Assertion: sponge's skeleton | /r/MadeOf | glass
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Source sentences:
text: As their skeletons are both made of glass rather than calcium, early classification schemes grouped hexactinellids with the demosponges; however, at present hexactinellids are separated from cellular sponges (the Calcarea and Demospongiae) in the subphylum Symplasma because of their unique (syncytial) structure.