Oligochaeta

        Arhynchobdellida 
 Haplotaxida 
 Rhynchobdellida  

The Oligochaeta or "few-bristled" worms (singular Oligochaete, IPA /ˈɒlɪgoʊˌkit/) are well-segmented Annelids, most with a spacious coelom that is used as a hydroskeleton . Their setae (chaetae) or "bristles" are generally few in number and they lack the parapodia of the polychaeta. They have external fertilization, but copulate and store sperm in a receptacle called a spermatheca . Like the leeches, they have a clitellum which secretes a "cocoon" into which both eggs and sperm are deposited and which acts as an incubator for the embryonic worms. They lack a trochophore larval stage.

This taxon contains mainly freshwater and semi-terrestrial forms, including the earthworms, the tubificids , pot worms , ice worms and many interstitial marine worms. Most are detritus feeders. Some genera are predaceous (Agriodrilus and Phagodrilus ).