Foraminifers:
Informally known as "forams", these single-celled protists construct skeletons (tests) of highly diverse form. Agglutinated tests are created from particles that the organism selects from the environment. These particles may be cemented together by an organic matrix or with secreted material such as opal or calcite. The most common test material however is Mg-calcite. Shell forms range from simple ovoid enclosures to complex multi-chambered spirals and colonial masses.