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Biology11 Notes for Biology Notes

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Phylum : Nematoda

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

Characters:

  • Body form:
    They have elongated, cylindrical and unsegmented body like threads, so often called thread worms.

  • Habitat:
    Most of them are parasites while few are free living found in soil or water.

  • Cellularity:
    They are multicellular organisms with organ-system grade of body organization.

  • Coelom:
    They are pseudocoelomates.

  • Nutrition:
    They exhibit parasitic or holozoic modes of nutrition.

  • Digestive system:
    Digestion is extracellular.

  • Alimentary canal:
    Alimentary canal is complete and straight with mouth, pharynx, intestine and anus.

  • Excretion:
    Excretory system is absent. Gaseous exchange occurs through the general body surface.

  • Circulatory system:
    Blood vascular system is absent. Transport of materials occurs with the help of hydrolymph or pseudocoelomic fluid.

  • Reproduction:
    They are usually dioceious i.e. sexes are separate with sexual dimorphism. No asexual reproduction occurs.

  • Fertilization:
    It is internal and cross.

  • Nervous system:
    Nervous system consists of a nerve ring and the anterior part of the alimentary canal with longitudinal nerve cords.

  • Germ layers:
    They are triploblastic.

  • Segmentation:
    Body is unsegmented but maybe wrinkles.

  • Cephalsation:
    Body lacks the distinct head. However, mouth is present in the anterior system.

  • Sensory Organs:
    Amphids and phasmids are the sensory organs present in them.

  • 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:

  • Most of them are free living forms.
  • Phasmids are present.
  • Amphids are of various types.
  • Excretory organs are either reduced or absent E.g. Enoplus, Mermis etc.

2. Class 2. Phasmidia:

  • Most of them are parasitic forms.
  • Phasmids are present.
  • Amphids are pore like.
  • Excretory organs are developed with paired lateral canals. E.g. Ascarics lumbricoidesetc.

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