Tubifex tubifex, also called the sludge worm, or sewage worm, is a species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 of tubificid

Tubificidae

The Naididae are a family of clitellate oligochaete worms like the sludge worm, Tubifex tubifex. They are key components of the benthic communities of many freshwater and marine ecosystems....

 segmented worm that inhabits the sediments of lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

s and river

River

A river is a natural watercourse, usually freshwater, flowing towards an ocean, a lake, a sea, or another river. In a few cases, a river simply flows into the ground or dries up completely before reaching another body of water. Small rivers may also be called by several other names, including...

s on several continents. Tubifex

Tubifex

Tubifex is a cosmopolitan genus of tubificid Annelids that inhabit the sediments of lakes, rivers and occasionally sewer lines. There are over ten known species of Tubifex, but the number is not certain, as the species are not easily distinguishable from each other.-Food and feeding:Tubifex worms...

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

Salinity

Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...

.