Columbus
| Columbus | ||||||||||||
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Columbidae in the Bay of Plenty region in the north of New Zealand's North Island |
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| Onychophora | ||||||||||||
| Pit , 1853 | ||||||||||||
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The velvet worms (Onychophora - literally claws support ) form a separate phylum from the group of molting animals (ecdysozoan) and can be simplified as worms describe with legs. Mainly distributed in the southern hemisphere, they predatory on smaller animals such as insects , which they often prey by spraying a sticky slime. In modern zoology they are known not least for their viviparous mode of reproduction and the unusual mating forms of many species. Possible ancestors of the animals, the lobopods from the geological epoch of the Cambrian , are of great interest in paleontology .
Poles are very likely closely related to arthropods (Arthropoda) and tardigrades (Tardigrada) and with these two form the taxon Panarthropoda . The first genus was scientifically described in 1825 by Lansdown Guilding (1797-1831), who still viewed the animals as modified snails (Gastropoda); the name Onychophora was first coined in 1853 by Adolf Eduard Grube .
construction
Colibus are worm-shaped, segmented animals with a cylindrical body cross-section that is flattened on the abdomen and unorganized body appendages arranged in rows, the stubby feet. The animals grow to be between 0.5 and 20 centimeters long, with an average of about 5 centimeters, and have between 13 and 43 pairs of legs. The color of their skin, covered by numerous fine transverse rings, is often inconspicuously orange, red or brown, but sometimes also bright green, blue-black, gold-colored or white, occasionally also with colored patterns.
The outwardly inconspicuous segmentation, which can only be recognized by the regular spacing of the pairs of legs, is shown in the regular sequence of skin openings ( ostia ), excretory organs ( nephridia ) and nerve cell concentrations ( ganglia ). The individual body sections are hardly specialized; even the head develops little differently than any other segment of the abdomen. The segmentation is apparently determined by the engrailed gene , which can also be detected in other animal groups, and which is activated during embryonic development at the posterior segment border and in the growth zone of the stump feet.
Body appendages
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The name-giving feature of the columbus is the conical, sack-like body appendages, technically known as oncopodia , which are hollow on the inside, have no joints and are provided with a pair of claws at the end. Although their number can vary considerably between species, they are fundamentally very similar in structure. Their strength is based on the hydrostatic pressure of the body fluid they contain.
The legs are usually moved passively by contracting and stretching the entire body. However, each leg also has internal muscles that can shorten and bend it; due to the lack of joints, this bending can occur at any point along the long side of the leg.
The outlets of two different organ formations of individual species are located near the base of the leg:
- Crural glands are glands located at the base of the legs, which extend into the body cavity. They open outwards on the crural papillae on the abdomen , small wart-shaped elevations, and secrete chemical messengers, so-called pheromones . Its name is derived from the Latin cruralis , "related to the legs".
- Coxalvesikel , also called Coxalsäcke or Coxalorgane be referred to, are the abdominal side located on the leg bag approach that can be turned inside out and presumably serve for water absorption. They only come within the family before peripatidae and after the Latin word for "hip" coxa named.
A pair of retractable hardened ( sclerotized ) claws made of chitin sit on each of the feet , from which the scientific name of the taxon derives, which is derived from the Greek onyches , “claws” and pherein , “to carry”. At the base of the claws there are three to six thorny "cushions" on which the leg rests in the resting position and on which the animals walk on a smooth surface; the claws are mainly used on uneven terrain to gain a firm grip.
In addition to the pairs of legs, there are three other body attachments that are located on the head, which consists of three segments:
- On the first head segment there is a pair of ringed antennas that are used for sensory perception. They probably do not correspond to the antennae of the arthropods , but perhaps their "lips" or labrum . Except for a few blind species, a pair of simple eyes can be found at their approach . Before that, many Australian species have created various dimples, stiletto-like structures or thorns, the function of which has not yet been clarified, but at least in some species it seems to serve to transfer sperm packets ( spermatophores ).
- On the abdomen side of the second head segment is the mouth opening surrounded by sensory "lips", the labrum; in the case of columbus, however, the structure is a muscular outgrowth of the throat and therefore, despite the same name, probably not homologous to the labrum of the arthropod. Deep within the oral cavity lie the sharp, sickle-shaped jaws or mandibles , which are severely hardened and resemble the claws of the legs, to which they are believed to be homologous. They are divided into outer and inner mandibles, are covered with fine teeth and move back and forth in the longitudinal direction; Hunted prey can be shredded into fine pieces.
- On the third segment, to the left and right of the mouth, there are two openings called oral papillae , into which a pair of large, internally heavily branched mucous glands open. These lie roughly in the middle of the body and secrete a milky-white slime intended for prey capture and defense. Sometimes the connecting “mucus conductor” is thickened to form a reservoir that can temporarily store the pre-produced mucus. Mucous glands are likely modified crural glands.
All three structures correspond to their evolutionary origin according to the pairs of legs of the other segments.
Skin and muscles
Unlike the arthropods have no fixed velvet worms exoskeleton (exoskeleton). Instead, like a water-filled balloon, their body shape is maintained by the slightly increased pressure of the inner fluid on the body wall: Because this fluid is incompressible under the pressures generated by muscle strength, i.e. it does not change its volume, muscles can do work on the other hand. Because of the skeletal material water, one speaks of a hydroskeleton .
The body wall itself consists of a non-cellular outer skin, the cuticle , a single layer of cells, the epidermis , as the inner skin and usually three muscle layers underneath that are embedded in connective tissue .
The cuticle is about a micrometer thin, covered with fine hairs and resembles the cuticle of the arthropod in composition and structure. It consists of α-chitin and various proteins, but not collagen, and can be divided into an outer epicuticle and an underlying procuticle , which in turn is made up of exo- and endocuticula . This multi-layer structure is responsible for the high flexibility of the outer skin, which enables the animals to squeeze into the narrowest crevices. Although it has a water-repellent effect on the outside, it is not able to prevent moisture loss, with the result that colobus can only live in a microclimate with high humidity.
The surface of the cuticle is covered with numerous fine papillae; the larger of them wear the sensory bristles that are visible as hairs. The papillae are in turn covered with small scales, giving the skin a velvety appearance. It also feels dry and velvety to the touch, which is due to its water-repellent nature. As part of the moulting ( ecdysis ) mediated by the hormone ecdysone , it is shed regularly, sometimes every two weeks.
It is renewed by the single-layer inner skin below, the epidermis, which, in contrast to the cuticle, consists of living cells. It is subject to a thick layer of connective tissue, which is primarily formed by collagen fibers that are either parallel or perpendicular to the longitudinal axis of the body. Within the connective tissue lie three uninterrupted layers of unspecialized smooth muscles. The outer, relatively thick layer is formed by the circular muscles, the innermost by the longitudinal muscles, which are also voluminous. In between are the thin diagonal muscles, which spiral back and forth along the axis of the body. There are fine blood channels between the circular and diagonal muscles, which lie below the superficially recognizable transverse rings of the skin and are responsible for this pattern called pseudo -segmentation.
The body cavity lies beneath the innermost muscle layer. It is divided into three regions in cross-section by what are known as dorsoventral muscles , which run from the middle of the abdomen through it to the edges of the back: a central middle section and two side sections on the left and right of the abdomen, which also include the legs.
Hemocoeloma and blood circulation
The body cavity itself is a so-called pseudocoeloma , which in this special case is also called a hemocoeloma . In contrast to a real coelom, a pseudocoel is not surrounded by a real cell layer derived from embryonic mesoderm . However, a coelom is formed around the gonads and the excretory nephridia.
As the name haemocoelom suggests, the body cavity is filled with a blood-like fluid in which all organs are embedded; in this way they can easily be supplied with nutrients circulating in the blood. This liquid is colorless because it does not contain any pigments; for this reason, it is only used to a small extent to transport oxygen. Two different types of blood cells, or hemocytes , move in the blood fluid , the amebocytes and nephrocytes . The former probably have a function in the defense against bacteria and other foreign bodies; in some species they also play a role in reproduction. In contrast, nephrocytes store toxins or convert them into a form that can be used by the excretory organs known as nephridia.
The hemocoeloma is divided into two parts by a horizontal septum, the diaphragm , the posterior pericardial sinus and the abdominal perivisceral sinus . The former surrounds the tubular heart , the latter the other organs. The diaphragm is perforated several times so that fluid can be exchanged between the two partial cavities.
The heart itself is a single-layer epithelial tube consisting of a circular muscle tube with two lateral openings ( ostia ) per segment. While it is unknown whether the rear end is open or closed, the front end opens directly into the body cavity. Since there are no veins apart from the fine blood vessels between the muscular layers of the body wall and a pair of arteries that supply the antennae, it is called an open blood circulation .
The timing of the pumping process can be divided into two parts, diastole and systole . During diastole, blood flows from the pericardial (near the heart) sinus through the ostia into the heart. As soon as the systole begins, the ostia close, the heart muscles contract, thus reducing the heart volume and in this way pumps blood from the front end of the heart into the perivisceral (organ-near) hemocoeloma. In this way, the various tissues there are supplied with nutrients before the blood finally returns to the pericardial sinus through the perforations in the diaphragm. In addition to the pumping action of the heart, body movements also have an influence on blood circulation.
breathing
The oxygen uptake of the animals occurs on the one hand through simple diffusion over the entire body surface, in some species possibly also through their coxal vesicles on the stump legs. Most important, however, is the gas exchange via fine, unbranched tubes, the trachea , which extend from the surface of the body deep into the body to the various organs, but especially to the heart.
The walls of these structures, which are usually less than three micrometers thick, only consist of an extremely thin membrane through which the oxygen can easily diffuse. They arise from tiny openings, the spiracles , which in turn are grouped into several in a dent-shaped indentation in the outer skin, the atrium . The number of tracheal bundles thus formed can be given as an average of 75 per body segment; they occur more frequently on the back of the animals.
Unlike arthropods, which can control their tracheal openings, the velvet worms are unable to do so. Their tracheas are always open, which results in large water losses when it is dry. For this reason, velvet worms depend on habitats with high humidity .
Digestive system
The digestive tract begins with the subterminal , i.e. a little behind the foremost point of the body on the stomach side of the mouth, in which prey can be mechanically chopped up by the fine-toothed jaws. Two salivary glands open through a common conductor into the adjoining throat, which is already the first part of the foregut. The saliva they produce contains mucus and hydrolytic enzymes that initiate digestion in and outside the oral cavity. In evolutionary terms, the salivary glands are probably derived from excretory organs known as nephridia , which are located as homologous organs in the other body segments.
The throat itself is very muscular and serves to suck in the predigested, partially liquefied food and to pump it through the esophagus, which forms the back of the foregut, into the midgut. Unlike the foregut, this is not lined with a cuticle and consists only of a single layer of epithelial tissue that, unlike other animals, has no noticeable indentations. When entering the midgut, food particles are enveloped by a mucous peritrophic membrane , which is supposed to prevent mechanical damage to the intestinal tissue by sharp-edged particles. The intestinal epithelium itself secretes further digestive enzymes and absorbs the digested nutrients; Most of the digestion takes place externally or in the oral cavity. Indigestible residues reach the hindgut, also known as the rectum , which in turn is lined with a cuticle and opens into the anus on the abdomen near the rear end of the animals.
Excretory organs
Almost every segment has a pair of excretory organs called nephridia , which are derived from coelomial tissue . They consist of a scourge that occupied transition, from a small bag, Nephridiodukt called with an abdominal side located at the nearest leg approach opening the Nephridiopore is connected. The sachet is filled with special cells, the podocytes , which allow the blood to be ultrafiltered through the dividing wall between the hemocoeloma and nephridium. The composition of the primary urine is changed through the selective recovery of nutrients and water as well as the secretion of toxins and waste substances in the nephridiopore before it is released to the outside world at the nephridiopore. The most important nitrogen-containing excretion product is water-insoluble uric acid ; it can be released into the environment in a solid state with only a little water. This elimination mode , known as Uricotelian , represents an adaptation to rural life and the need to use water sparingly.
A former pair of nephridia in the head was converted secondarily into the salivary glands, another in the last segment of the males now takes on a glandular function that is apparently used in reproduction.
Nervous system
The nervous system consists primarily of the brain and two abdominal nerve cords that are connected to one another by numerous cross connections. According to more recent findings, however, it is not a " rope ladder nervous system " as in the arthropods , since there are neither segmental ganglia nor the cross connections are segmental.
The front and back of the brain consists of a left and a right half, which together with nerve connections form a ring around the esophagus. Instead of a tripartite division of the brain in the direction of the longitudinal axis of the body, as found in the closely related arthropods (Arthropoda), there are only two brain sections.
About fifteen pairs of nerves emanate from the brain and contain both sensory and motor nerve cells . The main nerves of the head region, the optic nerves , eyes and brain connect and two sensory nerves that run in the antennas and are responsible for the mediation of contact and chemical stimuli. The most important body nerves, on the other hand, are two belly-side nerve cords that are widely spaced and that stretch along the length of the body to the rear. The body wall and appendages are supplied by mixed nerves that transmit both sensory, sensory perception and motor signals that appeal to the muscles.
Sense organs
The entire body, including the stumpy feet, is covered with numerous papillae, warty protrusions that have a ( mechanoreceptive ) bristle on the tip that responds to mechanical stimuli and is connected to sensory nerve cells further inside. The oral papillae, the outlets of the mucous glands, probably also have a function in sensory perception.
The sensory cells or sensils on the “lips”, the labrum, on the other hand, serve to perceive chemical stimuli, which is why one speaks of chemoreceptors . They can also be found on the two antennae, which can be considered the most important sensory organs of the velvet worms.
With the exception of a few, mostly cave-living species, there is a simply built eye ( ocellus ) at the base of the antennae , which consists of a chitin-containing spherical lens as well as cornea and retina and is connected to the brain by an optic nerve each . The retina is made up of numerous pigment cells and photoreceptors; The latter are slightly modified flagellum cells , the flagellum membrane of which has a light-sensitive pigment on the surface.
The eye of the Onychophora shows no relation to the compound eye of the arthropods, neither in structure nor in innervation. A homology between this eye and the forehead eyes (ocelles) z. B. insects.
Reproductive organs
Both sexes have paired gonads that open through ducts known as gonoducts into a common sexual opening, the gonopore , which is located on the back of the abdomen. Both gonads and gonoducts are derived from real coelom tissue .
In females, the two ovaries have grown together in the middle and are attached to the horizontal septum. The gonoducts are different depending on whether they are viviparous or egg-laying species. In the former, each duct is divided into a slender fallopian tube and a spacious "uterus", the uterus , in which the embryos grow. The vagina, which is simply present, with which both uteri are connected, opens outwards at the gonopore. In egg-laying species, whose gonoducts are structured uniformly, the genital opening is at the tip of a long egg-laying apparatus, the ovipositors . In addition, the females of many species have a seminal receptacle called Receptaculum seminis , in which the male's sperm cells can be stored temporarily or for a longer period.
Males have two separate testes and the corresponding seminal vesicles ( vesicula seminalis ) and ducts ( vasa efferentia ). The latter unite to form a common spermatic duct, the vas deferens , which in turn widens to the ejaculation duct and opens outwards at the gonopore. Directly next to or behind this are two pairs of special glands in the males, which presumably have an auxiliary function in reproduction; the glands in the rear pair are also known as the anal glands.
A penis-like structure has so far only been demonstrated in males of the Paraperipatus genus , but has not yet been observed in action. Males of numerous Australian species, on the other hand, as already mentioned, have special structures on their heads that seem to take on tasks in the transfer of seeds to the females.
distribution and habitat
Poles live in tropical habitats and in the temperate zone of the southern hemisphere; they have both a circumtropical and a circumaustral distribution. Individual species are found in Central and South America , on the Caribbean islands , in equatorial West Africa and in South Africa , furthermore in northern India , Indonesia and parts of Malaysia , on New Guinea , in Australia and in New Zealand . The totality of the present-day distribution areas indicates the likely origin of the taxon on the former supercontinent Gondwana , which is why one speaks of a Gondwana distribution.
All velvet worms live on land ( terrestrial ) and prefer dark habitats with high humidity. Particularly suitable for this are the rainforests of the tropics and temperate zones, where the animals can be found in moss pillows , in the leaf litter of the ground, under tree trunks and stones, in rotting wood or in termite passages . They can also be found in unforested grasslands if there are enough cracks in the ground into which they can retreat during the day.
Two species live in caves , a habitat to which they are excellently adapted due to their ability to squeeze into the smallest of cracks and which guarantees them constant living conditions. Since the decisive prerequisites for a cave life were probably already in place before the settlement of this habitat , one speaks of exaptation . Agriculture has apparently opened up new habitats for velvet worms; in any case, they can also be found in cocoa and banana plantations in Central America and the Caribbean .
Because the risk of dehydration is greatest during the day and in dry weather, it is not surprising that colobus are usually active at night and in rainy weather. In cold or dry environmental conditions, they seek out crevices in the ground where they put their bodies into a state of rest. They turn away from strong light sources and are therefore negative phototactic .
The largest population density measured so far is very low with around 10 individuals per square meter of soil; Poles are also very difficult to find in their habitats.
Locomotion
To get around, the animals use their legs to crawl forward; unlike the arthropods, they always move both legs of a pair at the same time. Contact of the stomach side with the substrate is avoided as far as possible, so the body is held above the ground by the stubby feet. The claws on the feet are only used on hard, rough ground where a firm grip is required; On soft substrates such as moss, the animals walk on the foot cushions at the base of their claws.
The actual locomotion occurs less through the use of the leg muscles, but rather through a local change in body length. This can be influenced by the animals through the use of the circular and longitudinal muscles: If the former is contracted, the body cross-section is reduced and the corresponding segment expands because its volume must remain constant due to the incompressible fluid contained in the hemocoeloma - this is the usual mode of action Hydroskeleton as it is used in various worms . As a result of the stretching, the legs of the affected part of the body lift and swing forward. Local contraction of the longitudinal muscles, on the other hand, shortens the corresponding segment and in this way moves the associated pair of legs, which are now in contact with the substrate, backwards; this part of the movement cycle is the effective kick, which is responsible for the actual movement. The individual muscle stretches and relaxations of the segments are coordinated by the nervous system in such a way that contraction waves run through the body, which cause one pair of legs to swing forwards after the other and then move backwards on the ground. The speeds achieved in this way vary between about one millimeter and a little more than one centimeter per second.
nutrition
Poles are predatory organisms that are able to prey on animals that are considerably larger than themselves. The range of prey includes woodlice (Isopoda), termites (Isoptera), real crickets (Gryllidae), dust lice (Psocoptera), cockroaches ( Blattodea), millipedes (Myriapoda), spiders (Araneae), various worms and even larger snails (Gastropoda). They are considered the ecological equivalent of the centipedes (Chilopoda).
They usually track down potential victims with the help of the antennas and then follow them down to the smallest cracks. While smaller prey is killed directly, in larger animals they use a white, glue-like and high- protein liquid that is produced by the two mucous glands to immobilize their victims. It is sprayed through the pores of the oral papillae over a distance of up to 30 centimeters and hardens very quickly in air, so that the prey gets caught in the sticky substance and ultimately becomes immobile. On the other hand, the substance does not stick to the water-repellent skin of the velvet worms themselves, so that the animals can safely approach their prey at the shortest possible distance. By injecting toxic saliva it is now killed and pre-digested; the sharp jaws then cut the prey into fine pieces that pass through the mouth into the digestive tract.
Predators and parasites
Important predators of the animals are primarily various spiders (Araneae) and centipedes (Chilopoda), but also rodents (Rodentia) and birds , such as the yellow owl ( Turdus grayi ) in Central America . The snake species Micrurus hemprichii, which belongs to the coral otters ( Micrurus ) , even feeds almost exclusively on velvet worms. As a defense, some species reflexively curl up in a spiral; In addition, the animals can defend themselves against smaller opponents by expelling slime.
Various mites (Acari) are known as ectoparasites , which attach themselves to the skin of the anthropods. In addition, skin injuries usually lead to bacterial infections , which are almost always fatal.
Reproduction and life cycle
Colobus reproduce sexually throughout . The only known exception is the species Epiperipatus inthurni , in which no males have been detected so far and which reproduces parthenogenetically .
In many cases, the species show a noticeable sexual dimorphism - the females are usually larger than the males and, if the number of legs within a species is variable, also have more legs. In many species, they are only inseminated once in a lifetime, which has led to the fact that mating sometimes takes place when the reproductive organs of the females are not yet mature, for example in Macroperipatus torquatus at the age of three months. In this case, the transferred sperm are stored in a special storage organ where they can remain fertile for longer periods of time.
In general, the pollutants are fertilized internally, but the type of semen transfer varies considerably. In most species, such as those of the genus Peripatus , a sperm packet, the spermatophore , is placed in the sexual opening of the female. How this happens in detail is in most cases still unknown, a real penis has only been proven in species of the genus Paraperipatus . Numerous Australian species have dimples or special stiletto or hook-shaped structures on the head; in Florelliceps stutchburyae, for example, the male presses a long thorn against the female's genital opening and very likely positions his spermatophores there in this way. It is supported by the female, who is holding the male with the claws of the last pair of legs. The sperm transfer is particularly unusual in two species of the genus Peripatopsis . Here the male places two millimeter-sized spermatophores on the back or the flanks of the female. Amebocytes from the female's blood then collect on the inside of the deposition site and, through the secretion of enzymes, break down the body wall underneath the sperm packet and the shell of the spermatophore. The sperm released as a result then move freely through the haemocoel to the ovaries, penetrate the outer wall and finally fertilize the egg cells. It is still not understood why the self-inflicted skin damage does not lead to bacterial infections.
In Stummelfüßern can be found both egg-laying ( oviparous ), eggs viviparous ( ovoviviparous ) and viviparous ( viviparous ) forms.
- The former are only found in the family of the Peripatopsidae , often in regions with unsteady food supplies or changeable climates. The approximately 1.3 to 2 millimeters large, yolk-containing eggs are in this case covered by a protective eggshell made of chitin; maternal care is unknown.
- The majority of the species is viviparous: the medium-sized eggs, only covered by a membrane, remain in the "uterus", the uterus, but the embryos feed on the moderate amount of yolk contained in the eggs and are not separated from the mother supplied, they are therefore also called lecitotrophic ; they only hatch shortly before birth. Presumably this represents the original mode of reproduction of the velvet worms, that is, both oviparous and viviparous species have developed from the ovoviviparous forms.
- Genuine viviparous species can be found in both families, especially in tropical regions with a stable climate throughout the year and an even food supply. The embryos, which develop from the micrometer-sized eggs, are nourished in the uterus by their mother; they are therefore called matrotrophic . The supply takes place either through maternal secretion into the uterus or through a real tissue connection between the uterine epithelium and the growing embryo, a so-called placenta . The former occurs only outside of the American continent, the latter is found primarily in America and the Caribbean , less often in the ancient world. The gestation period, at the end of which the young are largely developed, can be up to 15 months. The embryos found in the uterus of a female do not necessarily have to be of the same age; It is quite possible that there are numerous differently developed animals that also descend from different males.
The number of offspring of a female varies between 1 and 23 per year; the development from the fertilized egg cell to the adult stage, which lasts between 6 and 17 months, basically takes place directly without the larval stage; this is probably also the original development mode. Poles can live up to 6 years.
Danger
It is difficult to assess the global endangerment of the colobus diversity; many species are only known from the type locality, i.e. the place from which the first described specimen came. Low population densities, the mostly nocturnal way of life of the animals, possibly as yet unrecognized seasonal influences and gender-specific differences make the collection of trustworthy information even more difficult.
So far, only eleven species have been examined in sufficient detail to be able to provide information about their frequency. The International Union for the Conservation of Nature and Natural Resources lists with the 2009 edition on the Red List as follows:
- three species as critically endangered, whereby one species is presumably already extinct,
- two species as endangered ("Endangered"),
- four species as endangered ("Vulnerable"),
- one species was classified as endangered and is now listed as having insufficient data ("data deficient");
- a Caribbean species classified as "Near Threatened".
The main cause of the threat is the loss and fragmentation of habitat due to increasing industrialization, the drainage of wetlands and slash and burn for agricultural purposes. Many species also have a naturally low population density and a geographically narrow distribution area, which means that even minor local disturbances of the ecosystems affected can lead to the extinction of entire populations and possibly species. Collecting animals for universities or research institutes also plays a regional role.
The protection of the velvet worms is very different from region to region; in some countries such as South Africa there are collective restrictions and export controls, in others such as Australia only the latter; in many countries no specific protective measures are known at all. A protection program in Tasmania is unique in the world : In one of the forest districts there, a separate “ velvet worm conservation plan ” is being used, which is specially tailored to a species of velvet worm .
It has not yet been possible to breed columbus. Corresponding considerations exist not only with the ulterior motive of being able to provide populations for later reintroduction, but also to better educate the public concerned about the animals and the need to protect them. Due to the often bright colors and the great importance for the history of evolution, an attitude in “insect zoos” would be an option.
Tribal history
Modern forms
Among the present-day forms, the velvet worms are probably very closely related to the arthropods, a very extensive taxon that includes crustaceans (Crustacea), insects (Insecta) and arachnids (Arachnida). They share with them, among other things, the skinned exoskeleton made up of α-chitin and non-collagenous proteins, the gonads and excretory organs surrounded by real coelomic tissue, the open blood system with the tubular heart on the back, a body cavity divided into pericardial and perivisceral cavities, breathing through trachea and a similar embryonic development. In addition, there is segmentation with two body attachments per segment. Antennae, mandibles and oral papillae, on the other hand, are presumably not homologous to the corresponding body parts of the arthropods, that is, they have evolved independently of these.
Another closely related group are the relatively obscure tardigrades (tardigrada), which, however, due to their very small size, lack a number of features of the columbus and arthropods, such as blood circulation and separate respiratory structures. Colibuses, arthropods and tardigrades together form a monophyletic taxon, the Panarthropoda, which means that all three groups taken together include all descendants of their last common ancestor.
Within the Panarthropoda, the stumpworms were usually grouped together to form a taxon Protoarthropoda due to a certain similarity in shape with the tardigrade. This designation was intended to express that both columbus and tardigrade have not yet reached the full developmental level of arthropods. Modern systematic theories reject such notions of “primitive” and “highly developed” forms of life and instead orientate themselves exclusively to the phylogenetic relationships of the taxa involved. These have not yet been fully clarified; However, it is considered to be relatively likely that the sister group of the stumpworts is formed by a taxon that includes both tardigrade and arthropod and is called the Tactopoda (but sometimes also Tritocerebra):
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For a long time the annelid worms (Annelida) were placed in the further relationship of the velvet worms . They are linked to, among other things, the worm-like body, the thin and flexible outer skin, layered muscles, paired excretory organs and a simply built brain and simple eyes. The decisive factor, however, was the segmentation existing in both groups, which shows little specialization of the individual segments. The annex worms' body appendages, called parapodia , correspond to the limbs of the velvet worms.
In the Articulata concept coined by Georges Cuvier , the columbus is therefore considered to be an evolutionary link between annelids and arthropods: worm-like precursors first developed parapodia, which then developed via stubby legs as an intermediate link to the body appendages of the arthropods. The columbus themselves were consequently considered to be living fossils because of their structural conservatism . This view is paradigmatically expressed in the statement of 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.
- Onychophores can be viewed as highly developed annelids, adapted to rural life, which prophetically heralded the appearance of the arthropod. They are a side branch that has survived until today without any relevant changes.
Modern systematics endeavors to leave out criteria such as a “higher” or “lower” stage of development of the living beings under consideration, as well as distinctions between main and side branches - it only considers the relationships established on the basis of cladistic methodology to be real. From this point of view, the segmented body in all animal groups concerned, the presence of paired segment appendages, the paired arrangement of excretory organs in each segment and, above all, a rope ladder-like nervous system based on a ventrally located double nerve cord still speak in favor of the Articulata concept.
The representatives of an alternative and now mostly preferred concept, the so-called Ecdysozoa hypothesis, on the other hand, place annelid worms and panarthropoda in two very different large groups, the Lophotrochozoa on the one hand and the molting animals (Ecdysozoa) on the other. Since the concept was first drawn up, numerous additional data have been added to support it. Due to e.g. B. the similar expression of the Hox genes , the genes for the organization of the extremities and confirmed by a molecular family tree based on the mitochondrial DNA, there is hardly any doubt about the position of the stumpworms within the Ecdysozoa.
The aforementioned common features of annelids and columbus are either convergent or “primitive” features, so-called symplesiomorphies , which both affected groups have adopted unchanged from the common ancestors of Lophotrochozoa and molting animals. In the first case, the rope ladder system would have developed independently twice in the course of evolution, in the second it was a very old characteristic that cannot establish a particularly close relationship between annelid worms and Panarthropoda.
The ecdysozoan concept sees the extended family of the velvet worms in a Cycloneuralia called taxon to the one next thread (Nematoda) and string worms (Nematomorpha) three rather obscure groups kinorhyncha (Kinorhyncha) priapulida (Priapulida) and loricifera (Loricifera) counts.
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Particularly characteristic of the Cycloneuralia is a circumoral nerve ring around the mouth opening, which proponents of the Ecdysozoa concept recognize in a modified form in the innervation pattern , i.e. the details of the nerve arrangement, of the Panarthropoda. In addition, there is a moulting mechanism common to all groups , ecdysis , and similarities found in molecular biology. One problem with the Ecdysozoa hypothesis is the subterminal position of the mouth of the velvet worms: In contrast to the Cycloneuralia, the mouth is not located at the front end of the body, but lies behind it on the abdomen. There are, however, through developmental studies, especially with regard to the origin of the cranial nerves, evidence that this was not always the case and the mouth original terminal was so created terminal. This is supported by the fossil record.
Extinct forms
The stratigraphic range of the velvet worms includes fossils one of three different time periods, including the early periods of the Paleozoic Cambrian and Ordovician , the later Carboniferous and the Tertiary in the Cenozoic . The Cambrian and Ordovician forms, whose association with the stumpworts is disputed, all still lived in the sea, while the later species are already considered terrestrial.
Traditionally, all fossil forms are placed in a separate taxon Xenusia, while the modern ones are called Euonychophora. However, this classification says nothing about the actual phylogenetic relationships, which is why it is not accepted by cladist taxonomists.
Lobopods
The oldest almost completely preserved fossils come from the Lower Cambrian. The most noticeable features that these lobopods share with the modern columbus are the worm-like body shape and numerous stub legs. A special feature of many lobopods, on the other hand, are numerous armor plates, the sclerites , which often cover the back of the whole body including the head; since they end in long, pointed back spines, they most likely had a defensive function to protect against predators. Individual sclerites can be detected as part of the so-called “ Small Shelly Fauna ” (SSF) even before the onset of the Cambrian 543 million years ago - accordingly, the lobopods or their immediate ancestors developed and were already in the Ediacarian of the Proterozoic thus the first animals to walk on legs.
Aysheaia is one of the better-known genera , which is known from the Canadian Burgess slate and which of all lobopods looks most similar to the modern velvet worms - a pair of head appendages can be interpreted as the forerunners of the antennae. Xenusion was apparently able to curl up with the spines facing outwards, which allows insight into the defense strategy of the lobopods. By far the most famous lobopod genus is Hallucigenia . It got its scientific name from the paleontologist Simon Conway Morris because of its bizarre appearance. Reconstructed by him with long, stilt-shaped legs and enigmatic fleshy back protrusions, it has long been considered a prime example of the fact that nature in the Cambrian experimented with the most diverse and bizarre body plans. However, further finds showed that the reconstruction had reversed above and below; interpreting the "stilts" as spines on the back it became clear that the fleshy outgrowths on the back were actually legs. The second reconstruction, however, swapped the front and back of the animal; only after further investigation did it emerge that Conway-Morris had correctly oriented the fossils in this regard.
Lobopod fossils can now be found in North America, China and North or Northeast Europe. In the Cambrian, however, due to the continental drift , the sites were much further south in the tropical and temperate belt of the earth and at that time corresponded to flat lagoon waters.
The way of life of the lobopods is largely in the dark, some species apparently a carnivorous diet , i.e. ate other animals such as sponges (Porifera) or echinoderms (Holothuroidea) of the genus Eldonia . It has been proven that they shed their skin like modern velvet worms.
While they made up a considerable proportion of the biodiversity in the Cambrian, it was drastically reduced in the Ordovician; At the latest with the onset of the Silurian mountains , the lobopods are considered extinct in the narrower sense. The American paleontologist Stephen Jay Gould sees this as a symptom of a large-scale collapse of the species richness of the Cambrian, which heralded an end to the "experimental phase", but his theses are considered controversial.
Lobopods are traditionally associated with the velvet worms because of their very similar morphology; according to this view they were members of the lineage of this taxon:
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Occasionally the scientific name Onychophora only designates the land-living forms, while the larger group, which also includes the various lobopods, is then referred to as the lobopodia.
An important alternative view assumes that the lobopods were instead members of the lineage of the Panarthropoda and Tactopoda:
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The genus Aysheaia , which has a particularly large number of characteristics in common with the modern velvet worms, therefore forms the sister group of all other Panarthropoda, which is then divided into the velvet worms and all other forms. The remaining Lobopod genera are now considered to be representatives of the line of development that leads to the modern tardigrade and arthropod. From the cladistic analysis, a trend towards increased external segmentation and a coordination of the fine body and leg ringing with the position of the segment boundaries can be determined.
However, the result is difficult to reconcile with the Articulata hypothesis because it means that the pronounced segmentation of annelids and arthropods must have arisen independently of one another. For the velvet worms this would mean that the lack of clearly recognizable external segmentation is an original, "primitive" and not a secondary characteristic. The second hypothesis on the systematic position of the lobopods is therefore much more compatible with the Ecdysozoa concept mentioned above.
Rural forms
Four terrestrial forms of the velvet worms are known for which, in contrast to the lobopods, there is no great doubt about the systematic position.
When the transition to land took place is unknown; It is considered plausible that it took place in the intertidal zone between the Ordovician and the late Silurian , i.e. about 490 to 420 million years ago . The typical armor of the lobopods, if it was ever present in the ancestors of modern species, could have been lost at this point in time, perhaps in order to be able to move better in narrow habitats due to the flexibility gained. In the intertidal zone, the various adaptations to rural life could have developed, such as the trachea, which would therefore have arisen independently of those of the insects, or the internal fertilization mode, which enables the sperm cells to be transferred to a life-friendly environment even when there is a lack of water. One can only speculate about the point in time at which the nephridia were converted into salivary and genital auxiliary glands; In the case of the mucous glands used to catch prey, it is known that they are inoperable under water, from which it can be carefully concluded that they probably first developed on land, perhaps originally to ward off predators.
Wherever the transition ultimately took place, it did not appear to be an environment conducive to fossilization; in any case, not a single fossil exists from this early period. The species Helenodora inopinata , known from the tropical belt of the late Carboniferous, was very likely already living on land and differs only little from the modern forms. For this reason and the still great resemblance to lobopods like Aysheaia , pebbles are a prime example of evolutionary stasis and a Bradytelian rate of evolution in which the biological organization of the entire organism changes only very slowly because strong stabilizing selection moves the entire line of development in a narrow corridor morphological-anatomical "space" is limited and larger deviations from the "typical" columbus shape are not tolerated.
From the Mesozoic only a fossil is known, the Cretaceous kind cretoperipatus burmiticus that in amber from Southeast Asia Myanmar has been found and is dated to 100 million years ago. It can even be assigned to one of the modern families, the Peripatidae .
Since today's velvet worms have a so-called Gondwana distribution, there is much to suggest that their last common ancestor lived on this very primordial continent. The existence of Succinipatopsis balticus , a 44 million year old species that has survived in Baltic amber from the period of the lower Eocene , proves that land- dwelling columbus also existed outside of Gondwana or today's remnants of this continent . The range of the velvet worms must once have been much larger than it is today - when and for what reason all non-Gondwana species became extinct is unknown.
The fourth fossil species is Tertiapatus dominicanus , known from amber from the Caribbean island of Hispaniola . It is possible, but not provable, that she already belonged to one of the two modern families, the Peripatidae. The species can be dated to an age of 17 to 20 million years.
The mucous glands can be clearly seen on both fossils of the Tertiary, which must have formed at this point in time at the latest, but probably much earlier. Formally, the two tertiary forms are combined in a taxon Tertiapatoidea, which is probably only one form taxon.
Systematics
The modern velvet worms form a so-called monophyletic group, i.e. they include all descendants of their common ancestor. Important common derived features ( synapomorphies ) are, for example, the jaw tools (mandibles) of the second and the oral papillae with associated mucous glands of the third body segment, ventral nerve cords with numerous cross connections per segment and the special shape of the trachea .
Up to 2004 about 155 modern species were described, assigned to 47 genera; however, the actual number of species is probably about twice as large. The best known is the type genus Peripatus , which was described as early as 1825 and which in English-speaking countries is also representative of all velvet worms.
All genera are assigned to one of two families , the distribution areas of which do not overlap, but are separated from each other by arid areas or seas.
- The Peripatopsidae have a relatively large number of characteristics that are considered to be original or “primitive”. They have 13 to 25 pairs of legs; behind or between the last of them is the genital opening ( gonopore ). Both egg-laying ( oviparous ), egg- bearing ( ovoviviparous ) and genuinely live-bearing ( viviparous ) species occur, but Peripatopsidae generally do not have a placenta. Its distribution area is around Australia and Tasmania, New Zealand, New Guinea, South Africa and Chile.
- The Peripatidae show a number of inferred characteristics. With a greater average length than the Peripatopsidae, they also have more pairs of legs: Their number varies within the group between 22 and 43; the gonopore always lies between the penultimate one. There are no oviparous species among the Peripatidae, the vast majority of them are viviparous. In many of the females of the latter group, a placenta forms, which supplies the growing embryo with nutrients. Occurrences of the Peripatidae are limited to the tropical and subtropical zones; They occur in detail in Central and Northern South America and on several Caribbean islands, as well as in West Africa, North India, Malaysia and individual Indonesian islands.
swell
literature
- H. Ruhberg: Onychophora, velvet worms . in: W. Westheide, R. Rieger: Special Zoology. Part 1. Protozoa and invertebrates. Ulmer, Stuttgart 1974, Elsevier, Munich 2007, ISBN 3-8001-2429-7 , ISBN 3-8274-1575-6
- Edward E. Ruppert, RS Fox, RD Barnes: Invertebrate Zoology: A functional evolutionary approach . Chapter 15. Brooks / Cole, London 2004, p. 505, ISBN 0-03-025982-7
- RC Brusca, GJ Brusca: Invertebrates. Chapter 15. Sinauer Associates, Sunderland Mass 2003, p. 463, ISBN 0-87893-097-3
Scientific literature
- GE Budd: The morphology of Opabinia regalis and the reconstruction of the arthropod stem group in: Lethaia. Taylor & Francis, Oslo 29.1996, p. 1. ISSN 0024-1164
- J. Eriksson: Evolution and Development of the Onychophoran Head and Nervous System. in: Comprehensive summaries of Uppsala dissertations from the Faculty of Science and Technology. Upsala 833, 2003. ISSN 1104-232X
- X. Hou, J. Bergström: Cambrian lobopodians - ancestors of extant onychophorans. in: Zoological Journal of the Linnean Society. Blackwell, Oxford 114.1995, p. 3. ISSN 0024-4082
- J. Monge-Najera: Phylogeny, biogeography and reproductive trends in the onychophora. in: Zoological Journal of the Linnean Society. Blackwell, Oxford 114.1995, p. 21. ISSN 0024-4082
- TR New: Onychophora in invertebrate conservation: priorities, practice and prospects. in: Zoological Journal of the Linnean Society. Blackwell, Oxford 114.1995, p. 77. ISSN 0024-4082
- G. Poinar: Fossil onychophorans from Dominican and Baltic Amber: Tertiapatus dominicanus ng, n.sp. (Tertiapatidae n. Fam.) And Succinipatopsis balticus ng, n. Sp. (Succinipatopsidae n. Fam.) With a proposed classification of the subphylum Onychophora. in: Invertebrate Biology. Blackwell, Malden Mass 119.2000, p. 104. ISSN 1077-8306
- G. Poinar: Fossil Velvet Worms in Baltic and Dominican Amber: Onchychophoran Evolution and Biogeography. in: Science . Moses King, Cambridge Mass 273.1996, p. 1370. ISSN 0096-3771
- CJ Wedeen, RG Kostriken, D. Leach, P. Whitington: Segmentally iterated expression of an engrailed-class gene in the embryo of an Australian onychophoran. in: Development Genes and Evolution. Springer, Heidelberg 207.1997,4, p. 282. ISSN 0949-944X
Individual evidence
- ↑ Georg Mayer & Steffen Hartzsch (2007): Immunolocalization of serotonin in Onychophora argues against segmental ganglia being an ancestral feature of arthropods. BMC Evolutionary Biology 7: 118 doi : 10.1186 / 1471-2148-7-118 http://www.biomedcentral.com/1471-2148/7/118
- ^ Georg Mayer, Paul M. Whitington, Paul Sunnucks, Hans-Joachim Pflüger (2010): A revision of brain composition in Onychophora (velvet worms) suggests that the tritocerebrum evolved in arthropods. BMC Evolutionary Biology 10: 255. http://www.biomedcentral.com/1471-2148/10/255
- ^ Georg Mayer (2006): Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods? Arthropod Structure & Development Volume 35, Issue 4: 231–245. doi : 10.1016 / j.asd.2006.06.003
- ↑ J. Monge-Najera, Phylogeny, biogeography and reproductive trends in the onychophora, Zoological Journal of the Linnean Society, 114, 1995, p. 21
- ↑ Jennifer K. Grenier, Theodore L. Garber, Robert Warren, Paul M. Whitington, Sean Carroll (1997): Evolution of the entire arthropod Hox gene set predated the origin and radiation of the onychophoran / arthropod clade. Current Biology 7: 547-553. doi : 10.1016 / S0960-9822 (06) 00253-3
- ↑ Janssen, R., Eriksson, BJ, Budd, GE, Akam, M., Prpic, N.-M. (2010): Gene expression patterns in an onychophoran reveal that regionalization predates limb segmentation in panarthropods. Evolution & Development 12: 363-372. doi : 10.1111 / j.1525-142X.2010.00423.x
- ↑ Lars Podsiadlowski, Anke Braband, Georg Mayer (2008): The complete mitochondrial genome of the Onychophoran Epiperipatus biolleyi reveals a unique transfer RNA set and provides further support for the Ecdysozoa hypothesis. Molecular Biology and Evolution 25 (1): 42-51. doi : 10.1093 / molbev / msm223
Web links
- Onychophora website , with information from scientific sources (English)
