Leeches are related to earthworms, but most are a bit more vicious than their dirt-dwelling cousins. Some are ambush predators, lying in wait for victims like slugs and snails, while others are blood-sucking parasites.
The best-known of these is the European medical leech (pictured), which has been used in human health care for millennia. It fell out of favor in the 1800s along with bloodletting, but it's making a comeback now as a way to control blood flow in some medical procedures. 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 (see the video below for more).