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Scientific classification
Kingdom: Animalia
Phylum: Nematoda
Rudolphi, 1808

   Subclass Enoplia
   Subclass Chromadoria
   Subclass Rhabditia
   Subclass Spiruria
   Subclass Diplogasteria

The nematodes or roundworms ( Phylum Nematoda from Greek νῆμα (nema): "thread" + ode "like") are one of the most common phyla of animals, with over 20,000 different described species (over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants and animals, humans included. Only the Arthropoda are more diverse.

The Nematodes were originally named Nematoidea by Rudolphi ( 1808). They were renamed Nematodes by Burmeister 1837 (as a family; Leuckart 1848 and von Siebold 1848 both promoted them to the rank of order), then Nematoda ( Diesing 1861), though Nathan Cobb ( 1919) argued that they should be called Nemata or Nemates (and in English 'nemas' rather than 'nematodes'). After some confusion which saw the nematodes placed (often together with the horsehair worms, Nematomorpha) as a class or order in various groups such as Aschelminthes, Lankester ( 1877) definitively promoted them to the level of phylum.


Nematodes are triploblastic protostomes with a complete digestive system. Roundworms have no circulatory or respiratory systems so they use diffusion to breathe and for circulation of substances around their body. They are thin and are round in cross section, though they are actually bilaterally symmetric. Nematodes are one of the simplest animal groups to have a complete digestive system, with a separate orifice for food intake and waste excretion, a pattern followed by all subsequent, more complex animals. The body cavity is a pseudocoel (persistent blastula), which lacks the muscles of coelomate animals used to force food down the digestive tract. Nematodes thus depend on internal/external pressures and body movement to move food through their digestive tracts. The mouth is often surrounded by various flaps or projections used in feeding and sensation. The portion of the body past the anus or cloaca is called the "tail." The epidermis secretes a layered cuticle made of keratin that protects the body from drying out, from digestive juices, or from other harsh environments, as well as in some forms sporting projections such as cilia that aid in locomotion. Although this cuticle allows movement and shape changes via a hydrostatic skeletal system, it is very inelastic so does not allow the volume of the worm to increase. Therefore, as the worm grows, it has to moult and form new cuticles. The cuticles don't allow volume to increase so as to keep hydrostatic pressure inside the organism very high. For this reason, the roundworms do not possess circular muscles (just longitudinal ones) as they're not required. This hydrostatic pressure is the reason the roundworms are round.

Most free-living nematodes are microscopic, though a few parasitic forms can grow to several meters in length (typically as parasites of very large animals such as whales). There are no circular muscles, so the body can only undulate from side to side. Contact with solid objects is necessary for locomotion; its thrashing motions vary from mostly to completely ineffective at swimming.

Nematodes generally eat bacteria, fungi and protozoans, although some are filter feeders. Excretion is through a separate excretory pore.

Reproduction is usually sexual. Males are usually smaller than females (often very much smaller) and often have a characteristically bent tail for holding the female for copulation. During copulation, one or more chitinized spicules move out of the cloaca and are inserted into genital pore of the female. Amoeboid sperm crawl along the spicule into the female worm.

Eggs may be embryonated or unembryonated when passed by the female, meaning that their fertilized eggs may not yet be developed. In free-living roundworms, the eggs hatch into larva, which eventually grow into adults; in parasitic roundworms, the life cycle is often much more complicated.

Nematodes have a simple nervous system, with a main nerve cord running along the ventral side. Sensory structures at the anterior end are called amphids, while sensory structures at the posterior end are called phasmids.

Free-living species

In free-living species, development usually consists of four molts of the cuticle during growth. Different species feed on materials as varied as algae, fungi, small animals, fecal matter, dead organisms and living tissues. Free-living marine nematodes are important and abundant members of the meiobenthos. One roundworm of note is Caenorhabditis elegans, which lives in the soil and has found much use as a model organism. C. elegans has had its entire genome sequenced, as well as the developmental fate of every cell determined, and every neuron mapped.

Parasitic species

Parasitic forms often have quite complicated life cycles, moving between several different hosts or locations in the host's body. Infection occurs variously by eating uncooked meat with larvae in it, by entrance into unprotected cuts or directly through the skin, by transfer via blood-sucking insects, and so forth.

Nematodes commonly parasitic on humans include whipworms, hookworms, pinworms, ascarids, and filarids. The species Trichinella spiralis, commonly known as the trichina worm, occurs in rats, pigs, and humans, and is responsible for the disease trichinosis. Baylisascaris usually infests wild animals but can be deadly to humans as well. Haemonchus contortus is one of the most abundant infectious agents in sheep around the world, causing great economic damage to sheep farms. In contrast, entomopathogenic nematodes parasitize insects and are considered by humans to be beneficial.

One form of nematode is entirely dependent upon the wasps which are the sole source of fig fertilization. They prey upon the wasps, riding them from the ripe fig of the wasp's birth to the fig flower of its death, where they kill the wasp, and their offspring await the birth of the next generation of wasps as the fig ripens.

Plant parasitic nematodes include several groups causing severe crop losses. The most common genera are: Aphelenchoides (foliar nematodes), Meloidogyne (root-knot nematodes), Heterodera, Globodera (cyst nematodes) such as the potato root nematode, Nacobbus, Pratylenchus (lesion nematodes), Ditylenchus, Xiphinema, Longidorus, Trichodorus. Several phytoparasitic nematode species cause histological damages to roots, including the formation of visible galls (Meloidogyne) which are useful characters for their diagnostic in the field. Some nematode species transmit plant viruses through their feeding activity on roots. One of them is Xiphinema index, vector of GFLV (Grapevine Fanleaf Virus), an important disease of grapes.

Other nematodes attack bark and forest trees. The most important representative of this group is Bursaphelenchus xylophilus, the pine wood nematode, present in Asia and America and recently discovered in Europe.


Soybean cyst nematode and egg
Soybean cyst nematode and egg

Depending on the species, a nematode may be beneficial or detrimental to a gardener's cause.

From a gardening perspective, there are two categories of nematode, predatory ones which will kill garden pests like cutworms, and pest nematodes like the root-knot nematode, which attack garden plants.

Predatory nematodes can be bred by soaking a specific recipe of leaves and other detritus in water, in a dark, cool place, and can even be purchased as an organic form of pest control.


Current studies suggest that roundworms (nematodes) are related to the arthropods and priapulids in a newly recognized group, the Ecdysozoa (molting animals).


  • Hundreds of nematode worms (C. elegans), featuring in a research project on mission STS-107, survived the Space Shuttle Columbia Disaster .
  • In the Spongebob Squarepants episode Home Sweet Pineapple, his house is consumed by a swarm of nematodes.
  • On the BBC2 quiz show QI, when Clive Anderson was asked, "What lives in the Dead Sea?", he answered, "I'm tempted to say nematode worms because they live everywhere."
  • In the SpongeBob SquarePants episode Best Day Ever, a swarm of nematodes try to eat the Krusty Krab until SpongeBob plays his nose as a flute. That also referes to the SpongeBob SquarePants beginning of an episode.

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