Phylum Aschelminthes or Nemathelminthes
(Greek; nema = thread; helmins = worm)
In phylum Aschelminthes, the organisms are also called roundworms because the body is circular in cross-section. These organisms are endoparasites. They are cylindrical, unsegmented, and triploblastic animals. The roundworms are mostly parasitic and cause diseases in plants, animals, and human beings.
Characteristics Features of Aschelminthes/ Nematoda
They are bilaterally symmetrical, triploblastic, and unsegmented pseudocoelomates. The body is slender, vermiform, and rounded, or cylindrical in shape.
- Habitat– These animals exist as free-living, aquatic, and terrestrial or may be found as a parasite in plants and animals.
- Level of organization– In Nematods the organ-system level of body organization is found.
- Symmetry– They possess a bilaterally symmetrical body. the distinct head is absent in Aschelminthes.
- Germinal layer– Triploblastic animals
- The body cavity or Coelom– These animals are pseudo-coelomate.
Alimentary canal in nematodes:
- The alimentary canal is complete. The alimentary canal is straight and with two openings, the mouth and the anus, and a muscular pharynx.
- The mouth is bounded by three lips which have chemoreceptors, called amphids.
Excretion– Waste products are removed from the body cavity by an excretory tube through the excretory pore. In some forms protonephridia rennet cells or giant cells as excretory organs.
- Circulatory and respiratory system: absent.
- The sense organs are in the form of pits, papillae, bristles, and eyespots.
Cuticle: The body wall is highly muscular and is covered with a thick cuticle i.e. syncytial, it is flexible, non-living, and made up of protein (scleroprotein)
Reproduction– sexes are separate- dioecious
- Dioecious- males and females are distinct. Females are often longer than males.
- Fertilization takes place internally.
- The development may be direct (the young ones resemble the adults) or indirect.
Examples of Aschelminthes:
Some examples of Aschelminthes are Ascaris (roundworm), Wuchereria (Filarial worm), Ancylostoma (Hookworm), Brachiomus, Colotheca, Urodasys, Macrodasys, Echinoderes, Centroderes, etc.
Classification of Phylum Aschelminthes/ Nemathelmminthes
- The members are endoparasites
- The body is Bilaterally symmetrical and vermiform.
- A complex cuticle covers the body.
- Examples: Ascaris (roundworm), Ancyclostomata (hookworm), Enterobius (pinworm)
- The organisms are microscopic aquatic.
- sexes are separate
- Excretion through protonephridia tubes.
- Example: Brachiomus, Colotheca
- They are free-living aquatic organisms.
- Sexes are separate or united
- Example: Urodasys, Macrodasys
- These are marine and microscopic organisms.
- Internally segmented body
- separate sexes
- Development is indirect (with the larval form)
- Example: Echinoderes, Centroderes
- The body of these organisms is slender and unsegmented.
- Also called hairworm
- Sexes are found separate.
- example: Nectonema, Gordius
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