The oligochaetes include earthworms and a group of related, mostly freshwater, species of annelids. Over 3000 species are known. Oligochaetes all bear setae, but the number of setae present is much smaller than in polychaetes (their name means "few hairs"). Oligochaetes also differ from polychaetes in that they possess permanent sex organs. Most are hermaphroditic, and development is direct, resulting in young that resemble tiny adults. Aquatic forms may have gills.
Oligochaetes feed primarily on detritus and algae. Earthworms cycle huge quantities of soil through their guts, a process that speeds the turnover of nutrients in soil and increases productivity. They also help to aerate soil. Aquatic oligochaetes are important food for fishes and larger invertebrates. A few are ectoparastic.
Phil Myers (author), Museum of Zoology, University of Michigan-Ann Arbor.