We may also divide luminous forms into two groups according as the oxidation of luminous material goes on continuously, independently of any stimulation of the organism; or is intermittent, oxidation and luminescence occurring only as a result of stimulation, using the word "stimulation" in the same sense in which it is used in connection with nerve or muscle tissue. Bacteria, fungi, and a few fish produce light continuously and independently of stimulation. Its intensity varies only over long periods of time and is dependent on the nature of the nutrient medium or general physiological condition of the organism. All other forms give off no light until they are stimulated. Stimulation may of course come from the inside (nerves) or outside. Only under unfavorable conditions, such as will eventually lead to the destruction of the luminous cells, do these forms give off a continuous light. This has often been spoken of as the "death glow," and is to be compared with _rigor_ in muscle tissue. Some of the fish which produce a continuous light possess a movable screen similar to an eyelid which can be drawn across the organ, thus shutting off the light, so that the animal appears to belong to the group which flashes on stimulation. This is true of _Photoblepharon_, while _Anomalops_ can rotate the light organ itself downward, so as to bring the lighting surface against the body wall and thus cut off the light (Steche, 1909). Other fish (_Monocentris_) are unable to "turn off" their light. Animals which flash spontaneously on stimulation through nerves from within, possess a very varied rhythm. The different species of fireflies can be distinguished by the character of their flashing (McDermott, 1910-17; Mast, 1912). Fig. 16 shows the method of flashing of some common eastern North America species. The glowworm light lasts for many seconds and then dies out. This interval of darkness persists for some minutes and is then followed by another period of glowing. Some fireflies have a light which may be described as partially intermittent. It lasts for hours, but may become more dim or be intensified on stimulation. [Illustration: FIG. 16.--Chart showing relative intensities and durations of flashes of American fireflies (_after McDermott_). One cm. vertically = approximately 0.02 candle power; one cm. horizontally = approximately one second. The flash of the males (♂) is at the left; that of females (♀) at right of chart.] Some forms only produce light at certain seasons of the year. According to Giesbrecht (1895) this is true of the copepods, which only light in summer and autumn, and according to Greene (1899) in the toad-fish; _Porichthys_, which can only be stimulated to luminesce during the spawning season in spring and early summer.