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Phylum : Nematoda

It is made up of 2 Greek words, nema = thread & edios = form
Characters:

1. Body form:
   They have elongated, cylindrical and unsegmented body like threads, so often called thread worms.
2. Habitat:
   Most of them are parasites while few are free living found in soil or water.
3. Cellularity:
   They are multicellular organisms with organ-system grade of body organization.
4. Coelom:
   They are pseudocoelomates.
5. Nutrition:
   They exhibit parasitic or holozoic modes of nutrition.
6. Digestive system:
   Digestion is extracellular.
7. Alimentary canal:
   Alimentary canal is complete and straight with mouth, pharynx, intestine and anus.
8. Excretion:
   Excretory system is absent. Gaseous exchange occurs through the general body surface.
9. Circulatory system:
   Blood vascular system is absent. Transport of materials occurs with the help of hydrolymph or pseudocoelomic fluid.
10. Reproduction:
    They are usually dioceious i.e. sexes are separate with sexual dimorphism. No asexual reproduction occurs.
11. Fertilization:
    It is internal and cross.
12. Nervous system:
    Nervous system consists of a nerve ring and the anterior part of the alimentary canal with longitudinal nerve cords.
13. Germ layers:
    They are triploblastic.
14. Segmentation:
    Body is unsegmented but maybe wrinkles.
15. Cephalsation:
    Body lacks the distinct head. However, mouth is present in the anterior system.
16. Sensory Organs:
    Amphids and phasmids are the sensory organs present in them.
17. Development:
    Development is direct without any larval form.

On the basis of presence or absence of Phasmids (caudal sensory organ), phylum nematode is futher classified as :

1. Class 1. Aphasmidia:
   1. Most of them are free living forms.
   2. Phasmids are present.
   3. Amphids are of various types.
   4. Excretory organs are either reduced or absent

E.g. Enoplus, Mermis etc.

1. Class 2. Phasmidia:
   1. Most of them are parasitic forms.
   2. Phasmids are present.
   3. Amphids are pore like.
   4. Excretory organs are developed with paired lateral canals.

E.g. Ascarics lumbricoidesetc.

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