Thanks to Stephen Longster for this absolutely disgusting photograph, we're not sure if it's rude but it is disgusting.
A type of worm.
The species photographed is a freshwater leech of the round bodied type which carries its eggs around its body in a sac and then sloughs off the sac at hatching time.
I admit my knowledge of blood sucking worms is limited to a few individuals. So if there are any leech experts out there I am willing to be corrected/better informed
It was at the Loch Lomond NNR taken in July 2010. In the bottom left hand corner you can see another Leech. If memory serves; the sluice board that the Leeches were on, had quite a few of these fat blood suckers.
As a medieval pharmacy, it is quite well stocked, 800 years ago it could have been quite lucrative find.
By the look of the leech, it was only at the start of the egg laying process, and there were many more to go.