Oligochaeta is a subclass in the biological phylum Annelida and includes various earthworms. Specifically, it contains the terrestrial megadrile earthworms, and freshwater or semi-terrestrial microdrile forms including the tubificids, pot worms and ice worms (Enchytraeidae), blackworms (Lumbriculidae) and several interstitial marine worms. These worms usually have few setae, and lack a parapodia, unlike polychaeta.
Oligochaeta are well-segmented annelids and most have a spacious body cavity (coelom) that is used as a hydroskeleton. Their length ranges from centimetres up to 2-3 metres such as the Australian native giant Gippsland earthworm. Unlike Polychaetes, Oligochaetes do not have parapodia (appendages used for locomotion) or sensory appendages on their prostomium (the most anterior segment of the head). Oligochaetes have four bundles of chaete per segment (two lateral, one lateroventral, and one dorsoventral). Each bundle of chaete consists of 1 to 25 individual chaete. The Oligochaetes can extend or retract their chaete by protractor and retractor muscles. Generally, when the longitudinal muscles of the worm are contracted, the chaete are retracted, and when the circular muscles are contracted, the chaete are extended. Chaete action permits grasping of soil during peristaltic burrowing. Most Oligochaetes are detritus feeders, some genera are predaceous.
