The Bryozoa (plural: bryozoans). They are small colony forming creatures, who form a calcium carbonate (exo)skeleton. In each of the small rooms, there’s an invertebrate animal with tentacles,  filtering nutrients out of the water. The colonies can both branch-shaped, fan-shaped or encrusting the sea surface. Fossil bryozoans occur in sediments dating back to the lower Ordovician period, up to this day. Usually the colonies are quite small. Most species live in the sea, but there are some well-known freshwater species as well.

Example of a bryozoan colony.