text: The name scintillon was first used to describe cytoplasmic particles isolated from a bioluminescent species of dinoflagellate that were able to produce a flash of light in response to a decrease in pH. Scintillons were first observed in L. polyedra by fluorescence microscopy, where they appear as small blue dots close to the cell surface. This blue fluorescence is due to the presence of the bioluminescence reaction substrate, a naturally fluorescent molecule called luciferin. When light production is stimulated by addition of dilute acid to the cells under the microscope, the site of light production corresponds to the location of the scintillons. Furthermore, the natural luciferin fluorescence is reduced after the light producing reaction.