Saturday, June 30, 2012

Class XI, BIOLOGY, "Phylum Aschelminthes (Nematoda/Round worm)"

Phylum Aschelminthes (Nematoda/Round worm)

  • Nematoda have a very wide distribution and they seem to have mastered almost every habitat.
  • Free living nematodes are found in the sea, fresh water or in the soil in all kinds of environment.
  • There are also many Parasitic nematodes found in all groups of Plants and animals.
  • The Saprophagous species live in decomposing plant and animal bodies and in rotting fruits.
They have a bilaterally symmetrical, cylindrical body, glistening smooth surface. They are triploblastic.
  • They show no trace of segmentation.
  • Most of the free living nematodes are less then a millimeter length.
  • Some of the parasitic species attain a length of several meters e.g. Guinea worm (Dracunculus medinensis).
  • They are usually long, round, tapered at both ends showing very little morphological diversity from species to species.
  • The mouth of nematodes is modified for various modes of feeding such as cutting, tearing, piercing and sucking fluids from the host.
  • Body is covered by cuticle, which moults only during the period of growth.
  • The organs are packed in parenchyma when young, but later on it disappears in adult. So that organs lie in a fluid filled cavity. This cavity is termed as PSEUDOCOEL and it has not peritoneum.
  • Muscles are only longitudinal.
  • Excretory system has no flame cells.
  • Alimentary canal is straight with ectodermal fore and hind gut and an endodermal mid gut.
  • Sexes are generally separate.
  • Gonades are tubular and continues with their ducts.
  • Female organs are usually paired and open by vulva.
  • Male organs are single and open into a cloaca.
  • The life cycle of Parasitic species involves one, two or more hosts
Ascaris (Round worms), Hookworms and Thread worms etc.


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