Leeches (sing. leech) are segmented worms that belong to the phylum Annelida and comprise the subclass Hirudinea. Like earthworms, leeches are hermaphrodites but differ in the way that they are real predators with a more solid body and two suckers, one at each end.
The majority of the 700 known species of leeches live in freshwater environments, while some species can be found in terrestrial and marine environments, as well. Most leeches are hematophagous, as they are predominantly blood suckers that feed on blood from vertebrate and invertebrate animals.
Leeches, such as the Hirudo medicinalis, have been historically used in medicine to remove blood from patients. In modern times, the practice of leeching disappeared with bloodletting and has been replaced by other contemporary uses, such as plastic surgeries and treating osteoarthritis.
Since it injects anticoagulants as it bites, a leech can reduce clotting, relieve pressure and spur circulation after surgery. The blood thinner hirudin is taken from leeches' salivary glands, and synthetic versions have now been made with its chemical blueprints. Leeches are also used in traditional medicine in India, where many believe they remove tainted blood from the body.