Nematodes, also called roundworms, are members of the animal phylum Nematoda. These worms have a complete digestive system and are more complex than the flatworms (phylum Platyhelminthes) but lack a circulatory system and other advanced features found in the annelids (segmented worms). The Nematoda is one of the largest animal phyla, with over 15,000 described species. Many more species remain to be discovered because most nematodes are microscopic in size and not easily observed.
Nematodes are an extremely diverse group and are common in most habitats. These aquatic worms are abundant in freshwater and marine ecosystems but also inhabit the moisture film around soil particles. A small handful of soil may contain several thousand individuals. Nematodes even occur in desert soils and in Antarctica.
Many kinds of nematodes are parasites , inhabiting vertebrates (including humans) or invertebrates. Others are parasites of plants and feed on or live within roots, tubers, bulbs, and other below-ground plant parts. A few unusual species live inside leaves, stems, or seeds. Some of the nonparasitic, free-living nematodes are predators of other minute organisms. Most free-living nematodes feed on bacteria or fungi. Their activities are important in the decomposition of organic matter and recycling of nutrients.
Poinar, George O., Jr. The Natural History of Nematodes. Englewood Cliffs, NJ: Prentice Hall, 1983.
Ruppert, Edward E., and Robert D. Barnes. Invertebrate Zoology, 6th ed. Philadelphia, PA: Saunders College Publishing, 1994.