Fig. 2. Accumulation of acridine orange in an acidic membrane bound space: The photo covers approximately the anterior 50% of the animal. Acridine orange, a weak base, accumulates in acidic granules. Presumably the acridine orange migrates across the membrane and becomes protonated in the low pH. Protonation makes it cationic which prevents it from passing back through the membrane. The dye's color is concentration dependent with higher concentration emitting toward the orange and lower concentration emitting toward the green. Although the entire animal stains with the dye, much higher concentrations can be found in the intestinal granules (punctate yellow and orange staining). Since acridine orange has a wide excitation spectrum one can not selectively excite the dye. The orange strip to the upper left of the photo is the pharynx and mouth, which stain extensively with the dye.

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