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Chuck Pell
05-16-2001, 01:02 AM
Re radial "vs." bilateral:

> One thing that distinguishes bi-laterally symmetric
> animals from asymmetric ones is that the former move
> faster (relative to the viscosity of their media).
> Thomas A. Stoffregen, Associate Professor
> Department of Psychology
> University of Cincinnati
>

Not a tenable statement, really, and that depends on
whether you are speaking of dynamic or kinematic viscosity.
Even though they seem to be bilaterally symmetrical
molluscs, squid (while jetting) can be considered largely
radially symmetrical about their long axis --- a streamlined
spindle water jet reaction rocket --- and they are among
the fastest creatures, some are even capable of leaping
clear of the water.

A point of terminology: we should use the term "radially
symmetrical" instead of "asymmetrical" here...a paramecium
or coral reef is more properly asymmetrical, and there are
other forms of symmetry...

As for either/or arguments in general: it should be clear
that the existence of such an argument often precedes the
widespread realization that the "answer" is a mixture of
the two (witness genes/environment) or that insufficient
information exists about a situation, perhaps indicating
that it will prove to be more complex than a simply
"bilateral" breakdown. This would apply to bilateral /
radial symmetries, and dumb luck / function. As for the
latter pair, luck happens (dumb or smart) and function
(physics) matters.

> Why does bilateral symmetry predominate in the animal
> kingdom would perhaps be a better question.

Does it? Define your use of "predominate," or better,
what do you mean when you say "animal kingdom" ?

Chuck Pell

Rule of Functure [sic]:
"Function without structure is a ghost, and
structure without function is a corpse."
--- Wainwright and Vogel, 1968

Thomas Stoffregen wrote:

> Some comments on the origins and utility of bi-lateral symmetry
> (i.e., even numbers of limbs) in animals.
>
> >My solution: Bilateral symmetry in the animal body plan produces the even
> >number of limbs, but those animals that develop with radial symmetry (like
> >the starfish) can have odd numbers of limbs. Why does bilateral symmetry
> >predominate in the animal kingdom would perhaps be a better question. My
> >answer to that question is: pure dumb luck.
>
> Dumb luck is possible, but before we accept this non-functional
> explanation, we should consider whether bi-lateral symmetry might
> have a functional role that lead to its being selected for (rather
> than merely tolerated) in the evolutionary history of animals.
>
> Bi-lateral symmetry is locomotor appendages is not universal, cf.
> star fish, and snails (the latter have only one foot, an odd number).

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