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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 cnidocytes 
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. Spongin is vitreous, or glass-like.
- ^ Anderson, D. (2001). Invertebrate Zoology. Oxford University Press.
- ^ Brusca, R.; Brusca, G. (2003). Invertebrate Zoology. Sinauer Associates. p. 191.