Nematoda (Roundworms)


The Nematoda are a group of nonsegmented worm‐like invertebrates that occur worldwide in a wide range of habitats, including fresh and saltwaters, soil, plants and animals. They are one of the most abundant group of invertebrates on the face of the earth and rival the arthropods in biodiversity and species abundance. Their estimated numbers range from 1.5 billion in an upper 20 mm in 1.6 hectares of marine beach sand to 380 million in a square meter of leaf litter. Evidence from fossils and extant forms indicate that they evolved in the sea and came on land by probably moving up brackish estuaries into freshwater habitats. While many of the plant parasites are considered pests and controlled with nematicides, other nematodes are important in controlling agricultural and medically important insect pests, and still, others have been used as experimental model organisms in genetics.

Key Concepts

  • Nematodes are the most successful group of the Ecdysoma or pseudocoelomate bilateria.
  • Nematodes are one of the most abundant groups of invertebrates.
  • Some 22 000 nematode species have been described.
  • Estimates for species diversity range from 100 000 to 10 million.
  • Plant‐parasitic nematodes can be important agricultural pests.
  • Many nematodes are parasites of invertebrates, humans and other vertebrates.

Keywords: nematodes; roundworms; plant parasites; metazoans; invertebrates

Figure 1. Cluster of dauer stages of the free‐living Rhabditis coarctada removed from the dorsum of a scarab beetle.
Figure 2. Stem nematode, Ditylenchus sp., showing stylet (arrow) used to remove plant cell contents.
Figure 3. Swollen egg‐containing female of the plant parasitic nematode, Heterodera sp.
Figure 4. The mermithid, Empidomermis cozii, emerging from its malaria‐carrying mosquito, Anopheles funestus, in West Africa.
Figure 5. The human stomach worm, Ascaris lumbricoides. Insert shows egg removed from the faeces of an infected person.
Figure 6. Nodule (arrow) containing the river blindness parasite, Onchocerca volvulus, in West Africa. Insert shows nematodes excised from the nodule.


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Further Reading

Andrássy I (2005) Free‐living nematodes of Hungary (Nematoda errantia). I. 518pp. Hungarian Natural History Museum and Systematic Zoology Research Group of the Hungarian Academy of Sciences: Budapest. ISBN: 9637093907; 9789635085743.

Grasse PP (1965) Traite de Zoologie, Tome IV, Nemathelminthes (Nematodes). Masson et Cie, Fasc. 2 and 3: Paris.

Levine ND (1968) Nematode Parasites of Domestic Animals and of Man. Burgess Publishing: Minneapolis, MN.

Thorne G (1961) Principles of Nematology. McGraw‐Hill: New York.

Yamaguti S (1961) Systema Helminthum, The Nematodes of Vertebrates, Parts 1 and 2, vol. III. Interscience Publishers, Inc.: New York.

Zuckerman BM, Mai WF and Rohde RA (eds) (1971) Plant Parasitic Nematodes, vol. 1 and 2. Academic Press: New York.

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Poinar, George(Sep 2020) Nematoda (Roundworms). In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029191]