Nemathelminthes is composed of two words i.e. nematodes which mean thread and helminths which mean worm. Gegenbaur coined the term nemathelminths in 1859.
General characteristic features of Nemathelminthes:
- They are cylindrical, elongated, unsegmented and worm-like.
- They are mostly parasitic and some are free living found in soil and water.
- They are triploblastic.
- They are bilaterally symmetrical.
- Coelom is pseudo coelom type i.e body cavity s lined by mesodermal layer.
- Nutrition is a holozoic type.
- The alimentary canal is complete and straight. Mouth is terminal surrounded by lips.
- The respiratory system is absent. Respiration is anaerobic type in parasitic forms only.
- Transport of fluid nutrients takes place with the help of hydro lymph or pseudocoelomate fluid.
- The body is covered with the thick non-living cuticle.
- It consists of lateral excretory ducts which collect the excretory substances and expel out through the anus.
- It consists of nerve ring ana a longitudinal nerve cord. sense organs are poorly developed.
- They are unisexual or dioecious and sexual reproduction takes place.
- Fertilization is internal.
- Development may be direct or indirect with some larval stages.
- The body has not the distinct head.
It is classified into two classes on the basis of presence and absence of amphids.
- Caudal sense organ(phasmid) is present.
- Most of them are parasitic forms.
- Example: Ascaris, Trichuris.
- Caudal sense organ is absent.
- Most of them are free-living forms.
- Amphids or anterior sensory organs are present.
- Example: Dorylaimus, Rotifers.