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 .
Cell numbers in the organs are constant within a species. Nematodes have a fixed, genetically determined number of cells, a phenomenon known as eutely. The male C. elegans has 1031 cells, a number which does not change after cell division stops at the end of the larval period. Growth is solely due to an increase in the size of individual cells.