[Taxacom] Bivalved molluscs
kennethkinman at webtv.net
Fri Jul 10 21:39:03 CDT 2009
Richard Petit wrote:
Sorry, but I had a lapsus and forgot that many on this forum have no
sense of humor. If I had thought that the simple existence of a
gastropod with a bivalved shell was important to the discussion, I would
have stated why it was important. It was a non-relevant fact that might
have been unknown to some.
Why do you regard this fact as "non-relevant"? Actually
bivalved gastropods are an excellent candidate as transitional forms
which bridge the gap between bivalves and euthyneuran gastropods in
general. This transition would be marked by the evolution of the
simplest of radulae (and thus the abandonment of filter-feeding),
cleavage becoming unequal, and the beginnings of buccal development
(encephalization). From this perspective, euthyneuran gastropods are
not detorted, but simply show the beginnings of torsion that some
euthyneurans passed on to their fully torted gastropod descendants. On
the other hand, Hyman's view that bivalves are decephalized molluscs
seems merely to be unchallenged dogma with little to back it up.
P.S. Another case of improbable decephalization is Simon Conway
Morris's view that halkieriid "worms" folded up to become brachiopods.
Seems more likely to me that the opposite happened---that a brachiopod
"unfolded" as the body grew larger, with the relectual shells protecting
vulnerable areas on the head and tail regions of the animal. Without
the shells, they would just be classified as another group of peculiar
worms. Whether halkieriids then evolved into annelids or were just a
dead-end group remains to be seen.
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