Class Oligochaeta consist of earthworms and other worms that live in terrestrial or freshwater environments. As shown by the class name (meaning few setae, or appendages), these worms lack the bristles that allow movement and increase surface area. There are some that are marine, but most (94%) live out of sea water. Also, all oligochaetes are hermaphroditic, neither male nor female. One oligochaete, the earthworm, extracts nutrients from the soil as it burrows, excreted from the anus. Because the nutrients that earthworms dig up are necessary for fertile soil, these earthworms are an intricate part of farms, actually tilling the soil.

Oligochaetes are in the phylum Annelida, made up of segmented worms. Body segmentation, a hallmark of annelids, was a major step in the evolution of animals. Annelids are protosomes, meaning they have a coelom made from cell masses. This coelom is divided into a series of repeated parts. Except for the head and tail region, each with an opening of the digestive tract, making it a complete tract, each segment in an annelid is ringlike and very similar. Segmentation allows for flexibility and mobility because annelids can bend at segmented parts. Other hallmarks of the annelids are soft bodies that are round in cross section, repetition of organs in the segemented parts, and a body that is much longer than it is wide. There are three main classes in the phylum Annelida.