Abstract The origin and evolution of Clitellata are discussed by inductive method, taking into account the morphological, ecological and embryonic characters of the microdriles, hirudineans and megadriles. Special attention is focused on the vascular system and on the female and copulatory apparatuses. The two marine littoral families Capilloventridae and Randiellidae, evidently primitive, contribute with their morphology and anatomy to reconstructing the physiognomy of ancestral Clitellata, but their relationships to the other microdrile families are still obscure. The Naididae and the Enchytraeidae also remain by themselves, as do the two interrelated families Tubificidae and Phreodrilidae. The primitive looking Haplotaxidae appear to be related to the Lum‐briculidae and the Alluroididae; the latter are allied to the Monili‐gastridae. Lumping together Branchiobdellae, Acanthobdellae and Euhirudinea seems to be justified, and so does suggesting the derivation of the hirudineans from the Naididae. The thr...