Oligochaeta
Encyclopedia
Oligochaeta is a subclass of animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s in the biological phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Annelida, which is made up of many types of aquatic
Aquatic animal
An aquatic animal is an animal, either vertebrate or invertebrate, which lives in water for most or all of its life. It may breathe air or extract its oxygen from that dissolved in water through specialised organs called gills, or directly through its skin. Natural environments and the animals that...
and terrestrial
Terrestrial animal
Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...
worm
Worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...
s, and this includes all of the various earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
s. Specifically, it contains the terrestrial megadrile earthworm
Earthworm
Earthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
s (some of which are semi- or fully aquatic), and freshwater or semi-terrestrial microdrile forms including the tubificid
Tubificidae
The Naididae are a family of clitellate oligochaete worms like the sludge worm, Tubifex tubifex. They are key components of the benthic communities of many freshwater and marine ecosystems....
s, pot worms and ice worm
Ice worm
Ice worms are species of the worm genus Mesenchytraeus that live in glacial ice. They include Mesenchytraeus solifugus, M. harrimani, M. kuril, M. maculatus and M. obscurus....
s (Enchytraeidae
Enchytraeidae
The Enchytraeidae are a microdrile oligochaete family. They resemble small earthworms and include both terrestrial species known as potworms that live in highly organic terrestrial environments, as well as some that are marine...
), blackworms
Lumbriculus variegatus
Lumbriculus variegatus, sometimes known as blackworm , is the name given to at least three distinct species of worms which are identical in outward appearance...
(Lumbriculidae
Lumbriculidae
The Lumbriculidae are a family of microdrile oligochaetes common in fresh-water environments, including streams, lakes, marshes, wells and ground-water. They should not be confused with the earthworm family Lumbricidae. Many species and even genera are highly endemic, mainly in Siberia and the...
) and several interstitial
Interstitial
An interstitial space or interstice is an empty space or gap between spaces full of structure or matter.In particular, interstitial may refer to:-Physical sciences:...
marine worms.
With around 10,000 known species the Oligochaeta make up about one half of the phylum Annelida.
These worms usually have few seta
Seta
Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....
e (chaetae) or "bristles" on the outer body surface, and lack parapodia
Parapodium
Parapodia , singular parapodium, are paired, un-jointed lateral outgrowths from the bodies of two different invertebrate groups, which are primarily marine in habitat...
, unlike polychaeta.
Common characteristics
Oligochaetes are well-segmented worms and most have a spacious body cavityBody cavity
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism. However, the term usually refers to the space located between an animal’s outer covering and the outer lining of the gut cavity, where internal organs develop...
(coelom) that is used as a hydroskeleton. They range in length from less than 0.5 millimetre (0.0196850393700787 in) up to 2 to 3 m (6.6 to 9.8 ft) in the 'giant' species such as the giant Gippsland earthworm
Giant Gippsland earthworm
The giant Gippsland earthworm, Megascolides australis, is one of Australia's 1,000 native earthworm species. These Giant earthworms average 1 meter long and 2 cm in diameter and can reach 3 m in length...
and the Mekong Worm Amynthas mekongianus (Cognetti, 1922).
The first segment, or prostomium
Prostomium
Prostomium is the first body segment in annelid worms. It is in front of the mouth, being usually a small shelf- or lip-like extension over the dorsal side of the mouth. It sometimes bears antennae and eyes. It often functions like a kind of overlip when the animal is feeding...
, 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 seta
Seta
Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....
e. These tend to be longer in aquatic forms than in the burrowing earthworms, and can have a variety of shapes. Oligochaetes are able to reproduce via insertion of a penis into an orifice called a vagina.
Each segment has four bundles of setae, with two igloos on the underside, and the others on the sides. The bundles can contain anything from one to twenty-five setae, 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
Peristalsis
Peristalsis is a radially symmetrical contraction and relaxation of muscles which propagates in a wave down the muscular tube, in an anterograde fashion. In humans, peristalsis is found in the contraction of smooth muscles to propel contents through the digestive tract. Earthworms use a similar...
, 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
Clitellum
The clitellum is a thickened glandular section of the body wall in earthworms and leeches, that secretes a viscid sac in which the eggs are deposited...
, 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 pharynxPharynx
The human pharynx is the part of the throat situated immediately posterior to the mouth and nasal cavity, and anterior to the esophagus and larynx. The human pharynx is conventionally divided into three sections: the nasopharynx , the oropharynx , and the laryngopharynx...
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
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
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
Liver
The liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
. Some of these cells also float freely in the body cavity, where they are referred to as "eleocytes".
Most oligochaetes have no gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
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
Urea or carbamide is an organic compound with the chemical formula CO2. The molecule has two —NH2 groups joined by a carbonyl functional group....
, but the aquatic forms typically secrete ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
, 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 to 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.
Life cycle
Earthworms are hermaphroditeHermaphrodite
In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...
s, which means that each animal has both male and female reproductive organs. They have hova fertilization (except for some members of the African family Eudrilidae
Eudrilidae
Eudrilidae is a family of earthworms, mostly of Africa. One species, Eudrilus eugeniae , is widely distributed around the warmer parts of the world and cultured as the "African nightcrawler"....
), but copulate and store sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...
in a receptacle called a spermatheca
Spermatheca
The spermatheca , also called receptaculum seminis , is an organ of the female reproductive tract in insects, some molluscs, oligochaeta worms and certain other invertebrates and vertebrates...
. When two earthworms mate, both worms typically fertilize each other. Like leech
Leech
Leeches are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like other oligochaetes such as earthworms, leeches share a clitellum and are hermaphrodites. Nevertheless, they differ from other oligochaetes in significant ways...
es, they have a clitellum
Clitellum
The clitellum is a thickened glandular section of the body wall in earthworms and leeches, that secretes a viscid sac in which the eggs are deposited...
which secretes a "cocoon" or capsule into which both eggs and sperm are deposited and acts as an incubator for the embryonic worms. The cocoon is deposited in the soil. On hatching, the young worms resemble small adults and grow continually until they reach maturity. They lack a trochophore
Trochophore
A trochophore is a type of free-swimming planktonic marine larva with several bands of cilia.By moving their cilia rapidly, a water eddy is created. In this way they control the direction of their movement...
larval stage.
Habitat
Earthworms typically live in various types of soil or mud, as well as organic matter such as compostCompost
Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...
or even feces
Feces
Feces, faeces, or fæces is a waste product from an animal's digestive tract expelled through the anus or cloaca during defecation.-Etymology:...
. They are found on every continent except Antarctica. Native earthworm species are often eradicated from natural areas as people clear native vegetation and introduced species become more dominant in these disturbed habitats. Introduced earthworms are most common in disturbed environments such as suburban gardens and farmland paddocks.
Families
The following list of Oligochaeta families follows ICZNInternational Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...
conventionn so that family-group name (ending in -) is followed by authorship and date.
- Randiellidae (Erséus & Strehlow, 1986)
- 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 (MichaelsenWilhelm MichaelsenWilhelm Michaelsen was a German zoologist. His principal contributions were to the study of Oligochaeta.After finishing high school he studied mechanical engineering...
, 1900) - Tiguassuidae (Brinkhurst, 1988)
- Lumbriculidae (Vejdovsky, 1884)
- EnchytraeidaeEnchytraeidaeThe Enchytraeidae are a microdrile oligochaete family. They resemble small earthworms and include both terrestrial species known as potworms that live in highly organic terrestrial environments, as well as some that are marine...
(Vejdovsky, 1879) - Moniligastridae (Claus, 1880)
- Alluroididae (MichaelsenWilhelm MichaelsenWilhelm Michaelsen was a German zoologist. His principal contributions were to the study of Oligochaeta.After finishing high school he studied mechanical engineering...
, 1900) - Syngenodrilidae (Smith and Green, 1919)
- GlossoscolecidaeGlossoscolecidaeGlossoscolecidae is a large family of earthworms which has native representatives in South and Central America. The species Pontoscolex corethrurus has a circumtropical distribution....
(Michaelsen, 1900) - Tumakidae (Righi, 1995)
- Ailoscolecidae (Bouché, 1969) (including Komarekionidae Gates, 1974)
- Sparganophilidae (Michaelsen, 1918)
- Microchaetidae (Michaelsen, 1900)
- LumbricidaeLumbricidaeThe Lumbricidae is a family of earthworms which includes most of the earthworm species well known to Europeans. About 33 lumbricid species have become naturalized around the world, but the bulk of the species are in Holarctic: from Canada and the United States The Lumbricidae is a family of...
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
- CriodrilidaeCriodrilidaeThe family Criodrilidae is represented by genera Criodrilus and Biwadrilus that are limicolous and/or aquatic earthworms endemic to the Palaearctic currently known only from Europe and Japan, respectively...
Vejdovsky, 1884 (including Biwadrilidae Brinkhurst & Jamieson, 1971) - AlmidaeAlmidaeThe animal family Almidae includes about six genera of segmented worms.A notable peculiarity of some species in this family is a tendency to extensions of the body wall in the vicinity of or including the male pores. These extensions may be mere protuberances, as in some species of Drilocrius; or...
Duboscq, 1902 - Ocnerodrilidae Beddard, 1891 (including Malabariinae Gates, 1966)
- AcanthodrilidaeAcanthodrilidaeAcanthodrilidae is an ancient and widely distributed family of earthworms which has native representatives in Australia, New Zealand, South Africa, South America and North America. Interestingly, no native species are known from India nor Asia...
Claus, 1880 (including Diplocardiinae Michaelsen, 1900) - Octochaetidae Michaelsen, 1900 (including Benhamiinae Michaelsen, 1895/7)
- Exxidae Blakemore, 2000
- MegascolecidaeMegascolecidaeMegascolecidae is a large family of earthworms which has native representatives in Australia, New Zealand, Southeast and East Asia, and North America. The most ancient lineages of the family show a Gondwanan distribution and have been used as evidence of continental drift. Members of the Pheretima...
Rosa, 1891 (including Pontodrilinae Vejdovsky, 1884; Plutellinae Vejdovsky, 1884 and Argilophilinae Fender & McKey-Fender, 1990) - EudrilidaeEudrilidaeEudrilidae is a family of earthworms, mostly of Africa. One species, Eudrilus eugeniae , is widely distributed around the warmer parts of the world and cultured as the "African nightcrawler"....
Claus, 1880.