in medusae the only supporting structure is the jelly inside their cell layers. "hydra" and most sea anemones close their mouths when they are not feeding, and the water in the digestive cavity then acts as a kind of skeleton, rather like a water-filled balloon. other polyps such as "tubularia" use columns of water-filled cells for support. sea pens stiffen the mesogleal jelly with calcium carbonate spicules and tough fibrous proteins, rather like sponges.