ossicles
... that the spines of a sea urchin are connected to other ossicles by ball and socket joints?
... that the spines of a <a href="./Sea_urchin" id="mwCGI" rel="mw:WikiLink" title="Sea urchin">sea urchin</a> <i id="mwCGM">(two pictured)</i> are connected to other <b id="mwCGQ"><a href="./Ossicle_(echinoderm)" id="mwCGU" rel="mw:WikiLink" title="Ossicle (echinoderm)">ossicles</a></b> by <a class="mw-redirect" href="./Ball_and_socket_joint" id="mwCGY" rel="mw:WikiLink" title="Ball and socket joint">ball and socket joints</a>?