nematode

Any of a group of unsegmented worms that are pointed at both ends, with a tough, smooth outer skin. They include many free-living species found in soil and water, including the sea, but a large number are parasites, such as the roundworms and pinworms that live in humans, or the eelworms that attack plant roots. They differ from flatworms in that they have two openings to the gut (a mouth and an anus). The group includes Caenorhabditis elegans which is a model genetic organism and the first multicellular animal to have its complete genome sequenced. (Phylum Nematoda.)

Most nematode species are found in deep-sea sediment. Around 13,000 species are known, but a 1995 study by the Natural History Museum, London, based on the analysis of sediment from 17 seabed sites worldwide, estimated that nematodes may make up as much as 75% of all species, with there being an estimated 100 million species. Some are anhydrobiotic, which means they can survive becoming dehydrated, entering a state of suspended animation until they are rehydrated.