A class or subclass (depending on the classification system used) of worms, including the earthworms, within the phylum Annelida. These animals exhibit both external and internal segmentation—that is, furrows in the body wall and transverse partitions (septa) dividing the coelom into chambers. They usually possess chaetae (setae), or bristles, made of chitin, which are not borne on parapodia and are few in number (compared to the Polychaeta). Oligochaetes are hermaphroditic. The gonads are few in number and situated in the anterior part of the body, the male gonads being anterior to the female gonads. The gametes are discharged through the oviducts and sperm ducts. At maturity, a portion of the body wall is swollen into a secretory area called the clitellum (illus. a). There is no larval stage during the development of oligochaetes.