Annelid - Knowledge

2040:. 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. 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. 1535: 5053: 2545: 4856: 3710:. 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. 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. 2248:-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 a 4766: 4835: 5013: 4897: 4992: 5073: 4797: 136: 5111: 118: 1936: 3689:. 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 placed 5217: 8809: 1455: 1374: 4941: 2174: 2122: 5209:, 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. Hence this development pattern is often described as "spiral determinate 8797: 872: 865: 2054: 1967:, 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. 1891:

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 like

1692:(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. 1567:(body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips, whose contractions make the segment shorter and fatter. But several families have lost the circular muscles, and it has been suggested that the lack of circular muscles is a 4639:("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 bodies 1820: 1610:

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 with

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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

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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. In leeches there are no septa, the connective tissue layer of the body wall is so thick that it occupies much of

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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

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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,

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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

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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. The mesothelium is made of

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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

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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, and

1598:, 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. Hence annelids' chetae are structurally different from the 1546:

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, and

1479:(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 the 995:

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 a

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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. Two

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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; Tzetlin

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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 their

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to crawl or swim. 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". However, the septa are often incomplete in annelids that are

2646:, 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. Then 2728:, 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. 1882:

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.

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into higher-level groups. 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:

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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 is

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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. Earthworms are also important prey for birds ranging in size from

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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, most textbooks use the traditional classification into the following sub-groups:

2264:, 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. 707:, 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 by 1826: 1771:

fibers in the body wall. 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 form

2109:, can regenerate from a single segment, and others can regenerate even if their heads are removed. Annelids are the most complex animals that can regenerate after such severe damage. On the other hand, leeches cannot regenerate. 2255:

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 full

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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).

2086:, 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. However, 2036:. 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 as 1474:

and Polygordius have lost their segmentation, but these are the exceptions from the rule. Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external

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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 of

1873:. 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 of 1695:

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. Nuchal ("on the neck") organs are paired,

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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 called

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of arthropods' eyes. 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. Some burrowing and tube-dwelling polychaetes have

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among the clitellates. Rouse accepted the analyses based on molecular phylogenetics, and their main conclusions are now the scientific consensus, although the details of the annelid family tree remain uncertain.

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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.

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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".

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Guo, Jin; Parry, Luke A.; Vinther, Jakob; Edgecombe, Gregory D.; Wei, Fan; Zhao, Jun; Zhao, Yang; Béthoux, Olivier; Lei, Xiangtong; Chen, Ailin; Hou, Xianguang; Chen, Taimin; Cong, Peiyun (2022).

1658:, 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 each 5654:

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".

2688:. 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. By the end of the 8081:

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".

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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.

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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".

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as it is to assume that the ancestral protostome or bilaterian was segmented and that segmentation disappeared in many descendant phyla. The current view is that annelids are grouped with

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that inject sperm into the female. There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge.

3685:, of which nine were outgroups, 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 larger 964:

because of their simple body structure, but are now regarded as polychaetes. Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:

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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 of

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Dunn, CW; Hejnol, A; Matus, DQ; Pang, K; Browne, WE; Smith, SA; Seaver, E; Rouse, GW; Obst, M (2008). "Broad phylogenomic sampling improves resolution of the animal tree of life".

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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; the

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a branching system of local nerves runs into the body wall and then encircles the body. However, in most polychaetes the two main nerve cords are fused, and in the tube-dwelling

3693:, 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 their 1642:(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. The brains of 5874:

Renne, Paul R.; Deino, Alan L.; Hilgen, Frederik J.; Kuiper, Klaudia F.; Mark, Darren F.; Mitchell, William S.; Morgan, Leah E.; Mundil, Roland; Smit, Jan (7 February 2013).

1075:). 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. 1586:("limbs") of annelids that have them often bear more complex chetae at their tips – for example jointed, comb-like or hooked. Chetae are made of moderately flexible β- 914:

that stores and nourishes fertilized eggs until they hatch or, in moniligastrids, yolky eggs that provide nutrition for the embryos. The clitellates are sub-divided into:

2313: 2240:, 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 similar 1646:

are generally in the prostomium, while those of clitellates are in the peristomium or sometimes the first segment behind the prostomium. In some very mobile and active

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research since 1997 has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the

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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 of

1822: 2279:) 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 all 1025:, but in 1998 it was suggested that they are a sub-group of polychaetes. However, another analysis in 2002 suggested that myzostomids are more closely related to 1594:, 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 forming 1857:

The fluid in the coelomata contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain a

1563:, 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 the 2329:

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 the

7035: 2025:– 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. 3887: 3726: 1764: 256: 6545:"A close phylogenetic relationship between Sipuncula and Annelida evidenced from the complete mitochondrial genome sequence of Phascolosoma esculenta" 3678:, were placed on other branches of the polychaete tree. Subsequent molecular phylogenetics analyses on a similar scale presented similar conclusions. 4265: 327: 10421: 4018: 3901: 3815: 3739: 3722: 2824: 1163:, and molt until the arthropods reach their full size. Most annelids have closed circulatory systems, where the blood makes its entire circuit via 735:. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of 272: 251: 11065: 1854:(fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themselves into it. 942:, mainly on fish, while most freshwater species are predators. They have suckers at both ends of their bodies, and use these to move rather like 6981: 6654:

Mitochondrial Genome Evolution in Annelida—A Systematic Study on Conservative and Variable Gene Orders and the Factors Influencing its Evolution

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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.

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were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate

8311: 1606:, 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. 1059:

characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the

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A group of worms classified by some as polychaetes and by others as clitellates, see Rouse & Fauchald (1997) "Cladistics and polychaetes"

7803: 2221: 8035:"Palaeosols in the upper Narrabeen Group of New South Wales as evidence of Early Triassic palaeoenvironments without exact modern analogues" 7829:"Inconsistencies in proposed annelid affinities of early biomineralized organism Cloudina (Ediacaran): structural and ontogenetic evidences" 2225: 7591: 1950:

Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular

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in others (dissolved in the plasma) – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the

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have a different system, in which one neuron controls a group of muscle fibers. Most annelids' longitudinal nerve trunks include giant

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and other capabilities. The simplest are unjointed and form paired bundles near the top and bottom of each side of each segment. The

7957: 2667:, and concluded that it was the oldest annelid known to date. There has been vigorous debate about whether the Burgess Shale fossil 11039: 7828: 1986:

The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the

11078: 10127: 1483:(Greek προ- meaning "in front of" and στομα meaning "mouth") contains the brain and sense organs, while the rearmost, called the 2517:

salts. These advantages have attracted the attention of engineers. Investigations showed that ragworm jaws are made of unusual

8701: 656:, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the 8713: 8011: 7732: 7680: 7603: 7568: 7541: 7514: 7470: 7411: 7386: 7352: 7321: 7287: 7256: 7225: 7145: 7058: 6921: 6890: 6859: 6828: 6803: 6772: 6703: 6373: 6335: 6304: 6270: 6082: 1571:

character in Annelida. Some annelids also have oblique internal muscles that connect the underside of the body to each side.

7668: 1499:(Greek περι- meaning "around" and στομα meaning "mouth"), is regarded by some zoologists as not a true segment, but in some 11083: 1927:

in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.

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Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.

11183: 7070:. 2002). Tzetlin and Filippova (2005) suggest that absence of circular muscles is possibly plesiomorphic in the Annelida. 2799:, have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed. 2699:, fossils of most of the modern mobile polychaete groups had appeared. Many fossil tubes look like those made by modern 8841: 5956: 2445:, and there have been proposals to farm them in order to reduce over-fishing of their natural populations. Some marine 924:. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or partly decomposed 844:(about 12,000 species). As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes have 7558: 8515: 7531: 7502: 5197:

The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids,

11150: 11018: 8878: 5726:"The earliest annelids: Lower Cambrian polychaetes from the Sirius Passet Lagerstätte, Peary Land, North Greenland" 2093:

Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete

3151:, which are distinguished by having long grooved palps that they use for feeding, and most of which live in tubes. 703:

show no obvious signs of segmentation. In species with well-developed septa, the blood circulates entirely within

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the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections. The rest of the

7436: 597:. The species exist in and have adapted to various ecologies – some in marine environments as distinct as 8957: 8947: 8930: 2181: 1373: 8426:"Combined-data phylogenetics and character evolution of Clitellata (Annelida) using 18S rDNA and morphology" 771:

fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about

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Budd, G. E.; Jensen, S. (May 2000). "A critical reappraisal of the fossil record of the bilaterian phyla".

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Struck, T.H.; Schult, N.; Kusen, T.; Hickman, E.; Bleidorn, C.; McHugh, D.; Halanych, K.M. (5 April 2007).

1684:, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons. 10974: 10961: 10120: 5818: 764: 8168: 6653: 7696: 5829: 2260:

or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent

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Scaps, P. (February 2002). "A review of the biology, ecology and potential use of the common ragworm

2652: 1462: 821: 135: 10979: 8502:. Vol. 1: Annelida Basal Groups and Pleistoannelida, Sedentaria I. De Gruyter. pp. 37–68. 8307: 7084:"Challenging received wisdoms: Some contributions of the new microscopy to the new animal phylogeny" 5875: 808:, are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the 8161:"Annelida. Annelida. Segmented worms: bristleworms, ragworms, earthworms, leeches and their allies" 6504:"Spiralian Phylogenomics Supports the Resurrection of Bryozoa Comprising Ectoprocta and Entoprocta" 2534: 1975:

that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine

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Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which are

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There are over 22,000 living annelid species, ranging in size from microscopic to the Australian

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Paxton, H. (June 2005). "Molting polychaete jaws—ecdysozoans are not the only molting animals".

3747:, the clade containing nearly all of annelid diversity, divided into two highly diverse groups: 10901: 10113: 8834: 8034: 3788: 3703: 3686: 2241: 1767:

cells; in other words, their bodies form part of the epithelium but their bases extend to form

1651: 1579: 1112: 680: 11132: 8737:"A Cambrian tommotiid preserving soft tissues reveals the metameric ancestry of lophophorates" 7996: 5725: 11127: 10577: 10227: 8925: 8287: 8268: 5636: 5237: 4636: 3157:, the most active polychaetes, which have parapodia reinforced by internal spines (aciculae). 2869: 2582:, but the similarities are too vague for these fossils to be classified with confidence. The 2557: 1841:(waves of contraction and expansion that sweep along the body), or flex the body while using 856:. Most are marine animals, although a few species live in fresh water and even fewer on land. 760: 476: 228: 11142: 11119: 2350:, and in some cases conserving earthworms may be essential for conserving endangered birds. 1515:, have fixed maximum numbers of segments, while others add segments throughout their lives. 10987: 10948: 10242: 8748: 8654: 8599: 8548: 8345: 8214: 8090: 8049: 7761: 6819:

Anderson, D. T. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.).

6456: 6158: 5890: 5779: 5663: 4640: 2280: 1717: 938:-shaped" and whose best known members are leeches. Marine species are mostly blood-sucking 826: 11070: 7378: 7372: 7344: 7338: 7313: 7307: 7279: 7273: 7248: 7242: 7217: 7211: 7137: 7131: 6913: 6907: 6882: 6876: 6851: 6845: 6795: 6789: 6764: 6758: 6365: 6359: 6327: 6321: 6074: 6068: 2610:

belong). Until 2008 the earliest fossils widely accepted as annelids were the polychaetes

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that tunnels through coral, detaches in order to spawn at the surface, and the people of

1858: 1847: 1747: 879: 751:. Scientists observe annelids to monitor the quality of marine and fresh water. Although 609: 10105: 8752: 8658: 8603: 8552: 8458: 8349: 8218: 8094: 8053: 7844: 7765: 6460: 6162: 5894: 5849: 5783: 5667: 2367:

Marine annelids may account for over one-third of bottom-dwelling animal species around

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phyla, but they have a distinctive combination of features. Their bodies are long, with

10939: 10572: 10254: 9390: 8942: 8920: 8908: 8883: 8813: 8623: 8572: 8521: 8480: 8406: 8371: 8136: 8132: 7938: 7926: 7895: 7777: 7750:"Decay and preservation of polychaetes; taphonomic thresholds in soft-bodied organisms" 7649: 7004: 6962: 6736: 6620: 6595: 6571: 6544: 6425: 6181: 6146: 5914: 5748: 5687: 4791: 3718: 2470: 2014: 1659: 1655: 1591: 1128: 992: 832: 712: 695:

separate the segments of many species, but are poorly defined or absent in others, and

130: 7723:

Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).

7461:

Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).

6694:

Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).

6261:

Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D. T. (ed.).

11178: 11114: 10364: 9812: 9732: 9082: 8952: 8915: 8903: 8893: 8827: 8774: 8766: 8709: 8682: 8677: 8642: 8615: 8564: 8525: 8511: 8442: 8425: 8363: 8242: 8237: 8202: 8007: 7930: 7891: 7728: 7676: 7599: 7564: 7537: 7510: 7466: 7432: 7407: 7382: 7348: 7317: 7283: 7252: 7221: 7192: 7184: 7141: 7105: 7054: 7000: 6954: 6950: 6917: 6906:

Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Onychophora and Tardigrada".

6886: 6855: 6824: 6799: 6768: 6757:

Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Introduction to Arthropoda".

6699: 6625: 6576: 6525: 6484: 6479: 6444: 6417: 6369: 6331: 6300: 6295:

Rouse, G. (1998). "The Annelida and their close relatives". In Anderson, D.T. (ed.).

6266: 6186: 6078: 5952: 5906: 5797: 5679: 5210: 3714: 2612: 2510: 2491: 2356: 1751: 1560: 1014:, mainly as parasites. In the past they have been regarded as close relatives of the 688: 602: 8484: 8375: 8140: 7942: 7899: 7781: 7653: 6966: 6740: 6429: 5752: 4855: 4765: 2375:. Burrowing species increase the penetration of water and oxygen into the sea-floor 10640: 10519: 10411: 10354: 10294: 10289: 10217: 10202: 8867: 8801: 8756: 8672: 8662: 8627: 8607: 8576: 8556: 8503: 8470: 8437: 8410: 8398: 8353: 8283: 8232: 8222: 8128: 8098: 8057: 7972: 7922: 7887: 7836: 7769: 7641: 7174: 7095: 7047:

Minelli, Alessandro (2009), "Chapter 6. A gallery of the major bilaterian clades",

7008: 6996: 6946: 6728: 6615: 6607: 6566: 6556: 6515: 6474: 6464: 6407: 6220: 6176: 6166: 5944: 5918: 5898: 5787: 5740: 5691: 5671: 4699: 4140: 2832: 2831:. For many years there was no clear arrangement of the approximately 80 polychaete 2719: 2406: 2380: 2300:

they are developing. All clitellates hatch as miniature adults rather than larvae.

2006: 1956: 1943: 1773: 1670: 1611: 981: 925: 782:

period. Fossils of most modern mobile polychaete groups appeared by the end of the

530: 492: 427: 293: 9086: 8589: 8424:

Marotta, Roberto; Ferraguti, Marco; Erséus, Christer; Gustavsson, Lena M. (2008).

8389:

Weigert, Anne; Bleidorn, Christoph (2015). "Current status of annelid phylogeny".

8196: 8194: 4834: 1990:

on the underside of the pygidium. However, in members of the tube-dwelling family

1495:, generally on the underside. The first section behind the prostomium, called the 1043:

was originally classified as annelids, despite the complete lack of segmentation,

683:. Each segment has the same sets of organs and, in most polychates, has a pair of 10953: 10813: 10718: 10680: 10635: 10567: 10324: 10284: 10269: 10162: 10093: 9999: 9850: 9753: 9610: 9143: 8334:"The Evolution of Annelids Reveals Two Adaptive Routes to the Interstitial Realm" 8074: 7990: 7988: 7986: 7272:

Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Introduction to Bilateria".

7048: 5766:

Bobrovskiy, I.; Nagovitsyn, A.; Hope, J. M.; Luzhnaya, E.; Brocks, J. J. (2022).

5233: 5012: 4954: 4896: 4461: 4071: 3744: 2747: 2569: 1759: 1313:

Coelom; but this is reduced or missing in many leeches and some small polychaetes

1120: 961: 957: 830:, which can both grow up to 3 meters (9.8 ft) long to the largest annelid, 364: 278: 10966: 7797:"A Revised Morphology of Cloudina with Ecological and Phylogenetic Implications" 6788:

Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Echiura and Sipuncula".

6719:

Cutler, B. (August 1980). "Arthropod cuticle features and arthropod monophyly".

5833:. Vol. 2 (9th ed.). New York: Charles Scribner's Sons. pp. 65–72. 5072: 4796: 11026: 11013: 10873: 10818: 10808: 10650: 10464: 10416: 10329: 10304: 10299: 10279: 10274: 10177: 10167: 9758: 9736: 9554: 9466: 9444: 9407: 8898: 8647:

Proceedings of the National Academy of Sciences of the United States of America

8498:

Struck TH (2019). "Phylogeny". In Purschke G, Böggemann M, Westheide W (eds.).

8207:

Proceedings of the National Academy of Sciences of the United States of America

8191: 7210:

Ruppert, E.E.; Fox, R.S. & Barnes, R.D. (2004). "Introduction to Metazoa".

6449:

Proceedings of the National Academy of Sciences of the United States of America

5274: 5264: 4878: 4810: 4660: 3939: 3924: 3404: 2746:, and it has been suggested that these animals evolved around the same time as 2724: 2703:

polychaetes, but the first tubes clearly produced by polychaetes date from the

2618: 2549: 2418: 2335: 2318: 2317:(1881) presented the first scientific analysis of earthworms' contributions to 2308: 2033: 2022: 2010: 1968: 1939: 1140: 1132: 1034: 977: 902:

or parapodia. However, they have a unique reproductive organ, the ring-shaped

744: 657: 213: 8761: 8736: 8507: 8402: 8358: 8333: 8061: 7983: 7773: 7645: 7179: 7162: 5792: 5767: 3694: 898:(about 10,000 species ). These have few or no chetae per segment, and no 11172: 10597: 10582: 10544: 10469: 10405: 10391: 10339: 10334: 10248: 10237: 10232: 10212: 10207: 10182: 9903: 9807: 9306: 8770: 7701: 7402:

Hickman, Cleveland; Roberts, L.; Keen, S.; Larson, A.; Eisenhour, D. (2007).

7188: 6561: 5854: 5418: 5225: 5110: 4991: 4722: 4624: 4476: 4153: 3148: 2945: 2715: 2689: 2657: 2623: 2495: 2464: 2330: 2257: 2029: 1964: 1892: 1866: 1725:(tilt and balance sensors) that indicate which way is down. A few polychaete 1701: 1568: 1124: 1060: 1052: 1047:

and other annelid characters. The phylum Sipuncula was later allied with the

953: 943: 853: 783: 752: 369: 70: 8643:"Evolution of the bilaterian body plan: What have we learned from annelids?" 8227: 8114: 8112: 7976: 6520: 6503: 6469: 5948: 5902: 2321:. Some burrow while others live entirely on the surface, generally in moist 731:

both as prey and in some regions are important in aeration and enriching of

10924: 10793: 10788: 10763: 10738: 10728: 10655: 10524: 10398: 10359: 10344: 10319: 10314: 10259: 10192: 10009: 10004: 9892: 9748: 9634: 9520: 9423: 9402: 9272: 9223: 9179: 9160: 8778: 8686: 8667: 8568: 8367: 7934: 7196: 7109: 6958: 6629: 6580: 6529: 6421: 6412: 6395: 6190: 6171: 5910: 5801: 5683: 5456: 5245: 5241: 4964: 4726: 4691: 4399: 4280: 4168: 4119: 3780: 3764: 3707: 3667: 3572: 3022: 2889: 2762: 2483: 2217: 2099: 1991: 1870: 1713: 1164: 1108: 1080: 1072: 969: 899: 849: 748: 704: 645: 621: 613: 605:, others in fresh water, and yet others in moist terrestrial environments. 467: 357: 332: 303: 8619: 8246: 8203:"Molecular evidence that echiurans and pogonophorans are derived annelids" 6488: 6445:"Molecular evidence that echiurans and pogonophorans are derived annelids" 6396:"Unsegmented Annelids? Possible Origins of Four Lophotrochozoan Worm Taxa" 5863:. Vol. 2 (11th ed.). Cambridge University Press. pp. 72–73. 5744: 5652:

Since this section was written, a new paper has revised the 2007 results:

2564:' fossil record consists mainly of the jaws that some species had and the 2360:

and at least one state (Wisconsin) has listed it as a prohibited species.

1935: 11052: 10933: 10848: 10823: 10768: 10743: 10665: 10660: 10625: 10615: 10309: 10222: 9970: 9941: 9927: 9917: 9502: 9318: 9231: 9013: 8109: 7840: 7100: 7083: 6611: 6225: 6208: 5876:"Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary" 5822: 5563: 5253: 4987: 4891: 4743: 4656: 4612: 4505: 4386: 4204: 3995: 3843: 3784: 3730: 3301: 2973: 2904: 2816: 2794: 2785: 2779: 2732: 2714:. In 2012, a 508 million year old species of annelid found near the 2683: 2634: 2607: 2593: 2574: 2450: 2442: 2422: 2395: 2322: 2284: 2206: 2105: 2076: 2001:, which can make up 15% of the worms' total weight. The bacteria convert 1838: 1830: 1755: 1708:("little eyes") that detect the direction from which light is coming and 1496: 1471: 1470:

In addition to Sipuncula and Echiura, also lineages like Lobatocerebrum,

1213: 1096: 1064: 1003: 917: 907: 803: 794: 708: 633: 459: 414: 347: 283: 266: 261: 45: 8560: 8475: 8459:"Phylogeny of basal descendants of cocoon-forming annelids (Clitellata)" 5675: 2807:

Traditionally the annelids have been divided into two major groups, the

2756: 2598:, has been classified by some authors as an annelid, but by others as a 2075:

off a new individual while the parent remains a complete organism. Some

1503:

the peristomium has chetae and appendages like those of other segments.

1116: 10881: 10843: 10773: 10753: 10748: 10733: 10703: 10675: 10630: 10620: 10592: 10587: 10529: 10504: 10197: 10172: 10067: 10052: 10047: 10028: 10023: 9951: 9898: 9868: 9860: 9802: 9676: 9668: 9651: 9431: 9343: 9296: 9236: 9198: 9172: 9018: 9003: 7874:

Dzik, J. (2004). "Anatomy and relationships of the Early Cambrian worm

6732: 5547: 5474: 5349: 5216: 5068: 5048: 4974: 4652: 4648: 4628: 4620: 4616: 4501: 4216: 3783:

polychaetes, and several archiannelids. Some small groups, such as the

3760: 3756: 3748: 3277: 2812: 2808: 2789: 2751: 2678: 2579: 2561: 2503: 2446: 2426: 2399: 2391: 2372: 2368: 2268: 2237: 2173: 2081: 2068: 2058: 1980: 1916: 1912: 1862: 1793: 1709: 1685: 1647: 1643: 1595: 1500: 1480: 1292: 1208: 1022: 1011: 921: 895: 841: 798: 728: 669: 661: 641: 629: 628:

for locomotion. Most textbooks still use the traditional division into

625: 598: 408: 394: 374: 317: 193: 117: 90: 55: 8819: 6389: 6387: 6385: 6202: 6200: 2189:

It is thought that annelids were originally animals with two separate

1115:

that are divided externally by shallow ring-like constrictions called

11057: 11000: 10798: 10778: 10670: 10549: 10484: 10474: 10459: 10454: 10435: 10187: 10033: 9980: 9975: 9946: 9935: 9884: 9879: 9831: 9743: 9530: 9525: 9487: 9336: 9328: 9257: 9209: 9130: 9117: 9098: 9093: 9052: 8611: 7370: 7336: 7305: 7129: 7125: 7123: 7121: 7119: 6905: 6875:

Ruppert, E. E.; Fox, R. S. & Barnes, R. D. (2004). "Arthropoda".

6874: 6843: 6787: 6756: 6319: 6066: 5504: 5257: 5202: 5007: 4940: 4830: 4760: 4756: 4678: 4664: 4632: 4374: 4312: 4229: 4091: 4034: 3828: 3768: 3734: 3690: 3326: 3245: 3154: 3139:, less than 1,000 burrowing species that look rather like earthworms. 3136: 3112: 3070: 3009: 2958: 2820: 2709: 2694: 2662: 2642: 2628: 2603: 2599: 2288: 2272: 2228:

becomes the prostomium (head); a small area round the trochophore's

2202: 2002: 1995: 1960: 1924: 1904: 1879: 1874: 1842: 1813: 1722: 1677: 1603: 1583: 1508: 1488: 1317: 1198: 1156: 1144: 1092: 1040: 1026: 1015: 931: 903: 887: 883: 845: 809: 788: 773: 768: 736: 700: 684: 673: 653: 590: 443: 432: 420: 400: 337: 322: 298: 183: 170: 147: 107: 95: 10895: 8102: 6357: 6290: 6288: 6286: 6284: 6282: 2741: 2394:

that can be very dangerous to their hosts. Some small tube-dwelling

1083:, a group of extremely simplified parasites traditionally placed in 11005: 10918: 10853: 10833: 10828: 10803: 10783: 10758: 10723: 10713: 10708: 10539: 10514: 10494: 10154: 10057: 10041: 9964: 9956: 9911: 9874: 9855: 9844: 9826: 9820: 9681: 9622: 9598: 9495: 9376: 9265: 9149: 9040: 9031: 8937: 8808: 7741: 7371: