Catenulida is a class of small, aquatic worms found primarily in freshwater environments. They belong to the phylum Platyhelminthes, which includes flatworms. Catenulids are characterized by their elongated bodies and a unique chain-like arrangement of segments, which gives them their name, meaning "little chains" in Latin. They are typically less than a few millimeters long and are often transparent. These worms play a role in the ecosystem by feeding on microorganisms and organic matter in the sediment. Catenulida reproduce asexually through a process called fragmentation, where a part of the worm can grow into a new individual. Their simple body structure and lifestyle make them important for studying evolutionary biology and ecology.