The tough protein fibers sometimes found in sponge skeletons are of a substance called _____________.
Sponge skeletons are comprised of calcareous or siliceous spicules or spongin.

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The Answer of The tough protein fibers sometimes found in sponge skeletons are of a substance called _____________.
Sponge skeletons are comprised of calcareous or siliceous spicules or spongin.

The Tough Protein Fibers Sometimes Found in Sponge Skeletons Are