They are segmented worms with the body formed by rings (from Latin annelus = rings), being among the largest micrometazoa of activated sludge. The easy adaptation allows them to inhabit a wide variety of ecological niches, feeding mainly on algae and other invertebrates. They have high regenerative capacity and a genre in particular (Aeolosoma sp.) The presence of orange glands in the skin can alter the color of the effluent, when they are found in large numbers. Occur only in stabilized sludge with excess of dissolved oxygen and high sludge age. It is rare and slightly dominant.