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I would like to offer an alternative interpretation of the behavior of the leech nervous system when severed from the leech body.  What if the absence of signals from the body is actually a signal to the nervous system.  Presumably, the nervous system is used to receiving inputs from the body and the absence of signal marks a departure from the normal state.  I think something similar happens with phantom limbs.  The nervous system is unsure of how to interpret the lack of input or response from a missing limb and so generates sensations.