Nematodes are simple worms that are found simply everywhere. Buchsbaum (p 175) quotes 'a leading student of the nematodes', who said: "If all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their estwhile nematode parasites."
Meglitsch (p231) puts it this way: "[It is] estimated that there may be as many as 500,000 nematode species, of which 10,000 are known. Nematode specialists will be kept busy for some time to come. Perhaps more truthfully than with any other group, one must say that nematodes occur everywhere, from hot springs to polar regions, and from deserts to ocean depths. They are almost unbelievably abundant: 90,000 in a single rotting apple; 36 species and 1074 individuals in 6.7 cc of mud from the Italian coast; 527,000 per acre in the top 3 inches of beach sand in Massachusetts; and from three to nine billion per acre in good farm land from northern China, Austria and the United States! They abound in soils containing dead plant and animal material; they parasitize plants and animals and they feed on the roots of plants."
On first looking at these worms, it is hard to see what makes them so successful. They are usually very small, white and tube shaped with pointed ends. They cannot crawl but simply thrash around in whatever medium you find them. The reason they are thrashing about is simply that they only have one kind of muscles, longitudinal muscles. There are no circular muscles to pull them forward and back. Instead they thrash, pushing against the water or the soil in order to move. Very inefficient it would seem and rather boring to watch, compared to the antics of copepods or rotifers.
Why then are they everywhere? Meglitsch thinks part of the answer is that they can resist factors toxic to other animals and by their adaptability. He mentions the vinegar eel, Turbatrix aceti, that can endure a pH of 1.5 and can live for several hours in mercuric chloride concentrations that would be instantly fatal to any other animal. Their success is not due to specialisation but just minor modifications to a very simple but effective body plan. This also means that identifying them is tedious, difficult and best left to the specialists.
The nematode body is basically a system of tubes. The body wall is a tube. The gut is a simple tube. Even the excretory system is a system of tubes. Sexes are separate and, yes, the gonads are tubes. There is no definite head but there is an anterior brain and the sense organs are grouped around the mouth. They have a body cavity that is pseudocoelomate in origin. It is covered in a tough cuticle that readily lets in water but protects the worm from all sorts of toxic compounds.
Most nematodes are free-living and are important members of their ecosystems. Many are herbivorous, others are decomposers and still others are predatory. A few are parasitic and some of these have become economically important because they infect humans and our pets and important agricultural species. One of the largest is Ascaris lumbricoides, which can be a few centimetres up to a third of a meter in length and lives in the human intestine. It is a common item of dissection because of its size. Like most parasites the emphasis is on reproduction and a female ascaris can lay up to two hundred thousand eggs a day, which pass out in the faeces and then infect other people in contaminated water, food and soils. Luckily most die before this happens, especially where there is modern sanitation and people wash their hands after using the toilet. On hatching in a new host, the little worms take a grand tour of the body, burrowing through the intestinal wall and travelling in the blood stream to the lungs where they bore through the bronchial tubes, go back to the mouth and then are swallowed back to the stomach and intestine. They stay there this time and grow rapidly by feeding on their host's digested food. They secrete an enzyme to keep the host from digesting them too. It is the migration through the body that can cause the most damage to the unfortunate host.
Another dangerous nematode is the hookworm, Necator americanus (which translates as American killer). It uses hooks to attach to the intestinal lining and it sucks blood and fluids from the damaged flesh. The eggs pass out with the feces and hatch in the soil where they are picked up by walking barefoot through contaminated soil. Wearing shoes and decent sanitation can reduce infections, which are then susceptible to treatment with vermicides.
Another dreaded nematode is the Trichina worm, Trichinella spiralis, which is spread to humans through undercooked pork and may have been a reason for religious bans on eating pig meat. Adult worms do little harm but larvae wander through the body and encyst in muscle tissue, causing excruciating pain and weakness, fever, anaemia and swelling. A third of sufferers will die from the effects.
In tropical and subtropical countries, the Filaria roundworm is a problem (Wuchereria bancrofti). They inhabit lymph glands and lead to a swelling of limbs known as elephantiasis. There is an intermediate host, the mosquito, which picks up eggs from the blood of an infected human. The eggs hatch in the mosquite and the larvae migrate to the biting apparatus where they are injected into a new human host.
Last but not least is the Guinea Worm (Dracunculus medinensis) which occurs in India, the Middle East and North Africa. The male is small but the female can be up to a meter long. It lives under the skin like a coiled varicose vein. The mature female approaches the surface and secretes toxins to cause a blister which bursts when put in cold water, such as when washing clothes in a river. The larvae then come out and swim until they can get into their invertebrate host, Cyclops, a type of copepod. Drinking unfiltered water gets the copepod and the worm back in another human being. Symptoms are vomiting, diarrhea and dizziness. Medicine men remove adult worms by wrapping them slowly and painfully around a stick but if done improperly this can lead to loss of a limb or even death. It is preventable by keeping infected people out of waterways and filtering drinking water. There has been a huge effort by the UN to break this cycle and reduce infections in countries where up to a fourth of the population are incapacitated by this parasite.
References: Buchsbaum, R. 1968. Animals without Backbones. Penguin Press Ed. Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press.