The body of the aschelminthes is circular in cross-section, hence, the name roundworms. These are the first group of animlas has a tubular gut with a mouth and anus. They may be free-living, aquatic and terrestrial or parasitic in plants and animals. Roundworms have organ-system level of body organisation. They are bilaterally symmetrical, triploblastic and pseudocoelomate animals. Alimentary canal is complete with a welldeveloped muscular pharynx. An excretory tube removes body wastes from the body cavity through the excretory pore. Sexes are separate (dioecious), i.e., males and females are distinct. Often females are longer than males. Fertilisation is internal and development may be direct (the young ones resemble the adult) or indirect.
Examples  of Aschelminthes: 
Ascaris lumbricoids (Round Worm):
  • Ascaris lumbricoids (Round Worm): It is common round worm found in intestine of children. The diseases caused by ascarias is called ascariasis. It is characterised by malnutrition, anaemia and intestinal obstruction. 
  • Wuchereria bancrofti (Filaria worm): It is an endoparasite living in the lympahatic system of man causing disease elephantiasis.
  • Ancylostoma duodenale (Hookworm): It is an intestinal parasite of man feeding on blood and tissues. The disease caused by the hook worm is called ancylostomiasis.
Ancylostoma duodenale (Hookworm)
  • Dracunculus
  • Rhabditis: They are free living small nematodes inhabiting the moist soil feeding on decaying matter.
  • Enterobius
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