Class Hirudinea is a group of annelid worms commonly referred to as leeches. Synonyms for this class of organisms include rhynchobdellida, arhynchobdellida, and hirudinomorpha. These synonyms are used to describe different characteristics of the leeches, such as the presence or absence of a proboscis, the structure of their mouthparts, and their overall body shape. Despite these variations, all members of Class Hirudinea share a unique ability to secrete anticoagulants and anesthetic compounds in their saliva that enable them to feed on the blood of other animals without detection. This remarkable adaptation has made leeches valuable tools in medicine, as well as in ecological and evolutionary studies.