Main characteristics of Nematoda:

(i) They are bilaterally symmetrical, triploblastic, pseudocoelomate and unsegmented animals.

(ii) They have a cylindrical or flattened, worm-like body.

(iii) Sexes are separate.

(iv) Body is covered with a tough, resistant cuticle.

Examples: Ascaris (round worm) and Wuchereria bancrofti (filarial worm).