Oligochaeta

Bristles of an earthworm in the microscope

The Little Borster or Oligochaeta ( from Ancient Greek ὀλίγος oligos, little ' and χαίτη Chaite, hair ') are one of the two orders of the belt worms ( Clitellata ) belonging to turn the segmented worms ( Annelida ). They include about 3,500 species, of which about 600 live in the sea, the other in fresh water or on land.

Physique

Little Borster consist of 6 to 600 homo autonomous segments. Furthermore, bristles are without parapodia (lateral leg -like growths ) directly to the body wall. Annelids have a very muscular skin muscular tube that they need help with lateral bristles for movement in the earth.

Reproduction

The Little Borster are hermaphrodites and have the belt for worms ( Clitellata ) reproductive characteristic way: they form a kind of thick belt ( clitellum ), which is formed by secretion and which develop the fertilized eggs. Practically lie down thereto during copulation two individuals in parallel so that the clitellum of an animal to the seed bag ( spermathecae ) of another animal comes to rest. Then the two seed bags are alternately filled with the sperm of the other animal. The animals run beneath the hardening shell of the secretion Clitellums with the front end out and give it into their eggs. In the forming from the cocoon (outer) fertilization between the egg and sperm takes place.

However, there are in different families and asexual (vegetative ) reproduction by division. In part, this is done in the form that initially form small animal chains that make up the new individual animals gradually cut off and solve ( Tubificidae family, subfamily Naidinae ), partly by pinch without animal chain formation (Family Lumbriculidae ).

Occurrence and ecological significance

The original distribution areas, in particular the bottom-dwelling species are partially greatly expanded through procrastination. So today, coming from the northern hemisphere earthworms ( Lumbricidae ) also live largely in the southern hemisphere, where they displace some native worms ( locally as the Megascolidae ( "giant earthworms " ) ).

Ecological function both in fresh water and on the land many ways as important decomposers, which participate in the degradation of organic matter and on remodeling are involved in new substances ( clay-humus complexes). Mix the soil-dwelling species and ventilate them and also optimize the nutrient supply (see bioturbation ).

Furthermore, some species are used as bio-indicators for water quality assessment.

System

The Little Borster not form a natural ( monophyletic ) unit, as no single shared derived character is known that it distinguishes itself from its sister group, the Hirudinea. Therefore, you are considered paraphyletic unit.

The best-known families are the following:

• Aeolosomatidae - small (about 1-10 mm ) molds with oil droplets in the body; living in the soil or fresh water.
• Enchytraeidae - about 650 species, whitish; in the soil often en masse; partially alive in fresh water (eg genus Pachydrilus ).
• Haplotaxidae - about 30 species living in fresh water (often in groundwater ).
• Lumbricidae ( earthworms ) - about 320 species, mostly living in the soil, sometimes in fresh water. For details see under earthworms.
• Lumbriculidae - live in fresh water in the northern hemisphere. This family is considered to be the closest related group ( sister taxon ) of the leech based on morphological and molecular genetic characteristics. A well-known, up to 10 cm long species Lumbriculus variegatus is a kind used as test organism in environmental toxicology, the va in stagnant water occurs at the base or between aquatic plants and is reminiscent of an earthworm. This species can reproduce asexually by transverse division (without chain formation).
• Megascolecidae - in the ground or on trees in the southern hemisphere alive; these include the Australian "giant earthworms " with a maximum of 2-3 m in length, with a thickness of up to 3 cm, eg the type Megascolides australis.
• Tubificidae - about 800 species in the sea and in fresh water. The best-known representatives are the Tubifex species used as feed for fish (mud tube worm). The Tubificidae includes the subfamily Naidinae, that were once mainly considered as a separate family " Naididae ". These are mainly smaller transparent worms with a striking manner in the form of vegetative propagation of animal chains. - More on this family see under Tubificidae and Naidinae.

Fossil evidence

Fossil Little Borster are very rare. In addition to some uncertain evidence from the Malm of Ulyanovsk (Russia) and from Paleocene deposits in the U.S., some from the Eocene Baltic amber specimens originating have been identified as belonging to the family Enchytraeidae.