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I had the exact same thought about the leech's NS Nelly. I thought that the reason for the "twitching" of the NS was because that was the natural state of its NS and that it needed some sort of inhibitor/regulator (ie the brain) to control these spasms. I mean it is very possible. Look at people who have epilepsy. They have these uncontrollable spasms. Although it's not the same because in this situation we have our brain connected to our NS, we still consider this as a brain disease. So there must be something going on up there (or lack of).