With more than 10,000 species described, roundworms (phylum Nematoda) are among the most numerous and widespread animals. They occur in all habitats, including freshwater, marine, and terrestrial ecosystems, from the tropics to the polar regions. They often occur in staggering numbers: 10.8 sq ft (1 sq m) of mud has been found to contain more than four million nematodes. Because of their distribution and ability to adapt to different situations, it is not surprising to find that nematodes have adapted to a wide range of living conditions. Many are free-living, but others are parasitic on both plants and animals.
All nematodes are characterized by their slender, elongate body, in which the two ends are slightly tapered to form a head and anal region. Many species measure less than 0.04 in (1 mm) in length; most are microscopic. The body is enclosed in a thin layer of collagen which represents the body wall, and is also supplied with a layer of muscle, enabling the worm to move in a sideways manner by contracting and expanding these muscles.
Among the free-living species, many roundworms are carnivorous, feeding on a wide range of protozoans as well as other nematodes; aquatic species feed largely on bacteria, algae, and microscopic diatoms. Some terrestrial species attack the roots of plants, extracting nutrients and essential fluids.
Most nematodes are dioecious (either male or female), with males commonly being smaller than females. When ready to breed, females of some species are thought to give off a pheromone that serves to attract potential suitors. During copulation, the male inserts its sperm into the female and fertilization takes place. The egg then develops a toughened outer coating and may either be held within the body for a short period or released to the outside. In hermaphrodite species, the sperm develop ahead of the eggs and are stored in special chambers until the eggs are ready for fertilization to take place. The young larvae that emerge progress through a series of body molts until they develop adult characteristics.
Many species of parasitic nematodes are unable to complete their life cycle without the presence of another animal. Commonly eggs are deposited on plants, which are then ingested or absorbed into the body in some other manner. Once within the host animal, the eggs hatch and burrow their way into the flesh (often the intestine or lungs), where they attach firmly to the lining of the chamber and begin to mature. From there the nematodes absorb nutrients from the host animal and release additional eggs, which pass out of the body in the feces.
Although some nematodes are beneficial in the manner in which they break down dead or decaying matter, many are of considerable economic importance: a great number are pests of animals and plant crops, while others are the cause of serious illnesses in humans. The tiny hookworms, for example, are believed to affect millions of people worldwide, causing serious bleeding and tissue damage. Larvae of the guinea worm (Dracunculus medinensis), which lives in freshwater streams in parts of Africa and Asia, seek an open wound in the body through which they pass and become installed in the connective tissue. Females of this species may develop to a length exceeding 3.3 ft (1 m), causing considerable discomfort.
See also Parasites.