On Sun, 19 Oct 2003 00:51:43 +0000 (UTC), Anthony Cerrato
 wrote:

>
>"Lane Lewis"  wrote in
>message news:bmn9q0$kg4$1{at}darwin.ediacara.org...
>>
>> "Wirt Atmar"  wrote in message
>> news:bmilro$2e88$1{at}darwin.ediacara.org...
>> > Trantis asks:
>> >
>> > >Why did animals develop two Kidneys and two
>testicles/ovaries?
>> >
>> > Once evolution has settled on a bilaterally symmetric
>design plan during
>> > embryogenesis, duplicating any structure simultaneously
>on each side of
>> the
>> > midline is probably the simplest design possible. The
>same question could
>> be
>> > asked of why do we have two eyes, two ears, two arms,
>two lungs, two brain
>> > lobes or two nares?
>> >
>> > The more difficult question to answer is why do we have
>only one heart,
>> one
>> > liver, one pancreas, one stomach, one gut, each
>asymmetrically placed
>> against
>> > the midline?
>> >
>> > Wirt Atmar
>> >
>>
>> Perhaps those organs that were close to the center did not
>need to have a
>> double but those out side the center needed two to be
>symmetrical. The
>> doubling of organs might not be needed for survival just
>symmetry. It may be
>> just easier to produce. The one organ that might bring
>this down is the
>> brain which even though is along the center has two
>complete sides.
>>
>> Lane
>
>Perhaps an even deeper question that might be asked is, why
>are all larger animals bilaterally symetric, rather than
>symetric from front to back, or even "tetra-symetric?" The
>latter should have even greater survival value (other things
>being equal. :))   ...tonyC
>

It is easy to see why animals with an anterior/posterior axis have a
clear "advantage" over radially symmetric ones.  Locomotion
specialized for movement in one direction can be much more efficient.
The anterior part then encounters new environments first and
specialized sensory structures are concentrated there as well as
central nervous structures to process the sensory information and
prepare appropriate behavior.  Given a one-way complete digestive
tract, it is also useful to put the mouth at the front and anus at the
rear where the anterior brain and sensory structures can be associated
with feeding.  A dorsal/ventral axis (contrasted with a cylindrical
body) also allows for postural and locomotory specalization.  Given
anterior/posterior and dorsal/ventral, then left/right follows
logically.  The duplication of internal organs provides some
redundancy and the duplication of external organs is important for
balanced sensation and locomotion. Other systems of symmetry, or total
lack of symmetry, do not allow for such structural and functional
specialization.

Of course, you do have to take into account many of the specifics. The
flatworms are bilateral without the complete digestive system.  The
gastropod molluscs have modified the strict anterior/posterior axis
and the scaphopod and cephalopod molluscs have modified the strict
bilaterality.  The echinoderms have become secondarily radial.  Still,
the bilateral planula larva of the Cndaria, with an anterior/posterior
axis, does go way back evolutionarily and may form the basic pattern
for the development of the bilateral animals. And the bliateral plan
(anterior/posterial, dorsal/ventral, left/right) has clearly been
enormously successful for the arthropods and the vertebrates, the
"major advanced" animals we now see.

Incidentally, the heart is no exception to the general plan of
bilateral symmetry.  This leaves the digestive system is the sole
unpaired system. According to the Nature Encyclopedia of Life
Sciences:

"Heart development starts with progenitor cells that are allocated
from mesoderm at the anterior domain of the embryo and become
committed to a  cardiogenic fate (cardioblast, heart specification).
These clusters of cells (cardiac precursors) form a bilaterally
symmetric cardiogenic field that develops further into bilateral
cardiac primordia. The primordia then come closer together and
eventually fuse at the midline to form a single linear heart ."

(Wang, Qing and Chen, Qiuyun (December  1999 ) Cardiovascular Disease
and Congenital Heart Defects. In: Nature Encyclopedia of Life
Sciences. London: Nature Publishing Group. http://www.els.net/
[doi:10.1038/npg.els.0001907] )
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