Oligochaeta (/ˌɒlɪɡəˈkiːtə, -ɡoʊ-/)[1] is a subclass of soft-bodied animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms.

These worms usually have few setae (chaetae) or "bristles" on their outer body surfaces, and lack parapodia, unlike polychaeta.

They range in length from less than 0.5 mm (0.02 in) up to 2 to 3 metres (6.6 to 9.8 ft) in the 'giant' species such as the giant Gippsland earthworm (Megascolides australis) and the Mekong worm (Amynthas mekongianus).

A few species are found in trees, among damp moss and in the debris that accumulates in leaf axils and crevices; some others make their homes in the rosettes of bromeliads.

[3] The first segment, or prostomium, of oligochaetes is usually a smooth lobe or cone without sensory organs, although it is sometimes extended to form a tentacle.

The digestive tract is essentially a tube running the length of the body, but has a powerful muscular pharynx immediately behind the mouth cavity.

The remainder of the digestive tract may include a crop for storage of food, and a gizzard for grinding it up, although these are not present in all species.

A number of yellowish chloragogen cells surround the intestine and the dorsal blood vessel, forming a tissue that functions in a similar fashion to the vertebrate liver.

Oligochaetes can taste their surroundings using chemoreceptors located in tubercles across their body, and their skin is also supplied with numerous free nerve endings that presumably contribute to their sense of touch.

[5] Movement and burrowing of earthworms is performed by peristalsis, with the alternation of contraction and relaxation of the circular and longitudinal muscles.

[3] Burrowing is performed by forcing the front end of the worm into a crevice and widening the gap by body expansion.

Members of the Naididae reproduce asexually, primarily by paratomy, in which the body breaks into two pieces after the "pregeneration" of certain anterior structures by the posterior portion.

[7] The name Protoscolex was given to a genus of segmented worms without bristles found in the Upper Ordovician of Kentucky, United States.

Because of its ability to colonise new areas and become dominant, the Lumbricidae has followed humans round the world and displaced many native species of earthworm.

[8] An early but now outdated classification system was to divide the oligochaetes into "Megadrili", the larger terrestrial species, and "Microdili", the smaller, mostly aquatic ones.