Scanning and transmission electron microscopic analysis of the shell and cytoplasm of Assulina muscorum Greef, 1888 show that the siliceous scales of the test are characteristic of other euglyphid species and originate in the posterior portion of the cytoplasm near Golgi bodies. The scales are deposited within vesicles, mature as they move anteriorly, and become stored within enclosing Vesicles in the vicinity of pigment granules. A frothy layer of vacuolated cytoplasm, containing digestive vacuoles with bacteria, extends anteriorly toward the aperture, and occasionally includes small amoebae (c. 2-3 mu m) that appear to be commensals. Reserve bodies, possibly lipid, are more abundant in the posterior cytoplasm surrounding the nucleus. The nucleus varies in shape from approximately circular in sections to elongate or lobate and contains a centrally located nucleolus. In overall view, the fine structure of A. muscorum resembles similar terrestrial euglyphid species such as Tracheleuglypha dentata. Membrane-bound, electron-opaque granules with less dense cylindrical cores may be secretory granules, and appear to be different from those reported in other euglyphid species. The doubling time in laboratory culture is 3.8 to 4.3 days, and the natural growth coefficient is 0.19 to 0.17 depending on the kind of growth medium.
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