The horny or fibrous substance found in the skeleton of many sponges.
- ‘Demospongea are the dominant modern group with 600 genera; they have a skeleton formed of spongin or silica, or a mixture of the two.’
- ‘They are bleached and marketed; the spongin gives the sponge its softness and absorbency.’
- ‘Some spicules are formed of the mineralized substances calcium carbonate and silica, while others are made of an organic substance called spongin.’
- ‘First of all, however, spongin is not a feature consistent of all Porifera, but may be a constituent character of the Demospongiae only.’