Re: antiquity of segmentation

[Bill Shear <bills@xxxxxxxxxxxxx>]

The radial symmetry of echinoderms is clearly secondary, based on their
ontogeny, and if one accepts carpoids, etc., as early echinoderms, perhaps
on the phylogeny of adults as well.  Interesting that many living taxa of
echinoderms now seem to be imposing secondary (tertiary?) bilateral
symmetry on the pentamerous radial symmetry--as in biscuit urchins and many
sea cukes.

My reason for asking about the segmentation genes in echinoderms was of
course the usual postulate that echinoderms and hemichordates are close to
chordates (of course there is the proposal of Jeffries that the
'calcichordates' are the common ancestor of both groups).

If these genes do not occur in echinoderms or have a different function......

My understanding has always been that the three-part coelom found in
deuterostomes and in some pseudocoelomates was not the same thing as
segmentation.  The referenced article in Science last week points out that
the segmentation genes operate only in the anteriormost 8 or 10 chordate
segments which form as coelomic pouches fromthe gut.  The more posterior
segments form in blocks of mesoderm without coelomic components.

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