The Oligochaeta includes the well-known earthworms and many small species of freshwater worms. They are part of the Phylum Annelida; the segmented worms, and if magnified the body is visibly segmented. Oligochaetes are also distinguished by the presence of hair-like or bristle-like setae (or chetae) along the body. Some species have visible eyes, some have a nose-like proboscis, and many are bright red when live.
Oligochaetes are found in almost all freshwater habitats, especially where there are fine sediments to burrow into.
They are deposit feeders, collecting fine organic matter from bottom sediments.
While oligochaetes can be found in pristine streams, they can also be abundant in the sediment of sewage treatment ponds. Their ability to tolerate highly polluted waters results in low tolerance values of 1 (hard bottom sites) and 3.8 (soft bottom sites).