Oligochaeta

Oligochaeta [‚äl·ə·gō′kēd·ə]

(invertebrate zoology)

A class of the phylum Annelida including worms that exhibit both external and internal segmentation, and setae which are not borne on parapodia.

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Oligochaeta

A class of the phylum Annelida including worms such as the earthworms. There are 21 families with over 3000 species. These animals exhibit both external and internal segmentation. They usually possess setae which are not borne on parapodia. 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 special ducts, the oviducts and sperm ducts. A clitellum is present at maturity. There is no larval stage during development.

The oligochaetes are primarily fresh-water and burrowing terrestrial animals. A few are marine and several species occur in the intertidal zone.

Oligochaetes are cylindrical, elongated animals with the anterior mouth usually overhung by a fleshy lobe, the prostomium, and the anus terminal. The body plan is that of a tube within a tube. Externally, the segments are marked by furrows. The setae or bristles are borne on most segments. Other external features are the pores of the reproductive systems opening on certain segments, the openings of the nephridia, and in many earthworms dorsal pores which open externally from the coelom. Some aquatic species have extensions of the posterior part of the body which function as gills.

The oligochaetes have been used in studies of physiology, regeneration, and metabolic gradients. Some aquatic forms are important in studies of stream pollution as indicators of organic contamination. Earthworms are important in turning over the soil and reducing vegetable material into humus. It is likely that fertile soil furnishes a suitable habitat for earthworms, rather than being a result of their activity. See Annelida