Original: Skeleton  The calcium carbonate or silica spicules of most sponge genera make them too rough for most uses, but two genera, Hippospongia and Spongia, have soft, entirely fibrous skeletons.
Simplified: The calcium carbonate or silica spicules of most sponges make them too rough for most uses, but two genera, Hippospongia and Spongia, have soft, entirely fibrous skeletons.