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.
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.
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.
Enrique
