Phylum Nemathelminthes / Aschelminthes : General Characteristics and Classification

  • The phylum ‘Nemathelminthes’ (Gr; nematos = thread ; helminths = worm) or Nematoda comprises the roundworms.
  • Triploblastic, bilaterally symmetrical, vermiform, unsegmented cylindrical true worms with pseudocoel (i.e pseudocoelomate or blastocoelomate)
  • Pseudocoelom / False coelom derived from embryonic blastocoel.
  • Tube within a tube body plan, organ-system grade of body organization.
  • Body round in cross section and covered with transparent  cuticle composed of scleroprotein.
  • A syncytial epidermis generally without cilia.
  • Body wall has only longitudinal muscles.
  • No distinct head, digestive tract complete usually surrounded by lips bearing sense organs.
  • Alimentary canal of roundworms have both mouth and anus.
  • Both respiratory and circulatory system are absent.
  •  Excretory system comprised of one or two renette cells or protonephridia.
  • Nervous system consists of circumpharyngeal nerve ring and six longitudinal nerve cords.
  • Presence of sensory papillae. (Amphids : in mouth, Phasmids : in anus) 
  • All are dioecious ; i.e. sexes are separate with sexual dimorphism. [1st unisexual phylum]
  • Eggs with chitinous shell, cleavage determinate and spiral.
  • Larval forms are : Rhabditiform, Filariform, Microfilariae etc.
  • Classification : 5 classes [Rotifera, Gastrotricha, Kinorhyncha, Nematomorpha and Nematoda]


  1. Ascaris lumbricoides (Round worm)
  2. Wuchheria bancrofti (Filarial worm)
  3. Enterobius vermicularis (pin / thread / seat worm / churna)
  4. Necator americana (Hook worm)
  5. Trichinella spiralis (Trichina worm)
  6. Loa loa (eye worm)
  7. Dracunculus medinensis (Guinea worm)

Binod G C

I'm Binod G C (MSc), a PhD candidate in cell and molecular biology who works as a biology educator and enjoys scientific blogging. My proclivity for blogging is intended to make notes and study materials more accessible to students.

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