Extremely ancient animals, sponges have displayed following Precambrian times, 540 million years ago, and have changed little since that time. The familiar bathroom sponge is made from a sponge skeleton.
Sponges exist principally connected to the seabed. They are upheld by a skeleton lying among the external and inward layer of units of the form. The skeleton is made of calcium carbonate, spongin (a protein) or glasslike silica, depending to the type of sponge. The large number of hard skeletal parts in sponge bodies means there are many fossils to explore.
