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Sponges are the simplest multicellular animals. In fact, a sponge is a kind of 'partnership' of a limited number of specialised unicellular organisms that cannot live without each other. A sponge is strengthened by a skeleton consisting of spicules of silica, calcium carbonate or a horn-like substance (as in our 'bath sponges'). Since sponges can be very variable in shape (even within a single species a wide range of variation occurs) they cannot be used as guide fossils. In the Maastricht Chalks the isolated, four-pointed 'needles' of Propagastella may be found, but you need a microscope for this: they measure only one millimetre in length. Larger, up to c. 5 cm, are species of the boring sponge Cliona. These sponges make excavations in substrates (shells, corals etc.). These bubble-shaped chambers connect with sea water via 'chimneys'.