Phylum Aschelminthes

Phylum Aschelminthes (Greek ascus = sac, helminth = worm), also called Nemathelminthes (nema = thread), includes animals such as Ascaris (roundworm), Wuchereria (filarial worm) and Ancylostoma (hookworm). Most members are parasitic, though a few are free-living. The body is long, cylindrical, thread-like and circular in cross-section, which is why the group is commonly called roundworms.

These animals are triploblastic, bilaterally symmetrical, and pseudocoelomate, following the tube within tube body plan. The body is covered by a tough, resistant cuticle. The body wall contains only longitudinal muscles, with circular muscles absent.

The alimentary canal is complete, running from mouth to anus at opposite ends, with a well-developed, muscular pharynx. Excretion occurs through canals or specialised gland cells, and waste is eliminated via an excretory pore. The nervous system consists of a nerve ring with nerves radiating from it. …