Re: antiquity of segmentation

[Bill Shear <bills@xxxxxxxxxxxxx>]

Peter Roopnarine--the mesodermal involvement should work.  Another
question: is the segmentation of chordates really coelomic?  Evidently the
first several segments (that become part of the head) are formed as
coelomic pouches (are gill pouches coelomic?) but the others originate in
the neuromuscular system (myotomes).  Certainly in any living chordates
there are no signs of coelomic compartmentalization.  This is what has
always convinced me that the segmentation of annelids/arthropods and
chordates cannot be homologous--hence the interest in the original report
in Nature (not Science as I originally posted).  Annelid segmentation may
have originated as an adaptation to burrowing in soft substrates and that
of chordates for swimming--the myotomes acting against the elasticity of
the notochord.

What do we know of molluscan segmentation (ie, chitons and Neopalina)?  One
sees some recent molecular trees that place molluscs closer to annelids
than are arthropods, suggesting that molluscs have reduced or lost
segmentation.

Bill Shear
Department of Biology
Hampden-Sydney College
Hampden-Sydney VA 23943
(804)223-6172
FAX (804)223-6374
email<bills@tiger.hsc.edu>