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Oligochaeta () is a subclass of animals in the
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature ...
Annelida, which is made up of many types of aquatic and terrestrial
worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...
s, including all of the various
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. T ...
s. Specifically, oligochaetes comprise the terrestrial megadrile earthworms (some of which are semiaquatic or fully aquatic), and freshwater or semiterrestrial microdrile forms, including the tubificids,
pot worm Enchytraeidae is a family of microdrile oligochaeta, oligochaetes. They resemble small earthworms and include both terrestrial species known as potworms that live in highly organic Ecoregion#Terrestrial, terrestrial environments, as well as some ...
s and ice worms ( Enchytraeidae), blackworms ( Lumbriculidae) and several interstitial marine worms. With around 10,000 known species, the Oligochaeta make up about half of the phylum Annelida. These worms usually have few
seta In biology, setae (singular seta ; from the Latin word for "bristle") are any of a number of different bristle- or hair-like structures on living organisms. Animal setae Protostomes Annelid setae are stiff bristles present on the body. ...
e (chaetae) or "bristles" on their outer body surfaces, and lack
parapodia In invertebrates, the term parapodium ( Gr. ''para'', beyond or beside + ''podia'', feet; plural: parapodia) refers to lateral outgrowths or protrusions from the body. Parapodia are predominantly found in annelids, where they are paired, unjointed ...
, unlike
polychaeta Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ...
.


Diversity

Oligochaetes are well-segmented worms and most have a spacious
body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...
(coelom) used as a hydroskeleton. They range in length from less than up to in the 'giant' species such as the
giant Gippsland earthworm The giant Gippsland earthworm, ''Megascolides australis'', is one of Australia's 1,000 native earthworm species. Description These giant earthworms average long and in diameter and can reach in length; however, their body is able to expand a ...
(''Megascolides australis'') and the
Mekong worm ''Amynthas mekongianus'', the Mekong worm or Mekong giant earthworm, previously known as ''Megascolex mekongianus'', is a species of earthworm in the family Megascolecidae. It is native to the vicinity of the River Mekong in southeastern Asia an ...
(''Amynthas mekongianus'').Blakemore, Robert J., Csaba Csuzdi, Masamichi T. Ito, Nobuhiro Kaneko, Maurizio G. Paoletti, Sergei E. Spiridonov, Tomoko Uchida & Beverley D. Van Praagh (2007). Megascolex (Promegascolex) mekongianus Cognetti, 1922: its extent, ecology and allocation to Amynthas (Oligochaeta: Megascolecidae). Opuscula Zoologica. 36: 19-30 (Aug. 2007

Terrestrial oligochaetes are commonly known as earthworms and burrow into the soil. The four main families with large numbers of species are Glossoscolecidae, Lumbricidae, Megascolecidae and Moniligastridae. Earthworms are found in all parts of the world except for
desert A desert is a barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...
s. They have a requirement for moist surroundings and the larger species create burrows that may go down several metres (yards) while young individuals and smaller species are restricted to the top few centimetres of soil. The largest numbers are found in humus-rich soils and acid soils. A few species are found in trees, among damp
moss Mosses are small, non-vascular flowerless plants in the taxonomic division Bryophyta (, ) '' sensu stricto''. Bryophyta ('' sensu lato'', Schimp. 1879) may also refer to the parent group bryophytes, which comprise liverworts, mosses, and ...
and in the debris that accumulates in leaf axils and crevices; some others make their homes in the rosettes of bromeliads. The majority of aquatic oligochaetes are small, slender worms, whose organs can be seen through the transparent body wall. They burrow into the sediment or live among the vegetation mostly in shallow, freshwater environments. Some are transitional between terrestrial and aquatic habitats, inhabiting swamps, mud or the borders of water bodies. About two hundred species are marine, mostly in the families Enchytraeidae and Naididae; these are found largely in the tidal and shallow subtidal zones, but a few are found at abyssal depths.


Anatomy

The first segment, or
prostomium The prostomium (From Ancient Greek, meaning "before the mouth"; plural: prostomia; sometimes also called the "acron") is the cephalized first body segment in an annelid worm's body at the anterior end. It is in front of (but does not include) t ...
, of oligochaetes is usually a smooth lobe or cone without sensory organs, although it is sometimes extended to form a tentacle. The remaining segments have no appendages, but they do have a small number of bristles, or chaetae. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes. Each segment has four bundles of chaetae, with two on the underside, and the others on the sides. The bundles can contain one to 25 chaetae, and include muscles to pull them in and out of the body. This enables the worm to gain a grip on the soil or mud as it burrows into the substrate. When burrowing, the body moves peristaltically, alternately contracting and stretching to push itself forward. A number of segments in the forward part of the body are modified by the presence of numerous secretory glands. Together, they form the
clitellum The clitellum is a thickened glandular and non-segmented section of the body wall near the head in earthworms and leeches, that secretes a viscid sac in which eggs are stored. It is located near the anterior end of the body, between the fourteent ...
, which is important in reproduction.


Internal anatomy

Most oligochaetes are detritus feeders, although some genera are predaceous, such as '' Agriodrilus'' and '' Phagodrilus''. The digestive tract is essentially a tube running the length of the body, but has a powerful muscular
pharynx The pharynx (plural: pharynges) is the part of the throat behind the mouth and nasal cavity, and above the oesophagus and trachea (the tubes going down to the stomach and the lungs). It is found in vertebrates and invertebrates, though its st ...
immediately behind the mouth cavity. In many species, the pharynx simply helps the worm suck in food, but in many aquatic species, it can be turned inside out and placed over food like a suction cup before being pulled back in. The remainder of the digestive tract may include a crop for storage of food, and a gizzard for grinding it up, although these are not present in all species. The
oesophagus The esophagus ( American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to ...
includes "calciferous glands" that maintain calcium balance by excreting indigestible calcium carbonate into the gut. A number of yellowish
chloragogen cell Chloragogen cells, also called ''y cells'', are star-shaped cells in annelids involved with excretory functions and intermediary metabolism. They function similarly to the liver in vertebrates. Chloragogen tissue is most extensively studied in ea ...
s surround the intestine and the dorsal blood vessel, forming a tissue that functions in a similar fashion to the vertebrate
liver The liver is a major organ only found in vertebrates which performs many essential biological functions such as detoxification of the organism, and the synthesis of proteins and biochemicals necessary for digestion and growth. In humans, it i ...
. Some of these cells also float freely in the body cavity, where they are referred to as "eleocytes". Most oligochaetes have no
gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they ar ...
s or similar structures, and simply breathe through their moist skin. The few exceptions generally have simple, filamentous gills. Excretion is through small ducts known as metanephridia. Terrestrial oligochaetes secrete
urea Urea, also known as carbamide, is an organic compound with chemical formula . This amide has two amino groups (–) joined by a carbonyl functional group (–C(=O)–). It is thus the simplest amide of carbamic acid. Urea serves an important ...
, but the aquatic forms typically secrete
ammonia Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous ...
, which dissolves rapidly into the water. The vascular system consists of two main vessels connected by lateral vessels in each segment. Blood is carried forward in the dorsal vessel (in the upper part of the body) and back through the ventral vessel (underneath), before passing into a sinus surrounding the intestine. Some of the smaller vessels are muscular, effectively forming hearts; from one to five pairs of such hearts is typical. The blood of oligochaetes contains haemoglobin in all but the smallest of species, which have no need of
respiratory pigments A respiratory pigment is a metalloprotein that serves a variety of important functions, its main being O2 transport. Other functions performed include O2 storage, CO2 transport, and transportation of substances other than respiratory gases. There ar ...
. The nervous system consists of two ventral nerve cords, which are usually fused into a single structure, and three or four pairs of smaller nerves per body segment. Only a few aquatic oligochaetes have eyes, and even then they are only simply
ocelli A simple eye (sometimes called a pigment pit) refers to a form of eye or an optical arrangement composed of a single lens and without an elaborate retina such as occurs in most vertebrates. In this sense "simple eye" is distinct from a multi-l ...
. Nonetheless, their skin has several individual photoreceptors, allowing the worm to sense the presence of light, and burrow away from it. Oligochaetes can taste their surroundings using chemoreceptors located in tubercles across their body, and their skin is also supplied with numerous free nerve endings that presumably contribute to their sense of touch.


Distribution and habitat

Oligochaetes occur in every continent in the world occupying terrestrial, freshwater and marine habitats. Of the 1700 known aquatic species, about 600 are marine and 100 inhabit groundwater. Aquatic oligochaetes occur in most groups, with the Naididae being the most speciose.


Locomotion

Movement and burrowing of earthworms is performed by
peristalsis Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whi ...
, with the alternation of contraction and relaxation of the circular and longitudinal muscles. To move forward, the anterior portion of the worm is extended forward by the contraction of the circular muscles, while the portion just behind this is made shorter and fatter by the contraction of longitudinal muscles. Next the anterior circular muscles relax, and a wave of circular contraction moves backwards along the worm. At the same time, the cheatae expand to grip the ground as the body shortens and are retracted as it lengthens. The steps are typically long and the worm moves at the rate of seven to ten steps per minute. The worm is able to reverse its direction of travel with the tail leading. Aquatic species use a similar means of locomotion to work their way through sediment and massed vegetation, but the tiny Aeolosomatids swim by means of the cilia on their prostomia. Burrowing is performed by forcing the front end of the worm into a crevice and widening the gap by body expansion. Large quantities of soil are swallowed in the process. This is mixed with
mucus Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It ...
as it passes through the gut, being used to plaster the tunnel walls, forming a lining. Excess material is extruded on the ground surface, forming a faecal casting. The burrow may have two entrances and several vertical and horizontal tunnels.


Reproduction

Whereas in general, polychaetes are marine and have separate sexes, external sperm transfer and external fertilisation, oligochaetes live on land or in fresh water, are
hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...
s, have no external sperm transfer and fertilisation takes place in the clitellum or cocoon. However there are exceptions to this, with some polychaetes inhabiting non-marine environments and a few species of oligochaetes being marine. Development of the offspring also differs between the two subclasses. The eggs of polychaetes are deposited in the sea where they develop into trochophore larvae that disperse as part of the
plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cruc ...
, while the
yolk Among animals which produce eggs, the yolk (; also known as the vitellus) is the nutrient-bearing portion of the egg whose primary function is to supply food for the development of the embryo. Some types of egg contain no yolk, for example ...
y eggs of oligochaetes do not have a larval stage and develop directly into juvenile worms in the cocoon. Reproduction among oligochaetes is mainly by sexual means but clonal reproduction is common in some genera, especially among aquatic species. Members of the Naididae reproduce asexually, primarily by paratomy, in which the body breaks into two pieces after the "pregeneration" of certain anterior structures by the posterior portion. Other species undergo fragmentation, in which the worm breaks into several pieces, each of which develops into a new worm.
Parthenogenesis Parthenogenesis (; from the Greek grc, παρθένος, translit=parthénos, lit=virgin, label=none + grc, γένεσις, translit=génesis, lit=creation, label=none) is a natural form of asexual reproduction in which growth and developmen ...
also occurs in some species.


Evolution and taxonomy

With their soft bodies, earthworms do not fossilize well, though they may form trace fossils. The name '' Protoscolex'' was given to a genus of segmented worms without bristles found in the Upper Ordovician of Kentucky, United States. Another species placed in the same genus was found in Herefordshire, England, but it is unclear whether these worms are in fact oligochaetes. Stephenson postulated in 1930 that the common ancestor of oligochaetes came from the primitive aquatic family Lumbriculidae. The more advanced families such as Glossoscolecidae, Hormogastridae, Lumbricidae and Microchaetidae may have evolved later than the other families. Because of its ability to colonise new areas and become dominant, the Lumbricidae has followed humans round the world and displaced many native species of earthworm. An early but now outdated classification system was to divide the oligochaetes into "Megadrili", the larger terrestrial species, and "Microdili", the smaller, mostly aquatic ones.


Families

* Acanthodrilidae Claus, 1880 (including Diplocardiinae Michaelsen, 1900) * Ailoscolecidae Bouché, 1969 (including Komarekionidae Gates, 1974) * Alluroididae Michaelsen, 1900 * Almidae Duboscq, 1902 * Criodrilidae Vejdovsky, 1884 (including Biwadrilidae Brinkhurst & Jamieson, 1971) * Dorydrilidae Cook, 1971 * Enchytraeidae Vejdovsky, 1879 * Eudrilidae Claus, 1880 * Exxidae Blakemore, 2000 * Glossoscolecidae Michaelsen, 1900 *
Haplotaxidae Haplotaxidae is a family of earthworms of the Haplotaxida The Haplotaxida are one of two orders within the annelid subclass Oligochaeta, the other being the Lumbriculida. No real common name exists, but they are simply referred to as haplo ...
Michaelsen, 1900 * Hormogastridae Michaelsen, 1900 (including Vignysinae Bouché, 1970 and Xaninae Diaz Cosin ''et al.'', 1989) * Kynotidae Brinkhurst & Jamieson, 1971 * Lumbricidae Claus, 1876 (including Diporodrilinae Bouché, 1970; Eiseniinae Omodeo, 1956; Spermophorodrilinae Omodeo & Rota, 1989; Postandrilinae Qiu & Bouché, 1998; Allolobophorinae Kvavadze, 2000 and Helodrilinae Kvavadze, 2000) * Lumbriculidae Vejdovsky, 1884 * Lutodrilidae McMahan, 1978 * Megascolecidae Rosa, 1891 (including Pontodrilinae Vejdovsky, 1884; Plutellinae Vejdovsky, 1884 and Argilophilinae Fender & McKey-Fender, 1990) * Microchaetidae Michaelsen, 1900 * Moniligastridae Claus, 1880 * Naididae / Tubificidae Vejdovsky, 1884 (including Naidinae Ehrenberg, 1831) * Narapidae Righi, 1983 * Ocnerodrilidae Beddard, 1891 (including Malabariinae Gates, 1966) * Octochaetidae Michaelsen, 1900 (including Benhamiinae Michaelsen, 1895/7) * Opistocystidae Cernosvitov, 1936 * Parvidrilidae Erséus, 1999 * Phreodrilidae Beddard, 1891 * Propappidae Coates, 1986 * Randiellidae Erséus & Strehlow, 1986 * Sparganophilidae Michaelsen, 1918 * Syngenodrilidae Smith & Green, 1919 * Tiguassuidae Brinkhurst, 1988 * Tritogeniidae Plisko, 2013 * Tumakidae Righi, 1995


References


Bibliography

* Blakemore, R. J. (2005). Whither Octochaetidae? – its family status reviewed. In: ''Advances in Earthworm Taxonomy II''. Eds. A. A. & V. V. Pop. Proceedings IOTM2, Cluj University Press. Romania. Pp. 63–84. https://web.archive.org/web/20071210202216/http://www.oligochaeta.org/ITOM2/IOTM2.htm. * Blakemore, R. J. (2006). Revised Key to Earthworm Families (Ch. 9). In: ''A Series of Searchable Texts on Earthworm Biodiversity, Ecology and Systematics from Various Regions of the World'' – 2nd Edition (2006). Eds.: N. Kaneko & M. T. Ito. COE Soil Ecology Research Group, Yokohama National University, Japan. CD-ROM Publication. Website: https://web.archive.org/web/20080105055856/http://bio-eco.eis.ynu.ac.jp/eng/database/earthworm/. * * Michaelsen, W. (1900). ''Das Tierreich 10: Vermes, Oligochaeta''. Friedländer & Sohn, Berlin. Pp. xxix+575, figs. 1-13. Online here: http://mail2web.com/cgi-bin/redir.asp?lid=0&newsite=https://archive.org/details/oligochaeta10mich. *Plisko, J.D. (2013). A new family Tritogeniidae for the genera ''Tritogenia'' and ''Michalakus'', earlier accredited to the composite Microchaetidae (Annelida: Oligochaeta). ''
African Invertebrates ''African Invertebrates'' is a peer-reviewed open access scientific journal that covers the taxonomy, systematics, biogeography, ecology, conservation, and palaeontology of Afrotropical invertebrates, whether terrestrial, freshwater, or ma ...
'' 54 (1): 69–9

* Siddall, M. E., Apakupakul, K, Burreson, E. M., Coates, K. A., Erséus, C, Gelder, S. R., Källersjö, M, & Trapido-Rosenthal, H. (2001). Validating Livanow's Hypothesis: Molecular Data Agree that Leeches, Branchiobdellidans and Acanthobdella peledina form a Monophyletic Group of Oligochaetes. Molecular Phylogenetics and Evolution, 21: 346-351. http://research.amnh.org/~siddall/pub/livanow.pdf. * Stephenson, J. (1930). ''The Oligochaeta''. Clarendon Press, Oxford. Pp. 978.


External links

* {{Authority control Composting Annelids Clitellata