On Monday, March 29, 1999 at 9:24:12 am VUT,
"NORA BRYAN" <[log in to unmask]> wrote:
>As a landlocked shell collector I've rarely seen living shells and so
 
Come now, what about those wonderful land and freshwater shells!!!  A few
random thoughts on your questions:
 
>About Periostraca - what function does the periostracum have, and do all
>gastropods and bivalves have one?.  I suspect that cones and cowries
>never do.
 
The periostracum is a tanned protein coat which probably protects the
underlying shell from mechanical abrasion, acid etching and maybe from boring
organisms.  It's produced by cells of the mantle close to those that produce
the rest of the shell.  Periostracum is essentially pre-fabricated: it's made
up of small fragments manufactured within the cells and assembled on the
surface.
 
>About Opercula - these little doors are clearly for defense and possibly
>also to prevent dehydration in the case of exposure.  Any other uses?
>In families where the operculum is present, is it present in all
>species? For example, do all trochids and turbinids have opercula?
 
Loss of the operculum is related to habit, so limpets of all sorts (including
keyhole limpets) do not possess them.  The shell covers the soft parts when
the animal is active so there is no use for an operculum to seal the
aperture.  Marine pulmonates of the family Amphibolidae possess an operculum,
and some tropical pulmonate land snails have "pseudo-opecula", which are
thickened areas of skin on the upper surface of the tail.
 
Bronwen
 
Melbourne, Australia