The segmented worms or annelids (Annelida) include earthworms, polychaete worms, and leeches. All members of the group are to some extent segmented, in other words, made up of segments that are formed by subdivisions that partially transect the body cavity. Members of the Phylum Annelida can be found throughout the world, in marine, freshwater, and terrestrial environments. Ecologically, they range from passive filter feeders to voracious and active predators.

The 2008 IUCN red list of threatened species has only listed one segmented worm species, the Lake Pedder Earthworm. One giant earthworm from the U.S. Pacific Northwest is probably also extinct! The Oregon Giant Earthworm (Driloleirus macelfreshi) was last documented in 1985. The Giant Palouse Earthworm (Driloleirus americanus) was also thought to be extinct after it was last documented in 1988. However, in May 2005, University of Idaho graduate student Yaniria Sanchez-de Leon became the first person in nearly two decades to find a specimen of the giant Palouse earthworm. Sanchez-de Leon said she plans to look for more worms at the test plots.

Reconstruction of the Lake Pedder Earthworm. Based on an image from Threatened Species Unit (2000). Created by Peter Maas for The Extinction Website. This image has been released under the Creative Commons Attribution Non-commercial No Derivatives 3.0 Licence.