The Collaborative International Dictionary of English v.0.48:

Spongin \Spon"gin\ (sp[u^]n"j[i^]n), n. (Physiol. Chem.)
   The chemical basis of sponge tissue, a nitrogenous, hornlike
   substance which on decomposition with sulphuric acid yields
   leucin and glycocoll.
   [1913 Webster]