Annelid

Annelida Temporal range: 520–0  Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N Possible Ediacaran record, 567 Ma[2]
Glycera sp.
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
(unranked):	Spiralia
Superphylum:	Lophotrochozoa
Phylum:	Annelida Lamarck, 1809
Classes and subclasses
Cladistic view Palaeoannelida Chaetopteriformia Amphinomida Sipunculida Lobatocerebrum Pleistoannelida Myzostomida Errantia Protodriliformia Aciculata Phyllodocida Eunicida Sedentaria Orbiniida Cirratuliformia Siboglinidae Sabellida Spionida Capitellida Echiura Capitellidae Terebelliformia Maldanomorpha Clitellata Traditional view Class " Polychaeta" (paraphyletic ) Class Clitellata "Oligochaeta" (paraphyletic) Branchiobdellida Hirudinea – leeches Sipuncula (old phylum) Echiura (old phylum) Myzostomida (old phylum) Pogonophora (old phylum) Class †Machaeridia

The annelids

There are over 22,000 living annelid species,[6]^[7] ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus, which can both grow up to 3 meters (9.8 ft) long ^[7][8][9] 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,^[10][11] most textbooks use the traditional classification into the following sub-groups:^[8][12]

• parapodia that function as limbs, and nuchal organs that are thought to be chemosensors.^[8] Most are marine animals, although a few species live in fresh water and even fewer on land.^[13]

An earthworm's clitellum

• Clitellates (about 10,000 species ^[7]). These have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped clitellum ("pack saddle") around their bodies, which produces a cocoon that stores and nourishes fertilized eggs until they hatch ^[12][14] or, in moniligastrids, yolky eggs that provide nutrition for the embryos.^[7] The clitellates are sub-divided into:^[8]
  • organic materials.^[13]
  • parasites, mainly on fish, while most freshwater species are predators.^[13] They have suckers at both ends of their bodies, and use these to move rather like inchworms.^[15]

• Pogonophora /
  Vestimentifera. More recently they have been re-classified as a family, Siboglinidae, within the polychaetes.^[13][16]
• 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.^[6][16][17]
• flatworms or to rotifers and acanthocephales.^[16]
• Sipuncula was originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters. The phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida.^[18] Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Myzostomida and Annelida (including echiurans and pogonophorans).^[19] It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.^[20]

Mitogenomic and phylogenomic analysis also implies that Orthonectida, a group of extremely simplified parasites traditionally placed in Mesozoa, are actually reduced annelids.^[21] Research suggest that also nemerteans are annelids, with Oweniidae and Magelonidae as their closest relatives.^[22]

Distinguishing features

No single feature distinguishes Annelids from other

  Prostomium

  Peristomium

O Mouth

  Growth zone

  Pygidium

O Anus

Diagram of segments of an annelid^[8][12]

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.^[33] 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 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 polychaetes the peristomium has chetae and appendages like those of other segments.^[8]

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.^[8] Some groups of annelids, including all leeches,^[15] have fixed maximum numbers of segments, while others add segments throughout their lives.^[12]

The phylum's name is derived from the Latin word annelus, meaning "little ring".^[6]

Body wall, chaetae and parapodia

Annelids' cuticles are made of

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.^[8] 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 the giant axons prevents this escape response but does not affect normal movement.^[8]

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.

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 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.^[8] The mesothelium is made of modified epitheliomuscular cells;^[8] in other words, their bodies form part of the epithelium but their bases extend to form muscle fibers in the body wall.^[38] 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 the oxygen-carrier hemoglobin; breaking down proteins; and turning nitrogenous waste products into ammonia and urea to be excreted.^[8]

Many annelids move by

However, leeches 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.^[15] 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 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.^[15]

Respiration

In some annelids, including

Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete genera, Chaetopterus and Dodecaceria, can regenerate from a single segment, and others can regenerate even if their heads are removed.^[12][41] Annelids are the most complex animals that can regenerate after such severe damage.^[44] On the other hand, leeches cannot regenerate.^[43]

Sexual reproduction

Apical tuft (cilia)

Prototroch (cilia)

Stomach

Mouth

Metatroch (cilia)

Mesoderm

Anus

/// = cilia

Trochophore larva^[45]

It is thought that annelids were originally animals with two separate

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 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.^[46] There is no guarantee that this is a representative sample of polychaetes' reproductive patterns, and it simply reflects scientists' current knowledge.^[46]

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 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.^[46]

Most mature

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

Scientists study aquatic annelids to monitor the oxygen content, salinity and pollution levels in fresh and marine water.[47]

Accounts of the use of

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 proteins that bind strongly to zinc.^[53]

Evolutionary history

Fossil record

Since annelids are

• Scolecida, less than 1,000 burrowing species that look rather like earthworms.^[66]
• Palpata, the great majority of polychaetes, divided into:
  • Canalipalpata, which are distinguished by having long grooved palps that they use for feeding, and most of which live in tubes.^[66]
  • Aciculata, the most active polychaetes, which have parapodia reinforced by internal spines (aciculae).^[66]

Also in 1997 Damhnait McHugh, using

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 invertebrates. 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.^[10][68] The 2007 study also noted that the ladder-like nervous system, which is associated with segmentation, is less universal than previously thought in both annelids and arthropods.^[10][b]

The updated

Bilateria	Acoelomorpha (Acoela and Nemertodermatida) Deuterostomia (Ambulacrarians and chordates ) Protostomia Ecdysozoa (Arthropods, etc.) Spiralia Gnathifera Platytrochozoa Rouphozoa Mesozoa Lophotrochozoa Cycliophora Annelida Mollusca Kryptotrochozoa Lophophorata Brachiozoa Brachiopoda Phoronida Bryozoa s.l. Entoprocta Ectoprocta Nemertea

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

Fossil discoveries lead to the hypothesis that Annelida and the

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External links

The Wikibook Dichotomous Key has a page on the topic of: Annelida

• Media related to Annelida at Wikimedia Commons
• Data related to Annelida at Wikispecies