Hello Conch-l'ers
I posted a similar message this morning from my computer at work, but it seems
to have taken a wrong turn somewhere in cyberspace.  If it does eventually
arrive, please forgive the more-or-less double posting.
A couple of thoughts generated by previous posts on this topic -
On the matter of glass sponges attached to the upper surface of Xenophora
shells, I believe that attachment is the work of the sponge, not the snail.
First of all, a carrier shell cannot attach objects near its apex.
Attachments are made at the lip of the shell, during the normal growth
process.  Once an item is secured in place, the lip continues to grow away
from that item, effectively moving it out of the snail's reach.  Those little
objects attached near the apex of the shell were fastened into place when the
shell was very small, at which time that was the location of the growing lip.
Secondly, such sponges typically attach to hard surfaces by a tough stalk
which is very difficult to tear loose (have you ever tried to remove one from
a Xenophora?).  Therefore, where would the carrier get such a sponge to
attach, even if it could do so, and why would it attach something so radically
different from its usual choices?  I think the carrier shell is just one more
rock or dead coral head from the sponge's perspective (does a sponge have
perspectives??), a hard surface to attach to.
This matter of a sponge attached to two carrier shells is difficult for me to
visualize, I must say.  The type of sponge I am picturing usually has one
attachment stalk, and grows branches upward from that site, like a small bush.
How can it attach to a second site?  Question to Monica and Debbie - does the
method of attachment to both carriers look similar?  Is there a stalklike
portion of the sponge at both attachment sites?  Is it possible that each
carrier had a sponge attached, whose branches became entangled and then grew
together?
Lastly, on the question of why an animal would cover one layer of camouflage
with another - carrier shells attach their usual array of objects (of course
there are varying opinions regarding the purpose of such attachments - not
everyone agrees that camouflage is the objective), and sometimes these
attachments become covered over with a secondary growth of bryozoans, tube
worms, barnacles, algae, and other growth.  However, in these cases, it is not
the Xenophora which is applying a second layer of covering - it is the other
organisms, which are simply using the carrier shell as a substrate for
attachment.
Paul M.