Cellular Ultrastructure

Abstract

Although the shell is one of the most obvious features of the living and dead planktonic foraminifers, the cytoplasm is clearly of central biological importance as the site of the dynamic properties of life including: Transformation of the information stored in the genetic code into substantive morphology, metabolism, sensitivity, and activity; and shell deposition and reproduction. Moreover, comparative cytoplasmic fine structural studies of living planktonic and benthic foraminifera and their relations to other Sarcodina may provide evidence of phylogenetic affinities and help to elucidate evolutionary origins in concert with data on shell morphology. To a large extent, the shell and cytoplasm are complementary in form and dynamically interrelated both in the process of calcite deposition, which is clearly determined by the activity of the cytoplasm, and subsequently in the intimate topological association of the cytoplasm with the surfaces and enclosing spaces of the shell. The close morphological complementarity of shell and cytoplasm is clearly visualized in an overall view of a light microscopic and electron microscopic image of a section through a planktonic foraminifer (Fig. 4.1). Much of the inner shell space is filled by the vacuolated cytoplasm, but the final chamber may be incompletely filled with cytoplasm, owing either to poor nutrition or lack of health, thus reducing the total cytoplasmic mass.