The shell of the cuttlefish is what is used for casting. The
cuttlefish is a member of the family which includes squid and
octopus. It looks kind of like a short-armed squid. The shell
(and I believe that this is the only one of the family to have a
shell) is totally internal, and what it’s use is I have no idea.
If you look in the small pet stores that specialize in birds, you
can find some giant specimens of these shells.
Richard