What Are Roundworms?


Roundworms, or nematodes, are a phylum of invertebrates. The nematodes are a very diverse phylum, with tens of thousands of species known to science. Some roundworms are parasitic, while others are free-living. Nematodes are very small, with many no more than 1/10 inch long. Only a few species can reach large sizes.


  • Roundworms inhabit almost every habitat on Earth. Nematode species are found in both salt and fresh water, as well as on land. In many biomes, such as the ocean floor, nematodes make up the majority of living organisms. Nematodes inhabit both desert environments and deep oceanic trenches. Free-living nematodes are scavengers; they feed on dead organisms and organic wastes, as well as fungus and algae. Countless tiny nematodes play a role in the decomposition process, each removing a small amount of dead tissue as its feeds.


  • Nematodes are small, thin animals. Their small size and slenderness gives them their name, which comes from a Greek word meaning "thread." The many species of nematodes are diverse. Some carnivorous species have teeth, while others have a needlelike projection called a stylet, used to suck out fluids and others have other methods of feeding. Food is drawn into the gut, which runs down the body's central cavity, is digested and expelled from the anus. Nematodes reproduce in many different ways. Some are hermaphroditic, while others reproduce sexually. In some species, larval nematodes consume the parent.

Nematode Parasites

  • Many species of nematodes are parasitic. Species that can carry nematode parasites include rats, dogs, cats, pigs, birds, wasps, sheep and humans. Hookworms, pinworms and whipworms are all parasitic nematodes found in humans. Parasitic roundworms draw nutrition from the tissues of the host. The life cycle of the parasite can be very complex. For instance, one species of roundworm, Myrmeconema neotropicum, infects ants, causing them to develop bright red swellings. Birds mistake these for berries and eat the ants, ingesting the nematodes, which lay their eggs in the bird's intestines. The larvae are then excreted and scavenged up by ants, which in turn become infected.

Health Risks

  • Infection by these parasites, as well as the Ascaris or giant intestinal roundworm, is a major public health concern. Nematode parasites can lead to malnutrition, and although they do not often directly cause death, they can weaken hosts and make them more vulnerable to other diseases. Parasitic nematodes in pigs can cause trichinosis in humans if they eat pork that has not been sufficiently cooked. Some nematodes can also harm plants, putting crops at risk. However, other nematodes play an important role in the reproductive cycle of plants.

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