropods, the velvet worms are unable to control the openings of their tracheae; the tracheae are always open, entailing considerable water loss inarid conditions. Water is lost twice as fast as in earthworms and forty times faster than in caterpillars.[35] For this reason, velvet worms are dependent uponhabitats with high air humidity.[36][37]
Oxygen transport is helped by the oxygen carrierhemocyanin.[38]
The digestive tract begins slightly behind the head, the mouth lying on the underside a little way from the frontmost point of the body. Here, prey can be mechanically dismembered by the mandibles with their covering of fine toothlets. Twosalivary glands discharge via a common conductor into the subsequent "throat", which makes up the first part of the frontintestine. The saliva that they produce contains mucus andhydrolyticenzymes, which initiatedigestion in and outside the mouth.
The throat itself is very muscular, serving to absorb the partially liquified food and to pump it, via theoesophagus, which forms the rear part of the front intestine, into the central intestine. 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.
On entering the central intestine, food particles are coated with a mucus-basedperitrophic membrane, which serves to protect the lining of the intestine from damage by sharp-edged particles. The intestinal epithelium secretes further digestive enzymes and absorbs the released nutrients, although the majority of digestion has already taken place externally or in the mouth. Indigestible remnants arrive in the rear intestine, orrectum, which is once again lined with a cuticula and which opens at theanus, located on the underside near to the rear end.[citation needed]
In almost every segment is a pair of excretory organs called nephridia, which are derived from coelom tissue. Each consists of a small pouch that is connected, via aflagellated conductor called a nephridioduct, to an opening at the base of the nearest leg known as a nephridiopore. The pouch is occupied by special cells calledpodocytes, which facilitateultrafiltration of the blood through the partition between haemocoelom and nephridium.
The composition of theurinary 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. The most important nitrogenous excretion product is the water-insolubleuric acid; this can be excreted in solid state, with very little water. This so-calleduricotelic excretory mode represents an adjustment to life on land and the associated necessity of dealing economically with water.[citation needed]
A pair of former nephridia in the head were converted secondarily into the salivary glands, while another pair in the final segment of male specimens now serve as glands that apparently play a role in reproduction.[39]
Both sexes possess pairs ofgonads, opening via a channel called a gonoduct into a common genital opening, thegonopore, which is located on the rear ventral side. Both the gonads and the gonoduct are derived from true coelom tissue.[citation needed]
A dissectedEuperipatoides kanangrensis. The two ovaries, full of stage II embryos, are floating to the bottom of the image.
In females, the twoovaries are joined in the middle and to the horizontal diaphragm. The gonoduct appears differently depending on whether the species is live-bearing oregg-laying. In live-bearing species, each exit channel divides into a slender oviduct and a roomy "womb", theuterus, in which the embryos develop. The singlevagina, to which both uteri are connected, runs outward to the gonopore. In egg-laying species, whose gonoduct is uniformly constructed, the genital opening lies at the tip of a long egg-laying apparatus, theovipositor. The females of many species also possess a sperm repository called thereceptacle seminis, in which sperm cells from males can be stored temporarily or for longer periods.[citation needed]
Males possess two separatetestes, along with the corresponding sperm vesicle (thevesicula seminalis) and exit channel (thevasa efferentia). The two vasa efferentia unite to a common sperm duct, thevas deferens, which in turn widens through the ejaculatory channel to open at the gonopore. Directly beside or behind this lie two pairs of special glands, which probably serve some auxiliary reproductive function; the rearmost glands are also known as anal glands.[40] Apenis-like structure has so far been found only in males of the genusParaperipatus but has not yet been observed in action.[citation needed]
There are different mating procedures: in some species males deposit theirspermatophore directly into the female's genital opening, while others deposit it on the female's body, where the cuticle will collapse, allowing the sperm cells to migrate into the female. There are also Australian species where the male place their spermatophore on top of their head, which is then pressed against the female's genitals. In these species the head have elaborate structures like spikes, spines, hollow stylets, pits, and depressions, whose purpose is to either hold the sperm and / or assist in the sperm transfer to the female. The males of most species also secrete a pheromone from glands on the underside of the legs to attract females.[36]
Fossils have been found inBaltic amber, indicating that they were formerly more widespread in theNorthern Hemisphere when conditions were more suitable.[42]
Velvet worms always sparsely occupy the habitats where they are found: they are rare among the fauna of which they are a part.[citation needed]
All extant velvet worms are terrestrial (land-living) and prefer dark environments with high air humidity. They are found particularly in therainforests of the tropics and temperate zones, where they live amongmoss cushions andleaf litter, under tree trunks and stones, in rotting wood or intermite tunnels. They also occur in unforestedgrassland, if there exist sufficientcrevices in the soil into which they can withdraw during the day,[citation needed] and in caves.[43] Two species live incaves, a habitat to which their ability to squeeze themselves into the smallest cracks makes them exceptionally well-adapted and in which constant living conditions are guaranteed. Since the essential requirements for cave life were probably already present prior to the settlement of these habitats, this may be described asexaptation.[citation needed]
Some species of velvet worms are able to occupy human-modified land-uses, such ascocoa andbananaplantations in South America and the Caribbean, but for others, conversion of rainforests is likely one of the most important threats to their survival[citation needed] (seeConservation).[clarification needed]
Velvet worms are photophobic: They are repelled by bright light sources.[36] Because the danger ofdesiccation is greatest during the day and in dry weather, it is not surprising that velvet worms are usually most active at night and during rainy weather. Under cold or dry conditions, they actively seek out crevices in which they shift their body into a resting state.[44]
Velvet worms/Onychophora move in a slow and gradual motion that makes them difficult for prey to notice.[24] Their trunk is raised relatively high above the ground, and they walk with non-overlapping steps.[45]To move from place to place, the velvet worm crawls forward using its legs; unlike in arthropods, both legs of a pair are moved simultaneously. The claws of the feet are used only on hard, rough terrain where a firm grip is needed; on soft substrates, such as moss, the velvet worm walks on the foot cushions at the base of the claws.[citation needed]
Actual locomotion is achieved less by the exertion of the leg muscles than by local changes of body length. This can be controlled using the annular and longitudinal muscles. 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. Due to the stretching, the legs of the segment concerned are lifted and swung forward. Local contraction of the longitudinal muscles then shortens the appropriate segment, and the legs, which are now in contact with the ground, are moved to the rear. This part of the locomotive cycle is the actual leg stroke that is responsible for forward movement. The individual stretches and contractions of the segments are coordinated by the nervous system such that contraction waves run the length of the body, each pair of legs swinging forward and then down and rearward in succession.Macroperipatus can reach speeds of up to four centimetres per second,[24] although speeds of around 6 body-lengths per minute are more typical.[46] The body gets longer and narrower as the animal picks up speed; the length of each leg also varies during each stride.[46]
The brains of Onychophora, though small, are very complex; consequently, the organisms are capable of rather sophisticated social interactions.[47] Behaviour may vary from genus to genus, so this article reflects the most-studied genus,Euperipatoides.[47]
TheEuperipatoides form social groups of up to fifteen individuals, usually closely related, which will typically live and hunt together. Groups usually live together; in drier regions an example of a shared home would be the moist interior of a rotting log. Group members are extremely aggressive towards individuals from other logs.[47] Dominance is achieved through aggression and maintained through submissive behaviour.[47] After a kill, the dominant female always feeds first, followed in turn by the other females, then males, then the young.[47]
When assessing other individuals, individuals often measure one another up by running their antennae down the length of the other individual.[47] Once hierarchy has been established, pairs of individuals will often cluster together to form an "aggregate"; this is fastest in male-female pairings, followed by pairs of females, then pairs of males.[47]
Social hierarchy is established by a number of interactions: Higher-ranking individuals will chase and bite their subordinates while the latter are trying to crawl on top of them.[47] Juveniles never engage in aggressive behaviour, but climb on top of adults, which tolerate their presence on their backs.[47]Hierarchy is quickly established among individuals from a single group, but not among organisms from different groups; these are substantially more aggressive and very rarely climb one another or form aggregates.[47] Individuals within an individual log are usually closely related; especially so with males. This may be related to the intense aggression between unrelated females.[47]
Unidentified onychophoran species feeding on acockroach
Velvet worms areambush predators, huntingonly by night,[24] and are able to capture animals at least their own size, although capturing a large prey item may take almost all of theirmucus-secreting capacity.[48] 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),[48] various worms, and even large snails (Gastropoda). Depending on their size, they eat on average every one to four weeks.[24] They are considered to beecologically equivalent to centipedes (Chilopoda).
The most energetically favourable prey are two-fifths the size of the hunting onychophoran.[24] Ninety percent of the time involved in eating prey is spent ingesting it; re-ingestion of the slime used to trap the insect is performed while the onychophoran locates a suitable place to puncture the prey, and this phase accounts for around 8% of the feeding time, with the remaining time evenly split between examining, squirting, and injecting the prey.[24] In some cases, chunks of the prey item are bitten off and swallowed; undigestable components take around 18 hours to pass through the digestive tract.[13]
Onychophora probably do not primarily use vision to detect their prey; although their tiny eyes do have a good image-forming capacity, their forward vision is obscured by their antennae;[24] their nocturnal habit also limits the utility of eyesight. Air currents, formed by prey motion, are thought to be the primary mode of locating prey; the role of scent, if any, is unclear.[24] Because it takes so long to ingest a prey item, hunting mainly happens around dusk; the onychophorans will abandon their prey at sunrise.[24] This predatory way of life is probably a consequence of the velvet worm's need to remain moist. Due to the continual risk of desiccation, often only a few hours per day are available for finding food. This leads to a strong selection for a low cost-benefit ratio, which cannot be achieved with a herbivorous diet.[citation needed]
Velvet worms literally creep up on their prey, with their smooth, gradual and fluid movement escaping detection.[24] Once they reach their prey, they touch it very softly with their antennae to assess its size and nutritional value. After each poke, the antenna is hastily retracted to avoid alerting the prey.[24] This investigation may last anywhere upwards of ten seconds, until the velvet worm makes a decision as to whether to attack it, or until it disturbs the prey and the prey flees.[24] Hungry Onychophora spend less time investigating their prey and are quicker to apply their slime.[24] Once slime has been squirted, Onychophora are determined to pursue and devour their prey, in order to recoup the energy investment. They have been observed to spend up to ten minutes searching for removed prey, after which they return to their slime to eat it.[24] In the case of smaller prey, they may opt not to use slime at all.[24] Subsequently, a soft part of the prey item (usually a joint membrane in arthropod prey) is identified, punctured with a bite from the jaws, and injected with saliva. This kills the prey very quickly and begins a slower process of digestion.[24] While the onychophoran waits for the prey to digest, it salivates on its slime and begins to eat it (and anything attached to it). It subsequently tugs and slices at the earlier perforation to allow access to the now-liquefied interior of its prey.[24] 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.[13] 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.[13] In social groups, the dominant female is the first to feed, not permitting competitors access to the prey item for the first hour of feeding. Subsequently, subordinate individuals begin to feed. The number of males reaches a peak after females start to leave the prey item.[47] After feeding, individuals clean their antennae and mouth parts before re-joining the rest of their group.[47]
The detailed process by which this is achieved is in most cases still unknown, a truepenis having been observed only in species of the genusParaperipatus. In many Australian species, there existdimples or specialdagger- oraxe-shaped structures on the head; the male ofFlorelliceps stutchburyae presses a longspine against the female's genital opening and probably positions its spermatophore there in this way. During the process, the female supports the male by keeping him clasped with the claws of her last pair of legs. The mating behavior of two species of the genusPeripatopsis is particularly curious. Here, the male places two-millimetre spermatophores on the back or sides of the female.Amoebocytes from the female'sblood collect on the inside of thedeposition site, and both the spermatophore's casing and the body wall on which it rests are decomposed via thesecretion ofenzymes. This releases the sperm cells, which then move freely through thehaemocoel, penetrate the external wall of the ovaries and finally fertilize theova. Why this self-inflicted skin injury does not lead to bacterial infections is not yet understood (though likely related to the enzymes used to deteriorate the skin or facilitate the transfer of viable genetic material from male to female). Velvet worms are found in egg-laying (oviparous), egg-live-bearing (ovoviviparous) and live-bearing (viviparous) forms.
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. This finding challenges the conventional understanding of reproductive behavior in invertebrates and highlights the diversity of parenting strategies in the animal kingdom.[50]
Ovipary occurs solely in the Peripatopsidae, often in regions with erratic food supply or unsettled climate. In these cases, theyolk-richeggs measure 1.3 to 2.0 mm and are coated in a protective chitinous shell.Maternal care is unknown.
The majority of species are ovoviviparous: the medium-sized eggs, encased only by a double membrane, remain in theuterus. Theembryos do not receive food directly from the mother, but are supplied instead by the moderate quantity of yolk contained in the eggs—they are therefore described aslecithotrophic. The young emerge from the eggs only a short time before birth. This probably represents the velvet worm's original mode of reproduction, i.e., both oviparous and viviparous species developed from ovoviviparous species.
True live-bearing species are found in both families, particularly in tropical regions with a stable climate and regular food supply throughout the year. The embryos develop from eggs only micrometres in size and are nourished in the uterus by their mother, hence the description "matrotrophic". The supply of food takes place either via a secretion from the mother directly into the uterus or via a genuine tissue connection between the epithelium of the uterus and the developing embryo, known as aplacenta. The former is found only outside the American continents, while the latter occurs primarily in America and the Caribbean and more rarely in theOld World. Thegestation period can amount to up to 15 months, at the end of which theoffspring emerge in an advanced stage of development. The embryos found in the uterus of a single female do not necessarily have to be of the same age; it is quite possible for there to be offspring at different stages of development and descended from different males. In some species, young tend to be released only at certain points in the year.[51]
A female can have between 1 and 23 offspring per year; development from fertilized ovum to adult takes between 6 and 17 months and does not have alarval stage. This is probably also the original mode of development. Velvet worms have been known to live for up to six years.
The velvet worm's important predators are primarily various spiders and centipedes, along withrodents andbirds, such as, in Central America, the clay-coloured thrush (Turdus grayi). In South America, Hemprichi's coral snake (Micrurus hemprichii) feeds almost exclusively on velvet worms.[52] For defence, some species roll themselves reflexively into a spiral, while they can also fight off smaller opponents by ejecting slime. Variousmites (Acari) are known to beectoparasites infesting the skin of the velvet worm. Skin injuries are usually accompanied by bacterial infections, which are almost always fatal.[citation needed]
The globalconservation status of velvet worm species is difficult to estimate; many species are only known to exist at their type locality (the location at which they were first observed and described). The collection of reliable data is also hindered by low population densities, their typicallynocturnal behaviour and possibly also as-yet undocumented seasonal influences and sexual dimorphism. To date, the only onychophorans evaluated by theIUCN are:
The primary threat comes from destruction and fragmentation of velvet worm habitat due toindustrialisation, draining ofwetlands, andslash-and-burn agriculture. Many species also have naturally low population densities and closely restricted geographic ranges; as a result, relatively small localised disturbances of importantecosystems can lead to the extinction of entire populations or species. Collection of specimens for universities or research institutes also plays a role on a local scale.[55] There is a very pronounced difference in the protection afforded to velvet worms between regions: in some countries, such as South Africa, there are restrictions on both collecting andexporting, while in others, such as Australia, only export restrictions exist. Many countries offer no specific safeguards at all. Tasmania has a protection programme that is unique worldwide: one region of forest has its own velvet worm conservation plan, which is tailored to a particular velvet worm species.[55]
Below is a mostly genus-levelcladogram of the various types of velvet worm. Note that this phylogeny doesn't analyze every species.[56] As of 2023, there are around 232 total living species, meaning this phylogeny should eventually be updated.[41]
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, an exoskeleton consisting of α-chitin and non-collagenous proteins; gonads and waste-elimination organs enclosed in true coelom tissue; an open blood system with a tubular heart situated at the rear; an abdominal cavity divided into pericardial and perivisceral cavities; respiration via tracheae; and similar embryonic development. Segmentation, with two body appendages per segment, is also a shared feature.
However, the antennae, mandibles, and oral papillae of velvet worms are probablynot homologous to the corresponding features in arthropods; i.e., they probably developed independently.
Another closely related group are the comparatively obscure water bears (Tardigrada); however, due to their very small size, water bears have no need for—and hence lack—blood circulation and separate respiratory structures: shared characteristics that support common ancestry of velvet worms and arthropods.
Together, the velvet worms, arthropods, and water bears form amonophyletic taxon, thePanarthropoda, i.e., the three groups collectively cover all descendants of their last common ancestor. Due to certain similarities of form, the velvet worms were usually grouped with the water bears to form the taxon Protoarthropoda. This designation would imply that both velvet worms and water bears are not yet as highly developed as the arthropods. Modern systematic theories reject such conceptions of "primitive" and "highly developed" organisms and instead consider exclusively the historical relationships among the taxa. These relationships are not as yet fully understood, but it is considered probable that the velvet worms' sister groups form a taxon designatedTactopoda, thus:
For a long time, velvet worms were also considered related to theannelids. 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. Decisive, however, was the existence of segmentation in both groups, with the segments showing only minor specialisation. The parapodia appendages found in annelids therefore correspond to the stump feet of the velvet worms. Within theArticulata hypothesis developed byGeorges Cuvier, the velvet worms therefore formed an evolutionary link between the annelids and the arthropods: worm-like precursors first developed parapodia, which then developed further into stub feet as an intermediate link in the ultimate development of the arthropods' appendages. Due to their structural conservatism, the velvet worms were thus considered "living fossils". This perspective was expressed paradigmatically in the statement by the French zoologist A. Vandel:
Onychophorans can be considered highly evolved annelids, adapted to terrestrial life, which announced prophetically the Arthropoda. They are a lateral branch which has endured from ancient times until today, without important modifications.
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 bycladistic methods are considered relevant. From this point of view, several common characteristics still support the Articulata hypothesis — segmented body; paired appendages on each segment; pairwise arrangement of waste-elimination organs in each segment; and above all, a rope-ladder-like nervous system based on a double nerve strand lying along the belly. An alternative concept, most widely accepted today, is the so-calledEcdysozoahypothesis. This places the annelids and Panarthropoda in two very different groups: the former in theLophotrochozoa and the latter in the Ecdysozoa. Mitochondrial gene sequences also provide support for this hypothesis.[58] 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. For example, in the first case, the rope-ladder nervous system would have developed in the two groups independently, while in the second case, it is a very old characteristic, which does not imply a particularly close relationship between the annelids and Panarthropoda. The Ecdysozoa concept divides the taxon into two, the Panarthropoda into which the velvet worms are placed, and the sister groupCycloneuralia, containing the threadworms (Nematoda), horsehair worms (Nematomorpha) and three rather obscure groups: the mud dragons (Kinorhyncha); penis worms (Priapulida); and brush-heads (Loricifera).
Particularly characteristic of the Cycloneuralia is a ring of "circumoral" nerves around the mouth opening, which the proponents of the Ecdysozoa hypothesis also recognise in modified form in the details of the nerve patterns of the Panarthropoda. Both groups also share a common skin-shedding mechanism (ecdysis) and molecular biological similarities. One problem of the Ecdysozoa hypothesis is the velvet worms' subterminal position of their mouths: Unlike in the Cycloneuralia, the mouth is not at the front end of the body, but lies further back, under the belly. However, investigations into their developmental biology, particularly regarding the development of the head nerves, suggest that this was not always the case, and that the mouth was originally terminal (situated at the tip of the body). This is supported by thefossil record.
The "stem-group arthropod" hypothesis is very widely accepted, but some trees suggest that the onychophorans may occupy a different position; their brain anatomy is more closely related to that of thechelicerates than to any other arthropod.[59] The modern velvet worms form a monophyletic group, incorporating all the descendants of their common ancestor. Important common derivative characteristics (synapomorphies) include, for example, the mandibles of the second body segment and the oral papillae and associated slime glands of the third; nerve strands extending along the underside with numerous cross-linkages per segment; and the special form of the tracheae. By 2011, some 180 modern species, comprising 49 genera, had been described;[60] the actual number of species is probably about twice this. According to more recent study, 82 species of Peripatidae and 115 species of Peripatopsidae have been described thus far. However, among the 197 species, 20 arenomina dubia, due to major taxonomic inconsistencies.[61] The best-known is the type genusPeripatus, which was described as early as 1825 and which, in English-speaking countries, stands representative for all velvet worms. 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:
The Peripatopsidae exhibit relatively many characteristics that are perceived as original or "primitive". The number of leg pairs in this family range from 13 (inOoperipatellus nanus[10]) to 29 (inParaperipatus papuensis[62][63]).[64] Behind or between the last leg pair is the genital opening (gonopore).[64][65] Both oviparous and ovoviviparous, as well as genuinely viviparous, species exist, although the peripatopsids essentially lack a placenta. Their distribution is circumaustral, encompassingAustralasia,South Africa, andChile.[61]
The Peripatidae exhibit a range of derivative features. They are longer, on average, than the Peripatopsidae and also have more legs. The number of leg pairs in this family range from 19 (inTyphloperipatus williamsoni[66]) to 43 (inPlicatoperipatus jamaicensis[10]).[64][65] The gonopore is always between thepenultimate leg pair.[64] None of the peripatid species are oviparous, and the overwhelming majority are viviparous. The females of many viviparous species develop a placenta with which to provide the growing embryo with nutrients. Distribution of the peripatids is restricted to the tropical and subtropical zones; in particular, they inhabitCentral America, northernSouth America,Gabon,Northeast India, andSoutheast Asia.[61]
As of February 2025, velvet worms have had only twonuclear genomes sequenced.[67][68] These are ofEuperipatoides rowelli (aperipatopsid) andEpiperipatus broadwayi (aperipatid). The first one is highly fragmented, while the second is less so, but still needs improvement.[69] Velvet worms seem to displaygenome gigantism, with the more complete assembly (E. broadwayi) having an size of 5.60 giga-base pairs.[69] Around 70.92% of its genome arerepeat sequences, something that contributes to the bulk of its size. While less substantial, it also has very largeintrons, or parts of a gene that do not becomeproteins.[69]
Feature decay timeline. Yellow means "pristine", orange means "decaying", red means "some or all lost"
Due to being soft-bodied, onychophorans need excellent conditions tofossilize. However, even when this happens, their fossils can be subject totaphonomic bias. Experiments were done with modern velvet worms to analyze their decomposition in varioussaline solutions.[70] The study also investigated whether they experienced something calledstemward slippage. In this phenomenon, animals are falsely categorized as more primitive due to the decay of certain features.[70]
The researchers found that different featuresdecayed at significantly different rates.[70]Salinity and time ofmoult had little effect on decay, and the way things decomposed remained the same for different species (though it could happen at different speeds).[70] Before any degradation, velvet worms flex into a S, U, or circular shape. Most flexing happens in the first 24 hours, but the process can continue for around two more days.[70] In the early stages of decay, theepidermis and outer cuticle separate, causing a bloated appearance. The elongates while increasing in width by around 10–30%. Thelimbs do the same, increasing in length and width by around 10–25%.[70] Around the same time, the internal organs begin to degrade. This eventually culminates in thegut rupturing, destroying the other organs.[70] In later stages of decay, the body cuticle shrinks close to its original size. A similar trend was found with the limbs, but it was just short of being statistically significant.[70] It's around this time that many external features begin to deteriorate. These include the dermal papillae, leg rings,anus,gonopore,antenna, slime papillae, and eventuallyeyes. Interestingly, the dermal papillae on the trunk disappear faster than those on the limbs.[70] Even as decay progresses, the body is still recognizable. This stops once the outer cuticle finally ruptures. After that, the animal’s anatomy is extremely difficult to interpret. At this stage, the only identifiable features would be thechitinous jaws and claws.[70]
Onychophorans are unlikely to experience anystemward slippage since their defining features (jaws, feet, and slime papillae) are generally decay-resistant.[70] However,decomposition has a significant impact on fossil anatomy. For starters, the preserved body outline is probably somewhat inaccurate, as this gets bloated in the decay process. A similar thing happens with thelimbs, so this needs to be accounted for when analyzing locomotion and/or leg length.[70] While fossilized onychophorans can appear to displaypatterning, these aren't true to life.pigment granules are one of the first things to degrade and can easily move around in the body.[70] Another finding is that characters such asinternal organs or thebody cavity are highly unlikely to fossilize. If these appear preserved in a fossil, they should be treated with skepticism, especially if they’reunmineralized.[70] When it comes to placement of themouth, even moderate decay makes it hard to tell if it's at the front or underside of the head.[70] Additionally,fossils that lack decay-resistant features probably lacked them in life. This is even more plausible if the fossil preserves decay-prone features, as their presence indicates a better level of preservation. For example, an onychophoran (or related animal) witheyes but no claws likely never had them.[70]
Certain fossils from the early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as thelobopodians, were marine and represent a grade from which arthropods, tardigrades, and Onychophora arose.[71][72]Possible fossils of onychophorans are found in theCambrian,[73]Ordovician (possibly),[74]Silurian[75] andPennsylvanian[5][76] periods.
Historically, all fossil Onychophora and lobopodians were lumped into the taxonXenusia, further subdivided by some authors to the Paleozoic Udeonychophora and the Mesozoic/Tertiary Ontonychophora; living Onychophora were termed Euonychophora.[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.[78] Possible exceptions areHallucigenia and related taxa such asCollinsium ciliosum, which bear distinctly onychophoran-like claws.[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 – approximately490 to 430 million years ago – via the intertidal zone.[34]
Reconstruction of the Carboniferous possible onychophoranHelenodora
The lowpreservation potential of the non-mineralised onychophorans means that they have a sparse fossil record. The lobopodianHelenodora from the Carboniferous of North America has been suggested to be a member of Onychophora,[79] but other studies recover it as more closely related to other lobopodians. ALate Carboniferous fossil fromMontceau-les-Mines, France,Antennipatus has been suggested to have clear onychophoran affinities, likely the first terrestrial onychophoran, but its poor preservation prohibits differentiating between its placement on the stem or crown of the two extant families, or on the onychophoran stem-group.[5]
In 2018, the identification ofAntennipatus as the oldest onychophoran has been argued by Giribet and colleagues, who suggested that the minimum age ofAntennipatus would be during theGzhelian age around298.75 million years ago, and incorporated the taxon conservatively for the phylogenetic analysis of oncyhophorans based on the uncertainty of its placement within the order.[7] In 2021, Baker and colleagues conducted divergence analyses using molecular dating and treatingAntennipatus conservatively as a stem-group onychophoran with a minimum age of300 million years ago, resulting in a divergence date of376 million years ago for the crown group onychophorans.[80] Crown group representatives are known only from amber, the oldest beingCretoperipatus fromBurmese amber during theCenomanian-Turonian stages of theLate Cretaceous, around 100-90 million years old, assigned to the family Peripatidae.[77] The affinity of amber records from the Cenozoic, likeTertiapatus, andSuccinipatopsis, which form the suggested superfamily termed Tertiapatoidea,[79] has been considered doubtful by other authors.[5]
^Fishelson, L. (1978).Zoology. Vol. 1 (3rd ed.). Israel: Hakibutz Hameuchad Publishing. p. 430.[verification needed]
^abGiribet, Gonzalo; Buckman-Young, Rebecca S.; Costa, Cristiano Sampaio; Baker, Caitlin M.; Benavides, Ligia R.; Branstetter, Michael G.; et al. (2018). "The 'Peripatos' in Eurogondwana? – Lack of evidence that south-east Asian onychophorans walked through Europe".Invertebrate Systematics.32 (4):840–863.doi:10.1071/IS18007.
^Morera-Brenes, B.; Monge-Nájera, J. (2010). "A new giant species of placented worm and the mechanism by which onychophorans weave their nets (Onychophora: Peripatidae)".Revista de Biología Tropical.58 (4):1127–1142.arXiv:1511.00983.doi:10.15517/rbt.v58i4.5398.PMID21246983.
^abWright, Jonathan C.; Luke, Barbara M. (1989). "Ultrastructural and histochemical investigations ofperipatus integument".Tissue & Cell.21 (4):605–25.doi:10.1016/0040-8166(89)90012-8.PMID18620280.
^abcdefghiBenkendorff, Kirsten; Beardmore, Kate; Gooley, Andrew A.; Packer, Nicolle H.; Tait, Noel N. (December 1999). "Characterisation of the slime gland secretion from the peripatus,Euperipatoides kanangrensis (Onychophora: Peripatopsidae)".Comparative Biochemistry and Physiology B.124 (4):457–65.doi:10.1016/S0305-0491(99)00145-5.
^abBaer, Alexander; Mayer, Georg (October 2012). "Comparative anatomy of slime glands in onychophora (velvet worms)".Journal of Morphology.273 (10):1079–1088.doi:10.1002/jmor.20044.PMID22707384.S2CID701697.
^Morera-Brenes, Bernal; Monge-Nájera, Julián (December 2010). "A new giant species of placented worm and the mechanism by which onychophorans weave their nets (onychophora: Peripatidae)".Revista Biologìa Tropical.58:1127–1142.arXiv:1511.00983.
^Harmer, Sidney Frederic; Shipley, Arthur Everett; et al. (1922).Peripatus, Myriapods, Insects. The Cambridge Natural History. Vol. 5. Macmillan Company.[page needed]
^Eriksson, J. (2003).Evolution and Development of the Onychophoran Head and Nervous System. Acta Universitatis Upsaliensis.ISBN978-91-554-5613-9.[page needed]
^Maas, Andreas; Mayer, Georg; Kristensen, Reinhardt M.; Waloszek, Dieter (December 2007). "A Cambrian micro-lobopodian and the evolution of arthropod locomotion and reproduction".Chinese Science Bulletin.52 (24):3385–92.Bibcode:2007ChSBu..52.3385M.doi:10.1007/s11434-007-0515-3.S2CID83993887.
^abMonge-Najera, Julian (May 1995). "Phylogeny, biogeography and reproductive trends in the Onychophora".Zoological Journal of the Linnean Society.114 (1):21–60.doi:10.1111/j.1096-3642.1995.tb00111.x.
^Holt, Jack (2010-02-07)."Introduction to the Onychophora". Protostomes > Ecdysozoa > Onychophora. Biol. 202: Systematic Biology. Selinsgrove, PA: Susquehanna University. Archived fromthe original on Oct 5, 2022. Retrieved2022-11-01.Excretory system: Pared metanephridia opening by their own ducts at each internal "segment". Anterior nephridia modified to salivary glands; posterior nephridia modified to gonopores.
^abcdefghijklmReinhard, Judith; Rowell, David M. (September 2005). "Social behaviour in an Australian velvet worm,Euperipatoides rowelli (Onychophora: Peripatopsidae)".Journal of Zoology.267 (1):1–7.doi:10.1017/S0952836905007090.
^Read, V. M. St. J. (July 1988). "The Onychophora of Trinidad, Tobago, and the Lesser Antilles".Zoological Journal of the Linnean Society.93 (3):225–57.doi:10.1111/j.1096-3642.1988.tb01362.x.
^Smith, J., & Jones, M. (2022). Maternal care in Peripatus: evidence for a hitherto unknown reproductive strategy in onychophorans. Journal of Zoology,301 (2), 125–134.
^Walker, Muriel H.; Tait, Noel N. (December 2004). "Studies of embryonic development and the reproductive cycle in ovoviviparous Australian Onychophora (Peripatopsidae)".Journal of Zoology.264 (4):333–54.doi:10.1017/S0952836904005837.
^Mongenajera, J.; Barrientos, Z.; Aguilar, F. (1993). "Behaviour ofEpiperipatus-biolleyi (Onychophora, Peripatidae) laboratory conditions".Revista de Biología Tropical.41 (3A):689–696.
^abNew, Tim R (1995). "Onychophora in invertebrate conservation: priorities, practice and prospects".Zoological Journal of the Linnean Society.114 (1):77–89.doi:10.1006/zjls.1995.0017.
^van Roy, Peter; Orr, Patrick J.; Botting, Joseph P.; Muir, Lucy A.; Vinther, Jakob; Lefebvre, Bertrand; el Hariri, Khadija; Briggs, Derek E.G. (May 2010). "Ordovician faunas of Burgess Shale type".Nature.465 (7295):215–218.Bibcode:2010Natur.465..215V.doi:10.1038/nature09038.PMID20463737.S2CID4313285.
^von Bitter, Peter H.; Purnell, Mark A.; Tetreault, Denis K.; Stott, Christopher A. (2007). "Eramosa Lagerstätte — Exceptionally preserved soft-bodied biotas with shallow-marine shelly and bioturbating organisms (Silurian, Ontario, Canada)".Geology.35 (10):879–882.Bibcode:2007Geo....35..879V.doi:10.1130/G23894A.1.
^Thompson, Ida; Jones, Douglas S. (May 1980). "A possible Onychophoran from the middle Pennsylvanian Mazon Creek beds of northern Illinois".Journal of Paleontology.54 (3):588–596.JSTOR1304204.
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