Foraminiferan shells, or tests, are built of hollow chambers separated by partitions, with small openings called foramina that connect the chambers (they get their name from these foramina). The final chamber (the last one added) has an opening or openings to the exterior, called the aperture. The living organism fills all the chambers in its shell except for one or two of the most recently constructed chambers. Most species build shells with multiple chambers (multilocular) but some species build shells with only a single chamber (unilocular). Click here to learn more about the most common types of chamber arrangements.
Each of the major groups of foraminifera uses different materials to build their shells. The basic types of wall structures are: