1. Non-marine Nematoda ( Eelworms)
This group encompasses many hundreds of species; those that are free-living are usually less than 1mm long whilst some that are parasitic are very long. Many spend only part of their lives as parasites. All lifestyles include some species that are serious pests of humans, directly or indirectly by affecting livestock or crops. Most free-living forms are restricted to the soil or the bottom of standing water, but one, the Thunder Worm Mermis nigrescens (the largest species at over 200mm) climbs vegetation attracting insects which its young parasitise.
2. Nematomorpha ( Horsehair, Hair or Gordian Worms)
There are only four British species, the young of which parasitize insects whilst the adult are freeliving in freshwater or damp soils. They often attain lengths of over 200mm.