
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. True spongin is found only in members of the class Demospongiae. Researchers have found over 100,000 types of spongin. Spongin...
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http://en.wikipedia.org/wiki/Spongin

• (n.) The chemical basis of sponge tissue, a nitrogenous, hornlike substance which on decomposition with sulphuric acid yields leucin and glycocoll.
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http://thinkexist.com/dictionary/meaning/spongin/

(from the article `sponge`) A few Demospongiae (e.g., Oscarella, Halisarca, and Chondrosia) lack skeletons. One group (Ceractinomorpha) has a type of spongin, which, in certain ...
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http://www.britannica.com/eb/a-z/s/143
Spon'gin noun (Physiol. Chem.) The chemical basis of sponge tissue, a nitrogenous, hornlike substance which on decomposition with sulphuric acid yields leucin and glycocoll.
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http://www.encyclo.co.uk/webster/S/170

proteinacous compound of which the spicules in Demospongiae are composed.
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http://www.ucmp.berkeley.edu/glossary/glossary_3.html
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