Foraminiferans
The Foraminifera ("hole bearers"), or forams for short, are a large group of amoeboid protists with reticulating pseudopods, fine strands of cytoplasm that branch and merge to form a dynamic net.They typically produce a test, or shell, which can have either one or multiple chambers, some becoming quite elaborate in structure.These shells are made of calcium carbonate (CaCO3) or agglutinated sediment particles. About 275,000 species are recognized, both living and fossil. They are usually less than 1 mm in size, but some are much larger, and the largest species reaching up to 20 cm.
See: http://en.wikipedia.org/wiki/Foraminifera
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Česky 
Virtual museum of the Czech Geological Survey, www.geology.cz, (C) Czech Geological Survey, 2011, v.0.99 [13.12.2011]