A foraminifer in the broadest sense is a cluster of interconnected chambers bounded by a firm wall. The interior contains cytoplasm and the organelles of the organism. Chambers are separated by walls known as septa. These septa are reflected in the exterior of some foraminifera tests as suture lines, which themselves can be diagnostic morphologies for given groups. As the foraminifera grows, new and generally larger chambers are added to accomodate the developing organism. If earlier chambers are still visible the test is termed evolute. It is usually in such cases that suture lines are most likely to be preserved. In evolute test morphologies, earlier chambers are enveloped by later chambers thus preventing sutures from exhibiting themselves on the outer test surface.
Successive chambers may result in a single continuous linear shape , or it may produce complex intersecting shapes as in the arborescent varieties. More commonly, to minimize surface area and maintain structural integrity, the foram will grow into a coil. It may coil into a single symmetric plane described as planispiral. Add a degree of asymmetry to this though, and it will result in a spire-like form known as trochospiral. Each single coil that wraps around the test is known as a whorl.