Classification of free-living and sedentary polychaetes relies almost exclusively . Most authors accept the annelids as having three major classes: Polychaeta. The most relevant conclusions are: (1) Annelida and Polychaeta are non- monophyletic, even when Classe des annélides polychétes et oligochétes, p. Denkschriften der Kaiserlichen Akademie der Wissenschaften, Mathematisch- naturwissenschaftliche Classe, Wien. 41(2): , plates I-VI., available online.
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The annelids Annelida, from Latin anellus”little ring” [a] also known as the ringed worms or segmented wormsare a large phylumwith over 22, extant species including ragwormsearthwormsand leeches. The annelids are bilaterally symmetricaltriploblasticcoelomateinvertebrate organisms. They also have parapodia for locomotion. Most textbooks still polycbaeta the traditional division polychatea polychaetes almost all marineoligochaetes polychaaeta include earthworms and leech -like species. Cladistic research since has radically changed this scheme, viewing leeches as a sub-group of oligochaetes and oligochaetes as a sub-group of polychaetes.
In addition, the PogonophoraEchiura and Sipunculapreviously regarded as separate phyla, are now regarded as sub-groups of polychaetes. Annelids are considered members of the Lophotrochozoaa “super-phylum” of protostomes that also includes molluscsbrachiopodsflatworms and nemerteans.
The basic annelid form consists of multiple segments. Each segment has the same sets of organs and, in most polychates, has a pair of parapodia that many species use for locomotion. Septa separate popychaeta segments of many species, but are poorly defined or absent in others, and Echiura and Sipuncula pklychaeta no obvious signs of segmentation.
In species with well-developed popychaeta, the blood circulates entirely within blood vesselsand the vessels in segments near the front ends of these species are often built up with muscles that act as hearts. The septa of such species also enable them to change the shapes of individual segments, which facilitates movement by peristalsis “ripples” that pass along the body or by undulations that improve the effectiveness of the parapodia.
In species with incomplete septa or none, the blood circulates through the main body polychaeeta without any cpasse of pump, and there is a wide range of locomotory techniques — some burrowing species turn their pharynges inside out to drag themselves through the sediment.
Although many species can reproduce asexually and use similar mechanisms to regenerate after severe injuries, sexual reproduction is the normal method in species whose reproduction has been claase. The minority of living polychaetes whose reproduction and lifecycles are known produce trochophore larvaethat live as plankton and then sink and metamorphose into miniature adults.
Oligochaetes are full hermaphrodites and produce a ring-like cocoon around their bodies, in which the eggs and hatchlings are nourished until they are ready to emerge.
Earthworms are oligochaetes that support terrestrial food chains both as prey and in some regions are important in aeration and enriching of soil. The burrowing of marine polychaetes, which may constitute up to a third of all species in near-shore environments, encourages the development of ecosystems by enabling water and oxygen to penetrate the sea floor.
In addition to improving soil fertilityannelids polychawta humans as food and as bait. Scientists observe annelids to monitor the quality of marine and fresh water.
Polychaeta – Wikipedia, le encyclopedia libere
Although blood-letting is used less frequently by doctors, some leech species are regarded as endangered species because they have been over-harvested for this purpose in the last few centuries. Ragworms’ jaws are now being studied by engineers as they offer an exceptional combination of lightness and strength. Since annelids are soft-bodiedtheir fossils are rare — mostly jaws and the mineralized tubes that some of the species secreted. There are over 22, living annelid species,   ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus Cognetti,which can both grow up to 3 metres 9.
The Archiannelidaminute annelids that live in the spaces between grains of marine sedimentwere treated as a separate class because of their simple body structure, but are now regarded as polychaetes. No single feature distinguishes Annelids from other invertebrate phyla, but they have a distinctive combination of features.
Their bodies are long, with segments that are divided externally by shallow ring-like constrictions called annuli and internally by septa “partitions” at the same points, although in some species the septa are incomplete and in a few cases missing.
Most of the segments contain the same sets of organsalthough sharing a common gutcirculatory system and nervous system makes them inter-dependent. 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 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 polycheta zone while the peristomium is the oldest.
This pattern is called teloblastic growth. The phylum’s name is derived from the Latin word annelusmeaning “little ring”.
Annelids’ cuticles are made of collagen fibers, usually classf layers that spiral in alternating directions so that the fibers cross each other.
Annelid – Classification |
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 vlasse structure, and mucus -secreting glands in the epidermis protect their skins. Below this are two layers of muscles, which develop from the lining of the coelom body cavity: The setae “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 pilychaeta produce chetae by forming microvillifine 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. Nearly all polychaetes have parapodia that function as limbs, while other major annelid groups clxsse them.
Parapodia are unjointed paired extensions of the body wall, and their muscles are derived from polhchaeta 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. The brain generally forms a ring round the pharynx throatconsisting of a pair of ganglia local control centers above and in front of the pharynx, linked by nerve cords either side of the pharynx to another pair of ganglia just below and behind it.
From each segmental ganglion a branching system of local nerves runs into the body wall and then encircles the body. As in arthropodseach muscle fiber cell is controlled by more than one neuronand 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. 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. 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. 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: 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.
The fluid in the coelomata contains coelomocyte cells that defend the animals against parasites and infections. In some species coelomocytes may also contain a respiratory pigment — red hemoglobin in some species, green chlorocruorin in others dissolved in the plasma  — 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 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 Clitellata.
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. In some annelids, including earthwormsall 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 in polychaetes.
The gills of tube-dwellers and burrowers usually cluster around whichever end has the stronger water flow. 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 familiesthe Eunicidae and Phyllodocidaehave evolved jaws, which can be used for seizing prey, biting off pieces of vegetation, or grasping dead and decaying matter.
On the other hand, some predatory polychaetes have neither jaws nor eversible pharynges. Selective deposit feeders generally live in tubes on the sea-floor and use palps to find food particles in the sediment and then wipe them into their mouths. Filter feeders use “crowns” of palps covered in cilia that wash food particles towards their mouths.
Non-selective deposit feeders ingest soil or marine sediments via mouths that are generally unspecialized. Some clitellates have sticky pads in the roofs of their mouths, and some of these can evert the pads to capture prey.
Leeches often have an eversible proboscis, or a muscular pharynx with two or three teeth.
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The gut is generally an almost straight tube supported by the mesenteries vertical partitions within segmentsand ends with the anus on the underside of the pygidium. The bacteria convert inorganic matter — such as hydrogen sulfide and carbon dioxide from hydrothermal ventsor 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. 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.
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 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. Polychaetes can reproduce asexually, by dividing into two or more pieces or by budding off a new individual while the parent remains a complete organism.
Asexual reproduction in oligochaetes is always by dividing into two or more pieces, rather than by budding. Most polychaetes and oligochaetes also use similar mechanisms to regenerate after suffering damage. Two polychaete generaChaetopterus and Dodecaceriacan regenerate from a single segment, and others can regenerate even if their heads are removed.
It is thought that annelids were originally animals with two separate sexeswhich released ova and sperm into the water via their nephridia. The rest care for the fertilized eggs until they hatch — some by producing jelly-covered masses of eggs which they tend, some by attaching the eggs to their bodies and a few species by keeping the eggs within their bodies until they hatch.
These species use a variety of methods for sperm transfer; for example, in some the females collect sperm released into the water, while in others the males have a penis that inject sperm into the female. Some polychaetes breed only once in their lives, while others breed almost continuously or through several breeding seasons. While most polychaetes remain of one sex all their lives, a significant percentage of species are full hermaphrodites or change sex during their lives.
Most polychaetes whose reproduction has been studied lack permanent gonadsand it is uncertain how they produce ova and sperm.