They are everywhere! About 80 000 species of Nematode have been described so far, but some authorities estimate that there may be as many as 500 000 to 100 million species in all! They live in all environments and can parasitize nearly all animals and plants. In fact there is scarcely an animal alive that does not harbour a population of parasitic nematodes at some stage in its life, and that does include you! Around 50 different species are known parasites of humans (see below for more details). Luckily most of the time they are so harmless that we don't even know they are there. Parasitic species may be up to nine metres long, but most species are less than five centimetres long. The longest known nematode is found in the placenta of sperm whales. To give you an idea of just how ubiquitous they are, you could expect to find around 10 million in one square yard of soil. One apple left on the ground to decompose had 90 000 individual nematodes living on and in it. One fig had 50 000 individuals belonging to over 8 species on and in it. In 1914 Nathan Cobb famously wrote "If all the matter in the universe except nematodes were swept away, our world would still be recognizable, its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes."

Compared to other phyla the external structure of the Nematoda is very uniform. They all have slender, elongated bodies with tapered ends. Some have specialised mouthparts, and the only other projecting parts are concerned with reproduction. The cuticle can have as many as nine layers, with cross fibres forming a spiral network. The body is a high pressure pseudocoel within a cuticle covered body wall with only longitudinal muscles. Locomotion is basically wriggling or thrashing with the longitudinal muscles on one side contracting, while the other side expands. The cuticle prohibits radial expansion, so serves the purpose of circular muscles. Males are usually smaller than females and the posterior is curled into a hook. This can be easily seen in the drawing below right. This shows a male and female Ascaris sp. in the gut of a pig.

On the right is a cross-section of a female Ascaris sp. The muscle arm extends to

Above is a piece of a false-killer whale's gut which has a heavy infestation of Bolbosoma capitatum, the thorny-headed worm. There have been cases where infestations reach up to 600 worms per square metre of gut.

Far right is part of a whale's kidney infected with several Crassicauda boopis, the fin whale round worm. The head of the worm is found in the blood vessels in the liver, the middle in the whale's kidney, and the tail end in the reproductive and excretory system. The entire length of the worm can be as long as 8 m.

Common parasitic nematodes of humans

Hookworm, Ancylostoma duodenale and Necator americanus. Juveniles in the soil can burrow into human skin. Common in southern USA.

Pinworm, Enterobius vermicularis. The female pinworm creeps out of the human anus ans lays her eggs around the margin of the anus at night. This action causes itching and scratching by the human. The eggs are caught under the fingernails, and easily spread to other humans or back to the original host. Eggs can also be inhaled with dust or licked off fingers which have touched dusty surfaces. The most common worm parasite in the modern affluent areas of the world. 30% of US children are estimated to be infected, however infection causes little harm in a healthy human.

Intestinal roundworm, Ascaris lumbricoides. Eggs are eaten with contaminated food. Infection is common in some rural areas of the US with up to 60% of the population infected. For more details see above.

Trichina worm, Trichinella spiralis. Infection occurs when eating infected meat.

Whipworm, Trichuris thrichiura. Infection occurs when eating contaminated food, or through unhygienic habits. This parasite tends to be found in areas where Ascaris. sp. is also common. Adults live in the intestine, but the larva burrow through the gut wall into the muscles causing cysts which can cause great pain and even death.

Filarial worms, Wucheria bancrofti and Brugia malayi etc. Infection comes from infected female mosquitoes taking a blood meal. Repeated and long exposure can lead to elephantiasis. Another filarial worm carried by the blackfly causes onchocerciasis, river blindness.

Loa-loa, the African eye worm lives in the subcutaneous tissues of man. The worm migrates and if it passes across the carnea of the eye it can be seen - hence its common name. Some believe that this is origin of lovers gaze into each other's eyes as an infected partener would not make a good mate.

Guinea worm, Dracunulus medinensis, occurs mainly in Asia and Africa. In humans infection occurs from drinking water containing Cyclops which have been infected by the worm. The worm larvae in the cyclops hatch out and penetrate the human intestine. Eggs are passed out of the human when a gravid female migrates to the skin and causes an ulcer. When this ulcer comes into contact with water the free-swimming larvae are released. The larvae swim until they either die or are eaten by a cyclops. A mature female is 1 mm in diameter but can reach 120 cm in length.