kinman at HOTMAIL.COM
Mon Aug 19 19:30:51 CDT 2002
Yes, I'm trying to keep my "feet on the ground". I just hope to get
people to stop assuming that all worms primitively lack shells (and that
*all* shells and shields in lophotrochozoan groups are "ad hoc").
It is interesting to note that McHugh (1997) found Sternaspis
branching off with Echiuran worms (although I think he has them branching
too high in the tree). In order to have a holophyletic polychaete clade, I
predict that both sternaspids and echiurans will be excluded, or at least
split off very early (before the scolecids, which are now shown to be the
basal polychaetes at the Tree of Life site).
Needless to say, I doubt that nuchal organs will be found in
sternaspids. In fact, I suspect that one would actually be more likely to
find nuchal organs in pogonophorans (or at least their ancestors). I prefer
to reduce pogonophorans to being just another Order of polychaetes.
Echiurans are probably best maintained as a separate Class from the
polychaetes, but not as a separate Phylum from the annelids as has been done
in the past. Phylum Sipuncula, however, should probably be kept at phylum
rank as an intermediate between molluscs and annelids.
----- Cheers, Ken
>From: Geoff Read <g.read at NIWA.CO.NZ>
>Reply-To: Geoff Read <g.read at NIWA.CO.NZ>
>To: TAXACOM at USOBI.ORG
>Subject: Re: Sternaspid "shells"
>Date: Sat, 17 Aug 2002 16:45:29 +1200
> > Therefore, I will continue to refer to Sternaspid paired shields
> > "shells", and also assume that they are homologous to bivalve shells,
> > whether they are directly descended from bivalve molluscs or not (but my
> > intuition is that they are bivalve descendants). I may be rather
> > imaginative, but it is based in a very wide reading of the literature
> > have access to.
>Ken, don't forget the serpulid polychaetes - they have whole houses built
>"shell". Clearly, molluscs manque.
>Let's keep our feet on the ground - the ability to secrete and to dissolve
>calcium carbonate is probably universal across the animal kingdom. Ad hoc
>'opercular' or shield structures evolve when appropriate. In the case of
>sipunculan genera with shields or caps - they happen to live in rock &
> Geoff Read <g.read at niwa.co.nz>
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