The phylum includes bilaterally symmetrical, triploblastic, pseudocoelomate animals with organ system grade of organisation. They are circular in cross-section.

They are free living, aquatic or terrestrial or parasitic (on plants or animals).

Excretory system involves an excretory cell (a large giant H-shaped cell) called Renette cell (multinucleated).

They have tube-within tube type of body plan evolved along Protostomic evolutionary line.

Sexes separate, show sexual dimorphism, fertilisation internal, development direct or indirect.


On the basis of caudal receptor or phasmids, Aschelminthes can be divided into 2 classes-

 Important classes of this phylum are: 

Class : Nematoda. 
Examples: Ascaris, Wuchereria, Enterobius, Trichuris. Rhabditis is a free living roundworm. 
Class : Rotifera. 
Examples: Wheel worms