Theannelids (/ˈænəlɪdz/), also known as thesegmented worms, are animals that comprise thephylumAnnelida (/əˈnɛlɪdə/; from Latinanellus'little ring').[3][a] The phylum contains over 22,000 extantspecies, includingragworms,earthworms, andleeches. The species exist in and have adapted to various ecologies – some in marine environments as distinct astidal zones andhydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.
The basic annelid form consists of multiplesegments. Each segment has the same sets of organs and, in most polychaetes, has a pair ofparapodia that many species use forlocomotion.Septa separate the segments of many species, but are poorly defined or absent in others, andEchiura andSipuncula show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely withinblood vessels, and the vessels in segments near the front ends of these species are often built up with muscles that act as hearts. The septa of such species also enable them to change the shapes of individual segments, which facilitates movement byperistalsis ("ripples" that pass along the body) or byundulations that improve the effectiveness of the parapodia. In species with incomplete septa or none, the blood circulates through the main body cavity without any kind of pump, and there is a wide range of locomotory techniques – some burrowing species turn theirpharynges inside out to drag themselves through thesediment.
Earthworms are oligochaetes that support terrestrialfood chains both as prey and in some regions are important in aeration and enriching ofsoil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development ofecosystems by enabling water andoxygen to penetrate the sea floor. In addition to improvingsoil fertility, annelids serve humans as food and asbait. Scientists observe annelids to monitor the quality of marine and fresh water. Althoughblood-letting is used less frequently by doctors than it once was, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength.
Since annelids aresoft-bodied, their fossils are rare – mostly jaws and themineralized tubes that some of the species secreted. Although some lateEdiacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about518 million years ago in the earlyCambrian period. Fossils of most modern mobile polychaete groups appeared by the end of theCarboniferous, about299 million years ago. Palaeontologists disagree about whether somebody fossils from the midOrdovician, about472 to 461 million years ago, are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in thePaleogene period, which began 66 million years ago.[5]
There are over 22,000 living annelid species,[6][7] ranging in size from microscopic to the Australiangiant Gippsland earthworm andAmynthas mekongianus, which can both grow up to 3 meters (9.8 ft) long[7][8][9] to the largest annelid,Microchaetus rappi which can grow up to 6.7 m (22 ft). Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids,[10][11] most textbooks use the traditional classification into the following sub-groups:[8][12]
Polychaetes (about 12,000 species[6]). As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes haveparapodia that function as limbs, andnuchal organs that are thought to bechemosensors.[8] Most are marine animals, although a few species live in fresh water and even fewer on land.[13]
Clitellates (about 10,000 species[7]). These have few or no chetae per segment, and nonuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shapedclitellum ("pack saddle") around their bodies, which produces acocoon that stores and nourishes fertilized eggs until they hatch[12][14] or, in moniligastrids, yolky eggs that provide nutrition for the embryos.[7] The clitellates are sub-divided into:[8]
Oligochaetes ("with few hairs"), which includesearthworms. Oligochaetes have a sticky pad in the roof of the mouth.[8] Most are burrowers that feed on wholly or partly decomposedorganic materials.[13]
Hirudinea, whose name means "leech-shaped" and whose best known members are leeches.[8] Marine species are mostly blood-suckingparasites, mainly on fish, while most freshwater species are predators.[13] They have suckers at both ends of their bodies, and use these to move rather likeinchworms.[15]
TheArchiannelida, minute annelids that live in the spaces between grains ofmarine sediment, were treated as a separateclass because of their simple body structure, but are now regarded as polychaetes.[12] Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:
Pogonophora /Siboglinidae were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separatephylum, Pogonophora, or as two phyla, Pogonophora andVestimentifera. More recently they have been re-classified as afamily, Siboglinidae, within the polychaetes.[13][16]
TheEchiura have a checkeredtaxonomic history: in the 19th century they were assigned to the phylum "Gephyrea", which is now empty as its members have been assigned to other phyla; the Echiura were next regarded as annelids until the 1940s, when they were classified as a phylum in their own right; but amolecular phylogenetics analysis in 1997 concluded that echiurans are annelids.[6][16][17]
Sipuncula was originally classified as annelids, despite the complete lack of segmentation,bristles and other annelid characters. The phylum Sipuncula was later allied with theMollusca, mostly on the basis ofdevelopmental andlarval characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida.[18] Subsequent analysis of themitochondrion's DNA has confirmed their close relationship to theMyzostomida and Annelida (includingechiurans andpogonophorans).[19] It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.[20]
Mitogenomic and phylogenomic analysis also implies thatOrthonectida, a group of extremely simplified parasites traditionally placed inMesozoa, are actually reduced annelids.[21] Research suggest that alsonemerteans are annelids, withOweniidae andMagelonidae as their closest relatives.[22]
No single feature distinguishes Annelids from otherinvertebrate phyla, but they have a distinctive combination of features. Their bodies are long, withsegments that are divided externally by shallow ring-like constrictions calledannuli and internally by septa ("partitions") at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets oforgans, although sharing a commongut,circulatory system andnervous system makes them inter-dependent.[8][12] Their bodies are covered by acuticle (outer covering) that does not containcells but issecreted by cells in the skin underneath, is made of tough but flexiblecollagen[8] and does notmolt[23] – on the other handarthropods' cuticles are made of the more rigid α-chitin,[8][24] and molt until the arthropods reach their full size.[25] Most annelids have closed circulatory systems, where the blood makes its entire circuit viablood vessels.[23]
In addition to Sipuncula and Echiura, also lineages like Lobatocerebrum,Diurodrilus and Polygordius have lost their segmentation, but these are the exceptions from the rule.[33] Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and externalchaetae (Greek χαιτη, meaning "hair") and, in some species, appendages. The frontmost and rearmost sections are not regarded as true segments as they do not contain the standard sets of organs and do not develop in the same way as the true segments. The frontmost section, called theprostomium (Greek προ- meaning "in front of" and στομα meaning "mouth") contains the brain and sense organs, while the rearmost, called thepygidium (Greek πυγιδιον, meaning "little tail") orperiproct contains theanus, generally on the underside. The first section behind the prostomium, called theperistomium (Greek περι- meaning "around" and στομα meaning "mouth"), is regarded by some zoologists as not a true segment, but in somepolychaetes the peristomium has chetae and appendages like those of other segments.[8]
The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is calledteloblastic growth.[8] Some groups of annelids, including allleeches,[15] have fixed maximum numbers of segments, while others add segments throughout their lives.[12]
The phylum's name is derived from theLatin wordannelus, meaning "little ring".[6]
Annelids' cuticles are made ofcollagen fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis (outermost skin layer). A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, andmucus-secretingglands in the epidermis protect their skins.[8] Under the epidermis is thedermis, which is made ofconnective tissue, in other words a combination of cells and non-cellular materials such as collagen. Below this are two layers of muscles, which develop from the lining of thecoelom (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips,[23] whose contractions make the segment shorter and fatter.[8] But several families have lost the circular muscles, and it has been suggested that the lack of circular muscles is aplesiomorphic character in Annelida.[34] Some annelids also have oblique internal muscles that connect the underside of the body to each side.[23]
Thesetae ("hairs") of annelids project out from the epidermis to providetraction and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. Theparapodia ("limbs") of annelids that have them often bear more complex chetae at their tips – for example jointed, comb-like or hooked.[8] Chetae are made of moderately flexible β-chitin and are formed byfollicles, each of which has a chetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by formingmicrovilli, fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta.[8] Hence annelids' chetae are structurally different from thesetae ("bristles") ofarthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle.[8]
Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes withcirri (fused bundles ofcilia) andgills.[23]
Thebrain generally forms a ring round thepharynx (throat), consisting of a pair ofganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it.[8] The brains ofpolychaetes are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the prostomium.[35] In some very mobile and activepolychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections.[23] The rest of thecentral nervous system, theventral nerve cord, is generally "ladder-like", consisting of a pair of nerve cords that run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection. From eachsegmental ganglion a branching system of local nerves runs into the body wall and then encircles the body.[8] However, in most polychaetes the two main nerve cords are fused, and in the tube-dwellinggenusOwenia the single nerve chord has no ganglia and is located in theepidermis.[12][36]
As inarthropods, each muscle fiber (cell) is controlled by more than oneneuron, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons.Vertebrates have a different system, in which one neuron controls a group of muscle fibers.[8] Most annelids' longitudinal nerve trunks include giantaxons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement.[8]
The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body.[8] Nuchal ("on the neck") organs are paired,ciliated structures found only in polychaetes, and are thought to bechemosensors.[23] Some polychaetes also have various combinations ofocelli ("little eyes") that detect the direction from which light is coming andcamera eyes orcompound eyes that can probably form images.[36][37] The compound eyes probablyevolved independently of arthropods' eyes.[23] Some tube-worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withdraw into their tubes.[36] Some burrowing and tube-dwelling polychaetes havestatocysts (tilt and balance sensors) that indicate which way is down.[36] A few polychaetegenera have on the undersides of their heads palps that are used both in feeding and as "feelers", and some of these also have antennae that are structurally similar but probably are used mainly as "feelers".[23]
Most annelids have a pair ofcoelomata (body cavities) in each segment, separated from other segments bysepta and from each other by verticalmesenteries. Each septum forms a sandwich withconnective tissue in the middle andmesothelium (membrane that serves as a lining) from the preceding and following segments on either side. Each mesentery is similar except that the mesothelium is the lining of each of the pair of coelomata, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it.[8] The mesothelium is made of modifiedepitheliomuscular cells;[8] in other words, their bodies form part of the epithelium but their bases extend to formmuscle fibers in the body wall.[38] The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium, especially on the outside of the gut, may also formchloragogen cells that perform similar functions to thelivers of vertebrates: producing and storingglycogen andfat; producing theoxygen-carrierhemoglobin; breaking downproteins; and turningnitrogenous waste products intoammonia andurea to beexcreted.[8]
Many annelids move byperistalsis (waves of contraction and expansion that sweep along the body),[8] or flex the body while usingparapodia to crawl or swim.[39] In these animals the septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled "balloon".[8] However, the septa are often incomplete in annelids that are semi-sessile or that do not move by peristalsis or by movements of parapodia – for example some move by whipping movements of the body, some small marine species move by means ofcilia (fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themselves into it.[8]
The fluid in the coelomata contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain arespiratory pigment – redhemoglobin in some species, greenchlorocruorin in others (dissolved in the plasma)[23] – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in theblood plasma. Species with well-developed septa generally also have blood vessels running all long their bodies above and below the gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks ofcapillaries in the body wall and around the gut transfer blood between the main blood vessels and to parts of the segment that need oxygen and nutrients. Both of the major vessels, especially the upper one, can pump blood by contracting. In some annelids the forward end of the upper blood vessel is enlarged with muscles to form a heart, while in the forward ends of manyearthworms some of the vessels that connect the upper and lower main vessels function as hearts. Species with poorly developed or no septa generally have no blood vessels and rely on the circulation within the coelom for delivering nutrients and oxygen.[8]
However,leeches and their closest relatives have a body structure that is very uniform within the group but significantly different from that of other annelids, including other members of the Clitellata.[15] In leeches there are no septa, the connective tissue layer of the body wall is so thick that it occupies much of the body, and the two coelomata are widely separated and run the length of the body. They function as the main blood vessels, although they are side-by-side rather than upper and lower. However, they are lined with mesothelium, like the coelomata and unlike the blood vessels of other annelids. Leeches generally use suckers at their front and rear ends to move likeinchworms. The anus is on the upper surface of the pygidium.[15]
In some annelids, includingearthworms, allrespiration is via the skin. However, manypolychaetes and someclitellates (the group to which earthworms belong) havegills associated with most segments, often as extensions of theparapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.[23]
Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscularpharynx that can be everted (turned inside out to extend it). In these animals the foremost few segments often lack septa so that, when the muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Twofamilies, theEunicidae andPhyllodocidae, have evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter. On the other hand, some predatory polychaetes have neither jaws nor eversible pharynges. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths.Filter feeders use "crowns" of palps covered incilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marinesediments via mouths that are generally unspecialized. Someclitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth.[23]
The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with theanus on the underside of the pygidium.[8] However, in members of the tube-dwelling familySiboglinidae the gut is blocked by a swollen lining that housessymbioticbacteria, which can make up 15% of the worms' total weight. The bacteria convertinorganic matter – such ashydrogen sulfide andcarbon dioxide fromhydrothermal vents, ormethane fromseeps – to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria.[23]
Annelids with blood vessels usemetanephridia to remove soluble waste products, while those without useprotonephridia.[8] Both of these systems use a two-stage filtration process, in which fluid and waste products are first extracted and these are filtered again to re-absorb any re-usable materials while dumping toxic and spent materials asurine. The difference is that protonephridia combine both filtration stages in the same organ, while metanephridia perform only the second filtration and rely on other mechanisms for the first – in annelids special filter cells in the walls of the blood vessels let fluids and other small molecules pass into the coelomic fluid, where it circulates to the metanephridia.[40] In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment. As a result, the hindmost segment (before the growth zone and pygidium) has no structure that extracts its wastes, as there is no following segment to filter and discharge them, while the first segment contains an extraction structure that passes wastes to the second, but does not contain the structures that re-filter and discharge urine.[8]
Polychaetes can reproduce asexually, by dividing into two or more pieces or bybudding off a new individual while the parent remains a complete organism.[8][41] Someoligochaetes, such asAulophorus furcatus, seem to reproduce entirely asexually, while others reproduce asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding.[12][42] However,leeches have never been seen reproducing asexually.[12][43]
Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaetegenera,Chaetopterus andDodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.[12][41] Annelids are the most complex animals that can regenerate after such severe damage.[44] On the other hand, leeches cannot regenerate.[43]
It is thought that annelids were originally animals with two separatesexes, which releasedova andsperm into the water via theirnephridia.[8] The fertilized eggs develop intotrochophorelarvae, which live asplankton.[46] Later they sink to the sea-floor andmetamorphose into miniature adults: the part of the trochophore between theapical tuft and theprototroch becomes the prostomium (head); a small area round the trochophore'sanus becomes the pygidium (tail-piece); a narrow band immediately in front of that becomes the growth zone that produces new segments; and the rest of the trochophore becomes the peristomium (the segment that contains the mouth).[8]
However, the lifecycles of most livingpolychaetes, which are almost all marine animals, are unknown, and only about 25% of the 300+ species whose lifecycles are known follow this pattern. About 14% use a similarexternal fertilization but produceyolk-rich eggs, which reduce the time the larva needs to spend among the plankton, or eggs from which miniature adults emerge rather than larvae. The rest care for the fertilized eggs until they hatch – some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few species by keeping the eggs within their bodies until they hatch. These species use a variety of methods for sperm transfer; for example, in some the females collect sperm released into the water, while in others the males have apenis that inject sperm into the female.[46] There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge.[46]
Some polychaetes breed only once in their lives, while others breed almost continuously or through several breeding seasons. While most polychaetes remain of one sex all their lives, a significant percentage of species are fullhermaphrodites or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanentgonads, and it is uncertain how they produce ova and sperm. In a few species the rear of the body splits off and becomes a separate individual that lives just long enough to swim to a suitable environment, usually near the surface, and spawn.[46]
Most matureclitellates (the group that includesearthworms andleeches) are full hermaphrodites, although in a few leech species younger adults function as males and become female at maturity. All have well-developed gonads, and allcopulate. Earthworms store their partners' sperm inspermathecae ("sperm stores") and then theclitellum produces acocoon that collects ova from theovaries and then sperm from the spermathecae. Fertilization and development of earthworm eggs takes place in the cocoon. Leeches' eggs are fertilized in the ovaries, and then transferred to the cocoon. In all clitellates the cocoon also either produces yolk when the eggs are fertilized or nutrients while they are developing. All clitellates hatch as miniature adults rather than larvae.[46]
Charles Darwin's bookThe Formation of Vegetable Mould Through the Action of Worms (1881) presented the first scientific analysis of earthworms' contributions tosoil fertility.[47] Some burrow while others live entirely on the surface, generally in moistleaf litter. The burrowers loosen thesoil so that oxygen and water can penetrate it, and both surface and burrowing worms help to produce soil by mixing organic and mineral matter, by accelerating thedecomposition of organic matter and thus making it more quickly available to other organisms, and by concentrating minerals and converting them to forms that plants can use more easily.[48][49] Earthworms are also important prey for birds ranging in size fromrobins tostorks, and for mammals ranging fromshrews tobadgers, and in some cases conserving earthworms may be essential for conserving endangered birds.[50]
Terrestrial annelids can be invasive in some situations. In the glaciated areas of North America, for example, almost all native earthworms are thought to have been killed by the glaciers and the worms currently found in those areas are all introduced from other areas, primarily from Europe, and, more recently, from Asia. Northern hardwood forests are especially negatively impacted by invasive worms through the loss of leaf duff, soil fertility, changes in soil chemistry and the loss of ecological diversity. Especially of concern isAmynthas agrestis and at least one state (Wisconsin) has listed it as a prohibited species.
Earthworms migrate only a limited distance annually on their own, and the spread of invasive worms is increased rapidly by anglers and from worms or their cocoons in the dirt on vehicle tires or footwear.
Marine annelids may account for over one-third of bottom-dwelling animal species aroundcoral reefs and intidal zones.[47] Burrowing species increase the penetration of water and oxygen into the sea-floorsediment, which encourages the growth of populations ofaerobic bacteria and small animals alongside their burrows.[51]
Earthworms make a significant contribution tosoil fertility.[47] The rear end of thePalolo worm, a marinepolychaete that tunnels through coral, detaches in order to spawn at the surface, and the people ofSamoa regard these spawning modules as a delicacy.[47]Anglers sometimes find that worms are more effective bait than artificial flies, and worms can be kept for several days in a tin lined with damp moss.[52]Ragworms are commercially important as bait and as food sources foraquaculture, and there have been proposals to farm them in order to reduce over-fishing of their natural populations.[51] Some marinepolychaetes' predation on molluscs causes serious losses to fishery andaquaculture operations.[47]
Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.[47]
Accounts of the use ofleeches for the medically dubious practice ofblood-letting have come from China around 30 AD, India around 200 AD, ancient Rome around 50 AD and later throughout Europe. In the 19th century medical demand for leeches was so high that some areas' stocks were exhausted and other regions imposed restrictions or bans on exports, andHirudo medicinalis is treated as an endangered species by bothIUCN andCITES. More recently leeches have been used to assist inmicrosurgery, and theirsaliva has providedanti-inflammatory compounds and several importantanticoagulants, one of which also preventstumors fromspreading.[47]
Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which arebiomineralized withcalcium salts. These advantages have attracted the attention of engineers. Investigations showed that ragworm jaws are made of unusualproteins that bind strongly tozinc.[53]
Since annelids aresoft-bodied, their fossils are rare.[54]Polychaetes' fossil record consists mainly of the jaws that some species had and themineralized tubes that some secreted.[55] SomeEdiacaran fossils such asDickinsonia in some ways resemblepolychaetes, but the similarities are too vague for these fossils to be classified with confidence.[1] Thesmall shelly fossilCloudina, from549 to 542 million years ago, has been classified by some authors as an annelid, but by others as acnidarian (i.e. in the phylum to whichjellyfish andsea anemones belong).[56][57] Until 2008 the earliest fossils widely accepted as annelids were the polychaetesCanadia andBurgessochaeta, both from Canada'sBurgess Shale, formed about505 million years ago in theMiddleCambrian.[58]Myoscolex, found in Australia and a little older than the Burgess Shale, was possibly an annelid. However, it lacks some typical annelid features and has features which are not usually found in annelids and some of which are associated with other phyla.[58] ThenSimon Conway Morris and John Peel reportedPhragmochaeta fromSirius Passet, about518 million years old, and concluded that it was the oldest annelid known to date.[1] There has been vigorous debate about whether the Burgess Shale fossilWiwaxia was amollusc or an annelid.[58] Polychaetes diversified in the earlyOrdovician, about488 to 474 million years ago. It is not until the early Ordovician that the first annelid jaws are found, thus the crown-group cannot have appeared before this date and probably appeared somewhat later.[59] By the end of theCarboniferous, about299 million years ago, fossils of most of the modern mobile polychaete groups had appeared.[58] Many fossil tubes look like those made by modernsessile polychaetes,[60] but the first tubes clearly produced by polychaetes date from theJurassic, less than199 million years ago.[58] In 2012, a 508 million year old species of annelid found near theBurgess shale beds inBritish Columbia,Kootenayscolex, was found that changed the hypotheses about how the annelid head developed. It appears to have bristles on its head segment akin to those along its body, as if the head simply developed as a specialized version of a previously generic segment.
The earliest good evidence foroligochaetes occurs in theTertiary period, which began65 million years ago, and it has been suggested that these animals evolved around the same time asflowering plants in the earlyCretaceous, from130 to 90 million years ago.[61] Atrace fossil consisting of a convolutedburrow partly filled with smallfecal pellets may be evidence that earthworms were present in the earlyTriassic period from251 to 245 million years ago.[61][62]Body fossils going back to the midOrdovician, from472 to 461 million years ago, have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed.[61][63]
Traditionally the annelids have been divided into two major groups, thepolychaetes andclitellates. In turn the clitellates were divided intooligochaetes, which includeearthworms, andhirudinomorphs, whose best-known members areleeches.[8] For many years there was no clear arrangement of the approximately 80 polychaetefamilies into higher-level groups.[10] In 1997 Greg Rouse and Kristian Fauchald attempted a "first heuristic step in terms of bringing polychaete systematics to an acceptable level of rigour", based on anatomical structures, and divided polychaetes into:[64]
Annelid groups and phyla incorporated into Annelida (2007;simplified).[10] Highlights major changes to traditional classifications.
Also in 1997 Damhnait McHugh, usingmolecular phylogenetics to compare similarities and differences in one gene, presented a very different view, in which: the clitellates were an offshoot of one branch of the polychaete family tree; thepogonophorans andechiurans, which for a few decades had been regarded as a separatephyla, were placed on other branches of the polychaete tree.[67] Subsequent molecular phylogenetics analyses on a similar scale presented similar conclusions.[68]
In 2007 Torsten Struck and colleagues compared three genes in 81taxa, of which nine were outgroups,[10] in other words not considered closely related to annelids but included to give an indication of where the organisms under study are placed on the largertree of life.[69] For a cross-check the study used an analysis of 11 genes (including the original 3) in ten taxa. This analysis agreed that clitellates, pogonophorans and echiurans were on various branches of the polychaete family tree. It also concluded that the classification of polychaetes into Scolecida, Canalipalpata and Aciculata was useless, as the members of these alleged groups were scattered all over the family tree derived from comparing the 81 taxa. It also placedsipunculans, generally regarded at the time as a separate phylum, on another branch of the polychaete tree, and concluded that leeches were a sub-group of oligochaetes rather than theirsister-group among the clitellates.[10] Rouse accepted the analyses based on molecular phylogenetics,[12] and their main conclusions are now the scientific consensus, although the details of the annelid family tree remain uncertain.[11]
In addition to re-writing the classification of annelids and three previously independent phyla, the molecular phylogenetics analyses undermine the emphasis that decades of previous writings placed on the importance ofsegmentation in the classification ofinvertebrates. Polychaetes, which these analyses found to be the parent group, have completely segmented bodies, while polychaetes' echiurans and sipunculan offshoots are not segmented and pogonophores are segmented only in the rear parts of their bodies. It now seems that segmentation can appear and disappear much more easily in the course of evolution than was previously thought.[10][67] The 2007 study also noted that the ladder-like nervous system, which is associated with segmentation, is less universal than previously thought in both annelids and arthropods.[10][b]
Annelids are members of theprotostomes, one of the two majorsuperphyla ofbilaterian animals – the other is thedeuterostomes, which includesvertebrates.[68] Within the protostomes, annelids used to be grouped witharthropods under the super-groupArticulata ("jointed animals"), as segmentation is obvious in most members of both phyla. However, the genes that drive segmentation in arthropods do not appear to do the same in annelids. Arthropods and annelids both have close relatives that are unsegmented. It is at least as easy to assume that they evolved segmented bodiesindependently as it is to assume that the ancestral protostome or bilaterian was segmented and that segmentation disappeared in many descendant phyla.[68] The current view is that annelids are grouped withmolluscs,brachiopods and several other phyla that havelophophores (fan-like feeding structures) and/ortrochophore larvae as members ofLophotrochozoa.[75] Meanwhile, arthropods are now regarded as members of theEcdysozoa ("animals thatmolt"), along with some phyla that are unsegmented.[68][76]
The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids,molluscs,nemerteans andflatworms, follow a similar pattern in the fertilized egg's development. When their cells divide after the 4-cell stage, descendants of these four cells form a spiral pattern. In these phyla the "fates" of the embryo's cells, in other words the roles their descendants will play in the adult animal, are the same and can be predicted from a very early stage.[77] Hence this development pattern is often described as "spiral determinatecleavage".[78]
^Since this section was written, a new paper has revised the 2007 results:Struck, T. H.; Paul, C.; Hill, N.; Hartmann, S.; Hösel, C.; Kube, M.; Lieb, B.; Meyer, A.; Tiedemann, R.; Purschke, G. N.; Bleidorn, C. (2011). "Phylogenomic analyses unravel annelid evolution".Nature.471 (7336):95–98.Bibcode:2011Natur.471...95S.doi:10.1038/nature09864.PMID21368831.S2CID4428998.
^abcdefghijkRouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).Invertebrate Zoology. Oxford University Press. pp. 176–179.ISBN978-0-19-551368-4.
^abcdeRouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.).Invertebrate Zoology. Oxford University Press. pp. 179–183.ISBN978-0-19-551368-4.
^abcdefghijklmnoRouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).Invertebrate Zoology. Oxford University Press. pp. 183–196.ISBN978-0-19-551368-4.
^Cutler, B. (August 1980). "Arthropod cuticle features and arthropod monophyly".Cellular and Molecular Life Sciences.36 (8): 953.doi:10.1007/BF01953812.S2CID84995596.
^Anderson, D. T. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.).Invertebrate Zoology. Oxford University Press. pp. 183–196.ISBN978-0-19-551368-4.
^Minelli, Alessandro (2009), "Chapter 6. A gallery of the major bilaterian clades",Perspectives in Animal Phylogeny and Evolution, Oxford University Press,ISBN978-0-19-856620-5, p. 82:This is the case for circular muscles, which have been reported as absent in many families (Opheliidae, Protodrilidae, Spionidae, Oweniidae, Aphroditidae, Acoetidae, Polynoidae, Sigalionidae, Phyllodocidae, Nephtyidae, Pisionidae, and Nerillidae; Tzetlinet al. 2002). Tzetlin and Filippova (2005) suggest that absence of circular muscles is possibly plesiomorphic in the Annelida.
^abcdeRouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).Invertebrate Zoology. Oxford University Press. pp. 196–202.ISBN978-0-19-551368-4.
^abScaps, P. (February 2002). "A review of the biology, ecology and potential use of the common ragwormHediste diversicolor".Hydrobiologia.470 (1–3):203–218.doi:10.1023/A:1015681605656.S2CID22669841.
^Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).Invertebrate Zoology. Oxford University Press. p. 202.ISBN978-0-19-551368-4.
^Conway Morris, S.; Pickerill, R.K.; Harland, T.L. (1982). "A possible annelid from the Trenton Limestone (Ordovician) of Quebec, with a review of fossil oligochaetes and other annulate worms".Canadian Journal of Earth Sciences.19 (11):2150–2157.Bibcode:1982CaJES..19.2150M.doi:10.1139/e82-189.
^Weigert, Anne; Bleidorn, Christoph (2015). "Current status of annelid phylogeny".Organisms Diversity & Evolution.16 (2):345–362.doi:10.1007/s13127-016-0265-7.S2CID5353873.
^Struck TH (2019). "Phylogeny". In Purschke G, Böggemann M, Westheide W (eds.).Handbook of Zoology: Annelida. Vol. 1: Annelida Basal Groups and Pleistoannelida, Sedentaria I. De Gruyter. pp. 37–68.doi:10.1515/9783110291582-002.ISBN9783110291469.S2CID242569001.
^Aguinaldo, A. M. A.; J. M. Turbeville; L. S. Linford; M. C. Rivera; J. R. Garey; R. A. Raff; J. A. Lake (1997). "Evidence for a clade of nematodes, arthropods and other moulting animals".Nature.387 (6632):489–493.Bibcode:1997Natur.387R.489A.doi:10.1038/387489a0.PMID9168109.S2CID4334033.
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