Foraminifera compose a distinct, large taxonomic group within the rhizaria. Most foraminifera live on or within the seabed, but some species are also planktonic and drift in the watercolumn. Many foraminifera build multi-chambered, calcareous housings that can be several millimeters big (or small). These housings - also called tests - are an important part of chalk and can be found in ancient sea floor sediments. Geologists use them to date these sediments and to reconstruct the climate history of the earth.