These are pseudocoelomate worms, which are commonly called as round worms. They are mostly aquatic some are terrestrial and parasitic in their habitat.
The body of the worm is bilaterally symmetrical, unsegmented and covered by a cuticle. These are triploblastic with organ-system grade of organization. Sexes are separate and fertilized eggs have a thick wall and can survive in adverse condition eg. Ascaris, (roundworm), Ancylostoma (Hookworm), Wucheraria bancrofti (Filarial worm) etc.
The annelids are triploblastic, eucoelomate worms whose body is divided into segments. The outer covering of the body is covered by cuticle, which is secreted by the underlying epidermis.
Some groups under this phylum possess locomotory organs, which are arranged segmentally, they are paired seta and chaetae.
Respiration is through body surface. It possesses blood vascular system, which is of closed type. Certain classes, which include earthworms and leeches are hermaphrodites. But in polychaetes the sexes are separate. Eg. Neries (sandworm), Aphrodite (sea-mouse), Pheretima (earthworm) Hirudinaria (leech) etc.