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|Earthworm (Lumbricus terrestris)|
The Oligochaeta // are a subclass of animals in the biological phylum Annelida, which is made up of many types of aquatic and terrestrial worms, and including all of the various earthworms. Specifically, the oligochaetes contain the terrestrial megadrile earthworms (some of which are semiaquatic or fully aquatic), and freshwater or semiterrestrial microdrile forms, including the tubificids, pot worms 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 setae (chaetae) or "bristles" on their outer body surfaces, and lack parapodia, unlike polychaeta.
Oligochaetes are well-segmented worms and most have a spacious body cavity (coelom) used as a hydroskeleton. They range in length from less than 0.5 mm (0.020 in) up to 2 to 3 metres (6.6 to 9.8 ft) in the 'giant' species such as the giant Gippsland earthworm and the Mekong worm Amynthas mekongianus (Cognetti, 1922).
The first segment, or prostomium, 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.
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 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 includes "calciferous glands" that maintain calcium balance by excreting indigestible calcium carbonate into the gut. A number of yellowish chloragogen cells surround the intestine and the dorsal blood vessel, forming a tissue that functions in a similar fashion to the vertebrate liver. Some of these cells also float freely in the body cavity, where they are referred to as "eleocytes".
Most oligochaetes have no gills 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, but the aquatic forms typically secrete ammonia, 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.
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. 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.
- Randiellidae Erséus & Strehlow, 1986
- Naididae / Tubificidae Vejdovsky, 1884 (including Naidinae Ehrenberg, 1831)
- Narapidae Righi, 1983
- Opistocystidae Cernosvitov, 1936
- Dorydrilidae Cook, 1971
- Parvidrilidae Erséus, 1999
- Phreodrilidae Beddard, 1891
- Propappidae Coates, 1986
- Haplotaxidae Michaelsen, 1900
- Tiguassuidae Brinkhurst, 1988
- Lumbriculidae Vejdovsky, 1884
- Enchytraeidae Vejdovsky, 1879
- Moniligastridae Claus, 1880
- Alluroididae Michaelsen, 1900
- Syngenodrilidae Smith & Green, 1919
- Glossoscolecidae Michaelsen, 1900
- Tumakidae Righi, 1995
- Ailoscolecidae Bouché, 1969 (including Komarekionidae Gates, 1974)
- Sparganophilidae Michaelsen, 1918
- Microchaetidae Michaelsen, 1900
- Tritogeniidae Plisko, 2013
- 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)
- Kynotidae Brinkhurst & Jamieson, 1971
- Hormogastridae Michaelsen, 1900 (including Vignysinae Bouché, 1970 and Xaninae Diaz Cosin et al., 1989)
- Lutodrilidae McMahan, 1978
- Criodrilidae Vejdovsky, 1884 (including Biwadrilidae Brinkhurst & Jamieson, 1971)
- Almidae Duboscq, 1902
- Ocnerodrilidae Beddard, 1891 (including Malabariinae Gates, 1966)
- Acanthodrilidae Claus, 1880 (including Diplocardiinae Michaelsen, 1900)
- Octochaetidae Michaelsen, 1900 (including Benhamiinae Michaelsen, 1895/7)
- Exxidae Blakemore, 2000
- Megascolecidae Rosa, 1891 (including Pontodrilinae Vejdovsky, 1884; Plutellinae Vejdovsky, 1884 and Argilophilinae Fender & McKey-Fender, 1990)
- Eudrilidae Claus, 1880
- 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) .
- Barnes, Robert D. (1982). Invertebrate Zoology. Philadelphia, PA: Holt-Saunders International. pp. 528–547. ISBN 0-03-056747-5.
- 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. 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: http://bio-eco.eis.ynu.ac.jp/eng/database/earthworm/.
- Erséus, C.; Källersjö, M. (2003). "18S rDNA phylogeny of basal groups of Clitellata (Annelida)". Zoologica Scripta 33 (2): 187–196. doi:10.1111/j.1463-6409.2004.00146.x.
- 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=http://www.archive.org/details/oligochaeta10mich.
- Plisko, J. D. (2006). The Oligochaeta type material housed at the Natal Museum, South Africa. African Invertebrates 47: 57-61. 
- Plisko, J.D. (2013). A new family Tritogeniidae for the genera Tritogenia and Michalakus, earlier accredited to the composite Microchaetidae (Annelida: Oligochaeta). African Invertebrates 54 (1): 69–92.
- 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.
- Media related to Oligochaeta at Wikimedia Commons