Spongin, a modified type of collagen protein, forms the fibrous skeleton of most organisms among the phylum Porifera, the sponges. It is secreted by sponge cells known as spongocytes. Spongin gives a sponge its flexibility.Click to see full answer. Herein, what are spicules and Spongin?The exoskeleton of sponges (so, the parts that you see) are composed of a mixture of spongin and/or spicules. Spongin is a modified type of collagen protein, and forms the “fibers” or “mortar” that hold spicules together. Spicules are composed of either Calcium or Silica.Also, what is the skeleton of Poriferans made of? The Demosponge is the largest class in the phylum Porifera. Their skeletons are made of spicules consisting of fibers of the protein sponge, the mineral silica, or both. They have a different shape than the glass sponge. These sponges are commonly known for being large. Beside this, do all sponges have Spongin? Many sponges have internal skeletons of spongin and/or spicules of calcium carbonate or silica. Primarily, their body consists of a thin sheet of cells over a frame (skeleton).What are the skeletal material in sponges?Skeletal structures of sponges are spicules and spongin fibres. Spicules are formed by carbonates of lime or silica in the form of needle like pieces. Spongin fibres are composed of a silk-like scleroprotein.