Spongin is a modified type<span> of collagen protein, </span>forms<span> the fibrous </span>skeleton<span> of most </span>organisms<span> among the phylum Porifera, the sponges. It is </span>secreted<span> by sponge cells known as spongocytes. Spongin gives a sponge its </span>flexibility<span>.</span>
