Oligochaetology is the study of worms - specifically, the Class Oligochaeta within the Phylum Annelida. Other classes in the Phylum Annelida, the true segmented worms, include the Acanthobdellae (bristle worms), Aphononeura (suction-feeding worms), Branchiobdellae (crayfish worms), Hirudinea (leeches), and Polychaeta (sand worms, tube worms, and clam worms). Over 15,000 species of worms have been described worldwide; we now recognize 2,450 species in North America north of Mexico.

Worms occur in virtually all habitats where water is present, even in areas that are only slightly moist. Numerous species also occur exclusively on land. Most annelids are free-living, but many species are parasitic, mutualistic, or commensal during part of, or throughout, their life cycle.

Worms are an important and often dominant group in aquatic systems, providing a valuable food source for many other aquatic organisms; 131 species presently are known to occur in Illinois. As early as Aristotle, aquatic worms have been recognized for their ability to thrive in organically polluted waters, often forming dense colonies that resemble red waving carpets. Recent works have noted the presence of aquatic worms in almost every habitat that is associated with water, including pristine springs, streams, wells, seeps, and lakes, as well as industrially polluted harbors, large rivers, and waste retention ponds.

Earthworms, although numbering only about 30 species in Illinois, play an important role in the decomposition of organic matter, mineral cycling, and the aeration, drainage, and root penetration of the soil; through this activity, they also provide suitable habitat for smaller soil fauna, particularly micro-organisms. It has been estimated that earthworms can 'move' up to 18 tons of soil per acre each year. Abundance estimates of earthworms have been as high as three million per acre.

The INHS Annelida Collection is perhaps the largest state collection of freshwater oligochaetes in the country, holding more than 315,000 specimens (over 6,300 lots, or collections). Approximately 212,000 specimens are permanently mounted on microscope slides; the remaining specimens are stored in alcohol in vials and jars. With the exception of one monospecific order - the Acanthobdellida, or bristle worms - the collection includes representatives of the other major Annelida groups: Aphanoneura (the head-crawling, or suction-feeding worms, including representatives of one family, Aeolosomatidae); Branchiobdellida (the crayfish worms, including representatives of the families Bdellodrilidae, Cambarincolidae, and Xironodrilidae); Hirudinea (the leeches, including representatives of families Haemopidae, Hirudinidae, Erpobdellidae, Glossiphoniidae, and Piscicolidae); the Oligochaeta, with representatives of the aquatic microdrile worms (families Enchytraeidae, Haplotaxidae, Lumbriculidae, Naididae, Opistocystidae, and Tubificidae), and the terrestrial megadrile worms (families Acanthodrilidae, Komarekionidae, Lumbricidae, Megascolecidae, and Sparganophilidae); and Polychaeta (sand worms, tube worms, or clam worms - primarily marine) including a few representatives of the families Capitellidae and Sabellidae. The INHS Annelida Collection includes representatives of many worm species that have limited known distributions in North America. Currently, none of the annelids known or thought likely to occur in Illinois is listed as endangered or threatened by either the federal government or by the State of Illinois, nor are there any under consideration for such listing.

The geographic scope of the INHS Annelida Collection is about 74% from Illinois; 25% from elsewhere in North America (collections from the states of Alabama, Alaska, Arizona, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Georgia, Hawaii, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Vermont, Virginia, Washington, West Virginia, Wisconsin, Wyoming, and the District of Columbia, and from the Canadian provinces of Alberta, British Columbia, Manitoba, Ontario, and Prince Edward Island); and 1% from other countries, including the Bahamas, Colombia, Ecuador, Germany, India, Jamaica, Lesser Antilles (Antigua, Barbados, Jamaica, Granada, Nexis, St. Lucia), The Netherlands, Panama, Peru, Puerto Rico, Sweden, and Venezuela.