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99-533: Annelid setae are stiff bristles present on the body. They help, for example, earthworms to attach to the surface and prevent backsliding during peristaltic motion. These hairs make it difficult to pull a worm straight from the ground. Setae in oligochaetes (a group including earthworms) are largely composed of chitin . They are classified according to the limb to which they are attached; for instance, notosetae are attached to notopodia ; neurosetae to neuropodia . Diptera setae are bristles present throughout

198-569: A cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt – on the other hand arthropods ' cuticles are made of the more rigid α- chitin , and molt until the arthropods reach their full size. Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels . In addition to Sipuncula and Echiura, also lineages like Lobatocerebrum, Diurodrilus and Polygordius have lost their segmentation, but these are

297-406: A gecko 's feet are small hair-like processes that play a role in the animal's ability to cling to vertical surfaces. The micrometer-scale setae branch into nanometer-scale projections called spatulae . A Tokay gecko 's two front feet can sustain 20.1 N of force parallel to the surface using approximately 14,400 setae per mm. This equates to ~ 6.2 pN per seta, but does not sufficiently account for

396-412: A "first heuristic step in terms of bringing polychaete systematics to an acceptable level of rigour", based on anatomical structures, and divided polychaetes into: Echiura Euarthropoda Onychophora Phylum In biology , a phylum ( / ˈ f aɪ l əm / ; pl. : phyla ) is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany

495-551: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in

594-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make

693-994: A class of synthetic adhesives that detach at will, sometimes called resettable adhesives, yet display substantial stickiness. The development of such synthetic materials is a matter of current research. Annelid Cladistic view Traditional view The annelids ( / ˈ æ n ə l ɪ d z / ), also known as the segmented worms , comprise a large phylum called Annelida ( / ə ˈ n ɛ l ɪ d ə / ; from Latin anellus  'little ring'). The phylum contains over 22,000 extant species , including ragworms , earthworms , and leeches . The species exist in and have adapted to various ecologies – some in marine environments as distinct as tidal zones and hydrothermal vents , others in fresh water, and yet others in moist terrestrial environments. The Annelids are bilaterally symmetrical , triploblastic , coelomate , invertebrate organisms. They also have parapodia for locomotion. Most textbooks still use

792-482: A distinctive combination of features. Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli 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 organs , although sharing a common gut , circulatory system and nervous system makes them inter-dependent. Their bodies are covered by

891-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,

990-596: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since

1089-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by

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1188-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae

1287-689: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to

1386-456: A sandwich with connective tissue in the middle and mesothelium ( 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. The mesothelium is made of modified epitheliomuscular cells; in other words, their bodies form part of

1485-446: A separate fluid-filled "balloon". 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 of cilia (fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themselves into it. The fluid in

1584-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on

1683-802: A significant contribution to soil fertility . The rear end of the Palolo worm , a marine polychaete that tunnels through coral, detaches in order to spawn at the surface, and the people of Samoa regard these spawning modules as a delicacy. 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. Ragworms are commercially important as bait and as food sources for aquaculture , and there have been proposals to farm them in order to reduce over-fishing of their natural populations. Some marine polychaetes ' predation on molluscs causes serious losses to fishery and aquaculture operations. Scientists study aquatic annelids to monitor

1782-466: 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. It is thought that annelids were originally animals with two separate sexes , which released ova and sperm into the water via their nephridia . The fertilized eggs develop into trochophore larvae , which live as plankton . Later they sink to

1881-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display

1980-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with

2079-508: A third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility , annelids serve humans as food and as bait . Scientists observe annelids to monitor the quality of marine and fresh water. Although blood-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

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2178-518: A true segment, but in some polychaetes the peristomium has chetae and appendages like those of other segments. 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 teloblastic growth . Some groups of annelids, including all leeches , have fixed maximum numbers of segments, while others add segments throughout their lives. The phylum's name

2277-427: 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 urine . 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

2376-407: Is parasitic . Setae are not present in all mosses, but in some species they may reach 15 to 20 centimeters in height. In the diatom family Chaetocerotaceae , "seta" refers to the hairlike outgrowths of the valve, i.e. of the face of the cells. These setae have a different structure than the valve. Such setae may prevent rapid sinking and also protect the cells from grazing. Synthetic setae are

2475-467: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by

2574-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,

2673-464: Is derived from the Latin word annelus , meaning "little ring". Annelids' cuticles are made of collagen 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 mucus -secreting glands in

2772-420: Is down. A few polychaete genera 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". Most annelids have a pair of coelomata (body cavities) in each segment, separated from other segments by septa and from each other by vertical mesenteries . Each septum forms

2871-408: Is enlarged with muscles to form a heart, while in the forward ends of many earthworms 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. However, leeches and their closest relatives have a body structure that

2970-472: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish

3069-457: Is treated as an endangered species by both IUCN and CITES . More recently leeches have been used to assist in microsurgery , and their saliva has provided anti-inflammatory compounds and several important anticoagulants , one of which also prevents tumors from spreading . Ragworms' jaws are strong but much lighter than the hard parts of many other organisms, which are biomineralized with calcium salts. These advantages have attracted

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3168-565: 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. Most mature clitellates (the group that includes earthworms and leeches ) 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 copulate . Earthworms store their partners' sperm in spermathecae ("sperm stores") and then

3267-532: 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 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

3366-652: The Burgess shale beds in British 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 for oligochaetes occurs in the Tertiary period, which began 65  million years ago , and it has been suggested that these animals evolved around

3465-552: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to

3564-564: The Lophotrochozoa , a "super-phylum" of protostomes that also includes molluscs , brachiopods , and nemerteans . The basic annelid form consists of multiple segments . Each segment has the same sets of organs and, in most polychaetes, has a pair of parapodia that many species use for locomotion . Septa separate the segments of many species, but are poorly defined or absent in others, and Echiura and Sipuncula show no obvious signs of segmentation. In species with well-developed septa,

3663-595: The Middle Cambrian . 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. Then Simon Conway Morris and John Peel reported Phragmochaeta from Sirius Passet , about 518 million years old , and concluded that it

3762-523: The clitellum produces a cocoon that collects ova from the ovaries 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. Charles Darwin 's book The Formation of Vegetable Mould Through

3861-428: The family Hymenochaetaceae . Though mainly microscopic, the setae of some species may be sufficiently prominent to be visible with a hand lens. In botany , "seta" refers to the stalk supporting the capsule of a moss or liverwort (both closely related in a clade called "Setaphyta"), and supplying it with nutrients. The seta is part of the sporophyte and has a short foot embedded in the gametophyte on which it

3960-412: The integument of insects are unicellular, meaning that each is formed from a single epidermal cell of a type called a trichogen, literally meaning "bristle generator". They are at first hollow and in most forms remain hollow after they have hardened. They grow through and project through a secondary or accessory cell of a type called a tormogen, which generates the special flexible membrane that connects

4059-449: The oxygen -carrier hemoglobin ; breaking down proteins ; and turning nitrogenous waste products into ammonia and urea to be excreted . Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body), or flex the body while using parapodia 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

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4158-417: The parapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow. Feeding structures in the mouth region vary widely, and have little correlation with the animals' diets. Many polychaetes have a muscular pharynx that can be everted (turned inside out to extend it). In these animals the foremost few segments often lack septa so that, when

4257-422: The prostomium (Greek προ- meaning "in front of" and στομα meaning "mouth") contains the brain and sense organs, while the rearmost, called the pygidium (Greek πυγιδιον, meaning "little tail") or periproct contains the anus , generally on the underside. The first section behind the prostomium, called the peristomium (Greek περι- meaning "around" and στομα meaning "mouth"), is regarded by some zoologists as not

4356-401: The ventral 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 each segmental ganglion 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

4455-931: The Action of Worms (1881) presented the first scientific analysis of earthworms' contributions to soil fertility . Some burrow while others live entirely on the surface, generally in moist leaf litter . The burrowers loosen the soil 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 decomposition 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 robins to storks , and for mammals ranging from shrews to badgers , and in some cases conserving earthworms may be essential for conserving endangered birds. Terrestrial annelids can be invasive in some situations. In

4554-524: The Australian giant Gippsland earthworm and Amynthas mekongianus , which can both grow up to 3 meters (9.8 ft) long 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, most textbooks use the traditional classification into the following sub-groups: The Archiannelida , minute annelids that live in

4653-491: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista

4752-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine

4851-409: The annelids have been divided into two major groups, the polychaetes and clitellates . In turn the clitellates were divided into oligochaetes , which include earthworms , and hirudinomorphs , whose best-known members are leeches . For many years there was no clear arrangement of the approximately 80 polychaete families into higher-level groups. In 1997 Greg Rouse and Kristian Fauchald attempted

4950-413: 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 setae ("bristles") of arthropods , which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in

5049-409: The attention of engineers. Investigations showed that ragworm jaws are made of unusual proteins that bind strongly to zinc . Since annelids are soft-bodied , their fossils are rare. Polychaetes ' fossil record consists mainly of the jaws that some species had and the mineralized tubes that some secreted. Some Ediacaran fossils such as Dickinsonia in some ways resemble polychaetes , but

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5148-406: The base of the seta to the surrounding integument . Depending partly on their form and function, setae may be called hairs, macrotrichia , chaetae , or scales . The setal membrane is not cuticularized and movement is possible. Some insects, such as Eriogaster lanestris larvae, use setae as a defense mechanism, as they can cause dermatitis when they come into contact with skin. The pads on

5247-450: The blood circulates entirely within blood 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 by peristalsis ("ripples" that pass along the body) or by undulations that improve the effectiveness of the parapodia. In species with incomplete septa or none,

5346-445: 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 pharynges inside out to drag themselves through the sediment . Earthworms are oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil . The burrowing of marine polychaetes, which may constitute up to

5445-416: The body and function as mechanoreceptors . Crustaceans have mechano- and chemosensory setae. Setae are especially present on the mouthparts of crustaceans and can also be found on grooming limbs. In some cases, setae are modified into scale like structures. Setae on the legs of krill and other small crustaceans help them to gather phytoplankton . It captures them and allows them to be eaten. Setae on

5544-433: The coelomata and unlike the blood vessels of other annelids. Leeches generally use suckers at their front and rear ends to move like inchworms . The anus is on the upper surface of the pygidium. In some annelids, including earthworms , all respiration is via the skin. However, many polychaetes and some clitellates (the group to which earthworms belong) have gills associated with most segments, often as extensions of

5643-492: The coelomata contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain a respiratory pigment – red hemoglobin in some species, green chlorocruorin in others (dissolved in the plasma) – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the blood plasma . Species with well-developed septa generally also have blood vessels running all long their bodies above and below

5742-464: The cuticle. 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

5841-440: The direction from which light is coming and camera eyes or compound eyes that can probably form images. The compound eyes probably evolved independently 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 statocysts (tilt and balance sensors) that indicate which way

5940-645: The early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous , about 299  million years ago . Palaeontologists disagree about whether some body fossils from the mid Ordovician , about 472 to 461 million years ago , are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the Paleogene period, which began 66 million years ago. There are over 22,000 living annelid species, ranging in size from microscopic to

6039-450: The end of the Carboniferous , about 299  million years ago , fossils of most of the modern mobile polychaete groups had appeared. Many fossil tubes look like those made by modern sessile polychaetes, but the first tubes clearly produced by polychaetes date from the Jurassic , less than 199  million years ago . In 2012, a 508 million year old species of annelid found near

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6138-462: The epidermis protect their skins. Under the epidermis is the dermis , which is made of connective 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 the coelom (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

6237-422: The epithelium but their bases extend to form muscle 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 chloragogen cells that perform similar functions to the livers of vertebrates: producing and storing glycogen and fat ; producing

6336-444: 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 chaetae (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

6435-531: The female. There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge. 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 hermaphrodites or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent gonads , and it

6534-416: 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 penis that inject sperm into

6633-504: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia

6732-479: The giant axons prevents this escape response but does not affect normal movement. 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, ciliated structures found only in polychaetes, and are thought to be chemosensors . Some polychaetes also have various combinations of ocelli ("little eyes") that detect

6831-420: 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

6930-419: 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. Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while

7029-399: The gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks of capillaries 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

7128-450: The last few centuries. Ragworms' jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength. Since annelids are soft-bodied , their fossils are rare – mostly jaws and the mineralized tubes that some of the species secreted. Although some late Ediacaran fossils may represent annelids, the oldest known fossil that is identified with confidence comes from about 518  million years ago in

7227-515: The loss of ecological diversity. Especially of concern is Amynthas 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 around coral reefs and in tidal zones . Burrowing species increase

7326-437: The mouth). However, the lifecycles of most living polychaetes , 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 external fertilization but produce yolk -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

7425-506: The muscles in these segments contract, the sharp increase in fluid pressure from all these segments everts the pharynx very quickly. Two families , the Eunicidae and Phyllodocidae , 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

7524-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within

7623-528: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of

7722-478: The overall stickiness behavior shown by the foot pads. In 2017, a description of a new species of basal deuterostome called Saccorhytus was published. This animal appears to have seta in the pores along the side of its body. However, in 2022, Saccorhytus is considered to be an early ecdysozoan , and was described as having "lacked setae". In mycology , "setae" refer to dark brown, thick-walled, thorn-like cystidia found in corticioid and poroid fungi in

7821-479: The oxygen content, salinity and pollution levels in fresh and marine water. Accounts of the use of leeches for the medically dubious practice of blood-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, and Hirudo medicinalis

7920-454: The pads to capture prey. Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth. The gut is generally an almost straight tube supported by the mesenteries (vertical partitions within segments), and ends with the anus on the underside of the pygidium. However, in members of the tube-dwelling family Siboglinidae the gut is blocked by a swollen lining that houses symbiotic bacteria , which can make up 15% of

8019-404: 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 cirri (fused bundles of cilia ) and gills . The brain generally forms a ring round the pharynx (throat), consisting of a pair of ganglia (local control centers) above and in front of the pharynx, linked by nerve cords either side of

8118-560: The parent remains a complete organism. Some oligochaetes , such as Aulophorus 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. However, leeches have never been seen reproducing asexually. Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genera , Chaetopterus and Dodecaceria , can regenerate from

8217-454: The penetration of water and oxygen into the sea-floor sediment , which encourages the growth of populations of aerobic bacteria and small animals alongside their burrows. Although blood-sucking leeches do little direct harm to their victims, some transmit flagellates that can be very dangerous to their hosts. Some small tube-dwelling oligochaetes transmit myxosporean parasites that cause whirling disease in fish. Earthworms make

8316-416: The pharynx to another pair of ganglia just below and behind it. The brains of polychaetes 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 polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections. The rest of the central nervous system ,

8415-478: The relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as

8514-469: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On

8613-569: The same time as flowering plants in the early Cretaceous , from 130 to 90 million years ago . A trace fossil consisting of a convoluted burrow partly filled with small fecal pellets may be evidence that earthworms were present in the early Triassic period from 251 to 245 million years ago . Body fossils going back to the mid Ordovician , from 472 to 461 million years ago , have been tentatively classified as oligochaetes, but these identifications are uncertain and some have been disputed. Traditionally

8712-419: The sea-floor and metamorphose into miniature adults: the part of the trochophore between the apical tuft and the prototroch becomes the prostomium (head); a small area round the trochophore's anus 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

8811-412: 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 in cilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert

8910-513: 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 plesiomorphic character in Annelida. Some annelids also have oblique internal muscles that connect the underside of the body to each side. The setae ("hairs") of annelids project out from the epidermis to provide traction and other capabilities. The simplest are unjointed and form paired bundles near

9009-525: The similarities are too vague for these fossils to be classified with confidence. The small shelly fossil Cloudina , from 549 to 542 million years ago , has been classified by some authors as an annelid, but by others as a cnidarian (i.e. in the phylum to which jellyfish and sea anemones belong). Until 2008 the earliest fossils widely accepted as annelids were the polychaetes Canadia and Burgessochaeta , both from Canada's Burgess Shale , formed about 505  million years ago in

9108-708: The spaces between grains of marine sediment , were treated as a separate class 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: Mitogenomic and phylogenomic analysis also implies that Orthonectida , a group of extremely simplified parasites traditionally placed in Mesozoa , are actually reduced annelids. Research suggest that also nemerteans are annelids, with Oweniidae and Magelonidae as their closest relatives. No single feature distinguishes Annelids from other invertebrate phyla, but they have

9207-466: The term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla, the plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering

9306-490: The top and bottom of each side of each segment. The parapodia ("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- chitin and are formed by follicles , 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 microvilli , fine hair-like extensions that increase

9405-527: The traditional division into polychaetes (almost all marine), oligochaetes (which include earthworms) and leech -like species. Cladistic 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 Pogonophora , Echiura and Sipuncula , previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of

9504-729: The tube-dwelling genus Owenia the single nerve chord has no ganglia and is located in the epidermis . As in arthropods , each muscle fiber (cell) is controlled by more than one neuron , 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. Most annelids' longitudinal nerve trunks include giant axons (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

9603-413: 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

9702-478: The worms' total weight. The bacteria convert inorganic matter – such as hydrogen sulfide and carbon dioxide from hydrothermal vents , or methane from seeps – 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. Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia . Both of these systems use

9801-431: Was the oldest annelid known to date. There has been vigorous debate about whether the Burgess Shale fossil Wiwaxia was a mollusc or an annelid. Polychaetes diversified in the early Ordovician , about 488 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. By

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