The knobs on a lion's paw are hollow.  On many specimens there are some
knobs that are open, and show the hollow interior.  It seems to me that they
must be filled with water in the living animal, considering that they are
formed open, at the lip of the growing shell, and gradually become closed as
the shell continues to grow.  This process takes place underwater, so I
don't see how the completed knob could be other than water-filled.

Paul M.