Global range of Onychophora: extant
Peripatidae in green,
Peripatopsidae in red, and fossils in black (click to enlarge)
Onychophora (from Ancient Greek, onyches, "claws"; and pherein, "to
carry"), commonly known as velvet worms (due to their velvety texture
and somewhat wormlike appearance) or more ambiguously as peripatus
(after the first described genus, Peripatus), is a phylum of elongate,
soft-bodied, many-legged panarthropods. In appearance they have
variously been compared to worms with legs, caterpillars, and
slugs. They prey upon smaller animals such as insects, which they
catch by squirting an adhesive slime. Approximately 200 species have
been described, although the true number of species is likely greater.
The two extant families of velvet worms are
Peripatopsidae. 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 within
animalia that is wholly endemic to terrestrial environments. Velvet
worms are considered close relatives of the
Arthropoda and Tardigrada,
with which they form the taxon Panarthropoda. This makes them of
palaeontological interest, as they can help reconstruct the ancestral
arthropod. In modern zoology, they are particularly renowned for their
curious mating behaviour and for bearing live young.
1.2 Slime glands
Skin and muscle
2 Distribution and habitat
Reproduction and life-cycle
10 External links
Velvet worms are segmented animals with a flattened cylindrical body
cross-section and rows of unstructured body appendages known as
oncopods or lobopods (informally: stub feet). The animals grow to
between 0.5 and 20 cm (.2 to 8 in), with the average being
about 5 cm (2 in), and have between 13 and 43 pairs of
legs. Their skin 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
of nerve cells. The individual body sections are largely
unspecialised; 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.
The stub feet that characterise 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
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 are only
found within the family
Peripatidae and are named from coxa, the Latin
word for "hip".
A pair of claws from
On each foot is a pair of retractable, hardened (sclerotised) chitin
claws, which give the taxon its scientific name:
derived from the Greek onyches, "claws"; and 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,
like Russian nesting dolls. The outermost is shed during ecdysis,
which exposes the next element in — which is fully formed, 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.) Apart from the pairs of legs, there
are three further body appendages, which are at the head and comprise
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 Arthropoda, but perhaps rather with their "lips" or
labrum. At their base is found a pair of simple eyes, except in a few
blind species. In front of these, in many Australian 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).
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 the
Arthropoda 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 similar
position and shape 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 chinons,
and have a uniform composition – except that there is a higher
concentration of calcium towards the tip, presumably affording greater
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.
On the third head segment, to the left and right of the mouth, are two
openings designated "oral papillae". Within these are a pair of large,
heavily internally branched slime glands. These lie roughly in the
centre of the body and secrete a sort of milky-white slime, which is
used to ensnare prey and for defensive purposes. Sometimes the
connecting "slime conductor" is broadened into a reservoir, which can
buffer pre-produced slime. The slime glands themselves are probably
modified crural glands. All three structures correspond to an
evolutionary origin in the leg pairs of the other segments.[citation
Skin and muscle
Unlike the arthropods, velvet worms do not possess a rigid
exoskeleton. Instead, their fluid-filled body cavity acts as a
hydrostatic skeleton, similarly to many unrelated soft-bodied animals
that are cylindrically shaped, for example sea anemones and various
worms. Pressure of their incompressible 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, the cuticula; a
single layer of epidermis cells forming an internal skin; and beneath
this, usually three layers of muscle, which are embedded in connective
tissues. The cuticula is about a micrometer thick and covered with
fine villi. In composition and structure, it resembles the cuticula of
the arthropods, consisting of α-chitin and various proteins,
although not containing collagen. 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 outwardly
water-repellent, the cuticula is not able to prevent water loss by
respiration, and, as a result, velvet worms can only live in
microclimates with high humidity to avoid desiccation. 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 tiny scales, lending the skin a velvety
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, induced by the hormone ecdysone.
The inner surface of the skin bears a hexagonal pattern. 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 either parallel or
perpendicular 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.
Within the connective tissue lie three continuous layers of
unspecialised smooth 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 a spiral.
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] 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
The body cavity is known as a "pseudocoel", or haemocoel. Unlike a
true coelom, a pseudocoel is not fully enclosed by a cell layer
derived from the embryonic mesoderm. A coelom is, however, formed
around the gonads and the waste-eliminating nephridia.[citation
needed] As the name haemocoel suggests, the body cavity is filled with
a blood-like liquid in which all the organs are embedded; in this way,
they can be easily supplied with nutrients circulating in the blood.
This liquid is colourless as it does not contain pigments; for this
reason, it only serves a limited role in oxygen transport. Two
different types of blood cells (or haemocytes) circulate in the fluid:
amoebocytes and nephrocytes. The amoebocytes probably function in
protection from bacteria and other foreign bodies; in some species,
they also play a role in reproduction. Nephrocytes absorb toxins or
convert them into a form suitable for elimination by the
nephridia. The haemocoel is divided by a horizontal
partition, the diaphragm, into two parts: the pericardial 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. The heart itself is
a tube of annular muscles consisting of epithelial 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 an open
circulation. The timing of the pumping procedure can
be divided into two parts: diastole and systole. 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 have an influence on circulation.
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. However, of most importance is gas
exchange via fine unbranched tubes, the tracheae, 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 thin membrane
through which oxygen can easily diffuse. The tracheae originate at
tiny openings, the spiracles, which themselves are clustered together
in dent-like recesses of the outer skin, the atria. The number of
"tracheae bundles" thus formed is on average around 75 per body
segment; they accumulate most densely on the back of the
organism. Unlike the arthropods, the velvet worms are
unable to control the openings of their tracheae; the tracheae are
always open, entailing considerable water loss in arid conditions.
Water is lost twice as fast as in earthworms and forty times faster
than in caterpillars. For this reason, velvet worms are dependent
upon habitats with high air humidity.
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. Two salivary glands discharge via a common
conductor into the subsequent "throat", which makes up the first part
of the front intestine. The saliva that they produce contains mucus
and hydrolytic enzymes, which initiate in and outside the mouth
digestion. Historically, the salivary glands probably evolved from the
waste-elimination organs known as nephridia, which are found
homologously in the other body segments. The throat
itself is very muscular, serving to absorb the partially liquified
food and to pump it, via the oesophagus, 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-based
peritrophic 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, or rectum, which is once again lined with a cuticula and
which opens at the anus, located on the underside near to the rear
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 a flagellated 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 called
podocytes, which facilitate ultrafiltration of the blood through the
partition between haemocoelom and nephridium. 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. The most important nitrogenous excretion product is the
water-insoluble uric acid; this can be excreted in solid state, with
very little water. This so-called uricotelic excretory mode represents
an adjustment to life on land and the associated necessity of dealing
economically with water. 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.
The entire body, including the stub feet, is littered with numerous
papillae: warty protrusions that carry a mechanoreceptive bristle
(responsive to mechanical stimuli) at the tip, each of which is also
connected to further sensory nerve cells lying beneath. The mouth
papillae, the exits of the slime glands, probably also have a function
in sensory perception. Sensory cells known as "sensills" on the "lips"
or labrum respond to chemical stimuli and are known as chemoreceptors.
These are also found on the two antennae, which can be regarded as the
velvet worm's most important sensory organs. Except in a few
(typically subterranean) species, one simply constructed eye (ocellus)
lies laterally, just underneath the head, behind each antenna.
This consists of a chitinous ball lens, a cornea and a retina and is
connected to the centre of the brain via an optic nerve. The
retina comprises numerous pigment cells and photoreceptors; the latter
are easily modified flagellated cells, whose flagellum membranes carry
a photosensitive pigment on their surface. The rhabdomeric eyes of the
Onychophora are thought to be homologous with the median ocelli of
arthropods; this would imply that the last common ancestor of
arthropods bore only median ocelli. However, the innervation shows
that the homology is limited: the eyes of
Onychophora form behind the
antenna, whereas the opposite is true in arthropods.
Euperipatoides kanangrensis. The two ovaries, full of
stage II embryos, are floating to the bottom of the image.
Both sexes possess pairs of gonads, opening via a channel called a
gonoduct into a common genital opening, the gonopore, which is located
on the rear ventral side. Both the gonads and the gonoduct are derived
from true coelom tissue. In females, the two ovaries are joined in the
middle and to the horizontal diaphragm. The gonoduct appears
differently depending on whether the species is live-bearing or
egg-laying. In the former, each exit channel divides into a slender
oviduct and a roomy "womb", the uterus, in which the embryos develop.
The single vagina, 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, the ovipositor. The females of many species also possess a
sperm repository called the receptacle seminis, in which sperm cells
from males can be stored temporarily or for longer periods.[citation
needed] Males possess two separate testes, along with the
corresponding sperm vesicle (the vesicula seminalis) and exit channel
(the vasa efferentia). The two vasa efferentia unite to a common sperm
duct, the vas 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 an auxiliary
reproductive function; the rearmost glands are also known as anal
glands. A penis-like structure has so far only been
found in males of the genus
Paraperipatus but has not yet been
observed in action. As previously mentioned, males of many Australian
species exhibit special structures on the head, which apparently take
over certain tasks in transferring sperm to the females. In the
Euperipatoides rowelli, sperm is collected by these
structures, and, when a female is encountered, the male inserts its
head in the vagina.
Distribution and habitat
Velvet worms live in all tropical habitats and in the temperate zone
of the Southern Hemisphere, showing a circumtropical and circumaustral
distribution. Individual species are found in Central and South
Caribbean islands; equatorial
West Africa and Southern
Africa; northeastern India; Thailand;
Indonesia and parts
of Malaysia; New Guinea; Australia; and New Zealand. Fossils have been
found in Baltic amber, indicating that they were formerly more
widespread in the
Northern Hemisphere when conditions were more
suitable. All extant velvet worms are terrestrial (land-living)
and prefer dark environments with high air humidity. 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. They also occur in
unforested grassland, if there exist sufficient crevices in the soil
into which they can withdraw during the day, and in
subterranean caves. Two species live in caves, 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 as exaptation.
apparently made available new habitats for velvet worms; in any case,
they are found in man-made cocoa and banana plantations in South
America and the Caribbean. Because the danger of desiccation 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.
Velvet worms are
negatively phototactical: they are repelled by bright light
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 to 200 microns)
at a speed of 3 to 5 m/s (10 to 20 ft/s). 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 only control 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
centimetre, but the maximal range has variously been reported to
be ten centimetres, 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 immobilised by
smaller squirts targeted at the limbs; additionally, the fangs of
spiders are sometimes targeted. The slime, which can account for
up to 11% of the organism's dry weight, 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
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. 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 is appropriate, because it takes an onychophoran about 24
days to replenish an exhausted slime repository. 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.
Velvet worms move in a slow and gradual motion that makes them
difficult for prey to notice. Their trunk is raised relatively
high above the ground, and they walk with non-overlapping steps.
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 only used 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.
The 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, although speeds of around 6 body-lengths
per minute are more typical. The body gets longer and narrower as
the animal picks up speed; the length of each leg also varies during
The brains of Onychophora, though small, are very complex;
consequently, the organisms are capable of rather sophisticated social
interactions. Behaviour may vary from genus to genus, so this
article reflects the most studied genus, Euperipatoides. They form
social groups of up to fifteen individuals, usually closely related,
which will typically live and hunt together. Groups usually live
together; an example in drier regions would be in a region of the
moist interior of a rotting log. Group members are extremely
aggressive towards individuals from other logs. Dominance is
achieved through aggression and maintained through submissive
behaviour. After a kill, the dominant female always feeds first,
followed in turn by the other females, then males, then the young.
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. Juveniles
never engage in aggressive behaviour, but climb on top of adults,
which tolerate their presence on their backs. When assessing other
individuals, individuals often measure one another up by running their
antennae down the length of the other individual. Once hierarchy
has been established, paired 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. 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.
Individuals within an individual log are usually closely related;
especially so with males. This may be related to the intense
aggression between unrelated females.
Velvet worms are ambush predators, hunting only by night, and are
able to capture animals at least their own size, although it may take
almost all of their slime-secreting capacity to capture a large prey
item. 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), various worms, and even
large snails (Gastropoda). Depending on their size, they eat on
average every one to four weeks. They are considered to be
ecologically equivalent to centipedes (Chilopoda). The most
energetically favourable prey are two-fifths the size of the hunting
onychophoran. 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. 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. 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; 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. Because it takes so long to ingest a
prey item, hunting mainly happens around dusk; the onychophorans will
abandon their prey at sunrise. 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
Velvet worms literally creep up on their prey, with their smooth,
gradual and fluid movement escaping detection by predators. 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 detection by the prey individual.
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. Hungry
less time investigating their prey and are quicker to apply their
slime. Once slime has been squirted,
Onychophora are determined to
pursue and devour their prey, in order to recoup the energetic
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. In the case of smaller prey, they may opt not to use slime
at all. 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. 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. 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. 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 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. After feeding,
individuals clean their antennae and mouth parts before re-joining the
rest of their group.
Reproduction and life-cycle
Almost all species of velvet worm reproduce sexually. The sole
Epiperipatus imthurni, of which no males have been
observed; reproduction instead occurs by parthenogenesis. All
species are in principle sexually distinct and bear, in many cases, a
marked sexual dimorphism: the females are usually larger than the
males and have, in species where the number of legs is variable, more
legs. The females of many species are fertilized only once during
their lives, which leads to copulation sometimes taking place before
the reproductive organs of the females are fully developed. In such
cases, for example at the age of three months in Macroperipatus
torquatus, the transferred sperm cells are kept in a special
reservoir, where they can remain viable for longer periods.
Fertilization takes place internally, although the mode of sperm
transmission varies widely. In most species, for example in the genus
Peripatus, a package of sperm cells called the spermatophore is placed
into the genital opening of the female. The detailed process by which
this is achieved is in most cases still unknown, a true penis having
only been observed in species of the genus Paraperipatus. 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. 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
Peripatopsis is particularly curious. Here, the male places
two-millimetre spermatophores on the back or sides of the female.
Amoebocytes from the female's blood collect on the inside of the
deposition site, and both the spermatophore's casing and the body wall
on which it rests are decomposed via the secretion of enzymes. This
releases the sperm cells, which then move freely through the
haemocoel, penetrate the external wall of the ovaries and finally
fertilize the ova. 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.
Ovipary occurs solely in the Peripatopsidae, often in regions with
erratic food supply or unsettled climate. In these cases, the
yolk-rich eggs 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 the uterus. The embryos
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 as lecithotrophic. 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 a placenta. The former is found only
outside the American continents, while the latter occurs primarily in
America and the
Caribbean and more rarely in the Old World. The
gestation period can amount to up to 15 months, at the end of
which the offspring 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
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 a larval stage. This is probably also the original mode
Velvet worms have been known to live for up to six
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). In South America,
Hemprichi's coral snake (Micrurus hemprichii) feeds almost exclusively
on velvet worms. For defence, some species roll themselves
reflexively into a spiral, while they can also fight off smaller
opponents by ejecting slime. Various mites (Acari) are known to be
ectoparasites infesting the skin of the velvet worm.
Skin injuries are
usually accompanied by bacterial infections, which are almost always
The global conservation 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 typically nocturnal behaviour and possibly also
as-yet undocumented seasonal influences and sexual dimorphism. To
date, the only onychophorans evaluated by the
IUCN are: Mesoperipatus
tholloni (Data Deficient),
Plicatoperipatus jamaicensis (Near
Peripatoides indigo (Vulnerable),
Peripatopsis alba (Vulnerable),
Macroperipatus insularis (Endangered), Tasmanipatus
Opisthopatus roseus (Critically
Peripatopsis leonina (Critically Endangered), and
Speleoperipatus spelaeus (Critically Endangered). The primary
threat comes from destruction and fragmentation of velvet worm habitat
due to industrialisation, draining of wetlands, and slash-and-burn
agriculture. Many species also have naturally low population densities
and closely restricted geographic ranges; as a result, relatively
small localised disturbances of important ecosystems 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. 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 and exporting,
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.
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 shared. However, antennae, mandibles, and oral papillae are
probably not 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, these lack some characteristics of the
velvet worms and arthropods, such as blood circulation and separate
respiratory structures. Together, the velvet worms, arthropods, and
water bears form a monophyletic taxon, the Panarthropoda, 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 designated Tactopoda, thus:
Velvet worms (Onychophora)
Water bears (Tardigrada)
For a long time, velvet worms were also considered related to the
annelids. 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 the
Articulata hypothesis developed by
Georges 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 by cladistic 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-called
Ecdysozoa hypothesis. This places the annelids and
Panarthropoda in two very different groups: the former in the
Lophotrochozoa and the latter in the Ecdysozoa. Mitochondrial gene
sequences also provide support for this hypothesis. 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
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 group Cycloneuralia, containing the threadworms
(Nematoda), horsehair worms (Nematomorpha) and three rather obscure
groups: the mud dragons (Kinorhyncha); penis worms (Priapulida); and
Panarthropoda (arthropods, velvet worms, water bears)
Cycloneuralia (threadworms, horsehair worms, and others)
Lophotrochozoa (annelids, molluscs, and others)
Particularly characteristic of the
Cycloneuralia is a ring of
"circumoral" nerves around the mouth opening, which the proponents of
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 mouth position: 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 the fossil 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 the chelicerates than to any other
arthropod. 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; 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 these 197
species, 20 are nomina dubia due to major taxonomic
inconsistencies. The best-known is the type genus Peripatus, 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:
Peripatopsidae exhibit relatively many characteristics that are
perceived as original or "primitive". They have between 13 and 25
pairs of legs, behind or between the last of which is the genital
opening (gonopore). Both oviparous and ovoviviparous, as well as
genuinely viviparous, species exist, although the Peripatopsidae
essentially lack a placenta. Their distribution is circumaustral,
encompassing Australasia, South Africa, and Chile.
Peripatidae exhibit a range of derivative features. They are
longer, on average, than the
Peripatopsidae and also have more leg
pairs, numbering between 22 and 43—the gonopore is always between
the penultimate pair. There are no oviparous species—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
Peripatidae is restricted to the
tropical and subtropical zones; in particular, they inhabit Central
America, northern South America, Gabon, Northeast India, and Southeast
Fossils from the early
Cambrian bear a striking resemblance to the
velvet worms. These fossils, known collectively as the lobopodians,
were marine and represent a grade from which arthropods, tardigrades,
Onychophora arose. They are found in the Cambrian,
Ordovician (possibly), Silurian and Pennsylvanian periods.
Historically, all fossil
Onychophora and lobopods were lumped into the
taxon Xenusia, further subdivided by some authors to the Paleozoic
Udeonychophora and the Mesozoic/Tertiary Ontonychophora; living
Onychophora were termed Euonychophora. 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. The exceptions are
related taxa such as Collinsium ciliosum, which bear distinctly
onychophoran-like claws. 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.
The low preservation potential of the non-mineralised Onychophora
means that they have a sparse fossil record. Stem-group members
Tertiapatus dominicanus, and
Succinipatopsis balticus (Tertiary); crown group representatives
are known only from amber—there is a single, partial specimen from
the Cretaceous, and a more comprehensive record in
from 40 million years ago. But, in fact, some of these
amber-borne specimens lack slime papillae and separate feet, thus may
belong in the stem group. The vagaries of the preservation process
can make fossils difficult to interpret. Experiments on the decay and
compaction of onychophora demonstrate difficulties in interpreting
fossils; certain parts of living onychophora are only visible in
certain conditions. The mouth may or may not be preserved; claws may
be re-oriented or lost; leg width may increase or decrease; and mud
may be mistaken for organs. More significantly, features seen in
fossils may be artefacts of the preservation process: for instance,
"shoulder pads" may simply be the second row of legs compressed
coaxially onto the body; branching "antennae" may in fact be produced
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Wikispecies has information related to Onychophora
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I.S. Oliveira, L. Hering, and G. Mayer. "
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Youtube, The Slimy, Deadly
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Helenodora • †Tertiapatoidea:
Succinipatopsis • †Tertiapatus
Commons • Wikispecies
Ctenophora (comb jellies)
Cnidaria (jellyfish and relatives)
craniates / vertebrates
Echinodermata (starfish and relatives)
Kinorhyncha (mud dragons)
Priapulida (penis worms)
Nematomorpha (horsehair worms)
Onychophora (velvet worms)
Chaetognatha (arrow worms)
Gnathostomulida (jaw worms)
Dicyemida or Rhombozoa
Annelida (ringed worms)
Nemertea (ribbon worms)
Entoprocta or Kamptozoa
Ectoprocta (moss animals)
Brachiopoda (lamp shells)
Phoronida (horseshoe worms)
Anthozoa inc. corals
Medusozoa inc. jellyfish
Asterozoa inc. starfish
Phyla with ≥5000 extant species bolded
Monoblastozoa (nomen dubium)