Most species of foraminifera build shells with multiple chambers (multilocular) but some species build shells with only a single chamber (unilocular). The most common types of chamber arrangements are:
unilocular -- a single chamber
uniserial -- chambers added in a single linear series
biserial -- chambers added in a double linear series
triserial -- chambers added in a triple linear series
planispiral -- chambers added in a coil within a single plane. The center of the coil is called the umbilicus. The coil may be either involute (only the chambers of the last coil visible) or evolute (all chambers visible).
trochospiral -- chambers added in a coil that forms a spire like a snail shell. The side on which all chambers are visible (evolute) is called the spiral side. On the other side only the final coil is visible (involute) and this is called the umbilical side.
milioline -- chambers arranged in a series where each chamber extends the length of the test, and each successive chamber is placed at an angle of up to 180 degrees from the previous one.
fusuline -- a planispiral coil which is elongated along the axis of coiling. Typically each chamber is subdivided by a complex set of internal partitions.
tubular -- a simple hollow tube.
arborescent -- an erect, branching series of tubes. These forms may live attached to a solid surface or "rooted" in sediment.
irregular -- without any definite arrangement of the chambers. These forms usually live attached to a solid surface.
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