To give my two cents worth to the Xenophora discussion:
I have a Xenophora pallidula which consists of two of them attached to each
other by a glass sponge.  Now that I learned how fast a Xenophora can
attach objects to itself I wonder how fast glass sponges can attach
themselves to other objects?  The strange thing is that, although these
shells (or this shell, whatever) quite clearly was (or were) collected dead
-- there are small worm tubes in both apertures -- the two apertures are
aligned in exactly the same direction.  Unless this is a (rather unlikely)
coincidence, I envisage these two molluscs, each peacefully minding their
own business, feeding on something, probably the same rather large thing,
next to each other, parallel to each other, and then suddenly when they
want to leave they can't, without the other one leaving too, because the
glass sponge has done its deed, reaching from one over to the other...
And then always not only having to crawl but also to do all other things in
unison, like Siamese twins, bizarre thought.
Does anybody know how fast glass sponges can work indeed, so that this
might be a likely scenario?  And do other Siamese twin Xenophoras exist
somewhere, anywhere, attached how?
 
Monika