Explain in details Class Hirudenia - Leeches?

This group includes the leeches. Most leeches grow in tropical freshwater habitats, and are familiar to most people as bloodsuckers. The most distinguishing feature of leeches are the sucking organs that attach the leech to its host. Leeches have two suckers-one sucker at either end of the worm. Once a sucker penetrates the skin, it secretes an anti-coagulant saliva that prevents the blood from clotting. Some species even inject an anesthetic into the bite wound so that the host doesn't feel the parasite! The leech then gorges itself with blood and can increase its mass up to 10 times its normal weight. Leeches are widely used in some post-surgical procedures to help reduce swelling around the wound. These leeches are, of course, sterile and bred specifically for this task.