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The wall structures and surface features of several modern planktonic foraminiferal species were examined with an electron microscope for the purpose of evaluating the taxonomic and ecologic significance of such microstructural details.
Species belonging to spinose Globigerinoides, non-spinose Globoquadrina, and non-spinose Globorotalia were selected because they are three representative taxa among the planktonic Foraminifera.
Surface replicas were made using the Triafol method, which has been found superior to direct shadowing and Polystyrene techniques. Photographs have been taken stepwise from low magnifications with the light microscope to higher magnification electron micrographs in stereo pairs.
In the juvenile stages when the individuals live near the ocean surface, the test wall is thin and transparent and is composed of small calcite crystals with their c-and major growth axes normal to the shell surface. In the later stages when the organisms descend to lower water depths there is additional crystal growth in the form of a calcite crust producing columnar prisms whose free ends are rhombic pyramids. As some of the prisms increase in size, other prisms are squeezed out by differential growth; the rhombic pyramids are especially well developed along the keel and apertural side of Globorotalia menardii and G. truncatulinoides.
Transitional steps in the test thickening of G. sacculifer have been observed from specimens having large, open pores and distinct spine bases to specimens having constricted pores, obscure spine bases, and surficial encrustment of calcite crystals.
Distinguishing between variations in primary test features (e.g., keel, pores, apertural lip) and those due to developmental phases as observed with the electron microscope is complicated by variations due to environmental influences.
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