- The body of the aschelminthes is circular in cross-section, hence, the name roundworms.
- Roundworms have organ-system level of body organization, bilaterally symmetrical, they are triploblastic and pseudocoelomate animals.
- Alimentary canal is complete with a well-developed muscular pharynx.
- An excretory tube removes body wastes through the excretory pore.
- They are dioecious, but often females are longer than males.
- Fertilization is internal and development may be direct or indirect.
Examples- Ascaris (Round worm), Wuchereria (Filaria worm).
Fig. Round worm