Most foraminifers construct tests (shells) covering the cell body. Composition of tests varies from organic, through agglutinated (cemented foreign grains) to calcareous secreted from calcium carbonate.
The size of the test is associated with amount of the cytoplasm. "Feeding adds to the bulk of the cytoplasm" (Armstrong & Brasier 2005), therefore, the test has to enlarge. Foraminifera have developed several strategies for test enlargement during ontogeny (after Armstrong & Brasier 2005):
- contained growth (see unilocular tests forms)
- simple septate growth (see simple multilocular tests)
- complex septate growth (see complex multilocular tests)
These growth patterns result in a very diverse variety of test morphologies.