Annelid

Annelids Temporal range: 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, collectively called Annelida (from Latin anellus "little ring"), are a large phylum of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches. They 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).

Classification

Most experts divide the annelids into the following sub-groups:

• Polychaetes. As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes have parapodia that function as limbs, and nuchal organs ("nuchal" means "on the neck"). Most are marine animals, although a few species live in fresh water and even fewer on land.

An earthworm'sclitellum

• Clitellates. 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") round their bodies, in which they store and nourish fertilized eggs until they are ready for release. The clitellates are sub-divided into:
  • Oligochaetes ("with few hairs"), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or partly decomposed organic materials.
  • Hirudinomorpha, whose name means "leech-shaped" and whose best known members are leeches. Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators. They have suckers at both ends of their bodies, and use these to move rather like inchworms.

Some other groups of animals have been classified in various ways, but are now generally regarded as annelids:

• Myzostomida live on crinoids and other echinoderms, mainly as parasites. In the past they have been regarded as close relatives of the trematode flatworms or of the tardigrades, but are now regarded as a sub-group of polychaetes.
• Pogonophora / Sibloginidae 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, Sibloginidae, within the polychaetes.

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 and internally by septa ("partitions") at the same points, although in some species the septa are incomplete and in a few cases missing. Most of the segments contain the same sets of organs, although sharing a common gut, circulatory system and nervous system makes them inter-dependent. Their bodies are covered by a cuticle (outer covering) that does not contain cells but is secreted by cells in the skin underneath, is made of tough but flexible collagen and does not molt – on the other hand arthropods' cuticles are made of the more rigid α-chitin, and molt until these animals reach their full size. Most annelids have closed circulatory systems, where the blood makes its entire circuit via blood vessels.

Summary of distinguishing features

	Annelida	Closely-related phyla			Similar-looking phyla
		Echiura	Sipuncula	Nemertea	Arthropoda	Onychophora
External segmentation	Yes	no	no	Only in a few species	Yes, except in mites	no
Repetition of internal organs	Yes	no	no	Yes	In primitive forms	Yes
Septa between segments	In most species	no	no	No	No	No
Cuticle material	collagen	collagen	collagen	none	α-chitin	α-chitin
Molting	Generally no; but some polychaetes molt their jaws, and leeches molt their skins	no	no	no	Yes	Yes
Body cavity	Coelom; but this is reduced or missing in many leeches and some small polychaetes	2 coeloms, main and in proboscis	2 coeloms, main and in tentacles	Coelom only in proboscis	Hemocoel	Hemocoel
Circulatory system	Closed in most species	Open outflow, return via branched vein	Open	Closed	Open	Open

Description

Segmentation

    Prostomium

    Peristomium

O Mouth

    Growth zone

    Pygidium

O Anus

Segments of an annelid

Most of an annelid's body consists of segments that are practically identical, having the same sets of internal organs and external chetae (Greek χαιτα, meaning "hair") and, in some species, appendages. However 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") contains the anus. 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.

The segments develop one at a time from a growth zone just ahead of the pygidium, so that an annelid's youngest segment is just in front of the growth zone while the peristomium is the oldest. This pattern is called teloblastic growth. Some groups of annelids, including all leeches, have fixed maximum numbers of segments, while others add segments thoughout their lives.

Body wall, chetae and parapodia

1 O Nephridiopore

2     Nephridium

3     Skin

4     Circular muscle

5     Longitudinal muscle

6     Peritoneum

7     Gut

8     Blood vessel

9     Nerve cord(s)

10     Coelom

Cross-section through a typical annelid

Annelids' cuticles are made of collagen fibers, usually in layers that spiral in alternating directions so that the fibers cross each other. These are secreted by the one-cell deep epidermis (outermost skin layer). A few marine annelids that live in tubes lack cuticles, but their tubes have a similar structure, and mucus-secreting glands in the epidermis protect their skins. Under the epidermis is the dermis, which is made of connective tissue, in other words a combination of cells and non-cellular materials such as collagen. Below this are two layers of muscles, which develop from the lining of the coelom (body cavity): circular muscles make a segment longer and slimmer when they contract, while under them are longitudinal muscles, usually four distinct strips, whose contractions make the segment shorter and fatter. Some annelids also have oblique internal muscles that connect the underside of the body to each side.

The chetae ("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. Chetae are made of moderately flexible β-chitin and are formed by follicles, each of which has a chaetoblast ("hair-forming") cell at the bottom and muscles that can extend or retract the cheta. The chetoblasts produce chetae by forming microvilli, fine hair-like extensions that increase the area available for secreting the cheta. When the cheta is complete, the microvilli withdraw into the chetoblast, leaving parallel tunnels that run almost the full length of the cheta. Hence annelids' chetae are structurally different from the setae ("bristles") of arthropods, which are made of the more rigid α-chitin, have a single internal cavity, and are mounted on flexible joints in shallow pits in the cuticle.

Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups lack them. Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from the circular muscles of the body. They are often supported internally by one or more large, thick chetae. The parapodia of burrowing and tube-dwelling polychaetes are often just ridges whose tips bear hooked chetae. In active crawlers and swimmers the parapodia are often divided into large upper and lower paddles on a very short trunk, and the paddles are generally fringed with chetae and sometimes with cirri (fused bundles of cilia) and gills.

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 phyarynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it. In some very mobile and active polychaetes the brain is enlarged and more complex, with visible hindbrain, midbrain and forebrain sections. The rest of the central nervous system is generally "ladder-like", consisting of a pair of nerve cords than run through the bottom part of the body and have in each segment paired ganglia linked by a transverse connection. From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body. However in most polychaetes the two main nerve cords are fused, and in the tube-dwelling genus Owenia the single nerve chord has no ganglia and is located in the epidermis.

As in arthropods, each muscle fiber (cell) is controled by more than one neuron, and the speed and power of the fiber's contractions depends on the combined effects of all its neurons. Vertebrates have a different system, in which one neuron controls a group of muscle fibers. Most annelids' longitudinal nerve trunks include giant axons (the output signal lines of nerve cells). Their large diameter decreases their resistance, which allows them to transmit signals exceptionally fast. This enables these worms to withdraw rapidly from danger by shortening their bodies. Experiments have shown that cutting the giant axons prevents this escape response but does not affect normal movement.

The sensors are primarily single cells that detect light, chemicals, pressure waves and contact, and are present on the head, appendages (if any) and other parts of the body. Nuchal ("on the neck") organs are paired, ciliated structures found only in polychaetes, and are thought to be chemosensors. Some polychaetes also have various combinations of ocelli ("little eyes") that detect the direction from which light is coming and camera eyes or compound eyes that can probably form images; these compound eyes probably evolved independently of arthropods'. Some tube-worms use ocelli widely spread over their bodies to detect the shadows of fish, so that they can quickly withraw into their tubes. Some burrowing and tube-dwelling polychaetes have statocysts (tilt and balance sensors) that tell them which way is down. A few polychaete genera have on the undersides of their heads palps that are used both in feeding and as "feelers", and some of these also have antennae that are structurally similar but probably are used mainly as "feelers".

Coelom, locomotion and circulatory system

Most annelids have a pair of coeloms (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 coeloms, and the blood vessels and, in polychaetes, the main nerve cords are embedded in it. The mesothelium is made of modified epitheliomuscular cells, in other words their bodies form part of the epithelium but their bases extend to form muscle fibers in the body wall. The mesothelium may also form radial and circular muscles on the septa, and circular muscles around the blood vessels and gut. Parts of the mesothelium may also form chlorogogen 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.

Peristalsis moves this "worm" to the right

Many annelids move by peristalsis (waves of contraction and expansion that sweep along the body), or flex the body while using parapodia to crawl or swim. In these animals the septa enable the circular and longitudinal muscles to change the shape of individual segments, by making each segment a separate fluid-filled "balloon". However the septa are often incomplete in annelids that are semi-sessile or that do not move by peristalsis or by movements of parapodia – for example some move by whipping movements of the body, some small marine species move by means of cilia (fine muscle-powered hairs) and some burrowers turn their pharynges (throats) inside out to penetrate the sea-floor and drag themsleves into it.

The fluid in the coeloms 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 – and provide oxygen transport within their segments. Respiratory pigment is also dissolved in the blood plasma. Species with well-developed septa generally also have blood vessels running all long their bodies above and below the gut, the upper one carrying blood forwards while the lower one carries it backwards. Networks of capillaries (fine blood vessels) in the body wall and around the gut transfer blood between the main blood vessels and to parts of the segment that need oxygen and nutrients. Both of the major vessels, especially the upper one, can pump blood by contracting. In some annelids the forward end of the upper blood vessel is enlarged with muscles to form a heart, while in the forward ends of many earthworms some of the vessels that connect the upper and lower main vessels function as hearts. Species with poorly-developed or no septa generally have no blood vessels and rely on the circulation within the coelom for delivering nutrients and oxygen.

However leeches and their closest relatives have a body structure that is very uniform within the group but significantly different from that of other annelids, including other members of the

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 coeloms 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 coeloms and unlike the blood vessls 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.

Respiration

In some annelids, including earthworms, all respiration is via the skin. However many polychaetes and some clitellates (the group to which earthworms belong) have gills associated with most segments, often as extensions of the parapodia. 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. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey. Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and them wipe them into their mouths. Filter feeders use "crowns" of palps covered in cilia that wash food particles towards their mouths.

The gut is generally an almost straight tube supported by the mesenteries (vertical partions within segments), and ends with the anus on the underside of the pygidium. However in members of the tube-dwelling family Siblongidaue the gut is blocked by a swollen lining that houses symbiotic bacteria, which can make up 15% of the worms' total weight. The bacteria convert inorganic matter – such as hydrogen sulfide and carbon dioxide from hydrothermal vents, or methane from seeps – to organic matter that feeds themselves and their hosts, while the worms extend their palps into the gas flows to absorb the gases needed by the bacteria.

Annelids with blood vessels use metanephridia to remove soluble waste products, while those without use protonephridia. Both of these systems use a two-stage filtration process, in which fluid and waste products are first extracted and these are filtered again to re-absorb any re-usable materials while dumping toxic and spent materials as urine. The difference is that protonephridia combine both filtration stages in the same organ, while metanephridia perform only the second filtration and rely on other mechanisms for the first - in annelids special filter cells in the walls of the blood vessels let fluids and other small molecules pass into the coelomic fluid, where it circulates to the metanephridia. In annelids the points at which fluid enters the protonephridia or metanephridia are on the forward side of a septum while the second-stage filter and the nephridiopore (exit opening in the body wall) are in the following segment. As a result the hindmost segment (before the growth zone and pygidium) has no structure that extracts its wastes, as there is no following segment via which to filter and discharge them, while the first segment contains an extraction structure that passes wastes to the second, but does not contain the structures that re-filter and discharge urine.

Physiology

Annelids are bilaterally symmetric and triploblastic protostomes with a coelom (which makes them coelomates), closed circulatory system and true segmentation. Their segmented bodies and coelom have given them evolutionary advantages over other worms. Oligochaetes and polychaetes typically have spacious coeloms; in leeches, the coelom is filled in with tissue and reduced to a system of narrow canals; archiannelids may lack the coelom entirely. The coelom is divided into a sequence of compartments by walls called septa. In the most general forms each compartment corresponds to a triple segment of the body, which also includes a portion of the nervous and (closed) circulatory systems, allowing it to function relatively independently. The closed circulatory system consists of networks of vessels containing blood with oxygen-carrying hemoglobin. Dorsal and ventral vessels are connected by segmental pairs of vessels. The dorsal vessel and five pairs of vessels that circle the esophagus of an earthworm are muscular and pump blood through the circulatory system. Tiny blood vessels are abundant in the earthworm's skin, which function as its respiratory organ. Each segment (metamere) 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 and in daria 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.

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 (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 which 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 which 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 nerve cord from which lateral nerves come in contact with 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.

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 identical worm. The position of the break is usually determined by an epidermal growth. Lumbriculus and Aulophorus, for example, are known to reproduce by the worm breaking into such fragments. This complete regeneration is noteworthy as these Annelid species are the most highly organized animals to have this capability. 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 are gonochoristic, that is, they 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 clitella 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

Serpulid tubes on a marine shell; Duck Harbor Beach on Cape Cod Bay, Wellfleet, Massachusetts.

The annelid fossil record is sparse, but a few definite forms are known as early as the Cambrian - or perhaps even before. Because the creatures have soft bodies, fossilization of a body is an especially rare event. However, a few annelids, such as the living polychaetes in the Serpulidae, secrete calcareous tubes, and such tubes are fairly common as fossils (although these are not necessarily from annelida, as other animal phyla can also secrete tubes). A fossil group has recently been assigned to the annelids, the machaeridians. These forms were polychaetes with rows of dorsal overlapping shell plates. The hard jaws of certain polychaetes, known as scolecodonts, are known from the Ordovician onward, and are common enough to be used for stratigraphic correlation in some cases. The best-preserved and oldest annelid body fossils come from the 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 molluscs, which share with them the presence of trochophore larvae. Annelids and Molluscs are thus united as the Trochozoa, a taxon more strongly supported by molecular evidence.

Sipuncula, Echiura and Siboglinidae have traditionally been placed in their own phyla, while Clitellata has been considered separated from the polychaete annelids. But recent research indicates that all of them actually belongs within the Polychaete, even if some of these groups have lost their segmentation^[1].

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. As traditionally defined, the Oligochaeta are paraphyletic.
  • Leeches (Hirudinea) - These include both bloodsucking external parasites and predators of small invertebrates.
  • Acanthobdellidea and Branchiobdella - small leech-like clitellates formerly included with the Hirudinea.
• Aphanoneura
• 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

• "Annelid Fossils" (Web page). The Virtual Fossil Museum. 2006. Retrieved May 20 2006. {{cite web}}: Check date values in: |accessdate= (help); Unknown parameter |dateformat= ignored (help) – Descriptions and images of annelid fossils from Mazon Creek and the Utah House Range.

Further reading

• Dales, R. P. 1967. Annelids, 2nd edition. London: Hutchinson University Library.

External links

• Polychaete Larva - Guide to the Marine Zooplankton of south eastern Australia, Tasmanian Aquaculture & Fisheries Institute