Just a couple of thoughts in response to previous postings -
First, on the question of whether a Xenophora attaches a sponge near
its apex, or the sponge grows there on its own - A Xenophora cannot
attach anything to its apex.  Attachments are made at or near the lip
of the shell as it grows, and then gradually "move" away from the lip
(actually the lip moves away from the attachment) as the shell
continues to grow.  Those small attachments near the apex were put
there when the shell was very small, and that was the location of the
lip.
On the matter of why one means of camouflage or disguise would be
covered over by another - of course, the question of the purpose of
Xenophora attachments is still open.  Some people think it is for
camouflage, others don't.  But in any case, the additional growth of
barnacles, tubeworms, bryozoans, sponges, or algae on top of the
attachments is not due to any action of the mollusk.  These kinds of
growth are due entirely to the action of the secondary organism, and
the Xenophora is simply a hard surface on which to get a foothold and
grow.  So it is really not accurate to speak of the Xenophora first
covering itself with dead shells, and then covering itself with algae
or other growths.  The Xenophora creates the first covering - the
algal or other species creates the second.
 
Paul M.