The annelids (), also known as the segmented worms, are animals that comprise the phylum Annelida (; ).[ The phylum contains over 22,000 extant species, including , , and . The species exist in and have adapted to various ecologies – some in marine environments as distinct as and hydrothermal vents, others in fresh water, and yet others in moist terrestrial environments.
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The annelids are bilaterally symmetrical, Triploblasty, , invertebrate organisms. They also have Parapodium for locomotion. Most textbooks still use the traditional division into (almost all marine), (which include earthworms) and Leech (leech-like species). Cladistics 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 Siboglinidae, Echiura and Sipuncula, previously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoa, a "super-phylum" of that also includes , , and .
The basic annelid form consists of multiple segments called metameres. Each segment has the same sets of organs and, in most polychaetes, has a pair of parapodia that many species use for locomotion. Septum 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, the blood circulates entirely within , 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, 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 pharynx inside out to drag themselves through the sediment.
Earthworms are oligochaetes that support terrestrial both as prey and predators, and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of by enabling water and oxygen to penetrate the sea floor. In addition to improving soil fertility, annelids serve humans as food and as Fishing 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 because they have been over-harvested for this purpose in the last few centuries. Ragworms' jaws are 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 in the early Cambrian period. Fossils of most modern mobile polychaete groups appeared by the end of the Carboniferous, about . Palaeontologists disagree about whether some from the mid Ordovician, about , are the remains of oligochaetes, and the earliest indisputable fossils of the group appear in the Paleogene period, which began 66 million years ago.
Classification and diversity
There are over 22,000 living annelid species,
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(about 12,000 species
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(about 10,000 species
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("with few hairs"), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth.
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Hirudinea, whose name means "leech-shaped" and whose best known members are leeches.
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The Archiannelida, minute annelids that live in 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:
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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 separate phylum, Pogonophora, or as two phyla, Pogonophora and Vestimentifera. More recently they have been re-classified as a family, Siboglinidae, within the polychaetes.
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The Echiura have a checkered taxonomic 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 molecular phylogenetics analysis in 1997 concluded that echiurans are annelids.
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Myzostomida live on and other , mainly as parasites. In the past they have been regarded as close relatives of the trematode or of the , but in 1998 it was suggested that they are a sub-group of polychaetes.
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Sipuncula was originally classified as annelids, despite the complete lack of segmentation, and other annelid characters. The phylum Sipuncula was later allied with the Mollusca, mostly on the basis of morphogenesis and characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida.
Mitogenomic and phylogenomic analysis also implies that Orthonectida, a group of extremely simplified parasites traditionally placed in Mesozoa, are actually reduced annelids.[ Annelida] Research suggest that also are annelids, with Oweniidae and Magelonidae as their closest relatives.[ Mitochondrial Genome Evolution in Annelida—A Systematic Study on Conservative and Variable Gene Orders and the Factors Influencing its Evolution]
Distinguishing features
No single feature distinguishes annelids from other invertebrate phyla, but they have 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.
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Description
Segmentation
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.[ The Invertebrate Tree of Life] Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external (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 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 a true segment, but in some 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 teloblast.
The phylum's name is derived from the Latin word annelus, meaning "little ring".
Body wall, chaetae and parapodia
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 in the epidermis protect their skins.
The ("hairs") of annelids project out from the epidermis to provide traction and other capabilities. The simplest are unjointed and form paired bundles near 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.
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 with cirri (fused bundles of cilia) and .
Nervous system and senses
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 double ventral nerve cords either side of the pharynx to another pair of ganglia just below and behind it.
As in , 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. have a different system, in which one neuron controls a group of muscle fibers.
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.
Coelom, locomotion and circulatory system
Most annelids have a pair of (body cavities) in each segment, separated from other segments by septum and from each other by vertical mesenteries. Each septum forms a sandwich with connective tissue in the middle and mesothelium (Mucous 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.
Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body),
The fluid in 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)
However, 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.
Respiration
In some annelids, including , all respiration is via the skin. However, many and some (the group to which earthworms belong) have associated with most segments, often as extensions of the parapodia in polychaetes. The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow.
Feeding and excretion
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 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 the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths. Filter feeding use "crowns" of palps covered in cilia that wash food particles towards their mouths. Non-selective deposit feeders ingest soil or marine via mouths that are generally unspecialized. Some Clitellata 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.
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.
Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia.
Reproduction and life cycle
Asexual reproduction
can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism.
Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genus, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.
Sexual reproduction
It is thought that annelids were originally animals with two separate , which released ovum and sperm into the water via their nephridia.
However, the lifecycles of most living , 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 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 the female.
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 or change sex during their lives. Most polychaetes whose reproduction has been studied lack permanent , 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.
Most mature Clitellata (the group that includes 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 ("sperm stores") and then the clitellum produces a Pupa that collects ova from the ovary 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.
Ecological significance
Charles Darwin's book The Formation of Vegetable Mould Through the Action of Worms (1881) presented the first scientific analysis of earthworms' contributions to soil fertility.
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 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 and in .
Although blood-sucking do little direct harm to their victims, some transmit that can be very dangerous to their hosts. Some small tube-dwelling transmit that cause whirling disease in fish.
Interaction with humans
Earthworms make a significant contribution to soil fertility.
Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.
Accounts of the use of 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 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 , one of which also prevents from metastasis.
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 the attention of engineers. Investigations showed that ragworm jaws are made of unusual that bind strongly to zinc.
Evolutionary history
Fossil record
Since annelids are soft-bodied, their fossils are rare.[
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The earliest good evidence for occurs in the Tertiary period, which began , and it has been suggested that these animals evolved around the same time as in the early Cretaceous, from .
Internal relationships
Traditionally the annelids have been divided into two major groups, the and clitellata. In turn the clitellates were divided into , which include , and Hirudinomorpha, whose best-known members are .
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Scolecida, less than 1,000 burrowing species that look rather like earthworms.
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Palpata, the great majority of polychaetes, divided into:
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Canalipalpata, which are distinguished by having long grooved palps that they use for feeding, and most of which live in tubes.
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Aciculata, the most active polychaetes, which have parapodia reinforced by internal spines (aciculae).
Also in 1997 Damhnait McHugh, using molecular 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; the Siboglinidae and , which for a few decades had been regarded as a separate phylum, were placed on other branches of the polychaete tree.
In 2007 Torsten Struck and colleagues compared three genes in 81 taxon, of which nine were outgroups,
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 segmentation in the classification of . 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 updated phylogenetic tree of the annelid phylum is comprised by a grade of basal groups of polychaetes: Palaeoannelida, Chaetopteriformia and the Amphinomida/Sipuncula/ Lobatocerebrum clade. This grade is followed by Pleistoannelida, the clade containing nearly all of annelid diversity, divided into two highly diverse groups: Sedentaria and Errantia. Sedentaria contains the , Siboglinidae, echiurans and some archiannelids, as well as several polychaete groups. Errantia contains the eunicida and phyllodocida polychaetes, and several archiannelids. Some small groups, such as the Myzostomida, are more difficult to place due to long branching, but belong to either one of these large groups.
External relationships
Annelids are members of the protostomes, one of the two major superphyla of bilaterian animals – the other is the deuterostomes, which includes .
The "Lophotrochozoa" hypothesis is also supported by the fact that many phyla within this group, including annelids, , and , 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.
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