Assertion: sea urchin's spine | /r/HasProperty | difficult
Facets:
Source sentences:
text: Some echinoids (such as Tylocidaris clavigera, from the Cretaceous period's English Chalk Formation) had very heavy, club-shaped spines that would be difficult for an attacking predator to break through and make the echinoid awkward to handle.
text: The calcium-filled spines that a sting can leave behind can be difficult to remove from the skin.