Crab Canon (JS Bach)
(click above to listen)

Crab Canon is from the Musical Offering, using the King's theme as its basis. 
The rule for the canon is for the second part to play the same notes 
as the first part, but starting from the end and working backwards. 
Notice the texture switch parts at the midway point. 
You can also see this in the music in GEB (Fig.44, p.202).

MIDI sequence(s) from the Classical Archives, by permission