Eelworms

• Threadworms
• Unsegmented worms roundworms

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Nematoda \Nem`a*to"da\, prop. n. pl. (Zo["o]l.) A phylum of worms, having a long, round, and generally smooth body; the roundworms. They are mostly parasites, in plants and animals, but some are free-living in soil or water. Also called Nematoidea .

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
a class of worms, Modern Latin compound of nemat- "thread" (from Greek nema , genitive nematos "thread," from stem of nein "to spin;" see needle (n.)) + -odes "like, of the nature of" (see -oid ).