Spongin

Spongin is a type of collagenous protein that serves as a structural component in the skeleton of certain sponges. It is a key element in the anatomy of glass sponges and other sponge species, providing them with flexibility and strength. Unlike the siliceous (silica-based) spicules found in some sponges, spongin is organic and helps to maintain the sponge's shape and integrity.