Reason for crossed projections in the nervous system?

[Sandra L Wegert]

> @ucl.ac.uk wrote:
> : A hypothesis I came across last night, attributed to Marcel Kinsbourne by 
> : Steven Pinker in his book "Language as Instinct", runs as follows: 
> : bilaterally symmetrical invertebrates have an uncrossed nervous system, 
> : their spinal cords (or the invert. equivalent) ventrally and their hearts 
> : etc. dorsally, whereas vertebrates have a crossed nervous system, their 
> : spinal cords dorsally and their internal organs ventrally. Kinsbourne 
> : speculates that sometime during invert-vert evolution, the head was 
> : twisted around 180 degrees so that it points the other way (sort of like 
> : what happened to flatfish only worse). I thought that was a kind of cute 
> : idea. I have no idea whether it has any merit or not. 
> 
> That makes the assumption that there is a direct lineage between the two 
> which I regard as unlikely.
> 
> Kal

And we're talking about left/right crossing, not dorsal/ventral 
crossing.  Also the mammalian brain DOES bulge ventrally, but the spinal 
cord (where things are crossed) is still the same.  Nice ideas though.
Sandra

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sandraw at U.Arizona.EDU