Unsegmented worms with elongated rounded body pointed at both ends; mostly free-living but some are parasitic WordNet 3.0 & Open English WordNet

The nematodes roundworms or eelworms, constitute the phylum Nematoda. Wikipedia

One of the most common phyla of animals, with over 80,000 different described species (of which over 15,000 are parasitic). They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as Antarctica and oceanic trenches. OmegaWiki

A worm of the large phylum Nematoda, such as a roundworm or threadworm. Wiktionary

Small invertebrate animal. Wiktionary (translation)

An invertebrate animal of the phylum Nematoda and other similar phyla. Many species of roundworms are parasites. Wiktionary