Nature & Environment
Life in the Palaeozoic
...system. They have a skeleton of calcium carbonate, silica, or, as in some bath sponges, horny organic material. Water passes in through the sponge's many surface pores, often to the central cavity of a sack-like body, and out through a large hole at the top. Some have a stalk (Figure 1a), others are encrusting and irregular in shape. Sponges feed by filtering off minute...