IN your issue of January 9 (p. 223), an account of some observations of the Geophilus is given, from which it would appear that it used its power of emitting light as a means of protection. It might be well to point out that irritation or excitation of many luminous organisms has this result. Even in such low forms as the light-producing bacteria the same effect can be seen. In a paper on the “Physical Basis of Animal Phosphorescence,” by S. Watase, of the University of Chicago, published with the biological lectures delivered at Woods Holl, 1895, a very full account is given of the phenomenon as seen in the ordinary fire-fly, and the process is essentially the same in all light-producing organisms. In some the luminous product of cell metabolism is oxidised in situ, while in others it is thrown out in response to a stimulus as a liquid secretion.