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 spongioblasts.
Spongin gives a sponge its flexibility. True spongin is found only in members of the class Demospongiae. Researchers have found over 100,000 types of spongin.