Figure 30.17 Sponge Diversity (A) The majority of sponge species are demosponges, such as these yellow tube sponges. The system of pores and water canals (see Figure 30.2B) that is typical of the sponge body plan is apparent in this photograph. (B) The supporting structures of both demosponges and glass sponges are silicaceous spicules, seen here in the skeleton of a glass sponge. (C) The spicules of calcareous sponges are made of calcium carbonate.