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 .
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 .