Hirudinea is a class of segmented worms commonly known as leeches. They are primarily found in freshwater environments, although some species inhabit marine and terrestrial ecosystems. Leeches have a flattened body and can vary in size, typically ranging from a few millimeters to several inches long. They are known for their ability to attach to hosts using suckers at both ends of their bodies. Leeches are often recognized for their role in medical applications, particularly in hirudotherapy, where they are used to promote healing and improve blood circulation. They feed on the blood of various animals, including humans, and possess specialized saliva that contains anticoagulants, preventing blood from clotting while they feed.