87-509: Cycloneuralia is a proposed clade of ecdysozoan animals including the Scalidophora ( Kinorhynchans , Loriciferans , Priapulids ), the Nematoida ( nematodes , Nematomorphs ), and the extinct Palaeoscolecida . It may be paraphyletic , or may be a sister group to Panarthropoda . Or perhaps Panarthropoda is paraphyletic with respect to Cycloneuralia. The group has also been considered
174-409: A mechanoreceptive bristle 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 some 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 seem to be
261-755: A broad consensus has formed supporting the Ecdysozoa and in 2011 the Darwin–Wallace Medal was awarded to James Lake for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa , and the Deuterostomia. Before Aguinaldo's Ecdysozoa proposal, one of the prevailing theories for the evolution of the bilateral animals was based on the morphology of their body cavities . There were three types, or grades of organization:
348-463: A firm foothold on uneven terrain. Each claw is composed of three stacked elements, like Russian nesting dolls . The outermost is shed during ecdysis, which exposes the next element, which is fully formed and so does not need time to harden before it is used. This distinctive construction identifies many early Cambrian fossils as early offshoots of the onychophoran lineage. Apart from the pairs of legs, there are three further body appendages, which are at
435-651: A flattened cylindrical body cross-section and rows of unstructured body appendages 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 being Ooperipatellus nanus and the largest known is Mongeperipatus solorzanoi . The number of leg pairs ranges from as few as 13 (in Ooperipatellus nanus ) to as many as 43 (in Plicatoperipatus jamaicensis ). Their skin consists of numerous, fine transverse rings and
522-767: 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. Upon ejection, it forms a net of threads about twenty microns in diameter, with evenly spaced droplets of viscous adhesive fluid along their length. It subsequently dries, shrinking, losing its stickiness, and becoming brittle. Onychophora eat their dried slime when they can, which seems provident, since an onychophoran requires about 24 days to replenish an exhausted slime repository. The slime can account for up to 11% of
609-415: 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
696-409: A group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al. strongly supported the monophyly of Ecdysozoa. The group Ecdysozoa is supported by many morphological characters, including growth by ecdysis , with moulting of
783-581: A large prey item may take almost all of their mucus -secreting capacity. 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
870-603: 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, at least among extant members. Velvet worms are generally considered close relatives of the Arthropoda and Tardigrada , with which they form the proposed taxon Panarthropoda . This makes them of palaeontological interest, as they can help reconstruct
957-399: 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 . 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
SECTION 10
#17328698818301044-530: A range of pigments. 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
1131-465: A resting state. The Onychophora forcefully squirt glue-like slime from their oral papillae; they do so either in defense against predators or to capture prey. The openings of the glands that produce the slime are in the papillae, a pair of highly modified limbs on the sides of the head below the antennae. Inside, they have a syringe-like system that, by a geometric amplifier, allows for fast squirt using slow muscular contraction. High speed films show
1218-415: A rigid exoskeleton . Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many distantly related soft-bodied animals that are cylindrically shaped, for example sea anemones and various worms . 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,
1305-440: A single phylum, sometimes given the old name Nemathelminthes . The uniting character is the nervous system organization with a circumpharyngeal brain and somata–neuropil–somata pattern. The name derives from the position of the brain around the pharynx . This protostome -related article is a stub . You can help Misplaced Pages by expanding it . Ecdysozoa Ecdysozoa ( / ˌ ɛ k d ɪ s oʊ ˈ z oʊ ə / )
1392-399: A strong selection for a low cost-benefit ratio, which cannot be achieved with a herbivorous diet. Velvet worms literally creep up on their prey, with their smooth, gradual and fluid movement escaping detection. 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
1479-432: Is a group of protostome animals, including Arthropoda ( insects , chelicerata (including arachnids ), crustaceans , and myriapods ), Nematoda , and several smaller phyla . The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes . This clade, that is,
1566-477: Is a pair of retractable, hardened (sclerotised) chitin claws, which give the taxon its scientific name: Onychophora is derived from the Ancient Greek : ονυχής , onyches , "claws"; and φέρειν , 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
1653-477: Is a three-layered cuticle (four in Tardigrada ) composed of organic material, which is periodically molted as the animal grows. This process of molting is called ecdysis , and gives the group its name. The ecdysozoans lack locomotory cilia and produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes. Ancestrally, the group exhibited sclerotized teeth within
1740-400: 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
1827-485: Is almost universally accepted, replacing an older hypothesis that Panarthropoda should be classified with Annelida in a group called the Articulata , and that Ecdysozoa are polyphyletic . Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida , though later considered them unrelated. Inclusion of the roundworms within the Ecdysozoa was initially contested but since 2003,
SECTION 20
#17328698818301914-509: Is based on Nielsen et al. and Howard et al. . Xenacoelomorpha [REDACTED] Deuterostomia [REDACTED] [REDACTED] Loricifera [REDACTED] Priapulida [REDACTED] Kinorhyncha [REDACTED] Nematoda [REDACTED] Nematomorpha [REDACTED] Tardigrada [REDACTED] Onychophora [REDACTED] Arthropoda [REDACTED] Spiralia [REDACTED] [REDACTED] Kimberella † The grouping proposed by Aguinaldo et al.
2001-421: Is colourless as it does not contain pigments ; for this reason, it serves only 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
2088-411: 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. Beneath
2175-421: 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. Oxygen transport is helped by the oxygen carrier hemocyanin . 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
2262-423: 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
2349-420: Is provided by the hydrostatic pressure of their fluid contents, and movement is usually obtained passively by stretching and contraction of the animal's entire body. However, each leg can also be shortened and bent by internal muscles . Due to the lack of joints, this bending can take place at any point along the sides of the leg. In some species, two different organs are found within the feet: On each foot
2436-510: 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 capturing
2523-410: 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 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
2610-399: 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
2697-417: 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 live only in microclimates with high humidity to avoid desiccation . The surface of the cuticula is scattered with numerous fine papillae,
Cycloneuralia - Misplaced Pages Continue
2784-530: 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. 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 America ;
2871-521: 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
2958-940: The Acoelomata (no coelom ), the Pseudocoelomata (partial coelom), and the Eucoelomata (true coelom ). Adoutte and coworkers were among the first to strongly support the Ecdysozoa. With the introduction of molecular phylogenetics, the coelomate hypothesis was abandoned, although some molecular, phylogenetic support for the Coelomata continued until as late as 2005. Onychophora Onychophora / ɒ n ɪ ˈ k ɒ f ə r ə / (from Ancient Greek : ονυχής , onyches , "claws"; and φέρειν , pherein , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus / p ə ˈ r ɪ p ə t ə s / (after
3045-611: The 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. Velvet worms always sparsely occupy the habitats where they are found: They are rare among
3132-547: The cuticle – without mitosis in the epidermis – under control of the prohormone ecdysone , and internal fertilization . The group was initially contested by a significant minority of biologists . Some argued for groupings based on more traditional taxonomic techniques, while others contested the interpretation of the molecular data . The name Ecdysozoa is scientific Greek , derived from ἔκδυσις ( ékdusis ) "shedding" + ζῷον ( zôion ) "animal" . The most notable characteristic shared by ecdysozoans
3219-557: 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
3306-454: The gastrotrichs , have been considered possible members but lack the main characters of the group, and are now placed elsewhere. The Arthropoda, Onychophora, and Tardigrada have been grouped together as the Panarthropoda because they are distinguished by segmented body plans. Dunn et al. in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as
3393-430: 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
3480-446: 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. 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
3567-457: 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 bundles 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
Cycloneuralia - Misplaced Pages Continue
3654-549: The Caribbean, but for others, conversion of rainforests is likely one of the most important threats to their survival (see Conservation ). Velvet worms are photophobic: They are repelled by bright light sources. 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
3741-530: The ancestral arthropod. Only two fossil species are confidently assigned as onychophorans: Antennipatus from the Late Carboniferous , and Cretoperipatus from the Late Cretaceous , the latter belonging to Peripatidae. In modern zoology , they are particularly renowned for their curious mating behaviours and the bearing of live young in some species. Velvet worms are segmented animals with
3828-421: The animal expelling two streams of adhesive liquid through a small opening (50–200 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 control only
3915-442: The antenna, whereas the opposite is true in arthropods. 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
4002-408: 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 responsive to touch that carry
4089-472: 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
4176-407: The claws. 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
4263-470: 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,
4350-402: 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
4437-462: 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. A penis -like structure has so far been found only in males of the genus Paraperipatus but has not yet been observed in action. There are different mating procedures: In some species males deposit their spermatophore directly into
SECTION 50
#17328698818304524-441: The fauna which they are a part of. 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
4611-438: The female's genitals 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
4698-493: The first described genus, Peripatus ), is a phylum of elongate, soft-bodied, many-legged animals . In appearance they have variously been compared to worms with legs, caterpillars, and slugs. 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 of species is likely greater. The two extant families of velvet worms are Peripatidae and Peripatopsidae . They show
4785-509: The foregut, and a ring of spines around the mouth opening, though these features have been secondarily lost in certain groups. A respiratory and circulatory system is only present in onychophorans and arthropods (often absent in smaller arthropods like mites); in the rest of the groups, both systems are missing. The Ecdysozoa include the following phyla: Arthropoda , Onychophora , Tardigrada , Kinorhyncha , Priapulida , Loricifera , Nematoda , and Nematomorpha . A few other groups, such as
4872-423: 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. Oxygen uptake occurs to an extent via simple diffusion through the entire body surface, with
4959-434: 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
5046-416: The head and comprise three segments: 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
5133-422: 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
5220-400: 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. The body cavity is known as a "pseudocoel", or haemocoel . Unlike a true coelom ,
5307-442: 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
SECTION 60
#17328698818305394-485: 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 suggest that the last common ancestor of arthropods may have only had median ocelli. However, the innervation shows that the homology is limited: The eyes of Onychophora form behind
5481-555: The length of the other individual. 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. 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. Hierarchy
5568-420: 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
5655-423: 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 digestion in and outside the mouth. The throat itself is very muscular, serving to absorb the partially liquified food and to pump it, via
5742-430: 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. When assessing other individuals, individuals often measure one another up by running their antennae down
5829-405: 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
5916-495: 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 each stride. The brains of Onychophora, though small, are very complex; consequently,
6003-459: The organism's dry weight and is 90% water; its dry residue consists mainly of proteins—primarily a collagen -type protein. 1.3% of the slime's dry weight consists of sugars, mainly galactosamine . The slime also contains lipids and the surfactant nonylphenol . Onychophora are the only organisms known to produce this latter substance. It tastes "slightly bitter and at the same time somewhat astringent". The proteinaceous composition accounts for
6090-409: 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 . The Euperipatoides 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
6177-519: 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
6264-430: 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
6351-553: The prey. 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 Onychophora spend 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 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. In
6438-428: 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
6525-416: The rear end. 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
6612-417: The rest of their group. Almost all species of velvet worm reproduce sexually. The sole exception is 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
6699-562: The sister group to the arthropods. The non-panarthropod members of Ecdysozoa have been grouped as Cycloneuralia but they are more usually considered paraphyletic in representing the primitive condition from which the Panarthropoda evolved. A modern consensus phylogenetic tree for the protostomes is shown below. It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya); dashed lines show especially uncertain placements. The phylogenetic tree
6786-422: 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
6873-408: 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
6960-418: The slime gland known as the "slime conductor" is broadened into a reservoir, allowing it to hold pre-produced slime. Velvet worm slime glands and oral papilla are likely modified and repurposed limbs. The glands themselves are probably modified crural glands. All three structures correspond to an evolutionary origin in the leg pairs of the other segments. Unlike the arthropods, velvet worms do not possess
7047-434: The slime's high tensile strength and stretchiness. The lipid and nonylphenol constituents may serve one of two purposes: They may line the ejection channel, stopping the slime from sticking to the organism when it is secreted; or they may slow the drying process long enough for the slime to reach its target. Velvet worms/Onychophora move in a slow and gradual motion that makes them difficult for prey to notice. Their trunk
7134-628: The soil into which they can withdraw during the day, and in 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 . Some species of velvet worms are able to occupy human-modified land-uses, such as cocoa and banana plantations in South America and
7221-400: The species is live-bearing or egg-laying . In live-bearing species, 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,
7308-421: 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
7395-404: The third head segment, to the left and right of the mouth, are two openings called "oral papillae", with each containing a large, heavily branched slime gland. These slime glands lie roughly in the center of a velvet worm's body and secrete a sort of milky-white slime. The slime is used to both ensnare prey and act as a distraction for defensive purposes. In certain species, an organ connected to
7482-440: 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
7569-407: The velvet worm's most important sensory organs. Except in a few (typically subterranean ) species, one simply constructed eye (ocellus) lies behind each antenna, laterally, just underneath the head. 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;
#829170