Nematodes or roundworms are one of the most numerous animal groups. In just a handful of soil, one can find thousands of these mainly microscopic organisms. About 20,000 species have been described, with most of them only poorly understood, but estimates range from 40,000 to 10 million. Free-living species inhabit almost all terrestrial and marine environments and many nematodes are parasites to a wide range of animals, plants or fungi. These parasitic forms can do considerable damage by destroying crops and causing a number of diseases of humans as well as domestic animals (although some insect parasites can be used in biological pest control).

The best-known member of this group is Caenorhabditis elegans, a small free-living soil nematode that has found fame as the first multi-cellular organism with a fully sequenced genome. C. elegans has become one of the model organisms for developmental and genome research as it is relatively anatomically and genetically simple, has a short life cycle and an invariant cell lineage with a fixed number of cells, so the embryological fate of each one can be traced. In addition, the species is easy to keep in the laboratory and manipulate experimentally.

All nematodes are united by the same relatively simple morphology. Their unsegmented body basically consist of two tubes, an inner one comprising the intestine and an outer one consisting of cuticle, hypodermis, muscle and nerve cells, which are separated by a space, the pseudocoelom that contains the gonads. Reproduction is usually sexual - in many species, a male fertilises a female, while hermaphrodites and males occur in others. All nematodes develop through four larval stages, which has led to complex multi-stage life cycles in many of the parasitic species - different larval stages specialise on different hosts or host tissues. Also, as noted, nematodes are characterised by a precisely determined, invariant pattern of somatic cell divisions and one of the few phyla that include organisms with a fixed number of cells (e.g. C. elegans has exactly 959 cells).

Not much is known about the early evolutionary history of nematodes, but they are generally assumed to be an ancient group, having arisen in the Precambrian. Also their relationship to other invertebrates as well as their own phylogeny are still unclear, although with respect to the former it is generally agreed they belong to the Ecdysozoa and so are relatives of the arthropods and priapulids (all shed their cuticle at some point in the process of ecdysis). Traditionally, nematodes have been grouped into two classes according to their trophic ecology and habitat, the parasitic as well as free-living terrestrial Secernentea and the free-living marine Adenophorea. Recent phylogenetic studies, however, have suggested five major clades, which are also supported by re-evaluations of nematode biology and morphology.

One reason for the trouble with nematode systematics seems to be that convergence is ubiquitous in this group. In a recently published article on nematode phylogeny it says: "The extensiveness of convergent evolution is one of the most striking phenomena observed in the phylogenetic tree presented here - it is hard to find a morphological, ecological or biological characteristic that has not arisen at least twice during nematode evolution. Convergent evolution appears to be an important additional explanation for the seemingly persistent volatility of nematode systematics." (van Megen et al. 2009, Nematology, vol. 11, pp. 927-950)

There have been multiple transitions to parasitism in the group. Nine nematode families include insect parasites, while plant parasitism has evolved independently in at least three disparate clades. Many functional similarities in the feeding structures of plant parasites are in fact convergences as they trace back to different developmental origins. For example, sedentary endoparasitism (where the nematode enters the plant and loses its ability to move) and plant nurse cells (modified plant cells induced by the nematode on which it then feeds) show convergent evolution in cyst nematodes and root-knot nematodes.

Remarkably, several studies have provided evidence for the independent evolution of hermaphroditism in the closely related C. elegans and C. briggsae. These two species achieve self-fertility in different ways, relying on different gene expression and thus demonstrating significant developmental genetic flexibility.

An example of molecular convergence is provided by nematode antibacterial factors and invertebrate defensins, both of which are polypeptids and part of the innate immune system. Despite these structural and functional similarities, sequence data and genomic organisation indicate a separate origin.

Some groups of fungi (namely the ascomycetes, basidiomycetes and zygomycetes) have independently evolved the ability to trap and feed on nematodes (nematophagy). Trapping methods range from adhesive knobs on the underground hyphae to a lasso-like arrangement of cells that can constrict. Enzymes then attack the nematode's cuticle and even toxins and spiny balls or projections can be employed.