Assertion: sponge's skeleton | /r/HasProperty | formed of horny fibres
Facets:
value: exclusively
type: DEGREE
Source sentences:
text: Demospongiae may have followed a line of evolution originating in Tetractinellida (with four-rayed spicules) and advancing, after reduction of the tetraxon spicules into monoaxons, to Keratosa; in addition, the polyaxon microscleres (asters) evolved into specialized forms (sigmas and chelas), and spongin appeared to cement the spicules, finally developing into a skeleton formed exclusively of horny fibres.