After this belt has been
formed, the canals nearest to it proceed to darken, and those a
little farther off follow suit, and so the
wave of visibility rolls in regular routine down the disk.
The sponge's cells, its calcium carbonate or glasslike silica spicules, and the mass of collagen that
forms its visible body all create a network of tunnels and chambers, with
little flailing hairs called cilia on the walls that
wave the water through and filter out plankton and waste.