Annelid Subclass Hirudinea, famous for so very many things. Crawling out of the undergrowth towards freshly arrived parked cars, for example (A memorable incident from a fungus-collecting trip I went on. Glancing off the road to see a dozen big stripey bloodsuckers emerging - at head height - with all possible haste, made quite an impression). They're often quite pretty - with yellow stripes, or electric green spots, and so on.
But now for some less personal fun leech facts!
Charles Darwin's first scientific speech was about leech eggs - specifically, the black specks inside oyster shells that were the eggs of the skate leech (one of many marine leeches - next time you're being eaten by a shark, be sure to check inside its nostrils for interesting leech species)
Not all leechs are parasitic - some are very effective carnivores. You might find some on a field trip, gorging themselves on frog eggs ( another personal memory ).
These folks came all the way out to Australia, and travelled from Sydney to Darwin, solely to collect rare and interesting leechs.
The Giant Amazonian Leech Haementeria ghilianii can be up to a foot long, and is one of quite a few species that carefully looks after its babies. Happily, the world's biggest leech feeds off it's biggest snake and rodent.
The leeches in the movie The African Queen did NOT enjoy their brief acting career - and actively tried to escape whenever Hepburn and Bogart tried to attach them for the next scene. The ones in Corman's z-grade classic Attack of the Giant Leeches! weren't even real.
A jar full of leechs makes a handy-dandy weather forecaster! (at least according to Dr Merryweather's Tempest Prognosticator which came designed as cute little gold-plated Indian Temple. Just full of leeches.)
The medicinal leech is invaluable in microsurgery - the anticoagulants in its saliva, and unfailing appetite for blood, keep recovering tissue healthy and uncongested. Although using them to relieve vascular congestion of the penis, and their tendency to wander off where they're not wanted, such as into the interior of a breast reconstruction, might take some getting used to.
They're also great for treating unconventional sexual desire (at least, that's what Blackadder was told)
On the island of Mauritius, there exists a species of leech that feeds on snails, rather than the usual leech-food - frogs!
and... well that's about it really. I can't think of any more leech facts, offhand. But rest assured if I think of any more I'll let you all know :)