Porifera (sponges), class: Calcarea Collection Highlights | Updated 4 years ago Sycon cf. gelatinosum Photo by Clay Bryce, image copyright WA Museum Typically pale in colour and with a tendency to be quite small in size (less than 30cm), this class of sponges lacks spongin and their skeletons are composed entirely of calcium carbonate. As a result, calcareous sponges are quite brittle to touch. These sponges vary in shape from vase-shaped to individuals made up of mesh-like thin tubes, through to irregular masses. They are usually found in shallow water, though one species has been found at a depth of 4000 meters. Marine Invertebrates Section View the discussion thread.