With around 10,000 species the Oligochaetes make up about a half of the phylum Annelida. These worms usually have few chaetae or "bristles" on the outer body surface, unlike polychaetes. The best known is the garden earthworm.
Oligochaetes are segmented worms and most have a spacious body cavity (coelom) which is used as a hydroskeleton. They range in length from less than 0.5 millimetres (0.020 in) up to 2 to 3 metres (6.6 to 9.8 ft) in the 'giant' species such as the Giant Gippsland Earthworm and the Mekong Worm Amynthas mekongianus.