Difference between revisions of "Annelid" - New World Encyclopedia
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{{Taxobox_begin | color = pink | name = Annelids}} | {{Taxobox_begin | color = pink | name = Annelids}} | ||
| − | {{Taxobox_image | image = [[Image: | + | {{Taxobox_image | image = [[Image:Nerr0328.jpg|230px]] | caption = ''Glycera sp.''}} |
{{Taxobox_begin_placement | color = pink}} | {{Taxobox_begin_placement | color = pink}} | ||
{{Taxobox_regnum_entry | taxon = [[Animal]]ia}} | {{Taxobox_regnum_entry | taxon = [[Animal]]ia}} | ||
| Line 6: | Line 6: | ||
{{Taxobox_end_placement}} | {{Taxobox_end_placement}} | ||
{{Taxobox_section_subdivision | color = pink | plural_taxon = Classes and subclasses}} | {{Taxobox_section_subdivision | color = pink | plural_taxon = Classes and subclasses}} | ||
| − | Class [[Polychaeta]] | + | Class [[Polychaeta]]* <br> |
| − | Class [[Clitellata]]<br> | + | Class [[Clitellata]]*<br> |
| − | [[Oligochaeta]] - [[Earthworm]]s and others<br> | + | [[Oligochaeta]]* - [[Earthworm]]s and others<br> |
| − | [[Acanthobdellida]]<br> | + | [[Acanthobdellida]]*<br> |
| − | [[Branchiobdellida]]<br> | + | [[Branchiobdellida]]*<br> |
| − | [[Hirudinea]] - [[Leech]]es<br> | + | [[Hirudinea]]* - [[Leech]]es<br> |
| − | Class [[Myzostomida]]<br> | + | Class [[Myzostomida]]*<br> |
| − | Class [[Archiannelida]] | + | Class [[Archiannelida]]*<br> |
<small>*Some authors consider the subclasses<br/>under Clitellata to be classes</small> | <small>*Some authors consider the subclasses<br/>under Clitellata to be classes</small> | ||
{{Taxobox_end}} | {{Taxobox_end}} | ||
| − | The '''annelids | + | |
| + | The '''annelids''' (from [[Latin]] ''anellus'' "little ring") are a large [[taxonomy|phylum]] ('''Annelida''') of [[invertebrate]] [[animal]]s, comprising the segmented [[worm]]s, including the well-known [[earthworm]]s and [[leech]]es. There are about 15,000 known modern species of annelids. | ||
| + | |||
| + | Annelids are found in most wet environments and include many terrestrial, freshwater, and especially marine species (such as the [[polychaete]]s), as well as some which are [[parasite|parasitic]] or [[symbiosis|mutualistic]]. They range in length from under a millimeter to over 3 meters (the seep tube worm ''Lamellibrachia luymesi''). | ||
== Anatomy == | == Anatomy == | ||
| − | Annelids are | + | Annelids are triploblastic protostomes with a coelom (at least historically), closed circulatory system, and true segmentation. ''Protosomes'' are animals with bilaterial symmetry where the first opening in development, the blastophore, becomes its mouth. ''Triploblastic'' means that they have three primary tissue areas formed during embryogenesis. A ''coelom'' is a fluid-filled body cavity. |
| + | |||
| + | Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coleom is divided into a sequence of compartments by walls called ''septa''. In the most general forms, each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called ''annuli''. Each segment also has an outer layer of circular [[muscle]] underneath a thin cuticle and [[epidermis]], and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms, they also carry a varying number of bristles, called ''setae'', and among the polychaetes a pair of appendages, called ''parapodia''. | ||
| + | |||
| + | [[Image:Libr0409.jpg|200px|left|thumb|[[Polychaeta]]: "A variety of marine worms" plate from ''Das Meer'' by M. J. Schleiden (1804–1881)]] | ||
| + | Anterior to the true segments lies the prostomium and peristomium, which carries the [[mouth]], and posterior to them lies the pygidium, where the [[anus]] is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (internal fold of the intestine or intestine inner wall), to increase surface area, along much of its length. | ||
| − | + | Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers. | |
| − | |||
| − | The [[vascular system]] and the [[nervous system]] are separate from the digestive tract. The vascular system includes a dorsal | + | The [[vascular system]] and the [[nervous system]] are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel that conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall. |
| − | The nervous system has a solid, ventral | + | The nervous system has a solid, ventral nerve cord from which lateral [[nerve]]s arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units; in others, the number of segments is fixed in early development. |
== Reproduction == | == Reproduction == | ||
| Line 36: | Line 43: | ||
===Asexual reproduction=== | ===Asexual reproduction=== | ||
| − | [[Asexual reproduction]] by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. [[Lumbriculus]] and [[Aulophorus]], for example, are known to reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow amputated segments. | + | [[Asexual reproduction]] by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. ''[[Lumbriculus]]'' and ''[[Aulophorus]]'', for example, are known to reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow [[amputated]] segments. |
===Sexual reproduction=== | ===Sexual reproduction=== | ||
| − | [[Sexual reproduction]] allows a species to better adapt to its environment. | + | [[Sexual reproduction]] allows a species to better adapt to its environment. Some annelida species are [[hermaphrodite|hermaphroditic]], while others have distinct sexes. |
| − | Most polychaete worms have separate males and females and external fertilization. The earliest [[larva]]l stage, which is lost in some groups, is a ciliated [[trochophore]], similar to those found in other phyla. | + | Most polychaete worms have separate males and females and external fertilization. The earliest [[larva]]l stage, which is lost in some groups, is a ciliated [[trochophore]], similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size. |
| − | Earthworms and other [[oligochaeta|oligochaetes]], as well as the [[leech]]es, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by [[copulation]]. Two worms which are attracted by each other's [[secretion]]s lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of [[sperm]] transference have been observed in different genera, and may involve internal [[spermatheca|spermathecae]] (sperm storing chambers) or [[spermatophore]]s that are attached to the outside of the other worm's body. | + | Earthworms and other [[oligochaeta|oligochaetes]], as well as the [[leech]]es, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by [[copulation]]. Two worms which are attracted by each other's [[secretion]]s lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of [[sperm]] transference have been observed in different genera, and may involve internal [[spermatheca|spermathecae]] (sperm storing chambers) or [[spermatophore]]s that are attached to the outside of the other worm's body. The clitellata lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic worms developing in a fluid-filled "cocoon" secreted by the [[clitellum]]. |
== Fossil record == | == Fossil record == | ||
| Line 50: | Line 57: | ||
The annelid [[fossil]] record is sparse, but a few definite forms are known as early as the [[Cambrian]], and there are some signs they were around in the later [[Precambrian]], but the earliest unequivocal annelid fossils are only known from the [[Cambrian]]. Because the creatures have soft bodies, fossilization is an especially rare event. The best-preserved and oldest annelid fossils come from Cambrian [[Lagerstätten]] such as the [[Burgess Shale]] of Canada, and the Middle Cambrian strata of the House Range in Utah. The Annelids are also diversely represented in the [[Pennsylvanian]]-age [[Mazon Creek]] fauna of Illinois. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family [[Siboglinidae]], and the [[Echiura]]. | The annelid [[fossil]] record is sparse, but a few definite forms are known as early as the [[Cambrian]], and there are some signs they were around in the later [[Precambrian]], but the earliest unequivocal annelid fossils are only known from the [[Cambrian]]. Because the creatures have soft bodies, fossilization is an especially rare event. The best-preserved and oldest annelid fossils come from Cambrian [[Lagerstätten]] such as the [[Burgess Shale]] of Canada, and the Middle Cambrian strata of the House Range in Utah. The Annelids are also diversely represented in the [[Pennsylvanian]]-age [[Mazon Creek]] fauna of Illinois. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family [[Siboglinidae]], and the [[Echiura]]. | ||
| − | == Relationships == | + | == Relationships == |
| − | + | ||
| − | + | The [[arthropod]]s and their kin have long been considered the closest relatives of the annelids, on account of their common segmented structure, giving rise to the grouping of [[Articulata]]. However, a number of differences between the two groups suggest this may be [[convergent evolution]]. The other major phylum which is of definite relation to the annelids are the [[mollusk]]s, which share with them the presence of [[trochophore]] larvae. Annelids and Mollusks are thus united as the [[Trochozoa]], a taxon more strongly supported by molecular evidence. | |
=== Classes and subclasses of Annelida === | === Classes and subclasses of Annelida === | ||
| Line 59: | Line 66: | ||
**[[Oligochaeta]] - The class Oligochaeta includes the megadriles ([[earthworm]]s), which are both aquatic and terrestrial, and the microdrile families such as [[tubificid]]s, which include many marine members as well. | **[[Oligochaeta]] - The class Oligochaeta includes the megadriles ([[earthworm]]s), which are both aquatic and terrestrial, and the microdrile families such as [[tubificid]]s, which include many marine members as well. | ||
**[[Leech]]es (Hirudinea) - These include both bloodsucking external parasites and predators of small invertebrates. | **[[Leech]]es (Hirudinea) - These include both bloodsucking external parasites and predators of small invertebrates. | ||
| − | *[[Polychaeta]] - This is the largest group of the | + | *[[Polychaeta]] - This is the largest group of annelids and the majority are marine. All segments are identical each with a pair of [[parapodia]]. The parapodia are used for swimming, burrowing and the creation of a feeding current. |
| + | |||
| + | == References == | ||
| + | * {{cite web | ||
| + | | year=2006 | url= http://www.fossilmuseum.net/fossils/Annelid-Fossils.htm | ||
| + | | title= Annelid Fossils | format= Web page | ||
| + | | publisher=The Virtual Fossil Museum | ||
| + | | accessdate=May 20 | accessyear=2006 }} – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range. | ||
== References == | == References == | ||
Annelid Fossils.[[http://www.fossilmuseum.net/fossils/Annelid-Fossils.htm]] (web page) The Virtual Fossil Museum: (2006). Retrieved on May 20, 2006. – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range. | Annelid Fossils.[[http://www.fossilmuseum.net/fossils/Annelid-Fossils.htm]] (web page) The Virtual Fossil Museum: (2006). Retrieved on May 20, 2006. – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range. | ||
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| − | {{credit| | + | {{credit|87907651}} |
[[Category:Life sciences]] | [[Category:Life sciences]] | ||
Revision as of 01:52, 16 November 2006
| Annelids | ||||
|---|---|---|---|---|
| 230px Glycera sp. | ||||
| Scientific classification | ||||
| ||||
| Classes and subclasses | ||||
|
Class Polychaeta *Some authors consider the subclasses |
The annelids (from Latin anellus "little ring") are a large phylum (Annelida) of invertebrate animals, comprising the segmented worms, including the well-known earthworms and leeches. There are about 15,000 known modern species of annelids.
Annelids are found in most wet environments and include many terrestrial, freshwater, and especially marine species (such as the polychaetes), as well as some which are parasitic or mutualistic. They range in length from under a millimeter to over 3 meters (the seep tube worm Lamellibrachia luymesi).
Anatomy
Annelids are triploblastic protostomes with a coelom (at least historically), closed circulatory system, and true segmentation. Protosomes are animals with bilaterial symmetry where the first opening in development, the blastophore, becomes its mouth. Triploblastic means that they have three primary tissue areas formed during embryogenesis. A coelom is a fluid-filled body cavity.
Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is largely filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coleom is divided into a sequence of compartments by walls called septa. In the most general forms, each compartment corresponds to a single segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. Each segment is marked externally by one or more rings, called annuli. Each segment also has an outer layer of circular muscle underneath a thin cuticle and epidermis, and a system of longitudinal muscles. In earthworms, the longitudinal muscles are strengthened by collagenous lamellae; the leeches have a double layer of muscles between the outer circulars and inner longitudinals. In most forms, they also carry a varying number of bristles, called setae, and among the polychaetes a pair of appendages, called parapodia.
Anterior to the true segments lies the prostomium and peristomium, which carries the mouth, and posterior to them lies the pygidium, where the anus is located. The digestive tract is quite variable but is usually specialized. For example, in some groups (notably most earthworms) it has a typhlosole (internal fold of the intestine or intestine inner wall), to increase surface area, along much of its length.
Different species of annelids have a wide variety of diets, including active and passive hunters, scavengers, filter feeders, direct deposit feeders that simply ingest the sediments, and blood-suckers.
The vascular system and the nervous system are separate from the digestive tract. The vascular system includes a dorsal vessel conveying the blood toward the front of the worm, and a ventral longitudinal vessel that conveys the blood in the opposite direction. The two systems are connected by a vascular sinus and by lateral vessels of various kinds, including in the true earthworms, capillaries on the body wall.
The nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion. Growth in many groups occurs by replication of individual segmental units; in others, the number of segments is fixed in early development.
Reproduction
Depending upon the species, annelids can reproduce both sexually and asexually.
Asexual reproduction
Asexual reproduction by fission is a method used by some annelids and allows them to reproduce quickly. The posterior part of the body breaks off and forms a new individual. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to reproduce by the body breaking into such fragments. Many other taxa (such as most earthworms) cannot reproduce this way, though they have varying abilities to regrow amputated segments.
Sexual reproduction
Sexual reproduction allows a species to better adapt to its environment. Some annelida species are hermaphroditic, while others have distinct sexes.
Most polychaete worms have separate males and females and external fertilization. The earliest larval stage, which is lost in some groups, is a ciliated trochophore, similar to those found in other phyla. The animal then begins to develop its segments, one after another, until it reaches its adult size.
Earthworms and other oligochaetes, as well as the leeches, are hermaphroditic and mate periodically throughout the year in favored environmental conditions. They mate by copulation. Two worms which are attracted by each other's secretions lay their bodies together with their heads pointing opposite directions. The fluid is transferred from the male pore to the other worm. Different methods of sperm transference have been observed in different genera, and may involve internal spermathecae (sperm storing chambers) or spermatophores that are attached to the outside of the other worm's body. The clitellata lack the free-living ciliated trochophore larvae present in the polychaetes, the embryonic worms developing in a fluid-filled "cocoon" secreted by the clitellum.
Fossil record
The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian, and there are some signs they were around in the later Precambrian, but the earliest unequivocal annelid fossils are only known from the Cambrian. Because the creatures have soft bodies, fossilization is an especially rare event. The best-preserved and oldest annelid fossils come from Cambrian Lagerstätten such as the Burgess Shale of Canada, and the Middle Cambrian strata of the House Range in Utah. The Annelids are also diversely represented in the Pennsylvanian-age Mazon Creek fauna of Illinois. A few small groups have been treated as separate phyla: the Pogonophora and Vestimentifera, now included in the family Siboglinidae, and the Echiura.
Relationships
The arthropods and their kin have long been considered the closest relatives of the annelids, on account of their common segmented structure, giving rise to the grouping of Articulata. However, a number of differences between the two groups suggest this may be convergent evolution. The other major phylum which is of definite relation to the annelids are the mollusks, which share with them the presence of trochophore larvae. Annelids and Mollusks are thus united as the Trochozoa, a taxon more strongly supported by molecular evidence.
Classes and subclasses of Annelida
- Clitellata
- Oligochaeta - The class Oligochaeta includes the megadriles (earthworms), which are both aquatic and terrestrial, and the microdrile families such as tubificids, which include many marine members as well.
- Leeches (Hirudinea) - These include both bloodsucking external parasites and predators of small invertebrates.
- Polychaeta - This is the largest group of annelids and the majority are marine. All segments are identical each with a pair of parapodia. The parapodia are used for swimming, burrowing and the creation of a feeding current.
ReferencesISBN links support NWE through referral fees
- Annelid Fossils (Web page). The Virtual Fossil Museum (2006). Retrieved May 20, 2006. – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range.
References
Annelid Fossils.[[1]] (web page) The Virtual Fossil Museum: (2006). Retrieved on May 20, 2006. – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range.
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