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THE POPULAR SCIENCE MONTHLY. 



In the pearl-like organs, also, if we have understood Professor Ley- 

 dig'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 pre- 

 sumption is confirmed, have here not a simple illuminating organ, but 

 a complete optical illuminating apparatus in different degrees of per- 

 fection, 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 ap- 

 paratuses. 



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 struct- 

 ure, 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 Extkemitt of Scopslus Humboldiii, with "glass-pearly" organs, and a large 



pearl- spot. 



why may she not also have produced a much simpler light-house lan- 

 tern 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 ("Kosnios," vol. vii, p. 

 479), and have endeavored to show that their principal service is proba- 

 bly 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 clear- 

 ing up the purple darkness below, or, as some have thought, of pro- 

 ducing 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 lu- 

 minous 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 protect- 

 ive colors of the animals of the upper world, especially if the appear- 

 ance were associated with a disagreeable taste or smell. Only in some 

 such manner as this can we account for the luminous organs, for ex- 

 ample, of a crustacean that was brought up by the Challenger Expedi- 



