Nematodes are small, usually microscopic, unsegmented roundworms with a narrow cylindrical body. They are incredibly numerous in soil, water, and inside other animals and plants throughout the world. Approximately 10,000 species are known. Four of every five animals on the planet are nematodes, outnumbering even insects. A single gram of soil can contain more than 1,000 plant parasitic nematodes.

Nematodes are simple animals with a body plan that resembles a tube within a tube with a head, mouth, tail, and anus. The outer tube is comprised of the cuticle, hypodermis, and muscle; and the inner tube contains the pharynx and intestine. Nematodes also have several hundred nerve cells and a prolific reproductive system. They vary in size from a fraction of a millimeter to many centimeters for some animal/human parasitic species.

Many nematodes are free-living in terrestrial and aquatic environments where they feed on bacteria. Numerous other species are parasitic, specializing in the invasion of plants, mammals, or even insects. Parasitic nematodes are major challenges to human health and agriculture. Parasitic nematodes, including whipworm, Ascaris, hookworm, and filarial worms, currently infect about 3 billion people. Plant parasitic nematodes, such as root knot nematode, cause an estimated 80 billion dollars in crop damage annually. Animal parasites infect livestock and pets and their treatment is a mainstay of veterinary practice (e.g. heartworm prevention in dogs and cats). Nematodes are an ancient phylum dating back hundreds of millions of years and their diversification likely predates the arrival of life on land. Parasites are likely to have evolved at least seven separate times from free-living ancestors.

Animal and human parasitic nematodes have adapted complex life cycles to take advantage of their hosts. Hookworm larvae, for instance, can enter the skin from the soil, burrow into the blood stream, travel through the circulation to the lungs, transit up the bronchial tree and down the esophagus, finally reaching the small intestine where they drink blood from capillaries, and reproduce by laying eggs in the feces. They take several weeks to complete this cycle. Filarial worms that cause river blindness have a very different life cycle, entering humans by the bite of a fly, growing to adulthood in nodules under the skin where they can live for more than 8 years, generating millions of larva which migrate through the skin and eyes. Other filarial worms are transmitted by mosquitoes.

Plant parasitic nematodes such as root knot and cyst nematodes are sedentary endoparasites, invading plant roots, establishing feeding sites where they divert plant nutrients toward their own growth. Once established at a feeding site, the female worm gradually changes to a pear shape and fills with hundreds of eggs. The life cycle generally takes several weeks. Other plant parasites migrate through roots, stems, or leaves. Many parasites have long-lived forms; for instance, soybean cyst nematode can survive five or more winters, making crop rotation ineffective.

The free-living nematode C. elegans has become an incredibly useful model organism for biologists and medical scientists. Selected in the mid-1960's by Sydney Brenner as a genetic specimen for the general study of animal development and behavior, the worm is now studied by nearly 500 laboratories worldwide. The advantages of C. elegans as a model include its short generation time (4 days), large reproductive size (350 offspring), and relatively simple body structure (959 somatic cells). It was the first animal for which the complete cell lineage was determined and the first to have its entire neuronal wiring deciphered. C. elegans was also the first multicellular organism to have its genome fully sequenced and annotated (www.wormbase.org). An international collaboration between Washington University, St. Louis and the Sanger Institute near Cambridge, England completed the 100 megabase genome in 1998, identifying ~20,000 genes. In parallel with these descriptive efforts, thousands of genetic mutations have been characterized affecting intercellular signaling, cell migration and death, sex determination, dauer formation, germ line development, muscle structure, mechanosensation, and developmental pathways involved in cell fate and timing. Numerous genes involved in these processes are highly conserved with human orthologs.

The importance of research using C. elegans has been widely recognized. Six nematologists studying C. elegans have been awarded the Nobel Prize this decade. The 2002 Nobel Prize in physiology or medicine was awarded to Sydney Brenner, John Sulston, and Robert Horvitz for their use of C. elegans in understanding cell development and programmed cell death. The 2006 Nobel prize in physiology or medicine was awarded to Craig Mello and Andrew Fire for their use of C. elegans in the discovery of gene expression silencing by RNA interference (RNAi). The 2008 Nobel prize in chemistry was awarded to three scientists including Martin Chalfie who introduced the use of green fluorescent protein (GFP) as a luminous genetic tag for tracking individual cells in C. elegans.

"If all the matter in the universe except nematodes were swept away, our world would still be 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 erstwhile nematode parasites."
~ Nathan Cobb, 1914