On the basis of number and presence or absence of setae, phylum Annelida is divided into three classes.
Class 1: Polychaeta: (a) They have well developed parapodia with numerous setae.
(b) The development is indirect, undergoing metamorphosis with a free swimming trochophore larva.
(c) Examples : Nereis, Aphrodite (Sea mouse),Chaetopterus (Paddle worm), Sabella.
Class 2: Oligochaeta : (a) The body has simple and few setae. There is ootheca formation in which ova get fertilized.
(b) There is no metamorphosis e.g., Pheretima, (Indian earthworm), Lumbricus, (European earthworm), Tubifex.
Class 3 : Hirudinea : (a) They are ectoparasitic leeches. There are no parapodia or setae.
(b) Beneath the muscular layer of the body wall and surrounding the alimentary canal is found botryoidal tissue (peculiar connective tissue).
(c) Leeches have haemocoelomic system.
(d) Examples : Hirudinaria,Pontobdella, Hirudo, Acanthobdella.