"Least evolved" and "most evolved" is problematic to define, though.
Patellogastropod limpets and pulmonate landsnails by definition have had an
equal amount of time to evolve since they split from a common ancestor.
The limpets and the landsnails changed in different ways; one changed some
things more and the other changed other things more.  So it does actually
boil down to the order selected by someone.  In the case of gastropods, the
modern sequence is largely following Thiele, plus all the changes due to
findings indicating that particular groups are more or less closely related
than Thiele thought.


-- 
Dr. David Campbell
Associate Professor, Geology
Department of Natural Sciences
110 S Main St, #7270
Gardner-Webb University
Boiling Springs NC 28017

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