Figure 4: Clear vesicle of Aplysia californica ((a)–(c)) and white vesicle of A. juliana ((d)–(f)). (a) An electron-lucent lumen surrounded by a thin layer of cytoplasm bordered by muscle. The cytoplasm frequently had an electron dense material (arrows) against it. ((b) and (c)) Parallel arrays of sheets (arrows) contained within a cytoplasmic vacuole (c) within a clear vesicle. (d) Vesicle contents are frequently separated into three zones of decreasing density, the one closest to the muscle wall containing protein crystals. (e) Vesicle cell membrane highly infolded with the folds (white arrows) containing filamentous, low electron dense material. (f) Protein crystals are not enclosed by a membrane. m, muscle; mi, mitochondrion; p, crystalline protein; v, vacuole. Scale bars: (a) = 2 μm; ((b) and (c)) = 200 nm; (d) = 2 μm; (e) = 0.5 μm; (f) = 0.25 μm.