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 .