Mollusc nervous systems
kinman at HOTMAIL.COM
Wed Aug 28 21:20:49 CDT 2002
Well, it may be Barry's impression that I am getting the worst of it
(since I am fighting a rather "lonely" battle). However, I'm just getting
started, and I've already found yet another (particularly troublesome)
problem with the conventional thinking about molluscs (the nervous system).
Such problems can be solved by using my "bivalves-first" phylogeny. The
"real state of affairs" is that this debate is still hanging in the balance.
I'm trying to inform and challenge (not using diversionary tactics nor
seeking sympathy). See below for the latest.
Speaking of polarity problems, here's another one I am looking at
carefully. Contrary to conventional wisdom, why couldn't the epiathroid
nervous system be the primitive state in molluscs, with the hypoathroid
nervous system being derived from it? Here are two of my reasons for
thinking so (the second is especially important):
(1) The epiathroid nervous system is found at the base of my cladogram,
and then transformed into a hypoathroid nervous system in several lines
independently. This seems a more parsimonious solution than conventional
phylogenies, which require various reversals (extra steps) such as
hypoathroid to epiathroid and back to hypoathroid again.
(2) Even MORE importantly, the extreme form of epiathroid nervous
system (namely cerebropleural fusion) occurs in very few
groups---lamellibranch bivalves, sacoglossans (incl. bivalved gastropods),
and couple of other opisthobranch gastropod groups. What are the odds that
bivalves and bivalved gastropods would convergently evolve both bivalved
shells and a rare type of nervous system? I don't think this is likely at
all, and this just strengthens my belief that they are NOT convergent and
that they are closely related groups.
------ Ken Kinman
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