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Science Forum Index » Bio Evolution Forum » Is "bilateria" monophyletic?
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| Paul Ciszek |
 Posted: Thu Feb 01, 2007 12:46 pm |
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Someone in another newsgroup said he'd heard a theory that bilateralism
had evolved more than once in different phyla. Is there any basis for
that?
It led me to wonder, though; is Vernanimalcula supposed to be a prostome?
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pciszek at panix dot com | for variety, not superiority."
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| Posted: Fri Feb 02, 2007 8:19 am |
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Sorry. I meant to say that the ctenophora are supposed to have evolved
separately from the other bilateria, not the other ctenophora |
Most of what a biologist would call radial symmetry are really
what a mathematician would call n-fold symmetry. A true radial animal
would be absolutely and completely round. I think only some hydrozoans
and sponges follow a strict radial symmetry. In any case, a bilateral
symmetry is embedded in both radial and n-fold symmetry. The larva of
many bilaterals have radial or n-fold symmetry. My guess is that
radial symmetry is probably a primitive ancestral feature, and that
bilateralism may have formed independently in some phyla.
Quote: On Feb 1, 5:46 pm, nos...@nospam.com (Paul Ciszek) wrote:
Someone in another newsgroup said he'd heard a theory that bilateralism
had evolved more than once in different phyla. Is there any basis for
that?
It led me to wonder, though; is Vernanimalcula supposed to be a prostome?
--
Please reply to: | "Evolution is a theory that accounts
pciszek at panix dot com | for variety, not superiority."
Autoreply has been disabled | -- Joan Pontius |
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| Guest |
| Posted: Fri Feb 02, 2007 8:19 am |
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I think that there is some conjectures that ctenophora evolved
their bilateral symmetry separately from other bilaterals. However,
some literature places ctenophores at the root of the bilateral tree.
The ctenophora (i.e., the sea combs) have a bilateral symmetry
but don't have any organs associated with other trioblasts. They look
a little like cnidaria, which are diblastic, but I think ctenophora
are triboblastic.Ctenophora really are divided into four quarters.
However, one side is a little compressed relative to the other side
which makes them bilateral. Ctenophora don't show an easily recognized
coelum, or a digestive track. Some articles that I read claim
ctenophora is a primitive bilateral animal, and articles claim that
the ctenophora are completely separate from the other ctenophora.
Also, creatures classified as having a "radial symmetry" have an
embedded bilateral symmetry. For instance, a starfish can be divided
into to mirror halfs by a plane cutting through both the madreporite
and the center of the starfish. Many sea anenomes have 6-fold symmetry
that can be divided into two mirror parts. A true radial symmetry, as
is found in some sponges and cnidaria, by definition has an embedded
bilateralism. This type of embedded bilateralism may not be a
primitive feature, but evolved independently in several lineages.
I don't think that we can assume that such a common symmetry as
bilateralism is automatically a primitive feature.
Quote: On Feb 1, 5:46 pm, nos...@nospam.com (Paul Ciszek) wrote:
Someone in another newsgroup said he'd heard a theory that bilateralism
had evolved more than once in different phyla. Is there any basis for
that?
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| Paul Ciszek |
| Posted: Wed Feb 14, 2007 8:49 am |
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In article <epvvak$1n6s$1@darwin.ediacara.org>, <drosen0000@wahoo.com> wrote:
Quote: The ctenophora (i.e., the sea combs) have a bilateral symmetry
but don't have any organs associated with other trioblasts. They look
a little like cnidaria, which are diblastic, but I think ctenophora
are triboblastic.
I have seen nothing to support this.
It does raise and interesting point, though: I would think triploblasty
would be a more fundamental difference than bilateralness. Why isn't
that considered the defining feature of the clade?
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pciszek at panix dot com | that want to deceive, you're never going to
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