A free-living transparent nematode that lives in temperate soil environments. It survives by feeding on microbes, primarily bacteria. C. elegans is an unsegmented pseudocoelomate and lacks respiratory or circulatory systems.

Most of these nematodes are hermaphrodites and a few are males (about 1 in 1000). Males have a single-lobed gonad, a vas deferens, and a tail specialized for mating, which incorporates spicules. Hermaphrodites have two ovaries, oviducts, and spermatheca, and a single uterus.

The hermaphroditic worm is considered to be a specialized form of self-fertile female, as its soma is female. The hermaphroditic germline produces male gametes first, and lays eggs through its uterus after internal fertilization.