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A nematode is an unsegmented pseudocoelomate worm (such as Caenorhabditis elegans) belonging to the phylum Nematoda which is generally round, one to two millimeters in length, and pointed at both ends. There are both parasitic and free-living species. The parasites can be found in all types of animals (both vertebrates and invertebrates) and in plants. The free-living species can be found in shallow water environments (both sea water and freshwater) and in soils.

Over 12,000 species of nematodes have been named and described by scientists; a single scoop of garden soil contains about a million microscopic nematodes.

Over 50 species of parasitic nematodes can infect humans. They are a major cause of disease and death worldwide, particularly in tropical areas. Some of the most common human parasites include:

Reference: Part of the information here was found in the book Biology by Helena Curtis (Worth Publishers, Inc.) The rest was taken from personal notes.