Nemathelminthes

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.

Classification:

It is classified into two classes on the basis of presence and absence of amphids.

Phasmida:

• Caudal sense organ(phasmid) is present.
• Most of them are parasitic forms.
• Example: Ascaris, Trichuris.

Aphasmida:

• Caudal sense organ is absent.
• Most of them are free-living forms.
• Amphids or anterior sensory organs are present.
• Example: Dorylaimus, Rotifers.