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Annelids: The first segmented bodies. Megan Van Der Bank Department of Biodiversity and Conservation Biology, University of the Western Cape Contents. 1 The major groups of annelids living today 2 Conservation status of annelids 3 Why are annelids so successful

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Annelids: The first segmented bodies

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Annelids: The first segmented bodies

Megan Van Der Bank

Department of Biodiversity and Conservation Biology, University of the Western Cape


  • 1 The major groups of annelids living today

  • 2 Conservation status of annelids

  • 3 Why are annelids so successful

  • Medicinal use of Hirudo medicinalis

  • The ecological role of earthworms

  • The first appearance of annelids

  • The environmental conditions during periods of diversification

  • Evolutionary advantage of segmentation

  • Myzostomida as the link between flatworms and polychaetes

  • 10 The relationship between annelids and arthropods

Three major classes can be distinguished, namely Polychaeta, Hirudinea, Oligochaeta (Branch and Branch, 1981)

These groups vary significantly in the habitat and niche that they occupy

The annelids are highly successful and ubiquitous, occupying mostly moist environments

All members show true segmentation and have chaeta, are protostome and triploblastic (Hickman et al, 2004)

Major annelid groups living today

Class: Polychaeta

Also known as bristle worms

The largest annelid group, containing as many as 10 000 species (Hickman et al, 2004)

Traditionally, free living forms (planktonic) are called Errantaria, while sedentary forms (tube- dwelling) are called Sedentaria (Branch and Branch, 1981)

Paddle-like parapodia with chaeta, trochophore larval stage, definite head (Hickman et al, 2004)

Fig 1. Ciliated trochophore larval stage

Fig 2.Free living polychaete with parapodia

Fig 3. Free living polychaete with bristle containing parapodia

Earthworms are predominantly detritus feeders that are mainly terrestrial, but can be freshwater or occupy the interstices of marine sediments (Branch and Branch, 1981).

They are hermaphroditic and secretes a cocoon into which eggs and sperm are deposited, namely a clitellum (

Oligochaetes lack the cilliated trochophore larvae present in polychaetes (Hickman et al, 2004).

Class: Oligochaeta

Fig 4.The morphology and anatomy of the oligochaete

Also known as leeches

The organisms contain a posterior and anterior sucker used to attach to the exterior surface of vertebrates such as amphibians and even humans.

However most are free-living, preying on small invertebrates and tend to lack appendages such as parapodia and chaeta.

Species such as Hirudo medicinalis supply heparin, a natural anticoagulant (

Class: Hirudinea

Fig 5. The leech with visible anterior and posterior suckers

Fig 6. The leech

The annelids are highly successful, however some vulnerable species such as Driloleirus americanus have been identified (

Extinction can mainly be contributed to habitat loss due to development and industrialization.

Hypolimnus pedderensis as an example of an extinct annelid species (

Conservation status of the annelids

The success of the annelids can mainly be contributed to their mode of reproduction.

Sexual reproduction allows better adaptation to the environment.

Asexual reproduction via fission and regeneration allows a fast rate of reproduction (

Segments have their own autonomy but unite to form a common body function.

Coelomic compartments serve as a supportive hydrostatic skeleton.

Annelids have a wide range of adaptive features.

Why are the annelids so successful?

The therapeutic use Hirudo medicinalis dates back to ancient Egypt where it was used in bloodletting.

The medical use of leeches lost its popularity by the end of the 19th century.

In 1884 it was discovered that the leech saliva contains a natural anticoagulant, heparin.

With the advent of genetic engineering in 1986 heparin could be produced in relatively large quantities.

Recently it has been used to relieve blood congestion in compromised tissue

Researchers are currently developing a mechanical leech (Whitaker et al, 2004)

Medical use of Hirudo medicinalis

Increases soil fertility

Plays an important role in the cycling of soil organic matter

Plays a role in soil mixing, porosity, aeration and water holding capacity

Affects the overall soil structure (Edwards and Lofty, 1972)

Ecological role of earthworms

The annelids, like many other soft bodied animals, are sparsely represented in the fossil record.

Some polychaetes leave a calcareous cement to their tube walls allowing these tubes to be preserved in marine sediment (

The polychaete Canadia is the oldest fossil found in Burgess shale, dating back as far as the Late Precambrian, Early Cambrian (

The fossil record: When did annelids first appear?

Fig 7. Calcareous tube fossils of polychaetes

Fig 8. Ichnofossil of a segmented worm found during the late Cambrian

Members of the Sepulidae, Spionidae, and Eunicida were recovered, dating back to the Ordovician (

By the end of the Carboniferous most polychaete lineages had appeared.

Archarenicola, a member of the group Scolecida dates back to the Triassic.

Oligochaetes evolved during the Jurassic and diversified during the Cretaceous

Cambrian 542 million years ago (

Explosive adaptive radiation of most metazoan phyla

Warmer climate and higher oxygen levels.

Four major continents, Laurentia, West Eurasia, East Eurasia and Gondwanaland were concentrated around the equator (

Conditions during periods of diversification.

Fig 9. The major four continents around the equator during the cambrain period

Fig 10. Global temperatures and CO2 over geologic time

Mesozoic Cretaceous 142-65 million years ago

Period of extensive sea floor spreading along the oceanic ridges and Gondwana fragmentation

Increased carbon dioxide levels leading to increased global temperatures caused by the greenhouse effect

Rise in sea levels

Early Cretaceous was dominated with conifers, ferns and cycads

Appearance of the first angiosperms leads to diversification of oligochaetes (Hickman et al, 2004).

True metamerism is shared by annelids, arthropods and chordates (Davis et al, 1999)

The advent of segmentation allowed the development of greater complexity in structure of function.

Segments are a repetition of body units and are able to function independently.

Segmentation allow better flexibility and increased the efficiency of burrowing in annelids (Hickman et al, 2004).

The evolutionary significance of segmentation.

Myzostomida is frequently classified within annelida but are actually more closely related to flatworms.

Ultrastructural evidence suggests that the segmentation, chaeta and trochophore larvae of Myzostomida are homologous to those of annelids.

The ancestor of myzostomids, flatworms and trochozoans is segmented, worm-like with chaeta and a trochophore larval stage (Eeckhaut et al, 2000)

Myzostomida as the link between polychaetes and flatworms.

Arthropods and annelids evolved segmentation separately.

The last common ancestor of arthropods and annelids was unsegmented but possibly had repeating organ systems resembling that of some large flatworms or nemerteans.

The arthropods became segmented and evolved jointed appendages while annelids evolved segmentation and retained their flexible epidermal cuticle (Valentine, 1990).

The relationship between annelids and arthropods

Branch G, Branch M (1981) Living Shores of Southern Africa. Struik Publishers, Cape Town.ISBN 0869771159, pp272.

Davis G and Patel N (1999) The origin and evolution of segmentation. Trends in Genetics 15(12)M68-M72

Edwards C, Lofty J (1972) Biology of earthworms. Chapman and Hall LTD, London. ISBN 412110601,pp 283

Eeckhaut I, McHugh D, Mardulyn P, Tiedemann R, Monteyne D, Jangoux M, Milinkovitch C (2000) Myzostomida: A link between Trochozoans and Flatworms. Biological Sciences 267(1451)1383-1392

Hickman C, Roberts L, Larson A, I’Anson H (2004) Integrated principles of Zoology. McGraw Hill, New York. ISBN 0072439408, pp 872

Valentine J (1990) Molecules and the Early Fossil Record. Paleobiology 16(1)94-95

Whitaker J (2004) Historical Article:Hirudo medicinalis : ancient origins of, and trends in the use of medicinal leeches throughout history. British Journal of Oral and Maxiillofacial surgery 42(2)133-137


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