Nematoda : Characteristics, classification and example of Nematoda

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).

Characteristics of Nematoda
1. Body long, cylindrical, bilaterally symmetrical, narrow at both ends and wide in the middle. Known as roundworms.
2. Body unsegmented, ciliated and covered with thick cuticle.
3. The body is flexible and made of elastin.
4. Abnormal cells are present in the body.
5. They are unisexual and sexually dimorphic.
6. The alimentary canal is complete, straight, unbranched and extends from the mouth to the anus. So the structure of the body looks like a tube inside a tube.
7. The mouth opening is surrounded by characteristic mucus.
8. Respiratory and circulatory systems are absent.
9. A rhabditiform or microfilariae larval stage exists in the life cycle.
10. They are terrestrial or aquatic, free-living or parasitic.

[Key words to remember characteristics : Dimorphism, tube-within-tube, ostra, rhabditiform, microfilaria]

Classification of Nematoda
The order Nematoda is divided into 2 classes.
1. Secernentia/Phasmedia: Their phasmids are present. Having pore-like amphids. Excretory system present. For example – round worm (Ascaris lumbricoides).
2. Adenophores/Aphasmedia: Their phasmids are absent. Amphids are not pore-like. Caudal gland present. Eg – Whipworm (Trichinella spiralis).

[Sexual dimorphism: When the male and female members of the same species differ in size, shape and color, it is called sexual dimorphism]

Some animals of Nematoda
Roundworm – Ascaris lumbricoides
Eye worm – Loa loa
Hook worm – Ancylostoma doudenale
Hook worm – Necator americanus
Mealworm – Enterobius vermicularis
Ringworm- Wuchereria bancroftis
Whipworm – Trichinella spiralis
Trichuris trichiura
Dioctophyme – Dioctophyme renale
Free-living roundworm – Caenorhabditis elegans