Annelid

Annelida Fossil range: 518–0 Ma PreЄ Є O S D C P T J K Pg N Cambrian - Recent
Glycera sp.
Scientific classification
Kingdom:	Animalia
Superphylum:	Lophotrochozoa
Phylum:	Annelida Lamarck, 1809
Classes and subclasses
Class Polychaeta (paraphyletic?) Class Clitellata (see below)    Oligochaeta - earthworms, etc.    Branchiobdellida    Hirudinea - leeches Class Myzostomida Class Archiannelida (polyphyletic)

The annelids (also declared "ringed worms"), formally declared Annelida (from French annelés "ringed ones", ultimately from Latin anellus "little ring"¹), are a ample phylum of anecdotal worms, with over 17,000 avant-garde brand including ragworms, earthworms and leeches. They are begin in abyssal environments from tidal zones to hydrothermal vents, in freshwater, and in clammy earthbound environments. Although a lot of textbooks still use the adequate assay into polychaetes (almost all marine), oligochaetes (which cover earthworms) and leech-like species, assay aback 1997 has radically afflicted this scheme, examination leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes. In addition, the Pogonophora, Echiura and Sipuncula, advanced admired as abstracted phyla, are now admired as sub-groups of polychaetes. Annelids are advised associates of the Lophotrochozoa, a "super-phylum" of protostomes that aswell includes molluscs, brachiopods, flatworms and nemerteans.

The basal annelid assay consists of assorted segments, anniversary of which has the aforementioned sets of organs and, in a lot of polychaetes, a brace of parapodia that abounding brand use for locomotion. Septa abstracted the segments of abounding species, but are poorly-defined or absent in some, and Echiura and Sipuncula appearance no accessible signs of segmentation. In brand with well-developed septa, the claret circulates absolutely aural blood vessels, and the argosy in segments abreast the foreground ends of these brand are about congenital up with assay to act as hearts. The septa of these brand aswell accredit them to change the shapes of alone segments, which facilitates movement by peristalsis ("ripples" that canyon forth the body) or by undulations that advance the capability of the parapodia. In brand with abridged septa or none, the claret circulates through the capital physique atrium afterwards any affectionate of pump, and there is a advanced ambit of locomotory techniques – some burrowing brand about-face their pharynges central out to annoyance themselves through the sediment.

Although abounding brand can carbon asexually and use agnate mechanisms to change afterwards astringent injuries, sexual reproduction is the accustomed adjustment in brand whose reproduction has been studied. The boyhood of alive polychaetes whose reproduction and lifecycles are accustomed aftermath trochophore larvae, which reside as plankton and afresh bore and metamorphose into miniature adults. Oligochaetes are abounding hermaphrodites and aftermath a ring-like cocoon annular their bodies, in which the eggs and hatchlings are nourished until they are accessible to emerge.

Earthworms abutment earthbound food chains both as casualty and by aerating and adorning soil. The burrowing of abyssal polychaetes, which may aggregate up to a third of all brand in near-shore environments, encourages the development of ecosystems by enabling baptize and oxygen to access the sea floor. In accession to convalescent soil fertility, annelids serve bodies as aliment and as bait. Scientists beam annelids to adviser the superior of abyssal and beginning water. Although blood-letting is no best in favor with doctors, some drain brand are admired as endangered brand because they accept been over-harvested for this purpose in the endure few centuries. Ragworms' aperture are now getting advised by engineers as they action an aberrant aggregate of animation and strength.

Since annelids are soft-bodied, their fossils are attenuate – mostly aperture and the mineralized tubes that some of the brand secreted. Although some backward Ediacaran fossils may represent annelids, the oldest accustomed drop that is articular with aplomb comes from about 518 actor years ago in the aboriginal Cambrian period. Fossils of a lot of avant-garde adaptable polychaete groups appeared by the end of the Carboniferous, about 299 actor years ago. Scientists disagree about whether some body fossils from the mid Ordovician, about 472 to 461 actor years ago, are the charcoal of oligochaetes, and the age-old assertive fossils of the accumulation arise in the Tertiary period, which began 65 actor years ago.

Contents 1 Classification and diversity 2 Distinguishing features 3 Description 3.1 Segmentation 3.2 Body wall, chetae and parapodia 3.3 Nervous adjustment and senses 3.4 Coelom, locomotion and circulatory system 3.5 Respiration 3.6 Feeding and excretion 3.7 Reproduction and activity cycle 3.7.1 Asexual reproduction 3.7.2 Sexual reproduction 4 Ecological significance 5 Interaction with humans 6 Evolutionary history 6.1 Fossil record 6.2 Family tree 7 References 8 Further reading 9 External links

Classification and diversity

There are over 17,000 living annelid species,² alignment in admeasurement from diminutive to the Australian giant Gippsland earthworm, which can abound up to 3 metres (9.8 ft) long.³⁴ Although assay aback 1997 has radically afflicted scientists' angle about the evolutionary ancestors timberline of the annelids,⁵⁶ a lot of textbooks use the adequate allocation into the afterward sub-groups:³⁷

• Polychaetes (about 12,000 species²). As their name suggests, they accept assorted chetae ("hairs") per segment. Polychaetes accept parapodia that action as limbs, and nuchal organs ("nuchal" agency "on the neck") that are anticipation to be chemosensors.³ A lot of are abyssal animals, although a few brand reside in beginning baptize and even beneath on land.⁸

• Clitellates (about 5,000 species²). These accept few or no chetae per segment, and no nuchal organs or parapodia. However, they accept a altered changeable organ, the ring-shaped clitellum ("pack saddle") annular their bodies, which produces a cocoon that aliment and nourishes fertilized eggs until they hatch.⁷⁹ The clitellates are sub-divided into:³
  • Oligochaetes ("with few hairs"), which includes earthworms. Oligochaetes accept a adhesive pad in the roof of the mouth.³ A lot of are burrowers that augment on wholly or partly addle organic materials.⁸
  • Hirudinea, whose name agency "leech-shaped" and whose best accustomed associates are leeches.³ Abyssal brand are mostly blood-sucking parasites, mainly on fish, while a lot of freshwater brand are predators.⁸ They accept suckers at both ends of their bodies, and use these to move rather like inchworms.¹⁰

The Archiannelida, minute annelids that reside in the spaces amid grains of sediment, were advised as a abstracted class because of their simple physique structure, but are now admired as polychaetes.⁷ Some added groups of animals accept been classified in assorted ways, but are now broadly admired as annelids:

• Pogonophora / Siboglinidae were aboriginal apparent in 1914, and their abridgement of a apparent gut fabricated it difficult to allocate them. They accept been classified as a abstracted phylum, Pogonophora, or as two phyla, Pogonophora and Vestimentifera. Added afresh they accept been re-classified as a family, Siboglinidae, aural the polychaetes.⁸¹¹
• The Echiura accept a checky taxonomic history: in the 19th century they were assigned to the phylum "Gephyrea", which is now abandoned as its associates accept been assigned to added phyla; the Echiura were next admired as annelids until the 1940s, if they were classified as a phylum in their own right; but a molecular phylogenetics assay in 1997 assured that Echiurans are annelids.²¹¹¹²
• Myzostomida reside on crinoids and added echinoderms, mainly as parasites. In the accomplished they accept been admired as abutting ancestors of the trematode flatworms or of the tardigrades, but in 1998 it was appropriate that they are a sub-group of polychaetes.⁸ However, accession assay in 2002 appropriate that myzostomids are added carefully accompanying to flatworms or to rotifers and acanthocephales.¹¹

Distinguishing features

No alone affection distinguishes Annelids from added invertebrate phyla, but they accept a characteristic aggregate of features. Their bodies are long, with segments that are disconnected evidently by bank ring-like constrictions declared annuli and internally by septa ("partitions") at the aforementioned points, although in some brand the septa are abridged and in a few cases missing. A lot of of the segments accommodate the aforementioned sets of organs, although administration a accustomed gut, circulatory system and nervous system makes them inter-dependent.³⁷ Their bodies are covered by a cuticle (outer covering) that does not accommodate cells but is secreted by beef in the derma underneath, is fabricated of boxy but adjustable collagen³ and does not molt¹³ – on the added duke arthropods' cuticles are fabricated of the added adamant α-chitin,³¹⁴ and afford until the arthropods ability their abounding size.¹⁵ A lot of annelids accept bankrupt circulatory systems, breadth the claret makes its absolute ambit via blood vessels.¹³

Summary of appropriate features

	Annelida3	Recently alloyed into Annelida5		Closely-related	Similar-looking phyla
		Echiura16	Sipuncula17	Nemertea18	Arthropoda19	Onychophora20
External segmentation	Yes	no	no	Only in a few species	Yes, except in mites	no
Repetition of centralized organs	Yes	no	no	Yes	In archaic forms	Yes
Septa amid segments	In a lot of species	no	no	No	No	No
Cuticle material	collagen	collagen	collagen	none	α-chitin	α-chitin
Molting	Generally no;13 but some polychaetes afford their jaws, and leeches afford their skins21	no22	no22	no22	Yes15	Yes
Body cavity	Coelom; but this is bargain or missing in abounding leeches and some baby polychaetes13	2 coeloms, capital and in proboscis	2 coeloms, capital and in tentacles	Coelom alone in proboscis	Hemocoel	Hemocoel
Circulatory system	Closed in a lot of species	Open outflow, acknowledgment via angled vein	Open	Closed	Open	Open

Description

Segmentation

Most of an annelid's physique consists of segments that are about identical, accepting the aforementioned sets of centralized organs and alien chaetae (Greek χαιτα, acceptation "hair") and, in some species, appendages. However, the frontmost and aback sections are not admired as accurate segments as they do not accommodate the accustomed sets of organs and do not advance in the aforementioned way as the accurate segments. The frontmost section, declared the prostomium (Greek προ- acceptation "in foreground of" and στομα acceptation "mouth") contains the academician and faculty organs, while the rearmost, declared the pygidium (Greek πυγιδιον, acceptation "little tail") contains the anus, about on the underside. The aboriginal breadth abaft the prostomium, declared the peristomium (Greek περι- acceptation "around" and στομα acceptation "mouth"), is admired by some zoologists as not a accurate segment, but in some polychaetes the peristomium has chetae and appendages like those of added segments.³

The segments advance one at a time from a advance breadth just advanced of the pygidium, so that an annelid's youngest articulation is just in foreground of the advance breadth while the peristomium is the oldest. This adjustment is declared teloblastic growth.³ Some groups of annelids, including all leeches,¹⁰ accept anchored best numbers of segments, while others add segments throughout their lives.⁷

The phylum's name is acquired from the Latin chat annelus, acceptation "little ring".²

Body wall, chetae and parapodia

Annelids' cuticles are fabricated of collagen fibers, usually in layers that circling in alternating admonition so that the fibers cantankerous anniversary other. These are buried by the one-cell abysmal covering (outermost derma layer). A few abyssal annelids that reside in tubes abridgement cuticles, but their tubes accept a agnate structure, and mucus-secreting glands in the covering assure their skins.³ Beneath the covering is the dermis, which is fabricated of connective tissue, in added words a aggregate of beef and non-cellular abstracts such as collagen. Beneath this are two layers of muscles, which advance from the lining of the coelom (body cavity): annular assay accomplish a articulation best and slimmer if they contract, while beneath them are longitudinal muscles, usually four audible strips,¹³ whose contractions accomplish the articulation beneath and fatter.³ Some annelids aswell accept angled centralized assay that affix the base of the physique to anniversary side.¹³

The chetae ("hairs") of annelids activity out from the covering to accommodate traction and added capabilities. The simplest are unjointed and assay commutual bundles abreast the top and basal of anniversary ancillary of anniversary segment. The parapodia ("limbs") of annelids that accept them about buck added circuitous chetae at their tips – for archetype jointed, comb-like or hooked.³ Chetae are fabricated of moderately adjustable β-chitin and are formed by follicles, anniversary of which has a chaetoblast ("hair-forming") corpuscle at the basal and assay that can extend or abjure the cheta. The chetoblasts aftermath chetae by basic microvilli, accomplished hair-like extensions that access the breadth accessible for secreting the cheta. If the cheta is complete, the microvilli abjure into the chetoblast, abrogation alongside tunnels that run about the abounding breadth of the cheta.³ Hence annelids' chetae are structurally altered from the setae ("bristles") of arthropods, which are fabricated of the added adamant α-chitin, accept a alone centralized cavity, and are army on adjustable joints in bank pits in the cuticle.³

Nearly all polychaetes accept parapodia that action as limbs, while added aloft annelid groups abridgement them. Parapodia are unjointed commutual extensions of the physique wall, and their assay are acquired from the annular assay of the body. They are about accurate internally by one or added large, blubbery chetae. The parapodia of burrowing and tube-dwelling polychaetes are about just ridges whose tips buck absorbed chetae. In alive crawlers and swimmers the parapodia are about disconnected into ample top and lower paddles on a actual abbreviate trunk, and the paddles are about belted with chetae and sometimes with cirri (fused bundles of cilia) and gills.¹³

Nervous adjustment and senses

The brain about forms a ring annular the pharynx (throat), consisting of a brace of ganglia (local ascendancy centers) aloft and in foreground of the pharynx, affiliated by assumption cords either ancillary of the pharynx to accession brace of ganglia just beneath and abaft it.³ The accuracy of polychaetes are about in the prostomium, while those of clitellates are in the peristomium or sometimes the aboriginal articulation abaft the peristomium.²³ In some actual adaptable and alive polychaetes the academician is continued and added complex, with arresting hindbrain, midbrain and forebrain sections.¹³ The blow of the central afraid system is about "ladder-like", consisting of a brace of assumption cords that run through the basal allotment of the physique and accept in anniversary articulation commutual ganglia affiliated by a axle connection. From anniversary segmental ganglion a aberration adjustment of bounded fretfulness runs into the physique bank and afresh encircles the body.³ However, in a lot of polychaetes the two capital assumption cords are fused, and in the tube-dwelling genus Owenia the alone assumption ambit has no ganglia and is amid in the epidermis.⁷²⁴

As in arthropods, anniversary beef cilia (cell) is controlled by added than one neuron, and the acceleration and ability of the fiber's contractions depends on the accumulated furnishings of all its neurons. Vertebrates accept a altered system, in which one neuron controls a accumulation of beef fibers.³ A lot of annelids' longitudinal assumption trunks cover behemothic axons (the achievement arresting curve of assumption cells). Their ample bore decreases their resistance, which allows them to address signals awfully fast. This enables these worms to abjure rapidly from crisis by abridgement their bodies. Experiments accept apparent that acid the behemothic axons prevents this escape acknowledgment but does not affect accustomed movement.³

The sensors are primarily alone beef that ascertain light, chemicals, burden after-effects and contact, and are present on the head, appendages (if any) and added locations of the body.³ Nuchal ("on the neck") organs are paired, ciliated structures begin alone in polychaetes, and are anticipation to be chemosensors.¹³ Some polychaetes aswell accept assorted combinations of ocelli ("little eyes") that ascertain the administration from which ablaze is advancing and camera eyes or compound eyes that can apparently assay images.²⁴ The admixture eyes apparently evolved independently of arthropods' eyes.¹³ Some tube-worms use ocelli broadly advance over their bodies to ascertain the caliginosity of fish, so that they can bound withraw into their tubes.²⁴ Some burrowing and tube-dwelling polychaetes accept statocysts (tilt and antithesis sensors) that acquaint them which way is down.²⁴ A few polychaete genera accept on the undersides of their alive palps that are acclimated both in agriculture and as "feelers", and some of these aswell accept antennae that are structurally agnate but apparently are acclimated mainly as "feelers".¹³

Coelom, locomotion and circulatory system

Most annelids accept a brace of coeloms (body cavities) in anniversary segment, afar from added segments by septa and from anniversary added by vertical mesenteries. Anniversary septum forms a sandwich with connective tissue in the average and mesothelium (membrane that serves as a lining) from the above-mentioned and afterward segments on either side. Anniversary mesentery is agnate except that the mesothelium is the lining of anniversary of the brace of coeloms, and the claret argosy and, in polychaetes, the capital assumption cords are anchored in it.³ The mesothelium is fabricated of adapted epitheliomuscular cells,³ in added words their bodies assay allotment of the epithelium but their bases extend to assay muscle fibers in the physique wall.²⁵ The mesothelium may aswell assay adorable and annular assay on the septa, and annular assay about the claret argosy and gut. Locations of the mesothelium, abnormally on the alfresco of the gut, may aswell assay chloragogen beef that accomplish agnate functions to the livers of vertebrates: bearing and autumn glycogen and fat; bearing the oxygen-carrier hemoglobin; breaking down proteins; and axis nitrogenous decay articles into ammonia and urea to be excreted.³

Many annelids move by peristalsis (waves of abbreviating and amplification that ambit forth the body),³ or angle the physique while application parapodia to clamber or swim.²⁶ In these animals the septa accredit the annular and longitudinal assay to change the appearance of alone segments, by authoritative anniversary articulation a abstracted fluid-filled "balloon".³ However, the septa are about abridged in annelids that are semi-sessile or that do not move by peristalsis or by movements of parapodia – for archetype some move by whipping movements of the body, some baby abyssal brand move by agency of cilia (fine muscle-powered hairs) and some burrowers about-face their pharynges (throats) central out to access the sea-floor and annoyance themselves into it.³

The aqueous in the coeloms contains coelomocyte beef that avert the animals adjoin parasites and infections. In some brand coelomocytes may aswell accommodate a respiratory pigment – red hemoglobin in some species, blooming chlorocruorin in others¹³ – and accommodate oxygen carriage aural their segments. Respiratory colorant is aswell attenuated in the blood plasma. Brand with well-developed septa about aswell accept claret argosy alive all continued their bodies aloft and beneath the gut, the top one accustomed claret assiduously while the lower one carries it backwards. Networks of capillaries (fine claret vessels) in the physique bank and about the gut alteration claret amid the capital claret argosy and to locations of the articulation that charge oxygen and nutrients. Both of the aloft vessels, abnormally the top one, can pump claret by contracting. In some annelids the advanced end of the top claret barge is continued with assay to assay a heart, while in the advanced ends of abounding earthworms some of the argosy that affix the top and lower capital argosy action as hearts. Brand with poorly-developed or no septa about accept no claret argosy and await on the apportionment aural the coelom for carrying nutrients and oxygen.³

However, leeches and their abutting ancestors accept a physique assay that is actual compatible aural the accumulation but decidedly altered from that of added annelids, including added associates of the Clitellata.¹⁰ In leeches there are no septa, the affiliation tissue bandage of the physique bank is so blubbery that it occupies abundant of the body, and the two coeloms are broadly afar and run the breadth of the body. They action as the capital claret vessels, although they are side-by-side rather than top and lower. However, they are lined with mesothelium, like the coeloms and clashing the claret argosy of added annelids. Leeches about use suckers at their foreground and rear ends to move like inchworms. The anus is on the top apparent of the pygidium.¹⁰

Respiration

In some annelids, including earthworms, all respiration is via the skin. However, abounding polychaetes and some clitellates (the accumulation to which earthworms belong) accept gills associated with a lot of segments, about as extensions of the parapodia in polychaetes. The aspect of tube-dwellers and burrowers usually array about whichever end has the stronger baptize flow.¹³

Feeding and excretion

Feeding structures in the aperture arena alter widely, and accept little alternation with the animals' diets. Abounding polychaetes accept a able-bodied pharynx that can be everted (turned central out to extend it). In these animals the foremost few segments about abridgement septa so that, if the assay in these segments contract, the aciculate access in aqueous burden from all these segments everts the pharynx actual quickly. Two families, the Eunicidae and Phyllodocidae, accept acquired jaws, which can be acclimated for abduction prey, bitter off pieces of vegetation, or acquisitive asleep and corrupt matter. On the added duke some bloodthirsty polychaetes accept neither aperture nor eversible pharynges. Selective drop feeders about reside in tubes on the sea-floor and use palps to acquisition aliment particles in the debris and afresh clean them into their mouths. Filter feeders use "crowns" of palps covered in cilia that ablution aliment particles arise their mouths. Non-selective drop feeders blot clay or abyssal sediments via mouths that are about unspecialized. Some clitellates accept adhesive pads in the roofs of their mouths, and some of these can belie the pads to abduction prey. Leeches about accept an eversible proboscis, or a able-bodied pharynx with two or three teeth.¹³

The gut is about an about beeline tube accurate by the mesenteries (vertical partitions aural segments), and ends with the anus on the base of the pygidium.³ However, in associates of the tube-dwelling ancestors Siboglinidae the gut is blocked by a bloated lining that houses symbiotic bacteria, which can accomplish up 15% of the worms' absolute weight. The bacilli catechumen inorganic amount – such as hydrogen sulfide and carbon dioxide from hydrothermal vents, or methane from seeps – to amoebic amount that feeds themselves and their hosts, while the worms extend their palps into the gas flows to blot the gases bare by the bacteria.¹³

Annelids with claret argosy use metanephridia to abolish acrid decay products, while those afterwards use protonephridia.³ Both of these systems use a two-stage filtration process, in which aqueous and decay articles are aboriginal extracted and these are filtered afresh to re-absorb any re-usable abstracts while auctioning baneful and spent abstracts as urine. The aberration is that protonephridia amalgamate both filtration stages in the aforementioned organ, while metanephridia accomplish alone the additional filtration and await on added mechanisms for the aboriginal - in annelids appropriate clarify beef in the walls of the claret argosy let fluids and added baby molecules canyon into the coelomic fluid, breadth it circulates to the metanephridia.²⁷ In annelids the credibility at which aqueous enters the protonephridia or metanephridia are on the advanced ancillary of a septum while the second-stage clarify and the nephridiopore (exit aperture in the physique wall) are in the afterward segment. As a aftereffect the afterwards articulation (before the advance breadth and pygidium) has no assay that extracts its wastes, as there is no afterward articulation to clarify and acquittal them, while the aboriginal articulation contains an abstraction assay that passes wastes to the second, but does not accommodate the structures that re-filter and acquittal urine.³

Reproduction and activity cycle

Asexual reproduction

Polychaetes can carbon asexually, by adding into two or added pieces or by budding off a new alone while the ancestor charcoal a complete organism.³²⁸ Some oligochaetes, such as Aulophorus furcatus, accept to carbon absolutely asexually, while others carbon asexually in summer and sexually in autumn. Asexual reproduction in oligochaetes is consistently by adding into two or added pieces, rather than by budding.⁷²⁹ However, leeches accept never been apparent ancestry asexually.⁷³⁰

Most polychaetes and oligochaetes aswell use agnate mechanisms to change afterwards adversity damage. Two polychaete genera, Chaetopterus and Dodecaceria, can change from a alone segment, and others can change even if their alive are removed.⁷²⁸ Annelids are the a lot of circuitous animals that can change afterwards such astringent damage.³¹ On the added duke leeches cannot regenerate.³⁰

Sexual reproduction

It is anticipation that annelids were originally animals with two abstracted sexes, which arise ova and sperm into the baptize via their nephridia.³ The fertilized eggs advance into trochophore larvae, which reside as plankton.³³ Afterwards they bore to the sea-floor and metamorphose into miniature adults: the allotment of the trochophore amid the apical tuft and the prototroch becomes the prostomium (head); a baby breadth annular the trochophore's anus becomes the pygidium (tail-piece); a attenuated bandage anon in foreground of that becomes the advance breadth that produces new segments; and the blow of the trochophore becomes the peristomium (the articulation that contains the mouth).³

However, the lifecycles of a lot of alive polychaetes, which are about all abyssal animals, are unknown, and alone about 25% of the 300+ species whose lifecycles are accustomed chase this pattern. About 14% use a agnate external fertilization but aftermath yolk-rich eggs, which abate the time the larva needs to absorb a part of the plankton, or eggs from which miniature adults arise rather than larvae. The blow affliction for the fertilized eggs until they buck – some by bearing jelly-covered masses of eggs which they tend, some by adhering the eggs to their bodies and a few brand by befitting the eggs aural their bodies until they hatch. These brand use a array of methods for agent transfer; for example, in some the females aggregate agent arise into the water, while in others the males accept penes that inject agent into the female.³³ There is no agreement that this is a adumbrative sample of polychaetes' changeable patterns, and it artlessly reflects scientists' accustomed knowledge.³³

Some polychaetes brand alone already in their lives, while others brand about continuously or through several ancestry seasons. While a lot of polychaetes abide of one sex all their lives, a cogent allotment of brand are abounding hermaphrodites or change sex during their lives. A lot of polychaetes whose reproduction has been advised abridgement abiding gonads, and it is ambiguous how they aftermath ova and sperm. In a few brand the rear of the physique splits off and becomes a abstracted alone that lives just continued abundant to bathe to a adequate environment, usually abreast the surface, and spawn.³³

Most complete clitellates (the accumulation that includes earthworms and leeches) are abounding hermaphrodites, although in a few drain brand adolescent adults action as males and become changeable at maturity. All accept well-developed gonads, and all copulate. Earthworms abundance their partners' agent in spermathecae ("sperm stores") and afresh the clitellum produces a cocoon that collects ova from the ovaries and afresh agent from the spermathecae. Fertilization and development of earthworm eggs takes abode in the cocoon. Leeches' eggs are fertilized in the ovaries, and afresh transferred to the cocoon. In all clitellates the cushion aswell either produces yolk if the eggs are fertilized or nutrients while they are developing. All clitellates buck as miniature adults rather than larvae.³³

Ecological significance

Charles Darwin's book The Formation of Vegetable Mould through the Action of Worms (1881) presented the aboriginal accurate assay of earthworms' contributions to soil fertility.³⁴ Some couch while others reside absolutely on the surface, about in clammy leaf litter. The burrowers alleviate the soil so that oxygen and baptize can access it, and both apparent and burrowing worms advice to aftermath clay by bond amoebic and mineral matter, by accelerating the decomposition of amoebic amount and appropriately authoritative it added bound accessible to added organisms, and by apperception minerals and converting them to forms that plants can use added easily.³⁵³⁶ Earthworms are aswell important casualty for birds alignment in admeasurement from robins to storks, and for mammals alignment from shrews to badgers, and in some cases absorption earthworms may be capital for absorption endangered birds.³⁷

Marine annelids may annual for over one-third of bottom-dwelling beastly brand annular coral reefs and in tidal zones.³⁴ Burrowing brand access the assimilation of baptize and oxygen and baptize into the sea-floor sediment, which encourages the advance of populations of bacilli and baby animals alongside their burrows.³⁸

Although blood-sucking leeches do little absolute abuse to their victims, some address flagellates that can be actual alarming to their hosts. Some baby tube-dwelling oligochaetes address myxosporean parasites that could could cause whirling disease in fish.³⁴

Interaction with humans

Earthworms accomplish a cogent accession to soil fertility.³⁴ The rear end of the Palolo worm, a abyssal polychaete that tunnels through coral, detaches in adjustment to spawn at the surface, and the humans of Samoa absorption these breeding modules as a delicacy.³⁴ Anglers sometimes acquisition that worms are added able allurement than bogus flies, and worms can be kept for several canicule in a tin lined with clammy moss.³⁹ Ragworms are commercially important as allurement and as aliment sources for aquaculture, and there accept been proposals to acreage them in adjustment to abate over-fishing of their accustomed populations.³⁸ Some abyssal polychaetes' predation on molluscs causes austere losses to fishery and aquaculture operations.³⁴

Scientists abstraction amphibian annelids to adviser the oxygen content, salinity and abuse levels in beginning and abyssal water.³⁴

Accounts of the use of leeches for the medically arguable practise of blood-letting accept arise from China about 30 AD, India about 200 AD, age-old Rome about 50 AD and afterwards throughout Europe. In the 19th century medical appeal for leeches was so top that some areas' stocks were beat and added regions imposed restrictions or bans on exports, and Hirudo medicinalis is advised as an endangered brand by both IUCN and CITES. Added afresh leeches accept been acclimated to abetment in microsurgery, and their saliva has provided anti-inflammatory compounds and several important anticoagulants, one of which aswell prevents tumors from spreading.³⁴

Ragworms' aperture are able but abundant lighter than the harder locations of abounding added organisms, which are biomineralized with calcium salts. These advantages accept admiring the absorption of engineers. Investigations showed that ragworm aperture are fabricated of abnormal proteins that bind acerb to zinc.⁴⁰

Evolutionary history

Fossil record

Since annelids are soft-bodied, their fossils are rare.⁴¹ Polychaetes' drop almanac consists mainly of the aperture that some brand had and the mineralized tubes that some secreted.⁴² Some Ediacaran fossils such as Dickinsonia in some agency resemble polychaetes, but the similarities are too ambiguous for these fossils to be classified with confidence.⁴³ The small shelly fossil Cloudina, from 549 to 542 actor 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).⁴⁴ Until 2008 the age-old fossils broadly accustomed as annelids were the polychaetes Canadia and Burgessochaeta, both from Canada's Burgess Shale, formed about 505 actor years ago in the aboriginal Cambrian.⁴⁵ Myoscolex, begin in Australia and a little earlier than the Burgess Shale, was possibly an annelid. However, it lacks some archetypal annelid appearance and has appearance which are not usually begin in annelids and some of which are associated with added phyla.⁴⁵ Afresh Simon Conway Morris and John Peel arise Phragmochaeta from Sirius Passet, about 518 actor years old, and assured that it was the oldest annelid accustomed to date.⁴³ There has been alive agitation about whether the Burgess Shale drop Wiwaxia was a mollusc or an annelid.⁴⁵ Polychaetes adapted in the aboriginal Ordovician, about 488 to 474 actor years ago. By the end of the Carboniferous, about 299 actor years ago, fossils of a lot of of the avant-garde adaptable polychaete groups had appeared.⁴⁵ Abounding drop tubes attending like those fabricated by avant-garde sessile polychaetes, but the aboriginal tubes acutely produced by polychaetes date from the Jurassic, beneath than 199 actor years ago.⁴⁵

The age-old adequate affirmation for oligochaetes occurs in the Tertiary period, which began 65 actor years ago, and it has been appropriate that these animals acquired about the aforementioned time as flowering plants in the aboriginal Cretaceous, from 130 to 90 actor years ago.⁴⁶ A trace fossil consisting of a bizarre burrow partly abounding with baby fecal pellets may be affirmation that earthworms were present in the aboriginal Triassic aeon from 251 to 245 actor years ago.⁴⁶⁴⁷ Body fossils traveling aback to the mid Ordovician, from 472 to 461 actor years ago, accept been tentatively classified as oligochaetes, but these identifications are ambiguous and some accept been disputed.⁴⁶⁴⁸

Family tree

Traditionally the annelids accept been disconnected into two aloft groups, the polychaetes and clitellates. In about-face the clitellates were disconnected into oligochaetes, which cover earthworms, and hirudinomorphs, whose best-known associates are leeches.³ For abounding years there was no bright adjustment of the about 80 polychaete families into higher-level groups.⁵ In 1997 Greg Rouse and Kristian Fauchald attempted a "first heuristic footfall in agreement of bringing polychaete systematics to an adequate akin of rigour", based on anatomical structures, and disconnected polychaetes into:⁵⁰

• Scolecida, beneath than 1,000 burrowing brand that attending rather like earthworms.⁵¹
• Palpata, the abundant majority of polychaetes, disconnected into:
  • Canalipalpata, which are acclaimed by accepting continued acclimatized palps that they use for feeding, and a lot of of which reside in tubes.⁵¹
  • Aciculata, the a lot of alive polychaetes, which accept parapodia able by centralized spines (aciculae).⁵¹

Also in 1997 Damhnait McHugh, application molecular phylogenetics to analyze similarities and differences in one gene, presented a actual altered view, in which: the clitellates were an off-shoot of one annex of the polychaete ancestors tree; the pogonophorans and echiurans, which for a few decades had been admired as a abstracted phyla, were placed on added branches of the polychaete tree.⁵² Subsequent atomic phylogenetics analyses on a agnate calibration presented agnate conclusions.⁵³

In 2007 Torsten Struck and colleagues compared 3 genes in 81 taxa, of which 9 were outgroups,⁵ in added words not advised carefully accompanying to annelids but included to accord an adumbration of breadth the bacilli beneath abstraction are placed on the beyond tree of life.⁵⁴ For a cross-check the abstraction acclimated an assay of 11 genes (including the aboriginal 3) in 10 taxa. This assay agreed that clitellates, pogonophorans and echiurans were on assorted branches of the polychaete ancestors tree. It aswell assured that the allocation of polychaetes into Scolecida, Canalipalpata and Aciculata was useless, as the associates of these declared groups were broadcast all over the ancestors timberline acquired from comparing the 81 taxa. In addition, it aswell placed sipunculans, about admired at the time as a abstracted phylum, on accession annex of the polychaete tree, and assured that leeches were a sub-group of oligochaetes rather than their sister-group a part of the clitellates.⁵ Rouse accustomed the analyses based on atomic phylogenetics,⁷ and their capital abstracts are now the accurate consensus, although the data of the annelid ancestors timberline abide uncertain.⁶

In accession to re-writing the allocation of annelids and 3 advanced absolute phyla, the atomic phylogenetics analyses attenuate the accent that decades of antecedent writings placed on the accent of segmentation in the allocation of invertebrates. Polychaetes, which these analyses begin to be the ancestor group, accept absolutely anecdotal bodies, while polychaetes' echiurans and sipunculan offshoots are not anecdotal and pogonophores are anecdotal alone in the rear locations of their bodies. It now seems that assay can arise and abandon abundant added calmly in the advance of change than was advanced thought.⁵⁵² The 2007 abstraction aswell acclaimed that the ladder-like afraid system, which is associated with segmentation, is beneath accepted than advanced anticipation in both annelids and arthropods.⁵

Annelids are associates of the protostomes, one of the two aloft superphyla of bilaterian animals - the added is the deuterostomes, which includes vertebrates.⁵³ Aural the protostomes, annelids acclimated to be aggregate with arthropods beneath the super-group Articulata ("jointed animals"), as assay is accessible in a lot of associates of both phyla. However, the genes that drive assay in arthropods do not arise to do the aforementioned in annelids. Arthropods and annelids both accept abutting ancestors that are unsegmented. It is at atomic as simple to accept that they acquired anecdotal bodies independently as it is to accept that the affiliated protostome or bilaterian was anecdotal and that assay abolished in abounding brood phyla.⁵³ The accustomed appearance is that annelids are aggregate with molluscs, brachiopods and several added phyla that accept lophophores (fan-like agriculture structures) and/or trochophore larvae as associates of Lophotrochozoa.⁵⁵ Bryzoa may be the a lot of basal phylum (the one that aboriginal became distinctive) aural the Lophotrochozoa, and the relationships amid the added associates are not yet known.⁵³ Arthropods are now admired as associates of the Ecdysozoa ("animals that molt"), forth with some phyla that are unsegmented.⁵³⁵⁶

The "Lophotrochozoa" antecedent is aswell accurate by the actuality that abounding phyla aural this group, including annelids, molluscs, nemerteans and flatworms, chase a agnate adjustment in the fertilized egg's development. If their beef bisect afterwards the 4-cell stage, birth of these 4 beef assay a circling pattern. In these phyla the "fates" of the embryo's cells, in added words the roles their birth will play in the developed animal, are the aforementioned and can be predicted from a actual aboriginal stage.⁵⁷ Hence this development adjustment is about declared as "spiral belted cleavage".⁵⁸

References

Further reading

• Dales, R. P. 1967. Annelids, 2nd edition. London: Hutchinson University Library.
• "Annelid Fossils" (Web page). The Virtual Drop Museum. 2006. http://www.fossilmuseum.net/fossils/Annelid-Fossils.htm. Retrieved May 20, 2006.  – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range.

Wikimedia Commons has media accompanying to: Annelida

Wikispecies has advice accompanying to: Annelida

The Wikibook Dichotomous Key has a page on the affair of Annelida

External links

• Polychaete Larva - Guide to the Abyssal Zooplankton of south eastern Australia, Tasmanian Aquaculture & Fisheries Institute
• Malaysia Medicinal Leeches

v • d • e Extant phyla of commonwealth Animalia by subkingdom Parazoa Porifera (Calcarea, Demospongiae, Hexactinellida) · Placozoa (Trichoplax) Mesozoa Orthonectida · Rhombozoa Eumetazoa Radiata Ctenophora · Cnidaria (Anthozoa, Hydrozoa, Scyphozoa, Cubozoa, Staurozoa, Myxozoa) Bilateria Protostomia Ecdysozoa Cycloneuralia: Scalidophora (Kinorhyncha, Loricifera, Priapulida) · Nematoida (Nematoda, Nematomorpha) Panarthropoda: Onychophora · Tardigrada · Arthropoda Spiralia Platyzoa Platyhelminthes · Gastrotricha Gnathifera: Rotifera · Acanthocephala · Gnathostomulida · Micrognathozoa · Cycliophora Lophotrochozoa Trochozoa (Sipuncula, Nemertea, Mollusca, Annelida) Lophophorata (Bryozoa, Entoprocta, Phoronida, Brachiopoda) Deuterostomia Ambulacraria Hemichordata · Echinodermata · Xenoturbellida Chordata Craniata (Vertebrata, Myxini) · Cephalochordata · Tunicata Basal/disputed Acoelomorpha (Acoela, Nemertodermatida) · Chaetognatha