Phylum : Aschelminthes/Nemathelminthes (round or thread worms), its classification and characteristics

  • multicellular, triploblastic, bilaterally symmetrical with system grade of body organization.
  • tube within tube body plan.
  • mostly parasitic, few are free living found in sea water, fresh water or damp soil.
  • body is elongated, cylindrical, unsegmented without lateral appendages.
  • body is covered by cuticle, formed by syncytial epidermis.
  • body cavity is pseudocoel. Thus, they are pseudocoelomate.

* coelom (true body cavity)- present between body wall and gut wall formed by splitting of mesoderm.

* pseudocoel– cavity present in body is persistent blastocoel.

  • complete alimentary canal with both mouth and anus.
  • respiratory and circulatory organs are absent.
  • excretion occurs through proto-nephridia or H- shaped renette cells.
  • unisexual (dioecious). They have sexual dimorphism.
  • nutrition is holozoic or parasitic.
  • digestion is extracellular.
  • development is mostly indirect.
  • mostly small sized. Some are microscopic, a meter or more long.
  • sense organs are present in the form of papillae which are of two types. They are amphids (mouth) and phasmids (Anus).

Biology - Ctenophora,Aschelminthes & Platyhelminthes | askIITians

                                                                             Image source: askiitians



Roundworms are classified into 5 classes. They are:

Class 1: Nematoda

Commonly known as roundworms, they are aquatic or terrestrial.

  • may be free-living or parasitic.
  • body wall is with syncytial cuticle and longitudinal muscles in four bands.
  • digestive system is complete, with muscular pharynx and glands.
  • lack respiratory and circulatory system.
  • sexes are generally separate.
  • examples: Ascaris, Rhabtites, Oxyrius, Ancylostoma, etc.

Describe on Phylum Aschelminthes - QS Study                                                                                        Image source: qsstudy

Class 2: Rotifera

  • microscopic and aquatic animals.
  • eye spots and antennae act as sensory organs.
  • commonly called wheel animalcules having a trunk and a tail.
  • body wall is thickened into plates or lorica.
  • anteriorly, they have a ciliated trochial disc.
  • parthenogenesis usually occurs, dioecious.
  • examples: Limnias, Rotara (Rotifera) etc.

Class 3: Nematomorpha

  • commonly known as hair worms, found in spring (aquatic).
  • body is unsegmented and covered with a thick cuticle.
  • pseudocoel is present which is filled with parenchyma.
  • lack circulatory, respiratory and excretory system.
  • digestive system is complete.
  • examples: Paragordius, Nectonema etc.

Class 4: Gastrotricha

  • microscopic and may be fresh water forms or marine.
  • body is covered by cuticle which bears spines.
  • body long and ventrally flattened.
  • mouth is surrounded by bristles.
  • ventral surface bears cilia which provide gliding movement.
  • excretory system consists of two protonephridia.
  • pharynx is tri-radiate and muscular.
  • posterior end of the body is forked.
  • dioecious or monoecious.
  • examples:  Macrodasys, Chaetonotus etc.

Class 5: Kinorhyncha

  • marine, microscopic and worm like forms.
  • body is superficially segmented.
  • dioecious.
  • nerve ring and ventral cord present.
  • alimentary canal with salivary gland.
  • body surface is spherical and consists of spiny cuticle.
  • development includes several larval forms.
  • examples: TrachydemusEchinoderes, etc.





Phylum : Aschelminthes/Nemathelminthes (round or thread worms), its classification and characteristics