General characteristics of phylum Aschelminthes (Nemathelminthes)

  • They are cylindrical, unsegmented and triploblastic animals commonly known as round worms.
  • They are mostly aquatic, but few are terrestrial.
  • They are free-living as well as parasitic.
  • Body is pseudo-coelomate (false coelom), derived from embryonic blastocoel and they lack peritoneal lining.
  • Body is bilaterally symmetrical. A distinct head is absent, but internal cephalization is present.
  • They don’t have the power of regeneration.
  • Body is covered with a thick cuticle (syncytial) which is flexible, non-living and made up of scleroprotein.
  1. Body systems:
  • Digestive system is complete with mouth and anus.
  • Pharynx is muscular and highly specialized. Cloaca occurs in some forms.
  • Circulatory system and respiratory system are absent.
  • Protonephridia (in some forms) or Rennet cells (‘H’ shaped) or Giant cells are the excretory organs which also help in
  • Sense organs are in the form of pits, papillae, bristles and eye spots. The papillae are believed to be tango-receptors (sensitive to touch) and the amphids (pits) are chemo or olfacto-receptors.
  1. Reproduction:
  • They show sexual dimorphism (sexes are separate).
  • Male is smaller than the female. In male, testis is single (monodelphic) whereas in female, a pair of ovary are present (didelphic).
  • Male possesses a pair of pineal setae or spicules. The testis is thread like and the sperm is amoeboid and without a tail.
  • Eggs are embryonated or mammilated with chitinous shell.

Phylum Aschelminthes or Nemathelminthes is divided into following five classes.

Class: Nematoda

  • Commonly known as roundworms, they are aquatic or terrestrial.
  • They 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.
  • They lack respiratory and circulatory system.
  • e.g. Ascaris, Rhabtites, Oxyrius etc.

Also see Differences between Male and Female Ascaris

Class: Rotifera

  • They are microscopic and aquatic animals.
  • Eye spots and antennae act as sensory organs.
  • They are 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.
  • e.g. Limnias, Rotara (Rotifera) etc.

Class: Nematomorpha

  • They are 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.
  • They lack circulatory, respiratory and excretory system.
  • Digestive system is complete.
  • e.g. Paragprdius, Nectonema etc.

Class: Gastrotricha

  • They are microscopic and may be fresh water forms or marine.
  • Body is covered by cuticle which bears spines.
  • Excretory system consists of two protonephridia.
  • Pharynx is tri-radiate and muscular.
  • The posterior end of the body is forked.
  • e.g. Macrodasys, Chaetonotus etc.

Class: Kinorhyncha

  • They are marine, microscopic and worm like forms.
  • Body is superficially segmented.
  • Body surface is spherical and consists of spiny cuticle.
  • e.g. Trachydemus, Echinoderes etc.

General characteristics of phylum Aschelminthes (Nemathelminthes)