Assertion: sponge's skeleton | /r/HasProperty | spongin
Facets:
Source sentences:
text: The structure of the commercial sponges (within the class Demospongiae) is based on a skeleton composed of spongin as opposed to the fragile glass sponges in the class Hyalospongiae that feature silicic acid, and the Calcispongiae, which have spicules made of calcium carbonate.