Nematodes are the most numerous multicellular animals on earth. A handful of soil will contain thousands of the microscopic worms, many of them parasites of insects, plants or animals. Free-living species are abundant, including nematodes that feed on bacteria, fungi, and other nematodes, yet the vast majority of species encountered are poorly understood biologically. There are nearly 20,000 described species classified in the phylum Nemata .

The word Nematoda comes from the Greek words nematos, meaning thread, and eidos, meaning form.

If all the matter in the universe except nematodes were swept away, our world would still be recognizable, and if 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 erstwhile nematode parasites.

3 EXAMPLES OF PARASITES IN HUMANS:

Ascaris
Ascaris lumbricoides is one of the largest and most common parasites found in humans.  The adult females of this species can measure up to 18 inches long (males are generally shorter), and it is estimated that 25% of the world's population is infected with this nematode.  The adult worms live in the small intestine and eggs are passed in the feces.  A single female can produce up to 200,000 eggs each day!  About two weeks after passage in the feces the eggs contain an infective larval or juvenile stage, and humans are infected when they ingest such infective eggs.  The eggs hatch in the small intestine, the juvenile penetrates the small intestine and enters the circulatory system, and eventually the juvenile worm enters the lungs.  In the lungs the juvenile worm leaves the circulatory system and enters the air passages of the lungs.  The juvenile worm then migrates up the air passages into the pharynx where it is swallowed, and once in the small intestine the juvenile grows into an adult worm.  Why Ascaris undergoes such a migration through the body to only end up where it started is unknown.  Such a migration is not unique to Ascaris, as its close relatives undergo a similar migration in the bodies of their hosts

Trichinella
Unlike many parasites that demonstrate a high degree of host specificity, the trichina worm, can be found in many species of carnivores and omnivores.  Animals are infected with T. spiralis when they ingest infective larvae (juveniles) in raw or undercooked meat.  The larvae mature into adults in the host's small intestine in a few weeks, and the female worms give birth to larvae.  (The males die after fertilizing the females, and the females die after producing larvae.)  The larvae enter the blood stream of the host and, eventually, end up in the host's muscles.  Here the larvae mature into infective larvae, and the next host is infected when it eats these larvae.  In the muscles the larvae cause a severe host reaction that results in soreness and tenderness of the muscles (view diagram of the life cycle).  Although this parasite probably only rarely causes fatalities in humans, it can cause extreme discomfort.

Trichinosis is probably best known as a parasite that humans contract from eating raw or undercooked pork.  Through an aggressive program of meat inspection, the incidence of trichinosis in pigs in the United States has been lowered to less than 1%, so it is unlikely (but not impossible) that pork products purchased in your local supermarket will contain Trichinella larvae.  Most recent outbreaks of trichinosis in the United States have been traced to pork products from pigs that have not been inspected and that have been slaughtered privately.  Because of its low host-specificity, almost any "wild" meat should be considered suspect, and hunters should be careful when preparing meat from their kills.  In particular, a number of infections have been traced to contaminated bear meat.

Hookworms
There are many species of hookworms that infect mammals.  The most important, at least from the human standpoint, are the human hookworms, Ancylostoma duodenale and Necator americanus, which infect an estimated 800,000,000 persons, and the dog and cat hookworms, A. caninum and A. braziliense, respectively.  Hookworms average about 10 mm in length and live in the small intestine of the host.  The males and females mate, and the female produces eggs that are passed in the feces. Depending on the species, female hookworms can produce 10,000-25,000 eggs per day.  About two days after passage the hookworm egg hatches, and the juvenile worm (or larva) develops into an infective stage in about five days.  The next host is infected when an infective larva penetrates the host's skin.  The juvenile worm migrates through the host's body and finally ends up in the host's small intestine where it grows to sexual maturity.  The presence of hookworms can be demonstrated by finding the characteristic eggs in the feces; the eggs can not, however, be differentiated to species (view diagram of the life cycle).

The mouthparts of hookworms are modified into cutting plates.  Attachment of hookworms to the host's small intestine causes hemorrhages, and the hookworms feed on the host's blood.  Hookworm disease can have devastating effects on humans, particularly children, due to the loss of excessive amounts of blood.
 
 
 
 
 

http://nematode.unl.edu/wormgen.htm