Leeches pack thirty-two segments into a solid, suckered body
About six hundred eighty leech species have been named, mostly freshwater worms with suckers at both ends. Unlike earthworms, their body cavity shrinks to slim channels, and many species still earn modern medical roles thanks to a clotting-blocking peptide called hirudin.
Leeches belong to the subclass Hirudinea inside the phylum Annelida, close kin of oligochaetes such as earthworms. Soft muscular rings lengthen and shorten, yet ring marks on the skin rarely match the true internal segments. Most live in freshwater; fewer occupy land or the sea. The famous medicinal leech Hirudo medicinalis attaches with a sucker, pierces skin with jaws, and releases hirudin so a blood meal will not clot. Other species push a proboscis instead of jaws, and a minority hunt small invertebrates rather than drink blood.
Roughly one hundred marine, four hundred eighty freshwater, and the rest terrestrial forms make up the known roster. The giant Amazonian Haementeria ghilianii can stretch to thirty centimetres, while the smallest true leeches hover near one centimetre. Densities can explode: more than ten thousand individuals per square metre once clustered under Illinois rocks in organically polluted water. During droughts some bury themselves, shedding up to ninety percent of body weight and still surviving. Glossiphoniidae brood eggs and ferry young on their undersides—rare parental care among annelids.
Jean-Baptiste Lamarck coined Les hirudinées in 1818; the Latin hirudo and Greek bdella both simply mean leech. Fossils reach the middle Permian around two hundred sixty-six million years ago, and a 2025 study named Macromyzon siluricus from Silurian Wisconsin rock as a stem-leech, arguing early forms were marine predators or parasites before freshwater life. Bloodletting used leeches into the nineteenth century; today they aid joint and vein care and microsurgery, while hirudin treats clotting disorders. Proverbs cast the animal as greed incarnate—an image that outlived the lancet.
Source: Leech
