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Nematoda (nematode worms or roundworms)

Life > Eukaryotes > Opisthokonta > Bilateria > Ecdysozoa > Introverta > Nematoida

About 80 000 species known, but an estimated 200000+ species extant, mostly microscopic. Considered the second most diverse animal phylum after the arthropods.


The Nematoda is a highly diverse group of free-living or parasitic worms that are difficult to distinguish from their external appearance because they tend to all have a uniform worm-like appearance with no external appendages. The morphological taxonomy of the group has been based on examination of the internal structures, especially the oesophagus and the male and female reproductive organs. Life cycle patterns are also important in distinguishing groups. Molecular phylogenetic anaylysis is transforming the systematics of the Nematoda, but still has a way to go before some stability in classification can be attained.

Nematodes are found in abundance in marine, freshwater and terrestrial habitats. Densities can exceed 1 million individuals per square metre and it has been claimed that about 80% of all individual animals on earth are nematodes!

A breakdown of named nematodes (worldwide) by life style reveals the following distribution of species (from Hugot et al. 2001).

Life style No. species
Free-living marine 4070
Free-living terrestrial 6610
Plant parasites 4110
Invertebrate parasites 3500
Vertebrate parasites 8360
Total 26650


Based on the Nematode classification presented on the website of the De Ley Lab, University of California Riverside.








Free-living nematodes that inhabit marine and brackish water where they feed on algae, diatoms and other similar organisms.



Plant-feeding nematodes.






Inhabit soil and freshwater. Also includes the family Longidoridae, members of which are plant parasites and transmit plant viruses.



Inhabit soil and freshwater. Predators. 



One family: Isolaimidae. Inhabit soil.



Two families: Dioctophymatidae and Soboliphymatidae.



Two families: Muspiceidae and Robertdollfusiidae.



One family: Marimermithidae.



Two families: Mermithidae and Tetradonematidae.












Three families: Desmoscolecidae, Meyliidae and Cyartonematidae



Five families: Chromadoridae, Ethmolaimidae, Neotonchidae, Achromadoridae and Cyatholaimidae









Four families: Axonolaimidae, Comesomatidae, Diplopeltidae and Coninckiidae








Uncertain position



One family: Benthimermithidae.



One family: Rhaptothyreidae.






  • Anderson, R.C., 2000. Nematode Parasites of Vertebrates. Their Development and Transmission, second ed. CABI Publishing, Wallingford, Oxon.

  • Blaxter, M.L., 2003. Nematoda: genes, genomes and the evolution of parasitism. Advances in Parasitology 54:  102197.

  • Chilton, N.B., Newton, L.A., Beveridge, I., Gasser, R.B., 2001. Evolutionary relationships of trichostrongyloid nematodes (Strongylida) inferred from ribosomal DNA sequence data. Molecular Phylogenetics and Evolution 19: 367386.

  • De Ley, P., Blaxter, M.L., 2002. Systematic position and phylogeny. In: Lee, D.L. (Ed.), The Biology of Nematodes. Taylor & Francis, London, pp. 130.

  • Holterman, M., van der Wurff, A., van den Elsen, S., van Megen, H., Bongers, T., Holovachov, O., Bakker, J. and Helder, J. 2006. Phylum-Wide Analysis of SSU rDNA Reveals Deep Phylogenetic Relationships among Nematodes and Accelerated Evolution toward Crown Clades. Molecular Biology and Evolution 23(9): 1792-1800.

  • Hugot, J-P., Baujard, P. & Morand, S. 2001. Biodiversity in helminths and nematodes as a field of study: an overview. Nematology, 3: 199-208.

  • Liu, J., Poinar, G.O. & Berry, R.E. 2000. Control of insects pests with entomopathogenic nematodes: the impact of molecular biology and phylogenetic reconstruction. Annual Review of Entomology 45: 287-306.


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