Cnidarians and metazoan evolution

J. David Spafford jspaffor at gpu.srv.ualberta.ca
Wed Mar 30 05:05:14 EST 1994

My lab uses molecular biological techniques to study the nervous systems of
an extant cnidarian, a hydrozoan jellfish.  Our assumption in this work is
that cnidarians have a nervous system with "primitive" features.  Of all
extant groups, cnidarians are the lowest phylum to contain animals with a
recognizable nervous system.  Now I ask, how valid is the assumption that
ancestral cnidarians (or their close relatives) were the first metazoans? 
What is the best evidence for this?

Hydrozoans have generally been considered the most "primitive" of the
cnidarians.  Scyphozoans (true jellyfish), cubozoa, anthozoans (sea anenome,
corals) appear more advanced because they often have an incomplete
"alternation of generations" and/or radial symmetry.  Mitochondial evidence
has been somewhat controversial; in one study anthozoa are considered more
primitive and others suggest the hydrozoans are most primitive.  Am I right
in suggesting that the hydrozoa are the most primitive of the cnidarians?

We are beginning to do some work with Trichoplax, a placozoan.  Where do
they fit into the evolutionary picture?  What about the cnidarian relative,
the ctenophores?  What about the poriferans (sponges) and the mesozoans? 
Are the metazoans mono- or polyphyletic (did they arise once or many times)?
Was there a eukaryotic (protozoan) ancestor?  Was the prokaryotic ancestor
most likely a relative of the flagellate bacteria rather than a ciliate?

I need an expert opinion to help me wade through the literature.
Thanks in advance,
J. David Spafford
Department of Zoology, University of Alberta
jspaffor at gpu.srv.ualberta.ca

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