IUBio

Cross - Wired Eyes

George Hammond ghammond at mediaone.net
Wed May 16 08:34:04 EST 2001


Enrique wrote:
> 
> George Hammond wrote:
> >
> > Richard L. Hall wrote:
> > >
> > > How many vertebrates lack eye lenses?  Fish have them as do all
> > > tetrapods that I know of.  So maybe the answer is zero.  There are
> > > some species that have greatly reduced eyes and presumably greatly
> > > reduced optic tracts.  Still, so many other central systems
> > > decussate, that it seems unlikely that something like the formation
> > > of a lens could account for crossed visual pathways.
> >
> > [Hammond]
> > So far as I know, animals that can see but without lenses,
> > e.g. "compound eyes", have ipsilateral CNS structure...
> > there is no decussation... further support for the theory.
> > Take a fruit fly for instance.
> >
> 
> This only means that there is a significant difference in
> design between a fly eye and a vertebrate eye (and brains!).
> Each little facet in a compound eye do have a lens. I think
> the point is, probably, that compound eyes are not imaging
> devices, but light detection devices.

GH:  Iyuh, iyuh

> 
> As others has posted before, a pinhole will produce an
> inverted image. So, evolving a whole lens-ed eye from a
> primitive pinhole/retina projection eye will not imply a
> sudden image inversion when the lens come to function.

GH: Who said it was a pinhole, Sadie Hawkins?  More likely
    it was just a light receptive surface.

> 
> The decussation of CNS connections was probably a completely
> independent event in the evolution of the antecessors of
> vertebrates. Once taken, almost impossible to go back.

GH: Really



> 
> Enrique

-- 
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-----------------------------------------------------------
George Hammond, M.S. Physics
Email:    ghammond at mediaone.net
Website:  http://people.ne.mediaone.net/ghammond/index.html
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