Illustrate why the shell is called the shell

There would be plenty of illustration available for this particular
metaphor (think
http://en.wikipedia.org/wiki/The_Birth_of_Venus_(Botticelli)), but
that is to the speaker to decide upon.
added gdevenyi comment
fixed wording
Merge remote-tracking branch 'psteinb/why_the_shell_is_called_the_shell'

* psteinb/why_the_shell_is_called_the_shell:
  fixed wording
  added gdevenyi comment
  Illustrate why the shell is called the shell