Annelid worms

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Annelid worms
Common earthworm (Lumbricus terrestris)

Common earthworm ( Lumbricus terrestris )

without rank: Multicellular animals (Metazoa)
without rank: Tissue animals (Eumetazoa)
without rank: Bilateria
without rank: Primordial mouths (protostomia)
Over trunk : Lophotrochozoa (Lophotrochozoa)
Trunk : Annelid worms
Scientific name
Lamarck , 1809

The annelids (annelids, annelida) or articulated worms form a trunk within the trunk group of the lophotrochozoa (Lophotrochozoa), which belong to the original mouths (protostomia) , due to their independent structural plan . Annelids be divided into two classes: polychaete (Polychaeta) and clitellata (Clitellata) extending in oligochaetes (Oligochaeta) and flukes split (Hirudinea). There are around 18,000 different species in total.

The largest known species are in Australia -based Giant Worm Giant Gippsland Earthworm , which can reach a length of about 3 meters, and the marine worm Eunice Aphroditois with a similar length and up to 1000 individual segments. The smallest species live in the groundwater and in the sand gap fauna and reach lengths of 300 micrometers (genus Diurodrilus ), the dwarf males of the species Dinophilus gyrociliatus are even only 50  micrometers long.


The most important evolutionary changes compared to the blueprints of simpler built bilaterally symmetrical animals such as the roundworms or the flatworms are the acquisition of a real coelom and a segmentation for the annelids . The Coelom offers space for complex organs and allows novel, differentiated forms of movement. The skin muscle tube and the viscera can perform independent movements. The segmentation makes it possible to specialize individual segment sections for certain tasks (heteronomous segmentation). Overall, the body of an annelid can be divided into three regions:

  1. Head ( prostomium and peristomium )
  2. Fuselage (similar segments)
  3. Rear end ( pygidium )

This classification can no longer be clearly recognizable in some subgroups of the annelids.

Segments and parapodies

Historical detailed drawing of the complex parapodium of a polychaete with different bristles and gill leaves (Brehms Thierleben, 1887)
Schematic representation of the anatomy of the trunk segments of an annelid (here a representative of the polychaetes)

The German name "Ringelwurm" goes back to the fact that the body of these animals is made up of numerous so-called segments , which appear from the outside as a ring pattern. In fact, these segments were historically referred to as "rings" or "ringlets". see. z. B.  On the individual trunk segments, especially well-developed in the case of the polychaetes, there are pair-shaped lateral stubby feet ( parapodia ) in which bristles, the so-called chaetae , are anchored. The parapodia are protuberances on the body wall into which the body cavity (coelom) expands and into which muscles are drawn. The segments are formed one after the other at the sprouting zone in front of the rear end of the body ( teloblastia ). In some species the number of segments is constant for all individuals, in other species the number fluctuates by a few segments. Most annelids live in water, some species (especially larger ones) also have gills , these are usually located on the parapodia.

Body cavity and locomotion

The secondary body cavity ( coelom ) of the segmented body, which appears for the first time in evolution in the annelid worms, is completely filled with fluid and is traversed by a straight, tubular intestine . The hydrostatic skeleton , in conjunction with the circular muscles, the longitudinal muscles and the bristles, enables an effective locomotion system. " Errante ", ie annelid worms that do not live in tubes or structures, receive their typical locomotion pattern through this system. Depending on the habitat and sometimes also after different phases of life, annelids can crawl, swim, meander or undulate (rhythmic movement to ensure that water flows through living tubes and thus for ventilation).

blood circulation

The blood vessel system is closed and consists of a back and an abdominal vessel . The abdominal vessel carries the blood from the front to the back, the back system lies above the intestine and carries the blood from the back to the front. The dorsal vessel is usually contractile and sometimes also double. The longitudinal vessels are often connected by side vessels. There is no endothelium , the blood flows in the spaces between adjacent tissues. As respiratory pigments were hemoglobins , chlorocruorin (green) and Hemerythrin detected (purple). The gas exchange and thus also breathing takes place through the skin .

Nervous system

Especially when originally built annelids a place cephalization instead. The neural centers are originally concentrated in the head area ( prostomium ) together with the most important sensory cells as the upper pharyngeal ganglion . The prostomium is equipped with light and chemoreceptor organs and movable appendages. On these appendages ( palps and antennae) there are also receptors. To control the individual segments, however, there are still clusters of nerve cells , so-called ganglia , in pairs in the individual segments. Here, too, there may be a concentration of ganglia.

The rope ladder nervous system typical of the annelids is found as the abdominal marrow on the ventral side of these animals. The nerve rope ladder, which is often shown schematically in textbooks, cannot be easily recognized in the prepared animal. With the naked eye, even in large annelids, you can only see a collection of nerve fibers , which at first glance does not look like a rope ladder. This is because in these taxa the nerve cords are fused into a single cord, or the nerve cell bodies are distributed over the connective .

The peristomium joins the prostomium with the mouth opening and other appendages. These attachments may be missing as a secondary feature. The rear end is also originally provided with appendages on which there are receptors. Eyes come in many forms - from the two-celled ocelles to the complex lenticular eye ( tomopteris ). Most often they are on the prostomium, but they can also occur in other places (e.g. the segments). Nuchal organs occur within the annelids only in the polychaetes . There are paired, strongly ciliated structures at the rear end of the prostomium, which mainly contain chemoreceptors.

Epidermis and skin muscle tube

The soft epidermis of the annelids has many glands. These glands produce, for example, mucus, secretions for building tubes or even luminous secretions. The epidermis is surrounded on the outside by a cuticle which usually contains a lattice of collagen fibers . The bristles characteristic of most annelids, which are mobile, are also deposits of the epidermis. This means that these bristles are arranged segmentally in deep epidermal pockets. The number and shape of the bristles is very constant for each type and are an important distinguishing feature.

There are two layers of muscle beneath the epidermis: a circular muscle layer and a longitudinal muscle layer. Diagonal fibers can lie between these layers. In addition to these muscles, there are various other muscles such as those that move the bristles, parapodial muscles or dorsoventral muscles. A transfer of forces takes place through the water-filled coelom, which forms the hydrostatic skeleton with the collagen fibers of the cuticle. The muscle cells are mononuclear and belong to the type of sloping muscles .

Intestinal tract and excretory organs

The intestinal tract of the annelids is primarily a tube covered with a single layer of epithelium, which is straight and divided into three sections. It extends from the ventral opening of the mouth at the peristomium to the anus at the pygidium. Based on this, there are many variations depending on the diet and lifestyle.

The excretion takes place through the nephridia . Originally, these are available in pairs in each segment. The majority of annelids have metanephridia , but protonephridia also occur.

Habitats, nutrition

Left: Polychaetes from the group of "Sedentaria" - sessile tubular calcareous worms (here the Christmas tree worm, Spirobranchus giganteus) in a coral reef.  Right: A representative of the belt worms from the group of leeches (here probably a horse leech, Haemopis saguisuga) in a freshwater body. Left: Polychaetes from the group of "Sedentaria" - sessile tubular calcareous worms (here the Christmas tree worm, Spirobranchus giganteus) in a coral reef.  Right: A representative of the belt worms from the group of leeches (here probably a horse leech, Haemopis saguisuga) in a freshwater body.
Left: Polychaetes from the group of "Sedentaria" - sessile tube worms (here the Christmas tree worm , Spirobranchus giganteus ) in a coral reef .
Right: A representative of the belt worms from the group of leeches (here probably a horse leech , Haemopis saguisuga ) in a freshwater body.

The poly-bristles are mostly marine animals. The sedentary species (" Sedentaria ") feed on plankton , the free-living species (" Errantia ") mostly feed as predators or as grazers. Some polychaete are basically free to move, but are counted as Sedentaria because they live in tubes and are adapted to a sedentary lifestyle, such as the Pier or Wattwurm ( Arenicola marina ). They and also the earthworms like Lumbricus terrestris , which belong to the little bristle group, feed on bacteria and organic matter by eating the surrounding substrate and digesting the nutrient-rich parts. Earthworms and lugworms are therefore significantly involved in the upheaval and loosening of the inhabited soils. The leeches , which predominantly occur in fresh water, also belong to the annelids. They are either blood-sucking ectoparasites (e.g. adult animals of the medical leech , fish leech and snail leech ) or are predatory ( e.g. horse leech , roller gel ).


The reproduction of annelids is extremely varied. There is hermaphroditic and sexual reproduction, live-giving species, larval development, sprouting, generation change. Depending on the way of life, suitable forms of reproduction have developed.

Reparative regeneration

The possibility of repairing amputations through new sprouting zones is well developed among the annelids and is also used by them for reproduction. In some species, a single segment is enough to fill in the rest of the missing ones. Regenerating earthworms go into rigidity.

Sexual reproduction

Sexual reproduction of the annelids originally takes place freely in water. This means that the gametes are released into the water and fuse there. The direct transmission of the sperm to the sexual partner has developed independently of this several times, some species also have external sex organs.

Larval development

Originally larval development takes place via a trochophora larva. However, a number of polychaetes and all clitellates evolve directly.


For a long time, annelid worms were divided into multi - bristle - mostly sea creatures with parapodia on which numerous bristles are attached - and belt worms - mostly freshwater or land dwellers without parapodia and with a clitellum (belt) and few or no bristles. In turn, the polychaete disintegrate in sessile or limited movable Sedentaria and mobile Errantia that clitellata other hand, in oligochaetes with few bristles and predatory or blood-sucking leeches without bristles, with suction cups, one traversed by powerful muscles body and reduced coelom .

The majority of the species of annelid worms are divided in the traditional system into the polychaeta with around 10,000 species and the oligochaeta with around 7,700 species, while the leeches only count around 300 species - the girdle worms (clitellata) together, for example 8000 species. There is also a systematically controversial group of echinoderms of parasitic forms, the myzostomida with around 140 species.

Phylogenetic studies on the basis of the anatomy of the multi-bristle lead to the systematics according to Rouse & Fauchald (1998). While the Errantia are monophyletic and belong to the Aciculata group, i.e. multi-bristle bristles with strong, muscular parapodia supported by an inner skeleton of aciculae, the Sedentaria are broken down into the primitive, mostly restrictedly mobile Scolecida and the sessile Canalipalpata - including the former Beard worms - with palps, some of which have developed into extensive tentacle systems . According to Rouse & Fauchald (1998), the Canalipalpata are more closely related to the Aciculata than to the Scolecida and together with the Aciculata they form the common taxon Palpata . When considering all annelids as well as the later included former own tribes Sipuncula and Echiura , the following cladogram can be drawn:




 Annelida  s. stricto 










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The splashworms (Sipuncula), the hedgehog worms (Echiura) and the beard worms (formerly Pogonophora, today only the Siboglinidae family within the Sabellida ) were until recently considered to be independent tribes, and their systematic classification was long controversial. On the basis of more recent phylogenetic studies on a molecular genetic basis, such as by Andrade and others (2015), Parry and others (2016), Torsten Hugo Struck and others (2015) and Anne Weigert and Christoph Bleidorn (2016), these taxa must be assigned to annelid worms from a cladistic point of view become. Like all annelids, beard worms have ciliated trochophoric larvae . In addition, the segmentation of the end piece and the oxygen transport molecule hemoglobin point to a classification in the annelid worms. The previous taxa Oligochaeta ( few bristles ) and Polychaeta ( many bristles ) no longer exist because they are paraphyletic . The old taxa " Errantia " and " Sedentaria " are being revived here , but to a completely different extent than before.

Cladogram according to Struck et al. 2015 and Weigert & Bleidorn 2016:

 Annelida  s. lato 











Protodriliformia (with Protodrilidae , part of the former Archiannelida )







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Orbiniida (with Orbiniidae , part of the former Archiannelida )


Canalipalpata ( Sabellida incl. Former Pogonophora , Cirratulida , Terebellida )


Scolecida (paraphyletic, with Arenicolida , Opheliida ) , Clitellata , Echiura

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  1. From the point of view of classical Latin, the name A nell ida or A nell iden should be written, corresponding to ā nell us “small ring” in classical Latin, cf. Charlton T. Lewis and Charles Short, A Latin Dictionary , keyword ānellus . In post-classical Latin, the version made ā nn ellus out, to which the notation A nn elida or A nn eliden can be traced back; see. annellus in the English Wiktionary.

Individual evidence

  1. ^ Duden online: Annelids
  2. Cf. Meyers Großes Konversations-Lexikon (1908), keyword Ringelwürmer
  3. ^ Oscar Schmidt: Brehms Thierleben. General customer of the animal kingdom. Great edition. Second revised and enlarged edition. Fourth Division: Invertebrates. Second volume: The lower animals. Verlag des Bibliographisches Institut, Leipzig 1878, doi: 10.5962 / bhl.title.1067 , p. 68 ff.
  4. ^ Hermann Burmeister: Handbook of natural history. Second section: zoology . Published by Theodor Christian Friedrich Enslin, Berlin 1837 ( HathiTrust ), p. 531 ff.


Web links

Commons : Annelida  - album with pictures, videos and audio files