The ostracod is a marine or freshwater crustacean with a bivalved carapace that encloses the rest of the body. The valves composing the carapace are heavily calcified and, being an integral part of the exoskeleton, they are shed during each molt. The valves of the new exoskeleton are then calcified while the rest of the exoskeleton remains unmineralized.
SYNOPSIS. The composition of the test of Rosalina floridana (Cushman) was examined histochemically, and its structure was studied with the electron microscope by means of thin sections and carbon replicas. The test is composed of a thick organic lining overlain by one or more calcite layers bounded above and below by thin membranes. The membranes are fused to organic pore processes composed of coarse fibers that penetrate the calcite layers. The ***lining, consisting of coarse fibers matted into a laminated sheet, is considered a strengthening element of the test. The membranes covering each calcite layer are composed of fine, headed fibrils which in aggregate have a striated pattern; they are thought to be the crystal‐nucleating agent during calcification and to form a protective covering for the previously deposited calcite layers. The pore processes, which are devoid of an internal entrance for cytoplasm, are considered to be points of attachment for the membranes; they tie the organic test components into a unified whole. The calcite layers and the chambers lack this unity, being separated from each other and from the preceding chambers by membranes so that there are no calcite‐to‐calcite boundaries between them. An organic, sievelike structure of undetermined function has been found in the foramina of chambers near the prolocular region of the test.
Histochemical methods show that the lining contains proteins, polysaccharides, and unidentified substances; the membranes and the pore processes stain as a protein‐polysaccharide complex free of other substances.
A morphological study of crystal growth during chamber development in Rosalina floridatza was combined with a tracer study of calcium and carbonate uptake during the same developmental periods. The results of the morphological study show that crystallites form within compartments of a membrane complex surrounding the cytoplasmic anlage and coalesce as they increase in size to form larger crystalline units.The chamber wall is completed with a granular layer and a smooth, amorphous finishing layer. All the evidence indicates that calcification is under strict cellular control.The Ca" and C14 tracer studies showed that neither the calcium or carbonate ions used for calcification are stored in the protoplasm prior to calcification; both are extracted from sea water only during calcification.
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