General characteristics and classification of phylum Nematodes/ Nemathelminthes/ Aschelminthes

General characteristics and classification of phylum nemathelminthes/ aschelminthes/ nematodes

General Characteristics of phylum Nematodes/ Nemathelminthes/ Aschelminthes:

  1. Nemathelminthes or nematodes are mostly aquatic, but some may also be terrestrial.
  2. The phylum includes both free-living as well as parasitic organisms.
  3. They have unsegmented, bilaterally symmetrical and triploblastic body.
  4. A well-defined head is absent in them.
  5. Their whole body is covered with thick, non-cellular and flexible cuticle.
  6. Most of their phyla bear adhesive organs.
  7. Cilia and muscles are the locomotory organs in them.
  8. Their body cavity may either be pseudocoelomate or some may be acoelomate.
  9. They bear non-muscular, straight gut with a mouth and an anus.
  10. They have muscular and highly specialised pharynx.
  11. Respiratory and blood vascular systems are absent in them.
  12. Their nervous system consists of circumenteric nerve ring with longitudinal nerves.
  13. Excretory organs in them are protonephridia and flame cell.
  14. They are generally dioecious.
  15. Some of their phyla exhibit determinate cleavage.
  16. Their fertilisation is internal.
  17. They doesn’t have any larval stage in their life cycle.
  18. Sense organs in them are poorly developed which includes phasmids, amphids, bristles, pepeline.
  19. They are ubiquitous in nature.
  20. They reproduces sexually by the fusion of gametes.

Classification of phylum Nematodes/ Nemathelminthes/ Aschelminthes:

Based on the presence or absence of phasmids (sensory organ) nematodes are classified into two types along with their examples, as follows:


Aphasmidia/ Adenophoria

  1. Phasmids are absent in them.
  2. They doesn’t bear any excretory canal.
  3. Amphids are usually present behind their lips.
  4. They are mostly found in free living condition, but some are parasitic too.
  5. Free living species among them are usually marine inhabitants.
  6. Examples: Enoplus, Xiphinema and Mermis.

CLASS – ll

Phasmidia/ Secernentea

  1. Phasmids are present in them.
  2. They bear excretory canals.
  3. Amphids are usually present in their lateral lips.
  4. They are mostly parasitic in nature (thus they can also be called as parasitic organisms).
  5. Free living species in them are soil inhabitants.
  6. Examples: Encylostoma, Euthereria, Ascaris

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