"Hirudo medicinalis, the European medicinal leech, is a four-inch-long carnivorous, hermaphroditic, segmented worm with a sucker on each end, five pairs of eyes, and 32 nerve bundles, or "brains," in the middle."
and don't forget the 300 teeth - split between the three jaws.
" It is one of 650 leech species and is found mostly in ponds and bogs. Some species are highly specialized — one, in fact, feeds only on earthworms. Another feeds on fish in freezing polar seas. One dwells in the nostrils of Saharan camels, another inside the rectum of the African hippopotamus. "
No wonder they twitch their tails
"Nine years later, in a memorably bizarre case,leeches saved a life. During an operation for congenital facial abnormalities, an 8-year-old Dutch boy developed swelling so severe that his tongue filled with blood and protruded from his mouth,blocking his airway. Steroids and antibiotics didn't help. But six hours and 27 leeches later, the boy was out of danger, and leeches had been firmly re-established as good medical science."
Wiki also claims that the pure culture of Aeromonas hydrophila in their guts produces an antibiotic discovered 100 years before penicillin, but I can't find any references for that claim. I can easily believe the bacteria are doing it (they can be just as aggressive towards competing species as anybody else) but I doubt the antibiotic effect was realised that long-ago
Here's a drawing of a daddymummy (they are hermaphrodites, after all) glossiphoniid leech guarding hisher eggs, and later with hisher babies peering out from underneath.
Some genera, such as Marsupiobdella, go even further and have an internal pouch for their young.
See here for a long paper on the breeding of Giant Amazonian Leechs, and shaving rabbits.