Hirudinea or leeches are annelids with the unique ability to such the blood of a host. They are most commonly found in fresh water ponds, lakes, and rivers. They can be as small as 0.2in. or as large as 18 inches. To draw blood out of a host, it uses its suckers to attach on to it. The sucker surrounding its mouth has three sets of jaws that bite into the flesh of its host in the shape of a "Y". It then uses special saliva to open the blood vessels, thin the blood, and prevent pain from a bite. Leeches have had a history in being used for medical purposes and are still used today to remove excess or harmful blood.
http://www.faqs.org/health/topics/55/Leeches.html
http://www.pbs.org/wnet/nature/bloodysuckers/leech.html
http://www.faqs.org/health/topics/55/Leeches.html
http://www.pbs.org/wnet/nature/bloodysuckers/leech.html