The cytoplasm inside the shell of the monothalamous foraminifer Allogromia laticollaris is loosened and disrupted by a continuous vacuolar system (lacunary system), which extends through the entire cell. This system is separated from the cytoplasm by the cell-membrane and is directly connected to the outer medium (sea-water) across the shell-opening; it is therefore extracellular space. In extreme cases, most of the space inside the shell of cells with extended rhizopodia (reticulopodia) is filled with sea-water. The disruption of the cytoplasm is even more visible at a higher magnification: Numerous fine anastamosing cytoplasmic strands, partly with a diameter of only about 20 m μ, extend through these sea-water filled vacuolar spaces (lacunes). In this way an extreme enlargement of the cell-surface inside the shell is attained. Consequently, the lacunary system must be of importance for exchange processes (respiration, digestion) at this inner cell-surface. In addition, this system serves as a reservoir for the rhizopodia-material, not melted into a compact mass together with the cytoplasm inside the shell during rhizopodia withdrawal, but stored in the form of strands. Since the cytoplasm inside the shell, as well as within the reticulopodia, always exists in a vacuolar strand-like form, extension and withdrawal of the rhizopodia can take place without essential changes of the cytoplasmic structure and, therefore, can occur very fast. Within the shell A. laticollaris shows four differently structured cytoplasmic regions. In cells with extended reticulopodia these regions can be characterized by the different degree of extension of the lacunary system. The nucleus is surrounded by an extremely thin cytoplasmic layer and lies, often nearly isolated, within a large lacune of the vacuolar system. The transparent, flexible shell, consisting partly of acid mucopolysaccharides, is composed of filament-like structural elements embedded in a glue-like substance. © 1969 Biologischen Anstalt Helgoland.
