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Due to the usually poor preservation, detailed reconstructions of the head region are only available for a handful of lobopodian species. The head of a lobopodian is more or less bulbous, and sometime possesses a pair of pre-ocular, presumely protocerebral appendages – for example, primary antennae or well-developed frontal appendages, which are individualized from the trunk lobopods (with the exception of Antennacanthopodia, which have two pairs of head appendages instead of one). Mouthparts may consist of rows of teeth or a conical proboscis. The eyes may be represented by a single ocellus or by numerous pairs of simple ocelli, as has been shown in Luolishania (=Miraluolishania), Ovatiovermis, Onychodictyon, Hallucigenia, Facivermis, and less certainly Aysheaia as well. However, in gilled lobopodians like Kerygmachela, the eyes are relatively complex reflective patches that may had been compound in nature.
Trunk and lobopods
The trunk is elongated and composed of numerous body segments (somites), each bearing a pair of legs called lobopods or lobopodous limbs. The segmental boundaries are not as externally significant as those of arthropods, although they are indicated by heteronomous annulations (i.e., the alternation of annulation density corresponding to the position of segmental boundaries) in some species. The trunk segments may bear other external, segment-corresponding structures such as nodes (e.g. Hadranax, Kerygmachela), papillae (e.g. Onychodictyon), spine/plate-like sclerites (e.g. armoured lobopodians) or lateral flaps (e.g. gilled lobopodians). The trunk may terminate with a pair of lobopods (e.g. Aysheaia, Hallucigenia sparsa) or a tail-like extension (e.g. Paucipodia, Siberion, Jianshanopodia). | Lobopodia | Wikipedia | 462 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
The lobopods are flexible and loosely conical in shape, tapering from the body to tips that may or may not bear claws. The claws, if present, are hardened structures with a shape resembling a hook or gently-curved spine. Claw-bearing lobopods usually have two claws, but single claws are known (e.g. posterior lobopods of luolishaniids), as are more than two (e.g. three in Tritonychus, seven in Aysheaia) depending on its segmental or taxonomical association. In some genera, the lobopods bear additional structures such as spines (e.g. Diania), fleshy outgrowths (e.g. Onychodictyon), or tubercules (e.g. Jianshanopodia). There is no sign of arthropodization (development of a hardened exoskeleton and segmental division on panarthropod appendages) in known members of lobopodians, even for those belonging to the arthropod stem-group (e.g. gilled lobopodians and siberiids), and the suspected case of arthropodization on the limbs of Diania is considered to be a misinterpretation.
Differentiation (tagmosis) between trunk somites barely occurs, except in hallucigenids and luolishaniids, where numerous pairs of their anterior lobopods are significantly slender (hallucigenids) or setose (luolishaniids) in contrast to their posterior counterparts.
Internal structures | Lobopodia | Wikipedia | 333 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
The gut of lobopodians is often straight, undifferentiated, and sometimes preserved in the fossil record in three dimensions. In some specimens the gut is found to be filled with sediment. The gut consists of a central tube occupying the full length of the lobopodian's trunk, which does not change much in width - at least not systematically. However, in some groups, specifically the gilled lobopodians and siberiids, the gut is surrounded by pairs of serially repeated, kidney-shaped gut diverticulae (digestive glands). In some specimens, parts of the lobopodian gut can be preserved in three dimensions. This cannot result from phosphatisation, which is usually responsible for 3-D gut preservation, because the phosphate content of the guts is under 1%; the contents comprise quartz and muscovite. The gut of the representative Paucipodia is variable in width, being widest at the centre of the body. Its position in the body cavity is only loosely fixed, so flexibility is possible.
Not much is known about the neural anatomy of lobopodians due to the spare and mostly ambiguous fossil evidence. Possible traces of a nervous system were found in Paucipodia, Megadictyon and Antennacanthopodia. The first and so far the only confirmed evidence of lobopodian neural structures comes from the gilled lobopodian Kerygmachela in Park et al. 2018 — it presents a brain composed of only a protocerebrum (the frontal-most cerebral ganglion of panarthropods) that is directly connected to the nerves of eyes and frontal appendages, suggesting the protocerebral ancestry of the head of lobopodians as well as the whole Panarthropoda.
In some extant ecdysozoan such as priapulids and onychophorans, there is a layer of outermost circular muscles and a layer of innermost longitudinal muscles. The onychophorans also have a third, intermediate, layer of interwoven oblique muscles. Musculature of the gilled lobopodian Pambdelurion shows a similar anatomy, but that of the lobopodian Tritonychus shows the opposite pattern: it is the outermost muscles that are longitudinal and the innermost layer that consists of circular muscles.
Categories | Lobopodia | Wikipedia | 492 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Based on external morphology, lobopdians may fall under different categories — for example the general worm-like taxa as "xenusiid" or "xenusian"; xenusiid with sclerite as "armoured lobopodians"; and taxa with both robust frontal appendages and lateral flaps as "gilled lobopodians". Some of them were originally defined under a taxonomic sense (e.g. class Xenusia), but neither any of them are generally accepted as monophyletic in further studies.
Armoured lobopodians
Armoured lobopodians referred to xenusiid lobopodians which bore repeated sclerites such as spine or plates on their trunk (e.g. Hallucigenia, Microdictyon, Luolishania) or lobopods (e.g. Diania). In contrast, lobopodians without sclerites may be referred to as "unarmoured lobopodians". Function of the sclerites were interpreted as protective armor and/or muscle attachment points. In some cases, only the disarticulated sclerites of the animal were preserved, which represented as component of small shelly fossils (SSF). Armoured lobopodians were suggest to be onychophoran-related and may even represent a clade in some previous studies, but their phylogenetic positions in later studies are controversial. (see text)
Gilled lobopodians | Lobopodia | Wikipedia | 307 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Dinocaridids with lobopodian affinities (due to shared features like annulation and lobopods) are referred to as "gilled lobopodians" or "gilled lobopods". These forms sport a pair of flaps on each trunk segment, but otherwise no signs of arthropodization, in contrast to more derived dinocaridids like the Radiodonta that have robust and sclerotized frontal appendages. Gilled lobopodians cover at least four genera: Pambdelurion, Kerygmachela, Utahnax and Mobulavermis. Opabinia may also fall under this category in a broader sense, although the presence of lobopods in this genus is not definitively proven. Omnidens, a genus known only from Pambdelurion-like mouthparts and distal parts of the frontal appendages, may also be a gilled lobopodian. The body flaps may have functioned as both swimming appendages and gills, and are possibly homologous to the dorsal flaps of radiodonts and exites of Euarthropoda. Whether these genera were true lobopodians is still contested by some. However, they are widely accepted as stem-group arthropods just basal to radiodonts.
Siberion and similar taxa
Siberion, Megadictyon and Jianshanopodia may be grouped as siberiids (order Siberiida), jianshanopodians or "giant lobopodians" by some literatures. They are generally large — body length ranging between 7 and 22 centimeters (2¼ to 8⅔ inches) — xenusiid lobopodians with widen trunk, stout trunk lobopods without evidence of claws, and most notably a pair of robust frontal appendages. With the possible exception of Siberion, they also have digestive glands like those of a gilled lobopodian and basal euarthropod. Their anatomy represent transitional forms between typical xenusiids and gilled lobopodians, eventually placing them under the basalmost position of arthropod stem-group.
Paleoecology | Lobopodia | Wikipedia | 455 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Lobopodians possibly occupied a wide range of ecological niches. Although most of them had undifferentiated appendages and straight gut, which would suggest a simple sediment-feeding lifestyle, sophisticated digestive glands and large size of gilled lobopodians and siberiids would allow them to consume larger food items, and their robust frontal appendages may even suggest a predatory lifestyle. On the other hand, luolishaniids such as Luolishania and Ovatiovermis have elaborate feather-like lobopods that presumably formed 'baskets' for suspension or filter-feeding. Lobopods with curved terminal claws may have given some lobopodians the ability to climb on substrances.
Not much is known about the physiology of lobopodians. There is evidence to suggest that lobopodians moult just like other ecdysozoan taxa, but the outline and ornamentation of the harden sclerite did not vary during ontogeny. The gill-like structures on the body flaps of gilled lobopodians and ramified extensions on the lobopods of Jianshanopodia may provide respiratory function (gills). Pambdelurion may control the movement of their lobopods in a way similar to onychophorans.
Distribution
During the Cambrian, lobopodians displayed a substantial degree of biodiversity. One species is known from each of the Ordovician and Silurian periods, with a few more known from the Carboniferous (Mazon Creek) — this represents the paucity of exceptional lagerstatten in post-Cambrian deposits.
Phylogeny
The overall phylogenetic interpretation on lobopodians has changed dramatically since their discovery and first description. The reassignments are not only based on new fossil evidence, but also new embryological, neuroanatomical, and genomic (e.g. gene expression, phylogenomics) information observed from extant panarthropod taxa. | Lobopodia | Wikipedia | 409 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Based on their apparently onychophoran-like morphology (e.g. annulated cuticle, lobopodous appendage with claws), lobopodians were originally thought to be present a group of paleozoic onychophorans. This interpretation was challenged after the discovery of lobopodians with arthropod and tardigrade-like characteristics, suggesting that the similarity between lobopodians and onychophorans represents deeper panarthropod ancestral traits (plesiomorphies) instead of onychophoran-exclusive characteristics (synapomorphies). For example, The British palaeontologist Graham Budd sees the Lobopodia as representing a basal grade from which the phyla Onychophora and Arthropoda arose, with Aysheaia comparable to the ancestral plan, and with forms like Kerygmachela and Pambdelurion representing a transition that, via the dinocaridids, would lead to an arthropod body plan. Aysheaia's surface ornamentation, if homologous with palaeoscolecid sclerites, may represent a deeper link connecting it with cycloneuralian outgroups. Lobopodians are paraphyletic, and include the last common ancestor of arthropods, onychophorans and tardigrades.
Stem-group arthropods | Lobopodia | Wikipedia | 299 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Compared to other panarthropod stem-groups, suggestion on the lobopodian members of arthropod stem-group is relatively consistent — siberiid like Megadictyon and Jianshanopodia occupied the basalmost position, gilled lobopodians Pambdelurion and Kerygmachela branch next, and finally lead to a clade compose of Opabinia, Radiodonta and Euarthropoda (crown-group arthropods). Their positions within arthropod stem-group are indicated by numerous arthropod groundplans and intermediate forms (e.g. arthropod-like digestive glands, radiodont-like frontal appendages and dorso-ventral appendicular structures link to arthropod biramous appendages). Lobopodian ancestry of arthropods also reinforced by genomic studies on extant taxa — gene expression support the homology between arthropod appendages and onychophoran lobopods, suggests that modern less-segmented arthropodized appendages evolved from annulated lobopodous limbs. On the other hand, primary antennae and frontal appendages of lobopodians and dinocaridids may be homologous to the labrum/hypostome complex of euarthropods, an idea support by their protocerebral origin and developmental pattern of the labrum of extant arthropods. | Lobopodia | Wikipedia | 308 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Diania, a genus of armoured lobopodian with stout and spiny legs, were originally thought to be associated within the arthropod stem-group based on its apparently arthropod-like (arthropodized) trunk appendages. However, this interpretation is questionable as the data provided by the original description are not consistent with the suspected phylogenic relationships. Further re-examination even revealed that the suspected arthropodization on the legs of Diania was a misinterpretation — although the spine may have hardened, the remaining cuticle of Diania's legs were soft (not harden nor scleritzed), lacking any evidence of pivot joint and arthrodial membrane, suggest the legs are lobopods with only widely spaced annulations. Thus, the re-examination eventually reject the evidence of arthropodization (sclerotization, segmentation and articulation) on the appendages as well as the fundamental relationship between Diania and arthropods.
Stem-group onychophorans
While Antennacanthopodia is widely accepted as a stem-group onychophoran, the position of other xenusiid genera that were previously thought to be onychophoran-related is controversial — in further studies, most of them were either suggested to be stem-group onychophorans or basal panarthropods, with a few species (Aysheaia or Onychodictyon ferox) occasionally suggested to be stem-group tardigrades. A study in 2014 suggested that Hallucigenia are stem-group onychophorans based on their claws, which have overlapped internal structures resembling those of an extant onychophoran. This interpretation was questioned by later studies, as the structures may be a panarthropod plesiomorphy.
Stem-group tardigrades | Lobopodia | Wikipedia | 401 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Lobopodian taxa of the tardigrade stem-group is unclear. Aysheaia or Onychodictyon ferox had been suggest to be a possible member, based on the high claw number (in Aysheaia) and/or terminal lobopods with anterior-facing claws (in both taxa). Although not widely accepted, there are even suggestions that Tardigrada itself representing the basalmost panarthropod or branch between the arthropod stem-group. However, a paper in 2023 found luolishaniids to be the closest relatives of tardigrades using various morphological characteristics.
Stem-group panarthropods
It is unclear that which lobopodians represent members of the panarthropod stem-group, which were branched just before the last common ancestor of extant panarthropod phyla. Aysheaia may have occupied this position based on its apparently basic morphology; while other studies rather suggest luolishaniid and hallucigenid, two lobopodian taxa which had been resolved as members of stem-group onychophorans as well.
Described genera
As of 2018, over 20 lobopodian genera have been described. The fossil materials being described as lobopodians Mureropodia apae and Aysheaia prolata are considered to be disarticulated frontal appendages of the radiodonts Caryosyntrips and Stanleycaris, respectively. Miraluolishania was suggested to be synonym of Luolishania by some studies. The enigmatic Facivermis was later revealed to be a highly specialized genus of luolishaniid lobopodians.
Acinocricus
Antennacanthopodia
Aysheaia
Carbotubulus
Cardiodictyon
Collinsium
Collinsovermis
Diania
Entothyreos
Facivermis
Fusuconcharium
Hadranax
Hallucigenia
Jianshanopodia
Kerygmachela?
Lenisambulatrix
Luolishania (=Miraluolishania)
Megadictyon
Microdictyon
Mobulavermis?
Omnidens?
Onychodictyon
Orstenotubulus
Ovatiovermis
Pambdelurion?
Parvibellus?
Paucipodia
Quadratapora
Siberion
Thanahita
Tritonychus
Utahnax?
Xenusion
Youti? | Lobopodia | Wikipedia | 497 | 43184 | https://en.wikipedia.org/wiki/Lobopodia | Biology and health sciences | Ecdysozoa | Animals |
Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size from , reaching in extreme cases, and in diameter. Horsehair worms can be discovered in damp areas, such as watering troughs, swimming pools, streams, puddles, and cisterns. The adult worms are free-living, but the larvae are parasitic on arthropods, such as beetles, cockroaches, mantises, orthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorphs often coil themselves in tight balls that resemble knots.
Description and biology
Nematomorphs possess an external cuticle without cilia. Internally, they have only longitudinal muscle and a non-functional gut, with no excretory, respiratory or circulatory systems. The nervous system consists of a nerve ring near the anterior end of the animal, and a ventral nerve cord running along the body.
Reproductively, they have two distinct sexes, with the internal fertilization of eggs that are then laid in gelatinous strings. Adults have cylindrical gonads, opening into the cloaca. The larvae have rings of cuticular hooks and terminal stylets that are believed to be used to enter the hosts. Once inside the host, the larvae live inside the haemocoel and absorb nutrients directly through their skin. Development into the adult form takes weeks or months, and the larva moults several times as it grows in size.
The adults are mostly free-living in freshwater or marine environments, and males and females aggregate into tight balls (Gordian knots) during mating. | Nematomorpha | Wikipedia | 413 | 43196 | https://en.wikipedia.org/wiki/Nematomorpha | Biology and health sciences | Ecdysozoa | Animals |
In Spinochordodes tellinii and Paragordius tricuspidatus, which have grasshoppers and crickets as their hosts, the infection acts on the infected host's brain. This causes the host insect to seek water and drown itself, thus returning the nematomorph to water. P. tricuspidatus is also remarkably able to survive the predation of their host, being able to wiggle out of the predator that has eaten the host. The nematomorpha parasite affects host Hierodula patelliferas light-interpreting organs so the host is attracted to horizontally polarized light. Thus the host goes into water and the parasite's lifecycle completes. Many of the genes the parasites use for manipulating their host have been acquired through horizontal gene transfer from the host genome.
There are a few cases of accidental parasitism in vertebrate hosts, including dogs, cats, and humans. Several cases involving Parachordodes, Paragordius, or Gordius have been recorded in human hosts in Japan and China.
Community ecology
Owing to their use of orthopterans as hosts, nematomorphs can be significant factors in shaping community ecology. One study conducted in a Japanese riparian ecosystem showed that nematomorphs can cause orthopterans to become 20 times more likely to enter water than non-infected orthopterans; these orthopterans constituted up to 60% of the annual energy intake for the Kirikuchi char. Absence of nematomorphs from riparian communities can thus lead to char predating more heavily on other aquatic invertebrates, potentially causing more widespread physiological effects.
Taxonomy
Nematomorphs can be confused with nematodes, particularly mermithid worms. Unlike nematomorphs, mermithids do not have a terminal cloaca. Male mermithids have one or two spicules just before the end apart from having a thinner, smoother cuticle, without areoles and a paler brown colour. | Nematomorpha | Wikipedia | 415 | 43196 | https://en.wikipedia.org/wiki/Nematomorpha | Biology and health sciences | Ecdysozoa | Animals |
The phylum is placed along with the Ecdysozoa clade of moulting organisms that include the Arthropoda. Their closest relatives are the nematodes. The two phyla make up the group Nematoida in the clade Cycloneuralia. During the larval stage, the animals show a resemblance to adult kinorhyncha and some species of Loricifera and Priapulida, all members of the group Scalidophora. The earliest Nematomorph could be Maotianshania, from the Lower Cambrian; this organism is, however, very different from extant species; fossilized worms resembling the modern forms have been reported from mid Cretaceous Burmese amber dated to 100 million years ago.
Relationships within the phylum are still somewhat unclear, but two classes are recognised. The five marine species of nematomorph are contained in Nectonematoida. This order is monotypic containing the genus Nectonema Verrill, 1879: adults are planktonic and the larvae parasitise decapod crustaceans, especially crabs. They are characterized by a double row of natotory setae along each side of the body, dorsal and ventral longitudinal epidermal cords, a spacious and fluid-filled blastocoelom and singular gonads.
The approximately 320 remaining species are distributed between two families, within the monotypic class Gordioida. Gordioidean adults are free-living in freshwater or semiterrestrial habitats and larvae parasitise insects, primarily orthopterans. Unlike nectonematiodeans, gordioideans lack lateral rows of setae, have a single, ventral epidermal cord and their blastocoels are filled with mesenchyme in young animals but become spacious in older individuals. | Nematomorpha | Wikipedia | 370 | 43196 | https://en.wikipedia.org/wiki/Nematomorpha | Biology and health sciences | Ecdysozoa | Animals |
Onychophora (from , , "claws"; and , , "to carry"), commonly known as velvet worms (for their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, Peripatus), is a phylum of elongate, soft-bodied, many-legged 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 to be much greater than that.
The two extant families of velvet worms are Peripatidae and Peripatopsidae. They show a peculiar distribution, with the peripatids being predominantly equatorial and tropical, while the peripatopsids are all found south of the equator. 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 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. | Onychophora | Wikipedia | 342 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Anatomy and physiology
Velvet worms are segmented animals with a flattened cylindrical body cross-section and rows of unstructured body appendages known as oncopods or lobopods (informally: stub feet). They reach lengths between 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 is often inconspicuously coloured orange, red or brown, but sometimes also bright green, blue, gold or white, and occasionally patterned with other colours. Segmentation is outwardly inconspicuous, and identifiable by the regular spacing of the pairs of legs and in the regular arrangement of skin pores, excretion organs and concentrations of nerve cells. The individual body sections are largely unspecialised; even the head develops only a little differently from the abdominal segments. Segmentation is apparently specified by the same gene as in other groups of animals, and is activated in each case, during embryonic development, at the rear border of each segment and in the growth zone of the stub feet. Although onychophorans fall within the protostome group, their early development has a deuterostome trajectory, (with the mouth and anus forming separately); this trajectory is concealed by the rather sophisticated processes which occur in early development.
Appendages
The stub feet that characterise the velvet worms are conical, baggy appendages of the body, which are internally hollow and have no joints. Although the number of feet can vary considerably between species, their structure is basically very similar. Rigidity is provided by the hydrostatic pressure of their fluid contents, and movement is usually obtained passively by stretching and contraction of the animal's entire body. However, each leg can also be shortened and bent by internal muscles. Due to the lack of joints, this bending can take place at any point along the sides of the leg. In some species, two different organs are found within the feet: | Onychophora | Wikipedia | 459 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Crural glands are situated at the shoulder of the legs, extending into the body cavity. They open outwards at the crural papillae—small wart-like bumps on the belly side of the leg—and secrete chemical messenger materials called pheromones. Their name comes from the Latin cruralis meaning "of the legs".
Coxal vesicles are pouches located on the belly side of the leg, which can be everted and probably serve in water absorption. They belong to the family Peripatidae and are named from , the Latin word for "hip".
On each foot is a pair of retractable, hardened (sclerotised) chitin claws, which give the taxon its scientific name: Onychophora is derived from the , , "claws"; and , , "to carry". At the base of the claws are three to six spiny "cushions" on which the leg sits in its resting position and on which the animal walks over smooth substrates. The claws are used mainly to gain a firm foothold on uneven terrain. Each claw is composed of three stacked elements, like Russian nesting dolls. The outermost is shed during ecdysis, which exposes the next element, which is fully formed and so does not need time to harden before it is used. This distinctive construction identifies many early Cambrian fossils as early offshoots of the onychophoran lineage. Apart from the pairs of legs, there are three further body appendages, which are at the head and comprise three segments: | Onychophora | Wikipedia | 321 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
On the first head segment is a pair of slender antennae, which serve in sensory perception. They probably do not correspond directly to the antennae of the Arthropoda, but perhaps rather with their "lips" or labrum. At their base is a pair of simple eyes, except in a few blind species. In front of these, in many Australian species, are various dimples, the function of which is not yet clear. It appears that in at least some species, these serve in the transfer of sperm-cell packages (spermatophores).
On the belly side of the second head segment is the labrum, a mouth opening surrounded by sensitive "lips". In the velvet worms, this structure is a muscular outgrowth of the throat, so, despite its name, it is probably not homologous to the labrum of the Arthropoda and is used for feeding. Deep within the oral cavity lie the sharp, crescent-shaped "jaws", or mandibles, which are strongly hardened and resemble the claws of the feet, with which they are serially homologous; early in development, the jaw appendages have a position and shape similar to the subsequent legs. The jaws are divided into internal and external mandibles and their concave surface bears fine denticles. They move backward and forward in a longitudinal direction, tearing apart the prey, apparently moved in one direction by musculature and the other by hydrostatic pressure. The claws are made of sclerotised α-chitin, reinforced with phenols and quinones, and have a uniform composition, except that there is a higher concentration of calcium towards the tip, presumably affording greater strength.
The surface of the mandibles is smooth, with no ornamentation. The cuticle in the mandibles (and claws) is distinct from the rest of the body. It has an inner and outer component; the outer component has just two layers (whereas body cuticle has four), and these outer layers (in particular the inner epicuticle) are dehydrated and strongly tanned, affording toughness.
Slime glands | Onychophora | Wikipedia | 439 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
On the third head segment, to the left and right of the mouth, are two openings called "oral papillae", with each containing a large, heavily branched slime gland. These slime glands lie roughly in the center of a velvet worm's body and secrete a sort of milky-white slime. The slime is used to both ensnare prey and act as a distraction for defensive purposes. In certain species, an organ connected to the slime gland known as the "slime conductor" is broadened into a reservoir, allowing it to hold pre-produced slime.
Velvet worm slime glands and oral papilla are likely modified and repurposed limbs. The glands themselves are probably modified crural glands. All three structures correspond to an evolutionary origin in the leg pairs of the other segments. | Onychophora | Wikipedia | 170 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Unlike the arthropods, velvet worms do not possess a rigid exoskeleton. Instead, their fluid-filled body cavity acts as a hydrostatic skeleton, similarly to many 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, the cuticula; a single layer of epidermis cells forming an internal skin; and beneath this, usually three layers of muscle, which are embedded in connective tissues. The cuticula is about a micrometer thick and covered with fine villi. In composition and structure, it resembles the cuticula of the arthropods, consisting of α-chitin and various proteins, although not containing collagen. It can be divided into an external epicuticula and an internal procuticula, which themselves consist of exo- and endo-cuticula. This multi-level structure is responsible for the high flexibility of the outer skin, which enables the velvet worm to squeeze itself into the narrowest crevices. Although outwardly water-repellent, the cuticula is not able to prevent water loss by respiration, and, as a result, velvet worms can live only in microclimates with high humidity to avoid desiccation. The surface of the cuticula is scattered with numerous fine papillae, the larger of which carry visible villi-like sensitive bristles. The papillae themselves are covered with tiny scales, lending the skin a velvety appearance (from which the common name is likely derived). It also feels like dry velvet to the touch, for which its water-repellent nature is responsible. Moulting of the skin (ecdysis) takes place regularly, around every 14 days, induced by the hormone ecdysone. The inner surface of the skin bears a hexagonal pattern. At each moult, the shed skin is replaced by the epidermis, which lies immediately beneath it; unlike the cuticula, this consists of living cells. Beneath this lies a thick layer of connective tissue, which is composed primarily of collagen fibres aligned either parallel or perpendicular to the body's longitudinal axis. The colouration of Onychophora is generated by a range of pigments | Onychophora | Wikipedia | 511 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
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 | Onychophora | Wikipedia | 67 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Within the connective tissue lie three continuous layers of unspecialised smooth muscular tissue. The relatively thick outer layer is composed of annular muscles, and the similarly voluminous inner layer of longitudinal muscles. Between them lie thin diagonal muscles that wind backward and forward along the body axis in a spiral. Between the annular and diagonal muscles exist fine blood vessels, which lie below the superficially recognisable transverse rings of the skin and are responsible for the pseudo-segmented markings. Beneath the internal muscle layer lies the body cavity. In cross-section, this is divided into three regions by so-called dorso-ventral muscles, which run from the middle of the underbelly through to the edges of the upper side: a central midsection and on the left and right, two side regions that also include the legs.
Circulation
The body cavity is known as a "pseudocoel", or haemocoel. Unlike a true coelom, a pseudocoel is not fully enclosed by a cell layer derived from the embryonic mesoderm. A coelom is, however, formed around the gonads and the waste-eliminating nephridia. As the name haemocoel suggests, the body cavity is filled with a blood-like liquid in which all the organs are embedded; in this way, they can be easily supplied with nutrients circulating in the blood. This liquid is colourless as it does not contain pigments; for this reason, it 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 a form suitable for elimination by the nephridia. | Onychophora | Wikipedia | 392 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The haemocoel is divided by a horizontal partition, the diaphragm, into two parts: The pericardial sinus along the back and the perivisceral sinus along the belly. The former encloses the tube-like heart, and the latter, the other organs. The diaphragm is perforated in many places, enabling the exchange of fluids between the two cavities. The heart itself is a tube of annular muscles consisting of epithelial tissues, with two lateral openings (ostia) per segment. While it is not known whether the rear end is open or closed, from the front, it opens directly into the body cavity.
Since there are no blood vessels, apart from the fine vessels running between the muscle layers of the body wall and a pair of arteries that supply the antennae, this is referred to as an open circulation. The timing of the pumping procedure can be divided into two parts: Diastole and systole. During diastole, blood flows through the ostia from the pericardial sinus (the cavity containing the heart) into the heart. When the systole begins, the ostia close and the heart muscles contract inwards, reducing the volume of the heart. This pumps the blood from the front end of the heart into the perivisceral sinus containing the organs. In this way, the various organs are supplied with nutrients before the blood finally returns to the pericardial sinus via the perforations in the diaphragm. In addition to the pumping action of the heart, body movements also influence circulation.
Respiration
Oxygen uptake occurs to an extent via simple diffusion through the entire body surface, with the coxal vesicles on the legs possibly being involved in some species. However, of most importance is gas exchange via fine unbranched tubes, the tracheae, which draw oxygen from the surface deep into the various organs, particularly the heart.
The walls of these structures, which are less than three micrometers thick in their entirety, consist only of an extremely thin membrane through which oxygen can easily diffuse. The tracheae originate at tiny openings, the spiracles, which themselves are clustered together in dent-like recesses of the outer skin, the atria. The number of "tracheae bundles" thus formed is on average around 75 bundles per body segment; they accumulate most densely on the back of the organism. | Onychophora | Wikipedia | 503 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Unlike the arthropods, the velvet worms are unable to control the openings of their tracheae; the tracheae are always open, entailing considerable water loss in arid conditions. Water is lost twice as fast as in earthworms and forty times faster than in caterpillars. For this reason, velvet worms are dependent upon habitats with high air humidity.
Oxygen transport is helped by the oxygen carrier hemocyanin.
Digestion
The digestive tract begins slightly behind the head, the mouth lying on the underside a little way from the frontmost point of the body. Here, prey can be mechanically dismembered by the mandibles with their covering of fine toothlets. Two salivary glands discharge via a common conductor into the subsequent "throat", which makes up the first part of the front intestine. The saliva that they produce contains mucus and hydrolytic enzymes, which initiate digestion in and outside the mouth.
The throat itself is very muscular, serving to absorb the partially liquified food and to pump it, via the oesophagus, which forms the rear part of the front intestine, into the central intestine. Unlike the front intestine, this is not lined with a cuticula but instead consists only of a single layer of epithelial tissue, which does not exhibit conspicuous indentation as is found in other animals.
On entering the central intestine, food particles are coated with a mucus-based peritrophic membrane, which serves to protect the lining of the intestine from damage by sharp-edged particles. The intestinal epithelium secretes further digestive enzymes and absorbs the released nutrients, although the majority of digestion has already taken place externally or in the mouth. Indigestible remnants arrive in the rear intestine, or rectum, which is once again lined with a cuticula and which opens at the anus, located on the underside near to the rear end. | Onychophora | Wikipedia | 412 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
In almost every segment is a pair of excretory organs called nephridia, which are derived from coelom tissue. Each consists of a small pouch that is connected, via a flagellated conductor called a nephridioduct, to an opening at the base of the nearest leg known as a nephridiopore. The pouch is occupied by special cells called podocytes, which facilitate ultrafiltration of the blood through the partition between haemocoelom and nephridium.
The composition of the urinary solution is modified in the nephridioduct by selective recovery of nutrients and water and by isolation of poison and waste materials, before it is excreted to the outside world via the nephridiopore. The most important nitrogenous excretion product is the water-insoluble uric acid; this can be excreted in solid state, with very little water. This so-called uricotelic excretory mode represents an adjustment to life on land and the associated necessity of dealing economically with water.
A pair of former nephridia in the head were converted secondarily into the salivary glands, while another pair in the final segment of male specimens now serve as glands that apparently play a role in reproduction.
Sensation
The entire body, including the stub feet, is littered with numerous papillae: warty protrusions responsive to touch that carry 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 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; the latter are easily modified flagellated cells, whose flagellum membranes carry a photosensitive pigment on their surface. | Onychophora | Wikipedia | 499 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
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 the antenna, whereas the opposite is true in arthropods.
Reproduction
Both sexes possess pairs of gonads, opening via a channel called a gonoduct into a common genital opening, the gonopore, which is located on the rear ventral side. Both the gonads and the gonoduct are derived from true coelom tissue.
In females, the two ovaries are joined in the middle and to the horizontal diaphragm. The gonoduct appears differently depending on whether the species is live-bearing or egg-laying. In 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, 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 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. | Onychophora | Wikipedia | 457 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
There are different mating procedures: In some species males deposit their spermatophore directly into 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 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.
Distribution and habitat
Distribution
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; 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.
Habitat
Velvet worms always sparsely occupy the habitats where they are found: they are rare among the fauna of which they are a part.
All extant velvet worms are terrestrial (land-living) and prefer dark environments with high air humidity. They are found particularly in the rainforests of the tropics and temperate zones, where they live among moss cushions and leaf litter, under tree trunks and stones, in rotting wood or in termite tunnels. They also occur in unforested grassland, if there exist sufficient crevices in the soil into which they can withdraw during the day, and in 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 the Caribbean, but for others, conversion of rainforests is likely one of the most important threats to their survival (see Conservation). | Onychophora | Wikipedia | 492 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
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 a resting state.
Slime
The Onychophora forcefully squirt glue-like slime
from their oral papillae; they do so either in defense against predators or to capture prey. The openings of the glands that produce the slime are in the papillae, a pair of highly modified limbs on the sides of the head below the antennae. Inside, they have a syringe-like system that, by a geometric amplifier, allows for fast squirt using slow muscular contraction. High speed films show the animal expelling two streams of adhesive liquid through a small opening (50–200 microns) at a speed of . The interplay between the elasticity of oral papillae and the fast unsteady flow produces a passive oscillatory motion (30–60 Hz) of the oral papillae. The oscillation causes the streams to cross in mid air, weaving a disordered net; the velvet worms can control only the general direction where the net is thrown. | Onychophora | Wikipedia | 281 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The slime glands themselves are deep inside the body cavity, each at the end of a tube more than half the length of the body. The tube both conducts the fluid and stores it until it is required. The distance that the animal can propel the slime varies; usually it squirts it about a centimetre, but the maximal range has variously been reported to be ten centimetres, or even nearly a foot, although accuracy drops with range. It is not clear to what extent the range varies with the species and other factors. One squirt usually suffices to snare a prey item, although larger prey may be further immobilised by smaller squirts targeted at the limbs; additionally, the fangs of spiders are sometimes targeted. Upon ejection, it forms a net of threads about twenty microns in diameter, with evenly spaced droplets of viscous adhesive fluid along their length. It subsequently dries, shrinking, losing its stickiness, and becoming brittle. Onychophora eat their dried slime when they can, which seems provident, since an onychophoran requires about 24 days to replenish an exhausted slime repository.
The slime can account for up to 11% of the organism's dry weight and is 90% water; its dry residue consists mainly of proteins—primarily a collagen-type protein. 1.3% of the slime's dry weight consists of sugars, mainly galactosamine. The slime also contains lipids and the surfactant nonylphenol. Onychophora are the only organisms known to produce this latter substance. It tastes "slightly bitter and at the same time somewhat astringent". The proteinaceous composition accounts for the slime's high tensile strength and stretchiness. The lipid and nonylphenol constituents may serve one of two purposes: They may line the ejection channel, stopping the slime from sticking to the organism when it is secreted; or they may slow the drying process long enough for the slime to reach its target.
Behaviour
Locomotion | Onychophora | Wikipedia | 436 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Velvet worms/Onychophora move in a slow and gradual motion that makes them difficult for prey to notice. Their trunk is raised relatively high above the ground, and they walk with non-overlapping steps.
To move from place to place, the velvet worm crawls forward using its legs; unlike in arthropods, both legs of a pair are moved simultaneously. The claws of the feet are used only on hard, rough terrain where a firm grip is needed; on soft substrates, such as moss, the velvet worm walks on the foot cushions at the base of the claws.
Actual locomotion is achieved less by the exertion of the leg muscles than by local changes of body length. This can be controlled using the annular and longitudinal muscles. If the annular muscles are contracted, the body cross-section is reduced, and the corresponding segment lengthens; this is the usual mode of operation of the hydrostatic skeleton as also employed by other worms. Due to the stretching, the legs of the segment concerned are lifted and swung forward. Local contraction of the longitudinal muscles then shortens the appropriate segment, and the legs, which are now in contact with the ground, are moved to the rear. This part of the locomotive cycle is the actual leg stroke that is responsible for forward movement. The individual stretches and contractions of the segments are coordinated by the nervous system such that contraction waves run the length of the body, each pair of legs swinging forward and then down and rearward in succession. Macroperipatus can reach speeds of up to four centimetres per second, although speeds of around 6 body-lengths per minute are more typical. The body gets longer and narrower as the animal picks up speed; the length of each leg also varies during each stride.
Sociality
The brains of Onychophora, though small, are very complex; consequently, the organisms are capable of rather sophisticated social interactions. Behaviour may vary from genus to genus, so this article reflects the most-studied genus, Euperipatoides.
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 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. | Onychophora | Wikipedia | 511 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
When assessing other individuals, individuals often measure one another up by running their antennae down the length of the other individual. Once hierarchy has been established, 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 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.
Feeding
Velvet worms are ambush predators, hunting only by night, and are able to capture animals at least their own size, although capturing 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 the size of the hunting onychophoran. Ninety percent of the time involved in eating prey is spent ingesting it; re-ingestion of the slime used to trap the insect is performed while the onychophoran locates a suitable place to puncture the prey, and this phase accounts for around 8% of the feeding time, with the remaining time evenly split between examining, squirting, and injecting the prey. In some cases, chunks of the prey item are bitten off and swallowed; undigestable components take around 18 hours to pass through the digestive tract. | Onychophora | Wikipedia | 483 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Onychophora probably do not primarily use vision to detect their prey; although their tiny eyes do have a good image-forming capacity, their forward vision is obscured by their antennae; their nocturnal habit also limits the utility of eyesight. Air currents, formed by prey motion, are thought to be the primary mode of locating prey; the role of scent, if any, is unclear. Because it takes so long to ingest a prey item, hunting mainly happens around dusk; the onychophorans will abandon their prey at sunrise. This predatory way of life is probably a consequence of the velvet worm's need to remain moist. Due to the continual risk of desiccation, often only a few hours per day are available for finding food. This leads to a strong selection for a low cost-benefit ratio, which cannot be achieved with a herbivorous diet. | Onychophora | Wikipedia | 177 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
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 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 the case of smaller prey, they may opt not to use slime at all. Subsequently, a soft part of the prey item (usually a joint membrane in arthropod prey) is identified, punctured with a bite from the jaws, and injected with saliva. This kills the prey very quickly and begins a slower process of digestion. While the onychophoran waits for the prey to digest, it salivates on its slime and begins to eat it (and anything attached to it). It subsequently tugs and slices at the earlier perforation to allow access to the now-liquefied interior of its prey. The jaws operate by moving backwards and forwards along the axis of the body (not in a side-to-side clipping motion as in arthropods), conceivably using a pairing of musculature and hydrostatic pressure. The pharynx is specially adapted for sucking, to extract the liquefied tissue; the arrangement of the jaws about the tongue and lip papillae ensures a tight seal and the establishment of suction. In social groups, the dominant female is the first to feed, not permitting competitors access to the prey item for the first hour of feeding. Subsequently, subordinate individuals begin to feed. The number of males reaches a peak after females start to leave the prey item. After feeding, individuals clean their antennae and mouth parts before re-joining the rest of their group. | Onychophora | Wikipedia | 472 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Reproduction and life-cycle
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 is variable, more legs. The females of many species are fertilized only once during their lives, which leads to copulation sometimes taking place before the reproductive organs of the females are fully developed. In such cases, for example at the age of three months in Macroperipatus torquatus, the transferred sperm cells are kept in a special reservoir, where they can remain viable for longer periods. Fertilization takes place internally, although the mode of sperm transmission varies widely. In most species, for example in the genus Peripatus, a package of sperm cells called the spermatophore is placed into the genital opening of the female. | Onychophora | Wikipedia | 216 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The detailed process by which this is achieved is in most cases still unknown, a true penis having been observed only in species of the genus Paraperipatus. In many Australian species, there exist dimples or special dagger- or axe-shaped structures on the head; the male of Florelliceps stutchburyae presses a long spine against the female's genital opening and probably positions its spermatophore there in this way. During the process, the female supports the male by keeping him clasped with the claws of her last pair of legs. The mating behavior of two species of the genus Peripatopsis is particularly curious. Here, the male places two-millimetre spermatophores on the back or sides of the female. Amoebocytes from the female's blood collect on the inside of the deposition site, and both the spermatophore's casing and the body wall on which it rests are decomposed via the secretion of enzymes. This releases the sperm cells, which then move freely through the haemocoel, penetrate the external wall of the ovaries and finally fertilize the ova. Why this self-inflicted skin injury does not lead to bacterial infections is not yet understood (though likely related to the enzymes used to deteriorate the skin or facilitate the transfer of viable genetic material from male to female). Velvet worms are found in egg-laying (oviparous), egg-live-bearing (ovoviviparous) and live-bearing (viviparous) forms.
In a recent peer-reviewed paper published in the "Journal of Zoology," researchers discovered that certain species of Peripatus exhibit a unique form of parental care. Unlike most invertebrates, where parental involvement is minimal, female Peripatus were observed actively guarding their eggs and even providing protection to their offspring after hatching. This finding challenges the conventional understanding of reproductive behavior in invertebrates and highlights the diversity of parenting strategies in the animal kingdom. | Onychophora | Wikipedia | 411 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Ovipary occurs solely in the Peripatopsidae, often in regions with erratic food supply or unsettled climate. In these cases, the yolk-rich eggs measure 1.3 to 2.0 mm and are coated in a protective chitinous shell. Maternal care is unknown.
The majority of species are ovoviviparous: the medium-sized eggs, encased only by a double membrane, remain in the uterus. The embryos do not receive food directly from the mother, but are supplied instead by the moderate quantity of yolk contained in the eggs—they are therefore described as lecithotrophic. The young emerge from the eggs only a short time before birth. This probably represents the velvet worm's original mode of reproduction, i.e., both oviparous and viviparous species developed from ovoviviparous species.
True live-bearing species are found in both families, particularly in tropical regions with a stable climate and regular food supply throughout the year. The embryos develop from eggs only micrometres in size and are nourished in the uterus by their mother, hence the description "matrotrophic". The supply of food takes place either via a secretion from the mother directly into the uterus or via a genuine tissue connection between the epithelium of the uterus and the developing embryo, known as a placenta. The former is found only outside the American continents, while the latter occurs primarily in America and the Caribbean and more rarely in the Old World. The gestation period can amount to up to 15 months, at the end of which the offspring emerge in an advanced stage of development. The embryos found in the uterus of a single female do not necessarily have to be of the same age; it is quite possible for there to be offspring at different stages of development and descended from different males. In some species, young tend to be released only at certain points in the year.
A female can have between 1 and 23 offspring per year; development from fertilized ovum to adult takes between 6 and 17 months and does not have a larval stage. This is probably also the original mode of development. Velvet worms have been known to live for up to six years. | Onychophora | Wikipedia | 468 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Ecology
The velvet worm's important predators are primarily various spiders and centipedes, along with rodents and birds, such as, in Central America, the clay-coloured thrush (Turdus grayi). In South America, Hemprichi's coral snake (Micrurus hemprichii) feeds almost exclusively on velvet worms. For defence, some species roll themselves reflexively into a spiral, while they can also fight off smaller opponents by ejecting slime. Various mites (Acari) are known to be ectoparasites infesting the skin of the velvet worm. Skin injuries are usually accompanied by bacterial infections, which are almost always fatal.
The South African species Peripatopsis capensis has been inadvertently introduced to Santa Cruz Island in the Galapagos Islands, where it co-occurs with native velvet worms.
Conservation
The global conservation status of velvet worm species is difficult to estimate; many species are only known to exist at their type locality (the location at which they were first observed and described). The collection of reliable data is also hindered by low population densities, their typically nocturnal behaviour and possibly also as-yet undocumented seasonal influences and sexual dimorphism. To date, the only onychophorans evaluated by the IUCN are:
Mesoperipatus tholloni (Data Deficient)
Plicatoperipatus jamaicensis (Near Threatened)
Peripatoides indigo (Vulnerable)
Peripatoides suteri (Vulnerable)
Peripatopsis alba (Vulnerable)
Peripatopsis clavigera (Vulnerable)
Macroperipatus insularis (Endangered)
Leucopatus anophthalmus (Endangered)
Opisthopatus roseus (Critically Endangered)
Peripatopsis leonina (Critically Endangered)
Speleoperipatus spelaeus (Critically Endangered) | Onychophora | Wikipedia | 384 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The primary threat comes from destruction and fragmentation of velvet worm habitat due to industrialisation, draining of wetlands, and slash-and-burn agriculture. Many species also have naturally low population densities and closely restricted geographic ranges; as a result, relatively small localised disturbances of important ecosystems can lead to the extinction of entire populations or species. Collection of specimens for universities or research institutes also plays a role on a local scale. There is a very pronounced difference in the protection afforded to velvet worms between regions: in some countries, such as South Africa, there are restrictions on both collecting and exporting, while in others, such as Australia, only export restrictions exist. Many countries offer no specific safeguards at all. Tasmania has a protection programme that is unique worldwide: one region of forest has its own velvet worm conservation plan, which is tailored to a particular velvet worm species.
Phylogeny
In their present forms, the velvet worms are probably very closely related to the arthropods, a very extensive taxon that incorporates, for instance, the crustaceans, insects, and arachnids. They share, among other things, an exoskeleton consisting of α-chitin and non-collagenous proteins; gonads and waste-elimination organs enclosed in true coelom tissue; an open blood system with a tubular heart situated at the rear; an abdominal cavity divided into pericardial and perivisceral cavities; respiration via tracheae; and similar embryonic development. Segmentation, with two body appendages per segment, is also a shared feature.
However, the antennae, mandibles, and oral papillae of velvet worms are probably not homologous to the corresponding features in arthropods; i.e., they probably developed independently.
Another closely related group are the comparatively obscure water bears (Tardigrada); however, due to their very small size, water bears have no need for—and hence lack—blood circulation and separate respiratory structures: shared characteristics that support common ancestry of velvet worms and arthropods. | Onychophora | Wikipedia | 428 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Together, the velvet worms, arthropods, and water bears form a monophyletic taxon, the Panarthropoda, i.e., the three groups collectively cover all descendants of their last common ancestor. Due to certain similarities of form, the velvet worms were usually grouped with the water bears to form the taxon Protoarthropoda. This designation would imply that both velvet worms and water bears are not yet as highly developed as the arthropods. Modern systematic theories reject such conceptions of "primitive" and "highly developed" organisms and instead consider exclusively the historical relationships among the taxa. These relationships are not as yet fully understood, but it is considered probable that the velvet worms' sister groups form a taxon designated Tactopoda, thus:
For a long time, velvet worms were also considered related to the annelids. They share, among other things, a worm-like body; a thin and flexible outer skin; a layered musculature; paired waste-elimination organs; as well as a simply constructed brain and simple eyes. Decisive, however, was the existence of segmentation in both groups, with the segments showing only minor specialisation. The parapodia appendages found in annelids therefore correspond to the stump feet of the velvet worms. Within the Articulata hypothesis developed by Georges Cuvier, the velvet worms therefore formed an evolutionary link between the annelids and the arthropods: worm-like precursors first developed parapodia, which then developed further into stub feet as an intermediate link in the ultimate development of the arthropods' appendages. Due to their structural conservatism, the velvet worms were thus considered "living fossils". This perspective was expressed paradigmatically in the statement by the French zoologist A. Vandel:
Onychophorans can be considered highly evolved annelids, adapted to terrestrial life, which announced prophetically the Arthropoda. They are a lateral branch which has endured from ancient times until today, without important modifications. | Onychophora | Wikipedia | 420 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Modern taxonomy does not study criteria such as "higher" and "lower" states of development or distinctions between "main" and "side" branches—only family relationships indicated by cladistic methods are considered relevant. From this point of view, several common characteristics still support the Articulata hypothesis — segmented body; paired appendages on each segment; pairwise arrangement of waste-elimination organs in each segment; and above all, a rope-ladder-like nervous system based on a double nerve strand lying along the belly. An alternative concept, most widely accepted today, is the so-called Ecdysozoa hypothesis. This places the annelids and Panarthropoda in two very different groups: the former in the Lophotrochozoa and the latter in the Ecdysozoa. Mitochondrial gene sequences also provide support for this hypothesis. Proponents of this hypothesis assume that the aforementioned similarities between annelids and velvet worms either developed convergently or were primitive characteristics passed unchanged from a common ancestor to both the Lophotrochozoa and Ecdysozoa. For example, in the first case, the rope-ladder nervous system would have developed in the two groups independently, while in the second case, it is a very old characteristic, which does not imply a particularly close relationship between the annelids and Panarthropoda. The Ecdysozoa concept divides the taxon into two, the Panarthropoda into which the velvet worms are placed, and the sister group Cycloneuralia, containing the threadworms (Nematoda), horsehair worms (Nematomorpha) and three rather obscure groups: the mud dragons (Kinorhyncha); penis worms (Priapulida); and brush-heads (Loricifera). | Onychophora | Wikipedia | 369 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Particularly characteristic of the Cycloneuralia is a ring of "circumoral" nerves around the mouth opening, which the proponents of the Ecdysozoa hypothesis also recognise in modified form in the details of the nerve patterns of the Panarthropoda. Both groups also share a common skin-shedding mechanism (ecdysis) and molecular biological similarities. One problem of the Ecdysozoa hypothesis is the velvet worms' subterminal position of their mouths: Unlike in the Cycloneuralia, the mouth is not at the front end of the body, but lies further back, under the belly. However, investigations into their developmental biology, particularly regarding the development of the head nerves, suggest that this was not always the case, and that the mouth was originally terminal (situated at the tip of the body). This is supported by the fossil record. | Onychophora | Wikipedia | 178 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The "stem-group arthropod" hypothesis is very widely accepted, but some trees suggest that the onychophorans may occupy a different position; their brain anatomy is more closely related to that of the chelicerates than to any other arthropod. The modern velvet worms form a monophyletic group, incorporating all the descendants of their common ancestor. Important common derivative characteristics (synapomorphies) include, for example, the mandibles of the second body segment and the oral papillae and associated slime glands of the third; nerve strands extending along the underside with numerous cross-linkages per segment; and the special form of the tracheae. By 2011, some 180 modern species, comprising 49 genera, had been described; the actual number of species is probably about twice this. According to more recent study, 82 species of Peripatidae and 115 species of Peripatopsidae have been described thus far. However, among the 197 species, 20 are nomina dubia, due to major taxonomic inconsistencies. The best-known is the type genus Peripatus, which was described as early as 1825 and which, in English-speaking countries, stands representative for all velvet worms. All genera are assigned to one of two families, the distribution ranges of which do not overlap but are separated by arid areas or oceans:
The Peripatopsidae exhibit relatively many characteristics that are perceived as original or "primitive". The number of leg pairs in this family range from 13 (in Ooperipatellus nanus) to 29 (in Paraperipatus papuensis). Behind or between the last leg pair is the genital opening (gonopore). Both oviparous and ovoviviparous, as well as genuinely viviparous, species exist, although the peripatopsids essentially lack a placenta. Their distribution is circumaustral, encompassing Australasia, South Africa, and Chile. | Onychophora | Wikipedia | 414 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The Peripatidae exhibit a range of derivative features. They are longer, on average, than the Peripatopsidae and also have more legs. The number of leg pairs in this family range from 19 (in Typhloperipatus williamsoni) to 43 (in Plicatoperipatus jamaicensis). The gonopore is always between the penultimate leg pair. None of the peripatid species are oviparous, and the overwhelming majority are viviparous. The females of many viviparous species develop a placenta with which to provide the growing embryo with nutrients. Distribution of the peripatids is restricted to the tropical and subtropical zones; in particular, they inhabit Central America, northern South America, Gabon, Northeast India, and Southeast Asia. | Onychophora | Wikipedia | 166 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Evolution
Onychophoran paleontology is plagued by the vagaries of the preservation process that makes fossils difficult to interpret. Experiments on the decay and compaction of onychophora demonstrate difficulties in interpreting fossils; certain parts of living onychophora are visible only in certain conditions:
The mouth may or may not be preserved;
The claws may be re-oriented or lost;
The leg width may increase or decrease; and
The mud may be mistaken for organs.
More significantly, features seen in fossils may be artifacts of the preservation process. For instance, "shoulder pads" may simply be the second row of legs coaxially compressed onto the body; branching "antennae" may in fact have been created during decay.
Certain fossils from the early Cambrian bear a striking resemblance to the velvet worms. These fossils, known collectively as the lobopodians, were marine and represent a grade from which arthropods, tardigrades, and Onychophora arose.
Possible fossils of onychophorans are found in the Cambrian, Ordovician (possibly), Silurian and Pennsylvanian periods.
Historically, all fossil Onychophora and lobopodians were lumped into the taxon Xenusia, further subdivided by some authors to the Paleozoic Udeonychophora and the Mesozoic/Tertiary Ontonychophora; living Onychophora were termed Euonychophora. Importantly, few of the Cambrian fossils bear features that distinctively unite them with the Onychophora; none can be confidently assigned to the onychophoran crown or even stem group. Possible exceptions are Hallucigenia and related taxa such as Collinsium ciliosum, which bear distinctly onychophoran-like claws. It is not clear when the transition to a terrestrial existence was made, but it is considered plausible that it took place between the Ordovician and late Silurian – approximately – via the intertidal zone. | Onychophora | Wikipedia | 407 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
The low preservation potential of the non-mineralised onychophorans means that they have a sparse fossil record. The lobopodian Helenodora from the Carboniferous of North America has been suggested to be a member of Onychophora, but other studies recover it as more closely related to other lobopodians. A Late Carboniferous fossil from Montceau-les-Mines, France, Antennipatus has been suggested to have clear onychophoran affinities, likely the first terrestrial onychophoran, but its poor preservation prohibits differentiating between its placement on the stem or crown of the two extant families, or on the onychophoran stem-group.
In 2018, the identification of Antennipatus as the oldest onychophoran has been argued by Giribet and colleagues, who suggested that the minimum age of Antennipatus would be during the Gzhelian age around , and incorporated the taxon conservatively for the phylogenetic analysis of oncyhophorans based on the uncertainty of its placement within the order. In 2021, Baker and colleagues conducted divergence analyses using molecular dating and treating Antennipatus conservatively as a stem-group onychophoran with a minimum age of , resulting in a divergence date of for the crown group onychophorans. Crown group representatives are known only from amber, the oldest being Cretoperipatus from Burmese amber during the Cenomanian-Turonian stages of the Late Cretaceous, around 100-90 million years old, assigned to the family Peripatidae. The affinity of amber records from the Cenozoic, like Tertiapatus, and Succinipatopsis, which form the suggested superfamily termed Tertiapatoidea, has been considered doubtful by other authors. | Onychophora | Wikipedia | 381 | 43198 | https://en.wikipedia.org/wiki/Onychophora | Biology and health sciences | Ecdysozoa | Animals |
Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of about 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many have patterns of yellow, orange, red and green coloration.
The foregut, stomach and intestine run a little below the midline of the body, the anus is at the tip of the tail, and the mouth is under the front. A little above the gut is the , a cavity which mostly runs above the midline and ends a little short of the rear of the body. All species have a proboscis which lies in the rhynchocoel when inactive but everts to emerge just above the mouth to capture the animal's prey with venom. A highly extensible muscle in the back of the rhynchocoel pulls the proboscis in when an attack ends. A few species with stubby bodies filter feed and have suckers at the front and back ends, with which they attach to a host.
The brain is a ring of four ganglia, positioned around the rhynchocoel near the animal's front end. At least a pair of ventral nerve cords connect to the brain and run along the length of the body. Most nemerteans have various chemoreceptors, and on their heads some species have a number of pigment-cup ocelli, which can detect light but can not form an image. Nemerteans respire through the skin. They have at least two lateral vessels which are joined at the ends to form a loop, and these and the rhynchocoel are filled with fluid. There is no heart, and the flow of fluid depends on contraction of muscles in the vessels and the body wall. To filter out soluble waste products, flame cells are embedded in the front part of the two lateral fluid vessels, and remove the wastes through a network of pipes to the outside. | Nemertea | Wikipedia | 415 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
All nemerteans move slowly, using their external cilia to glide on surfaces on a trail of slime, while larger species use muscular waves to crawl, and some swim by dorso-ventral undulations. A few live in the open ocean while the rest find or make hiding places on the bottom. About a dozen species inhabit freshwater, mainly in the tropics and subtropics, and another dozen species live on land in cool, damp places. Most nemerteans are carnivores, feeding on annelids, clams and crustaceans. Some species of nemerteans are scavengers, and a few live commensally inside the mantle cavity of molluscs.
In most species the sexes are separate, but all the freshwater species are hermaphroditic. Nemerteans often have numerous temporary gonads (ovaries or testes), and build temporary gonoducts (ducts from which the ova or sperm are emitted) opening to a gonopore, one per gonad, when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, and others protect their eggs in various ways. The fertilized egg divides by spiral cleavage and grows by determinate development, in which the fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or larvae that resemble the planulas of cnidarians. However, some form a pilidium larva, in which the developing juvenile has a gut which lies across the larva's body, and usually eats the remains of the larva when it emerges. The bodies of some species fragment readily, and even parts cut off near the tail can grow full bodies. | Nemertea | Wikipedia | 389 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Traditional taxonomy divides the phylum in two classes, Anopla ("unarmed" – their proboscises do not have a little dagger) with two orders, and Enopla ("armed" with a dagger) also with two orders. However, it is now accepted that Anopla are paraphyletic, as one order of Anopla is more closely related to Enopla than to the other order of Anopla. The phylum Nemertea itself is monophyletic, its main synapomorphies being the rhynchocoel and eversible proboscis. Traditional taxonomy says that nemerteans are closely related to flatworms, but both phyla are regarded as members of the Lophotrochozoa, a very large clade, sometimes viewed as a superphylum that also includes molluscs, annelids, brachiopods, bryozoa and many other protostomes.
History
In 1555 Olaus Magnus wrote of a marine worm which was apparently long ("40 cubits"), about the width of a child's arm, and whose touch made a hand swell. William Borlase wrote in 1758 of a "sea long worm", and in 1770 Gunnerus wrote a formal description of this animal, which he called Ascaris longissima. Its current name, Lineus longissimus, was first used in 1806 by Sowerby. In 1995, a total of 1,149 species had been described and grouped into 250 genera.
Nemertea are named after the Greek sea-nymph Nemertes, one of the daughters of Nereus and Doris. Alternative names for the phylum have included Nemertini, Nemertinea, and Rhynchocoela. The Nemertodermatida are a separate phylum, whose closest relatives appear to be the Acoela.
Description
Body structure and major cavities | Nemertea | Wikipedia | 406 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
The typical nemertean body is very thin in proportion to its length. The smallest are a few millimeters long, most are less than , and several exceed . The longest animal ever found, at long, may be a specimen of Lineus longissimus, Ruppert, Fox and Barnes refer to a Lineus longissimus long, washed ashore after a storm off St Andrews in Scotland. Other estimates are about . Zoologists find it extremely difficult to measure this species. For comparison:
The longest recorded blue whale was .
The dinosaurs Argentinosaurus and Patagotitan are estimated at approximately and respectively.
A specimen of the Arctic giant jellyfish Cyanea capillata arctica was long.
L. longissimus, however, is usually only a few millimeters wide. The bodies of most nemerteans can stretch a lot, up to 10 times their resting length in some species, but reduce their length to 50% and increase their width to 300% when disturbed. A few have relatively short but wide bodies, for example Malacobdella grossa is up to long and wide, and some of these are much less stretchy. Smaller nemerteans are approximately cylindrical, but larger species are flattened dorso-ventrally. Many have visible patterns in various combinations of yellow, orange, red and green.
The outermost layer of the body has no cuticle, but consists of a ciliated and glandular epithelium containing rhabdites, which form the mucus in which the cilia glide. Each ciliated cell has many cilia and microvilli. The outermost layer rests on a thickened basement membrane, the dermis. Next to the dermis are at least three layers of muscles, some circular and some longitudinal. The combinations of muscle types vary between the different classes, but these are not associated with differences in movement. Nemerteans also have dorso-ventral muscles, which flatten the animals, especially in the larger species. Inside the concentric tubes of these layers is mesenchyme, a kind of connective tissue. In pelagic species this tissue is gelatinous and buoyant.
They are unsegmented, but at least one species, Annulonemertes minusculus, is segmented. But this is assumed to be a derived trait. The segmentation does not include the coelom and body wall, and is therefore referred to as pseudosegmentation. | Nemertea | Wikipedia | 507 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
The mouth is ventral and a little behind the front of the body. The foregut, stomach and intestine run a little below the midline of the body and the anus is at the tip of the tail. Above the gut and separated from the gut by mesenchyme is the rhynchocoel, a cavity which mostly runs above the midline and ends a little short of the rear of the body. The rhynchocoel of class Anopla has an orifice a little to the front of the mouth, but still under the front of the body. In the other class, Enopla, the mouth and the front of the rhynchocoel share an orifice. The rhynchocoel is a coelom, as it is lined by epithelium.
Proboscis and feeding
The proboscis is an infolding of the body wall, and sits in the rhynchocoel when inactive. When muscles in the wall of the rhynchocoel compress the fluid inside, the pressure makes the proboscis jump inside-out along a canal called the rhynchodeum and through an orifice, the proboscis pore. The proboscis has a muscle which attaches to the back of the rhynchocoel, can stretch up to 30 times its inactive length and acts to retract the proboscis.
The proboscis of the class Anopla exits from an orifice which is separate from the mouth, coils around the prey and immobilizes it by sticky, toxic secretions. The Anopla can attack as soon as the prey moves into the range of the proboscis. Some Anopla have branched proboscises which can be described as "a mass of sticky spaghetti". The animal then draws its prey into its mouth. | Nemertea | Wikipedia | 384 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
In most of the class Enopla, the proboscis exits from a common orifice of the rhynchocoel and mouth. A typical member of this class has a stylet, a calcareous barb, with which the animal stabs the prey many times to inject toxins and digestive secretions. The prey is then swallowed whole or, after partial digestion, its tissues are sucked into the mouth. The stylet is attached about one-third of the distance from the end of the everted proboscis, which extends only enough to expose the stylet. On either side of the active stylet are sacs containing back-up stylets to replace the active one as the animal grows or an active one is lost. Instead of one stylet, the Polystilifera have a pad that bears many tiny stylets, and these animals have separate orifices for the proboscis and mouth, unlike other Enopla. The Enopla can only attack after contacting the prey.
Some nemerteans, such as L. longissimus, absorb organic food in solution through their skins, which may make the long, slim bodies an advantage. Suspension feeding is found only among the specialized symbiotic bdellonemerteans, which have a proboscis but no stylet, and use suckers to attach themselves to bivalves.
Respiration and circulatory system
Nemerteans lack specialized gills, and respiration occurs over the surface of the body, which is long and sometimes flattened. Like other animals with thick body walls, they use fluid circulation rather than diffusion to move substances through their bodies. The circulatory system consists of the rhynchocoel and peripheral vessels, while their blood is contained in the main body cavity. The fluid in the rhynchocoel moves substances to and from the proboscis, and functions as a fluid skeleton in everting the proboscis and in burrowing. The vessels circulate fluid round the whole body and the rhynchocoel provides its own local circulation. The circulatory vessels are a system of coeloms. | Nemertea | Wikipedia | 446 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
In the simplest type of circulatory system, two lateral vessels are joined at the ends to form a loop. However, many species have additional long-wise and cross-wise vessels. There is no heart nor pumping vessels, and the flow of fluid depends on contraction of both the vessels and the body wall's muscles. In some species, circulation is intermittent, and fluid ebbs and flows in the long-wise vessels. The fluid in the vessels is usually colorless, but in some species it contains cells that are yellow, orange, green or red. The red type contain hemoglobin and carry oxygen, but the function of the other pigments is unknown.
Excretion
Nemertea use organs called protonephridia to excrete soluble waste products, especially nitrogenous by-products of cellular metabolism. In nemertean protonephridia, flame cells which filter out the wastes are embedded in the front part of the two lateral fluid vessels. The flame cells remove the wastes into two collecting ducts, one on either side, and each duct has one or more nephridiopores through which the wastes exit. Semiterrestrial and freshwater nemerteans have many more flame cells than marines, sometimes thousands. The reason may be that osmoregulation is more difficult in non-marine environments.
Nervous-system and senses
The central nervous-system consists of a brain and paired ventral nerve cords that connect to the brain and run along the length of the body. The brain is a ring of four ganglia, masses of nerve cells, positioned round the rhynchocoel near its front end – while the brains of most protostome invertebrates encircle the foregut. Most nemertean species have just one pair of nerve cords, many species have additional paired cords, and some species also have a dorsal cord. In some species the cords lie within the skin, but in most they are deeper, inside the muscle layers. The central nervous-system is often red or pink because it contains hemoglobin. This stores oxygen for peak activity or when the animal experiences anoxia, for example while burrowing in oxygen-free sediments. | Nemertea | Wikipedia | 450 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Some species have paired cerebral organs, sacs whose only openings are to the outside. Others species have unpaired evertible organs on the front of their heads. Some have slits along the side of the head or grooves obliquely across the head, and these may be associated with paired cerebral organs. All of these are thought to be chemoreceptors, and the cerebral organs may also aiding osmoregulation. Small pits in the epidermis appear to be sensors. On their head, some species have a number of pigment-cup ocelli, which can detect light but not form an image. Most nemerteans have two to six ocelli, although some have hundreds. A few tiny species that live between grains of sand have statocysts, which sense balance.
Paranemertes peregrina, which feeds on polychaetes, can follow the prey's trails of mucus, and find its burrow by backtracking along its own trail of mucus.
Movement
Nemerteans generally move slowly, though they have occasionally been documented to successfully prey on spiders or insects. Most nemerteans use their external cilia to glide on surfaces on a trail of slime, some of which is produced by glands in the head. Larger species use muscular waves to crawl, and some aquatic species swim by dorso-ventral undulations. Some species burrow by means of muscular peristalsis, and have powerful muscles. Some species of the suborder Monostilifera, whose proboscis have one active stylet, move by extending the proboscis, sticking it to an object and pulling the animal toward the object.
Reproduction and life-cycle
Larger species often break up when stimulated, and the fragments often grow into full individuals. Some species fragment routinely and even parts near the tail can grow full bodies. But this kind of extreme regeneration is restricted to only a few types of nemerteans, and is assumed to be a derived feature. All reproduce sexually, and most species are gonochoric (the sexes are separate), but all the freshwater forms are hermaphroditic. | Nemertea | Wikipedia | 441 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Nemerteans often have numerous temporary gonads (ovaries or testes), forming a row down each side of the body in the mesenchyme. Temporary gonoducts (ducts from which the ova or sperm are emitted), one per gonad, are built when the ova and sperm are ready. The eggs are generally fertilised externally. Some species shed them into the water, some lay them in a burrow or tube, and some protect them by cocoons or gelatinous strings. Some bathypelagic (deep sea) species have internal fertilization, and some of these are viviparous, growing their embryos in the female's body.
The zygote (fertilised egg) divides by spiral cleavage and grows by determinate development, in which the fate of a cell can usually be predicted from its predecessors in the process of division. The embryos of most taxa develop either directly to form juveniles (like the adult but smaller) or to form planuliform larvae. The planuliform larva stage may be short-lived and lecithotrophic ("yolky") before becoming a juvenile, or may be planktotrophic, swimming for some time and eating prey larger than microscopic particles. However, many members of the order Heteronemertea and the palaeonemertean family Hubrechtiidae form a pilidium larva, which can capture unicellular algae and which Maslakova describes as like a deerstalker cap with the ear flaps pulled down. It has a gut which lies across the body, a mouth between the "ear flaps", but no anus. A small number of imaginal discs form, encircling the archenteron (developing gut) and coalesce to form the juvenile. When it is fully formed, the juvenile bursts out of the larva body and usually eats it during this catastrophic metamorphosis. This larval stage is unique in that there are no Hox genes involved during development, which are only found in the juveniles developing inside the larvae.
The species Paranemertes peregrina has been reported as having a life span of around 18 months.
Ecological significance | Nemertea | Wikipedia | 466 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Most nemerteans are marine animals that burrow in sediments, lurk in crevices between shells, stones or the holdfasts of algae or sessile animals. Some live deep in the open oceans, and have gelatinous bodies. Others build semi-permanent burrows lined with mucus or produce cellophane-like tubes. Mainly in the tropics and subtropics, about 12 species appear in freshwater, and about a dozen species live on land in cool, damp places, for example under rotting logs.
The terrestrial Argonemertes dendyi is a native of Australia but has been found in the British Isles, in Sao Miguel in the Azores, in Gran Canaria, and in a lava tube at Kaumana on the Island of Hawaii. It can build a cocoon, which allows it to avoid desiccation while being transported, and it may be able to build populations quickly in new areas as it is a protandrous hermaphrodite. Another terrestrial genus, Geonemertes, is mostly found in Australasia but has species in the Seychelles, widely across the Indo-Pacific, in Tristan da Cunha in the South Atlantic, in Frankfurt, in the Canary Islands, in Madeira and in the Azores. Geonemertes pelaensis has been implicated in the decline of native arthropod species on the Ogasawara Islands, where it was introduced in the 1980s.
Most are carnivores, feeding on annelids, clams and crustaceans, and may kill annelids of about their own size. They sometimes take fish, both living and dead. Insects and myriapods are the only known prey of the two terrestrial species of Argonemertes.
A few nemerteans are scavengers, and these generally have good distance chemoreception ("smell") and are not selective about their prey. A few species live commensally inside the mantle cavity of molluscs and feed on micro-organisms filtered out by the host.
Near San Francisco the nemertean Carcinonemertes errans has consumed about 55% of the total egg production of its host, the dungeness crab Metacarcinus magister. C. errans is considered a significant factor in the collapse of the dungeness crab fishery. Other coastal nemerteans have devastated clam beds. | Nemertea | Wikipedia | 498 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
The few predators on nemerteans include bottom-feeding fish, some sea birds, a few invertebrates including horseshoe crabs, and other nemerteans. Nemerteans' skins secrete toxins that deter many predators, but some crabs may clean nemerteans with one claw before eating them. The American Cerebratulus lacteus and the South African Polybrachiorhynchus dayi, both called "tapeworms" in their respective localities, are sold as fish bait.
Taxonomy | Nemertea | Wikipedia | 105 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Traditional taxonomic classification has divided the group into two classes and four orders:
Class Anopla ("unarmed"). Includes animals with proboscis without stylet, and a mouth underneath and behind the brain.
Order Palaeonemertea. Comprises 100 marine species. Their body wall has outer circular and inner length-wise muscles. In addition, Carinoma tremaphoros has circular and inner length-wise muscles in the epidermis; the extra muscle layers seem to be needed for burrowing by peristalsis.
Order Heteronemertea. Comprises about 400 species. The majority are marine, but three are freshwater. Their body-wall muscles are disposed in four layers, alternately circular and length-wise starting from the outermost layer. The order includes the strongest swimmers. Two genera have branched proboscises.
Class Enopla ("armed"). All have stylets except order Bdellonemertea. Their mouth is located underneath and ahead of the brain. Their main nerve cords run inside body-wall muscles.
Order Bdellonemertea. Includes seven species, of which six live as commensals in the mantle of large clams and one in that of a freshwater snail. The hosts filter feed and all the hosts steal food from them. These nemerteans have short, wide bodies and have no stylets but have a sucking pharynx and a posterior stucker, with which they move like inchworms.
Order Hoplonemertea. Comprises 650 species. They live in benthic and pelagic sea water, in freshwater and on land. They feed by commensalism and parasitism, and are armed with stylet(s)
Suborder Monostilifera. Includes 500 species with a single central stylet. Some use the stylet for locomotion as well as for capturing prey.
Suborder Polystilifera. Includes about 100 pelagic and 50 benthic species. Their pads bear many tiny stylets. | Nemertea | Wikipedia | 413 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Recent molecular phylogenetic studies divided the group into two superclasses, three classes, and eight orders:
Superclass Pronemertea
Class Palaeonemertea
Order Carinomiformes
Order Tubulaniformes
Order Archinemertea
Superclass Neonemertea
Class Pilidiophora
Order Hubrechtiiformes
Order Heteronemertea
Class Hoplonemertea (= Enopla)
Order Polystilifera
Order Monostilifera (includes Bdellonemertea)
incertae sedis
Order Arhynchonemertea (provisionally has been separated its own class Arhynchocoela in 1995)
Evolutionary history
Fossil record
As nemerteans are mostly soft-bodied, one would expect fossils of them to be extremely rare. One might expect the stylet of a nemertean to be preserved, since it is made of calcium phosphate, but no fossil stylets have yet been found. reported nemertean fossils and traces from the Middle Triassic of Germany.
The Middle Cambrian fossil Amiskwia from the Burgess Shale has been classed as a nemertean, based on a resemblance to some unusual deep-sea swimming nemerteans, but few paleontologists accept this classification as the Burgess Shale fossils show no evidence of rhynchocoel nor intestinal caeca.
reported fossils of vermiform organisms with a wide range of morphologies occurring on bedding planes from the Late Ordovician (Katian) Vauréal Formation (Canada). In the specimens preserving the anterior end of the body, this end is pointed or rounded, bearing a rhynchocoel with the proboscis, which is characteristic for nemerteans. The authors attributed these fossils to nemerteans and interpreted them as the oldest record of the group reported so far. However, Knaust & Desrochers cautioned that partly preserved putative nemertean fossils might ultimately turn out to be fossils of turbellarians or annelids.
It has been suggested that Archisymplectes, one of the Pennsylvanian-age animals from Mazon Creek in northern and central Illinois, may be a nemertean.
This fossil, however, only preserves the outline of the "worm", and there is no evidence of a proboscis,
so there is no certainty that it represents a nemertean.
Within Nemertea | Nemertea | Wikipedia | 494 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
There is no doubt that the phylum Nemertea is monophyletic (meaning that the phylum includes all and only descendants of one ancestor that was also a member of the phylum). The synapomorphies (trait shared by an ancestor and all its descendants, but not by other groups) include the eversible proboscis located in the rhynchocoel.
While treat the Palaeonemertea as monophyletic, regard them as paraphyletic and basal (contains the ancestors of the more recent clades). The Anopla ("unarmed") represent an evolutionary grade of nemerteans without stylets (comprising the Heteronemertea and the Palaeonemerteans), while Enopla ("armed") are monophyletic, but find that Palaeonemertea is doubly paraphyletic, having given rise to both the Heteronemertea and the Enopla. treat the Bdellonemertea as a clade separate from the Hoplonemertea, while believe the Bdellonemertea are a part of the Monostilifera (with one active stylet), which are within the Hoplonemertea – which implies that "Enopla" and "Hoplonemertea" are synonyms for the same branch of the tree. The Polystilifera (with many tiny stylets) are monophyletic. | Nemertea | Wikipedia | 303 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Relationships with other phyla
English-language writings have conventionally treated nemerteans as acoelomate bilaterians that are most closely related to flatworms (Platyhelminthes). These pre-cladistics analyses emphasised as shared features: multiciliated (with multiple cilia per cell), glandular epidermis; rod-shaped secretory bodies or rhabdites; frontal glands or organs; protonephridia; and acoelomate body organization. However, multiciliated epidermal cells and epidermal gland cells are also found in Ctenophora, Annelida, Mollusca and other taxa. The rhabdites of nemertea have a different structure from those of flatworms at the microscopic scale. The frontal glands or organs of flatworms vary a lot in structure, and similar structures appear in small marine annelids and entoproct larvae. The protonephridia of nemertea and flatworms are different in structure, and in position – the flame cells of nemertea are usually in the walls of the fluid vessels and are served by "drains" from which the wastes exit by a small number of tubes through the skin, while the flame cells of flatworms are scattered throughout the body.
Rigorous comparisons show no synapomorphies of nemertean and platyhelminth nephridia. | Nemertea | Wikipedia | 299 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
According to more recent analyses, in the development of nemertean embryos, ectomesoderm (outer part of the mesoderm, which is the layer in which most of the internal organs are built) is derived from cells labelled 3a and 3b, and endomesoderm (inner part of the mesoderm) is derived from the 4d cell. Some of the ectomesoderm in annelids, echiurans and molluscs is derived from cells 3a and 3b, while the ectomesoderm of polyclad flatworms is derived from the 2b cell and acoel flatworms produce no ectomesoderm. In nemerteans the space between the epidermis and the gut is mainly filled by well-developed muscles embedded in noncellular connective tissue. This structure is similar to that found in larger flatworms such as polyclads and triclads, but a similar structure of body-wall muscles embedded in noncellular connective tissue is widespread among the Spiralia (animals in which the early cell divisions make a spiral pattern) such as sipunculans, echiurans and many annelids.
Nemerteans' affinities with Annelida (including Echiura, Pogonophora, Vestimentifera and perhaps Sipuncula) and Mollusca make the ribbon-worms members of Lophotrochozoa, which include about half of the extant animal phyla. Lophotrochozoa groups: those animals that feed using a lophophore (Brachiopoda, Bryozoa, Phoronida, Entoprocta); phyla in which most members' embryos develop into trochophore larvae (for example Annelida and Mollusca); and some other phyla (such as Platyhelminthes, Sipuncula, Gastrotricha, Gnathostomulida, Micrognathozoa, Nemertea, Phoronida, Platyhelminthes, and Rotifera).
These groupings are based on molecular phylogeny, which compares sections of organisms DNA and RNA. While analyses by molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. | Nemertea | Wikipedia | 504 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Most protostome phyla outside the Lophotrochozoa are members of Ecdysozoa ("animals that molt"), which include Arthropoda, Nematoda and Priapulida. Most other bilaterian phyla are in the Deuterostomia, which include Echinodermata and Chordata. The Acoelomorpha, which are neither protostomes nor deuterostomes, are regarded as basal bilaterians. | Nemertea | Wikipedia | 102 | 43200 | https://en.wikipedia.org/wiki/Nemertea | Biology and health sciences | Lophotrochozoa | Animals |
Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. More than 10,000 species are described in this class. Common representatives include the lugworm (Arenicola marina) and the sandworm or clam worm Alitta.
Polychaetes as a class are robust and widespread, with species that live in the coldest ocean temperatures of the abyssal plain, to forms which tolerate the extremely high temperatures near hydrothermal vents. Polychaetes occur throughout the Earth's oceans at all depths, from forms that live as plankton near the surface, to a 2- to 3-cm specimen (still unclassified) observed by the robot ocean probe Nereus at the bottom of the Challenger Deep, the deepest known spot in the Earth's oceans. Only 168 species (less than 2% of all polychaetes) are known from fresh waters.
Description
Polychaetes are segmented worms, generally less than in length, although ranging at the extremes from to , in Eunice aphroditois. They can sometimes be brightly coloured, and may be iridescent or even luminescent. Each segment bears a pair of paddle-like and highly vascularized parapodia, which are used for movement and, in many species, act as the worm's primary respiratory surfaces. Bundles of bristles, called chaetae, project from the parapodia.
However, polychaetes vary widely from this generalized pattern, and can display a range of different body forms. The most generalised polychaetes are those that crawl along the bottom, but others have adapted to many different ecological niches, including burrowing, swimming, pelagic life, tube-dwelling or boring, commensalism, and parasitism, requiring various modifications to their body structures. | Polychaete | Wikipedia | 424 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
The head, or prostomium, is relatively well developed, compared with other annelids. It projects forward over the mouth, which therefore lies on the animal's underside. The head normally includes two to four pair of eyes, although some species are blind. These are typically fairly simple structures, capable of distinguishing only light and dark, although some species have large eyes with lenses that may be capable of more sophisticated vision, including the Alciopids' complex eyes which rival cephalopod and vertebrate eyes.
Many species show bioluminescence; eight families have luminous species.
The head also includes a pair of antennae, tentacle-like palps, and a pair of pits lined with cilia, known as "nuchal organs". These latter appear to be chemoreceptors, and help the worm to seek out food.
Internal anatomy and physiology
The outer surface of the body wall consists of a simple columnar epithelium covered by a thin cuticle. Underneath this, in order, are a thin layer of connective tissue, a layer of circular muscle, a layer of longitudinal muscle, and a peritoneum surrounding the body cavity. Additional oblique muscles move the parapodia. In most species the body cavity is divided into separate compartments by sheets of peritoneum between each segment, but in some species it is more continuous.
The mouth of polychaetes is located on the peristomium, the segment behind the prostomium, and varies in form depending on their diets, since the group includes predators, herbivores, filter feeders, scavengers, and parasites. In general, however, they possess a pair of jaws and a pharynx that can be rapidly everted, allowing the worms to grab food and pull it into their mouths. In some species, the pharynx is modified into a lengthy proboscis. The digestive tract is a simple tube, usually with a stomach part way along.
The smallest species, and those adapted to burrowing, lack gills, breathing only through their body surfaces. Most other species have external gills, often associated with the parapodia. | Polychaete | Wikipedia | 442 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
A simple but well-developed circulatory system is usually present. The two main blood vessels furnish smaller vessels to supply the parapodia and the gut. Blood flows forward in the dorsal vessel, above the gut, and returns down the body in the ventral vessel, beneath the gut. The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart. In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system. Conversely, some species have little or no circulatory system at all, transporting oxygen in the coelomic fluid that fills their body cavities.
The blood may be colourless, or have any of three different respiratory pigments. The most common of these is haemoglobin, but some groups have haemerythrin or the green-coloured chlorocruorin, instead.
The nervous system consists of a single or double ventral nerve cord running the length of the body, with ganglia and a series of small nerves in each segment. The brain is relatively large, compared with that of other annelids, and lies in the upper part of the head. An endocrine gland is attached to the ventral posterior surface of the brain, and appears to be involved in reproductive activity. In addition to the sensory organs on the head, photosensitive eye spots, statocysts, and numerous additional sensory nerve endings, most likely involved with the sense of touch, also occur on the body.
Polychaetes have a varying number of protonephridia or metanephridia for excreting waste, which in some cases can be relatively complex in structure. The body also contains greenish "chloragogen" tissue, similar to that found in oligochaetes, which appears to function in metabolism, in a similar fashion to that of the vertebrate liver.
The cuticle is constructed from cross-linked fibres of collagen and may be 200 nm to 13 mm thick. Their jaws are formed from sclerotised collagen, and their setae from sclerotised chitin.
Ecology | Polychaete | Wikipedia | 446 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
Polychaetes are predominantly marine, but many species also live in freshwater, and a few in terrestrial environments. They are extremely variable in both form and lifestyle, and include a few taxa that swim among the plankton or above the abyssal plain. Most burrow or build tubes in the sediment, and some live as commensals. A few species, roughly 80 (less than 0.5% of species), are parasitic. These include both ectoparasites and endoparasites. Ectoparasitic polychaetes feed on skin, blood, and other secretions, and some are adapted to bore through hard, usually calcerous surfaces, such as the shells of mollusks. These "boring" polychaetes may be parasitic, but may be opportunistic or even obligate symbionts (commensals).
The mobile forms (Errantia) tend to have well-developed sense organs and jaws, while the stationary forms (Sedentaria) lack them, but may have specialized gills or tentacles used for respiration and deposit or filter feeding, e.g., fanworms.
Underwater polychaetes have eversible mouthparts used to capture prey. A few groups have evolved to live in terrestrial environments, like Namanereidinae with many terrestrial species, but are restricted to humid areas. Some have even evolved cutaneous invaginations for aerial gas exchange. | Polychaete | Wikipedia | 298 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
Notable polychaetes
One notable polychaete, the Pompeii worm (Alvinella pompejana), is endemic to the hydrothermal vents of the Pacific Ocean. Pompeii worms are among the most heat-tolerant complex animals known.
A recently discovered genus, Osedax, includes a species nicknamed the "bone-eating snot flower".
Another remarkable polychaete is Hesiocaeca methanicola, which lives on methane clathrate deposits.
Lamellibrachia luymesi is a cold seep tube worm that reaches lengths of over 3 m and may be the most long-lived annelid, being over 250 years old.
A still unclassified multilegged predatory polychaete worm was identified only by observation from the underwater vehicle Nereus at the bottom of the Challenger Deep, the greatest depth in the oceans, near in depth. It was about an inch long visually, but the probe failed to capture it, so it could not be studied in detail.
The Bobbit worm (Eunice aphroditois) is a predatory species that can achieve a length of ), with an average diameter of .
Dimorphilus gyrociliatus has the smallest known genome of any annelid. The species shows extreme sexual dimorphism. Females measure ~1 mm long and have simplified bodies containing six segments, a reduced coelom, and no appendages, parapodia, or chaetae. The males are only 50 μm long and consist of just a few hundred cells. They lack a digestive system and have just 68 neurons, and only live for roughly a week.
Reproduction
Most polychaetes have separate sexes, rather than being hermaphroditic. The most primitive species have a pair of gonads in every segment, but most species exhibit some degree of specialisation. The gonads shed immature gametes directly into the body cavity, where they complete their development. Once mature, the gametes are shed into the surrounding water through ducts or openings that vary between species, or in some cases by the complete rupture of the body wall (and subsequent death of the adult). A few species copulate, but most fertilize their eggs externally. | Polychaete | Wikipedia | 462 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
The fertilized eggs typically hatch into trochophore larvae, which float among the plankton, and eventually metamorphose into the adult form by adding segments. A few species have no larval form, with the egg hatching into a form resembling the adult, and in many that do have larvae, the trochophore never feeds, surviving off the yolk that remains from the egg.
However, some polychaetes exhibit remarkable reproductive strategies. Some species reproduce by epitoky. For much of the year, these worms look like any other burrow-dwelling polychaete, but as the breeding season approaches, the worm undergoes a remarkable transformation as new, specialized segments begin to grow from its rear end until the worm can be clearly divided into two halves. The front half, the atoke, is asexual. The new rear half, responsible for breeding, is known as the epitoke. Each of the epitoke segments is packed with eggs and sperm and features a single eyespot on its surface. The beginning of the last lunar quarter is the cue for these animals to breed, and the epitokes break free from the atokes and float to the surface. The eye spots sense when the epitoke reaches the surface and the segments from millions of worms burst, releasing their eggs and sperm into the water.
A similar strategy is employed by the deep sea worm Syllis ramosa, which lives inside a sponge. The rear ends of the worm develop into "stolons" containing the eggs or sperm; these stolons then become detached from the parent worm and rise to the sea surface, where fertilisation takes place.
Fossil record
Stem-group polychaete fossils are known from the Sirius Passet Lagerstätte, a rich, sedimentary deposit in Greenland tentatively dated to the late Atdabanian (early Cambrian). The oldest found is Phragmochaeta canicularis. Many of the more famous Burgess Shale organisms, such as Canadia, may also have polychaete affinities. Wiwaxia, long interpreted as an annelid, is now considered to represent a mollusc. An even older fossil, Cloudina, dates to the terminal Ediacaran period; this has been interpreted as an early polychaete, although consensus is absent. | Polychaete | Wikipedia | 476 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
Being soft-bodied organisms, the fossil record of polychaetes is dominated by their fossilized jaws, known as scolecodonts, and the mineralized tubes that some of them secrete. Most important biomineralising polychaetes are serpulids, sabellids, and cirratulids. Polychaete cuticle does have some preservation potential; it tends to survive for at least 30 days after a polychaete's death. Although biomineralisation is usually necessary to preserve soft tissue after this time, the presence of polychaete muscle in the nonmineralised Burgess shale shows this need not always be the case. Their preservation potential is similar to that of jellyfish.
Taxonomy and systematics
Taxonomically, polychaetes are thought to be paraphyletic, meaning the group excludes some descendants of its most recent common ancestor. Groups that may be descended from the polychaetes include the clitellates (earthworms and leeches), sipunculans, and echiurans. The Pogonophora and Vestimentifera were once considered separate phyla, but are now classified in the polychaete family Siboglinidae.
Much of the classification below matches Rouse & Fauchald, 1998, although that paper does not apply ranks above family.
Older classifications recognize many more (sub)orders than the layout presented here. As comparatively few polychaete taxa have been subject to cladistic analysis, some groups which are usually considered invalid today may eventually be reinstated.
These divisions were shown to be mostly paraphyletic in recent years. | Polychaete | Wikipedia | 333 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
Basal or incertae sedis
Family Diurodrilidae
Family Histriobdellidae
Family Nerillidae
Family Parergodrilidae
Family Potamodrilidae
Family Psammodrilidae
Family Spintheridae
Family Protodriloididae
Family Saccocirridae
Order Haplodrili
Order Myzostomida
Family Endomyzostomatidae
Family Asteromyzostomatidae
Family Myzostomatidae
Subclass Palpata
Family Protodrilidae
Family Polygordiidae
Subclass Aciculata
Family Levidoridae
Order Amphinomida
Family Amphinomidae
Family Euphrosinidae
Order Eunicida
Family Dorvilleidae
Family Eunicidae
Family Hartmaniellidae
Family Ichthyotomidae
Family Lumbrineridae
Family Oenonidae
Family Onuphidae
Order Phyllodocida
Suborder Aphroditiformia
Family Acoetidae
Family Aphroditidae
Family Eulepethidae
Family Iphionidae
Family Pholoidae
Family Polynoidae
Family Sigalionidae
Suborder Glyceriformia
Family Glyceridae
Family Goniadidae
Family Lacydoniidae
Family Paralacydoniidae
Suborder Nereidiformia
Family Antonbruunidae
Family Chrysopetalidae
Family Hesionidae
Family Nereididae
Family Pilargidae
Family Syllidae
Suborder Phyllodocida incertae sedis
Family Iospilidae
Family Nautiliniellidae
Family Nephtyidae
Family Typhloscolecidae
Family Tomopteridae
Suborder Phyllodociformia
Family Alciopidae
Family Lopadorrhynchidae
Family Phyllodocidae
Family Pontodoridae
Subclass Sedentaria
Family Chaetopteridae
Infraclass Canalipalpata
Order Sabellida
Family Caobangidae
Family Fabriciidae
Family Oweniidae
Family Sabellariidae
Family Sabellidae
Family Serpulidae
Family Siboglinidae (formerly the phyla Pogonophora & Vestimentifera)
Order Spionida
Suborder Spioniformia
Family Apistobranchidae
Family Longosomatidae
Family Magelonidae
Family Poecilochaetidae
Family Spionidae
Family Trochochaetidae
Family Uncispionidae
Order Terebellida
Suborder Cirratuliformia
Family Acrocirridae (sometimes placed in Spionida) | Polychaete | Wikipedia | 503 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
Family Cirratulidae (sometimes placed in Spionida)
Family Ctenodrilidae (sometimes own suborder Ctenodrilida)
Family Fauveliopsidae (sometimes own suborder Fauveliopsida)
Family Flabelligeridae (sometimes suborder Flabelligerida)
Family Flotidae (sometimes included in Flabelligeridae)
Family Poeobiidae (sometimes own suborder Poeobiida or included in Flabelligerida)
Family Sternaspidae (sometimes own suborder Sternaspida)
Suborder Terebellomorpha
Family Alvinellidae
Family Ampharetidae
Family Pectinariidae
Family Terebellidae
Family Trichobranchidae
Infraclass Scolecida
Family Arenicolidae
Family Capitellidae
Family Cossuridae
Family Maldanidae
Family Opheliidae
Family Orbiniidae
Family Paraonidae
Family Scalibregmatidae
Order Capitellida (nomen dubium)
Order Cossurida (nomen dubium)
Order Opheliida (nomen dubium)
Order Orbiniida (nomen dubium)
Order Questida (nomen dubium)
Order Scolecidaformia (nomen dubium)
Subclass Echiura
Order Bonelliida
Family Bonelliidae
Family Ikedidae
Order Echiurida
Family Echiuridae
Family Thalassematidae
Family Urechidae | Polychaete | Wikipedia | 299 | 43207 | https://en.wikipedia.org/wiki/Polychaete | Biology and health sciences | Lophotrochozoa | null |
Priapulida (priapulid worms, from Gr. πριάπος, priāpos 'Priapus' + Lat. -ul-, diminutive), sometimes referred to as penis worms, is a phylum of unsegmented marine worms. The name of the phylum relates to the Greek god of fertility, because their general shape and their extensible spiny introvert (eversible) proboscis may resemble the shape of a human penis. They live in the mud and in comparatively shallow waters up to deep. Some species show a remarkable tolerance for hydrogen sulfide, anoxia and low salinity. Halicryptus spinulosus appears to prefer brackish shallow waters. They can be quite abundant in some areas. In an Alaskan bay as many as 85 adult individuals of Priapulus caudatus per square meter has been recorded, while the density of its larvae can be as high as 58,000 per square meter (5,390 per square foot).
Together with Echiura and Sipuncula, they were once placed in the taxon Gephyrea, but consistent morphological and molecular evidence supports their belonging to Ecdysozoa, which also includes arthropods and nematodes. Fossil findings show that the mouth design of the stem-arthropod Pambdelurion is identical with that of priapulids, indicating that their mouth is an original trait inherited from the last common ancestor of both priapulids and arthropods, even if modern arthropods no longer possess it. Among Ecdysozoa, their nearest relatives are Kinorhyncha and Loricifera, with which they constitute the Scalidophora clade named after the spines covering the introvert (scalids). They feed on slow-moving invertebrates, such as polychaete worms.
Some analyses suggest that Priapulida may represent a basal lineage within Ecdysozoa, leading to their classification as "living fossils". Priapulid-like fossils are known at least as far back as the Middle Cambrian. They were likely major predators of the Cambrian period. However, crown-group priapulids cannot be recognized until the Carboniferous. 22 extant species of priapulid worms are known, half of them being of meiobenthic size. | Priapulida | Wikipedia | 493 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
Anatomy
Priapulids are cylindrical worm-like animals, ranging from 0.2–0.3 to 39 centimetres (0.08–0.12 to 15.35 in) long, with a median anterior mouth quite devoid of any armature or tentacles. The body is divided into a main trunk or abdomen and a somewhat swollen proboscis region ornamented with longitudinal ridges. The body is ringed and often has circles of spines, which are continued into the slightly protrusible pharynx. Family Priapulidae have species with a tail or a pair of caudal appendages. A slender tail or tail filament is also found in family Tubiluchidae. Appendages are absent in the remaining families. The body has a chitinous cuticle that is moulted as the animal grows. Members of the family Chaetostephanidae also secretes a gelatinous tube, open in both ends, which they live in.
There is a wide body-cavity, which has no connection with the renal or reproductive organs, so it is not a coelom; it is probably a blood-space or hemocoel. There are no vascular or respiratory systems, but the body cavity does contain phagocytic amoebocytes and cells containing the respiratory pigment haemerythrin.
The alimentary canal is straight, consisting of an eversible pharynx, an intestine, and a short rectum. The pharynx is muscular and lined by teeth. Three of the five extant families have gone through a significant miniaturization and become detritivores (Tubiluchidae and Meiopriapulidae) and filter feeders (Chaetostephanidae). The two remaining families Priapulidae and Halicryptidae are larger carnivores that feed on other animals, although some species also consume detritus as larvae. The shape of the teeth reflect these different lifestyles, and seem to be adapted mainly towards grasping prey or raking detritus from the sediment into the mouth. The anus is terminal, although in Priapulus one or two hollow ventral diverticula of the body-wall stretch out behind it. | Priapulida | Wikipedia | 458 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
The nervous system consists of a nerve ring around the pharynx and a prominent cord running the length of the body with ganglia and longitudinal and transversal neurites consistent with an orthogonal organisation. The nervous system retains a basiepidermal configuration with a connection with the ectoderm, forming part of the body wall. There are no specialized sense organs, but there are sensory nerve endings in the body, especially on the proboscis.
The priapulids are gonochoristic, having two separate sexes (i.e. male and female). Their male and female organs are closely associated with the excretory protonephridia. They comprise a pair of branching tufts, each of which opens to the exterior on one side of the anus. The tips of these tufts enclose a flame-cell like those found in flatworms and other animals, and these probably function as excretory organs. As the animals mature, diverticula arise on the tubes of these organs, which develop either spermatozoa or ova. These sex cells pass out through the ducts. The perigenital area of the genus Tubiluchus exhibit sexual dimorphism.
Reproduction and development
For the species Priapulus caudatus, the 80 μm egg undergoes a total and radial cleavage following a symmetrical and subequal pattern. Development is remarkably slow, with the first cleavage taking place 15 hours after fertilization, gastrulation after several days and hatching of the first 'lorica' larvae after 15 to 20 days. The species Meiopriapulus fijiensis have direct development. In current systematics, they are described as protostomes, despite having a deuterostomic development. Because the group is so ancient, it is assumed the deuterostome condition which appears to be ancestral for bilaterians have been maintained.
Fossil record | Priapulida | Wikipedia | 389 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
Stem-group priapulids are known from the Middle Cambrian Burgess Shale, where their soft-part anatomy is preserved, often in conjunction with their gut contents – allowing a reconstruction of their diets. In addition, isolated microfossils (corresponding to the various teeth and spines that line the pharynx and introvert) are widespread in Cambrian deposits, allowing the distribution of priapulids – and even individual species – to be tracked widely through Cambrian oceans. Trace fossils that are morphologically almost identical to modern priapulid burrows (Treptichnus pedum) officially mark the start of the Cambrian period, suggesting that priapulids, or at least close anatomical relatives, evolved around this time. Crown-group priapulid body fossils are first known from the Carboniferous.
Phylogeny
External phylogeny
Internal phylogeny
Classification
There are 22 known extant species: | Priapulida | Wikipedia | 186 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
Phylum Priapulida Théel 1906
Order Halicryptomorpha Salvini-Plawen 1974 [Adrianov & Malakhov 1995; Salvini-Plawen 1974; Eupriapulida Lemburg, 1999]
Family Halicryptidae Salvini-Plawen 1974
Genus Halicryptus
Species H. higginsi (Shirley & Storch, 1999)
Species H. spinulosus (von Siebold, 1849)
Order Meiopriapulomorpha
Family Meiopriapulidae
Genus Meiopriapulus
Species M. fijiensis (Morse, 1981)
Order Priapulomorpha Adrianov & Malakhov 1995 (assigned its own order by )
Family Priapulidae Gosse 1855 [Xiaoheiqingidae (sic) Hu 2002]
Genus Acanthopriapulus
Species A. horridus (Théel, 1911)
Genus Priapulopsis
Species P. australis (de Guerne, 1886)
Species P. bicaudatus (Danielssen, 1869)
Species P. cnidephorus (Salvini-Plawen, 1973)
Genus Priapulus
Species P. abyssorum (Menzies, 1959)
Species P. caudatus (Lamarck, 1816)
Species P. tuberculatospinosus (Baird, 1868)
Family Tubiluchidae van der Land 1970 [Meiopriapulidae Adrianov & Malakhov 1995]
Genus Tubiluchus
Species T. arcticus (Adrianov, Malakhov, Tchesunov & Tzetlin, 1989)
Species T. australensis (van der Land, 1985)
Species T. corallicola (van der Land, 1968)
Species T. lemburgi (Schmidt-Rhaesa, Rothe & Martínez, 2013)
Species T. pardosi (Scmidt-Rhaesa, Panpeng & Yamasaki, 2017)
Species T. philippinensis (van der Land, 1985)
Species T. remanei (van der Land, 1982)
Species T. soyoae (Scmidt-Rhaesa, Panpeng & Yamasaki, 2017)
Species T. troglodytes (Todaro & Shirley, 2003)
Species T. vanuatensis (Adrianov & Malakhov, 1991)
Order Seticoronaria | Priapulida | Wikipedia | 493 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
Family Chaetostephanidae Por & Bromley 1974 [Chaetostephanidae Salvini-Plawen 1974]
Genus Maccabeus
Species M. cirratus (Malakhov, 1979)
Species M. tentaculatus (Por, 1973) | Priapulida | Wikipedia | 54 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
Extinct groups
Stem-group †Scalidophora
Order †Ancalagonida Adrianov & Malakhov 1995 [Fieldiida Adrianov & Malakhov 1995]
Family †Ancalagonidae Conway Morris 1977
Genus †Ancalagon Conway Morris 1977
Family †Fieldiidae Conway Morris 1977
Genus †Fieldia Walcott 1912
Stem-group †Palaeoscolecida
Family †Selkirkiidae Conway Morris 1977
Genus †Selkirkia Walcott 1911 non Hemsley 1884
Order †Ottoiomorpha Adrianov & Malakhov 1995
Genus †Scolecofurca Conway Morris 1977
Family †Ottoiidae Walcott 1911
Genus †Ottoia Walcott 1911
Family †Corynetidae Huang, Vannier & Chen 2004
Genus †Corynetis Luo & Hu 1999 [Anningvermis Huang, Vannier & Chen 2004]
Family †Miskoiidae Walcott 1911
Genus †Miskoia Walcott 1911
Genus †Louisella Conway Morris 1977 | Priapulida | Wikipedia | 189 | 43209 | https://en.wikipedia.org/wiki/Priapulida | Biology and health sciences | Ecdysozoa | Animals |
Vetulicolia is a group of bilaterian marine animals encompassing several extinct species from the Cambrian, and possibly Ediacaran, periods. As of 2023, the majority of workers favor placing Vetulicolians in the stem group of the Chordata, but some continue to favor a more crownward placement as a sister group to the Tunicata. It was initially erected as a monophyletic clade with the rank of phylum in 2001, with subsequent work supporting its monophyly. However, more recent research suggests that vetulicolians may be paraphyletic and form a basal evolutionary grade of stem chordates.
Etymology
The taxon name, Vetulicolia, is derived from the type genus, Vetulicola, which is a compound Latin word composed of vetuli "old" and cola "inhabitant". It was named after Vetulicola cuneata, the first species of the group described in 1987.
Description
The vetulicolian body plan comprises two parts: a voluminous rostral (anterior) forebody, tipped with an anteriorly positioned mouth and lined with a lateral row of five round to oval-shaped openings on each side, which have been interpreted as gills (or at least orifices in the vicinity of the pharynx); and a caudal (posterior) section that primitively comprises seven body segments and functions as a tail. All vetulicolians lack preserved appendages of any kind, having no legs, feelers or even eye spots. The area where the anterior and posterior parts join is constricted in most genera. Notochord-like structures have been found in some vetulicolian fossils.
Ecology and lifestyle
From their superficially tadpole-like forms, leaf or paddle-shaped tails, and various degrees of streamlining, it is assumed that all vetulicolians discovered to date were swimming animals that spent much, if not all, of their time living in water. Some groups, like the genus Vetulicola, were more streamlined (complete with ventral keels) than other groups, such as the tadpole-like Didazoonidae. | Vetulicolia | Wikipedia | 439 | 43217 | https://en.wikipedia.org/wiki/Vetulicolia | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Because all vetulicolians had mouths which had no features for chewing or grasping, it is assumed that they were not predators. Since vetulicolians possessed gill slits, many researchers regard these organisms as planktivores. The sediment infills in the guts of their fossils have caused some to suggest that they were deposit feeders. This idea has been contested, as deposit feeders tend to have straight guts, whereas the hindguts of vetulicolians were spiral-shaped. Some researchers propose that the vetulicolians were "selective deposit-feeders" which actively swam from one region of the seafloor to another, while supplementing their nutrition with filter-feeding.
The earliest vetulicolians appear to have been living in shallow water, with the first deeper water specimens appearing in the Balang Biota and some maybe in the Quingjiang Biota .
Taxonomy and evolution
The phylum Vetulicolia was erected in 2001 to group the genera Vetulicola, Didazoon, and Xidazoon (later deemed a junior synonym of Pomatrum). Prior to this the class Vetulicolida had been defined in 1997 to group Vetulicola with the previously enigmatic genus Banffia due to its similar two-part construction, as well as apparent gill slits in a newly discovered specimen. Further work split Banffia into a separate class called Banffozoa, which was soon expanded to encompass similar species such as Heteromorphus.
While subsequent studies supported the monophyly of Vetulicolia, it has also been noted that this would preclude vetulicolians representing a stepwise development of deuterostome characteristics, as the genus with the most such characteristics, Vetulicola, is one of the most derived in the group. | Vetulicolia | Wikipedia | 368 | 43217 | https://en.wikipedia.org/wiki/Vetulicolia | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
A 2024 phylogenetic analysis by Mussini and colleagues found vetulicolians to be a paraphyletic group of stem-chordates, lying outside a clade formed by Yunnanozoon, Cathaymyrus, Pikaia and crown-chordates. This is in part due to the Cambroernida, which are basal stem-ambulacrarians, being discovered to share characteristics such as a terminal anus with vetulicolians, despite such characteristics previously being believed to be present in the last common ancestor of deuterostomes. However, ascidian larvae have been noted to have endoderm extending to the terminal end, which could suggest that the ancestral tunicate also had a terminal anus.
Other possible placements are suggested by the Centroneuralia hypothesis, which features a paraphyletic Deuterostomia with chordates as the sister-group to protostomes. If proven true, pharyngeal slits would no longer require a deuterostome placement and vetulicolians could prove to be stem protostomes that lost the post-anal tail. In such a scenario, Banffozoa could be a more derived stem protostome group than Vetulicolida.
Cladograms
The following cladograms show two possible placements of the Vetulicolia.
First, on the left, a monophyletic Vetulicolia is shown as the sister group to Tunicata, but with all internal relationships unresolved.
Next, on the right, the two proposed classes are shown as the earlier (Banffozoa) and later (Vetulicolida) parts of the vetulicolian grade. Within the Vetulicolida, the family Vetulicolidae as defined by Li et al. (2018) is recovered as monophyletic, while the three widely accepted members of the Didazoonidae are in a polytomy with the clade of crownward chordates.
Classification
The following classification is taken from Li et al. (2018) except where noted. | Vetulicolia | Wikipedia | 423 | 43217 | https://en.wikipedia.org/wiki/Vetulicolia | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Phylum Vetulicolia
? Genus Alienum
A. velamenus
Genus Shenzianyuloma
S. yunnanense
Class Heteromorphida (= Banffozoa )
"Form A"
Order Banffiata
Family Banffiidae
Genus Banffia
B. constricta
B. episoma
Genus Heteromorphus
H. confusus (= Banffia confusa) ; = H. longicaudatus )
Genus Skeemella
S. clavula
Class Vetulicolida
Genus Nesonektris
N. aldridgei
Order Vetulicolata
Family Vetulicolidae
Genus Vetulicola
V. cuneata
V. rectangulata
V. gantoucunensis
V. monile
V. longbaoshanensis
Genus Ooedigera
O. peeli
Genus Beidazoon (= Bullivetula )
B. venustum (= B. variola )
Family Didazoonidae
Genus Didazoon
D. haoae
Genus Pomatrum (= Xidazoon )
P. ventralis (= X. stephanus )
Genus Yuyuanozoon
Y. magnificissimi
History of identification
The current consensus view is that vetulicolians are stem group chordates, although some researchers continue to raise other possibilities. The possible identification of an endostyle bolstered theories of a tunicate affinity, but was later retracted, while the tentative identification of a notochord in Nesonektris and Vetulicola has further supported overall chordate affinities. Other characters that have been used to support a tunicate affinity include the limiting of the notochord to the tail and the presence of a stiff cuticle (tunic).
Recent research has strengthened the arguments for placing vetulicolians in the chordate stem lineage rather than near the tunicates. Like vetulicolians, members of the basal ambulacrarian clade Cambroernida have a terminal anus rathre than a post-anal tail. Since Ambulacraria is the sister-group of the chordates within the deuterostomes, this suggests that the last common ancestor of both groups lacked a post-anal tail. However, ascidian larvae have been noted to have endoderm extending to the terminal end, which could suggest that tunicates also lacked post-anal tails ancestrally. | Vetulicolia | Wikipedia | 490 | 43217 | https://en.wikipedia.org/wiki/Vetulicolia | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
Some workers have questioned the inclusion of Banffozoa within this group due to their lack of gill slits and apparent gut diverticula, and have theorized that they may fit within Protostomia instead. Skeemella, in particular, has been noted as having striking arthropod-like characteristics. However, Herpetogaster, the most basal cambroernid, is thought to have non-serialized pores for pharyngial openings. If banffozoans are the most basal vetulicolians, This could explain why they also lack serialized pharyngeal structures. Additionally, a comprehensive review of the Vetulicolia in 2007 did not find evidence of gut diverticula in their material while acknowledging the previous report regarding Banffia. Shenzianyuloma has been interpreted as a vetulicolian with both a notochord (a definitively deuterostome trait) and gut diverticula. However, this fossil has is of unusual provenance (a "crystal and fossil vendor"), and has not yet been examined by other researchers.
Vetulicolians were thought to be stem arthropods when Vetulicola was first discovered, but around 2001 the focus of most theories shifted towards stem deuterostomes due to the discovery of pharyngial gill slits (a deuterostome characteristic), as well as the mounting evidence that vetuicolians have no appendages of any kind. A theory grouping both vetulicolians and vetulocystids with Saccorhytus was disproven when the alleged pharyngial openings of Saccorhytus were shown to be remnants of spines that had broken off; the saccorhytids are now considered to be ecdysozoans. | Vetulicolia | Wikipedia | 367 | 43217 | https://en.wikipedia.org/wiki/Vetulicolia | Biology and health sciences | Prehistoric agnathae and early chordates | Animals |
In probability theory, a probability space or a probability triple is a mathematical construct that provides a formal model of a random process or "experiment". For example, one can define a probability space which models the throwing of a .
A probability space consists of three elements:
A sample space, , which is the set of all possible outcomes.
An event space, which is a set of events, , an event being a set of outcomes in the sample space.
A probability function, , which assigns, to each event in the event space, a probability, which is a number between 0 and 1 (inclusive).
In order to provide a model of probability, these elements must satisfy probability axioms.
In the example of the throw of a standard die,
The sample space is typically the set where each element in the set is a label which represents the outcome of the die landing on that label. For example, represents the outcome that the die lands on 1.
The event space could be the set of all subsets of the sample space, which would then contain simple events such as ("the die lands on 5"), as well as complex events such as ("the die lands on an even number").
The probability function would then map each event to the number of outcomes in that event divided by 6 – so for example, would be mapped to , and would be mapped to .
When an experiment is conducted, it results in exactly one outcome from the sample space . All the events in the event space that contain the selected outcome are said to "have occurred". The probability function must be so defined that if the experiment were repeated arbitrarily many times, the number of occurrences of each event as a fraction of the total number of experiments, will most likely tend towards the probability assigned to that event.
The Soviet mathematician Andrey Kolmogorov introduced the notion of a probability space and the axioms of probability in the 1930s. In modern probability theory, there are alternative approaches for axiomatization, such as the algebra of random variables.
Introduction | Probability space | Wikipedia | 414 | 43325 | https://en.wikipedia.org/wiki/Probability%20space | Mathematics | Probability | null |
A probability space is a mathematical triplet that presents a model for a particular class of real-world situations.
As with other models, its author ultimately defines which elements , , and will contain.
The sample space is the set of all possible outcomes. An outcome is the result of a single execution of the model. Outcomes may be states of nature, possibilities, experimental results and the like. Every instance of the real-world situation (or run of the experiment) must produce exactly one outcome. If outcomes of different runs of an experiment differ in any way that matters, they are distinct outcomes. Which differences matter depends on the kind of analysis we want to do. This leads to different choices of sample space.
The σ-algebra is a collection of all the events we would like to consider. This collection may or may not include each of the elementary events. Here, an "event" is a set of zero or more outcomes; that is, a subset of the sample space. An event is considered to have "happened" during an experiment when the outcome of the latter is an element of the event. Since the same outcome may be a member of many events, it is possible for many events to have happened given a single outcome. For example, when the trial consists of throwing two dice, the set of all outcomes with a sum of 7 pips may constitute an event, whereas outcomes with an odd number of pips may constitute another event. If the outcome is the element of the elementary event of two pips on the first die and five on the second, then both of the events, "7 pips" and "odd number of pips", are said to have happened. | Probability space | Wikipedia | 341 | 43325 | https://en.wikipedia.org/wiki/Probability%20space | Mathematics | Probability | null |
The probability measure is a set function returning an event's probability. A probability is a real number between zero (impossible events have probability zero, though probability-zero events are not necessarily impossible) and one (the event happens almost surely, with almost total certainty). Thus is a function The probability measure function must satisfy two simple requirements: First, the probability of a countable union of mutually exclusive events must be equal to the countable sum of the probabilities of each of these events. For example, the probability of the union of the mutually exclusive events and in the random experiment of one coin toss, , is the sum of probability for and the probability for , . Second, the probability of the sample space must be equal to 1 (which accounts for the fact that, given an execution of the model, some outcome must occur). In the previous example the probability of the set of outcomes must be equal to one, because it is entirely certain that the outcome will be either or (the model neglects any other possibility) in a single coin toss. | Probability space | Wikipedia | 214 | 43325 | https://en.wikipedia.org/wiki/Probability%20space | Mathematics | Probability | null |
Not every subset of the sample space must necessarily be considered an event: some of the subsets are simply not of interest, others cannot be "measured". This is not so obvious in a case like a coin toss. In a different example, one could consider javelin throw lengths, where the events typically are intervals like "between 60 and 65 meters" and unions of such intervals, but not sets like the "irrational numbers between 60 and 65 meters".
Definition
In short, a probability space is a measure space such that the measure of the whole space is equal to one.
The expanded definition is the following: a probability space is a triple consisting of:
the sample space – an arbitrary non-empty set,
the σ-algebra (also called σ-field) – a set of subsets of , called events, such that:
contains the sample space: ,
is closed under complements: if , then also ,
is closed under countable unions: if for , then also
The corollary from the previous two properties and De Morgan's law is that is also closed under countable intersections: if for , then also
the probability measure – a function on such that:
P is countably additive (also called σ-additive): if is a countable collection of pairwise disjoint sets, then
the measure of the entire sample space is equal to one: .
Discrete case
Discrete probability theory needs only at most countable sample spaces . Probabilities can be ascribed to points of by the probability mass function such that . All subsets of can be treated as events (thus, is the power set). The probability measure takes the simple form
The greatest σ-algebra describes the complete information. In general, a σ-algebra corresponds to a finite or countable partition , the general form of an event being . | Probability space | Wikipedia | 369 | 43325 | https://en.wikipedia.org/wiki/Probability%20space | Mathematics | Probability | null |
Keratin () is one of a family of structural fibrous proteins also known as scleroproteins. Alpha-keratin (α-keratin) is a type of keratin found in vertebrates. It is the key structural material making up scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin among vertebrates. Keratin also protects epithelial cells from damage or stress. Keratin is extremely insoluble in water and organic solvents. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and form strong unmineralized epidermal appendages found in reptiles, birds, amphibians, and mammals. Excessive keratinization participate in fortification of certain tissues such as in horns of cattle and rhinos, and armadillos' osteoderm. The only other biological matter known to approximate the toughness of keratinized tissue is chitin.
Keratin comes in two types, the primitive, softer forms found in all vertebrates and harder, derived forms found only among sauropsids (reptiles and birds).
Spider silk is classified as keratin, although production of the protein may have evolved independently of the process in vertebrates.
Examples of occurrence
Alpha-keratins (α-keratins) are found in all vertebrates. They form the hair (including wool), the outer layer of skin, horns, nails, claws and hooves of mammals, and the slime threads of hagfish. The baleen plates of filter-feeding whales are also made of keratin. Keratin filaments are abundant in keratinocytes in the hornified layer of the epidermis; these are proteins which have undergone keratinization. They are also present in epithelial cells in general. For example, mouse thymic epithelial cells react with antibodies for keratin 5, keratin 8, and keratin 14. These antibodies are used as fluorescent markers to distinguish subsets of mouse thymic epithelial cells in genetic studies of the thymus. | Keratin | Wikipedia | 453 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
The harder beta-keratins (β-keratins) are found only in the sauropsids, that is all living reptiles and birds. They are found in the nails, scales, and claws of reptiles, in some reptile shells (Testudines, such as tortoise, turtle, terrapin), and in the feathers, beaks, and claws of birds. These keratins are formed primarily in beta sheets. However, beta sheets are also found in α-keratins.
Recent scholarship has shown that sauropsid β-keratins are fundamentally different from α-keratins at a genetic and structural level. The new term corneous beta protein (CBP) has been proposed to avoid confusion with α-keratins.
Keratins (also described as cytokeratins) are polymers of type I and type II intermediate filaments that have been found only in chordates (vertebrates, amphioxi, urochordates). Nematodes and many other non-chordate animals seem to have only type VI intermediate filaments, fibers that structure the nucleus.
Genes
The human genome encodes 54 functional keratin genes, located in two clusters on chromosomes 12 and 17. This suggests that they originated from a series of gene duplications on these chromosomes.
The keratins include the following proteins of which KRT23, KRT24, KRT25, KRT26, KRT27, KRT28, KRT31, KRT32, KRT33A, KRT33B, KRT34, KRT35, KRT36, KRT37, KRT38, KRT39, KRT40, KRT71, KRT72, KRT73, KRT74, KRT75, KRT76, KRT77, KRT78, KRT79, KRT8, KRT80, KRT81, KRT82, KRT83, KRT84, KRT85 and KRT86 have been used to describe keratins past 20. | Keratin | Wikipedia | 447 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
Protein structure
The first sequences of keratins were determined by Israel Hanukoglu and Elaine Fuchs (1982, 1983). These sequences revealed that there are two distinct but homologous keratin families, which were named type I and type II keratins. By analysis of the primary structures of these keratins and other intermediate filament proteins, Hanukoglu and Fuchs suggested a model in which keratins and intermediate filament proteins contain a central ~310 residue domain with four segments in α-helical conformation that are separated by three short linker segments predicted to be in beta-turn conformation. This model has been confirmed by the determination of the crystal structure of a helical domain of keratins.
Type 1 and 2 Keratins
The human genome has 54 functional annotated Keratin genes, 28 are in the Keratin type 1 family, and 26 are in the Keratin type 2 family.
Fibrous keratin molecules supercoil to form a very stable, left-handed superhelical motif to multimerise, forming filaments consisting of multiple copies of the keratin monomer.
The major force that keeps the coiled-coil structure is hydrophobic interactions between apolar residues along the keratins helical segments.
Limited interior space is the reason why the triple helix of the (unrelated) structural protein collagen, found in skin, cartilage and bone, likewise has a high percentage of glycine. The connective tissue protein elastin also has a high percentage of both glycine and alanine. Silk fibroin, considered a β-keratin, can have these two as 75–80% of the total, with 10–15% serine, with the rest having bulky side groups. The chains are antiparallel, with an alternating C → N orientation. A preponderance of amino acids with small, nonreactive side groups is characteristic of structural proteins, for which H-bonded close packing is more important than chemical specificity. | Keratin | Wikipedia | 430 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
Disulfide bridges
In addition to intra- and intermolecular hydrogen bonds, the distinguishing feature of keratins is the presence of large amounts of the sulfur-containing amino acid cysteine, required for the disulfide bridges that confer additional strength and rigidity by permanent, thermally stable crosslinking—in much the same way that non-protein sulfur bridges stabilize vulcanized rubber. Human hair is approximately 14% cysteine. The pungent smells of burning hair and skin are due to the volatile sulfur compounds formed. Extensive disulfide bonding contributes to the insolubility of keratins, except in a small number of solvents such as dissociating or reducing agents.
The more flexible and elastic keratins of hair have fewer interchain disulfide bridges than the keratins in mammalian fingernails, hooves and claws (homologous structures), which are harder and more like their analogs in other vertebrate classes. Hair and other α-keratins consist of α-helically coiled single protein strands (with regular intra-chain H-bonding), which are then further twisted into superhelical ropes that may be further coiled. The β-keratins of reptiles and birds have β-pleated sheets twisted together, then stabilized and hardened by disulfide bridges.
Thiolated polymers (=thiomers) can form disulfide bridges with cysteine substructures of keratins getting covalently attached to these proteins. Thiomers exhibit therefore high binding properties to keratins found in hair, on skin and on the surface of many cell types.
Filament formation
It has been proposed that keratins can be divided into 'hard' and 'soft' forms, or 'cytokeratins' and 'other keratins'. That model is now understood to be correct. A new nuclear addition in 2006 to describe keratins takes this into account.
Keratin filaments are intermediate filaments. Like all intermediate filaments, keratin proteins form filamentous polymers in a series of assembly steps beginning with dimerization; dimers assemble into tetramers and octamers and eventually, if the current hypothesis holds, into unit-length-filaments (ULF) capable of annealing end-to-end into long filaments.
Pairing | Keratin | Wikipedia | 493 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
Cornification
Cornification is the process of forming an epidermal barrier in
stratified squamous epithelial tissue. At the cellular level,
cornification is characterised by:
production of keratin
production of small proline-rich (SPRR) proteins and transglutaminase which eventually form a cornified cell envelope beneath the plasma membrane
terminal differentiation
loss of nuclei and organelles, in the final stages of cornification
Metabolism ceases, and the cells are almost completely filled by keratin. During the process of epithelial differentiation, cells become cornified as keratin protein is incorporated into longer keratin intermediate filaments. Eventually the nucleus and cytoplasmic organelles disappear, metabolism ceases and cells undergo a programmed death as they become fully keratinized. In many other cell types, such as cells of the dermis, keratin filaments and other intermediate filaments function as part of the cytoskeleton to mechanically stabilize the cell against physical stress. It does this through connections to desmosomes, cell–cell junctional plaques, and hemidesmosomes, cell-basement membrane adhesive structures.
Cells in the epidermis contain a structural matrix of keratin, which makes this outermost layer of the skin almost waterproof, and along with collagen and elastin gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis and form protective calluses, which are useful for athletes and on the fingertips of musicians who play stringed instruments. Keratinized epidermal cells are constantly shed and replaced.
These hard, integumentary structures are formed by intercellular cementing of fibers formed from the dead, cornified cells generated by specialized beds deep within the skin. Hair grows continuously and feathers molt and regenerate. The constituent proteins may be phylogenetically homologous but differ somewhat in chemical structure and supermolecular organization. The evolutionary relationships are complex and only partially known. Multiple genes have been identified for the β-keratins in feathers, and this is probably characteristic of all keratins.
Silk
The silk fibroins produced by insects and spiders are often classified as keratins, though it is unclear whether they are phylogenetically related to vertebrate keratins. | Keratin | Wikipedia | 482 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
Silk found in insect pupae, and in spider webs and egg casings, also has twisted β-pleated sheets incorporated into fibers wound into larger supermolecular aggregates. The structure of the spinnerets on spiders' tails, and the contributions of their interior glands, provide remarkable control of fast extrusion. Spider silk is typically about 1 to 2 micrometers (μm) thick, compared with about 60 μm for human hair, and more for some mammals. The biologically and commercially useful properties of silk fibers depend on the organization of multiple adjacent protein chains into hard, crystalline regions of varying size, alternating with flexible, amorphous regions where the chains are randomly coiled. A somewhat analogous situation occurs with synthetic polymers such as nylon, developed as a silk substitute. Silk from the hornet cocoon contains doublets about 10 μm across, with cores and coating, and may be arranged in up to 10 layers, also in plaques of variable shape. Adult hornets also use silk as a glue, as do spiders.
Clinical significance
Abnormal growth of keratin can occur in a variety of conditions including keratosis, hyperkeratosis and keratoderma.
Mutations in keratin gene expression can lead to, among others:
Alopecia areata
Epidermolysis bullosa simplex
Ichthyosis bullosa of Siemens
Epidermolytic hyperkeratosis
Steatocystoma multiplex
Keratosis pharyngis
Rhabdoid cell formation in large cell lung carcinoma with rhabdoid phenotype
Several diseases, such as athlete's foot and ringworm, are caused by infectious fungi that feed on keratin.
Keratin is highly resistant to digestive acids if ingested. Cats regularly ingest hair as part of their grooming behavior, leading to the gradual formation of hairballs that may be expelled orally or excreted. In humans, trichophagia may lead to Rapunzel syndrome, an extremely rare but potentially fatal intestinal condition. | Keratin | Wikipedia | 415 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
Diagnostic use
Keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas, thymomas, sarcomas and trophoblastic neoplasms. Furthermore, the precise expression-pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases. For example, hepatocellular carcinomas typically express CK8 and CK18, and cholangiocarcinomas express CK7, CK8 and CK18, while metastases of colorectal carcinomas express CK20, but not CK7. | Keratin | Wikipedia | 133 | 43377 | https://en.wikipedia.org/wiki/Keratin | Biology and health sciences | Proteins | Biology |
The ballista (Latin, from Greek βαλλίστρα ballistra and that from βάλλω ballō, "throw"), plural ballistae or ballistas, sometimes called bolt thrower, was an ancient missile weapon that launched either bolts or stones at a distant target.
Developed from earlier Greek weapons, it relied upon different mechanics, using two levers with torsion springs instead of a tension prod (the bow part of a modern crossbow). The springs consisted of several loops of twisted skeins. Early versions projected heavy darts or spherical stone projectiles of various sizes for siege warfare. It developed into a smaller precision weapon, the scorpio, and possibly the polybolos.
Greek weapon
The early ballistae in Ancient Greece were developed from two weapons called oxybeles and gastraphetes. The gastraphetes ('belly-bow') was a handheld crossbow. It had a composite prod and was spanned by bracing the front end of the weapon against the ground while placing the end of a slider mechanism against the stomach. The operator would then walk forward to arm the weapon while a ratchet prevented it from shooting during loading. This produced a weapon that, it was claimed, could be operated by a person of average strength but which had a power that allowed it to be successfully used against armored troops. The oxybeles were a bigger and heavier construction employing a winch and were mounted on a tripod. It had a lower rate of fire and was used as a siege engine.
With the invention of the torsion spring bundle, the first ballistae could now be built. The advantage of this new technology was the fast relaxation time of this system. Thus it was possible to shoot lighter projectiles with higher velocities over a longer distance. By contrast, the comparatively slow relaxation time of the bow or prod of a conventional crossbow such as the oxybeles meant that much less energy could be transferred to light projectiles, limiting the effective range of the weapon.
The earliest form of the ballista is thought to have been developed for Dionysius of Syracuse, 400 BC. | Ballista | Wikipedia | 448 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
The Greek ballista was a siege weapon. All components that were not made of wood were transported in the baggage train. It would be assembled with local wood, if necessary. Some were positioned inside large, armored, mobile siege towers or even on the edge of a battlefield. For all of the tactical advantages offered, it was only under Philip II of Macedon, and even more so under his son Alexander, that the ballista began to develop and gain recognition as both a siege engine and field artillery. Historical accounts, for instance, cited that Philip II employed a group of engineers within his army to design and build catapults for his military campaigns. There is even a claim that it was Philip II with his team of engineers who invented the ballista after improving Dionysius's device, which was merely an oversized slingshot. It was further perfected by Alexander, whose own team of engineers introduced innovations such as the idea of using springs made from tightly strung coils of rope instead of a bow to achieve more energy and power when throwing projectiles. Polybius reported about the usage of smaller, more portable ballistae, called scorpions, during the Second Punic War.
Ballistae could be easily modified to shoot both spherical and shaft projectiles, allowing their crews to adapt quickly to prevailing battlefield situations in real time.
As the role of battlefield artillery became more sophisticated, a universal joint (which was invented just for this function) was integrated into the ballista's stand, allowing the operators to alter the trajectory and firing direction of the ballista as required without a lengthy disassembly of the machine.
Roman weaponry
After the absorption of the Ancient Greek city-states into the Roman Republic in 146 BC, the highly advanced Greek technology began to spread across many areas of Roman influence. This included the great military machine advances the Greeks had made (most notably by Dionysus of Syracuse), as well as all the scientific, mathematical, political and artistic developments.
The Romans adopted the torsion-powered ballista, which had by now spread to several cities around the Mediterranean, all of which became Roman spoils of war, including one from Pergamon, which was depicted among a pile of trophy weapons in relief on a balustrade.
The torsion ballista, developed by Alexander, was a far more complicated weapon than its predecessor and the Romans developed it even further, especially into much smaller versions, that could be easily carried. | Ballista | Wikipedia | 497 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
Early Roman ballistae
The early Roman ballistae were made of wood, and held together with iron plates around the frames and iron nails in the stand. The main stand had a slider on the top, into which were loaded the bolts or stone shot. Attached to this, at the back, was a pair of 'winches' and a 'claw', used to ratchet the bowstring back to the armed firing position.
The slider passed through the field frames of the weapon, in which were located the torsion springs (rope made of animal sinew), which were twisted around the bow arms, which in turn, were attached to the bowstring.
Drawing the bowstring back with the winches twisted the already taut springs, storing the energy to fire the projectiles. The bronze or iron caps, which secured the torsion bundles were adjustable by means of pins and peripheral holes, which allowed the weapon to be tuned for symmetrical power and for changing weather conditions.
The ballista was a highly accurate weapon (there are many accounts of single soldiers being picked off by ballistarii), but some design aspects meant it could compromise its accuracy for range. The maximum range was over , but the effective combat range for many targets was far shorter.
The Romans continued the development of the ballista, and it became a highly prized and valued weapon in the army of the Roman Empire.
It was used, just before the start of the Empire, by Julius Caesar during his conquest of Gaul and on both of his campaigns in subduing Britain.
First invasion of Britain
The first of Caesar's invasions of Britain took place in 55 BC, after a rapid and successful initial conquest of Gaul, in part as an expedition, and more practical to try to put an end to the reinforcements sent by the native Britons to fight the Romans in Gaul.
A total of eighty means of transport, carrying two legions, attempted to land on the British shore, only to be driven back by the many British warriors assembled along the shoreline. The ships had to unload their troops on the beach, as it was the only one suitable for many miles, yet the massed ranks of British charioteers and javeliners were making it difficult. | Ballista | Wikipedia | 453 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
Seeing this, Caesar ordered the warships – which were swifter and easier to handle than the transports, and likely to impress the natives more by their unfamiliar appearance – to be removed a short distance from the others, and then be rowed hard and run ashore on the enemy’s right flank, from which position men on deck could use the slings, bows, and artillery to drive them back. This maneuver was highly successful. Scared by the strange shape of the warships, the motion of the oars, and the unfamiliar machines, the natives halted and retreated. (Caesar, The Conquest of Gaul, p.99)
Siege of Alesia
In Gaul, the stronghold of Alesia was under a Roman siege in 52 BC, and was completely surrounded by a Roman fortification including a wooden palisade and towers. As was standard siege technique at the time, small ballistae were placed in the towers with other troops armed with bows or slings. The use of the ballista in the Roman siege strategy was also demonstrated in the case of the Siege of Masada.
Ballistae in the Roman Empire
During the conquest of the Empire, the ballista proved its worth many times in sieges and battles, both at sea and on land. It is from the time of the Roman Empire that many of the archaeological finds of ballistae date. Accounts by the finders, including technical manuals and journals, are used today by archaeologists to reconstruct these weapons.
After Julius Caesar, the ballista was a permanent fixture in the Roman army and, over time, modifications and improvements were made by successive engineers. This included replacing the remaining wooden parts of the machine with metal, creating a much smaller, lighter and more powerful machine than the wooden version, which required less maintenance (though the vital torsion springs were still vulnerable to the strain). The largest ballistae of the 4th century could throw a dart further than 1200 yards (1,100 m). The weapon was named ballista fulminalis in De rebus bellicis: "From this ballista, darts were projected not only in great number but also at a large size over a considerable distance, such as across the width of the Danube River." Ballistae were not only used in laying siege: after AD 350, at least 22 semi-circular towers were erected around the walls of Londinium (London) to provide platforms for permanently mounted defensive devices.
Eastern Roman Empire | Ballista | Wikipedia | 496 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
During the 6th century, Procopius described the effects of this weapon:
But Belisarius placed upon the towers engines which they call "ballistae". Now these engines have the form of a bow, but on the under side of them a grooved wooden shaft projects; this shaft is so fitted to the bow that it is free to move, and rests upon a straight iron bed. So when men wish to shoot at the enemy with this, they make the parts of the bow which form the ends bend toward one another by means of a short rope fastened to them, and they place in the grooved shaft the arrow, which is about one half the length of the ordinary missiles which they shoot from bows, but about four times as wide...but the missile is discharged from the shaft, and with such force that it attains the distance of not less than two bow-shots, and that, when it hits a tree or a rock, it pierces it easily. Such is the engine which bears this name, being so called because it shoots with very great force...
The missiles were able to penetrate body-armour:
And at the Salarian Gate a Goth of goodly stature and a capable warrior, wearing a corselet and having a helmet on his head, a man who was of no mean station in the Gothic nation, refused to remain in the ranks with his comrades, but stood by a tree and kept shooting many missiles at the parapet. But this man by some chance was hit by a missile from an engine which was on a tower at his left. And passing through the corselet and the body of the man, the missile sank more than half its length into the tree, and pinning him to the spot where it entered the tree, it suspended him there a corpse.
Carroballista
The carroballista was a cart-mounted version of the weapon. There were probably different models of ballista under the cheiroballistra class, at least two different two-wheeled models and one model with four wheels. Their probable size was roughly width, i.e., 5 Roman feet. The cart system and structure gave it a great deal of flexibility and capability as a battlefield weapon, since the increased maneuverability allowed it to be moved with the flow of the battle. This weapon features several times on Trajan's Column.
Polybolos | Ballista | Wikipedia | 486 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
It has been speculated that the Roman military may have also fielded a 'repeating' ballista, also known as a polybolos. Reconstruction and trials of such a weapon carried out in a BBC documentary, What the Romans Did For Us, showed that they "were able to shoot eleven bolts a minute, which is almost four times the rate at which an ordinary ballista can be operated". However, no example of such a weapon has been found by archaeologists.
Cheiroballistra and manuballista
The cheiroballistra and the manuballista are held by many archaeologists to be the same weapon. The difference in name may be attributable to the different languages spoken in the Empire. Latin remained the official language in the Western Empire, but the Eastern Empire predominantly used Greek, which added an extra 'r' to the word ballista.
The manuballista was a handheld version of the traditional ballista. This new version was made entirely of iron, which conferred greater power to the weapon, since it was smaller, and less iron (an expensive material before the 19th century), was used in its production. It was not the ancient gastraphetes, but the Roman weapon. However, the same physical limitations applied as with the gastraphetes.
Archaeology and the Roman ballista
Archaeology, and in particular experimental archaeology has been influential on this subject. Although several ancient authors (such as Vegetius) wrote very detailed technical treatises, providing us with all the information necessary to reconstruct the weapons, all their measurements were in their native language and therefore highly difficult to translate.
Attempts to reconstruct these ancient weapons began at the end of the 19th century, based on rough translations of ancient authors. It was only during the 20th century, however, that many of the reconstructions began to make any sense as a weapon. By bringing in modern engineers, progress was made with the ancient systems of measurement. By redesigning the reconstructions using the new information, archaeologists in that specialty were able to recognise certain finds from Roman military sites, and identify them as ballistae. The information gained from the excavations was fed into the next generation of reconstructions and so on. | Ballista | Wikipedia | 450 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
Sites across the empire have yielded information on ballistae, from Spain (the Ampurias Catapult), to Italy (the Cremona Battleshield, which proved that the weapons had decorative metal plates to shield the operators), to Iraq (the Hatra Machine) and even Scotland (Burnswark siege tactics training camp), and many other sites between.
The most influential archaeologists in this area have been Peter Connolly and Eric Marsden, who have not only written extensively on the subject but have also made many reconstructions themselves and have refined the designs over many years of work.
Middle Ages
With the decline of the Roman Empire, resources to build and maintain these complex machines became very scarce, so the ballista was likely supplanted initially by the simpler and cheaper onager and the more efficient springald.
However, while it remained less and less popular as more efficient siege engines such as the trebuchet and the mangonel became widespread, the Ballista still retained some use in Medieval Siege Warfare, especially by city and castle garrisons, until it became finally extinguished by the more convenient medieval canons, already omnipresent in all major European Catholic cities by the first half of the 14th century.
The Littere Wallie records the existence of 4 "balistas ad turrimi" at "Duluithelan" [Dolwyddelan] Castle in 1280, one "balistam de tur" at "Rothelano"[Rhuddlan] castle and one "magnam ballistam" at "Bere Blada" Castle [Castell y Bere?] in 1286. These all being held under the authority of the English Crown.
In remote and seemingly "savage" places like Ireland, however, where cannons were rare and personal firearms were almost non-existent, ballistae had recorded use well into late 15th century.
While not a direct descendant mechanically, the concept and naming continues on as arbalest crossbows (arcus 'bow' + ballista). | Ballista | Wikipedia | 417 | 43379 | https://en.wikipedia.org/wiki/Ballista | Technology | Artillery and siege | null |
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