Leeches: Annelids of the class Hirudinea. Some species, the bloodsuckers, may become temporarily parasitic upon animals, including man. Medicinal leeches (HIRUDO MEDICINALIS) have been used therapeutically for drawing blood since ancient times.Hirudo medicinalis: A species of European freshwater LEECHES used for BLOODLETTING in ancient times and also for LEECHING in modern times.Leeching: The application of LEECHES to the body to draw blood for therapeutic purposes. Such medicinal leeching, an ancient medical practice, is still being used in microsurgery and the treatment of venous congestion or occlusion.Ganglia: Clusters of multipolar neurons surrounded by a capsule of loosely organized CONNECTIVE TISSUE located outside the CENTRAL NERVOUS SYSTEM.Ganglia, Invertebrate: Clusters of neuronal cell bodies in invertebrates. Invertebrate ganglia may also contain neuronal processes and non-neuronal supporting cells. Many invertebrate ganglia are favorable subjects for research because they have small numbers of functional neuronal types which can be identified from one animal to another.Invertebrate Hormones: Hormones produced by invertebrates, usually insects, mollusks, annelids, and helminths.