When I was young there was one bit of Arthur Ransome's "Secret Water" I didn't get. Nancy Blacket, pirate, terror of the seas and captain of the Amazon is well known in the books for her exclamations in nautical phrases such as "jibbooms and bobstays".
Then in "Secret Water" they changed, due it was suggested, to the eel blood in her. All of a sudden there was "suffering lampreys". Eh? I think was my reaction, what is a lamprey?
The word came up again to describe the death of King Henry I due to him eating an excess of lampreys, though the repository of all of England's history as it should be remembered, the classic "1066 and all that", described it as a "surfeit of palfreys".
However I never really knew what they were which is just as well as they sound horrid. According to this story on the BBC web site the "parasitic creature attaches its circular mouth to larger creatures before sucking out their insides" (see photo above).
And these have just been found in the Thames where I go paddling and sailing!
Not sure I wholly agree with the environmental perspective that this is a good thing as they only like really clean water.