sponge (spŭnj)
n.
Any of numerous aquatic, chiefly marine invertebrate animals of the phylum Porifera, characteristically having a porous skeleton composed of fibrous material or siliceous or calcareous spicules and often forming irregularly shaped colonies attached to an underwater surface.
Hmm..does'nt say anything about being soft, in fact it says calcareous.
cal·car·e·ous (kăl-kâr'ē-əs)
adj.
Composed of, containing, or characteristic of calcium carbonate, calcium, or limestone; chalky.