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  • Leeches.

  • For most people, they're the stuff of nightmares.

  • But there's a lot more to them than just slimy bloodsuckers.

  • Now, the leeches in this tank are medicinal leeches, and

  • they've got this name for a really good reason.

  • So for about 2,000 years, up until quite recently, it was

  • thought that the body's health was kept in balance by

  • substances called humours which flowed through the body.

  • Now blood was one of those humours, and if you had bad

  • blood or too much blood, then it needed to be drained.

  • And that is where these guys came in.

  • Draining the blood from the body was known as

  • bloodletting.

  • And using leeches to do this was the height of fashion in

  • Victorian times.

  • In fact, millions were used every year across Europe.

  • Leeches are actually worms related to the ones you find

  • in your back garden.

  • But these are parasites that suck the

  • blood of other animals.

  • And they find a host by detecting shadows and movement

  • in the water.

  • So if I dip my fingers in the tank and wiggle them around,

  • the leeches should come to me.

  • And there we are.

  • This is Larry the leech--

  • or it could be Lucia.

  • Leeches are hermaphrodites.

  • They've got both male and female body parts.

  • Now this one is hungry.

  • You can see it's desperately trying to find

  • somewhere to attach.

  • So I'm going to let it feed on my arm.

  • When leeches are feeding, they inject anti-inflammatories and

  • an anticoagulant called hirudin that stops the blood

  • from clotting.

  • And it's because of these that they're still

  • used in medicine today.

  • They drain blood clots and promote blood

  • flowing skin grafts.

  • People say that leeches inject a kind of anaesthetic so the

  • host can't feel it feeding.

  • Now, there's actually no good evidence to support this.

  • And I can actually feel this.

  • It feels a bit like a mild nettle sting.

  • And what it's actually doing is using three rows of teeth

  • in a kind of Y shape.

  • There's about 100 teeth in each row, and it's using these

  • to slice through the skin to suck the blood.

  • It could take up to a couple of hours until it's fully

  • engorged, so all we can do is sit and wait.

  • The leech has been feeding for about two hours.

  • And as you can see it's absolutely massive, but it's

  • showing no sign of stopping yet.

  • Now, I don't want to try to pull it off, because if the

  • leech feels threatened it might regurgitate is stomach

  • contents back into the wound.

  • So after nearly three hours the leech has

  • finally dropped off.

  • And the wound is going to continue to bleed for at least

  • two hours afterwards.

  • Now the leech can last on this single

  • meal for over 12 months.

  • So I'll see you the same time next year.

  • The human volcano--

  • fire breathing.

  • It's the most dangerous stunt in the fire eater's

  • repertoire.

  • But how does it work?

  • Well, it all hinges on the oxygen in the air.

Leeches.

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