Tubifex tubifex, also called the sludge worm, or sewage worm, is a species of tubificid segmented worm that inhabits the sediments of lakes and rivers on several continents. Tubifex probably includes several species, but distinguishing between them is difficult because the reproductive organs, commonly used in species identification, are resorbed after mating, and because the external characteristics of the worm vary with changes in salinity. These worms ingest sediments, selectively digest bacteria, and absorb molecules through their body walls.