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We explored reconstructing the internal structure of an individual of the extant alveolinid larger benthic foraminifer Borelis schlumbergeri Reichel (1937) using high-resolution hard X-ray microtomography (µCT). Chamber segmentation analysis revealed a complex void architecture once filled by the living protoplasm beginning with a subsphaerical embryo (proloculus), surrounded by a streptospiral nepionic structure. It begins with Saturn-ring like, curved tubes perpendicular to each other. In late nepionic stage, they split up into an increasing number of chamberlets. Adult growth begins when streptospiral winding changes to planispiral coiling and the test starts stretching into the typical alveolinid fusiform shape. In the investigated individual the terminal chamber #48 is subdivided in 71 chamberlets. In the terminal stage, chamber volume is reduced and chamber growth becomes irregular. The cell volume increases 7,584-fold from a prolocular size of 13,592 µm 3 to reach a cell volume of 103,077,248 µm 3 , at a length of the protoplasmic body of 1,995 µm. µCT impressively illustrates the adaptive advantage of planispiral fusiform winding for squeezing a giant protoplasmic sheet into a compact and mechanically robust body without losing cellular connection from the interior to its periphery and still allowing endosymbionts to maintain photosynthetical function under high-energy shallow marine environments. The µCT data are compared with historical drawings and plasticine models of early alveolinid growth stages fabricated by micropaleontological pioneer Manfred Reichel in the 1930s, highlighting the unprecedented quality of structural analysis during those early days.
