The word Nematoda is formed from the Greek words nematos meaning thread, eidos meaning shape and helminth meaning worm. Those organisms whose bodies are cylindrical, cuticular, segmentless, bilaterally symmetrical and have irregular cilia are called nematodes. The animals of this phase are known as threadworms or roundworms. The structure of the body is like a ‘tube within a tube’. They cause ascariasis, elephantiasis, conjunctivitis, abdominal pain, trichuriasis etc. in human body. They play an important role in the nitrogen cycle. 90% of species under the sea are nematodes. The number of species in this category is about 25,033. Previously they were known as Nemathelminthes or Aschelminthes. The order Nemathelminthes was named by scientist Gagenbaur (1851).