Figure 23.6: Sponge Diversity (A) The great majority of sponge species are demosponges, such as this barrel sponge. (B) As in the demosponges, the supporting structures of glass sponges are silicaceous spicules. The pores and water canals “typical” of the sponge body plan are apparent in both (A) and (B). (C) The spicules of calcareous sponges are made of calcium carbonate.