Spongin
From Infogalactic: the planetary knowledge core
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 spongioblast[1]
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.[2] Spongin is vitreous, or glass-like.
References
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