In the pearl-like organs, also, if we have understood Professor Leydig's description aright, a curved, refracting body seems to lie on the side of the organ that is turned outward. We should thus, if our presumption is confirmed, have here not a simple illuminating organ, but a complete optical illuminating apparatus in different degrees of perfection, throwing out in an extremely concentrated condition, by means of a concave mirror and lenses, the phosphorescent light generated within it; and the fishes under consideration would be fully equipped with a series of little, button-shaped illuminating apparatuses.
I may assert here that there is nothing hazardous in this idea. As Professor Leydig has maintained, the "eye-like," and the" pearl-like," and the really luminous organs, are of thoroughly homologous structure, and we know of the latter, the only ones that have been observed in a living animal, that they emit a star-clear light. If, now, Nature has provided us with a most wonderful camera-obscura in our eyes,
Fig. 7.—Caudal Extremity of Scopelus Humboldtii, with "glass-pearly" organs, and a large pearl-spot.
why may she not also have produced a much simpler light-house lantern—provided, of course, that such an apparatus could be useful to the animal? I have already had something to say concerning the uses of their luminous apparatus to different animals ("Kosmos," vol. vii, p. 479), and have endeavored to show that their principal service is probably as a means of exciting fear. At any rate, the opinion may be given up that the light diffused by the deep-sea animals is a means of clearing up the purple darkness below, or, as some have thought, of producing the diversified hues of the deep-sea animals. Animals living in the dark do not require light for their existence, as is demonstrated by the numerous blind cave-animals. The opinion, also, that the luminous fish make their prey in any way visible by means of the organs subsidiary to their eyes could not in any degree help to account for the existence of luminous apparatus on the lower part of their bodies, for their eyes would not be able to see what those organs lighten up; but such organs might very well make the animal more visible from a distance, and might thereby serve a similar purpose with the protective colors of the animals of the upper world, especially if the appearance were associated with a disagreeable taste or smell. Only in some such manner as this can we account for the luminous organs, for example, of a crustacean that was brought up by the Challenger Expedi-
