Classification of Nematoda
The order Nematoda is divided into 2 classes.
1. Secernentia/Phasmedia: Their phasmids are present. Having pore-like amphids. Excretory system present. For example – round worm (Ascaris lumbricoides).
2. Adenophores/Aphasmedia: Their phasmids are absent. Amphids are not pore-like. Caudal gland present. Eg – Whipworm (Trichinella spiralis).

[Sexual dimorphism: When the male and female members of the same species differ in size, shape and color, it is called sexual dimorphism]