Reason for crossed projections in the nervous system?

[at ucl.ac.uk]

rubinsnk at is2.nyu.edu (Kalman Rubinson) wrote:
>Matt Jones (jonesmat at ohsu.edu) wrote:
>: Here's one hypothesis I heard in an undergraduate class. I have no idea
>: whether there's any evidence.
>
>: Early swimming animals evolved crossed projections from the visual system
>: to the motor systems, so that an object perceived on one side of the body
>: would cause contraction of the muscles on the opposite side, leading to a
>: swimming movement away from the object (escape).
>
>That seems derived from Coghill's thesis in 'Anatomy and the Problem of 
>Behavior' (1929).  He posited the need for crossed connections in a 
>multicelled pre-vertebrate to convey chemical or tactile stimuli to 
>contralateral effectors for avoidance.  From this, he builds a model of 
>spinal cord organization.

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. 

Kate Jeffery
k.jeffery at ucl.ac.uk